Stable exposure of the coreceptor-binding site in a CD4-independent HIV-1 envelope protein

We recently derived a CD4-independent virus from HIV-1/IIIB, termed IIIBx, which interacts directly with the chemokine receptor CXCR4 to infect cells. To address the underlying mechanism, a cloned Env from the IIIBx swarm (8x ) was used to produce soluble gp120, 8x gp120 bound directly to cells expre ssing only CXCR4, whereas binding of IIIB gp120 required soluble CD4, Using an optical biosensor, we found that CD4-induced (CD4i) epitopes recognized by mAbs 17b and 48d were more exposed on 8x than on IIIB gp120, The abilit y of 8x gp120 to bind directly to CXCR4 and to react with mAbs 17b and 48d in the absence of CD4 indicated that this gp120 exists in a partially trigg ered but stable state in which the conserved coreceptor-binding site in gp1 20, which overlaps with the 17b epitope, is exposed. Substitution of the 8x V3 loop with that from the R5 virus strain Bat resulted in an Env (8x-V3Ba L) that mediated CD4-independent CCR5-dependent virus infection and a gp120 that bound to CCRS in the absence of CD4. Thus, in a partially triggered E nv protein, the V3 loop can change the specificity of coreceptor use but do es not alter CD4 independence, indicating that these properties are dissoci able, Finally, IIIBx was more sensitive to neutralization by HIV-positive h uman sera, a variety of anti-IIIB gp120 rabbit sera, and CD4i mAbs than was IIIB, The sensitivity of this virus to neutralization and the stable expos ure of a highly conserved region of gp120 suggest new strategies for the de velopment of antibodies and small molecule inhibitors to this functionally important domain.