GCl2, 25 mM Tris, pH 7.5 and 140 mM NaCl). For pulse activation experiments, the COS-1 cells were incubated with sCD4 (40 mg/ml, 0.8 mM) or 191 (360 mM) suspended in blocking buffer for three minutes, washed three times with blocking buffer and incubated for different time periods until the C34-Ig or 48d antibodies were added (at 40 mg/ml or 1 mg/ml, respectively, for 30 minutes). To study the tem