The following reagents were obtained through the NIH AIDS Reagent Program, Division of AIDS, NIAID, NIH: TZM-bl cells from Dr. cells failed despite passaging virus for five months, demonstrating that Vif is a critical viral accessory protein. Keywords: APOBEC3G, cytidine deaminase, hypermutation, HIV-1, Vif, antiviral resistance, deamination, A3.01 cells, viral evolution, purifying selection 1. Introduction The viral infectivity factor (Vif) is an HIV accessory protein that is critical for viral replication in vivo. It primarily antagonizes the antiviral activity of APOBEC3G (A3G) (Sheehy et al., 2002). A3G is a cytidine deaminase that is packaged into retroviral particles where it can mutagenize the viral genome during reverse transcription (reviewed in (Goila-Gaur and Strebel, 2008)). Vif inhibits the packaging of A3G into progeny virions at least in part by inducing proteasomal degradation of the deaminase (Conticello et al., 2003; Kao et al., 2003; Kao et al., 2004; Marin et al., 2003; Mehle et al., 2004; Sheehy et al., 2003; Stopak et al., 2003; Yu et al., 2003). A3G is not ubiquitously expressed in all cell lines and Vif-dependence of HIV-1 replication is therefore, at (Z)-Capsaicin least in vitro, cell line-dependent. Based on the level of restriction of Vif-null HIV-1, cell types are categorized as non-permissive (e.g. PBMC, macrophages, H9, MT2), semi-permissive (e.g. A3.01, CEMx174), or permissive (e.g. Jurkat, CEM-SS, SupT1) (Borman et al., 1995; Gabuzda et al., 1992; Hoglund et al., 1994; Ma et al., 1994; Sakai et al., 1993). Previous reports indicate that expression of A3G in vivo can vary in a donor-specific manner (Cho et al., 2006; Jin et al., 2005). Also, A3F, A3DE (also referred to as A3D), and A3H have been shown to affect HIV-1 replication in a Vif-sensitive manner (Chaipan et al., 2013; Dang et al., 2006; Li et al., 2010; OhAinle et al., 2006; Wiegand et al., 2004; Zhen et al., 2010; Zheng et al., 2004). It is therefore conceivable that variation in their expression contributes to the non-permissive or semi-permissive phenotype of the host cells. The identification of natural Vif variants with reduced A3G antagonizing potency could be an indication of donor- or tissue-specific variations in the expression of A3G and other cytidine deaminases (Binka et al., 2012; Fourati et al., 2010). Nevertheless, as far as cell line-specific differences in Vif dependence observed in tissue culture are concerned, it is currently not clear whether they are due to differences in the relative expression of A3G, differential expression of additional cytidine deaminases, or a combination of both. While natural Vif variants can differ in their ability to target A3G, A3F, or A3H (Kataropoulou et al., 2009; Peng et al., 2013; Porcellini et al., 2009; Simon et al., 2005; Vallanti et al., 2005), there are no known primary replication competent viruses that completely lack expression of a Vif protein. This suggests that Vif-null viruses are replication incompetent in vivo making Vif an interesting target for antiviral therapy. Yet, there are currently no drugs in clinical use that specifically target Vif. Here, we studied replication of Vif-null HIV-1 NL4-3 in A3.01 cells to understand in more detail the reasons for the semi-permissive phenotype of these cells. Among possible contributing factors we explored (i) heterogeneous expression of A3G (i.e. mixed population), (ii) polymorphisms in the A3G gene potentially affecting (Z)-Capsaicin its catalytic activity, and (iii) differences in cellular expression and packaging TFR2 of A3G into progeny virions. We found that A3.01 cells represent (Z)-Capsaicin a homogeneous population with virtually all of the cells expressing A3G. Furthermore, sequence comparison of A3G expressed in A3.01 cells and H9 cells failed to reveal sequence polymorphisms. In contrast, we found that the cellular expression of A3G protein in A3.01 cells was somewhat lower than in H9 cells, and progeny virions produced in A3.01 cells contained approximately 1/3 of the A3G packaged into virus produced from H9 cells. To understand the impact of (Z)-Capsaicin these differences on HIV-1 replication we either reduced A3G expression in A3.01 cells by shRNA-mediated gene silencing or increased A3G production by transduction of cells with an A3G-expression vector. Interestingly, silencing of A3G rendered A3.01 cells fully permissive for Vif-null HIV-1 suggesting that the semi-permissive nature of A3.01 cells is primarily, if not exclusively, associated with A3G expression. Importantly, increasing the levels of A3G in A3.01 cells to levels similar to those in H9 cells rendered the cells fully non-permissive. Our results indicate that A3.01 cells express (Z)-Capsaicin sub-lethal levels of A3G that cause mutation of proviral sequences but allow the virus to survive by a mechanism.