Signal-regulatory protein alpha (SIRPalpha) but not SIRPbeta 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 (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.

Association of signal-regulatory proteins beta with KARAP/DAP-12

The signal-regulatory proteins (SIRP) are Ig-like cell surface receptors detected in hematopoietic and non-hematopoietic cells. SIRP are classified as SIRPalpha molecules, containing a 110- to 113-amino acid long, or SIRPbeta molecules, with a 5-amino acid long intracytoplasmic domain. SIRPalpha molecules belong to inhibitory immunoreceptor tyrosine-based inhibition motif (ITIM)-bearing molecules. The majority of ITIM-bearing receptors are paired with activating isoforms, which share highly related extracytoplasmic domains but harbor a shorter cytoplasmic domain devoid of ITIM and contain a charged amino acid residue in their transmembrane domain. Activating receptors are associated with immunoreceptor tyrosine-based activation motif (ITAM)-bearing proteins, such as KARAP/DAP-12 and FcRgamma. In this report, we show that human SIRPbeta1 is included in an oligomeric complex with KARAP/DAP-12 in hematopoietic and non-hematopoietic transfectant cells as well as in human monocytes. The physical association between SIRPbeta1 and KARAP/DAP-12 results in the functional coupling of SIRPbeta1 engagement to the recruitment of the protein tyrosine kinase Syk and to serotonin release in RBL cell transfectants. Therefore our results show that SIRPbeta1 acts as an activating isoform of SIRPalpha molecules, confirming the co-existence of inhibitory ITIM-bearing molecules, recruiting SHP-1 and SHP-2 protein tyrosine phosphatases, and activating counterparts, whose engagement couples to protein tyrosine kinases via ITAM-bearing molecules.

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

Signal-regulatory proteins (SIRPs) comprise a novel transmembrane glycoprotein 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 regions of SIRPalpha1 and SIRPalpha2, as well as a variety of monoclonal antibodies (MoAbs) against these domains. The antibodies reacted predominantly with monocytes, granulocytes, dendritic cells, and their precursors, as well as with bone marrow CD34(+), AC133(+), CD90(+) hematopoietic stem/progenitor cells. In contrast, SIRP expression was absent or significantly reduced on the majority of myeloid blasts from patients with acute myeloid leukemia (AML) or chronic myeloid leukemia (CML). Functional studies showed that the extracellular domains of SIRPalpha1 and SIRPalpha2 support adhesion of a number of primary hematopoietic cells and cell lines. This interaction could be blocked by 4 of 7 SIRPalpha1-reactive MoAbs. In addition, SIRPalpha1 and SIRPalpha2 competed for the same cell binding site, suggesting 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 binding to SIRPalpha1. Further analysis showed that this antibody recognized CD47, a ubiquitously expressed plasma membrane protein previously implicated in integrin function, host defense action, and neutrophil migration. In this study, we identify CD47 as the extracellular ligand for human SIRP and show that these two counterreceptors are involved in cellular adhesion.