BCL6 Inhibitor-Mediated Downregulation of Phosphorylated SAMHD1 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 , reduces GC formation, and limits immune activation 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 and tonsillar Tfh/non-Tfh CD4 T cells ( < 0.05) and total elongated and multispliced HIV-1 RNA production during the first round of viral life cycle ( < 0.01). Using purified circulating CD4 T cells from uninfected donors, we demonstrate that FX1 treatment resulted in downregulation pSAMHD1 expression ( < 0.05) and T cell activation (HLA-DR, CD25, and Ki67;  < 0.01) corresponding with inhibition of HIV-1 and HIV-2 replication. HIV-1 reactivation using purified peripheral CD4 T cells from HIV-infected ART-suppressed donors was also blocked by FX1 treatment ( < 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. 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: (i) antiretroviral drugs have limited tissue distribution, resulting in insufficient tissue concentration and lower efficacy in controlling HIV replication in lymphoid tissues, and (ii) 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 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 toward HIV remission beyond ART cessation.