Abstract: A vertebrate animal, such as a human, is immunized with a bacterial host cell harboring a recombinant replicon which provides a stochastically expressed cell killing function whereby the cells are biologically contained. The replicon also comprises a gene encoding an antigen which is to be displayed on the outer surface of the host cell, so that it can elicit an immune response from the immunized animal. This antigen comprises one or more epitopes from a pathogenic agent.

Abstract: A replicon, in which a nucleotide sequence encoding a cell killing function is regulatably expressed when the replicon is harbored in one type of host cell (primary host cell), so that cells harboring the replicon are killed under conditions under which the cell killing function is expressed, and the nucleotide sequence encoding the cell killing function is regulatably or constitutively expressed when the replicon is harbored in another type of host cell (secondary host cell), so that cells harboring the replicon are invariably killed or killed under conditions under which the cell killing function is expressed, may be used in a method of active biological containment of cells under defined environmental conditions. The biological containment principle may be utilized in the industrial production of a biosynthetic product by recombinant DNA techniques, when deliberately releasing a genetically engineered microorganism to the natural environment or in the preparation of a live vaccine.

Abstract: A replicon, in which a nucleotide sequence encoding a cell killing function is regulatably expressed when the replicon is harboured in one type of host cell (primary host cell), so that cells harbouring the replicon are killed under conditions under which the cell killing function is expressed, and the nucleotide sequence encoding the cell killing function is regulatably or constitutively expressed when the replicon is harboured in another type of host cell (secondary host cell), so that cells harbouring the replicon are invariably killed or killed under conditions under which the cell killing function is expressed, may be used in a method of active biological containment of cells under defined environmental conditions.The biological containment principle may be utilized in the industrial production of a biosynthetic product by recombinant DNA techniques, when deliberately releasing a genetically engineered microorganism to the natural environment or in the preparation of a live vaccine.

Abstract: The stable maintenance of a replicon in a population of growing cells is ensured by providing the replicon with a sequence which encodes a product capable of killing the cell harboring the replicon or the progeny of the cell (or encodes a precursor for the product) and a sequence encoding an antagonist for the killing product (or a precursor for the antagonist). The antagonist is one which suppresses the killing product (or a precursor for the killing product) in cells harboring the replicon, whereas the antagonist activity decays when the replicon is lost from the cell so that the antagonist (or its precursor) is no longer continuously expressed. This means that the killing product (or its precursor) present in the now replicon-free cell is no longer suppressed by the antagonist, resulting in cell death.Cells containing the thus stabilized replicon may be grown on a large scale without any significant loss of the replicon from the cell population even when no selection pressure is applied.