Selective BCL-X-L Antagonists Eliminate Infected Cells from a Primary-Cell Model of HIV Latency but Not from Ex Vivo Reservoirs

HIV persists, despite immune responses and antiretroviral therapy, in vi -ral reservoirs that seed rebound viremia if therapy is interrupted. Previously, we showed that the BCL-2 protein contributes to HIV persistence by conferring a sur-vival advantage to reservoir-harboring cells. Here, we demonstrate that many of the BCL-2 family members are overexpressed in HIV-infected CD4(+) T cells, indicating increased tension between proapoptotic and prosurvival family members-and sug-gesting that inhibition of prosurvival members may disproportionately affect the sur-vival of HIV-infected cells. Based on these results, we chose to study BCL-X-L due to its consistent overexpression and the availability of selective antagonists. Infection of primary CD4(+) T cells with HIV resulted in increased BCL-X-L protein expression, and treatment with two selective BCL-X-L antagonists, A-1155463 and A-1551852, led to selective death of productively infected CD4(+) T cells. In a primary cell model of la-tency, both BCL-X-L antagonists drove reductions in HIV DNA and in infectious cell frequencies both alone and in combination with the latency reversing agent bryosta-tin-1, with little off-target cytotoxicity. However, these antagonists, with or without bryostatin-1 or in combination with the highly potent latency reversing agent combi-nation phorbol myristate acetate (PMA) + ionomycin, failed to reduce total HIV DNA and infectious reservoirs in ex vivo CD4(+) T cells from antiretroviral therapy (ART)-sup-pressed donors. Our results add to growing evidence that bona fide reservoir-harbor-ing cells are resistant to multiple "kick and kill" modalities-relative to latency mod-els. We also interpret our results as encouraging further exploration of BCL-X-L antagonists for cure, where combination approaches, including with immune effec-tors, may unlock the ability to eliminate ex vivo reservoirs. IMPORTANCE Although antiretroviral therapy (ART) has transformed HIV infection into a manageable chronic condition, there is no safe or scalable cure. HIV persists in "reservoirs" of infected cells that reinitiate disease progression if ART is interrupted. Whereas most efforts to eliminate this reservoir have focused on exposing these cells to immune-mediated clearance by reversing viral latency, recent work shows that these cells also resist being killed. Here, we identify a "prosurvival" factor, BCL-X-L, that is overexpressed in HIV-infected cells, and demonstrate selective toxicity to these cells by BCL-X-L antagonists. These antagonists also reduced reservoirs in a pri-mary-cell latency model but were insufficient to reduce "natural" reservoirs in ex vivo CD4(+) T cells-adding to growing evidence that the latter are resilient in a way that is not reflected in models. We nonetheless suggest that the selective toxicity of BCL-X-L antagonists to HIV-infected cells supports their prioritization for testing in combinations aimed at reducing ex vivo reservoirs.