266 : Host restriction factor SAMHD1 limits human retrovirus infection of primary monocytes via the innate sensor STING

Abstract

Human T-lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia (ATL) and other HTLV-1 associated neurological disorders. Unlike most retroviruses, cell-free HTLV-1 virions are poorly infectious and do not stably infect its primary CD4+ T lymphocyte target. However, HTLV-1 efficiently infects cells of the myeloid lineage, leading to productive infection of myeloid and plasmacytoid dendritic cells or abortive infection of primary monocytic cells. Here, we investigate the mechanisms underlying monocyte depletion driven by HTLV-1 abortive infection and demonstrate that de novo HTLV-1 infection rapidly induced apoptosis of primary monocytes in a SAMHD1-dependent manner. SAMHD1, a deoxynucleoside triphosphate triphosphohydrolase, functions as a host restriction factor that limits HIV-1 replication by reducing the availability of deoxynucleoside triphosphates (dNTPs) required for reverse transcription. Silencing of SAMHD1 by RNA interference resulted in the inhibition of monocyte apoptosis; addition of exogenous dNTPs to the infected monocyte culture, or pre-treatment with azidothymidine (AZT) to block reverse transcription also inhibited apoptosis. To investigate a role for reverse transcription intermediates (RTI) in triggering apoptosis, a biotinylated 90 nucleotide RTI from the U5 region of HTLV-1 was introduced into monocytes; strikingly, the biotinylated HTLV-1 DNA intermediate induced apoptosis in monocytes and bound to the endoplasmic reticulum resident DNA sensor STING - which mediates activation of the host antiviral response via IRF3. Using siRNA knockdown and co-immunoprecipitation, we confirmed that STING-mediated apoptosis in HTLV-1 infected monocytes required the generation of pro-apoptotic complex between IRF3 and the Bcl-2 family member Bax. These studies provide a mechanistic explanation for HTLV-1 abortive infection of monocytes and report for the first time a link between SAMHD1 restriction of reverse transcription, sensing of retroviral reverse transcription intermediates by STING, and the initiation of IRF3-Bax driven apoptosis.