Abstract: The present invention relates to a polypeptide having a sequence of 7 to 20 amino acid residues, which is capable of modulating the immunosuppressive properties of a viral protein or a fragment thereof, against the host in which it is expressed (immunosuppression-modulatory sequence) when it substitutes the homologous sequence of the viral protein or fragment, the polypeptide including the minimum following consensus amino acid sequence: X1Y9Y10Y11CY12X2 wherein, X1 and X2 are selected to impact on the immunosuppressive properties, and Y9 to Y12 represent variable amino acid residues.

Abstract: The invention relates to a defective recombinant retrovirus containing a sequence coding for a specific protein in the genome of the cells. The DNA of the recombinant retrovirus contains the sequence coding for the protein intercalated between an acceptor splicing site and a donor splicing site, the whole assembly being placed under the control of a suitable promoter under conditions such that the infection of the said cells by this recombinant retrovirus is accompanied by the incorporation of the sequence coding for the protein into the genome of the infected cells.