Immunohistochemical analysis of murine mononuclear phagocytes that express class II major histocompatibility antigens

Expression of class II major histocompatibility antigen(s) was analyzed in mouse tissue sections using an immunocytochemical technique. Parallel sections were stained for the macrophage-specific antigen F4/80. MHCII antigen(s) were absent from the majority of F4/80+ cells in liver, bone marrow, splenic red pulp, and brain (microglia) but were present on intraepithelial, periepithelial, and free (e.g. alveolar) cells in gastrointestinal, urogenital, respiratory, and endocrine tissues. The staining pattern in such tissues was not distinguishable from that of F4/80. The results suggest that a major subpopulation of mononuclear phagocytes express MHCII antigen(s) constitutively.