HIV-1 Binding to CD4 T Cells Does Not Induce a Ca 2+ Influx or Lead to Activation of Protein Kinases

The penetration of CD4⁺ cells by human immunodeficiency virus (HIV) involves a high affinity interaction between the viral attachment protein, gp120, and the cellular receptor, CD4. The mechanism by which the virus penetrates the host cell subsequent to viral binding is unknown. We have investigated the possibility that HIV penetration induces changes in the metabolic state of the infected cell similar to those seen with the perturbation of CD4 cells by monoclonal antibodies (MAb) directed against the CD4 molecule, or with specific antigenmediated activation. The activation of cellular protein kinases was examined. The basal level of activity was not altered in the presence of HIV. Kinase activity was markedly increased in cells stimulated with phytohemagglutinin (PHA), and was qualitatively and quantitatively changed by a brief exposure to the phorbol ester TPA (12-o-tetradecanoyl phorbol-13-acetate). The phosphorylation state of the CD4 molecule was examined by radioimmunoprecipitation and found to be unaltered by the binding of HIV under conditions in which TPA induced rapid CD4 phosphorylation. The activity of the CD4-associated protein tyrosine kinase p56 ^lck was measured by in vitro assays of ³²PO₄ incorporation in CD4 immunoprecipitates from HIV-incubated cells. TPA incubation resulted in a rapid loss of CD4-associated p56 ^lck activity, presumably due to dissociation of the enzyme from CD4. Concanavalin A stimulation resulted in a similar change but with a slower time course. However, no change in CD4-associated activity was detected in HIV-incubated cells. We found that Ca²⁺ influx was not induced by the binding of HIV to CD4⁺ cells. However, HIV binding (and CD4 monoclonal antibody binding) did interfere with the Ca²⁺ influx mediated by attachment and crosslinking of CD3 monoclonal antibody.