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 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 and tonsillar Tfh/non-Tfh CD4 T cells (P 0.05) and total elongated and multispliced 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 downregulation pSAMHD1 expression (P 0.05) and T cell activation (HLA-DR, CD25, and Ki67; P 0.01) 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: (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 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 toward HIV remission beyond ART cessation.