Studies of Ia Antigens on Murine Peritoneal Macrophages

Murine peritoneal cells, both induced and noninduced, were examined for Ia antigens by a variety of techniques. Complement-mediated cytotoxicity and indirect immunofluorescence, analyzed by both visual microscopy and the fluorescence-activated cell sorter, detected Ia antigens on the surface of an average of 8% to 15% of cells with the morphologic and functional characteristics of macrophages. Internal radioisotope labeling studies showed that these antigens were actually synthesized by the macrophages. The antigens were borne on molecules which consisted of two coponents with apparent molecular weights of roughly 33,000 and 25,000 daltons. At least some of these molecules existed as a two-chain structure of 58,000 daltons linked by disulfide bonds. Although macrophage Ia antigens appeared to be structurally similar to the Ia antigens found on spleen cells, the radioisotope labeling studies indicated that the quantity of labeled Ia-bearing molecules isolated from peritoneal macrophages was at most 1/15 that found for B lymphocytes. In addition, anti-Ia antisera failed to inhibit the binding of heat-aggregated immunoglobulin to the Fc receptor of macrophage populations, similar to the low levels of Ia antigens found in T-lymphocyte populations. These studies suggest that Ia antigens exist only a subpopulation of peritoneal macrophages. Alternatively, all cells in the population might bear small amounts of Ia antigens with only a fraction having sufficient numbers of molecules to be detected by the assay systems used.