HIV-1 Env sequence and structure contribute to both coreceptor tropism and susceptibility to antibody neutralization

HIV-1 Env is the only viral protein expressed on the surface of the virus, making Env the sole target of selective pressure that favors mutated variants capable of evading the immune response. The surface subunit of Env, gp120, binds CD4 on the surface of the target cell, followed by binding of a coreceptor, usually chemokine receptors CCR5 or CXCR4. The virus that is normally transmitted during infection is a CCR5-using virus (R5-tropic), which can mutate to acquire CXCR4 usage (X4-tropism) later in infection. The coreceptor switch is manifested in some HIV-1 subtypes, whereas other subtypes rarely make the coreceptor switch. env contains regions of conserved and variable sequence, and the variable region 3 (V3) is the major determinant of coreceptor usage. In this dissertation, we describe subtype-specific conformational differences within the V3 region that permit coreceptor switching in subtype B, but not in subtype C. Furthermore, the backbone of Env interacts with V3 to shelter it from immune pressures; when V3 is exposed, Env is hyper-sensitive to neutralization by anti-V3 monoclonal antibodies, suggesting interactions between different regions of Env. In vivo coreceptor usage evolution of subtype B env variants indicates that dual-tropic variants are always generated in the transition to X4-tropism, suggesting that coreceptor evolution begins with R5-monotropic variants, transitions into dual-tropic variants and finally generates X4-monotropic variants. The coreceptor switch is associated with mutations in V3 and other areas of the env backbone, again suggesting the interaction of the entire Env in coreceptor interaction. Finally, the transmembrane subunit of Env, gp41, contains the epitopes of two broadly neutralizing antibodies, 2F5 and 4E10, located in the membrane proximal external region (MPER). A single mutation in the MPER confers increased neutralization sensitivity to both antibodies. The effect of the mutation is contained to the MPER, and there is no apparent effect on expression, folding, infection or the global structure, suggesting that the MPER can be modified for efficient antigen presentation as part of a comprehensive vaccine for long-term control of viremia. Our data collectively suggest that HIV-1 env sequence and structure contribute to both coreceptor tropism and susceptibility to antibody neutralization