Immunohistological studies with a mouse anti-rat macrophage mAb (BMAC-5) demonstrated the presence of numerous positive cells in the interstitial connective tissues of many organs. The pattern resembled that seen with anti-MHC class II antibodies, with the striking exception that BMAC-5+ cells were rare or absent in the portal triad, the islets of Langerhans, and the kidney. Double-labeling fluorescence studies were therefore performed in rat heart using the BMAC-5 mAb in combination with rabbit antisera to pure rat class II MHC antigens and pure rat leukocyte common (CD45) antigens. The tissue macrophages in heart were identified as BMAC-5+, MHC class II-negative, leukocyte common antigen-positive cells. They could be distinguished from the BMAC-5-, MHC class II-positive, leukocyte common antigen-positive interstitial dendritic cells. Moreover, 7 d after lethal irradiation, the class II-positive interstitial dendritic cells had completely disappeared from heart, whereas the BMAC-5+ macrophages were present in undiminished numbers. These studies strongly suggest that the interstitial dendritic cell and the tissue macrophage represent two distinct populations of leukocytes within the connective tissues of antigenically secluded organs such as the heart. They have potentially important implications for the physiology of the immune system, as well as for autoimmunity and transplantation.

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