Regulation by Src Homology 2 Domain-Containing Protein Tyrosine Phosphatase Substrate-1 of α-Galactosylceramide-Induced Antimetastatic Activity and Th1 and Th2 Responses of NKT Cells

Imaging α-GalCer-activated iNKT cells in a hepatic metastatic environment

Colorectal cancer patients frequently develop liver metastases after, and perhaps as a consequence of, lifesaving surgical resection of the primary tumor. This creates a potential opportunity for prophylactic metastatic treatment with novel immunostimulatory molecules. Here, we used state-of-the-art intravital imaging of an experimental liver metastasis model to visualize the early behavior and function of invariant (i)NKT cells stimulated with α-galactosylceramide (α-GalCer). Intravenous α-GalCer prior to tumor cell seeding in the liver significantly inhibited tumor growth. However, some seeding tumor cells survived. A multiple dosing regimen reduced tumor burden and prolonged the life of mice, whereas tumors returned within 5 days after a single dose of α-GalCer. With multiple doses of α-GalCer, iNKT cells increased in number and granularity (as did NK cells). As a result, the total number of contacts and time in contact with tumors increased substantially. In the absence of iNKT cells, the beneficial effect of α-GalCer was lost. Robust cytokine production dissipated over time. Repeated therapy, even after cytokine dissipation, led to reduced tumor burden and prolonged survival. Serial transplantation of tumors exposed to α-GalCer-activated iNKT cells did not induce greater resistance, suggesting no obvious epigenetic or genetic immunoediting in tumors exposed to activated iNKT cells. Very few tumor cells expressed CD1d in this model, and as such, adding monomers of CD1d-α-GalCer further reduced tumor growth. The data suggest early and repeated stimulation of iNKT cells with α-GalCer could have direct therapeutic benefit for colorectal cancer patients that develop metastatic liver disease.

CD47 Deficiency Ameliorates Ocular Autoimmune Inflammation

Autoimmune uveitis is a sight-threatening ocular inflammatory condition in which the retina and uveal tissues become a target of autoreactive immune cells. The CD47 is a ubiquitously expressed transmembrane protein which plays multiple roles in fundamental cellular functions including phagocytosis, proliferation, and adhesion. Signal regulatory protein alpha (SIRPα), one of the CD47 ligands, is predominantly expressed in myeloid lineage cells such as dendritic cells (DCs) or macrophages, and CD47-SIRPα signaling pathway is implicated in the development of autoimmune diseases. Our current study demonstrates how CD47 depletion is effective in the prevention of experimental autoimmune uveitis (EAU), an animal model of human autoimmune uveitis, in animals deficient of CD47 (CD47-/- ). Systemic suppression of SIRPα+ DCs in animals deficient in CD47 resulted in the inability of autoreactive CD4+ T cells to develop, which is crucial to induction of EAU. Of interest, retinal microglia, the resident immune cell of the retina, express SIRPα, however these cells were not operative in EAU suppression in response to CD47 depletion. These results identify CD47 as a significant regulator in the development of SIRPα+ DCs that is vital to disease induction in EAU.

Macrocyclic Peptide-Mediated Blockade of the CD47-SIRPα Interaction as a Potential Cancer Immunotherapy

Medium-sized macrocyclic peptides are an alternative to small compounds and large biomolecules as a class of pharmaceutics. The CD47-SIRPα signaling axis functions as an innate immune checkpoint that inhibits phagocytosis in phagocytes and has been implicated as a promising target for cancer immunotherapy. The potential of macrocyclic peptides that target this signaling axis as immunotherapeutic agents has remained unknown, however. Here we have developed a macrocyclic peptide consisting of 15 amino acids that binds to the ectodomain of mouse SIRPα and efficiently blocks its interaction with CD47 in an allosteric manner. The peptide markedly promoted the phagocytosis of antibody-opsonized tumor cells by macrophages in vitro as well as enhanced the inhibitory effect of anti-CD20 or anti-gp75 antibodies on tumor formation or metastasis in vivo. Our results suggest that allosteric inhibition of the CD47-SIRPα interaction by macrocyclic peptides is a potential approach to cancer immunotherapy.

α-GalCer and iNKT Cell-Based Cancer Immunotherapy: Realizing the Therapeutic Potentials

NKT cells are CD1d-restricted innate-like T cells expressing both T cell receptor and NK cell markers. The major group of NKT cells in both human and mice is the invariant NKT (iNKT) cells and the best-known function of iNKT cells is their potent anti-tumor function in mice. Since its discovery 25 years ago, the prototype ligand of iNKT cells, α-galactosylceramide (α-GalCer) has been used in over 30 anti-tumor clinical trials with mostly suboptimal outcomes. To realize its therapeutic potential, numerous preclinical models have been developed to optimize the scheme and strategies for α-GalCer-based cancer immunotherapies. Nevertheless, since there is no standard protocol for α-GalCer delivery, we reviewed the preclinical studies with a focus on B16 melanoma model in the goal of identifying the best treatment schemes for α-GalCer treatment. We then reviewed the current progress in developing more clinically relevant mouse models for these preclinical studies, most notably the generation of new mouse models with a humanized CD1d/iNKT cell system. With ever-emerging novel iNKT cell ligands, invention of novel α-GalCer delivery strategies and significantly improved preclinical models for optimizing these new strategies, one can be hopeful that the full potential of anti-tumor potential for α-GalCer will be realized in the not too distant future.

CD47-signal regulatory protein α signaling system and its application to cancer immunotherapy

Tumor cells evade immune surveillance through direct or indirect interactions with various types of immune cell, with much recent attention being focused on modifying immune cell responses as the basis for the development of new cancer treatments. Signal regulatory protein α (SIRPα) and CD47 are both transmembrane proteins that interact with each other and constitute a cell-cell communication system. SIRPα is particularly abundant in myeloid cells such as macrophages and dendritic cells, whereas CD47 is expressed ubiquitously and its expression level is elevated in cancer cells. Recent studies have shown that blockade of CD47-SIRPα interaction enhances the phagocytic activity of phagocytes such as macrophages toward tumor cells in vitro as well as resulting in the efficient eradication of tumor cells in a variety of xenograft or syngeneic mouse models of cancer. Moreover, CD47 blockade has been shown to promote the stimulation of tumor-specific cytotoxic T cells by macrophages or dendritic cells. Biological agents, such as Abs and recombinant proteins, that target human CD47 or SIRPα have been developed and are being tested in preclinical models of human cancer or in clinical trials with cancer patients. Preclinical studies have also suggested that CD47 or SIRPα blockade may have a synergistic antitumor effect in combination with immune checkpoint inhibitors that target the adaptive immune system. Targeting of the CD47-SIRPα signaling system is thus a promising strategy for cancer treatment based on modulation of both innate and acquired immune responses to tumor cells.

Anti-human SIRPα antibody is a new tool for cancer immunotherapy

Interaction of signal regulatory protein α (SIRPα) expressed on the surface of macrophages with its ligand CD47 expressed on target cells negatively regulates phagocytosis of the latter cells by the former. We recently showed that blocking Abs to mouse SIRPα enhanced both the Ab‐dependent cellular phagocytosis (ADCP) activity of mouse macrophages for Burkitt's lymphoma Raji cells opsonized with an Ab to CD20 (rituximab) in vitro as well as the inhibitory effect of rituximab on the growth of tumors formed by Raji cells in nonobese diabetic (NOD)/SCID mice. However, the effects of blocking Abs to human SIRPα in preclinical cancer models have remained unclear given that such Abs have failed to interact with endogenous SIRPα expressed on macrophages of immunodeficient mice. With the use of Rag2^−/−γ_c^−/− mice harboring a transgene for human SIRPα under the control of human regulatory elements (hSIRPα‐DKO mice), we here show that a blocking Ab to human SIRPα significantly enhanced the ADCP activity of macrophages derived from these mice for human cancer cells. The anti‐human SIRPα Ab also markedly enhanced the inhibitory effect of rituximab on the growth of tumors formed by Raji cells in hSIRPα‐DKO mice. Our results thus suggest that the combination of Abs to human SIRPα with therapeutic Abs specific for tumor antigens warrants further investigation for potential application to cancer immunotherapy. In addition, humanized mice, such as hSIRPα‐DKO mice, should prove useful for validation of the antitumor effects of checkpoint inhibitors before testing in clinical trials.

Anti-SIRP antibodies as a potential new tool for cancer immunotherapy

Tumor cells are thought to evade immune surveillance through interaction with immune cells. Much recent attention has focused on the modification of immune responses as a basis for new cancer treatments. SIRPα is an Ig superfamily protein that inhibits phagocytosis in macrophages upon interaction with its ligand CD47 expressed on the surface of target cells. Here, we show that SIRPα is highly expressed in human renal cell carcinoma and melanoma. Furthermore, an anti-SIRPα Ab that blocks the interaction with CD47 markedly suppressed tumor formation by renal cell carcinoma or melanoma cells in immunocompetent syngeneic mice. This inhibitory effect of the Ab appeared to be mediated by dual mechanisms: direct induction of Ab-dependent cellular phagocytosis of tumor cells by macrophages and blockade of CD47-SIRPα signaling that negatively regulates such phagocytosis. The antitumor effect of the Ab was greatly attenuated by selective depletion not only of macrophages but also of NK cells or CD8⁺ T cells. In addition, the anti-SIRPα Ab also enhances the inhibitory effects of Abs against CD20 and programmed cell death 1 (PD-1) on tumor formation in mice injected with SIRPα-nonexpressing tumor cells. Anti-SIRPα Abs thus warrant further study as a potential new therapy for a broad range of cancers.

The CD47-SIRPα signalling system: its physiological roles and therapeutic application

Signal regulatory protein α (SIRPα), also known as SHPS-1/BIT/ CD172a, is an immunoglobulin superfamily protein that binds to the protein tyrosine phosphatases SHP-1 and SHP-2 through its cytoplasmic region. CD47, another immunoglobulin superfamily protein, is a ligand for SIRPα, with the two proteins constituting a cell-cell communication system (the CD47-SIRPα signalling system). SIRPα is particularly abundant in the myeloid-lineage hematopoietic cells such as macrophages or dendritic cells (DCs), whereas CD47 is expressed ubiquitously. Interaction of CD47 (on red blood cells) with SIRPα (on macrophages) is thought to prevent the phagocytosis by the latter cells of the former cells, determining the lifespan of red blood cells. Recent studies further indicate that this signalling system plays important roles in engraftment of hematopoietic stem cells as well as in tumour immune surveillance through regulation of the phagocytic activity of macrophages. In the immune system, the CD47-SIRPα interaction is also important for the development of a subset of CD11c(+)DCs as well as organization of secondary lymphoid organs. Finally, the CD47-SIRPα signalling system likely regulates bone homeostasis by osteoclast development. Newly emerged functions of the CD47-SIRPα signalling system thus provide multiple therapeutic strategies for cancer, autoimmune diseases and bone disorders.

'Clustering' SIRPα into the plasma membrane lipid microdomains is required for activated monocytes and macrophages to mediate effective cell surface interactions with CD47

SIRPα, an ITIMs-containing signaling receptor, negatively regulates leukocyte responses through extracellular interactions with CD47. However, the dynamics of SIRPα-CD47 interactions on the cell surface and the governing mechanisms remain unclear. Here we report that while the purified SIRPα binds to CD47 and that SIRPα is expressed on monocytes and monocytic THP-1 or U937, these SIRPα are ineffective to mediate cell binding to immobilized CD47. However, cell binding to CD47 is significantly enhanced when monocytes transmigrating across endothelia, or being differentiated into macrophages. Cell surface labeling reveals SIRPα to be diffused on naïve monocytes but highly clustered on transmigrated monocytes and macrophages. Protein crosslink and equilibrium centrifugation confirm that SIRPα in the latter cells forms oligomerized complexes resulting in increased avidity for CD47 binding. Furthermore, formation of SIRPα complexes/clusters requires the plasma membrane ‘lipid rafts’ and the activity of Src family kinase during macrophage differentiation. These results together suggest that ‘clustering’ SIRPα into plasma membrane microdomains is essential for activated monocytes and macrophages to effectively interact with CD47 and initiate intracellular signaling.

Signal regulatory protein-α protects against cardiac hypertrophy via the disruption of toll-like receptor 4 signaling

Signal regulatory protein-α (SIRPA/SIRPα) is a transmembrane protein that is expressed in various tissues, including the heart. Previous studies have demonstrated that SIRPA is involved in multiple biological processes, including macrophage multinucleation, skeletal muscle differentiation, neuronal survival, protection against diabetes mellitus, and negative regulation of immune cells. However, the role of SIRPA in cardiac hypertrophy remains unknown. To examine the role of SIRPA in pathological cardiac hypertrophy, we used SIRPA knockout mice and transgenic mice that overexpressed mouse SIRPA in the heart. Cardiac hypertrophy was evaluated by echocardiographic, hemodynamic, pathological, and molecular analyses. We observed downregulation of SIRPA expression in dilated cardiomyopathy human hearts and in animal hearts after aortic banding surgery. Accordingly, SIRPA(-/-) mice displayed augmented cardiac hypertrophy, which was accompanied by increased cardiac fibrosis and reduced contractile function, as compared with SIRPA(+/+) mice 4 weeks after aortic banding. In contrast, transgenic mice with the cardiac-specific SIRPA overexpression exhibited the opposite phenotype in response to pressure overload. Likewise, SIRPA protected against angiotensin II-induced cardiomyocyte hypertrophy in vitro. Mechanistically, we revealed that SIRPA-mediated protection during cardiac hypertrophy involved inhibition of the Toll-like receptor 4/nuclear factor-κB signaling axis. Furthermore, we demonstrated that the disruption of Toll-like receptor 4 rescued the adverse effects of SIRPA deficiency on pressure overload-triggered cardiac remodeling. Thus, our results identify that SIRPA plays a protective role in cardiac hypertrophy through negative regulation of the Toll-like receptor 4/nuclear factor-κB pathway.

Autoimmune animal models in the analysis of the CD47-SIRPα signaling pathway

Signal regulatory protein α (SIRPα), also known as SHPS-1/SIRPA, is an immunoglobulin superfamily protein that binds to the protein tyrosine phosphatases Shp1 and Shp2 through its cytoplasmic region and is predominantly expressed in dendritic cells and macrophages. CD47, a widely expressed transmembrane protein, is a ligand for SIRPα, with the two proteins constituting a cell-cell communication system. It was previously demonstrated that the CD47-SIRPα signaling pathway is important for prevention of clearance by splenic macrophages of red blood cells or platelets from the bloodstream. In addition, this signaling pathway is also implicated in homeostatic regulation of dendritic cells and development of autoimmunity. Here we describe the detailed protocols for methods that were used in our recent studies to study the role of the CD47-SIRPα signaling pathway in autoimmunity. We also demonstrate that hematopoietic SIRPα as well as nonhematopoietic CD47 are important for development of experimental autoimmune encephalomyelitis. Thus, we here strengthen the importance of experimental animal models as well as other methods for the study of molecular pathogenesis of autoimmunity.

Increased susceptibility to Salmonella infection in signal regulatory protein α-deficient mice

Recent studies have shed light on the connection between elevated erythropoetin production/spleen erythropoiesis and increased susceptibility to Salmonella infection. In this article, we provide another mouse model, the SIRPα-deficient (Sirpα⁻/⁻) mouse, that manifests increased erythropoiesis as well as heightened susceptibility to Salmonella infection. Sirpα⁻/⁻ mice succumbed to systemic infection with attenuated Salmonella, possessing significantly higher bacterial loads in both the spleen and the liver. Moreover, Salmonella-specific Ab production and Ag-specific CD4 T cells were reduced in Sirpα⁻/⁻ mice compared with wild-type controls. To further characterize the potential mechanism underlying SIRPα-dependent Ag-specific CD4 T cell priming, we demonstrate that lack of SIRPα expression on dendritic cells results in less efficient Ag processing and presentation in vitro. Collectively, these findings demonstrate an indispensable role of SIRPα for protective immunity to Salmonella infection.

Signal regulatory protein α regulates the homeostasis of T lymphocytes in the spleen

The molecular basis for formation of lymphoid follicle and its homeostasis in the secondary lymphoid organs remains unclear. Signal regulatory protein α (SIRPα), an Ig superfamily protein that is predominantly expressed in dendritic cells or macrophages, mediates cell-cell signaling by interacting with CD47, another Ig superfamily protein. In this study, we show that the size of the T cell zone as well as the number of CD4(+) T cells were markedly reduced in the spleen of mice bearing a mutant (MT) SIRPα that lacks the cytoplasmic region compared with those of wild-type mice. In addition, the expression of CCL19 and CCL21, as well as of IL-7, which are thought to be important for development or homeostasis of the T cell zone, was markedly decreased in the spleen of SIRPα MT mice. By the use of bone marrow chimera, we found that hematopoietic SIRPα is important for development of the T cell zone as well as the expression of CCL19 and CCL21 in the spleen. The expression of lymphotoxin and its receptor, lymphotoxin β receptor, as well as the in vivo response to lymphotoxin β receptor stimulation were also decreased in the spleen of SIRPα MT mice. CD47-deficient mice also manifested phenotypes similar to SIRPα MT mice. These data suggest that SIRPα as well as its ligand CD47 are thus essential for steady-state homeostasis of T cells in the spleen.

Regulation by SIRPα of dendritic cell homeostasis in lymphoid tissues

The molecular basis for regulation of dendritic cell (DC) development and homeostasis remains unclear. Signal regulatory protein α (SIRPα), an immunoglobulin superfamily protein that is predominantly expressed in DCs, mediates cell-cell signaling by interacting with CD47, another immunoglobulin superfamily protein. We now show that the number of CD11c(high) DCs (conventional DCs, or cDCs), in particular, that of CD8-CD4+ (CD4+) cDCs, is selectively reduced in secondary lymphoid tissues of mice expressing a mutant form of SIRPα that lacks the cytoplasmic region. We also found that SIRPα is required intrinsically within cDCs or DC precursors for the homeostasis of splenic CD4+ cDCs. Differentiation of bone marrow cells from SIRPα mutant mice into DCs induced by either macrophage-granulocyte colony-stimulating factor or Flt3 ligand in vitro was not impaired. Although the accumulation of the immediate precursors of cDCs in the spleen was also not impaired, the half-life of newly generated splenic CD4+ cDCs was markedly reduced in SIRPα mutant mice. Both hematopoietic and nonhematopoietic CD47 was found to be required for the homeostasis of CD4+ cDCs and CD8-CD4- (double negative) cDCs in the spleen. SIRPα as well as its ligand, CD47, are thus important for the homeostasis of CD4+ cDCs or double negative cDCs in lymphoid tissues.

Leishmania donovani-induced expression of signal regulatory protein alpha on Kupffer cells enhances hepatic invariant NKT-cell activation

Signal regulatory protein α (SIRPα) and its cognate ligand CD47 have been documented to have a broad range of cellular functions in development and immunity. Here, we investigated the role of SIRPα–CD47 signalling in invariant NKT (iNKT) cell responses. We found that CD47 was required for the optimal production of IFN-γ from splenic iNKT cells following exposure to the αGalCer analogue PBS-57 and in vivo infection of mice with Leishmania donovani. Surprisingly, although SIRPα was undetectable in the liver of uninfected mice, the hepatic iNKT-cell response to infection was also impaired in CD47^−/− mice. However, we found that SIRPα was rapidly induced on Kupffer cells following L. donovani infection, via a mechanism involving G-protein-coupled receptors. Thus, we describe a novel amplification pathway affecting cytokine production by hepatic iNKT cells, which may facilitate the breakdown of hepatic tolerance after infection.

Functions and molecular mechanisms of the CD47-SIRPalpha signalling pathway

Signal regulatory protein (SIRP)alpha, also known as SHPS-1 or SIRPA, is a transmembrane protein that binds to the protein tyrosine phosphatases SHP-1 and SHP-2 through its cytoplasmic region and is predominantly expressed in neurons, dendritic cells and macrophages. CD47, a widely expressed transmembrane protein, is a ligand for SIRPalpha, with the two proteins constituting a cell-cell communication system. The interaction of SIRPalpha with CD47 is important for the regulation of migration and phagocytosis. Recent studies have implicated the CD47-SIRPalpha signalling pathway in immune homeostasis and in regulation of neuronal networks. Advances in the structural and functional analyses of the CD47-SIRPalpha signalling pathway now provide exciting hints of the therapeutic benefits of manipulating this signalling system in autoimmune diseases and neurological disorders.

Expression of Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 in pancreatic beta-Cells and its role in promotion of insulin secretion and protection against diabetes

Insulin secretion by beta-cells of pancreatic islets is regulated by various soluble factors including glucose and hormones. The importance of direct cell-cell communication among beta-cells or between beta-cells and other cell types for such regulation has remained unclear, however. Transmembrane proteins Src homology 2 domain-containing protein tyrosine phosphatase substrate-1 (SHPS-1) and its ligand CD47 interact through their extracellular regions and contribute to intercellular communication. We now show that both SHPS-1 and CD47 are prominently expressed in beta-cells of the pancreas. The plasma insulin level in the randomly fed state was markedly reduced in mice that express a mutant form of SHPS-1 lacking most of the cytoplasmic region compared with that in wild-type (WT) mice, although the blood glucose concentrations of the two types of mice were similar. This reduction in the plasma insulin level of SHPS-1 mutant mice was even more pronounced in animals maintained on a high-fat diet. Glucose tolerance was also markedly impaired in SHPS-1 mutant mice on a high-fat diet, whereas both peripheral insulin sensitivity and the insulin content of the pancreas in the mutant animals were similar to those of WT mice. Glucose-stimulated insulin secretion was similar for islets isolated from WT or SHPS-1 mutant mice. The impaired glucose tolerance of SHPS-1 mutant mice was ameliorated by treatment with the alpha2-adrenergic antagonist yohimbine. These results suggest that SHPS-1 promotes insulin secretion from beta-cells and thereby protects against diabetes. Preventing of alpha2-adrenergic receptor-mediated inhibition of insulin secretion may partly participate in such a function of SHPS-1.