Human signal-regulatory protein is expressed on normal, but not on subsetsof leukemic myeloid cells and mediates cellular adhesion involving its counterreceptor CD47

Signal-regulatory proteins (SIRPs) comprise a novel transmembrane glycoprot ein family involved in the negative regulation of receptor tyrosine kinase- coupled signaling pathways. To analyze the expression and function of SIRPs , we prepared soluble recombinant fusion proteins of the extracellular regi ons of SIRP alpha 1 and SIRP alpha 2, as well as a variety of monoclonal an tibodies (MoAbs) against these domains. The antibodies reacted predominantl y with monocytes, granulocytes, dendritic cells, and their precursors, as w ell as with bone marrow CD34(+), AC133(+), CD90(+) hematopoietic stem/proge nitor cells. In contrast, SIRP expression was absent or significantly reduc ed on the majority of myeloid blasts from patients with acute myeloid leuke mia (AML) or chronic myeloid leukemia (CML). Functional studies showed that the extracellular domains of SIRP alpha 1 and SIRP alpha 2 support adhesio n of a number of primary hematopoietic cells and cell lines. This interacti on could be blocked by 4 of 7 SIRP alpha 1-reactive MoAbs. In addition, SIR P alpha 1 and SIRP alpha 2 competed for the same cell binding site, suggest ing a common widely expressed SIRP ligand. In an approach to identify this molecule, MoAbs were generated against the SIRP-binding cell line CCRF-CEM, and MoAb CC2C6 was selected because of its capacity to inhibit cell bindin g to SIRP alpha 1. Further analysis showed that this antibody recognized CD 47, a ubiquitously expressed plasma membrane protein previously implicated in integrin function, host defense action, and neutrophil migration. In thi s study, we identify CD47 as the extracellular ligand for human SIRP and sh ow that these two counterreceptors are involved in cellular adhesion. (C) 1 999 by The American Society of Hematology.