A Role for the Anti-Viral Factor APOBEC3G in NK Cell Recognition of HIV-1 Infected Primary T Lymphocytes

Abstract

Natural killer (NK) cells recognize virally infected cells through a balance of activating and inhibitory signals received from target cells. The NK cell recognition of human immunodeficiency virus type 1 (HIV-1) infected cells is inefficient due to multiple viral immune evasion strategies. The HIV negative factor (Nef) selectively downmodulates MHC-I surface expression, where expression of the NK cell inhibitory allotypes HLA-C and -E is not affected. Here I show NK cell recognition of HIV-infected cells is also limited by insufficient NK cell activating ligand expression. NK cell activating ligand expression is modulated by three HIV-1 accessory proteins: viral infectivity factor (Vif), viral protein R (Vpr) and Nef. Vpr upregulates NK cell activating ligand expression. However, the Vif and Nef accessory proteins limit these detrimental effects of Vpr expression. The cellular cytidine deaminase APOBEC3G is an intrinsic restriction factor that suppresses HIV infection. It acts by causing G to A hypermutation and inhibiting viral genome synthesis. HIV counteracts APOBEC3G through the activity of Vif, which promotes the degradation of APOBEC3G. In other contexts, DNA damage like that caused by APOBEC3G is known to activate NK cell lysis by upregulating NK cell activating ligands. However, a possible effect of APOBEC3G-induced DNA damage on the cellular immune response has not yet been examined. This dissertation shows that APOBEC3G expression levels correlate with activation of DNA repair pathways and subsequent expression of NK cell activating ligands in primary T lymphocytes. Expression of Vif mitigates this effect and partially rescues infected cells from NK cell lysis. A Vif mutant unable to bind APOBEC3G was less effective at counteracting the effects of APOBEC3G on the DNA damage response and NK cell activating ligand expression. Interestingly, APOBEC3G and HIV infection activated an early stage in the DNA damage response; however downstream DNA repair signaling and upregulation of NK cell activating ligands required HIV Vpr expression. These results define a new role for APOBEC3G acting not just as an intrinsic antiviral factor but also as an activator of the innate cellular immune response.