Recognition of Two Overlapping CTL Epitopes in HIV-1 p17 by CTL from a Long-Term Nonprogressing HIV-1-Infected Individual

HIV-1 infection has been shown to elicit strong CTL responses in some infected persons, but few data are available regarding the relationship between targeted epitopes and in vivo viral quasispecies. In this study, we examined the CTL response in a person infected for 15 yr with a CD4 count persistently >500 cells/μl. The dominant in vivo activated CTL response was directed against two overlapping Gag CTL epitopes in an area of p17 known to be essential for viral replication. The 9-mer SLYNTVATL (amino acids 77–85) was recognized in conjunction with HLA-A2, whereas the overlapping 8-mer TLYCVHQR (amino acids 83–91) was recognized by HLA-A11-restricted CTL. Analysis of in vivo virus sequences both in PBMC and plasma revealed the existence of sequence variation in this region, which did not affect viral replication in vitro, but decreased recognition by the A11-restricted CTL response, with maintenance of the A2-restricted response. These results indicate that an essential region of the p17 protein can be simultaneously targeted by CTL through two different HLA molecules, and that immune escape from CTL recognition can occur without impairing viral replication. In addition, they demonstrate that Ag processing can allow for presentation of overlapping epitopes in the same infected cell, which can be affected quite differently by sequence variation.

TAP-Independent MHC Class I Peptide Antigen Presentation to Alloreactive CTL Is Enhanced by Target Cell Incubation at Subphysiologic Temperatures

We investigated the peptide dependency of a group of CD8+ anti-HLA-B7 alloreactive CTL. The CTL killed target cells after acid denaturation of more than 98% of target cell surface peptide/MHC class I complexes. The CTL also killed TAP− HLA-B7-transfected T2 (T2B7) cells. The killing was enhanced by target cell incubation at 26°C. Despite these findings, which suggested peptide-independent allorecognition, CTL-mediated cytolysis was reduced or abolished by several point mutations affecting the HLA-B7 peptide-binding groove. Acid denaturation of HLA complexes on T2B7 cells prohibited CTL recognition. CTL recognition was restored by T2B7 cell incubation with β2-microglobulin and a single HPLC fraction containing peptides extracted from TAP+HLA-B7+ cells, but not by any of a panel of 17 synthetic HLA-B7-binding peptides. These findings indicated that CTL allorecognition was peptide specific. Sensitizing peptide was extracted from T2B7 cells only after incubation at 26°C. The amount of peptide detected in TAP+ cells was at least 10-fold and 100-fold greater than that detected in TAP− cells incubated at 26°C and at 37°C, respectively. TAP-independent peptide epitope presentation was sensitive to treatment with brefeldin A, but not sensitive to treatment with chloroquine, consistent with an endogenous peptide source. We propose that subphysiologic temperature incubation can enhance peptide/MHC class I presentation in the total absence of TAP function.