Jacklyn Johnson, Yinjie Zhai, Hamid Salimi, Nicole Espy, Noah Eichelberger, Orlando DeLeon, Yunxia O'Malley, Joel Courter, Amos B. Smith, Navid Madani, Joseph Sodroski, Hillel Haim

The envelope glycoproteins (Envs) on the surface of HIV-1 particles are targeted by host antibodies. Primary HIV-1 isolates demonstrate different global sensitivities to antibody neutralization; Tier-1 isolates are sensitive whereas Tier-2 isolates are more resistant. Single-site mutations in Env can convert Tier-2 into Tier-1-like viruses. We hypothesized that such global change in neutralization sensitivity results from weakening of intra-molecular interactions that maintain Env integrity. Three strategies commonly applied to perturb protein structure were tested for their effect on global neutralization sensitivity; exposure to low temperature, Env-activating ligands and a chaotropic agent. A large panel of diverse Tier-2 isolates from clades B and C was analyzed. Incubation at 0°C, which globally weakens hydrophobic interactions, causes gradual and reversible exposure of the coreceptor-binding site. In the cold-induced state, Envs progress at isolate-specific rates to unstable forms that are sensitive to antibody neutralization and then gradually lose function. Agents that mimic the effects of CD4 (CD4Ms) also induce reversible structural changes to states that exhibit isolate-specific stabilities. The chaotropic agent urea (at low concentrations) does not affect structure or function of native Env. However, urea efficiently perturbs metastable states induced by cold and CD4Ms and increases their sensitivity to antibody neutralization and their inactivation rate. Therefore, chemical and physical agents can guide Env from the stable native state to perturbation-sensitive forms and modulate their stability, to bestow Tier-1-like properties to primary Tier-2 strains. These concepts can be applied to enhance the potency of vaccine-elicited antibodies and microbicides at mucosal sites of HIV-1 transmission.IMPORTANCE An effective vaccine to prevent transmission of HIV-1 is a primary goal of the scientific and healthcare communities. Vaccine-elicited antibodies target the viral envelope glycoproteins (Envs) and can potentially inhibit infection. However, the potency of such antibodies is generally low. Single-site mutations in Env can enhance the global sensitivity of HIV-1 to neutralization by antibodies. We found that such a hyper-sensitivity phenotype can also be induced by agents that destabilize protein structure. Exposure to 0°C or low concentrations of Env-activating ligands gradually guides Env to metastable forms that expose cryptic epitopes and are highly sensitive to neutralization. Low concentrations of the chaotropic agent urea do not affect native Env but destabilize perturbed states induced by cold or CD4Ms and increase their neutralization. The concept of enhancing antibody sensitivity by chemical agents that affect structural stability of proteins can be applied to increase the potency of topical microbicides and vaccine-elicited antibodies.