SEQUESTRATION OF P56(ICK) BY GP120, A MODEL FOR TCR DESENSITIZATION

Ligation of the CD4 receptor by HIV envelope glycoprotein gp120 inhibi ts T cell activation and signaling through the TCR complex. Recent rep orts suggest CD4 ligation by gp120 + anti-gp120 Abs uncouples protein tyrosine kinases (PTKs) from the TCR signal-transduction cascade. This finding and other observations led us to hypothesize that the effects of gp120 are mediated through p56(lck), a PTK noncovalently associate d with CD4. To test this hypothesis, we first examined the kinetics of gp120/anti-gp120-induced TCR signaling defects in the Jurkat T cell l ine. Pretreating cells with gp120/anti-gp120 for 1 to 4 h before stimu lation prevented TCR-directed PTK activation. Coincident with TCR dese nsitization, pretreatment with gp120/anti-gp120 also decreased the amo unt of p56(lck) that could be immunoprecipitated from the Nonidet P-40 detergent-soluble fraction of cellular lysates, while simultaneously increasing the recovery of p56(lck) from the Nonidet P-40 detergent-in soluble fraction (or cytoskeleton). To assess the potential role of th e actin in this process, experiments were conducted in the presence of cytochalasin D. Cytochalasin D restored TCR signaling in cells previo usly desensitized with gp120/anti-gp120 and prevented translocation of p56(lck) from the Nonidet P-40 detergent-soluble fraction of cell lys ates. Furthermore, p56(lck) was found to coimmunoprecipitate with anti -actin. These data suggest that gp120/anti-gp120 may inhibit TCR signa ling by sequestering p56(lck) to the cytoskeleton.