Exposure of V3 neutralization epitopes in the native HIV-1 Env trimer. The exposure of V3 epitopes from the primary viruses HIV-1YU2, HIV-1BORI d9-4F8_1413, HIV-111006-11, and HIV-163068-05 (49) was tested by 447-52D and F425 B4e8 neutralization in three contexts: primary HIV-1 Env (open symbol; solid line), primary HIV-1 Env after sCD4 triggering (open symbol; dotted line), and in the HIV-2KR.X7/HIV-1 V3 Env scaffold (closed symbol). The HIV-2KR.X7 BORI V3 chimera (B) was constructed similarly to the HIV-2KR.X7 YU2 V3 chimera (A) (see Materials and Methods). Virus entry was measured by luciferase production 48 h after infection of TZM-bl reporter cells and normalized to luciferase expression in the absence of MAb.