sMer is shed from primary cells in culture and is detected in mouse and human blood. (A) Elicited peritoneal macrophages and dissociated splenocytes from wild-type mice were cultured in serum-free DMEM for 7 hours. Cell lysates and conditioned medium from the macrophages and splenocytes, and 5 μL serum from wild-type mice or mice with the Mer gene disrupted (KD) were analyzed by SDS-PAGE and immunoblotting. (B) Human monocyte-derived macrophages (Μφ), MVECs, platelets, and plasma samples were examined for the presence of Mer by Western blotting. Cultured monocyte-derived macrophages and MVECs express 205- and 185-kDa Mer glycoform, respectively, and sMer was present in CM from these cells. A 165-kDa Mer protein was detected in pelleted platelets, and sMer extracellular domain proteins of 110 to 140 kDa were abundant in pooled plasma from healthy donors (George King Bio-Medical, Overland Park, KS) and in plasma from free-flowing blood (plasma), and subsequently after clotting, in serum from the same donor (serum).