An immunodominant Kb-restricted peptide from the p15E transmembrane protein of endogenous ecotropic murine leukemia virus (MuLV) AKR623 that restores susceptibility of a tumor line to anti-AKR/Gross MuLV cytotoxic T lymphocytes

H-2b tumor cells expressing the endogenous ecotropic murine leukemia virus (EMV) induce an anti-AKR/Gross murine leukemia virus (MuLV) cytotoxic T-lymphocyte (CTL) response in the C57BL/6 mouse strain. The EMV clone AKR623 has been used to infect SC.Kb fibroblast cells, resulting in SC.Kb/623 targets that are lysed by bulk anti-AKR/Gross MuLV CTL with a profile that is similar to that for the EMV+ AKR.H-2b SL1 tumor target. Anti-AKR/Gross MuLV CTL are restricted by the class I Kb antigen and do not cross-react with Friend-Moloney-Rauscher virus-positive targets. The AKR623 genome was searched by computer for coding sequences that fit the motif XXXX(FY)XX(VIML) for Peptides that bind Kb. Of 30 octameric Peptides identified, 12 that were unique to AKR623 and different from published Friend-Moloney-Rauscher sequences were synthesized and bound to EMV-negative SC.Kb cells, which were then assayed as targets against anti-AKR/Gross MuLV CTL. One peptide, peptide 12 (KSPWFTTL) from the p15E transmembrane protein, sensitized SC.Kb target cells to lysis by anti-AKR/Gross MuLV CTL with a profile similar to those seen for AKR.H-2b SL1 tumor targets and SC.Kb/623 fibroblast targets. Low concentrations of peptide were sufficient, the half-maximal lysis occurring at 10 to 100 pg/ml. SC.Kb/peptide 12 targets were recognized by the H-2b-restricted bulk CTL in a conventional class I Kb-restricted fashion. Unlabeled SC.Kb/peptide 12-pulsed targets were effective in competing with radiolabeled SC.Kb/623 targets for lysis by anti-AKR/Gross MuLV CTL. This finding is consistent with the notion that peptide 12 represents the dominant endogenously processed epitope recognized by these Antiviral CTL. In addition, peptide 12 is immunogenic in that it could stimulate the in vitro generation of an anti-AKR/Gross MuLV CTL response from tumor-primed C57BL/6 responder spleen cells. Finally, the physiological relevance of peptide 12 was suggested by its ability to fully restore the recognition and lysis of AKR.H-2b SL1 clone 18-5 tumor cells, a naturally occurring variant tumor clone that is insusceptible to lysis by anti-AKR/Gross MuLV CTL. These data indicate that a virus-encoded antigen, represented by peptide 12, and not a nonviral tumor antigen, is the immunodominant epitope responsible for the recognition of EMV+ tumor cells by C57BL/6-derived anti-AKR/Gross MuLV CTL.