FIG. 7.

Dynamics of the transition from six-helix bundles to membrane fusion: loss of sensitivity to neutralization by antibundle antibodies after transfer of E/T cells from 31.5°C (2 h) to 37°C in the presence of anti-CD4 monoclonal antibody. TF228 (IIIB Env)/PM1 cells were incubated for 2 h at 31.5°C in the absence of antibodies (Abs). Immediately prior to transfer to 37°C (time zero), anti-CD4 monoclonal antibody Q4120 was added at 3 μg/ml. Six-helix bundle-specific IgG from R948 was added at 30 μg/ml at the indicated times. Syncytia were scored 4 h after transfer to 37°C.