A role for Ia antigens in thymocyte binding by macrophages

To investigate the membrane structures involved in cellular interactions between thymocytes and macrophages, the relative ability of different murine macrophage populations to spontaneously bind thymocytes was compared. Macrophages derived from the spleen or thymus bound three to four times the number of thymocytes than macrophages from peripheral blood, peritoneum, or bone marrow. This reflects differences both in the number of macrophages binding thymocytes and in the number of thymocytes bound per macrophage. The extent of binding seems to positively correlate with the number of Ia-positive macrophages contained in these populations, as based on previously published values. This was confirmed by showing that elimination of splenic Ia-positive macrophages with anti-Ia and complement treatment dramatically reduced thymocyte binding. In addition, mouse peritoneal washout macrophages incubated for several days with supernatant fluid from concanavalin A-stimulated spleen cells, which induce Ia-antigen expression, exhibited a marked increase in the number of macrophages that bound thymocytes and the number of thymocytes bound per macrophage. To determine if Ia antigens were directly involved in binding, spleen, thymus, or Ia-induced peritoneal macrophages were treated with a monoclonal anti-Ia antibody prior to the addition of thymocytes. Treatment with anti-Ia reduced binding by around 50%, whereas treatment with anti-H-2D antibody had no effect. Monoclonal anti-I-A and anti-I-E antibody treatments of macrophages both inhibited thymocyte binding to similar extents, and treatment of macrophages with both reagents together reduced thymocyte binding by 80%. These results indicate that thymocyte binding is in part dependent on macrophage Ia expression.