Interfering overlapping epitopes contribute to the subdominance of an HLA-A2-Restricted HIV Gag-specific Epitope (132.18)

Recapitulating immunodominant A2-restricted HIV-specific CTLs that do not suppress virus in vivo is unlikely to be an effective vaccine strategy. To detect novel subdominant determinants in HIV-1 Gag, we primed CD8+ T cells from five seronegative donors with autologous dendritic cells transduced to express Gag. T cells were re-stimulated weekly with monocytes pulsed with 123 15-mer overlapping peptides (OLPs) spanning Gag. Responses were identified with OLP matrix pools followed by interrogation at the single OLP level in responsive pools using IFN-γ ELISPOT assays. One OLP (Gag145-159) was recognized by all donors. Of note, this reactivity predominated in three of five T cell cultures. Fine mapping with progressively truncated peptides revealed three overlapping epitopes: RTLNAWVKV (RV9), RTLNAWVKVV (RV10) and TLNAWVKVV (TV9). TV9 is a known epitope that is rarely recognized in vivo. In contrast, RV9 and RV10 are novel. Although the latter bound with lower affinities to HLA-A2 than TV9, RV9- and RV10-cultures were readily generated. RV9- and RV10-T cells were cytotoxic, capable of secreting many cytokines, and suppressive of HIV replication in vitro. In sum, we report two novel naturally processed and presented epitopes in HIV p24 that are recognized by pre-infection T cell repertoires. Further studies are needed to explain why these reactivities are rare during infections and more importantly, whether this conserved region of the HIV proteome has value as a prophylactic vaccine for A2 individuals.