All human monocytes have the capability of expressing HLA-DQ and HLA-DP molecules upon stimulation with interferon-γ

We have simultaneously studied expression of all three classes of human Ia (HLA-DR, DP, and DQ) on normal human B cells and monocytes (MΦ) by using two-color immunofluorescence and flow cytometry. Expression was investigated on freshly isolated cells and after incubation of cells for 48 and 96 hr in interferon-γ (IFN-γ). All freshly isolated B cells express high levels of DR, DQ, and DP, and these levels are unchanged by incubation with IFN-γ for 48 hr and 96 hr. In contrast, freshly isolated MΦ are on the average 91% DR⁺, 32% DQ⁺, and 15% DP⁺. Incubation with IFN-γ increases Ia expression on MΦ to 98% DR⁺, 75% DQ⁺, and 58% DP⁺ at 48 hr, with virtually all cells becoming positive for all three Ia antigens at 96 hr. Furthermore, after the 96-hr incubation, antigen density increases 10-fold for DR, 15-fold for DQ, and 15-fold for DP in MΦ to reach levels of expression comparable with B cells. These studies demonstrate that all peripheral blood monocytes have the capacity to become HLA-DQ and HLA-DP positive; IFN-γ regulates expression of all three classes of human Ia in MΦ; and IFN-γ does not significantly modulate Ia expression in B cells.