Epigenetic Mechanisms Underlying HIV-Infection Induced Susceptibility of CD4+ T Cells to Enhanced Activation-Induced FasL Expression and Cell Death

Background: Chronic immune activation and CD4(+) T cell depletion are significant pathogenic features of HIV infection. Expression of Fas ligand (FasL), a key mediator of activation-induced cell death in T cells, is elevated in people living with HIV-1 infection (PLWH). However, the epigenetic mechanisms underlying the enhanced induction of FasL expression in CD4(+) T lymphocytes in PLWH are not completely elucidated. Hence, the current work examined the effect of HIV infection on FasL promoter-associated histone modifications and transcriptional regulation in CD4(+) T lymphocytes in PLWH. Method: Flow cytometric analysis was performed to examine the Fas-FasL expression on total CD4(+) T cells and naive/memory CD4(+) T cell subsets. Epigenetic FasL promoter histone modifications were investigated by chromatin immunoprecipitation-quantitative real-time polymerase chain reaction analysis using freshly isolated total CD4(+) T lymphocytes from HIV-1 infected and noninfected individuals. Results: All naive/memory CD4(+) T cell subsets from PLWH showed markedly greater frequency of FasL expression. Notably, examination of functional outcome of FasL/Fas co-expression demonstrated the preferential susceptibility of Tcm and Tem subsets to activation-induced apoptosis. Importantly, these CD4(+) T cells collectively demonstrated a distinct FasL promoter histone profile involving a coordinated cross-talk between histone H3 modifications leading to enhanced FasL gene expression. Specifically, levels of transcriptionally permissive histone H3K4-trimethylation (H3K4Me3) and histone H3K9-acetylation (H3K9Ac) were increased, with a concomitant decrease in the repressive H3K9-trimethylation (H3K9Me3). Conclusion: The present work demonstrates that epigenetic mechanisms involving promoter-histone modifications regulate transcriptional competence and FasL expression in CD4(+) T cells from PLWH and render them susceptible to activation-induced cell death.