PNEUMOCOCCAL SEROTYPE 6D
Disclosed is a new and emerging serotype of Streptococcus pneumoniae designated serotype 6D, and assays and monoclonal antibodies useful in identifying same. Also disclosed is a novel pneumococcal polysaccharide with the repeating unit→2) glucose 1 (1→3) glucose 2 (1→3) rhamnose (1-4) ribitol (5→phosphate. This new serotype may be included in pneumococcal vaccines.
This application is a divisional of U.S. patent application Ser. No. 13/866,857, filed Apr. 19, 2013, now U.S. Pat. No. 8,945,568, which is a divisional of U.S. patent application Ser. No. 12/601,896, filed May 4, 2010, which is related to and claims the benefit of PCT Application No. U.S. Ser. No. 08/064,951, filed May 28, 2008, and U.S. Provisional Patent Applications No. 60/924,703 and No. 60/924,704, both filed May 29, 2007.
FEDERAL FUNDINGThis invention was made with U.S. governmental support under Contracts No. AI 30021 and No. AI-031473 awarded by the National Institutes of Health. The U.S. government has certain rights in this invention.
FIELD OF THE INVENTIONThis invention relates to bacteriology, immunology, and epidemiology. More specifically, this invention relates to new and emerging serotypes of Streptococcus pneumoniae and assays and monoclonal antibodies useful in identifying these serotypes.
BACKGROUNDStreptococcus pneumoniae is a well known human pathogen and a major etiologic agent for pneumonia, meningitis, otitis media as well as sepsis, among primarily young children and older adults. S. pneumoniae has been divided into ninety serotypes based on its expression of serologically distinct carbohydrate capsules. Antibodies to a capsular polysaccharide (PS) may provide protection against pneumococci expressing the same capsular serotype. Currently available pneumococcal vaccines contain a mixture of capsular PS of multiple serotypes. For example, one pneumococcal vaccine (called PS vaccine) contains capsular PS from twenty-three commonly found serotypes. The most recently developed type of vaccine (called conjugate vaccine) contains capsular PS from seven to thirteen serotypes that are conjugated to a protein molecule. A seven-valent conjugate vaccine was introduced in 2000 for clinical use in the USA and has reduced the incidence of invasive pneumococcal diseases in children and in adults.
The distribution of pneumococcal serotypes is useful in estimating vaccine efficacy. Ideally, an effective pneumococcal vaccine would reduce the prevalence of pneumococci expressing the serotypes included in the vaccine and leave the prevalence of the pneumococci expressing non-vaccine serotypes the same. In reality, the prevalence of the pneumococci expressing non-vaccine types increases to replace those expressing the vaccine serotypes. Further, the prevalence of specific serotypes may change over time for unknown reasons. Consequently, accurate and efficient serotyping of pneumococcal isolates is important for monitoring the efficacy of pneumococcal vaccines. Indeed, identifying emerging pneumococcal serotypes and producing more efficacious pneumococcal vaccines remain crucial goals in public health.
SUMMARY OF THE INVENTIONAn embodiment of the present invention provides for the identification of a new and emerging pneumococcal serotype and means for identifying same. More specifically, the present invention provides for a novel pneumococcal serotype closely related to serotypes 6A, 6B, and 6C, identified herein as serotype 6D.
An additional feature provides for an isolated culture of a bacterium designated Streptococcus pneumoniae 6D.
Another embodiment provides for a novel pneumococcal capsular polysaccharide with the repeating unit {→2) glucose 1 (1→3) glucose 2 (1→3) rhamnose (1→4) ribitol (5→phosphate}, which corresponds to S. pneumonia serotype 6D. A related embodiment provides for an antigenic composition comprising the novel 6D polysaccharide. Another embodiment relates to antigen-binding proteins, such as antibodies, specific for serotype 6D or serotype 6D polysaccharide.
Another feature provides for monoclonal antibodies (mAbs) useful in identifying emerging pneumococcal serotypes, particularly monoclonal antibodies that distinguish serotype 6D, identified here as mAb Hyp6BM6, mAb Hyp6BM7, and mAb Hyp6BM8.
In step A, wchA/wciNα/wciO-P region of TIGR6A4 was replaced with Cassette 1. Cassette 1 has three parts (central core and two flanking regions) and each part is about 1 kb long. The central core has antibiotic susceptibility genes, kanR and rpsL+. The two flanking regions were made with wchA and wciO-P regions from AAU33 strain. In step B, Cassette 1 in TIGR6AX was replaced with Cassette 2. Cassette 2 has wciNβ gene, wchA and wciO-P regions from a 6C strain (CHPA388, regions labeled Aβ). TIGR6C4 shows the final product that is obtained after Cassette 2 is inserted. XbaI and BamHI sites in the PCR primers, which were introduced to simplify genetic manipulations, are shown.
It should be understood that this invention is not limited to the particular methodology, protocols, and reagents, etc., described herein and as such may vary. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention, which is defined solely by the claims.
As used herein and in the claims, the singular forms “a,” “an,” and “the” include the plural reference unless the context clearly indicates otherwise. Thus, for example, the reference to an antibody is a reference to one or more such antibodies, including equivalents thereof known to those skilled in the art. Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein should be understood as modified in all instances by the term “about.” All patents and other publications identified are expressly incorporated herein by reference for the purpose of describing and disclosing, for example, the methodologies described in such publications that might be used in connection with the present invention. These publications are provided solely for their disclosure prior to the filing date of the present application. Nothing in this regard should be construed as an admission that the inventors are not entitled to antedate such disclosure by virtue of prior invention or for any other reason. All statements as to the date or representation as to the contents of these documents is based on the information available to the applicants and does not constitute any admission as to the correctness of the dates or contents of these documents.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as those commonly understood to one of ordinary skill in the art to which this invention pertains. Although any known methods, devices, and materials may be used in the practice or testing of the invention, the methods, devices, and materials in this regard are described here.
The present invention provides for Streptococcus pneumoniae with a novel polysaccharide capsule layer. Briefly, serogroup 6 of S. pneumoniae includes two serotypes named 6A and 6B with highly homologous capsule gene loci. Recently, and as described herein, serotype 6C was identified. The 6A and 6B capsule gene loci consistently differ from each other by one nucleotide in the wciP gene. Additionally, the 6A capsule gene locus has a galactosyl transferase, but the 6C capsule gene locus has a glucosyl transferase. The present invention provides for a new serotype named “6D”, the in which the galactosyl transferase of the 6B capsule gene locus has been replaced with the glucosyl transferase of 6C. The gene transfer yields a viable pneumococcal strain, and the capsular polysaccharide from this strain has the predicted chemical structure and serologic similarity to the capsular polysaccharide of the 6B serotype. The new strain (i.e., serotype 6D) is typed as 6B by quellung reaction, but it can be distinguished from 6B strains with monoclonal antibodies to 6B polysaccharides. Reexamination of 264 pneumococcal isolates that were previously typed as 6B with classical typing methods revealed no isolates expressing serotype 6D.
The novel 6D isolate provided for herein has a chemically distinct capsular polysaccharide (PS) structure. More specifically, the repeating units of the 6D PS apparently contain one ribitol, one rhamnose, and two glucose moieties. This is the same content as serotype 6C, however the chemical linkages of the carbohydrate moities is different in these serotypes. Further, the distinct chemical structure of the PSs are antigenically distinct. Serotype 6D thus represents the 92nd pneumococcal serotype, with 90 pneumococcal serotypes having been previously recognized, (Henrichsen, 33 J. Clin. Microbiol. 2759-62 (1995)), and the 6C serotype representing the 91st serotype.
Importantly, the novel 6D serotype provided for herein offers an important alternative PS for vaccine development. More specifically, the current pneumococcal vaccines contain 6B PS rather than 6A PS because, in part, 6A PS was not stable enough in vaccine preparations. This chemical instability was due to the 1→3 linkage between rhamnose and ribitol in the 6A PS. Because the 1→4 linkage of 6B PS is more stable than the 1→3 linkage of 6A PS, 6B PS is more stable than 6A PS. The 6C linkage, provided for herein, has the more unstable 1→3 linkage whereas the 6D PS has the more stable 1→4 linkage. Thus, 6D PS may prove even more useful in a vaccine than 6C PS.
Regarding the importance of vaccine, S. pneumoniae is a well known human pathogen and a major etiologic agent for pneumonia, meningitis, otitis media as well as sepsis, primarily among young children and older adults. Fedson, V
S. pneumoniae has been divided into 91 serotypes based on its expression of serologically distinct chemistry of the PS capsules. Henrichsen, 1995; Park et al., 45 J. Clin. Microbiol. 1225-33 (2007). Antibodies against the PS provide serotype-specific protection from infection, and current vaccines against pneumococcus incorporate capsular PS of the most prevalent strains. Cole, 61 JAMA, 663-66 (1913). Serogroup 6 strains are very common in invasive pneumococcal disease and the current vaccines are formulated to protect against serogroup 6 infections. Hausdorff et al., 30 Clin. Infect. Dis. 100-21 (2000).
Accurate efficient serotyping pneumococcal isolates is important for measuring the efficacy of pneumococcal vaccines. Following the introduction of a new pneumococcal vaccine, pneumococci expressing the serotypes included in the vaccine become less common while the prevalence of the pneumococci expressing non-vaccine types may stay the same. In some cases, pneumococci expressing the non-vaccine types replace those expressing the vaccine serotypes and the prevalence of non-vaccine types may become higher. Pelton, 19(1) Vaccine S96-S99 (2000). Further, the prevalence of serotypes can change over time for unknown reasons. Finland & Barnes, 5 J. Clin. Microbiol. 154-66 (1977). Because these changes influence the clinical effectiveness of a vaccine, serotyping of a large number of pneumococcal isolates is an important part of monitoring pneumococcal vaccines.
Moreover, regarding S. pneumoniae serotype 6A, current vaccine formulations do not carry a 6A PS, but carry the 6B PS, and the antibodies raised against 6B are thought to cross react against 6A. This phenomenon, is not 100%, however: Some vaccines that include the 6B PS do not raise antibodies against 6A. Yu et al., 180(5) J. Infect. Dis. 1569-76 (1999). Indeed, it appears that non-vaccine serotypes such as 6A are still causing disease in vaccinated children. Clover & Klein, Strategies for Prevention & Treatment of Pneumococcal Disease, 44th Ann. ICAAC Meeting (Washington, DC, 2004). Hence, the emergence of additional group 6 serotypes may become even more important.
Further, the 6A and 6B serotypes account for 4.7% and 7%, respectively, of invasive pneumococcal diseases. Robbins et al., 148 J. Infect. Dis. 1136-59 (1983). Biochemical studies found that serotypes 6A and 6B PS include linear polymers of a repeating unit containing four monosaccharides: rhamnose, ribitol, galactose, and glucose. Kamerling, in S. pneumoniae: M
Although there are various serotyping methods well known in the art, they are largely manual, slow, and tedious to perform. An improved serotyping “multibead assay”, based on a multiplexed immunoassay that can be performed semi-automatically with a flow cytometer has been described. Park et al., 7 Clin. Diagn. Lab. Immunol. 486-89 (2000). The multibead assay's specificity has been fully established using pneumococcal strains representing all ninety known serotypes. Yu et al., 43(1) J. Clin. Microbiol. 156-62 (2005). This assay provides superior specificity, using many mAbs specific for pneumococcal capsular PS, and is largely automated and can provide a high throughput. Consequently, the assay may be useful in many epidemiologic studies.
The multibead assay is particularly advantageous because monoclonal antibodies are more specific than polyclonal reagents. Regarding 6A serotypes, although most “6A” isolates (defined by quellung reaction and polyclonal reagents) reacted with 6A-specific monoclonal antibodies (Hyp6AG1, Hyp6AM6, and Hyp6AM3), some “6A” isolates reacted with one mAb (Hyp6AG1) but not others (Hyp6AM6 or Hyp6AM3). Other tests described herein confirmed that the 6A isolates that did not react with Hyp6AM6 or Hyp6AM3 were a previously unidentified 6A subtype. In other words, the monoclonal antibodies recognized subtypes within the 6A serotype. See Lin et al., 44(2) J. Clin. Microbiol. 383-88 (2006). Initially, the isolates reacting with both mAbs were identified as 6Aa and those reacting with only Hyp6AG1 were labeled as 6Aβ, but subsequently, but subsequently it was proposed that the 6Aa remain 6A, and the new serotype be identified as 6C rather than 6Aβ. Id.; Park et al., 45 J. Clin. Microbiol. 1225-33 (2007). Thus both 6Aβ and 6C may be used herein and are equivalent. Monoclonal antibodies useful for identifying 6D include those designated Ab Hyp6BM6, mAb Hyp6BM7, and mAb Hyp6BM8.
Further regarding 6C, a consideration in defining a new serotype is its binding characteristics with human antibodies. The various 6A, 6C, and 6B isolates were compared using an opsonization assay and a human serum with a high level of anti-6B antibodies. Although the human serum opsonized 6B and 6A (
Genetic studies report that pneumococci expressing either 6A or 6B serotype have almost identical capsule gene locus (CGL) of about 17.5Kb in size. Sequence information is available on-line at, for example, the Sanger Institute's Sequencing Genomics Projects site. A consistent difference exists in the wciP gene that encodes for rhamonosyl transferase. Mavroidi et al., 186 J. Bacteriol. 8181-92 (2004). The serotype 6A wciP gene encodes serine at residue 195 but the serotype 6B gene encodes asparagine at that residue. Id.
Genetic studies also confirmed that the 6C isolates were, indeed, members of the 6A serotype (rather than the closely related 6B serotype or some other unrelated serotype). In a study of ten isolates collected from Brazil, Korea, and the U.S., all ten isolates identified as 6C had the serine at residue 195, consistent with the wciP gene in serotype 6A. DNA sequences of the wciP gene of several pneumococcal isolates are presented in
The nucleotide sequences of the PCR products from one 6A isolate (AAU9) and one 6C isolate (ST745) were compared (
Note that the capsule gene locus of 6C is very similar to the 6A locus except for the wciN gene: 6A strains have the wciNα gene, but 6C strains have the wciNβ gene. The two genes differ in sizes, thus 6A and 6C serotypes can be readily distinguished by PCR. The wciNα gene encodes WCINα with 314 amino acids, while the wciNβ gene produces a 1125 base-long ORF and its product, WCINβ has 374 amino acids. Additionally, these two proteins have little homology at the amino acid level.
Sequences of the putative wciN gene products suggest their glycosyl transferase functions. WCINβ has similarity to the staphylococcal capH gene product and has a 160-amino acid-long transferase domain that belongs to glycosyl transferase group 1 family. In contrast, WCINα belongs to glycosyl transferase family 8 (ex), which includes many galactosyl transferases. Campbell et al., 326 Biochem. J. 929-39 (1997). The present studies show that wciN is responsible for the difference between the 6A and 6C bacteria, as substitution of wciN6C (wciN of 6C) for wciN6A (wciN of 6A) through homologous recombination appears to have caused the serotype switch from 6A to 6C. Park et al., 75 Infect. Immun. 4482-89 (2007). Indeed, a 6A strain can be converted to a 6C strain by replacing the wciNα gene with the wciNβ gene.
Interestingly, the galactose/glucose exchange observed for 6A and 6C is found for other pneumococcal serotypes. The 9L serotype PS of pneumococcus has a galactose molecule, but 9N PS has a glucose molecule. The capsule gene loci of the 9L and 9N serotypes resemble each other but differ in one gene, wcjA, which encodes a galactosyl transferase for 9L and a glucosyl transferase for 9N. The wcjA genes of the 9L and 9N serotypes are very similar; it is likely that one arose from the other by mutation.
In contrast, the wciNα and wciNβ genes are very different, and the wciNβ gene is not homologous to any other pneumococcal genes available in databases. Perhaps, the wciNβ gene may have originated from an organism other than pneumococci, a notion supported by the wciNβ gene's two flanking regions which may have participated in homologous recombination and which are known to be critical for homologous recombination in pneumococci. Prudhomme et al., 99 P.N.A.S. USA 2100-05 (2002). Additionally, studies of antibiotic-resistance genes have shown horizontal gene transfers between S. pneumoniae and another bacterial species. See, e.g., Feil et al., 151(6) Res. Microbiol. 465-69 (2000); Muller-Graf et al., 145(11) Microbiol. 3283-93 (1999); Coffey et al., 5(9) Mol. 2255-60 (1991).
Additionally, a part of the wciNβ gene is similar (81% homology) to the EpsG gene, a gene involved in the synthesis of exopolysaccharide by S. thermophilus. The homology is found for only a very short piece of DNA, however, thus, S. thermophilus may not be the source for wciNβ. The protein sequence of WCINβ resembles the waaG (rfaG) gene product of E. coli K-12 strain and some pneumococcal genes may have come from Gram-negative organisms. Thus, it is possible that the wciNβ gene could have come from a Gram-negative species as well. Nevertheless, S. salivarius, S. mitis, and S. oralis are the leading candidates because they co-exist in the oral cavity with pneumococci and many antibiotic-resistance genes have been linked to S. oralis.
When the wciNβ region was examined for multiple 6C isolates, their cross-over points and flanking region sequences were found to be identical. Also, their capsule gene locus profiles are highly limited to 9-10-1 in contrast to 6A isolates, which have many different capsule gene locus profiles. Mavroidi et al., 2004. In addition, the 9-10-1 capsule gene profile is unusual among, and largely segregated from, the capsule gene profiles of the 6A and 6B isolates, findings indicating that the capture of the wciNβ gene must have taken place, and that all the 6C isolates found through out the world, and causing many types of diseases have the capsule gene locus from the single bacterium that originally became 6C. Because 6C may provide a unique and clear example of foreign gene capture, it would be a good model for studying bacterial genetic evolution. This may also constitute a stable change, unlike antibiotic resistance genes.
The 6C serotype has only one or two capsule gene locus profile(s) whereas the 6A and 6B serotypes have diverse capsule gene locus profiles. Mavroidi et al., 2004. Thus, the 6C capsule gene locus may have appeared much more recently compared with the 6A or 6B capsule gene loci. Although 6C may have appeared more than twenty-seven years ago, these findings suggest the 6C serotype capsule gene locus appeared “recently” in one place and spread quickly through out the world. When a gene provides strong survival advantage, the gene can spread quickly throughout the world. For example, an antibiotic-resistance gene may spread worldwide within only years. Perhaps natural human antibodies are less effective against 6C than against 6A or 6B. Whether the 6C capsule gene locus provides more survival advantage than 6A or 6B may be investigated.
MLST studies show that 6C expresses multiple independent STs, thus the 6C capsule gene locus must have been exchanged among different pneumococcal isolates. Whether the 6C capsule gene locus may combine with a ST that provides additional survival advantages may be investigated. The spread of 6C and the emergence of the 6C capsule locus among international strains that have multiple resistance genes should be monitored.
As noted above, the 6C pneumococcal isolate has a chemically distinct PS structure. More specifically, monosaccharide analysis indicated that the galactose found in the 6A capsular PS is absent in the 6C PS, which contains glucose instead. The repeating units of the 6C PS apparently contain one ribitol, one rhamnose, and two glucose moieties. Serotype 6C is the third member of serogroup 6 in view of its serological and structural relation to serotype 6A, and represents the 91st pneumococcal serotype.
Galactose and glucose molecules differ only in the orientation of the hydroxyl group attached to their fourth carbon, and the repeating units of 6A and 6C PS differ only in the orientation of one hydroxyl group. This small structural difference explains why 6C was not identified previously with polyclonal antisera. With the elucidation of the chemical structure, 6C can be distinguished biochemically from 6A by carbohydrate composition analysis or by simple proton NMR of anomeric protons. Abeygunawardana et al., 279 Anal. Biochem. 226-40 (2000). Although 6A NMR and 6C NMR patterns do differ, the NMR pattern of the anomeric protons of 6C is very similar to that of 6A. Although chemical and genetic tests can be used, serological methods may be the most useful way to identify 6C using either monoclonal antibodies or polyclonal antisera made specific by absorption.
Serogroup 6 has been known to contain three epitopes: 6a, 6b, and 6c. Henrichsen, 1995. Epitope 6a is known to be present in both serotypes 6A and 6B whereas epitopes 6b and 6c are found only in either serotype 6A or 6B, respectively. Discovery of the 6C serotype indicates the presence of additional epitopes within serogroup 6. The mAb Hyp6AM3, which recognizes 6A and 6B but not 6C, should recognize epitope 6b. Because mAb Hyp6AG1 recognizes 6A and 6C, it may be defined as recognizing a new epitope “6d”. Another mAb binding to all three serotypes (6A, 6B, and 6C) and the shared epitope may be defined as “6e”. A confirmation-dependent epitope for serotypes 6A and 6B has also been described. Sun et al., 69 Infect. Immun. 336-44 (2001). The observation of so many epitopes for serogroup 6 is consistent with a previous observation that even a simple linear homopolymer of sialic acid can have at least three epitopes. Rubenstein & Stein, 141 J. Immunol. 4357-62 (1988). Indeed, pneumococcal PS have many more epitopes than previously defined (Henrichsen, 1995), and that the presence of many epitopes increases chances of altering epitopes during the manufacture of pneumococcal conjugate vaccines.
The discovery of serotype 6C was quite unexpected because serogroup 6 has been extensively studied following its discovery in 1929. Heidelberger & Rebers, 1960. One should therefore consider the possibility that additional subtypes (or serotypes) are present among even well-established and extensively characterized serogroups. For instance, one may need to consider the possible presence of subtypes among serotype 19A because two chemical structures for the 19A capsular PS have been reported. Kamerling, Pneumococcal polysaccharides: a chemical view, in M
The discovery of 6C increases the evolutionary potential of serogroup 6 capsule gene locus by the logical possibility for a new member of serogroup 6. The new member provided for herein, labeled “6D”, comprises the wciP of 6B and wciN6c. Chemically, the 6D PS has a glucose instead of galactose, and has a 1→4 rhamnose-ribitol linkage. It was unclear, before the present invention, if this novel serotype existed or might emerge in nature. Therefore, the serotype 6D strain was used to examine a laboratory collection of pneumococcal isolates for a 6D strain.
The 6C serotype is also useful to monitor vaccine efficacy: The 6A and 6C serotypes must be distinguished in epidemiological studies involving pneumococcal vaccines and in studies of pneumococcal vaccine efficacy. For example, if a pneumococcal vaccine is effective against 6A but not 6C, the vaccine may not be effective in areas where the 6C serotype is prevalent. This would be the case because pneumococcal vaccines elicit antibodies opsonizing 6C only occasionally. Also, usage of conventional pneumococcal vaccines may well alter the prevalence of 6C: the prevalence of 6C may increase although the prevalence of 6A decreases. Preliminary data shown below suggests that 6C prevalence is unchanged whereas 6A prevalence has decreased with the use of conjugate vaccines since 2000. Without distinguishing between the serotypes, it may be difficult to deploy a vaccine or assess its efficacy. At present, the new serotypes can be identified by the antibodies as disclosed herein, but additional genetic and biochemical tests may be devised and are envisioned by the present invention.
Moreover, the prevalence of the 6C serotype or the emergence of the 6D serotype should be monitored globally, providing valuable information on the emergence of new pneumococci in areas with and without pneumococcal vaccine distribution. The 6C serotype has also been identified in Brazil, Canada, China, Korea, Mexico, Europe (Hermans et al., 26 Vaccine 449-50 (2008)), and the U.S.
To that end, the monoclonal antibodies of the present invention are useful in identifying the 6C serotype. To wit, both 6A and 6C are identified by the mAb Hyp6AG1, but the 6C serotype does not react with the mAb Hyp6AM6 or mAb Hyp6AM3. Hence, Hyp6AM6 or Hyp6AM3 may be used as a negative control from which 6A and 6C can be identified. Using these monoclonal antibodies, the prevalence of 6A and 6C among the U.S. pneumococcal isolates submitted to the CDC were analyzed. Approximately the same number of pneumococcal isolates were submitted to the CDC from 1999 to 2006. Specimens typed as “6A” by the old method were reanalyzed using the monoclonal antibodies described herein. Almost all the “6A” specimens received in 1999, 2003, and 2004 were reanalyzed. Only a fraction of the samples the CDC received in 2005 and 2006 were reanalyzed. As seen in the table, the prevalence of 6A decreased but the prevalence of 6C remained the same. This suggests that the currently available pneumococcal vaccine may not be effective against 6C.
Serotype 6C has been deposited with the American Type Culture Collection in accord with the Budapest Treaty.
The actual synthesis of pneumococcal capsular polysaccharides requires cooperation among many different gene products. For instance, a new repeating unit made by a new glycosyl transferase must be compatible with the existing flippase as well as polymerase before it can be expressed as a new capsule. Thus, the new strain expressing serotype 6D (named TIGR6D) was produced by inserting wciN6C into 6B capsule gene locus. This novel strain produces capsular polysaccharide with predicted structure, it displays serological similarity to 6B, and it can grow as well as other members of serogroup 6 in various growth conditions. Thus serotype 6D is possible not only logically but also biologically, and it could exist in nature.
Just as 6C was previously typed as “6A” by the classical typing method (Lin et al., 2006; Park et al., 2007), the quellung reaction method typed the new 6D strain as serotype 6B. Thus, to identify natural isolates expressing serotype 6D, isolates that were classically defined as 6B were re-examined using mAbs. Despite testing more than 250 isolates that were previously typed as 6B, 6D isolates were not found to exist in nature. Additionally, wciN6c was not detected among the CDC isolates that were serotyped as “6B” with the classical typing method. Thus, pneumococcal isolates expressing serotype 6D have not been found to exist in nature. Alternatively, if serotype 6D exists in nature, its prevalence is extremely low (<1% of the 6B prevalence).
Despite its apparent absence in nature, the 6D serotype could emerge in nature by one of two possible mechanisms. One mechanism involves a mutation of the wciP gene of 6C, because the only difference between 6A and 6B serotype appears to be one nucleotide in the wciP gene. The mutation rate for pneumococci is ˜1×10−8 (del Campo et al., 43 J. Clin. Microbiol. 2207-14 (200); Fedson, 1988; Morosini et al., 47 Antimicrob. Agents. Chemother. 1464-67 (2003)), and a COPD patient with stable pneumonia may have 2.6×108 CFU of pneumococci per mL of sputum. Sethi et al., 176 Am. J. Respir. Crit. Care Med. 356-61 (2007). Thus, the correct mutation should arise in almost all cases of 6C pneumonia and often in other 6C infections with less bacterial load. The alternative mechanism involves the lateral gene transfer of wciN from a 6C strain into a 6B strain as provided for herein. The situation could actually occur in nature because carriage of multiple pneumococcal serotypes can be relatively high among children (Gratten et al., 50 Biol. Neonate 114-20 (1986); Hill et al., 46 Clin. Infect. Dis. 807-14 (2008)), and serotypes 6B and 6C are fairly common in some parts of the world (e.g., Brazil) (Lin et al., 2006; Park et al., 2007). Further, homologous recombination would occur easily because the 17 kb of the capsule gene locus of 6B and 6C are almost identical except for the wciN gene. These considerations strongly suggest that the circumstances for creating 6D serotype exist in nature.
Given that the circumstances for creating 6D do exist in nature, it is interesting to consider reasons for its absence. It is possible that there could be natural immune barriers against 6D, but apparently pre-immune human sera do not kill or opsonize TIGR6D. Alternatively, the 6C could have appeared so recently that there may not have been enough time for 6D to appear. Although it is difficult to estimate the physical time that would permit the appearance of 6D, 6C has existed for decades and is found in several continents. The most likely explanation is that there has not been enough biological pressure for selecting 6D over 6A, 6B, or 6C. In the absence of survival advantage, 6D serotype may have appeared in nature (as it should in almost every case of a 6C infection, as mentioned above) but was not propagated due to competition with more abundant 6A, 6B, or 6C serotypes. In an analogous manner, antibiotic resistant strains survive and propagate when antibiotics are clinically used and they disappear when antibiotics are discontinued. See Katsunuma et al., 102 J. Appl. Microbiol. 1159-66 (2007)
The 6A PS was included in the original 14-valent PS vaccine, but it was replaced with 6B PS when the 23-valent vaccine became available in 1983 because, inter alia, 6B PS is greatly more resistant to hydrolytic breakdowns than 6A PS. Zon et al., 37 Infect. & Immun. 89-103 (1982). The work provided herein shows that 6D PS is as chemically stable as 6B PS, and much more resistant to hydrolysis than 6C PS. Because 6D PS would likely elicit antibodies cross-reactive with 6C PS, 6D may be more useful as a vaccine than 6C PS.
It is interesting to understand the evolution of the capsule gene locus, which encodes pneumococci's most important virulence factor. Even when only two serotypes were known, the evolution of serogroup 6 has been extensively studied (Mavroidi et al., 2004; Robinson et al., 2002). Serogroup 6 has become more interesting following the discovery of serotype 6C: now the serogroup is even more interesting for evolution studies with serotype 6D.
Moreover, although the currently available 23-valent pneumococcal vaccine contains 6B PS, the old 14-valent pneumococcal vaccine contained 6A PS. The PS was replaced in the 23-valent pneumococcal vaccine because 6A PS was not stable in vaccine preparations, and because 6B PS often induced antibodies cross-reacting to 6A PS. Robbins et al., 1983. Investigations found that 6A's PS chemical instability was due to the 1→3 linkage between rhamnose and ribitol. Zon et al., 1982. The 1→4 linkage found in 6B PS is more stable than the 1→3 linkage of 6A PS. The putative structure of 6D PS is identical to that of 6C PS except that 6C PS has the unstable 1→3 linkage whereas 6D PS has the more stable 1→4 linkage. Thus, because of this stability, 6D PS may prove even more useful in a vaccine than 6C PS.
Importantly, the novel serotypes 6C and 6D provided herein may be useful in a vaccine or in pneumococcal vaccine development. For example the 6D PS, a portion of that PS, or a mimetic of the PS may be incorporated into a pneumococcal vaccine. Conjugate vaccines comprising streptococcal and pneumococcal PS are well-known in the art. See e.g., U.S. Pat. No. 6,248,570; U.S. Pat. No. 5,866,135; U.S. Pat. No. 5,773,007. PS mimotopes, such as protein or peptide mimetics of polysaccharide molecules, are also possible as alternative antigens or immunogens. See, e.g., Pincus et al., 160. J. Immunol. 293-98 (1998); Shin et al., 168 J. Immunol. 6273-78 (2002). Additionally, the proteins or nucleic acids of 6C and/or 6D may serve as antigens or immunogens in vaccine or vaccine development using any number of techniques known in the art. See, e.g., U.S. Pat. No. 6,936,252. One or more adjuvant agents may be included in such vaccines. The delivery of pneumococcal vaccines, either by parenteral, mucosal, or other administration, and the design, monitoring, and dosing regimens of such vaccines, are well-known in the art.
Additionally, the 6C and 6D serotypes may be useful in vaccine development because the bacteria would be used as the target in an opsonization or ELISA assays using sera or antibodies raised by test vaccines. The antigens of the 6C and 6D serotypes may also be used to raise antibodies that might be used for passive protection. Such methods are also well-known in the art.
Further, the identification of 6C and 6D provides for the production and isolation of anti-6C antibodies and anti-6D antibodies. These can be prepared by conventional means well known in the art in light of the current specification. In this regard, the term antibodies includes both intact immunoglobulin molecules as well as portions, fragments, peptides and derivatives thereof, such as, for example, Fab, Fab′, F(ab′)2, Fv, CDR regions, or any portion or peptide sequence of an immunoglobulin molecule that is capable of binding a 6C antigen, epitope, or mimotope, all of which may also be referred to as an “antigen binding molecule.” An antibody or antigen binding molecule is said to be “capable of binding” an antigen if it is capable of specifically reacting with the antigen to thereby bind the antigen to the antibody or antigen binding molecule. See, e.g., WO/US2006/014720; WO/US2006/015373.
The embodiments of the invention will now be described further by non-limiting examples.
EXAMPLES Example 1 Identification of Pneumococcal SerotypesCollection of pneumococcal lysates: The pneumococci serotype 6Aβ (See Lin et al., 44 J. Clin. Micro. 383-88 (2006)) was isolated in a blinded study using 495 clinical isolates: Fifty isolates were from Mexico, 100 from Denmark, and 345 from Brazil. Twenty-two isolates were from asymptomatic carriers of pneumococci in the nasopharynx and 475 isolates were from patients with invasive pneumococcal infections such as meningitis and sepsis. In addition, control pneumococcal strains expressing serotypes 11A, 11B, 11C, 11D, and 11F were purchased from Statens Serum Institut (Copenhagen, Denmark).
Lysates of the clinical isolates were prepared in the country of origin. Three hundred microliters of Todd-Hewitt medium with 0.5% yeast extract (THY medium) was inoculated with a single colony of pneumococci. After an overnight incubation at 37° C., cells were lysed with 50 μl of lysing solution (0.2% sodium deoxycholate, 0.02% SDS, 0.1% sodium azide, 0.3M sodium citrate, pH 7.8). In Brazil, 400 μl of THY medium was used for bacterial growth and 100 μl was removed to store the bacteria frozen before mixing the remaining 300 μl with 50 μl of lysing solution. In Denmark, 325 μl of THY medium and 25 μl of lysing solution were used. Bacteria were lysed by incubating the mixture at 37° C. The lysates were coded and shipped to the University of Alabama at Birmingham (UAB) laboratory for serotype testing by regular mail at ambient temperature.
To simplify the shipping of bacterial lysates from distant sites to UAB for the multibead assay, the stability of bacterial lysates was compared after storage at room temperature (RT) or 37° C. The work revealed that bacterial lysates can be stored at RT for up to one month or at 37° C. for several days without affecting the results of the multibead assay. Thus, the regular postal mail system was used to ship all the lysates in this study at ambient temperature without any thermal protection.
Serological Reagents: All the polyclonal serotyping sera were made in rabbits and were obtained from Statens Serum Institut. They include twelve serum pools for serogrouping and various type- or factor-specific antisera. Sorensen, 31 J. Clin. Microbiol. 2097-2100 (1993). All the mAbs were produced as described, and hybridoma culture supernatants were used. Yu et al., 2005.
Multibead assay: This assay was performed as described using two different sets of latex beads. Yu et al., 2005. One set of beads (Set 1) was a mixture of fourteen different latex beads, each coated with one pneumococcal PS antigen. The fourteen pneumococcal PS antigens were serotypes 1, 3, 4, 5, 6A, 6B, 7F, 9N, 9V, 14, 18C, 19A, 19F, and 23F. Bead Set 2 was created by coating each of ten bead types with one of ten different pneumococcal PS (serotypes 2, 8, 10A, 11A, 12F, 15B, 17F, 20, 22F, and 33F).
Set 1 beads were mixed with either 5× or 20× diluted bacterial lysate and a mixture of mAbs specific for the pneumococcal capsular PS contained on the beads. After incubation and washing, the bead mixture was reacted with fluorescein-conjugated anti-mouse immunoglobulin antibody. Set 2 beads were used the same as Set 1 beads except that a mixture of polyclonal rabbit antisera (Statens Serum Institut) and fluorescein-conjugated anti-rabbit immunoglobulin antibody were used. After incubation, the amount of fluorescence of each bead type was determined with a flow cytometer (FACSCalibur, Beckton Dickinson, San Jose Calif.). The fluorescence of each bead type was then used to determine its serotype. Fluorescence inhibitions greater than 67% were used as positives.
Neufeld's test: This assay was performed as described (Henrichsen, 33 J. Clin. Microbiol. 2759-62 (1995); Konradsen, 23 Vaccine 1368-73 (2005); Lund, 23 Bull.Wld Hlth Org. 5-13. (1960)) by the reference laboratories in Denmark, Brazil, and Mexico using standard serogrouping (Sorensen, 1993) and serotyping rabbit antisera from Statens Serum Institut.
Dot blot assay: To investigate discrepant results, this assay was performed as described (Fenoll et al., 35 J. Clin. Micro. 764-76 (1997)), using pneumococcal antisera from Statens Serum Institut to the following serogroups/serotypes: 1, 4, 5, 6, 7, 8, 9, 11, 12, 14, 18, and 23. Monoclonal antibodies specific for 6A (Hyp6AM3) and 18C (Hyp18CM1) were also used in some cases. Briefly, heat-killed pneumococci grown in THY medium were spotted on strips of nitrocellulose membranes. After drying, the strips were blocked and washed. Strips were then incubated in a diluted antiserum or mAb solution for 1 hour, washed and exposed to a diluted goat anti-rabbit or mouse immunoglobulin-peroxidase conjugate. After one hour incubation at room temperature, the strips were washed and exposed to 3-amino-9 ethylcarbazole solution. When the spots appeared, the strips were washed and evaluated.
PCR reactions: Pneumococci were grown in THY medium to an OD of 0.8 at 650 nm wavelength. Chromosomal DNA was prepared using the Invitrogen EASY-DNA kit and following the given instructions, beginning with a 4 ml sample of the THY-grown pneumococci concentrated to 1 ml (Invitrogen, Carlsbad, Calif.). For serogroup 6 determination, PCR was performed using chromosomal DNA as template and primers wciP-up, 5′-ATG GTG AGA GAT ATT TGT CAC-3′ (SEQ ID NO:1) and wciP-down, 5′-AGC ATG ATG GTA TAT AAG CC-3′ (SEQ ID NO:2). PCR thermocycling conditions were as described in Mavroidi et al., 2004. A Qiagen PCR cleanup column (Qiagen, Valencia, Calif.) was used to remove excess primer from the PCR reactions and the PCR was submitted as DNA template for automated DNA sequencing using the wciP-up primer. Results were analyzed with the aid of the Sequencher (GeneCodes, Inc., Ann Arbor, Mich.) and the MacVector Sequence Analysis (Accelyrs, San Diego, Calif.).
For serotype 11A determination, PCR for a part of the capsule gene locus was performed as described (Mavroidi et al., 2004), using chromosomal DNA as the template, 1 μl of forward primer (50 pmol), and 1 μl of reverse primer (50 pmol). Primers were 11A forward, 5′-GGA CAT GTT CAG GTG ATT TCC CAA TAT AGT G-3′ (SEQ ID NO:3) and 11A reverse 5′-GAT TAT GAG TGT AAT TTA TTC CAA CTT CTC CC-3′ (SEQ ID NO:4). PCR cycling began with 94° C. for 5 min, followed by thirty cycles of 94° C. for 1 min, 50° C. for 1 min and 72° C. for 2 min, followed by a final extension of 72° C. for 10 min. The PCR reaction products were analyzed by agarose gel electrophoresis (Tris-acetate buffer 0.8% agarose) to determine the amplicon size.
Study of fifty isolates from Mexico: The fifty isolates from Mexico were grown in THY medium, lysed, and sent to UAB for typing. When the multibead assay results were compared with the Neufeld's test results, results from ten samples were discrepant. When new lysates of eight of the discrepant samples were obtained and re-examined in a blind fashion, all results matched, suggesting that the discrepancies were largely due to mislabeling. Two isolates (MX24 and MX37) that were typed to be serotype 3 and 10A by the Neufeld's test were originally typed as non-typeable (NT) by the multibead assay. Because the two serotypes should have been identified by the multibead assay, the two bacterial isolates were sent to the UAB laboratory for further study. There they were found to grow well in THY medium, with the new lysates producing results matching the Neufeld's test results. Thus, the two isolates were initially falsely identified as negatives by multibead assay, most likely due to insufficient growth of pneumococci.
Study of 100 isolates from Denmark: When the multibead assay results of one hundred Denmark isolates were compared with the Neufeld's test results, four errors were found in transcribing the Neufeld's test results and one strain (DK94) was typed as serotype 20 by the Neufeld's test and as NT by the multibead assay (Table 1).
When the DK94 isolate was re-grown in THY and re-examined, it produced almost no inhibition (9%) at a 1:5 dilution, but it produced more inhibition at higher dilutions (35% at a 1:20 dilution and 50% at a 1:320 dilution). This unexpected behavior suggested the presence of non-specific binding material in the lysate of this specific isolate. When the PS in the lysate was precipitated with 70% ethanol and the ethanol precipitate was examined with the multibead assay, the precipitate produced a clear inhibition for serotype 20 (86% at a 1:5 dilution and 81% at a 1:20 dilution). Thus, the initial discrepancy was due to non-specific binding, which was occasionally observed with the assays performed with polyclonal sera, and there is no intrinsic problem in assay sensitivity and specificity with clinical isolates.
Study of 345 samples from Brazil: When the results of 345 Brazilian isolates obtained with the two assay methods were compared, there were thirty-eight mismatches. When these thirty-eight samples were re-examined by investigating test records and retesting by Neufeld's test in Brazil, seventeen of the mismatches could be explained as typing mistakes or sample misidentification. One of the seventeen mismatches was strain BZ652. This was initially typed as 18B, but was determined to be 6A because it was typed as weakly 6A by Neufeld's test and was typed as serogroup 6 by the dot blot assay using the polyclonal antisera and mAb Hyp6AM3. When the twenty-one remaining mismatched samples were regrown in THY medium and retested by multibead assay, the new results of thirteen isolates matched the Neufeld's test results. When the original multibead assay results of the thirteen isolates were re-examined, three isolates produced weak and incomplete inhibitions (inhibitions were less than 67%) for the appropriate serotype in the original multibead. Although twelve isolates were initially typed as NT, one isolate (BZ52) was initially typed as type 3. It was retyped as NT with the second sample and the result became consistent with the Neufeld's test result (Table 1) (above).
After these re-examinations, eight discrepancies were reproducible and still unexplained (Table 2 and Table 3): five isolates were typed as 6A by the Neufeld's test but as NT by the multibead assay, two isolates (BZ435 and BZ705) were typed as 11A by the Neufeld's test but as NT by the multibead assay, and one isolate (BZ438) was typed as NT by the Neufeld's test but as 18C by the multibead assay. By the Neufeld's test, BZ438 did not react with several lots of serogrouping Poolsera A and Q (Sorensen, 1993), which should react with serogroup 18 pneumococci. It also did not react with several different lots of antisera specific for serogroup 18 or specific for factors 18c, 18d, 18e, and 18f. BZ438 produced positive dot blot results, however, with a serogroup 18-specific polyclonal serum or with mAb Hyp18CM1 (Yu et al., 2005). Thus, the BZ438 isolate was considered to be 18C.
Strains BZ435 and BZ705 were considered to be 11A by the Neufeld's test but not 11A, 11D, or 11F by the multibead assay. Because the standard multibead assay uses a polyclonal antiserum against serogroup 11 (Yu et al., 2005), the two strains were examined with two mAbs (Hyp11AM1 and Hyp11AM2) that are specific for serotypes 11A, 11D, and 11F and that were recently produced in the UAB laboratory (Table 2). Interestingly, Hyp11AM1 recognizes BZ435 but not BZ705. Interestingly, Hyp11AM2 recognized neither strain, suggesting heterogeneity among the strains expressing the 11A serotype. A PCR test produces 463 base pair amplimers with strains for 11A, 11D, and 11F but not for 11B and 11C (Table 2). When both strains were tested by this PCR, BZ435 was positive, but BZ705 was not. Although the Neufeld's test showed that both strains reacted with antisera specific for factor 11c, the Neufeld's test also revealed differences between them: BZ435 but not BZ705 reacted with Poolserum T (Sorensen, 1993), with serogroup 11 antisera, or with 11f factor serum. BZ705 yielded ambiguous results for factor 11b expression and this suggested that it could be serotype 11D. In a dot blot test for serogroup 11 using rabbit typing serum, however, BZ435 was positive but that BZ705 was negative. Considering all of these results, it appeared that BZ435 is an 11A strain and that BZ705 is not 11A, 11D, or 11F. BZ705 may belong to the 11C serotype since BZ705 expresses the 11c epitope (and reacts with 11c antisera) that is not expressed on 11B strains.
To investigate the remaining discrepant strains that were 6A, the DNA sequence of the wciP gene was examined. A recent study reported that the capsular PS of 6A and 6B has repeating units with rhamnose linked to ribitol. The linkage is 1-3 for 6A and 1-4 for 6B. The study found that the rhamnosyl transferase is likely encoded by the wciP gene in the capsule locus, that wciP for 6A encodes serine at residue 195, and that wciP for 6B encodes asparagine at residue 195 (Mavroidi et al., 2004). Also, wciP alleles 1, 2, 7, 9, and 11 are exclusively associated with serotype 6A, and alleles 3, 4, 5, 6, 8, and 12 are associated with serotype 6B. (Mavroidi et al., 2004).
Bacterial DNA was obtained from the five isolates labeled 6A as well as BZ652, which was considered to be only weakly 6A by the Neufeld's test. This DNA was amplified a part of the wciP gene by PCR, sequenced the amplicon, and examined the sequence (645 base pairs). Five samples were amplified successfully, and their sequences were consistent with a 6A serotype because they expressed alleles associated with the 6A serotype (Table 3) and expressed serine at amino acid residue 195. Compared to the prototypic sequence of allele 2 wciP, the wciP sequence of BZ652 has five base pair changes with three potential amino acid replacements. Three isolates (BZ17, BZ39, and BZ86) express the identical wciP gene sequence with one identical nucleotide variation from the prototypic sequence for allele #9 and may, therefore, be clonally related (Table 3).
Because the DNA study suggested that these isolates may belong to the 6A serotype, these isolates were examined with the multibead assays using polyclonal antisera. All six isolates were typed as 6A (Table 3). When they were typed with nineteen different 6A-specific mAbs in addition to Hyp6AM3, one mAb (Hyp6AG1) identified the six isolates as 6A (Table 3). When Hyp6AG1 was used to retest forty-six 6A isolates (twenty-one from this study and twenty-four in the Univ. Alabama Birmingham laboratory collection), it was found that this mAb identified all of them as 6A and that it did not recognize any of the eighty-nine non-6A serotypes, including the forty-three isolates expressing the 6B serotype. Thus, it was clear that all these six isolates are 6A and that Hyp6AG1 recognizes all 6A isolates. Also, mAb Hyp6AM3 recognizes a subset of 6A isolates, although that subset is very large.
Example 2 Pneumococcal Serotype 6C Isolates from Different Countries have the Molecular Characteristics Associated with 6AAs described above, Brazilian isolates that did not react with both mAbs previously associated with serotype 6A were shown to belong to the 6A serotype by examining the wciP allele. Thus, the inventors examined wciP gene of ten 6C isolates from geographically diverse locations. Brazilian isolates collected in 2003 and in 2004, USA isolates and one isolate from Korea were examined. The sequences clearly showed that all ten isolates have the genetic characteristics associated with 6A serotype.
Example 3 6C Isolates from Different Areas have Uniform Serological CharacteristicsTo investigate serological characteristics of the 6C isolates in a quantitative manner, isolates were examined using an inhibition assay. The assay measured inhibition by bacterial lysates of various anti-6A antibodies binding to 6A PS-coated ELISA plates. Briefly, the wells of ELISA plates (Corning Costar Corp., Acton, Mass.) were coated at 37° C. with 5 μg/mL of 6A capsular PS (a gift of G. Schiffman, Brooklyn, N.Y.) overnight in PBS. After washing the plates with PBS containing 0.05% of Tween 20, previously diluted bacterial culture supernatant (or lysates) was added to the wells along with an anti-6A antibody. Pneumococcal lystates were prepared by growing pneumococci in 10 mL of Todd-Hewlett broth supplemented with 0.5% yeast extract (THY) without shaking until the tubes became turbid and then incubating the tubes for 15 minutes at 37° C. with a lysis buffer (0.1% sodium deoxycholate, 0.01% SDS, and 0.15M sodium citrate in deionized water). Hyp6AG1 mAb was used at a 1:250 dilution, and Hyp6AM3 mAb was used at 1:100 dilution. Pool Q and factor “6b” rabbit antisera from Staten Serum Institute (Copenhagen, DK) were used at a 1:500 dilution. After thirty minutes of incubation in a humid incubator at 37° C., the plates were washed and incubated for two hours with alkaline phosphatase-conjugated goat anti-mouse Ig (Sigma, St. Louis, Mo.) or alkaline phosphatase-conjugated-goat anti-rabbit Ig (Biosource, Camarillo, Calif.). The amount of the enzyme immobilized to the wells was determined with paranitrophenyl phosphate substrate (Sigma) in diethanolamine buffer. The optical density at 405 nm was read with a microplate reader (BioTek Instruments Inc. Winooski, Vt.).
Because the qualitative nature of the quellung reaction may have prevented detection of 6A subtypes, it was determined whether the subtypes might be distinguishable with a quantitative assay using the rabbit sera used for quellung reactions. This was determined by adapting the rabbit sera to an inhibition assay, in which pneumococcal lysates were allowed to inhibit the binding of rabbit antisera to 6A PS immobilized on ELISA plates (
When the pneumococcal lysates were examined for inhibiting Pool Q (a rabbit antiserum often used for serotyping (Sorensen, 31 J. Clin. Microbiol. 2097-2100 (1993)), both lysates of 6A and 6C could inhibit equally well, but the 6B lysates could not inhibit (
Various 6A, 6C, and 6B isolates were compared using an opsonization assay and a human serum with a high level of anti-6B antibodies. Although the human serum opsonized 6B as well as 6A (
Because a human antiserum can opsonize 6A and 6B but not 6C, serum samples from twenty adults for were examined for opsonizing 6A, 6B, and 6C serotypes (
To examine whether immunization with 6B induces antibodies cross-reacting with 6C, the sera from twenty adults who were immunized with a pneumococcal vaccine were studied (
Mouse hybridomas were produced as described previously. Yu et al., 2005 (citing Sun et al., 69 Infect. Immun. 336-44 (2001)). Briefly, BALB/c mice were immunized twice subcutaneously with PS-protein conjugate (days 0 and 21) and once intraperitoneally on day 59. The immunogen for seven serotypes (4, 6B, 9V, 14, 18C, 19F and 23F) was Prevnar (Wyeth Lederle Vaccines, Pearl River, NY). Conjugates used for serotypes 5 and 7F were prepared at the U.S. Food and Drug Administration (Bethesda, Md.), the 6A conjugate was a gift of Porter Anderson (Rochester, N.Y.), and conjugates of serotypes 1, 3, and 9N to ovalbumin were prepared as follows. Cyanogen bromide-activated PS was coupled to ovalbumin during an overnight incubation. The PS-protein conjugate was purified from the reaction mixture with a molecular weight sizing column. Each dose contained 1 μg of PS for serotypes 4, 9V, 18C, 19F, and 23F; 2 μg for serotypes 3 and 6B; and 10 μg for serotypes 1, 5, 6A, 7F, and 9N. The primary and secondary immunogens contained 10 μg of Quil A (Sigma Chemical, St. Louis, Mo.).
Three days after the last immunization, the mice were sacrificed, the spleens harvested, and the splenocytes fused with SP2/0 Ag-14 as described previously. Nahm et al., 129 J. Immunol. 1513-18 (1982). Primary culture wells were screened for the production of desirable antibodies, and the wells producing such antibodies were cloned twice by limiting dilution. A human-mouse hybridoma, Dob9, was produced by hybridizing peripheral blood lymphocytes from a person immunized with a 23-valent PB vaccine, as described previously. Sun et al., 67 Infect. Immun. 1172-79 (1999).
The human-mouse hybridoma is specific for pneumococcal serotypes 19A and 19F. All hybridomas produced either IgM or IgG antibodies, excepting one IgA producer. Hyp6AG1 is IgG and Hyp6AM6 is IgM.
A total of twenty-one hybridomas specific for 6A serotypes were isolated. Many have similar serological behavior and some may be sister clones (i.e., some may have the identical variable region structure). Names of 6A-specific hybridomas produced are Hyp6A1, Hyp6AM1, Hyp6AM2, Hyp6AM3, Hyp6AM4, Hyp6AM5, Hyp6AM6, Hyp6AM7, Hyp6AM8, Hyp6AM9, Hyp6AM10, Hyp6AM11, Hyp6AM12, Hyp6AM13, Hyp6AG1, Hyp6AG2, Hyp6AG3, Hyp6AG4, Hyp6AG5, Hyp6AG6, Hyp6AG7.
Example 6 Genetic Study of 6CA non-capsulated pneumococcal strain might be transformed with genes from a 6C isolate. This finding suggests that 6C capsule synthesis requires one (not multiple) gene fragment, most likely the capsule gene locus. To identify the gene(s) responsible for 6Aβ expression, three transferases (wciN, wciO, and wciP) were examined. The wciP gene may be identical between 6A and 6C isolates (as discussed above). When wciN region was examined by PCR using primers 5016 and 3101 (5106: 5′-TAC CAT GCA GGG TGG AAT GT (SEQ ID NO:5) and 3101: 5′-CCA TCC TTC GAG TAT TGC (SEQ ID NO:6)), all nine 6Aa isolates examined yielded about 200 base pair (bp) longer product than did all six 6C isolates examined (
The nucleotide sequences of the PCR products from one 6A isolate (AAU9) and one 6C isolate (ST745) were then determined (
Two 6C isolates (BZ17 and BZ650), four 6A strains (SP85, ST558, and CHPA378), and two 6B strains (ST400 and ST518) were compared. All pneumococcal isolates had colony morphologies typical of pneumococci, and were both optochin-sensitive and bile-soluble. Subtyping assays were conducted as described in Example 3, above.
Polysaccharide Isolation and Purification:
A pneumococcal strain (SP85 or BZ17) was grown in two liters of a chemically defined medium (van de Rijn et al., 27 Infect. Immun. 444-49 (1980)) from JRH Biosciences (Lenexa, Kans.), which was supplemented with choline chloride, sodium bicarbonate and cysteine-HCl, and lysed with 0.05% deoxycholate. After removing cell debris by centrifugation, PS was precipitated in 70% ethanol and was recovered by dissolving it in 120 mL of 0.2 M NaCl. After dialyzing the PS in 10 mM Tris-HCl (pH 7.4), the PS was loaded onto a DEAE-Sepharose (Amersham Biosciences, Uppsala, Sweden) column (50 ml) and eluted with a NaCl concentration gradient. The resulting fractions were tested for 6Aa or 6A13 PS with the inhibition assay described above. The PS-containing fractions were pooled, concentrated by ethanol precipitation (70%), dialyzed, and lyophilized. The lyophilized PS was dissolved in 3 ml of water and loaded onto a gel filtration column containing 120 ml of Sephacryl S-300 HR (Amersham Biosciences). The PS was eluted from the column with water and all the fractions were tested for 6Aβ PS with the inhibition assay. The fractions containing the first 6Aa or 6Aβ PS peak were pooled and lyophilized.
Monosaccharide Analysis:
The lyophilized capsular PS was subjected to methanolysis in 1.5 M HCl at 80° C. for 16 hr. After evaporating the methanolic HCl, the residue was treated with Tri-Sil reagent (Pierce Biotech. Inc. Rockford, Ill.) for 20 min at room temperature. The reaction products were analyzed on a GLC/MS (Varian 4000, Varian Inc. Palo Alto, Calif.) fitted with a 30 m (0.25 mm in diameter) VF-5 capillary column. Column temperate was maintained at 100° C. for 5 min, and then increased to 275° C. at 20° C./min, and finally held at 275° C. for 5 min. The effluent was analyzed by mass spectrometry using the electron impact ionization mode.
Oxidation, reduction, and hydrolysis: Capsular PS (1 mg/mL) was treated with 40 mM sodium periodate in 80 mM sodium acetate buffer (pH=4) for four days at 4° C. in the dark. After neutralizing the excess periodate with ethylene glycol, the sample was dialyzed and lyophilized. Stroop et al., 337 Carbohydr. Res. 335-44 (2002). The PS (1 mg/mL) was reduced with 200 mg/mL of sodium borohydride (NaBH4) or its deuterium form (NaBD4) for three hours at RT, dialyzed, and lyophilized. The oxidized/reduced 6C PS was hydrolyzed in 0.01M NaOH at 85° C. for thirty minutes, neutralized by adding 0.01M HCl, and then directly used for mass spectrometry without desalting.
Tandem Mass Spectrometry:
The tandem mass spectral analysis of native and oxidized/reduced 6C were performed in the Mass Spectrometry Shared Facility at the University of Alabama at Birmingham with Micromass Q-TOF2 mass spectrometer (Micromass Ltd. Manchester, UK) equipped with an electrospray ion source. The samples, dissolved in distilled water, were injected into the mass spectrometer along with running buffer (50/50 acetonitrile/water containing 0.1% formic acid) at the rate of 1 μL/min using a Harvard syringe pump. The injected sample was negatively ionized with electrospray (needle voltage=2.8 kV) and detected with a TOF mass spectrometer. The injected sample was negatively ionized with electrospray (needle voltage=2.8 kV) and detected with a TOF mass spectrometer. For MS/MS, the parent ion was fragmented into daughter ions by energizing it to 40 eV before collision with argon gas. The daughter ions were analyzed with a TOF mass spectrometer. The MS/MS spectra were processed using the Max-Ent3 module of MassLynx 3.5.
Smith Degradation and Glycerol Detection:
Periodate treated 6A and 6C PSs were reduced with 10 mg/ml Sodium borodeuteride in 1M ammonium hydroxide for 16 hr. Excess sodium borodeuteride was removed by addition of glacial acetic acid and 0.5 ml of methanol:acetic acid (9:1) was added. Samples were dried under a stream of nitrogen and washed twice with 0.25 ml of methanol. Dried samples were suspended in 0.5 ml of 1.5M methanolic HCl and incubated at 80° C. for 16 hr. Samples were dried under a stream of nitrogen and washed twice with 0.25 ml of methanol. Dried samples were suspended in 0.1 ml of Tri-Sil (Pierce) and incubated at 80° C. for 20 min. The 1 μl of samples were injected into a Varian 4000 gas chromatograph mass-spectrometer (Varian Inc., Palo Alto, Calif.) equipped with a 60 m VF-1 column. Helium was used as the carrier gas at a constant flow rate of 1.2 ml/min. The oven conditions were an initial temperature of 50° C. held for 2 min, temperature increase at 30° C./min to 150° C., then another increase at 3° C./min to 220° C., which was held for two minutes. The injector temperature was kept at 250° C. and the MS transfer-line at 280° C. MS data acquisition parameters included scanning from m/z 40 to 1000 in the electron impact (EI) mode or in the chemical ionization (CI) mode using acetonitrile.
The chromatography of 6A PS showed all the peaks that are characteristic of ribitol, rhamnose, glucose, and galactose (
To further investigate the two glucose molecules presumed to be present in 6C PS, 6A and 6C PS were treated with periodate, which selectively destroys vicinal glycols. As expected from the published structure of 6A PS, the galactose and ribitol peaks of 6A PS became undetectable while the glucose and rhamnose peaks were undisturbed. Kamerling, Pneumococcal polysaccharides: a chemical view, in Mol. Biol. & Mechanisms of Disease 81-114 (Mary Ann Liebert, Larchmont, 2000); Kim et al., 347 Anal. Biochem. 262-74 (2005); Rebers & Heidelberger, 83 J. Am. Chem. 3056-59 (1961). When 6C PS was periodate-treated, its ribitol became undetectable and its glucose peak was reduced by about half while its rhamnose peaks remained undisturbed (
A mild alkali hydrolysis of 6A PS breaks the phosphodiester bond in each repeating unit and produces a repeating unit with a negative charge, which can then be examined with tandem mass spectrometry. The hydrolysis product of 6A PS (from strain SP85) showed three well-defined peaks with a negative charge: peaks with 683.21, 701.21, and 759.19 mass to charge ratio (m/z) units (
Analysis of 6C PS, using the same procedure used for 6A PS, showed three major peaks at 683.24, 701.25, and 759.22 m/z units, corresponding to the three major peaks found for 6A PS (
To identify the 6C glucose that is periodate-sensitive, 6C PS was oxidized and reduced to repeating units by mild alkali hydrolysis, and the repeating units studied with tandem mass spectrometry. Their mass spectrum showed several major (and dominant) peaks between 650 and 700 m/z units (
Daughter ions were obtained by fragmenting the parent ion with 673.25 m/z units (
Periodate cleavage divided glucose 1 into two parts (which were named Gx and Gy in
The mass spectrum of daughter ions also provided information about the glycosidic linkages of 6Aβ PS. Glucose and rhamnose must be linked to the preceding carbohydrate at their first carbon. Rebers & Heidelberger, 1961. Also, they must be linked to the succeeding carbohydrate at the third carbon in order to be resistant to periodate. Rebers & Heidelberger, 1961 Thus, 6C PS must have glucose 1 (1→3) glucose 2 (1→3) rhamnose (1→). Further examination of the daughter ions shows that their glucose 1 has the phosphodiester bond at its second carbon. To be periodate sensitive, glucose 1 must have its phosphodiester link only at positions 2, 4, or 6. The phosphodiester bond linkage is not at position 6 because the linkage at 6 results in a loss of a carbon atom in glucose 1 (
Although hydrolysis cleaves the phosphodiester bond with glucose 1, it occasionally breaks the phosphodiester bond with ribitol instead. Examination of this reverse cleavage products further confirms that the phosphodiester linkage must be at the second carbon of glucose 1. The peaks with 150.95 and 243.00 m/z units are reverse cleavage products of glucose 1 (
Additional examination of the mass spectra showed that the rhamnose-ribitol linkage must be (1→3). Because pneumococci use CDP-5-ribitol that is produced for teichoic acid synthesis for their capsule synthesis as well (Pereira & Brown 43 Biochem. 11802-12 (2004)), the linkage between ribitol and glucose 1 must be ribitol (5→P→2) glucose 1. The peaks at 78.94 and 96.94 correspond to PO3− and H2PO4−, while the peaks at 182.98 and 200.99 (
When 6A PS was analyzed, peaks identical to the 6C PS peaks were found, which indicate that galactose and ribitol were destroyed by periodate but that glucose 2 and rhamnose remained intact. Thus, the structure of 6A PS must be {→2) galactose (1→3) glucose 2 (1→3) rhamnose (1→3) ribitol (5→P), which is identical to the 6A PS structure published in the literature. Kamerling, 2000; Rebers & Heidelberger, 1961. In summary, the structural difference between 6A and 6C PS is the orientation of the hydroxyl group at the fourth carbon of glucose 1 (or galactose).
Classically, the phosphodiester bond of 6A PS was determined to be at the second carbon of galactose by demonstrating that glycerol is released after a Smith degradation of the 6A PS that was oxidized and reduced. Rebers & Heidelberger, 1961. To confirm the position of the 6C phosphodiester bond using this classical approach, the Smith degradation of 6A and 6C PSs after oxidation and reduction was performed as described above. The reaction products of 6A and 6C PSs indicated glycerol from the two PSs. Thus, glucose 1 has a phosphodiester bond at the second carbon of glucose 1.
Example 10 Genetic Origin of Serotype 6CBacterial strains and culture: The pneumococcal strains used in the 6C study are listed in Table 4:
In addition to the 6C isolates from Brazil that were reported earlier (Lin et al., 2006), additional 6C strains were identified by retyping the preexisting pneumococcal isolates archived in the laboratory as the “6A” serotype. The collection includes 6A isolates used for studies by Robinson et al., 184 J. Bacteriol. 6367-75(2002) and Mavroidi, 2004. One strain (BGO-2197) was isolated in 1979 in Birmingham, Ala., U.S. The TIGR4JS4 strain is a non-capsulated variant of the TIGR4 strain (Tettelin et al., 293 Science 498-506 (2001)), and was produced by replacing type 4 capsule gene locus with Janus cassette (kanR-rpsL+) and backcrossing 3 times to wildtype TIGR4 (Trzcinski et al., 69 Micorbiol. 7364-70 (2003); Hollingshead (unpublished). TIGR6AX, TIGR6A4, and TIGR6C4 are TIGR4JS4 variants expressing, respectively, no, 6A, or 6C capsule types. These variants were produced as described below.
PCR and DNA Sequencing:
All the PCR primers used in this study are listed in Table 5. The primers used for multi-locus sequence typing (MLST) were as described by Enright & Spratt, 144(11) Microbiol. 3049-60 (1998), and the primers used to amplify the wciN, wciO, and wciP genes were described by Mavroidi et al., 2004. Additional primers were designed using the DNA sequences of the 6A and 6B capsule gene loci in GenBank (accession numbers CR931638 and CR931639, respectively).
For capsule gene locus PCR, the reaction mixture had 10 ng to 30 ng of chromosomal DNA, 1 μl of each primer from a 100-pmol stock, 2 μl of 10 mM dNTP, 5 μl of 10× buffer solution, 0.5 μl (2.5U) of Taq polymerase (Takara Biomedical, Shiga, Japan), and 39.5 μl of sterile water (Sigma, St Louis, Mich.). The reaction mixture for multi-locus sequence typing had 10 ng to 30 ng of chromosomal DNA, 1 μl of each primer from a 50-pmol stock, 2 μl of MgCl2, 5 μl of Q-solution (Qiagen, Chatsworth, Calif.), 12.5 μl of Master Mix (Qiagen), and 4 μl sterile water (Sigma). Chromosomal DNA was isolated with a Wizard Genomic DNA Purification Kit (Promega, Madison, Wis.) according to the manufacturer's instruction. Thermal cycling conditions were: initial denaturation at 95° C. for 3 min, 30 cycles of denaturation at 95° C. for 1 min, annealing at 52° C.-58° C. for 1 min, extension at 72° C. for 2 min, and a final extension at 72° C. for 10 min. Multi-locus sequence typing used 30 cycles, and capsule locus gene PCR used 35 cycles. The size of the PCR products was determined by electrophoresis in a 1%-1.5% agarose gel.
The DNA sequence of the PCR products was determined by the genomics core facility at the University of Alabama using an automated DNA sequencer, and the PCR products were purified with a Wizard PCR Cleanup Kit (Promega). DNA sequences were analyzed with Lasergene v. 5.1 software (DNASTAR, Madison, Wis.) and the Basic Local Alignment Search Tool (BLAST) located on-line at the NCBI NLM NIH website.
The sequences from the capsule gene locus were compared with the sequences previously reported. Mavroidi et al., 2004. Alleles of each sequence type were assigned using the on-line pneumococcal Multi Locus Sequence Typing (MLST) website. When the sequences were different, new allele numbers were assigned. All the wciNβ sequences are then deposited in the pneumococcal MLST. The entire capsule gene locus of the pneumococcal isolate CHPA388 is deposited in GenBank.
Genetic profiles of 6C strains collected from global sources are presented in Table 6:
Production of TIGR4 Variants with 6A and 6C Capsule Gene Loci:
To investigate the role of the wciN gene in 6C capsule expression, desired genes or gene fragments were inserted into the TIGR4JS4 strain, which is derived from TIGR4 but which has lost the capsule gene locus (
To transform TIGR4JS4, a frozen bacterial aliquot was thawed and mixed with 50 ng of competence-stimulating peptide variant 2. Trzcinski et al., 2003. After 14 min incubation at 37° C., 100 μl of TIGR4JS4 was mixed with 10 μl of bacterial lysate (AAU33 strain) or 100 ng of DNA cassettes. After 2 hr incubation at 37° C., the bacteria were plated on sheep blood agar plates containing 200 μg/ml kanamycin or 300 μg/ml streptomycin and incubated at 37° C. in a candle jar. Colonies of transformants growing in the antibiotic media were harvested and backcrossed three times with DNA-recipient competent bacteria.
To prepare a bacterial lysate of AAU33 for transformation, 10 ml of THY broth was inoculated with the AAU33 strain and cultured for about 5 hr at 37° C. until the optical density at 600 nm was ˜0.4-0.5. The THY broth was centrifuged to obtain a bacterial pellet, and the pellet was lysed by resuspending it in 0.1 ml of sodium citrate buffer (0.15M, pH 7.5) containing 0.1% sodium deoxycholate and 0.01% sodium dodecylsulfate and then incubating it for 10 min at 37° C. The lysate (0.1 ml) was then mixed with 0.9 ml of normal saline buffered with 0.015M sodium citrate (pH 7.0) and heat-inactivated at 65° C. for 15 min.
To replace the wciNα gene region of TIGR6A4 with the wciNβ gene region from CHPA388, two different DNA cassettes were prepared, labeled Cassette 1 and Cassette 2 in
Cassette 2 was used to replace the antibiotic selection genes with the wciNβ gene. The central core has the wciNβ gene from CHPA388. The wchA and wciO-P DNA fragments were obtained by PCR from AAU33 as described for Cassette 1 (
Identification of Additional 6C Strains Among “6A” Collections:
To obtain a representative collection of 6C serotypes from various locations, the preexisting collection of “6A” strains were re-tested by quellung reaction (Mavroidi et al., 2004; Robinson et al., 2002) and identified nine additional 6C isolates from five countries on three different continents (Table 4). These isolates were obtained from spinal fluid, blood, and the nasopharynx samples, indicating that 6C can be associated with invasive pneumococcal infections as well as asymptomatic carriage. One isolate (BGO2197) was obtained in 1979 at Birmingham, Ala. This finding shows that the 6C serotype, identified and isolated for the first time as described herein, may have been in existence for more than twenty-seven years and is now found throughout the world.
Many 6C strains have the identical capsule gene locus profile but different sequence types: To begin investigating the genetic basis for the serotype 6C, the capsule gene locus profiles and the sequence types (STs) of the twelve isolates were examined. Similar to what was observed previously for the Brazilian 6C isolates (Lin et al., 2006), all 6C isolates have allele 9 of the wciP gene with either no or one nucleotide difference. Similarly, all 6C isolates have allele 1 of the wzx gene with either no or one nucleotide difference. All 6C isolates have allele 10 for the wzy gene except for one isolate, which expresses allele 1. In contrast to the 6C isolates' restricted capsule gene locus profile, multi-locus sequence typing shows that 6C isolates express diverse STs. The fact that 6C is associated with multiple STs but with one single capsule gene locus profile (except for one isolate) suggests that the gene(s) responsible for the 6C serotype is probably in the capsule gene locus.
The capsule gene loci of 6A and 6C differ in the region between the wchA and wciO genes: It was hypothesized that the genetic difference between serotypes 6A and 6C is a glycosyl transferase gene, the same gene that is responsible for the difference between serotypes 6A and 6B. When PCR was used to compare the sizes of their glycosyl transferase genes, it was found that the sizes of their wciN genes were different. The wciN PCR products of all 6C isolates were about 1.8 kb long whereas the wciN PCR products of all 6A isolates were about 2 kb long (
To further investigate wciNβ gene, the DNA sequences of the wciNβ gene region including the wchA and wciO genes from five 6C strains (BZ17, BZ86, CHPA388, KK177, and ST 260) were compared. Because their sequences were almost identical, the actual DNA sequence is shown for only CHPA388 (
The sequence comparison revealed clear differences in wciNα and wciNβ genes: The 6C serotype has 1029-bp-long DNA in place of 1222-bp-long DNA in 6A (
The flanking regions may have been involved in the homologous recombination of the wciNβ gene to the 6A capsule locus. Furthermore, all 6C isolates have the identical flanking region sequences. This suggests that the genetic replacement took place only once and that all the 6C isolates must be progeny of this single founder bacterium.
With this gene replacement, wciNβ has a new open reading frame (ORF) that is 1125 bases long and encodes a peptide with 374 amino acids, which is named the WCINβ protein (
The sequences of the capsule gene loci of the 6A and 6C serotypes differ only slightly in regions other than the wciN gene: To determine if the 6A and 6C capsule gene loci differ only in the wciN region, the sequence of the entire capsule locus of a 6C isolate (CHPA388) was analyzed by PCR amplifying the entire capsule gene locus between dexB and aliA loci in six overlapping DNA fragments.
When the sequence was compared with the capsule gene locus of a 6A strain (GenBank accession No. CR31638), except for the wciN region described above, the capsule gene locus of 6C was very homologous (˜98%) to that of 6A. Also, homology was significantly low (about 78%) for about 60 bp in the middle of the cpsA ORF and the “tnp's” found at either end of the capsule gene loci were different between the 6A and 6C capsule gene loci. The 6C capsule gene locus did not have the INDEL that is present upstream to the wciO gene in some 6A or 6B capsule gene loci. Mavroidi et al., 2004. Despite these differences, the most prominent difference between 6A and 6C capsule gene loci is found in the wciN region.
The wciN gene region is responsible for conversion from the 6A to 6C serotype. Although the above comparison of the capsule gene loci showed that the major difference is in the wciN region, minor differences are present in the entire capsule gene region (e.g., cpsA ORF). It is possible that some other small genetic differences outside of the capsule locus could be involved in the 6C expression. To show that only the wciN region is involved, whether the interchange of the wciNα region with the wciNβ region could convert the 6A serotype to the 6C serotype (
Previous studies of the chemical structure of serotypes 6A and 6B capsular polysaccharides (PS) showed them to be polymers of repeating units containing galactose, glucose, rhamnose, and ribitol phosphate (
As discussed herein, the 6C PS has a glucose residue in place of the galactose residue of 6A PS (
Because the 6A and 6B capsule gene loci are also almost identical except for one nucleotide difference in the wciP gene, the 6B capsule gene locus has wciNα just as 6A does. The 6B capsule gene locus might capture the wciNβ gene and may form a new capsular PS, named 6D. It was unknown whether the 6D serotype exists in nature because there had been no search for 6D among natural isolates.
Although the currently available 23-valent pneumococcal vaccine contains 6B PS, the old 14-valent pneumococcal vaccine contained 6A PS. The PS was replaced in the 23-valent pneumococcal vaccine because 6A PS was not highly stable in vaccine preparations, and because 6B PS induced antibodies cross-reacting to 6A PS. Robbins et al., 1983. Investigations found the chemical instability to be due to the 1→3 linkage between rhamnose and ribitol. Zon et al., 1982. Because the 1→4 linkage found in 6B PS is more stable than the 1→3 linkage of 6A PS, 6B PS is more stable than 6A PS. The putative structure of 6D PS should be identical to that of 6C PS except that 6C PS has the unstable 1→3 linkage whereas 6D PS should have the more stable 1→4 linkage. Thus, it is likely that 6D PS would be more useful in a vaccine than 6C PS.
PCR and DNA Sequencing:
PCR reaction mixtures contained 38.80 of sterile water, 2 μl of each 5-pmol primer, 2 μl of 10 mM dNTP, 5 μl of 10× buffer solution, and 1 μl of LA Taq polymerase (2.5U/μl, Takara Biomedical, Shiga, Japan). For template, either chromosomal DNA isolated with Wizard genomic DNA purification kit (Promega, Madison, Wis.) or colonies grown on blood agar plates were used. Thermal cycling conditions varied depending on the primer set used. PCR products were analyzed by electrophoresis in 1% agarose gels. The primers used are listed in Table 7. PCR products were purified using Wizard PCR Clean-up System (Promega), and the DNA sequencing was performed by the genomics core facility at the University of Alabama at Birmingham (UAB). DNA sequences were analyzed with Lasergene v. 5.1 software (DNASTAR, Madison, Wis.) and were compared to the previously reported sequences of the 6B and 6C cps loci in GenBank (access numbers CR931639, EF538714, respectively). Mavroidi et al., 2004.
Creation of 6D by a Genetic Manipulation:
Capsule synthesis involves the cooperation of many genes. For instance, if the repeating unit of 6D PS cannot be exported to the outside of the bacteria to be assembled into a long chain of capsular PS, the repeating unit would accumulate in the bacteria, creating a lethal condition for bacteria with the capsule gene loci for the 6D serotype. If this is so, it is unlikely that the 6D serotype is present in nature. Thus, it is important to demonstrate that the 6D serotype is biologically possible.
The strategy for creating TIGR6D is depicted in
The TIGR6D cps locus sequence is deposited in Genbank (accession number EU714777). TIGR6D was morphologically indistinguishable from TIGR6B when grown on blood agar plates. Also, TIGR6D grew as well as other pneumococcal strains in THY broth. Serotype 6D was deposited in the American Type Culture Collection (10601 University Blvd., Manassas, Va. 20110) on Oct. 5, 2010, and is commercially available as BEI Number NR-20806.
Quellung Reaction:
Bacterial colonies from blood agar plates were suspended in a small volume of phosphate-buffered saline (PBS), and 20 of this broth was combined with 20 of serum and 20 of methylene blue dye solution (3 mg/mL methylene blue and 1.5 mg/mL NaCl in sterile water) on a glass microscope slide. After adding a coverslip, mixtures were examined under bright-field microscopy using a 100× oil immersion lens. The rabbit antisera specific for serotypes 6A and 6B were prepared by the CDC.
Example 12 Inhibition ELISA Used to Distinguish Serotypes 6B and 6DThe two serotypes were distinguished using an inhibition-type-ELISA. Briefly, the wells of ELISA plates (Corning Costar Corp., Acton, Mass.) were coated at 37° C. with 5 μg/mL of 6B capsular PS (ATCC, Manassas, Va.) overnight in PBS. After washing the plates three times with PBS containing 0.05% Tween 20, 500 of a previously diluted bacterial culture supernatant (or lysates) was added to the wells along with 500 of an anti-6B mAb. Pneumococcal lysates were prepared by growing pneumococci overnight in 1 ml of THY broth without shaking and then incubating the tubes for 15 min at 37° C. with a lysis buffer (0.1% sodium deoxycholate, 0.01% sodium dodecyl sulfate, and 0.15M sodium citrate in deionized water). Culture supernatants of 6B-specific hybridomas Hyp6BM7 and Hyp6BM8 were used at dilutions of 1:50 and 1:100, respectively. After 30 min of incubation in a humid incubator at 37° C., the plates were washed three times and incubated for 30 min with alkaline phosphatase-conjugated goat anti-mouse immunoglobulin (Sigma, St. Louis, Mo.). The plates were washed three times and then 1000 of paranitrophenyl phosphate substrate (Sigma) in diethanolamine buffer at a concentration of 1 mg/ml was added, and allowed to incubate at room temperature for 1-2 hours. The optical density at 405 nm was read with a microplate reader (BioTek Instruments Inc, Winooski, Vt.).
Example 13 Purification and Characterization of 6D Capsular PSCapsular PS expressing serotype 6D was purified in two different ways. One method (ethanol precipitation method) was to purify the PS by ethanol precipitation, ion exchange chromatography, and molecular weight sizing chromatography as described previously. Park et al., 45 J. Clin. Microbiol. 1225-33 (2007). The other method, which is faster than the first method and is to purify capsular PS after removing protoplasts, is described below. TIGR6D was grown in 1 liter of THY broth without shaking until the culture reached an OD600 of ˜0.4. The culture was then centrifuged at 15,000 g for 10 min. The cell pellet was washed twice with 1L PBS, and resuspended in 30 mL of protoplast buffer {20% sucrose, 5 mM Tris-HCl (pH 7.4), and 2.5 mM Mg2SO4 in deionized water} with mutanolysin (Sigma) at a concentration of 20 U/mL and allowed to incubate overnight at room temperature. The next day, the bacterial cells were examined under a phase contrast microscope to ensure “protoplasting” had occurred, then protoplasts were removed by centrifugation at 27,000 g for 15 min. The supernatant was sterilized through a 0.22 micron filter, diluted 1:1 in deionized water, and was loaded onto a DEAE-Sepharose column (Amersham Biosciences, Uppsala, Sweden) with a 2 ml bed volume. The column was washed with 4 ml of 50 mM ammonium acetate, and the PS was eluted from the column with 4 ml of 500 mM ammonium acetate. After lyophilization, the eluted PS was loaded on a Sephacryl S-300 HR column (Amersham Biosciences) with a bed volume of 130 mL and the PS was eluted with 10 mM Tris-HCl (pH 7.4). The fractions were tested for the presence of PS by the inhibition assay using Hyp6BM8. The first 1 mL of fractions, which contains majority of PS, were pooled and lyophilized.
Monosaccharide Composition Analysis of PS:
1 mg of lyophilized capsular PS prepared by the protoplast method was dissolved in 500 μl of 1M HCl and incubated at 80° C. for 16 hr. After drying the sample under a nitrogen stream, the remaining PS was washed twice with 250 μl of methanol. The sample was then incubated with 200 μl of Tri-Sil Reagent (Pierce Biotech Inc., Rockford, Ill.) to trimethylsilylate all the monosaccharides. The reaction products were analyzed on a gas-liquid chromatograph/mass spectrometer (GLC/MS) (Varian 4000; Varian Inc., Palo Alto, Calif.) fitted with a 30-m (0.25-mm-diameter) VF-5 capillary column. Column temperature was maintained at 100° C. for 5 min and then increased to 275° C. at 20° C./min and finally held at 275° C. for 5 min. The effluent was analyzed by mass spectrometry (MS) using the electron impact ionization mode. The areas of each monosaccharide peaks in GLC/MS were determined using Varian MS Workstation v6.5 software.
Analysis of PS by Tandem Mass Spectrometry:
Intact capsular PSs prepared by the ethanol precipitation method were hydrolyzed to their repeating units before analysis by mass spectrometry. A 2 mg aliquot of PS was hydrolyzed in 1 mL of 10 mM NaOH at 85° C. for 120 hours followed by another hydrolysis at 50 mM NaOH at 85° C. for 120 hr. At the end of hydrolysis, all samples were neutralized with 0.1M HCl.
Tandem mass spectrometry (MS/MS) was performed in the Mass Spectrometry Shared Facility at the UAB with a Micromass Q-TOF2 mass spectrometer (Micromass Ltd., Manchester, United Kingdom) equipped with an electrospray ion source. The samples dissolved in distilled water were injected into the mass spectrometer with running buffer (50/50 acetonitrile-water containing 0.1% formic acid) at a rate of lμ1/min using a Harvard syring pump. The injected sample was negatively ionized with electrospray and detected with a time-of-flight mass spectrometer. For MS/MS, the parent ion was fragmented into daughter ions by energizing it to either 35 eV or 40 eV before collision with argon gas. The daughter ions were analyzed with a time-of-flight mass spectrometer. The MS/MS spectra were processed using the Max-Ent3 module of MassLynx 2.5. The study showed that ribitol and glucose 1 are cleaved by periodate while glucose 2 and rhamnose are not. The mass of daughter ions showed that the phosphodiester bond is made to the 2nd position of glucose 1 and all other glycosidic bonds are same as 6B PS). The MS/MS studies supported the proposed structure shown in
More specifically, to determine if the monosaccharide sequence of the 6D PS is as proposed in
Additionally, the ion with 683 AMU (i.e., intact repeating unit) was subjected to argon collision and identified its daughter ions with MS/MS analysis. Daughter ions were observed at 521, 359, and 213 AMU, which respectively represents daughter ions that lost the first glucose, the second glucose, and rhamnose (
Oxidation and Reduction of PS:
Capsular PSs were dissolved in 80 mM sodium acetate buffer (pH 4) at a concentration of 1 mg/mL. Sodium periodate was added to the PS solution to a final concentration of 40 mM and the reaction mixture was incubated in the dark at 4° C. for 72 hours. Excess periodate was destroyed by adding ethylene glycol. To determine intact monosaccharides of the oxidized capsular PS, PS was then lyophilized then analyzed using GLC/MS as described above. To investigate the glycosidic bonds, sample was reduced with sodium borohydride or sodium tetradeuteroborate as previously described (Park et al., 45 J. Clin. Microbiol. 1225-33 (2007)), before subjecting to MS/MS as described above. 6D PS was prepared with protoplast method and 6B PS was obtained from ATCC.
Hydrolytic Stability Assay:
0.9 mL of 2 mg/mL PS in water was mixed with 0.1 mL of 0.1 M NaOH and this solution was split into two eppendorf tubes and incubated at 85° C. At the indicated times, 0.1 mL was removed from these samples, neutralized with 0.1M HCl, and then stored at 4° C. until used in the inhibition ELISA. Using the same buffers and incubation conditions described for the inhibition ELISA above, plates were coated with 100 μl of 5 μg/mL 6A, 6B, 6C, or 6D PS. The ELISA was performed with the hydrolyzed samples on plates coated with their respective PSs. For 6A and 6C PSs, Hyp6AG1 was used as the primary antibody (as performed in Park et al., 2007), and for 6B and 6D, Hyp6BM8 was used (as described above). Data shown is the average of samples run in duplicate (
During the alkali hydrolysis experiments for mass spectrometry, 6D PS was very resistant to alkali hydrolysis. To measure resistance to hydrolysis, the ability of 6A, 6B, 6C and 6D PSs to inhibit binding of Hyp6BM8 (for 6B and 6D PSs) or Hyp6AG1 (for 6A and 6C PSs) to target PS after alkali hydrolysis for various time periods were compared. The 6A and 6C PSs completely lost their ability to inhibit after only 1 hour of hydrolysis. In contrast, 6B PS lost 90% of its inhibitory ability in 8 hours and more than 100 hours of hydrolysis was needed for 6D PS to lose its inhibitory ability by 90% (
To determine whether 6D exists in nature, 264 pneumococcal isolates that were previously serotyped as “6B” by classical means were re-serotyped for serotypes 6B or 6D using mAbs. The isolates were a part of a University of Alabama, Birmingham, laboratory collection of 6B isolates, which have originated from Africa, Asia, Australia, South America, North America, and Europe. In addition to these, TIGR6A, TIGR6AX, and TIGR6C, which are isogenic strains of TIGR4 expressing the 6A-type capsule, no capsule, and 6C-type capsule (Park et al., 75. Infect. Immun. 4482-89 (2007)), were used as assay controls or a source of DNA. Additional TIGR4 variants, which are TIGR6B, TIGR6BX, and TIGR6D, were prepared as described below. All bacteria were grown in Todd-Hewitt broth (BD Biosciences, San Jose, Calif.) supplemented with 0.5% yeast extract (THY) and kept frozen at −80° C. until used.
When the serological properties of TIGR6D were examined by the quellung reaction using polyclonal rabbit antisera, it was typed as 6B. When TIGR6D PS was examined for binding various mAbs to 6A and 6B PS using an inhibition ELISA, it was reactive with many mAbs to 6B PS. For instance, TIGR6D inhibited Hyp6BM8 binding to 6B PS. These observations clearly demonstrated that 6D PS is serologically very close to 6B PS. In contrast, a mAb specific to 6B PS (Hyp6BM7) was not reactive with 6D PS (
Above serological studies showed that if pneumococcal isolates expressing 6D PS are present in nature, they would have been typed as 6B serotype. To look for the presence of serotype 6D isolates in nature, 264 pneumococcal isolates that were previously typed as serotype 6B were examined using an inhibition ELISA capable of distinguishing between the 6B and 6D serotypes (
Claims
1. An isolated bacterial strain designated Streptococcus pneumoniae 6D; said bacterial strain characterized as having a capsular polysaccharide having the repeating unit {→2) glucose 1 (1→3) glucose 2 (1→3) rhamnose (1→4) ribitol (5→phosphate}.
2. A composition comprising a purified polysaccharide comprising the repeating unit {→2) glucose 1 (1→3) glucose 2 (1→3) rhamnose (1→4) ribitol (5→phosphate}.
3. The composition of claim 2, wherein said polysaccharide is conjugated to a carrier.
4. The composition of claim 2, wherein the carrier is a protein.
5. The composition of claim 2, wherein the carrier is a bead.
6. The composition of claim 2, wherein the composition comprises an adjuvant.
7. The composition of claim 2, wherein the polysaccharide is produced by a recombinant bacterium that expresses a recombinant genome that produces a capsular polysaccharide having the repeating unit {→2) glucose 1 (1→3) glucose 2 (1→3) rhamnose (1→4) ribitol (5→phosphate}.
8. The composition of claim 2, wherein the polysaccharide is produced by an isolated Streptococcus pneumoniae 6D, wherein said Streptococcus pneumoniae 6D has a capsular polysaccharide having the repeating unit {→2) glucose 1 (1→3) glucose 2 (1→3) rhamnose (1→4) ribitol (5→phosphate}.
9. A method of generating monoclonal or polyclonal antibodies using the composition of claim 2.
10. The method of claim 9, wherein the composition further comprises an adjuvant.
11. The method of claim 9, wherein the polysaccharide is conjugated to a carrier.
Type: Application
Filed: Feb 3, 2015
Publication Date: Jun 11, 2015
Inventors: Moon H. Nahm (Mountain Brook, AL), In Ho Park (Seoul)
Application Number: 14/612,417
