The HIV-1 HLA-A2-SLYNTVATL Is a Help-Independent CTL Epitope

Abstract

The CTL response to the HLA-A*0201-restricted, HIV-1 p17 Gag77–85 epitope (SLYNTVATL; SL9) has been extensively studied in patients. Although this reactivity is exceptionally prominent in chronically infected patients and inversely correlated to viral load, SL9-specific CTLs (SL9-CTLs) are rarely detected in acute infection. To explore the cellular basis for this unusual manifestation, SL9-CTLs primed ex vivo from naive circulating CD8+ T cells of healthy, seronegative donors were generated and characterized. SL9 appeared to differ from other well-studied A*0201-restricted epitopes in several significant respects. In contrast to published reports for influenza and melanoma peptides and the HIV gag IV9 epitope studied here in parallel, SL9-CTLs were primed by immature but not mature autologous dendritic cells. Highly activated SL9-CTLs produce sufficient autocrine mediators to sustain clonal expansion and CTL differentiation for months without CD4+ T cells or exogenous IL-2. Moreover, SL9-CTLs were sensitive to paracrine IL-2-induced apoptosis. IL-2 independence and sensitivity to paracrine IL-2 were also characteristic of SL9-CTLs immunized by dendritic cells transduced by a nonreplicating lentiviral vector encoding full-length Gag. In vitro-primed SL9-CTLs resembled those derived from patients in degeneracy of recognition and functional avidities for both SL9 and its natural mutations. Together, these data show that SL9 is a highly immunogenic, help-independent HIV epitope. The scarcity of SL9-CTLs in acute infection may result from cytokine-induced apoptosis with the intense activation of the innate immunity. In contrast, SL9-CTLs that constitutively produce autocrine help would predominate during CD4-diminished chronic infection.

Availability of a Diversely Avid CD8+ T Cell Repertoire Specific for the Subdominant HLA-A2-Restricted HIV-1 Gag p2419–27 Epitope

“…HLA typing was possible for 280 of these samples, of which 121 were HLA-A2 ϩ . OLP21 (PRTLNAWVKVVEEKAP, p24 [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] ) was recognized by 24 subjects, with most of this reactivity (14 of 24) likely directed against the HLA-B*1503-restricted VKV VEEKAF epitope frequently seen in HLA-B*1503-expressing subjects (21). Reactivity to the nonameric TV9 peptide was confirmed in one of two HLA-A2 ϩ HLA-B*1503 Ϫ samples tested that reacted to OLP21 (L8 47, 500 sfc/10 6 PBMCs; Fig.…”

“…Cytotoxicity was determined by the chromium release assay (24). Target cells were T2 cells, Jurkat cells cotransfected with HLA-A*0201 and the HIV-1 proviral clone R7hyg (JA2/R7/Hyg) (49), and C1R cells expressing full-length wild-type HLA-A*0201 (C1R-A2 wt ) or the mutant D227K/ T228A HLA-A*0201 (C1R-A2 CD8null ) that does not bind the CD8 coreceptor (50).…”

“…Positively selected CD8 ϩ T cells (Dynabeads; Dynal Biotech) were primed with irradiated (4000 cGy) peptidepulsed or transduced DCs at a T cell-DC ratio of 5:1 in 48-well or 96-well cluster plates. Cells were cultured in complete medium containing 10 ng/ml IL-7 (Genzyme) and restimulated every 7-10 days with autologous monocytes pulsed with peptides or transduced DCs (24,25). IL-2 (20 U/ml) was added 1 and 4 days poststimulation.…”

“…With this approach, we have shown that CTLs to the HLA-A2-restricted immunodominant SL9 epitope in Gag p17 [77][78][79][80][81][82][83][84][85] are aberrantly sensitive to cytokine-mediated activation-induced cell death in vitro. At the same time, these T cells are capable of producing sufficient autocrine factors to support prolonged periods of proliferation (24,25). These characteristics provide a plausible explanation for the paucity of circulating SL9-specific CD8 ϩ T cell reactivity during the proinflammatory acute phase of infection, as well as its paradoxical dominance during the CD4-diminished chronic phase of infection (18,26,27).…”

“…HIV-1-specific CTLs primed directly ex vivo from healthy seronegative donors can provide complementary insights to findings in infected people, particularly into the pre-existing repertoire of T cell clonotypes and the immunological characteristics of T cells immediately after priming, in the absence of immune responses engendered to competing epitopes (24). With this approach, we have shown that CTLs to the HLA-A2-restricted immunodominant SL9 epitope in Gag p17 [77][78][79][80][81][82][83][84][85] are aberrantly sensitive to cytokine-mediated activation-induced cell death in vitro.…”