Historical Background, Structure, and Its Ligands
Signal regulatory protein alpha (SIRPα) was initially cloned as a substrate for Src homology region 2 (SH2) domain-containing phosphatase-1 (SHP-1) (Ptpn6) and SHP-2 (Ptpn11). SHP-1 and SHP-2 are cytoplasmic-type protein tyrosine phosphatases and SIRPα was initially termed SHPS-1 (SHP substrate-1) (reviewed in Matozaki et al. 2009; Barclay and van den Berg 2014). SIRPα was also named as brain immunoglobulin (Ig)-like molecule with tyrosine-based activation motifs (BIT), which was a highly phosphorylated glycoprotein in the brain, as well as macrophage fusion receptor (MFR) and MyD-1. SIRPα belongs to SIRP family members, and other two SIRP family members, namely SIRPβ1 and SIRPγ, have a similar structure of SIRPα in their extracellular regions but have different cytoplasmic regions from SIRPα (Matozaki et al. 2009; Barclay and van den Berg 2014).
The N-terminal IgV-like domain of SIRPα is highly polymorphic in both human and mouse. These polymorphisms of SIRPα are thought to affect its interaction with CD47. Indeed, the polymorphic variants of human SIRPα, as well as mouse SIRPα, showed different phagocytic activity toward CD47-expressing target cells (Nuvolone et al. 2013; Rodriguez et al. 2013). Thus, SIRPα polymorphisms might be crucial for regulation of phagocytosis by interaction with CD47, nevertheless crystal structure analysis of the N-terminal IgV-like domain of SIRPα suggested that the polymorphisms in SIRPα are unlikely to affect CD47 binding in humans.
SIRPα in Central Nervous System
SIRPα is expressed throughout the brain, especially abundant in the hippocampus and cerebellum as well as in the retina (Ohnishi et al. 2005). A detailed analysis of primary-cultured neurons revealed that SIRPα presents on both axons and dendrites, whereas the expression of CD47 was largely restricted to dendrites (Ohnishi et al. 2005). In addition, forced expression of CD47 on primary-cultured neurons promoted filopodia formation and spine formation of neurons (Murata et al. 2006), suggesting that the SIRPα-CD47 interaction between neighboring neurons plays an important role in neural network formation. Moreover, SIRPα is also expressed on microglia, which is important for demyelination of axons. Given that CD47 is expressed on myelin, the interaction of SIRPα on microglia with CD47 on myelin is thought to play a protective role in C3bi-dependent myelin phagocytosis (Gitik et al. 2011).
Several studies demonstrated that the BDNF receptor TrkB as well as Src family kinases (SFKs) mediate the tyrosine phosphorylation of SIRPα, which subsequently activate SHP-2 (Matozaki et al. 2009). Furthermore, the forced swimming test (an in vivo experimental model for stress-induced responses) as well as exposure of mice to cold stress revealed robust tyrosine phosphorylation of SIRPα, which was followed by activation of SHP-2 (Maruyama et al. 2012; Ohnishi et al. 2010). Thus, the function of SIRPα in the brain may be closely related to behavioral immobility. A recent study further documented that SIRPα also participated in the oxidative stress and the pathology of ischemic stroke in the brain. The size of cerebral ischemia was attenuated, and the neural damages were inhibited in SIRPα-deficient mice after cerebral artery occlusion (Wang et al. 2012).
SIRPα Inhibits Phagocytic Activity of Macrophages
Moreover, recent studies indicated that rejection of xenograft donor cells is critically regulated by SIRPα expressed on host macrophages in the generation of humanized mouse models for studying human Immunology and hematology. Of interest is that the nonobese diabetic (NOD) mouse strain expresses a polymorphic variant of SIRPα, which can bind human CD47 with a high affinity. The binding is thought to inhibit the elimination of human donor cells by NOD-derived macrophages (Takenaka et al. 2007). Breeding of the NOD SIRPα gene onto the immunodeficient animals, as well as introduction of a bacterial artificial chromosome encoding human SIRPα into the immunodeficient animals, showed significantly improved multilineage development of human cells after transplantation of human hematopoietic stem and progenitor cells into these mice (Strowig et al. 2011; Yamauchi et al. 2013). Taken together, the interaction between CD47 on donor cells and SIRPα on recipient macrophages is important for success in xenotransplantation.
SIRPα on Phagocytes Acts as a Potential Therapeutic Target Against Tumors
Furthermore, a recent study has revealed that the expression of SIRPα is remarkably increased in some tumors including renal cell carcinoma, malignant melanoma, and acute myelomonocytic leukemia compared with normal cells. Thus, in addition to anti-CD47 antibody therapy, an anti-SIRPα antibody therapy that prevents the SIRPα-CD47 interaction could be used for a potential cancer immunotherapy. The treatment of SIRPα expressing tumors with the blocking antibody against SIRPα alone indeed revealed to eliminate tumor cells by macrophages (Yanagita et al. 2017) (Fig. 3b). This therapeutic effect of the anti-SIRPα antibody could be mediated by dual mechanisms: direct induction of antibody-dependent cellular phagocytosis of tumor cells by macrophages and blockade of CD47-SIRPα signaling that negatively regulates such phagocytosis. Moreover, the anti-SIRPα antibody therapy also promotes the effect of anti-CD20 antibody (Rituximab) or anti-programmed cell death 1 (PD-1) antibody on tumor progression in mice inoculated with tumor cells, suggesting that anti-SIRPα antibody therapy has a therapeutic potential for a broad range of cancers (Yanagita et al. 2017).
SIRPα Regulates Homeostasis of Dendritic Cells and T Cells in the Secondary Lymphoid Organs
Interaction of CD47 with SIRPα is also thought to regulate homeostasis of T cells and stromal cells in the T cell zone, namely fibroblastic reticular cells (FRCs), in the spleen. FRCs produce homeostatic chemokines such as CCL19 and CCL21, both are crucial for the attraction and retention of T cells. FRCs also produce interleukin (IL)-7, which supports the survival of T cells. The amount of T cells and FRCs, as well as production of these homeostatic chemokines and cytokine, was significantly reduced in the spleen of both SIRPα- and CD47-deficient mice (Sato-Hashimoto et al. 2011). Studies from bone marrow chimeras indicated that hematopoietic SIRPα, likely on DCs or macrophages, might regulate the homeostasis of T cells and FRCs in SLOs (Sato-Hashimoto et al. 2011).
SIRPα Regulates Inflammation and Autoimmunity
The transmigration of neutrophils or monocytes from the circulation into tissue parenchyma is crucial for the development of inflammation. Previous in vitro studies revealed that the interaction between SIRPα on neutrophils or monocytes and CD47 on endothelial cells likely promotes the extravasation of these leukocytes during infection (Barclay and van den Berg 2014). Moreover, antibodies against SIRPα or CD47-Ig fusion protein inhibited in vitro migration of neutrophils, monocytes, or melanoma cells (Matozaki et al. 2009). Besides migration and extravasation of neutrophils or macrophages, SIRPα was also reported to be a negative regulator of the oxidative microbial killing by these cells. Inhibition of SIRPα indeed promoted NADPH oxidase production by neutrophils or macrophages, and CD47 was also required for such production (van Beek et al. 2012).
On the other hand, studies from SIRPα-deficient animals indicated that SIRPα was essential for the induction of Th17- or Th1-cell induced autoimmune diseases, such as experimental autoimmune encephalomyelitis (EAE), collagen-induced arthritis (CIA), 2,4-dinitro-1-fluorobenzene-induced contact hypersensitivity (CHS), and IL-10 deficiency-induced colitis (reviewed in Murata et al. 2014). In these models, the production of IL-17 or interferon-γ by Th cells against the disease-specific antigens was remarkably impaired in SIRPα-deficient mice. Of note, CD47-deficient mice were also shown to be resistant to EAE (Han et al. 2012), CHS, and colitis models, and the interaction between SIRPα and CD47 is thus likely required for the development of Th17- or Th1-cell induced autoimmune diseases. However, it remains an open question why SIRPα is essential for the development of autoimmunity. Given that DCs are thought to be crucial for the generation of autoreactive Th17- or Th1- cells, the reduced susceptibility to the development of autoimmune diseases in SIRPα-deficient mice is likely attributable to the impairment of DC functions.
In summary, SIRPα is important for the homeostatic regulation of myeloid cell functions, in particular, the phagocytic activity of macrophages and homeostasis of DCs in the immune system. SIRPα is also abundant in synapse-rich areas in the brain and the SIRPα-CD47 signaling regulates neural networks. Moreover, there is the possibility that SIRPα polymorphism is a key determinant of these functions in the immune system as well as in the brain. Targeting SIRPα (e.g. antibodies or recombinant proteins) might provide a novel therapeutic strategy against hematopoietic disorders, autoimmune disorders, organ transplantation, as well as cancer immunotherapy.
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