Cellular adhesion receptor LFA-1 and its ICAM ligands are known to play a role in HIV infection. The presence of these molecules on virions and target cells promotes virus infectivity and has previously been shown to hinder virus neutralization by anti-HIV antibodies. To delineate the effect of these molecules on neutralization of HIV-1, human monoclonal antibodies (MAbs) to V3 and the CD4-binding domain (CD4bd) of gp120 were examined in the presence of anti-LFA-1 MAbs. When either of two anti-LFA-1 MAbs was present, higher levels of virus neutralization were achieved by both anti-V3 and anti- CD4bd MAbs. This effect was observed with primary HIV-1 isolates as well as with a laboratory-adapted strain. However, this activity was seen only when an anti-LFA-1 MAb was combined with anti-gp120 MAbs that exhibited virus- specific neutralizing activities, demonstrating the specificity of both the anti-LFA-1 and anti-gp120 MAbs. Enhanced neutralization by anti-gp120 MAbs was observed if the anti-LFA-1 MAb was present during the initial 24 hr only, if added 24 hr after infection, or if present throughout the culture period. These data suggest that the anti-LFA-1 MAbs could act at different stages of HIV-1 infection, including the initial virus-cell interaction as well as during the amplification and spread of virus from cell to cell. These findings demonstrate the significant role of LFA-1 in HIV-1 infection and have important implications for evaluating the neutralizing activity of anti- HIV antibodies.
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