BCL6 Inhibitor-mediated Downregulation of pSAMHD1 and T Cell Activation Are Associated with Decreased HIV Infection and Reactivation.

Clearance of HIV-infected germinal center (GC) CD4+ follicular helper T cells (Tfh) after combination antiretroviral therapy (ART) is essential to an HIV cure. Blocking B cell lymphoma 6 (BCL6, the master transcription factor for Tfh cells) represses HIV infection of tonsillar CD4+ Tfh ex vivo , reduces GC formation and limits immune activation in vivo We assessed the anti-HIV activity of a novel BCL6 inhibitor, FX1, in Tfh/non-Tfh CD4+ T cells, and its impact on T cell activation and SAMHD1 phosphorylation (Thr592). FX1 repressed HIV-1 infection of peripheral CD4+ T cells, tonsillar Tfh/non-Tfh CD4+ T cells (p<0.05), total elongated-and multi-spliced HIV-1 RNA production during the first round of viral life cycle (p<0.01). Using purified circulating CD4+ T cells from uninfected donors, we demonstrate that FX1 treatment resulted in down-regulation pSAMHD1 expression (p<0.05) and T cell activation (HLA-DR, CD25, Ki67, p<0.05) ex vivo corresponding with inhibition of HIV-1 and HIV-2 replication. Ex vivo HIV-1 reactivation using purified peripheral CD4+ T cells from HIV-infected ART-suppressed donors was also blocked by FX1 treatment (p<0.01). Our results indicate that BCL6 function contributes to Tfh/non-Tfh CD4+ T cell activation and cellular susceptibility to HIV infection. BCL6 inhibition represents a novel therapeutic strategy to potentiate HIV suppression in Tfh/non-Tfh CD4+ T cells without reactivation of latent virus. IMPORTANCE The expansion and accumulation of HIV-infected BCL6+ Tfh CD4+ T cells are thought to contribute to the persistence of viral reservoirs in infected subjects undergoing ART. Two mechanisms have been raised for the preferential retention of HIV within Tfh CD4+ T cells: (a) antiretroviral drugs have limited tissue distribution, resulting in insufficient tissue concentration and lower efficacy in controlling HIV replication in lymphoid tissues; (b) cytotoxic CD8+ T cells within lymphoid tissues express low levels of chemokine receptor (CXCR5) thus limiting their ability to enter the GCs to control/eliminate HIV-infected Tfh cells. Our results indicate that the BCL6 inhibitor FX1 can not only repress HIV infection of tonsillar Tfh ex vivo , but also suppress HIV infection and reactivation in primary, non-Tfh CD4+ T cells. Our study provides a rationale for targeting BCL6 protein to extend ART-mediated reduction of persistent HIV and/or support strategies towards HIV remission beyond ART cessation.