Signal-regulatory protein alpha (SIRP alpha) but not SIRP beta is involvedin T-cell activation, binds to CD47 with high affinity, and is expressed on immature CD34(+)CD38(-) hematopoietic cells

Signal-regulatory proteins (SIRPs) represent a new family of inhibitory/act ivating receptor pairs. They consist of 3 highly homologous immunoglobulin (Ig)-like domains in their extracellular regions, but differ in their cytop lasmic regions by the presence (SIRP alpha) or absence (SIRP beta) of immun oreceptor tyrosine-based inhibitory motifs (ITIMs). To analyze the differen tial expression on hematopoietic cells, function and ligand binding capacit y of SIRP alpha and SIRP beta molecules, soluble fusion proteins consisting of the extracellular domains of SIRP alpha1, SIRP alpha2, and SIRP beta1, as well as SIRP alpha/beta -specific and SIRP beta -specific monoclonal ant ibodies (MoAbs) were generated. In contrast to SIRP alpha1 and SIRP alpha2, no adhesion of SIRP beta1 to CD47 could be detected by cell attachment ass ays and flow cytometry. Using deletion constructs of SIRP alpha1, the epito pe responsible for SIRP alpha1 binding to CD47 could be confined to the N-t erminal Ig-like loop. Flow cytometry analysis with SIRP alpha/beta- and SIR P beta -specific MoAbs revealed that SIRP alpha but not SIRP beta is expres sed on CD34(+)CD38(-) hematopoietic cells. In addition, a strong SIRP alpha expression was also observed on primary myeloid dendritic cells (DCs) from peripheral blood as well as on in vitro generated DCs. Analysis of the T-c ell stimulatory capacity of in vitro generated DCs in the presence of solub le SIRP alpha1 fusion proteins as well as SIRP alpha/beta -specific and CD4 7-specific MoAbs revealed a significant reduction of T-cell proliferation i n mixed lymphocyte reaction and inhibition of induction of primary T-cell r esponses under these conditions. In contrast, soluble SIRP alpha or SIRP be ta -specific antibodies had no effect. The data suggest that the interactio n of SIRP alpha with CD47 plays an important role during T-cell activation and induction of antigen-specific cytotoxic T-lymphocyte responses by DCs. (Blood. 2001;97:2741-2749) (C) 2001 by The American Society of Hematology.