Responses to Microbial Challenges by SLAMF Receptors.

The SLAMF family (SLAMF) of cell surface glycoproteins is comprised of nine glycoproteins and while SLAMF1, 3, 5, 6, 7, 8, and 9 are self-ligand receptors, SLAMF2 and SLAMF4 interact with each other. Their interactions induce signal transduction networks in trans, thereby shaping immune cell-cell communications. Collectively, these receptors modulate a wide range of functions, such as myeloid cell and lymphocyte development, and T and B cell responses to microbes and parasites. In addition, several SLAMF receptors serve as microbial sensors, which either positively or negatively modulate the function of macrophages, dendritic cells, neutrophils, and NK cells in response to microbial challenges. The SLAMF receptor-microbe interactions contribute both to intracellular microbicidal activity as well as to migration of phagocytes to the site of inflammation. In this review, we describe the current knowledge on how the SLAMF receptors and their specific adapters SLAM-associated protein and EAT-2 regulate innate and adaptive immune responses to microbes.

The cell surface receptor Slamf6 modulates innate immune responses during Citrobacter rodentium-induced colitis.

The homophilic cell surface receptors CD150 (Slamf1) and CD352 (Slamf6) are known to modulate adaptive immune responses. Although the Th17 response was enhanced in Slamf6(-/-) C57BL/6 mice upon oral infection with Citrobacter rodentium, the pathologic consequences are indistinguishable from an infection of wild-type C57BL/6 mice. Using a reporter-based binding assay, we show that Slamf6 can engage structures on the outer cell membrane of several Gram(-) bacteria. Therefore, we examined whether Slamf6, like Slamf1, is also involved in innate responses to bacteria and regulates peripheral inflammation by assessing the outcome of C. rodentium infections in Rag(-/-) mice. Surprisingly, the pathology and immune responses in the lamina propria of C. rodentium-infected Slamf6(-/-) Rag(-/-) mice were markedly reduced as compared with those of Rag(-/-) mice. Infiltration of inflammatory phagocytes into the lamina propria was consistently lower in Slamf6(-/-) Rag(-/-) mice than in Rag(-/-) animals. Concomitant with the reduced systemic translocation of the bacteria was an enhanced production of IL-22, suggesting that Slamf6 suppresses a mucosal protective program. Furthermore, administering a mAb (330) that inhibits bacterial interactions with Slamf6 to Rag(-/-) mice ameliorated the infection compared with a control antibody. We conclude that Slamf6-mediated interactions of colonic innate immune cells with specific Gram(-) bacteria reduce mucosal protection and enhance inflammation, contributing to lethal colitis that is caused by C. rodentium infections in Rag(-/-) mice.

Migration of myeloid cells during inflammation is differentially regulated by the cell surface receptors Slamf1 and Slamf8.

Previous studies have demonstrated that the cell surface receptor Slamf1 (CD150) is requisite for optimal NADPH-oxidase (Nox2) dependent reactive oxygen species (ROS) production by phagocytes in response to Gram- bacteria. By contrast, Slamf8 (CD353) is a negative regulator of ROS in response to Gram+ and Gram- bacteria. Employing in vivo migration after skin sensitization, induction of peritonitis, and repopulation of the small intestine demonstrates that in vivo migration of Slamf1-/- dendritic cells and macrophages is reduced, as compared to wt mice. By contrast, in vivo migration of Slamf8-/- dendritic cells, macrophages and neutrophils is accelerated. These opposing effects of Slamf1 and Slamf8 are cell-intrinsic as judged by in vitro migration in transwell chambers in response to CCL19, CCL21 or CSF-1. Importantly, inhibiting ROS production of Slamf8-/- macrophages by diphenyleneiodonium chloride blocks this in vitro migration. We conclude that Slamf1 and Slamf8 govern ROS-dependent innate immune responses of myeloid cells, thus modulating migration of these cells during inflammation in an opposing manner.

Glucocorticoid-Induced TNF Receptor Family-Related Protein Ligand is Requisite for Optimal Functioning of Regulatory CD4(+) T Cells.

Glucocorticoid-induced tumor necrosis factor receptor family-related protein (TNFRSF18, CD357) is constitutively expressed on regulatory T cells (Tregs) and is inducible on effector T cells. In this report, we examine the role of glucocorticoid-induced TNF receptor family-related protein ligand (GITR-L), which is expressed by antigen presenting cells, on the development and expansion of Tregs. We found that GITR-L is dispensable for the development of naturally occurring FoxP3(+) Treg cells in the thymus. However, the expansion of Treg in GITR-L (-/-) mice is impaired after injection of the dendritic cells (DCs) inducing factor Flt3 ligand. Furthermore, DCs from the liver of GITR-L (-/-) mice were less efficient in inducing proliferation of antigen-specific Treg cells in vitro than the same cells from WT littermates. Upon gene transfer of ovalbumin into hepatocytes of GITR-L (-/-)FoxP3(GFP) reporter mice using adeno-associated virus (AAV8-OVA) the number of antigen-specific Treg in liver and spleen is reduced. The reduced number of Tregs resulted in an increase in the number of ovalbumin specific CD8(+) T effector cells. This is highly significant because proliferation of antigen-specific CD8(+) cells itself is dependent on the presence of GITR-L, as shown by in vitro experiments and by adoptive transfers into GITR-L (-/-) Rag (-/-) and Rag (-/-) mice that had received AAV8-OVA. Surprisingly, administering αCD3 significantly reduced the numbers of FoxP3(+) Treg cells in the liver and spleen of GITR-L (-/-) but not WT mice. Because soluble Fc-GITR-L partially rescues αCD3 induced in vitro depletion of the CD103(+) subset of FoxP3(+)CD4(+) Treg cells, we conclude that expression of GITR-L by antigen presenting cells is requisite for optimal Treg-mediated regulation of immune responses including those in response during gene transfer.

Glucocorticoid-induced TNF receptor family-related protein ligand regulates the migration of monocytes to the inflamed intestine.

Glucocorticoid-induced TNF receptor family-related protein (GITR) regulates the function of both T cells and antigen-presenting cells (APCs), while the function of GITR ligand (GITR-L) is largely unknown. Here we evaluate the role of GITR-L, whose expression is restricted to APCs, in the development of enterocolitis. On injecting naive CD4(+) T cells, GITR-L(-/-)Rag(-/-) mice develop a markedly milder colitis than Rag(-/-) mice, which correlates with a 50% reduction of Ly6C(+)CD11b(+)MHCII(+) macrophages in the lamina propria and mesenteric lymph nodes. The same result was observed in αCD40-induced acute colitis and during peritonitis, suggesting an altered monocyte migration. In line with these observations, the number of nondifferentiated monocytes was approximately 3-fold higher in the spleen of GITR-L(-/-)Rag(-/-) mice than in Rag(-/-) mice after αCD40 induction. Consistent with the dynamic change in the formation of an active angiotensin II type 1 receptor (AT1) dimer in GITR-L(-/-) splenic monocytes during intestinal inflammation, the migratory capability of splenic monocytes from GITR-L-deficient mice was impaired in an in vitro transwell migration assay. Conversely, αGITR-L reduces the number of splenic Ly6C(hi) monocytes, concomitantly with an increase in AT1 dimers. We conclude that GITR-L regulates the number of proinflammatory macrophages in sites of inflammation by controlling the egress of monocytes from the splenic reservoir.

Signaling lymphocyte activation molecule regulates development of colitis in mice.

BACKGROUND & AIMS: Signaling lymphocyte activation molecule (Slamf)1 is a co-stimulatory receptor on T cells and regulates cytokine production by macrophages and dendritic cells. Slamf1 regulates microbicidal mechanisms in macrophages, therefore we investigated whether the receptor affects development of colitis in mice. METHODS: We transferred CD45RB(hi) CD4(+) T cells into Rag(-/-) or Slamf1(-/-)Rag(-/-) mice to induce colitis. We also induced colitis by injecting mice with an antibody that activates CD40. We determined the severity of enterocolitis based on disease activity index, histology scores, and levels of cytokine production, and assessed the effects of antibodies against Slamf1 on colitis induction. We quantified migration of monocytes and macrophage to inflamed tissues upon induction of colitis or thioglycollate-induced peritonitis and in response to tumor necrosis factor-α in an air-pouch model of leukocyte migration. RESULTS: Colitis was reduced in Slamf1(-/-)Rag(-/-) mice, compared with Rag(-/-) mice, after transfer of CD45RB(hi) CD4(+) T cells or administration of the CD40 agonist. The numbers of monocytes and macrophages were reduced in inflamed tissues of Slamf1(-/-)Rag(-/-) mice, compared with Rag(-/-) mice, after induction of colitis and other inflammatory disorders. An antibody that inhibited Slamf1 reduced the level of enterocolitis in Rag(-/-) mice. CONCLUSIONS: Slamf1 contributes to the development of colitis in mice. It appears to indirectly regulate the appearance of monocytes and macrophages in inflamed intestinal tissues. Antibodies that inhibit Slamf1 reduce colitis in mice, so human SLAMF1 might be a therapeutic target for inflammatory bowel disease.