Apoptotic cell phagocytosis is not associated with Syk activation. (A) Mouse peritoneal macrophages were incubated with apoptotic thymocytes (left) or IgG-SRBCs (right) for the indicated times at 37°C at a macrophage-target ratio of 1:20. (B) Rat alveolar macrophages were incubated with increasing amounts of apoptotic thymocytes (left) or IgG-SRBCs (right) for 7 minutes at 37°C. Incubations were terminated by addition of lysis buffer and lysates were subjected to immunoprecipitation and immunoblotting as described in “Materials and methods.” In panels A and B, immunoblots in upper panels represent phosphorylated Syk detected with antiphosphotyrosine antibody, and those in lower panels, the amounts of Syk protein evaluated with anti-Syk antibody. Results are representative of 3 separate experiments.