AKR/Gross virus-specific cytotoxic T lymphocytes (CTL) from C57BL/6 (B6) mice are H-2K-restricted and recognize epitopes encoded by the prototype endogenous ecotropic murine leukaemia virus (Emv) AKR623. Four CTL epitopes have been identified by the use of synthetic peptides corresponding to AKR623-encoded amino acid sequences. Here we present both functional and nucleotide sequence data indicating that three closely related Emv share all of these CTL epitopes. We also found that one other murine leukaemia virus (MuLV) was not susceptible to lysis by these CTL. This is the ecotropic component of the LP-BM5 virus complex that causes murine AIDS. Nucleotide sequencing revealed that three of the four epitopes, including the immunodominant peptide, are altered in this virus. The other epitope was unchanged. These data implied that the inability of anti-AKR/Gross virus CTL to lyse cells infected with the LP-BM5 ecotropic (BM5eco) MuLV was due to the functional loss of three of the four CTL epitopes. Using recombinant vaccinia and Sindbis virus vectors, we have shown that the BM5eco-encoded form of the immunodominant epitope, which differs only by an arginine for lysine substitution at the N-terminal residue, fails to induce a CTL response in B6 mice. Immunization with BM5eco-infected cells also failed to induce MuLV-specific CTL. In light of the long passage history of the LP-BM5 complex in B6 mice, our results are consistent with a contribution of CTL-mediated immune selection to the evolution of the BM5eco MuLV.


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