MxB targets HIV capsid to mediate its anti

MxB targets HIV-1 order LY335979 to mediate its anti-viral function. We have observed that HIV-1 resistant variants maintained the ability to be recognized by MxB, suggesting the existence of MxB co-factor(s) required for MxB mediated viral restriction. Consistent with this idea, Jurkat cells express high level of endogenous MxB proteins after interferon treatment (Fig. S10). However, interferon induced MxB expression did not alter HIV-1 infectivity in these Jurkat cells compared to the untreated Jurkat cells (Fig. S10). It has been reported that U937 cells express MxB, but this expression has limited impact on HIV-1 infection (Kane et al., 2013). CypA has been reported to be a critical factor for MxB mediated HIV-1 restriction. We have observed that addition of CypA binding to HIV-2 capsid did not alter HIV-2 sensitivity to MxB (Fig. S9). Factor(s) in addition to CypA may be required for MxB mediated HIV-1 restriction. Interestingly, the three MxB-resistant HIV-1 capsid variants are all located on the raised surface region formed by two subunits of the capsid hexamer (Fig. S11), suggesting that these sites could influence the putative MxB cofactor binding but may not alter intrinsic features of the viral core (Fig. 5). Identification of the critical role of these HIV-1 capsid mutations in overcoming a host restriction factor may provide opportunities for the development of therapeutic interventions.
Funding Sources This work was supported in part by funding from the Chinese Ministry of Science and Technology (No 2012CB911100), the Chinese Ministry of Education (No IRT1016). The funding sources had no role in the writing of the manuscript or the decision to submit for publication. We would like to acknowledge support for the statistical analysis from the National Center for Research Resources and the National Center for Advancing Translational Sciences (NCATS) of the National Institutes of Health through Grant Number 1UL1TR001079. The authors were not paid to write this article by any pharmaceutical company or agency.
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Introduction In nearly all cases HIV-1 infection leads to a progressive loss of CD4+ T cells, and both host immunologic factors and viral characteristics influence the rate of progression towards AIDS. Although HIV-1 can enter target cells by either using the C-C chemokine receptor 5 (CCR5) or CXC-chemokine receptor 4 (CXCR4) co-receptor after CD4 receptor engagement (Deng et al., 1996; Feng et al., 1996), presence of syncytium-inducing (SI) CXCR4-using viruses is associated with accelerated disease progression (Koot et al., 1993; Richman and Bozzette, 1994; Connor et al., 1997). In general, viruses that exclusively utilize the CCR5 receptor (termed R5 or non-syncytium-inducing (NSI)) establish initial infection, and these viruses persist throughout the course of disease (Connor et al., 1997; van\'t Wout et al., 1994; Schuitemaker et al., 1992). During the chronic phase of infection, some individuals have a dual-mixed (DM) population consisting of R5 viruses and CXCR4-using variants, which can either only use CXCR4 (termed X4) or be dual-tropic that can engage both co-receptors (termed dual or R5X4) (Schuitemaker et al., 1992; Connor et al., 1997). Previous studies have not clarified if CXCR4-using viruses accelerate disease progression or if faster CD4+ T cell decline facilitates the emergence of DM virus populations. Deciphering mechanisms for the emergence of CXCR4-utilizing variants will provide insights into this viral factor important for progression towards AIDS. It is generally agreed that R5 variants give rise to R5X4 viruses, which can further evolve to X4 HIV-1 (Pastore et al., 2004). Specific modifications in the envelope glycoprotein (env) variable loop 3 (V3), which binds the co-receptor, are required for the transition from R5 to X4 virus (Huang et al., 2007; Chesebro et al., 1996; Cocchi et al., 1996). The selective pressures which induce the transition from CCR5 to CXCR4 usage during the natural course of infection remain poorly understood. It has been suggested that CXCR4-using viruses potentially emerge because of a reduced number of susceptible CCR5+ target cells or replication differences in various immune cells among the variants that use different co-receptors (Regoes and Bonhoeffer, 2005; Ribeiro et al., 2006). Indeed, individuals heterozygous for a 32 base pair deletion in CCR5 gene (CCRΔ32), who have lower levels of the CCR5 receptor on their cells, have an increased likelihood of harboring CXCR4-using viruses compared to people with both wild-type alleles (Brumme et al., 2005; Henrich et al., 2014). Other non-mutually exclusive hypotheses argue that CXCR4-utilizing variants arise because of random mutations, and these new viruses persist because declining adaptive and innate immune responses fail to clear the less fit dual-tropic and/or X4 HIV-1. Previous studies have also suggested that neutralizing antibody (nAb) pressure do not drive co-receptor switching (Trkola et al., 1998; Montefiori et al., 1998; Cecilia et al., 1998; Lacasse et al., 1998). Other investigations have argued that CXCR4-using viruses are highly neutralization susceptible, and the variants that can use the CXCR4 receptor only emerge after collapse of the humoral immune response (Bunnik et al., 2007; Ho et al., 2007). Within weeks to months after HIV-1 acquisition, infected individuals develop nAbs against their autologous viruses, but the virus changes its env to escape this humoral response (Richman et al., 2003; Wei et al., 2003). Neutralization resistance is frequently associated with modifications in and around the env variable portions, such as the env V1–V2 and V3 loop (Sagar et al., 2006, Rong et al., 2009, Moore et al., 2009). In a cohort of HIV-1 subtype C (HIV-1C) infected individuals, we have previously shown that the env quasispecies diversity was significantly higher among individuals with DM as compared to strictly R5 viruses (Lin et al., 2012). In addition, we observed that all HIV-1C X4 as compared to R5 variants had either a 2 amino acid insertion prior to or basic amino acid substitutions in the generally invariant V3 loop crown. Together, these observations led us to hypothesize that difference in host antibody responses drive the greater env diversity observed in individuals with DM as compared to R5 virus populations, and the V3 loop genotypic signatures associated with X4 variants potentially modify neutralization susceptibility. We also postulated that the emergence of specific host antibodies in some but not all individuals force env sequence and structural changes. These env modifications influence co-receptor switching, and the continual presence of the specific host antibodies allows for the persistence of CXCR4-using viruses. In this study, we examined the role of nAbs on HIV-1C co-receptor switching.