The wild-type p15E short peptide is poorly immunogenic and does not inhibit tumor growth. C57BL/6 mice were vaccinated intramuscularly with 2 μg of E7 or p15E peptide displayed on CPQ liposomes (along with 8 μg CoPoP, 3.2 μg PHAD and 3.2 μg QS-21) on day 0 and 7; then blood was collected for Ag-specific CD8+ T-cell tetramer staining on day 14 and E7 vaccinated mice were challenged with 1 × 105 TC-1 cells and p15E vaccinated mice were challenged with 1 × 106 MC38 cells. (A) Ag-specific CD8+ T-cell percentages in CD8+ T cells in blood. TC-1 (B) and MC38 (C) tumor volume in non-vaccinated control (Ctrl) mice, or mice immunized with the E7 or p15E (wild-type) peptides. Statistical analysis shows unpaired t test. ****, P < 0.0001. Error bars show mean + SD for 3 mice per group.
ARTICLE ABSTRACT
Mimotopes of short CD8+ T-cell epitopes generally comprise one or more mutated residues, and can increase the immunogenicity and function of peptide cancer vaccines. We recently developed a two-step approach to generate enhanced mimotopes using positional peptide microlibraries and herein applied this strategy to the broadly used H-2Kb–restricted murine leukemia p15E tumor rejection epitope. The wild-type p15E epitope (sequence: KSPWFTTL) was poorly immunogenic in mice, even when combined with a potent peptide nanoparticle vaccine system and did not delay p15E-expressing MC38 tumor growth. Following positional microlibrary functional screening of over 150 mimotope candidates, two were identified, both with mutations at residue 3 (p15E-P3C; “3C,” and p15E-P3M; “3M”) that better induced p15E-specific CD8+ T cells and led to tumor rejection. Although 3M was more immunogenic, 3C effectively delayed tumor growth in a therapeutic setting relative to the wild-type p15E. As 3C had less H-2Kb affinity relative to both p15E and 3M, 15 additional mimotope candidates (all that incorporated the 3C mutation) were assessed that maintained or improved predicted MHC-I affinity. Valine substitution at position 2 (3C2V, sequence: KVCWFTTL) led to improved p15E-specific immunogenicity, tumor rejection, and subsequent long-term antitumor immunity. 3C, 3M, and 3C2V mimotopes were more effective than p15E in controlling MC38 and B16-F10 tumors. T-cell receptor (TCR) sequencing revealed unique TCR transcripts for mimotopes, but there were no major differences in clonality. These results provide new p15E mimotopes for further vaccine use and illustrate considerations for MHC-I affinity, immunogenicity, and functional efficacy in mimotope design.
The MHC-I–restricted p15E tumor rejection epitope is expressed in multiple murine cancer lines and is used as a marker of antitumor cellular immunity, but has seen limited success as a vaccine immunogen. An in vivo screening approach based on a positional peptide microlibraries is used to identify enhanced p15E mimotopes bearing amino acid mutations that induce significantly improved functional immunogenicity relative to vaccination with the wild-type epitope.
