Background
The precise mechanisms enabling ART-free control of HIV remain to be fully elucidated. While HIV-specific CD8 T cells of HIV controllers (HIVc) are highly proliferative, the acquisition of maximal classical cytolytic potential (granzyme B, GZMB/perforin) after antigen exposure may require several days. In addition, the expression of classical cytolytic molecules may be tightly regulated in sites of HIV persistence, such as lymph nodes (LN). Here, we hypothesized that LN HIV-specific CD8 T cells may harbor tissue-specific, fast-acting additional cytotoxic functions.
Methods
To test this hypothesis, we performed an in-depth characterization of the transcriptomic, phenotypic and functional profiles of HIV-specific CD8 T cells of blood and LNs collected simultaneously from 5 individuals with chronic HIV infection (CHI), 7 HIVc and 4 ART-treated individuals.
Results
These analyses revealed that LN HIV-specific CD8 T cells of HIVc are enriched in cells co-expressing granzyme K (GZMK), CD69 (LN-retention marker) and CXCR5 (follicular-homing receptor) when compared to CHI (P<0.001) or long-term ART-treated (P<0.05) individuals, indicating that GZMK+ HIV-specific CD8 T cells of HIVc can preferentially migrate to LN follicles. We next showed that GZMK can be rapidly released after HIV peptide stimulation (P<0.001), suggesting that GZMK can be secreted upon antigen exposure. Interestingly, we showed that exogenous GZMK can bind the surface of tonsillar CD4 T cells (P<0.01) in a process inhibited by heparin (P<0.001), suggesting that released GZMK may interact with tissue CD4 T cells’ membrane-associated glycosaminoglycans. Notably, GZMK bound significantly more to tonsillar T-follicular helper (Tfh) than non-Tfh cells (P<0.01). Finally, GZMK exposure in presence of C1q-deficient serum induced a dose-dependent deposition of activated complemented fragment C3b and cell death, significantly higher on Tfh than non-Tfh cells (P<0.05). Notably, this effect was abolished by heparin or EDTA (P<0.05), indicating a process involving GZMK surface binding and subsequent local activation of the C3 convertase.
Conclusions
LN HIV-specific CD8 T cells of HIVc could position to follicular areas and express GZMK. GZMK is rapidly released after TCR engagement, can bind to CD4 T cells, mediate C3b deposition and ultimately lead to cell death. This mechanism may represent an additional, fast-acting antiviral response complementing the canonical GZMB/perforin-based cytotoxicity in lymphoid tissues.