Commentary Inhibitory receptors abound ?

Receptor biologists have traditionally focused on receptors that are physiologically coupled to inductive responses such as gene expression, cytokinesis, secretion, or entry into the cell cycle. It has generally been assumed that such inductive responses were terminated by ligand decay or receptor desensitization, which lead to cessation of signaling. While such mechanisms are clearly operative, a number of recent experiments have demonstrated that inhibitory receptors exist that function to attenuate inductive signals and further indicate that these receptors play a very important regulatory role in biology. Relatively few inhibitory receptors have been defined in part because the effects of these receptors can only be detected against the backdrop of an inductive signal. However, this situation is rapidly changing as recent pioneering studies have provided new approaches to isolation of such receptors. Three reports published in recent months (1–3), one by Kubagawa et al. in the Proceedings (1), have identified two new extended families of cell surface proteins that may function as inhibitory receptors. These findings are discussed below in the context of known inhibitory receptors and their mode of action (Table 1). One of the first important insights regarding mechanisms of inhibitory signaling came from observations that signal transduction by tyrosine kinase-coupled receptors can be terminated by receptor association with phosphotyrosine phosphatases. A notable example is the termination of erythropoietin receptor signaling as a consequence of receptor phosphotyrosine binding to the hematopoietic lineage restricted phosphatase SHP-1 (previously known as HCP, SHPTP1, PTP1C, and SHP) (4). It was subsequently shown that FcgRIIB, a receptor for immunoglobulin G constant (Fc) regions known to mediate inhibition of antigen receptor activation of B cells, could recruit SHP-1 as well as the closely related and ubiquitiously expressed phosphotyrosine phosphatase SHP-2 (previously known as SHPTP2, PTP1D, and Syp) to the receptor complex upon coligation with the antigen receptor (5, 6). SHP-1 expression was found to be necessary for FcgRIIB inhibition of antigen receptor activation of B cell proliferation. Based in part on these findings, the role of these phosphatases in inhibitory signaling by CD22 (7), the newly described killer inhibitory receptors (KIRs) (8, 9), and CTLA4 (10) was explored. Activated receptors andyor phosphopeptides containing the cytoplasmic sequences of these molecules were found to bind SHP-1, SHP-2 or both phosphatases. More recently, Fujioka et al. (2) and subsequently Kharitonenkov et al. (3) isolated and cloned potential new inhibitory receptors based on their ability to coimmunoprecipitate with SHP-2. Fujioka et al. (2) isolated a protein they named SHP substrate 1 (SHPS-1) from v-src-transformed rat fibroblasts and subsequently cloned human and mouse SHPS-1 homologues. Using the same strategy Kharitonenkov et al. (3) isolated a family of proteins they named SIRPs (signal-regulatory proteins) of which SHPS-1 appears to be a member. SIRPs appear to be a broadly expressed multigene family with more than 15 members. SIRPa1 was shown to be tyrosine phosphorylated following cell stimulation with epidermal growth factor, insulin, or platelet-derived growth factor. Similarly, SHPS-1 was shown to be phosphorylated upon stimulation with insulin, serum, or lysophosphatidic acid. In their phosphorylated state, SIRPa1 and SHPS-1 bind SHP-2 and SHP-1 and act as SHP substrates. Overexpression of SIRPa1 led to decreased responsiveness to epidermal growth factor, insulin, and platelet-derived growth factor, suggesting that SIRPs have inhibitory function and indicating that multiple receptor-tyrosine kinase coupled pathways are SIRP targets. In the most recent chapter of this quest for novel inhibitory receptors, Kubagawa et al. (1) have cloned genes encoding two novel surface molecules, PIR-A and PIR-B, expressed on B lymphocytes and myeloid lineage cells, based on homology to the mouse Fca receptor. The supposition that PIR-B and SIRP are receptors is based on their content of extracellular domains and the fact that these extracellular domains exhibit sequence variability consistent with their being determinants of ligand specificity. Although the ligand specificity of SIRPs, PIR-A and PIR-B, are unknown, activation of SIRP phosphorylation by growth factors and lysophosphatidic acid (2, 3) is most consistent with the possibility that the SIRP ligands are the respective receptors themselves. In this regard the situation is similar to CD22, a known inhibitory receptor that is rapidly phosphorylated upon B cell antigen receptor aggregation and binds SHP-1 (7). Thus a component of the B cell antigen receptor (BCR) complex may be a CD22 ligand. The inhibitory receptors defined thus far fall into two structural families (Fig. 1). Most are monomeric proteins that contain multiple immunoglobulin super-family (IgSF) domains in their extracellular regions. Surprisingly, some of the KIRs are homodimers containing c-type lectin domains in their extracellular regions. Thus, KIR can recognize its ligand, major histocompatiblity complex (MHC) class I molecules, using very different structures (11). All of the inhibitory receptors contain single transmembrane spanning regions and cytoplasmic tails ranging from 35 to 178 amino acids in length. The contextual sequence surrounding the sites of tyrosine phosphorylation of the inhibitory receptors has proven to be one of the best predictors of the ability of candidate receptors to associate with SHP-1 and SHP-2 and to function in an inhibitory capacity. SHP-1 binding activity has been localized to specific tyrosines in FcgRIIB1, p58.183 (tyrosine 1 and 2), p58.EB6, p70 (tyrosine 1 and 2), and CD22 (tyrosine 2, 5, and 6) shown in Fig. 2 (7–9). Analysis of these sequences suggests a consensus for SHP-1 SH2 binding that is IyVxYxxL. This motif has been referred to as the immunoreceptor tyrosinebased inhibitory motif or ITIM (5, 12). This consensus is also found in gp49B, SIRPs, Ly49, and NG2A. Importantly, Vely et al. (13) have recently shown that mutation of the IyV position in the motif disrupts its ability to bind SHP-1 and SHP-2. The requirement for a hydrophobic residue in this position is consistent with the occurrence of a hydrophobic cleft in the c-SH2 domain of SHP-2 that may be properly positioned to