The adaptor protein SAP regulates type II NKT-cell development, cytokine production, and cytotoxicity against lymphoma.

CD1d-restricted NKT cells represent a unique lineage of immunoregulatory T cells that are divided into two groups, type I and type II, based on their TCR usage. Because there are no specific tools to identify type II NKT cells, little is known about their developmental requirements and functional regulation. In our previous study, we showed that signaling lymphocytic activation molecule associated protein (SAP) is essential for the development of type II NKT cells. Here, using a type II NKT-cell TCR transgenic mouse model, we demonstrated that CD1d-expressing hematopoietic cells, but not thymic epithelial cells, meditate efficient selection of type II NKT cells. Furthermore, we showed that SAP regulates type II NKT-cell development by controlling early growth response 2 protein and promyelocytic leukemia zinc finger expression. SAP-deficient 24αß transgenic T cells (24αß T cells) exhibited an immature phenotype with reduced Th2 cytokine-producing capacity and diminished cytotoxicity to CD1d-expressing lymphoma cells. The impaired IL-4 production by SAP-deficient 24αß T cells was associated with reduced IFN regulatory factor 4 and GATA-3 induction following TCR stimulation. Collectively, these data suggest that SAP is critical for regulating type II NKT cell responses. Aberrant responses of these T cells may contribute to the immune dysregulation observed in X-linked lymphoproliferative disease caused by mutations in SAP.

Inhibition of TYK2 and JAK1 ameliorates imiquimod-induced psoriasis-like dermatitis by inhibiting IL-22 and the IL-23/IL-17 axis.

Psoriasis is a chronic autoimmune disease affecting the skin and characterized by aberrant keratinocyte proliferation and function. Immune cells infiltrate the skin and release proinflammatory cytokines that play important roles in psoriasis. The Th17 network, including IL-23 and IL-22, has recently emerged as a critical component in the pathogenesis of psoriasis. IL-22 and IL-23 signaling is dependent on the JAK family of protein tyrosine kinases, making JAK inhibition an appealing strategy for the treatment of psoriasis. In this study, we report the activity of SAR-20347, a small molecule inhibitor with specificity for JAK1 and tyrosine kinase 2 (TYK2) over other JAK family members. In cellular assays, SAR-20347 dose dependently (1 nM-10 µM) inhibited JAK1- and/or TYK2-dependent signaling from the IL-12/IL-23, IL-22, and IFN-α receptors. In vivo, TYK2 mutant mice or treatment of wild-type mice with SAR-20347 significantly reduced IL-12-induced IFN-γ production and IL-22-dependent serum amyloid A to similar extents, indicating that, in these models, SAR-20347 is probably acting through inhibition of TYK2. In an imiquimod-induced psoriasis model, the administration of SAR-20347 led to a striking decrease in disease pathology, including reduced activation of keratinocytes and proinflammatory cytokine levels compared with both TYK2 mutant mice and wild-type controls. Taken together, these data indicate that targeting both JAK1- and TYK2-mediated cytokine signaling is more effective than TYK2 inhibition alone in reducing psoriasis pathogenesis.

SAP is required for the development of innate phenotype in H2-M3--restricted Cd8(+) T cells.

H2-M3--restricted T cells have a preactivated surface phenotype, rapidly expand, and produce cytokines upon stimulation, and, as such, are classified as innate T cells. Unlike most innate T cells, M3-restricted T cells also express CD8αß coreceptors and a diverse TCR repertoire: hallmarks of conventional MHC Ia-restricted CD8(+) T cells. Although invariant NKT cells are also innate T cells, they are selected exclusively on hematopoietic cells (HC), whereas M3-restricted T cells can be selected on either hematopoietic or thymic epithelial cells. Moreover, their phenotypes differ depending on what cells mediate their selection. Although there is a clear correlation between selection on HC and development of innate phenotype, the underlying mechanism remains unclear. Signaling lymphocyte activation molecule-associated protein (SAP) is required for the development of invariant NKT cells and mediates signals from signaling lymphocyte activation molecule receptors that are exclusively expressed on HC. Based on their dual selection pathway, M3-restricted T cells present a unique model for studying the development of innate T cell phenotype. Using both polyclonal and transgenic mouse models, we demonstrate that although M3-restricted T cells are capable of developing in the absence of SAP, SAP is required for HC-mediated selection, development of preactivated phenotype, and heightened effector functions of M3-restricted T cells. These findings are significant because they directly demonstrate the need for SAP in HC-mediated acquisition of innate T cell phenotype and suggest that, due to their SAP-dependent HC-mediated selection, M3-restricted T cells develop a preactivated phenotype and an intrinsic ability to proliferate faster upon stimulation, allowing for an important role in the early response to infection.

Two SHIPs passing in the middle of the immune system.

Immunity requires a complex, multiscale system of molecules, cells, and cytokines. In this issue of the European Journal of Immunology, Collazo et al. [Eur. J. Immunol. 2012. 42: 1785-1796] provide evidence that links the lipid phosphatase SHIP1 with the coordination of interactions between regulatory T (Treg) cells and myeloid-derived suppressor cells (MDSCs). Using conditional knockouts of SHIP1 in either the myeloid or T-cell-lineage of mice, the authors show that the regulated development of Treg cells is controlled directly by cell-intrinsic SHIP1, and indirectly by extrinsic SHIP1 control of an unknown myeloid cell. Regulation of MDSCs is also determined by SHIP1 in an extrinsic manner, again via an as-yet-unknown myeloid cell. Furthermore, this extrinsic control of Treg cells and MDSCs is mediated in part by increased production of G-CSF, a growth factor critical for the production of neutrophils, in SHIP1-deficient mice. Thus, a physiologically important implication of this report is the collaboration between the innate and adaptive immune systems in fine tuning of Treg cells as discussed in this commentary.

Fyn promotes Th17 differentiation by regulating the kinetics of RORγt and Foxp3 expression.

Th17 cells constitute a proinflammatory CD4(+) T cell subset that is important for microbial clearance, but also are implicated as propagators of various autoimmune pathologies. Evidence suggests that Th17 cells share common progenitors with immunosuppressive CD4(+) inducible regulatory T cells (T(REG)) and that the developmental pathways of these two subsets are reciprocally regulated. In this study, we show evidence that the Src family tyrosine kinase Fyn helps regulate this Th17/T(REG) balance. When placed under Th17-skewing conditions, CD4(+) T cells from fyn(-/-) mice had decreased levels of IL-17, but increased expression of the T(REG) transcription factor Foxp3. The defect in IL-17 expression occurred independently of the ectopic Foxp3 expression and correlated with a delay in retinoic acid-related orphan receptor γt upregulation and an inability to maintain normal STAT3 activation. Fyn-deficient Th17 cells also exhibited delayed upregulation of Il23r, Il21, Rora, and Irf4, as well as aberrant expression of Socs3, suggesting that Fyn may function upstream of a variety of molecular pathways that contribute to Th17 polarization. The fyn(-/-) mice had fewer IL-17(+)CD4(+) T cells in the large intestinal lamina propria compared with littermate controls. Furthermore, after transfer of either wild-type or fyn(-/-) naive CD4(+) T cells into Rag1(-/-) hosts, recipients receiving fyn(-/-) cells had fewer IL-17-producing T cells, indicating that Fyn may also regulate Th17 differentiation in vivo. These results identify Fyn as a possible novel regulator of the developmental balance between the Th17 cell and T(REG) subsets.

CD1d-expressing breast cancer cells modulate NKT cell-mediated antitumor immunity in a murine model of breast cancer metastasis.

BACKGROUND: Tumor tolerance and immune suppression remain formidable obstacles to the efficacy of immunotherapies that harness the immune system to eradicate breast cancer. A novel syngeneic mouse model of breast cancer metastasis was developed in our lab to investigate mechanisms of immune regulation of breast cancer. Comparative analysis of low-metastatic vs. highly metastatic tumor cells isolated from these mice revealed several important genetic alterations related to immune control of cancer, including a significant downregulation of cd1d1 in the highly metastatic tumor cells. The cd1d1 gene in mice encodes the MHC class I-like molecule CD1d, which presents glycolipid antigens to a specialized subset of T cells known as natural killer T (NKT) cells. We hypothesize that breast cancer cells, through downregulation of CD1d and subsequent evasion of NKT-mediated antitumor immunity, gain increased potential for metastatic tumor progression. METHODOLOGY/PRINCIPAL FINDINGS: In this study, we demonstrate in a mouse model of breast cancer metastasis that tumor downregulation of CD1d inhibits iNKT-mediated antitumor immunity and promotes metastatic breast cancer progression in a CD1d-dependent manner in vitro and in vivo. Using NKT-deficient transgenic mouse models, we demonstrate important differences between type I and type II NKT cells in their ability to regulate antitumor immunity of CD1d-expressing breast tumors. CONCLUSIONS/SIGNIFICANCE: The results of this study emphasize the importance of determining the CD1d expression status of the tumor when tailoring NKT-based immunotherapies for the prevention and treatment of metastatic breast cancer.

Lck regulates IL-10 expression in memory-like Th1 cells.

The Src family kinase Lck is thought to facilitate Th2 differentiation; however, its role in Th1 cells has not been well explored. Using mice that lack Lck in mature T cells, we find that lck(-/-) Th1 skewed cells have normal expression of T-bet and produce IFN-γ at WT levels. However, there is a 3-fold increase in IL-10 producing cells in the mutant cultures. These cells do not have elevated levels of IL-4, GATA3, IL-17 or Foxp3, indicating that they are not Th2, Th17, or Foxp3(+) T regulatory cells (Treg). Nor do these cells behave in a similar manner as the type 1 Treg. Most of the IL-10 in the lck(-/-) Th1 cultures is derived from the memory/activated subset, as the cytokine profile from Th1 cultures established from purified CD62L(+) (naïve) cells are similar to WT cells. Furthermore, this IL-10 expression appears to be dependent on IL-12 and correlates with elevated c-Maf. These data highlight a previously unappreciated role for Lck in regulating IL-10 in Th1 cells.

Lck mediates Th2 differentiation through effects on T-bet and GATA-3.

The Src family kinase Lck has been shown to be crucial in T cell signaling and development. However, its role in Th effector functions is not well understood. Lck has previously been shown to play a role in the cytokine expression of Th2 cells, but the mechanism by which Lck influences Th2 effector functions is unknown. Using a mouse model, we report that Lck is important in regulating the expression of IL-4 in Th2 skewed cells but is not as necessary for the expression of Th2 cytokines IL-5, IL-10, and IL-13. Furthermore, in the absence of Lck, T-bet and GATA-3 expression is aberrant. Moreover, this atypical expression pattern of T-bet and GATA-3 correlates with increased histone 3 acetylation at the Ifng locus and production of the Th1 cytokine IFN-gamma. We find overexpression of GATA-3 restores IL-4 expression in lck(-/-) Th2 cells; this indicates that the decreased IL-4 expression is due in part to reduced amounts of GATA-3. Taken together, these data imply that Lck mediates Th2 differentiation through effects on T-bet and GATA-3.

Phenotypic differences between mice deficient in XIAP and SAP, two factors targeted in X-linked lymphoproliferative syndrome (XLP).

Mutations in the X-linked inhibitor of apoptosis (XIAP) have recently been identified in patients with the rare genetic disease, X-linked lymphoproliferative syndrome (XLP), which was previously thought to be solely attributable to mutations in a distinct gene, SAP. To further understand the roles of these two factors in the pathogenesis of XLP, we have compared mice deficient in Xiap with known phenotypes of Sap-null mice. We show here that in contrast to Sap-deficient mice, animals lacking Xiap have apparently normal NKT cell development and no apparent defect in humoral responses to T cell-dependent antigens. However, Xiap-deficient cells were more susceptible to death upon infection with the murine herpesvirus MHV-68 and gave rise to more infectious virus. These differences could be rescued by restoration of XIAP. These data provide insight into the differing roles of XIAP and SAP in the pathogenesis of XLP.

The adaptor molecule signaling lymphocytic activation molecule-associated protein (SAP) regulates IFN-gamma and IL-4 production in V alpha 14 transgenic NKT cells via effects on GATA-3 and T-bet expression.

NKT cells comprise a rare regulatory T cell population of limited TCR diversity, with most cells using a Valpha14 Jalpha18 TCR. These cells exhibit a critical dependence on the signaling adapter molecule, signaling lymphocytic activation molecule-associated protein (SAP), for their ontogeny, an aspect not seen in conventional alphabeta T cells. Prior studies demonstrate that SAP enhances TCR-induced activation of NF-kappaB in CD4(+) T cells. Because NF-kappaB is required for NKT cell development, SAP might promote the ontogeny of this lineage by signaling to NF-kappaB. In this study, we demonstrate that forced expression of the NF-kappaB target gene, Bcl-x(L), or inhibitory NF-kappaB kinase beta, a catalytic subunit of the IkappaB kinase complex essential for NF-kappaB activation, fails to restore NKT cell development in sap(-/-) mice, suggesting that SAP mediates NKT cell development independently of NF-kappaB. To examine the role of SAP in NKT cell function, we generated NKT cells in sap(-/-) mice by expressing a transgene encoding the Valpha14 Jalpha18 component of the invariant TCR. These cells bound alpha-galactosylceramide-loaded CD1d tetramers, but exhibited a very immature CD24(+)NK1.1(-) phenotype. Although sap(-/-) tetramer-reactive cells proliferated in response to TCR activation, they did not produce appreciable levels of IL-4 or IFN-gamma. The reduction in cytokine production correlated with the near absence of GATA-3 and T-bet, key transcription factors regulating cytokine expression and maturation of NKT cells. Ectopic expression of GATA-3 partially restored IL-4 production by the NKT cells. Collectively, these data suggest that by promoting GATA-3 and T-bet expression, SAP exerts control over NKT cell development and mature NKT cell cytokine production.

Fyn kinase acts upstream of Shp2 and p38 mitogen-activated protein kinase to promote chemotaxis of mast cells towards stem cell factor.

The c-Kit receptor protein-tyrosine kinase plays a critical role in the differentiation, growth and survival of mast cells. Binding of its ligand stem cell factor (SCF), induces c-Kit dimerization, autophosphorylation, and recruitment of signaling proteins. The juxtamembrane sequence of c-Kit contains recruitment sites for the Src family kinases Fyn and Lyn, as well as Shp1 and Shp2 protein-tyrosine phosphatases. To characterize the role of Fyn in c-Kit signaling, we generated bone marrow-derived mast cells (BMMCs) from wild-type and Fyn knock-out mice. In contrast with previous studies of Lyn-deficient BMMCs, SCF treatment of Fyn-deficient BMMCs revealed no overt defects in the overall pattern of tyrosine phosphorylation, phosphatidylinositol 3' kinase recruitment to c-Kit, or phosphorylation of Stat3 transcription factor. However, Fyn-deficient mast cells showed a significant reduction in phosphorylation of Shp2 phosphatase and p38 mitogen-activated protein kinase. Defects in Shp2 and p38 phosphorylation were restored in Fyn-deficient mast cells transduced with a Fyn-expressing retrovirus (Fyn-rescue). Fyn-deficient BMMCs displayed reduced chemotaxis towards SCF, and this defect was corrected in Fyn-rescue cells. This study provides evidence that recruitment of both Shp2 and Fyn to juxtamembrane sites in c-Kit results in Shp2 phosphorylation, downstream signaling to p38 mitogen-activated protein kinase, and enhanced chemotaxis of mast cells.

Regulation of NKT cell development by SAP, the protein defective in XLP.

The adaptor molecule SAP is expressed in T lymphocytes and natural killer (NK) cells, where it regulates cytokine production and cytotoxicity. Here, we show that SAP, encoded by the SH2D1A gene locus, also has a crucial role during the development of NKT cells, a lymphocyte subset with immunoregulatory functions in response to infection, cancer and autoimmune disease. Following stimulation with the NKT cell-specific agonist alpha-galactosyl ceramide (alphaGC), Sh2d1a-/- splenocytes did not produce cytokines or activate other lymphoid lineages in an NKT cell-dependent manner. While evaluating the abnormalities in alphaGC-induced immune responses, we observed that Sh2d1a-/- animals lacked NKT cells in the thymus and peripheral organs. The defect in NKT cell ontogeny was hematopoietic cell autonomous and could be rescued by reconstitution of SAP expression within Sh2d1a-/- bone marrow cells. Seventeen individuals with X-linked lymphoproliferative disease (XLP), who harbored germline mutations in SH2D1A, also lacked NKT cells. Furthermore, a female XLP carrier showed completely skewed X chromosome inactivation within NKT cells, but not T or B cells. Thus, SAP is a crucial regulator of NKT cell ontogeny in humans and in mice. The absence of NKT cells may contribute to the phenotypes of SAP deficiency, including abnormal antiviral and antitumor immunity and hypogammaglobulinemia.

Restoration of NK T cell development in fyn-mutant mice by a TCR reveals a requirement for Fyn during early NK T cell ontogeny.

NK T cells are a unique lymphocyte population that have developmental requirements distinct from conventional T cells. Mice lacking the tyrosine kinase Fyn have 5- to 10-fold fewer mature NK T cells. This study shows that Fyn-deficient mice have decreased numbers of NK1.1(-) NK T cell progenitors as well. 5-Bromo-2'-deoxyuridine-labeling studies indicate that the NK T cells remaining in fyn(-/-) mice exhibit a similar turnover rate as wild-type cells. The fyn(-/-) NK T cells respond to alpha-galactosylceramide, a ligand recognized by NK T cells, and produce cytokines, but have depressed proliferative capacity. Transgenic expression of the NK T cell-specific TCR alpha-chain Valpha14Jalpha18 leads to a complete restoration of NK T cell numbers in fyn(-/-) mice. Together, these results suggest that Fyn may have a role before alpha-chain rearrangement rather than for positive selection or the peripheral upkeep of cell number. NK T cells can activate other lymphoid lineages via cytokine secretion. These secondary responses are impaired in Fyn-deficient mice, but occur normally in fyn mutants expressing the Valpha14Jalpha18 transgene. Because this transgene restores NK T cell numbers, the lack of secondary lymphocyte activation in the fyn-mutant mice is due to the decreased numbers of NK T cells present in the mutant, rather than an intrinsic defect in the ability of the other fyn(-/-) lymphoid populations to respond.

Lineage choices in the developing thymus: choosing the T and NKT pathways.

Thymic development proceeds through several defined stages that generate not only alpha beta and gamma delta T cells but can produce dendritic cells and B cells. The earliest thymocytes exist in the CD4(-)CD8(-) double negative compartment within a heterogeneous fraction termed DN1. Recent progress has identified several candidate populations that may be the bone fide T-cell progenitor population. The potential roles of these populations, which include hematopoietic stem cells, early lymphocyte precursors, common lymphoid progenitors, and early T lineage progenitors are being elucidated. The alpha beta T-cell lineage consists of distinct subsets, one of which is NKT cells. The developmental relationship of NKT cells to conventional T cells has been controversial. Recent work has shown that these cells are probably derived from CD4(+)CD8(+) thymocytes. The discovery and application of CD1d tetramers has made it possible to more fully describe NKT-cell development.

The mer receptor tyrosine kinase: expression and function suggest a role in innate immunity.

The mer receptor tyrosine kinase mediates phagocytosis of apoptotic cells and modulates cytokine production; it is also required for prevention of systemic autoimmune disease. Using a mer-specific antibody, we have confirmed the presence of mer on macrophages and now report its expression on NK cells, NKT cells, and dendritic cells (DC). We found that DC do not require mer for ingestion of apoptotic cells, as DC from mer-deficient mice phagocytose apoptotic cells normally. Mer was observed in splenic sections on cells outside follicular areas, probably representing DC and macrophages. Mer apparently participates in NKT-cell antigen-induced signaling, as NKT cells from mer-deficient mice evinced much lower cytokine production after in vivo alpha-galactosylceramide stimulation; this defect was intrinsic to the mer-deficient NKT cells. Taken together, these studies show mer expression on cells of the innate immune system. Mer, through its binding of lipid antigens, may not only mediate ingestion of apoptotic cells, but also signal events in NK cells, NKT cells, and DC.

NK T cell precursors exhibit differential cytokine regulation and require Itk for efficient maturation.

NK T cells are a lymphocyte lineage that is selected by CD1d and is characterized by the ability to rapidly secrete large amounts of both IFN-gamma and IL-4 after TCR stimulation. Using reactivity to CD1d tetramers to define presumptive NK T cells, several NK T cell progenitor populations were characterized based upon NK marker expression and CD4 vs CD8 expression. The earliest populations were found to be negative for NK markers and could proliferate to IL-7, while mature NK T cells did not. The NK1.1(-) NK T cell progenitors were capable of up-regulating NK1.1 when transferred in vivo. Upon stimulation, the NK1.1(-) populations secrete IL-4, but little IFN-gamma. As the cells mature and up-regulate NK1.1, they acquire the ability to secrete IFN-gamma. Finally, the Tec family tyrosine kinase Itk is necessary for optimal NK1.1 up-regulation and hence final maturation of NK T cells. The itk(-/-) mice also display a progressive decrease in NK T cells in older animals, suggesting a further role in peripheral maintenance.

'Srcasm: a novel Src activating and signaling molecule.

The Src family tyrosine kinase, Fyn, can facilitate regulation of cell proliferation and differentiation. Mice with mutations in the fyn gene have defects in the brain, immune system, and epidermal differentiation. To identify molecules that may interact with Fyn in the epidermis, we performed a yeast two-hybrid interaction screen of a murine keratinocyte library. A novel adaptor-like molecule was isolated and termed Srcasm for Src activating and signaling molecule. Murine Srcasm is a 52.7-kDa protein that contains a VHS membrane association domain and a number of tyrosine motifs suggesting that it may be a substrate for Src family kinases and serve as an adaptor protein. Northern blot analysis of murine tissues demonstrates that Srcasm expression is highest in brain and kidney. In situ hybridization analysis reveals that srcasm mRNA is expressed in regions of the epidermis and hair follicle where keratinocyte differentiation occurs. In the brain, srcasm mRNA distribution correlates with that of fyn, with both being highly expressed in the hippocampal and cerebellar Purkinje neurons. Fyn can phosphorylate Srcasm, and association of these molecules relies on cooperative binding between the SH2 and SH3 domains of Fyn and corresponding canonical binding sites in Srcasm. Srcasm is capable of interacting with Grb2 and the regulatory subunit of phosphoinositide 3-kinase, p85, in a phosphorylation-dependent manner. The evidence suggests that Srcasm may help promote Src family kinase signaling in cells.