Faster kinetics of viral entry for virions bearing host-derived MHC-II. Sup-T1 cells were incubated at 37°C for the indicated periods of time with virus stocks harvested from either RAJI-CD4 (▪) or RM3-CD4 (□) cells. Next, cells were washed several times with PBS and treated with trypsin to remove uninternalized viruses. Cells were washed and lysed using Triton X-100, and the amounts of internalized viral p24 were determined using a p24 enzymatic assay. Data shown are the mean of triplicate samples. Error bars indicate sample standard deviations. Levels of viral p24 in mock-infected cells were less than 20 pg/mL.