Signal Regulatory Proteins in the Immune System Ellen M. van Beek, Fiona Cochrane, A. Neil Barclay and Timo K. van den Berg This information is current as of October 1, 2021. J Immunol 2005; 175:7781-7787; ; doi: 10.4049/jimmunol.175.12.7781 http://www.jimmunol.org/content/175/12/7781 Downloaded from Supplementary http://www.jimmunol.org/content/suppl/2005/12/06/175.12.7781.DC1 Material References This article cites 46 articles, 27 of which you can access for free at: http://www.jimmunol.org/content/175/12/7781.full#ref-list-1 http://www.jimmunol.org/
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BRIEF REVIEWS
Signal Regulatory Proteins in the Immune System Ellen M. van Beek,* Fiona Cochrane,† A. Neil Barclay,† and Timo K. van den Berg1*
Signal regulatory proteins (SIRPs) constitute a family of are typically composed of a single V-set and two C1-set IgSF transmembrane glycoproteins with extracellular Ig-like domains, which, when compared with other members of the domains. Several SIRP family members have thus far been IgSF, display a particularly close structural relationship to rear- identified on myeloid and other cells in man, mouse, rat, ranging AgRs (TCR and BCR). Based on these similarities, we and cattle. In the present study, we provide a description have previously proposed an evolutionary relationship between of the SIRP multigene family, including a number of pre- AgRs and SIRPs (6). Moreover, it appears that the putative pri- mordial AgR that formed the target for the critical transposition viously undescribed SIRP genes, based on the complete ge- ϳ nome sequences of various mammalian and bird species. event (7) that led to the appearance, 500 million years ago, of
We discuss this information in the context of the known rearranging AgRs and thus adaptive immunity was possibly a Downloaded from SIRP-like molecule. Insight into the functional properties of immunological properties of the individual SIRP family SIRPs may provide clues about the innate origins of the adap- members. Our analysis reveals SIRPs as a diverse multi- tive immune system. As with many members of the other gene family of immune receptors, which includes inhibi- ␣  ␥ “paired receptors” mentioned above, SIRPs possess the poten- tory SIRP , activating SIRP , nonsignaling SIRP , and tial to signal through cytoplasmic tails that contain either IT- soluble SIRP␦ members. For each species, there appears to IMs (8) or transmembrane regions that have positively charged http://www.jimmunol.org/ ␣ be a single inhibitory SIRP member that, upon interac- residues that allow an association with adaptor proteins, such as tion with the “self” ligand CD47, controls “homeostatic” DAP12/KARAP, containing ITAMs (9). innate immune effector functions, such as host cell phago- cytosis. The activating SIRP proteins show considerable The biochemically characterized SIRP members variability in structure and number across species and do To date, three distinct human SIRP family members have been not bind CD47. Thus the SIRP family is a rapidly evolv- characterized at the protein level (for structural features of SIRP ing gene family with important roles in immune family members, see Fig. 1A). The first and best characterized member of this SIRP family, SIRP␣ (also known as SHPS-1,
regulation. The Journal of Immunology, 2005, 175: by guest on October 1, 2021 7781–7787. BIT, MFR, CD172a, or p84), was originally identified in rat cells by its association with cytoplasmic tyrosine phosphatase Src homology region 2 domain-containing phosphatase 2 he signal regulatory protein (SIRP ; CD172 and (SHP)-2 (8) and was later shown to exist in other mammals, SHPS) family belongs to an increasing number of fam- including humans, mice, and cattle (10–12). Two other SIRP T ilies of membrane proteins involved in the regulation of family genes, SIRP1 (CD172b) and SIRP␥ (CD172g), have leukocyte function. More especially, the SIRPs belong to a been described in humans (11, 13–15). SIRP␥ was originally group of receptors that have members with closely related ex- described as SIRP2 (14), but as outlined in the accompanying tracellular regions but different cytoplasmic regions giving con- letter, we believe the name SIRP␥ is more suitable. In the trasting types of signals—the “paired receptors” that are partic- mouse, two SIRP members (SIRP1 and SIRP2), which can ularly common on NK cells but also present on many myeloid both associate and signal via DAP12, have been characterized cells. These “paired receptors” include the killer Ig-like recep- (16, 17). The SIRP1 was previously termed mouse SIRP. tors, paired Ig-like receptor, Ly49, CD94/NKG2A, Ig-like However, protein sequences with similarity to the reported transcript, dendritic cell immunoreceptor, and CD200R fami- SIRPs are present in the genomes, and the availability of the lies (1–4). Some of these families have C-type lectin domains in complete genome sequences of various species and extensive ex- their extracellular regions (e.g., Ly49 and CD94), but the most pressed sequence tag (EST) data now permit characterization of common domain type in the paired receptors and indeed all the complete SIRP family in humans and other species. The leukocyte membrane proteins is the Ig superfamily domain hypothesis is that proteins related to the known SIRPs will have (IgSF) (5). The SIRPs are expressed mainly by immune cells evolved by gene duplication and have related functions as al- and more especially myeloid cells. Their extracellular regions ready shown for SIRP␥, which has the same ligand (i.e., CD47)
*Department of Molecular Cell Biology and Immunology, VU University Medical Center, 1 Address correspondence and reprint requests to Dr. Timo K. van den Berg, VU Univer- Amsterdam, The Netherlands; and †Sir William Dunn School of Pathology, University of sity Medical Center, P.O. Box 7057, 1007 MB Amsterdam, The Netherlands. E-mail ad- Oxford, Oxford, United Kingdom dress: [email protected] Received for publication July 13, 2005. Accepted for publication September 26, 2005. 2 Abbreviations used in this paper: SIRP, signal regulatory protein; IgSF, Ig superfamily domain; SHP, Src homology region 2 domain-containing phosphatase; EST, expressed The costs of publication of this article were defrayed in part by the payment of page charges. sequence tag; m, murine; DC, dendritic cell; r, rat; g, chicken. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Copyright © 2005 by The American Association of Immunologists, Inc. 0022-1767/05/$02.00 7782 BRIEF REVIEW: SIGNAL REGULATORY PROTEINS IN THE IMMUNE SYSTEM
FIGURE 1. The SIRP family in man (Homo sapiens), chimpanzee (Pan troglo- dytes), rat (Rattus norvegicus), mouse (Mus musculus), and chicken (Gallus gal- lus). A, Schematic structures of SIRP family members. The existence of some SIRP proteins, including human SIRP␣, SIRP1 and SIRP␥, rat SIRP␣, mouse SIRP␣, SIRP1, and SIRP2, is supported by direct biochemical evi- dence (see text for details), whereas that of the others is based on prediction. The gray and white circles represent V- and
C1-set Ig domains, respectively. The Y- Downloaded from marked black and white squares repre- sent ITIM and ITAM tyrosine residues, respectively. The positively charged ly- sine residues of the transmembrane re- gions of the SIRP members, which mediate or are proposed to mediate, an association with DAP12 are indicated http://www.jimmunol.org/ with a “ϩ”. In case of SIRP1, mSIRP1, and mSIRP2, an interac- tion with DAP12 is supported by direct evidence (16, 17, 38, 40) (͗www.ub. uni-heidelberg.de/archiv/4817͘); for the other SIRP members, it is pro- posed based on the presence and posi- tion of the transmembrane lysine resi- due. B, Genomic organization of the by guest on October 1, 2021 SIRP family. See supplementary Fig. 1 for database accession numbers and amino acid sequences.
as SIRP␣ (13, 15). Clearly, understanding the full range of The SIRP genes in humans and other primates genes in the SIRP family is essential for understanding its sig- The genes of the three identified SIRPs in human, SIRP␣, nificance in the immune system. SIRP1, and SIRP␥, are clustered on chromosome 20p13 (Fig. Novel genes related to the SIRPs were identified on the basis 1B). Adjacent to the known human SIRP genes on chromo- of sequence similarity and/or by systematic analysis of the re- some 20, we have identified two additional SIRP genes and a gions of the genome where the known SIRP genes were present pseudogene. Based on their structural properties, we termed or regions syntenic to these in other species. Similarity searches these novel SIRP molecules, SIRP2, SIRP3p (for the pseu- were performed using the Basic Local Alignment Search Tool/ ␦ Sequence Search and Alignment by Hashing algorithms in Na- dogene), and SIRP (Fig. 1A) (for nomenclature of the SIRP tional Center for Biotechnology Information or Ensembl data- family, see the accompanying Brief Review). These novel SIRP bases using the complete or partial sequences of known SIRPs. family members are clearly more divergent than the three pre- ␣  IgSF domains can often show considerable divergence even be- viously known SIRP molecules. In fact, SIRP , SIRP 1, and ␥ tween species orthologs (the extracellular region of human CD4 SIRP appear to form a subgroup that evolved relatively re- shows only 53% amino acid sequence identity with its mouse cently before the bifurcation of chimpanzee and man (see be- counterpart) while unrelated domains may show ϳ20% iden- low). SIRP2 appears to encode a cell surface receptor com- tity as residues key for the domain fold are conserved (5). There- posed of two extracellular IgSF V-type domains and, in analogy fore, a cutoff of 30% amino acid identity was used to search and to SIRP1, a transmembrane region with a positively charged identify related genes (see below). residue, suggesting a putative association with the adaptor The Journal of Immunology 7783 protein DAP12. Consistent with this, SIRP2 is expressed by encoding exon, suggesting that perhaps in early mammals there cells of the monocyte-macrophage lineage (as determined by was a SIRP family member with a C1-C1-C1-V-V-TM-Cyt PCR and analysis of EST data), and there is alternative splicing structure. Despite the low sequence similarity (ϳ23%) between of the extracellular IgSF domains (E. M. Van Beek et al., un- the C1-set domains of SIRP3 and the C1-set domains of, for published observations). SIRP3p has an unusual extracellular example, SIRP␣, a distant relationship can still be seen. It seems  region, with three NH2-terminal C1-type IgSF domains and that a putative primordial SIRP 3 arose by an incomplete du- two membrane proximal V-type IgSF domains. The human plication of the two tandem C1-set domains as present in the SIRP3p appears to be a pseudogene, but its orthologs in ro- SIRP␣, SIRP1, and SIRP␥ subgroup genes. As compared dents (i.e., rat (r)SIRP3 and murine (m)SIRP3) appear func- with primates and chicken, the organization of SIRP genes in tional (see below). Although gene prediction programs identify the rodent genome appears special in two aspects. Firstly, the an open reading frame, there is no evidence (from, for example, rodent cluster I is uniquely flanked by the IL-1␣ and -1 genes, EST) for expression. SIRP␦ encodes a putatively secreted mol- which is likewise the result of multiple translocations. Further- ecule with a single V-type IgSF domain. EST evidence suggests more, there is a second cluster of SIRP genes (cluster II), which that SIRP␦ may be expressed by sperm cells and in respiratory is located on corresponding regions of the rat chromosome tissue (T. K. van den Berg, unpublished observations). 2q23 and mouse chromosome 3A1. Cluster II shows a relatively The human SIRP gene cluster is flanked by the dynorphin high degree of divergence among mice and rats. In mice, but precursor (pDyn) gene, and this orientation is conserved in all not in rats, there is a SIRP member (SIRP1) representing the species investigated where a SIRP cluster is found in a syntenic SIRP molecule described by Hayashi et al. (16). It is clear that position in the genome. Outside the 20p13 SIRP cluster, we this mSIRP1 is closely related to members of the primate Downloaded from have not detected any other active human SIRP genes, apart SIRP␣-SIRP1-SIRP␥ subgroup as well as to the mouse from an intronless SIRP␣ gene (92% overall nucleotide iden- SIRP␣ (all identities ϳ55%), but because the homologies tity) on chromosome 22, which probably represents a retro- among the members of the primate subgroup are considerably transposon (Fig. 1A). Although this SIRP␣-like gene has an higher (ϳ75%), they probably arose later (i.e., independently), open reading frame, the lack of evidence for transcription and thus mSIRP1 cannot be considered a direct ortholog of strongly suggests it to be a pseudogene (termed SIRP␣2p). In any of these. However, because it contains the typical trans- http://www.jimmunol.org/ the chimpanzee, the productive SIRP␣ gene is localized in a membrane lysine residue and can also associate with and signal syntenic position on chromosome 23 while, the SIRP␣ like via DAP12 (16), it may serve a function analogous to the pri- gene within the chimpanzee SIRP-gene cluster is most probably mate SIRP1. The rodent rSIRP1, rSIRP2, rSIRP4, and a pseudogene. This illustrates the difficulty in assigning specific mSIRP2 molecules, i.e., with variable numbers of only V-set species homologues and reflects how, as with many other IgSF domains and a transmembrane region with a charged ly- “paired receptors,” these genes have been subject to extensive sine, are not particularly related to the primate SIRP2, which duplication, divergence, and loss. Furthermore, it seems possi- within the context of the whole SIRP family appears rather di- ble that the chimpanzee SIRP1 does not contain a V-type Ig- vergent. A phylogenetic tree for the individual V-like domains by guest on October 1, 2021 like domain, but the latter could also be a sequencing error or a strongly suggests that the various SIRP members from cluster II mistake of the gene prediction algorithm. Because expression arose independently in the rat and mouse lineage by either sin- data are lacking, as yet only genomic information has been used gle or tandem V-like domain exon duplication events (Fig. 2). for the chimpanzee, and thus SIRP genes and their products are All this appears consistent with the translocation of a cluster exclusively predicted here by gene prediction programs. One of I-derived SIRP␣- or SIRP1-like founder gene giving rise to the more important implications from the above is that there is cluster II. only a single human SIRP␣ family member, suggesting indi- rectly that most of the additional human SIRP␣ sequences re- SIRP genes in other species and the evolution of SIRPs ported (11) probably represent polymorphic variants. In fact, SIRP genes are broadly found among mammals and are not re- our own findings (A. Van Elsas and T. K. van den Berg, un- stricted to primates and rodents. We have also found SIRPs in published observations) suggest that the most frequent SIRP␣ dog (Canis familiaris), cow (Bos taurus) (10), pig (Sus scrofa), allotypes in the human population are represented by the two horse (Equus caballus), and the gray short-tailed opossum (Mo- ␣ ␣ ␣ deposited SIRP 1 and SIRP 2/BIT sequences (SIRP 1 (Na- nodelphis domestica), a marsupial mammal. The full SIRP rep- tional Center for Biotechnology Information Entrez protein ac- ertoire in these species can be described in detail once their ge- cession no. CAA71403) and SIRP␣2/BIT (protein accession nome sequences and expression data have become available, no. NP_542970)). and this may provide further insight into the evolution of SIRPs in mammals. The SIRP genes in rodents Considering the evolutionary relationship of SIRPs with In rodents, two clusters of SIRP genes are present (Fig. 1B). The AgRs, which has previously led us to propose a common non- SIRP␣ ortholog and rat and mouse SIRP3 (i.e., the orthologs rearranging SIRP-like predecessor in early vertebrates (6), it was of the human SIRP3p pseudogene) are in the first cluster of interest to know whether nonmammalian vertebrates such as (cluster I). Interestingly, these rSIRP3 and mSIRP3 repre- birds, reptiles, amphibians, and fish encode SIRP family mem- sent functional genes and not a pseudogene as in humans. Both bers. The availability of the chicken, xenopus, zebrafish, and have a C1-C1-C1-TM-Cyt structure and represent the only fugu (pufferfish) genome sequences provided a means to ex- family member that lacks a typical V-set IgSF domain. How- plore this. In chicken, we found three family members, termed ever, in the rodent SIRP3 genes and primate SIRP3 pseudo- chicken (g)SIRP␣, gSIRP␥, and gSIRP␦, which are encoded in gene, there is evidence for the remains of two V-like sequences a position syntenic to the mammalian cluster I. These chicken located between the third C1 domain exon and the TM region- SIRP-like molecules are not obvious orthologs of the primate 7784 BRIEF REVIEW: SIGNAL REGULATORY PROTEINS IN THE IMMUNE SYSTEM
FIGURE 2. Phylogenetic analysis of
SIRP family members in human (no Downloaded from prefix), chimpanzee (c), rat (r), mouse (m), and chicken (g), using predicted protein sequences. A, Phylogram of the V domains of the SIRP family members. B, Phylogram of the C1 domains of the SIRP family members. See Fig. 1 and the text for SIRP terminology. http://www.jimmunol.org/ by guest on October 1, 2021
SIRP␣, SIRP␥, or SIRP␦ genes but may still serve analogous in the other major vertebrate groups (i.e., amphibians and fish) functions. The presence of SIRPs with a V-C1-C1 structure as is perhaps surprising considering the previously suggested rela- well as a gene with ITIMs (i.e., gSIRP␣) suggests that these tionship between SIRPs and AgRs that predicted an ancient comprise perhaps the original ingredients of a primordial SIRP. SIRP-like molecule in early (jawed) vertebrates (6). It seems Of interest, our extensive searches have not provided evidence likely that SIRP family members did indeed exist in early ver- for the presence of typical SIRP family members in other major tebrates and continued to exist in birds and mammals but were phylogenetic groups, including amphibians, fish, and inverte- lost at some point(s) during fish and amphibian evolution or brates. Our analysis included the complete (draft) genome se- that they arose later. Clearly, it is more difficult to trace the evo- quences of zebrafish (Danio rerio), Torafugu pufferfish lution of proteins such as these as they are diverging very rapidly (Takifugu rubripes), spotted green pufferfish (Tetraodon nigro- compared with, for example, cytosolic enzymes where phyloge- viridis), clawed toad (Xenopus tropicalis), fruit fly (Drosophila netic analysis is simpler, and also it is likely that there has been melanogaster), mosquito (Anopheles gambiae), and helminth extensive duplication and gene loss within the species making (Caenorhabditis elegans). The closest homologues of SIRPs that direct orthologs difficult to identify (see discussion above on can be found by basic local alignment search tool searches in for mouse SIRP1 and primate SIRPs). instance the complete genomes of zebrafish and fugu are either AgR chains or members of the novel immune type receptor The functions of SIRP family members in immunity families (18) (at ϳ20–30% identity), which clearly represent The biochemically characterized SIRPs are all expressed by cells related but distinct IgSF families. This apparent lack of SIRPs of the immune system with SIRP␣ also expressed on neurons The Journal of Immunology 7785
Table I. Immunological properties of characterized human SIRP family members
SIRP␣ SIRP1 SIRP␥
Ligand CD47 Unknown CD47 Signaling motifs Four ITIMs; two proline-rich regions Lys in TM region Nonea Signaling proteins SHP-1, SHP-2, Jak2, Csk, Pyk2, DAP12 Nonea Grb2, FyB/SLAP-130, SKAP55hom Function Negative regulation of host cell Promotion of phagocytosis?b Mediates antigen phagocytosis/clearance; Regulation presentation and T cell of inflammatory mediator proliferation production a The cytoplasmic tail of SIRP␥ comprises four amino acids with no known signaling motifs or interactions. a The positive effect on phagocytosis is based on studies with the mouse mSIRP1, which also associates with DAP12 and may serve an analogous function.
(19, 20) (the relevant properties of human SIRPs are summa- SH3 domains in some of these proteins. Thus, SIRP␣ does not rized in Table I). SIRP␣ is relatively ubiquitously expressed on only function as a typical inhibitory receptor but may also act as myeloid cells, including macrophages, granulocytes, myeloid a scaffold for a variety of other signaling molecules at the plasma dendritic cells (DCs), mast cells, and their precursors, including membrane. Downloaded from hemopoietic stem cells (19–21). CD47, a broadly expressed Apart from the above, reported evidence suggests that transmembrane glycoprotein with a single Ig-like domain and SIRP␣-CD47 interactions play a role in macrophage fusion to five membrane spanning regions, functions as a cellular ligand form multinucleated cells in vitro (33, 34). It remains to be es- ␣ ␣ for SIRP (21, 22) with binding mediated through the NH2- tablished whether the SIRP -CD47 interaction is also relevant terminal V-like domain of SIRP␣ (23). CD47 can itself signal, for osteoclast fusion and multinucleated giant cell formation in and hence, engagement can potentially give two-way signaling vivo. http://www.jimmunol.org/ (24). The cytoplasmic region of SIRP␣ contains four ITIMs, In addition to the regulatory functions in macrophages, in- which upon ligand binding become phosphorylated, and me- teractions between CD47 on T cells and SIRP␣ on DCs have diate recruitment and activation of the tyrosine phosphatases been shown to regulate, in a bidirectional fashion, DC and T SHP-1 and SHP-2 (8, 11, 25). SHP-1 and SHP-2 can, in turn, cell activation. Furthermore, it seems that SIRP␣ provides sig- dephosphorylate specific protein substrates and thereby regu- nals that can modulate DC maturation (35). The contributions late cellular functions, generally in a negative fashion. Probably of the different signals are difficult to discern as these in vitro the best documented role of SIRP␣ is its inhibitory role in the studies with human cells did not really clarify whether the ef- phagocytosis of host cells by macrophages (26). In particular, fects observed were the result of agonistic or antagonistic action the ligation of SIRP␣ on macrophages by CD47 expressed on of the reagents used. Apart from this, it will be interesting to by guest on October 1, 2021 the host target cell generates an inhibitory signal mediated by investigate more directly the possible role of SIRP␣ in adaptive SHP-1 that negatively regulates phagocytosis. The role of immunity. SIRP␣ in host cell phagocytosis is supported by in vivo studies Finally, SIRP␣ appears to control myeloid cell migration. with target cells from CD47-deficient mice (27, 28), as well as Studies using SIRP␣-mutant mice provide evidence for a role of by using macrophages from SIRP␣-mutant mice (26, 27). Mice SIRP␣ in the emigration of Langerhans cells from the skin (36). that lack the SIRP␣ cytoplasmic domain are thrombocytope- Furthermore, interactions of SIRP␣ on monocytes or granulo- nic, which apparently results from an increased rate of clearance cytes with CD47 on endothelial or epithelial cells are important of circulating platelets (26, 27). Recent evidence also implies for transendothelial or epithelial migration (37–39). Collec- SIRP␣ in the clearance of aged erythrocytes (29). This strongly tively, these findings picture SIRP␣ as a versatile regulator of suggests that SIRP␣ acts to detect signals provided by “self,” in myeloid cell function. this case CD47 on host cells, to negatively control innate im- SIRP1 was reported originally by Kharitonenkov et al. (11) mune effector function against these cells. This is analogous to in humans and is expressed on myeloid cells, including mono- the “self” signals provided by MHC class I molecules to NK cytes, granulocytes, and DCs (40–43). Interestingly, SIRP1, cells via Ig-like or Ly49 receptors. Another example of negative although closely related to SIRP␣, does not appear to bind regulation through SIRP␣ is the inhibition of LPS-induced CD47 and lacks cytoplasmic ITIMs or any other recognizable TNF-␣ production in macrophages (30). However, the role of cytosolic motifs for signaling. Instead, SIRP1 contains a trans- SIRP␣ may be more complex than previously anticipated, and membrane region with a positively charged lysine residue that the molecule may not only provide negative signals. For in- mediates association with DAP12, an adaptor protein that car- stance, we have shown recently that engagement of SIRP␣ by ries an ITAM (41, 43). Phosphorylation of the DAP12 ITAM CD47 in macrophages can promote the production of NO via mediates recruitment of the protein tyrosine kinase Syk and the SIRP␣-associated Janus kinase (JAK2), suggesting that consequent activation of the MAPK pathway that regulates var- SIRP␣ can also activate certain effector functions (31). Finally, ious functions (9, 41, 43, 44). Triggering of the murine there is the potential for a variety of other components, which SIRP1 receptor, for instance, which also complexes with can also bind to SIRP␣, to regulate its signaling. These include DAP12, promotes phagocytosis in macrophages (16). This in- the tyrosine kinases CSK and PYK2 and the adaptor molecules dicates that mSIRP1 acts as an activating SIRP family mem- Grb2, FyB/SLAP-130, and SKAP55hom (11, 12, 25, 32). In ber. Further evidence for the rapid evolution of this family of fact, apart from the ITIMs, the SIRP␣ cytoplasmic tail contains proteins is given by the differences in SIRP1 sequences from two proline-rich regions that may form a docking site for the different mouse strains (16). 7786 BRIEF REVIEW: SIGNAL REGULATORY PROTEINS IN THE IMMUNE SYSTEM
SIRP␥, the third member of the human SIRP family, is ex- (46, 47). It will clearly be of interest to further explore the pressed on T cells and activated NK cells (13–15). It can bind interactions of SIRP family members with poxvirus vCD47 CD47, albeit with 10-fold lower affinity as compared with and other ligands expressed by pathogens. SIRP␣ (13). Moreover, SIRP␥-CD47 interaction mediates Note added in proof. Recent evidence suggests that apoptosis of cell-cell adhesion and supports APC-T cell contact, enhancing cells can be accompanied by a down-regulation of (functional) CD47 Ag presentation, the consequent T cell proliferation, and cyto- ␥ expression (48). As a result, the apoptotic cells will no longer provide kine secretion (15). It seems unlikely that SIRP itself generates an inhibitory signal for phagocytosis via SIRP␣ which will facilitate intracellular signals because it does not have any recognizable their uptake by macrophages. CD47 may thus act as a “viability” or ␥ signaling motifs. Instead, SIRP may trigger signaling of CD47 “don’t eat me” signal to the phagocyte. in APCs.
Concluding remarks Acknowledgments We thank Georg Kraal for critically reading the manuscript. The SIRP family comprises multiple genes in mammals and birds. Similar to what is seen in various related families of leu- References kocyte receptors, the genome includes SIRP family members 1. Borges, L., and D. Cosman. 2000. LIRs/ILTs/MIRs, inhibitory and stimulatory Ig- with either inhibitory or activating signaling potential. Of in- superfamily receptors expressed in myeloid and lymphoid cells. Cytokine Growth Fac- tor Rev. 11: 209–217. terest, for each species analyzed, we have found only a single, 2. Kanazawa, N., K. Tashiro, and Y. Miyachi. 2004. Signaling and immune regulatory relatively well-conserved, inhibitory SIRP␣ receptor member, role of the dendritic cell immunoreceptor (DCIR) family lectins: DCIR, DCAR, dec- tin-2 and BDCA-2. Immunobiology 209: 179–190. Downloaded from which is consistent with a homeostatic function. Indeed, 3. Lanier, L. L. 2005. NK cell recognition. Annu. 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