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 (SIRPalpha) or absence (SIRPbeta) of immunoreceptor tyrosine-based inhibitory motifs (ITIMs). To analyze the differential expression on hematopoietic cells, function and ligand binding capacity of SIRPalpha and SIRPbeta molecules, soluble fusion proteins consisting of the extracellular domains of SIRPalpha1, SIRPalpha2, and SIRPbeta1, as well as SIRPalpha/beta-specific and SIRPbeta-specific monoclonal antibodies (MoAbs) were generated. In contrast to SIRPalpha1 and SIRPalpha2, no adhesion of SIRPbeta1 to CD47 could be detected by cell attachment assays and flow cytometry. Using deletion constructs of SIRPalpha1, the epitope responsible for SIRPalpha1 binding to CD47 could be confined to the N-terminal Ig-like loop. Flow cytometry analysis with SIRPalpha/beta- and SIRPbeta-specific MoAbs revealed that SIRPalpha but not SIRPbeta is expressed on CD34(+)CD38(-) hematopoietic cells. In addition, a strong SIRPalpha 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 SIRPalpha1 fusion proteins as well as SIRPalpha/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 SIRPalpha or SIRPbeta-specific antibodies had no effect. The data suggest that the interaction of SIRPalpha with CD47 plays an important role during T-cell activation and induction of antigen-specific cytotoxic T-lymphocyte responses by DCs.