Flow cytometric analysis of MHC class I and p24 expression on HIV-infected primary CD4 T cells used as stimulator cells. CD4 PHA blasts from representative subject 307 were used as uninfected control (a) or were infected with HIV_IIIB (b), NL4-3 (c), or autologous virus (d) for 3 days and then enriched for infected cells by immunomagnetic depletion of cells that had not downmodulated CD4 expression. Cells were stained externally with PE-conjugated anti–HLA-ABC antibody (MHC class I framework) and internally with FITC-conjugated anti-p24 antibody.