This study shows that induction of tumor-specific CD4+ T cells by vaccination with a specific viral T helper epitope, contained within a synthetic peptide, results in protective immunity against major histocompatibility complex (MHC) class II negative, virus-induced tumor cells. Protection was also induced against sarcoma induction by acutely transforming retrovirus. In contrast, no protective immunity was induced by vaccination with an unrelated T helper epitope. By cytokine pattern analysis, the induced CD4+ T cells were of the T helper cell 1 type. The peptide-specific CD4+ T cells did not directly recognize the tumor cells, indicating involvement of cross-priming by tumor-associated antigen-presenting cells. The main effector cells responsible for tumor eradication were identified as CD8+ cytotoxic T cells that were found to recognize a recently described immunodominant viral gag-encoded cytotoxic T lymphocyte (CTL) epitope, which is unrelated to the viral env-encoded T helper peptide sequence. Simultaneous vaccination with the tumor-specific T helper and CTL epitopes resulted in strong synergistic protection. These results indicate the crucial role of T helper cells for optimal induction of protective immunity against MHC class II negative tumor cells. Protection is dependent on tumor-specific CTLs in this model system and requires cross-priming of tumor antigens by specialized antigen-presenting cells. Thus, tumor-specific T helper epitopes have to be included in the design of epitope-based vaccines.

CTL epitope (KSPWFTTL) encoded by AKV/MCF type of murine leukemia virus (MuLV) differs from the sequence in Friend/Moloney/Rauscher (FMR) type in one residue (RSPWFTTL). CTL experiments indicated defective processing of the FMR peptide in tumor cells. Proteasome-mediated digestion of AKV/MCF-type 26-mer peptides resulted in the early generation and higher levels of epitope-containing fragments than digestion of FMR-type peptides, explained by prominent cleavage next to R in the FMR sequence. The fragments were identified as 10- and 11-mer peptides and were efficiently translocated by TAP. The naturally presented AKV/MCF peptide is the 8-mer, indicating ER peptide trimming. In conclusion, a single residue exchange can cause CTL epitope destruction by specific proteasomal cleavage.

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Upon infection with the Moloney murine sarcoma virus-murine leukemia virus (MuLV) complex, H-2b C57BL/6 (B6) mice respond with a class I D'-restricted cytotoxic T-lymphocyte (CTL) response, which protects against virus-induced tumorigenesis. In the B6-derived Db mutant B6.CH-2bm13 (bml3) strain, part of the class I Dt antigen-presenting groove is shaped by a class I K'-encoded sequence. Like B6 mice, bml3 mice reject Moloney virus-induced tumors, but the protective CTL response is K' restricted. In this study we show enhanced levels of Moloney MuLV-specific CTLp with a restriction for K' in bml3 mice. Through the use of CTL clones from Moloney virus-immunized bml3 mice, the class I Kb-presented CTL epitope was identified. The epitope is located in the Moloney virus gp7O envelope protein region {Moloney envelope, amino acids 189 to 196 [Mol env (189-196)] }, SSWDFITV and has the K' allele-specific binding motif. The Dbm13 molecule does not present the env(189 to 196) epitope to Kb-restricted bml3 CTL. In B6 mice, Mol env(189-196)-specific CTL could be induced by peptide vaccination. B6 mice thus have CTL precursors specific for this epitope but at considerably lower levels than do bml3 mice. We hypothesize that additional positive selection of K?-restricted CTL on the Dbml3 molecule in bml3 mice explains this difference in precursor frequencies. We examined related strains of MuLV for the presence of Mol env(189-196) sequence equivalents. Rauscher, Friend, and AKV MuLV-encoded Mol env(189-196) epitope equivalents were properly recognized in cytotoxicity assays, both as synthetic and as endogenously expressed (Rauscher MuLV) peptides. In contrast, the mink cell focus-forming virus MuLV-encoded epitope equivalent, lacking a K' anchor residue, was not presented for CTL recognition and hence can be excluded as an important CTL epitope for mink cell focus-forming viruses.

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