Varying temperature-dependence of post-attachment neutralization of human immunodeficiency virus type 1 by monoclonal antibodies to gp120: identification of a very early fusion-independent event as a neutralization target

Neutralization of human immunodeficiency virus type 1 by adding antibody after the virus has attached to the host cell (post-attachment neutralization: PAN) was investigated using three rat monoclonal antibodies (MAbs) directed against the outer domain of the membrane protein, gp120. Two of the MAbs are specific for the CD4-binding site region and one for the V3 loop. MAb ICR39.13g (CD4-binding site region-specific; IgG2b) effected PAN efficiently at temperatures from 4 to 35 °C. MAb ICR41.1i (V3 loop-specific; IgG2c) effected PAN only at temperatures of 24 °C and below. This suggests that its V3 epitope is masked by a change in gp120 which occurs at temperatures ⩾ 26 °C, or that the virion function which is inhibited by ICR41.1i and is responsible for neutralization has already operated at ⩾ 26 °C. Resistance to neutralization by ICR41.1i occurred within 20 min of shifting the temperature up to 35 °C. Finally, MAb ICR39.3b (CD4-binding site region-specific; IgG2b) did not give PAN at any temperature, indicating that neutralization can only occur if this MAb binds virus before it attaches to the cell. Thus, these studies identify at least one novel fusion-independent event, the neutralization target of a V3 MAb, which occurs very early in the initial stages of virus-cell interaction.