Signal-regulatory protein α (SIRPα) but not SIRPβ is involved in 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/activating receptor pairs. They consist of 3 highly homologous immunoglobulin (Ig)-like domains in their extracellular regions, but differ in their cytoplasmic regions by the presence (SIRP alpha) or absence (SIRP beta) of immunoreceptor tyrosine-based inhibitory motifs (ITIMs). To analyze the differential expression on hematopoietic cells, function and ligand binding capacity 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 antibodies (MoAbs) were generated. In contrast to SIRP alpha1 and SIRP alpha2, no adhesion of SIRP beta1 to CD47 could be detected by cell attachment assays and flow cytometry. Using deletion constructs of SIRP alpha1, the epitope responsible for SIRP alpha1 binding to CD47 could be confined to the N-terminal Ig-like loop. Flow cytometry analysis with SIRP alpha/beta- and SIRP beta -specific MoAbs revealed that SIRP alpha but not SIRP beta is expressed 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-cell stimulatory capacity of in vitro generated DCs in the presence of soluble SIRP alpha1 fusion proteins as well as SIRP alpha/beta -specific and CD47-specific MoAbs revealed a significant reduction of T-cell proliferation in mixed lymphocyte reaction and inhibition of induction of primary T-cell responses under these conditions. In contrast, soluble SIRP alpha or SIRP beta -specific antibodies had no effect. The data suggest that the interaction 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.