The expression of Ia antigens on murine macrophages was studied by indirect immunofluorescence, microcytotoxicity, and biosynthetic labeling. The majority of peritoneal macrophages lacked Ia antigens. When splenic macrophages were examined, it was found that the majority of these cells expressed Ia antigens and that these were synthesized by the macrophages. All the splenic macrophages which reacted with alloantibody raised against the whole I region expressed I-A subregion antigens, while only a subset of these cells also expressed I-E or C subregion antigens.

When antigen-pulsed normal peritoneal and spleen macrophage-enriched cell populations were compared for their ability to stimulate antigen specific proliferation of primed lymph node T cells, a positive correlation was found between the number of Ia bearing phagocytic cells and T cell DNA synthesis. Spleen adherent cells and T cells were then treated with subregion restricted anti-Ia antisera and complement.

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