It is generally agreed that

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 orexin 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.
Materials and Methods
Results
Discussion Studies have suggested that dual-tropic strains evolve from R5 variants, and X4 viruses arise from the R5X4 strains (Pastore et al., 2004). It is unlikely that the emergence of a dual-tropic virus during the passage experiments occurred by chance because CXCR4-using variants were not detected in the absence of plasma and changes required for co-receptor switching are generally associated with diminished replication fitness (Pastore et al., 2004). The DM268 passage results were surprising because we observed no significant neutralization susceptibility difference to contemporaneous autologous plasma among co-existing R5 and dual-tropic strains (Fig. 1). Although previous studies have demonstrated that diverse glycosylation changes can render viruses neutralization resistant (Sagar et al., 2006, Wei et al., 2003), it remains unclear how the addition of PNGS at position 88 leads to low level CXCR4 usage. It has been shown that co-circulating R5 and R5X4 variants often have the same predicted V3 loop sequence (Huang et al., 2007; Lin et al., 2012), and thus it possible that modifications in other env segments lead to a structural change that can influence co-receptor usage. Interestingly, DM viruses did not emerge in the other 2 cases (DM8 and DM178) even though their X4 as compared to R5 envs were more neutralization resistant to autologous contemporaneous plasma. Our observations suggest that the passaged DM8 and DM178 env variants accommodated changes required for neutralization resistance to autologous plasma without co-receptor switching. Other R5 variants from these individuals, not examined in the passaging experiments, may acquire the ability to use the CXCR4 receptor as they become resistant to autologous antibodies.