Abstract. Transepithelial migration of neutrophils (PMN) is a defining characteristic of active inflammatory states of mucosal surfaces. The process of PMN transepithelial migration, while dependent on the neutrophi1132 integrin CDllb/CD18, remains poorly understood. In these studies, we define a monoclonal antibody, C5/D5, raised against epithelial membrane preparations, which markedly inhibits PMN migration across polarized monolayers of the human intestinal epithelial cell line T84 in a bidirectional fashion. In T84 cells, the antigen defined by C5/D5 is upregulated by epithelial exposure to IFN-% and represents a membrane glycoprotein of ~60 kD that is expressed on the basolateral membrane. While transepithelial migration of PMN was markedly inhibited by either C5/D5 IgG or C5/D5 Fab fragments, the antibody failed to inhibit both adhesion of PMN to T84 monolayers and adhesion of isolated T84 cells to the purified PMN integrin, CDllb/CD18. Thus, epithelial-PMN interactions blocked by C5/D5 appear to be downstream from initial CDllb/CD18-mediated adhesion of PMN to epithelial cells. Purification, microsequence analysis, and cross-blotting experiments indicate that the C5/D5 antigen represents CD47, a previously cloned integral membrane glycoprotein with homology to the immunoglobulin supeffamily. Expression of the CD47 epitope was confirmed on PMN and was also localized to the basolateral membrane of normal human colonic epithelial cells. While C5/D5 IgG inhibited PMN migration even in the absence of epithelia, preincubation of T84 monolayers with C5/D5 IgG followed by antibody washout also resulted in inhibition of transmigration. These results suggest the presence of both neutrophil and epithelial components to CD47-mediated transepithelial migration. Thus, CD47 represents a potential new therapeutic target for downregulating active inflammatory disease of mucosal surfaces. TIVE inflammation of surfaces lined by columnar epithelia is histologically defined by transmigration of neutrophils (PMN) 1 across such epithelial monolayers and subsequent collection of PMN in the lumen (26,53). Recently, neutrophils have been recognized not only to influence epithelial function during transmigration, but also to interact with biochemically distinct apical domains after translocation to the lumenal compartment, thus further modifying key epithelial processes (33, 51). For example, in intestinal epithelia it appears that PMN transepithelial migration may reversibly influence epithelial barrier function (19,37,44), while arrival in the lumenal space may result in interactions promoting electrogenic C1-secretion (32, 33), the known basis for secretory diarrhea