Mer extracellular domain is released into the medium of cultured cell lines. (A) J774 (mouse) or (B) U937 (human) monocytic cells were grown overnight in serum-free medium. CM was collected and concentrated 10-fold, cells were lysed, and a sample of each CM was deglycosylated with PNGase F. (C) Concentrated CM from overnight cultures of human cell lines. (D) Concentrated CM from HSB-2 and U937 cells untreated and digested with PNGase F. Cell lysates (cells), CM, and PNGase-digested medium (CM + PNG) were analyzed by SDS-PAGE and immunoblotting with antibodies against mouse (A) or human (B-D) Mer extracellular domain. Untreated sMer (sMer) and deglycosylated sMer (degly sMer) are indicated in panel D.