Expression of MHC class I molecules on primary T-cell blasts after infection with various strains of HIV. PHA-stimulated primary CD4⁺ T cells from HIV-uninfected donors were infected with HIV-1_SF162 (A, C), HIV-1_SF33 (A), HIV-1_SF128A (B), HIV-1_SF2 (B), or HIV-1_92US712 (C) for 5 days. Cells were stained with PE-conjugated anti-MHC class I antibody followed by human anti-HIV p24 antibody, together with FITC-conjugated goat anti-human IgG antibody. As controls, uninfected CD4⁺ T cells were also stained with the same antibody. The extent to which 10⁴ HIV p24 antigen-positive infected cells and uninfected cells expressed MHC class I molecules on the cell surface was determined.