Cutting Edge: Eosinophils Undergo Caspase-1-Mediated Pyroptosis in Response to Necrotic Liver Cells.

Many chronic liver disorders are characterized by dysregulated immune responses and hepatocyte death. We used an in vivo model to study the immune response to necrotic liver injury and found that necrotic liver cells induced eosinophil recruitment. Necrotic liver induced eosinophil IL-1ß and IL-18 secretion, degranulation, and cell death. Caspase-1 inhibitors blocked all of these responses. Caspase-1-mediated cell death with accompanying cytokine release is the hallmark of a novel form of cell death termed pyroptosis. To confirm this response in a disease model, we isolated eosinophils from the livers of Schistosoma mansoni-infected mice. S. mansoni eggs lodge in the hepatic sinusoids of infected mice, resulting in hepatocyte death, inflammation, and progressive liver fibrosis. This response is typified by massive eosinophilia, and we were able to confirm pyroptosis in the infiltrating eosinophils. This demonstrated that pyroptosis is a cellular pathway used by eosinophils in response to large-scale hepatic cell death.

Differential expression of interleukin-17A and -17F is coupled to T cell receptor signaling via inducible T cell kinase.

T helper 17 (Th17) cells play major roles in autoimmunity and bacterial infections, yet how T cell receptor (TCR) signaling affects Th17 cell differentiation is relatively unknown. We demonstrate that CD4(+) T cells lacking Itk, a tyrosine kinase required for full TCR-induced phospholipase C-gamma (PLC-gamma1) activation, exhibit decreased interleukin-17A (IL-17A) expression in vitro and in vivo, despite relatively normal expression of retinoic acid receptor-related orphan receptor-gammaT (ROR-gammaT) and IL-17F. IL-17A expression was rescued by pharmacologically induced Ca(2+) influx or constitutively activated nuclear factor of activated T cells (NFAT). Conversely, decreased TCR stimulation or calcineurin inhibition preferentially reduced IL-17A expression. We further found that the promoter of Il17a but not Il17f has a conserved NFAT binding site that bound NFATc1 in wild-type but not Itk-deficient cells, even though both exhibited open chromatin conformations. Finally, Itk(-/-) mice also showed differential regulation of IL-17A and IL-17F in vivo. Our results suggest that Itk specifically couples TCR signaling to Il17a expression and the differential regulation of Th17 cell cytokines through NFATc1.

Targeting the GM-CSF receptor for the treatment of CNS autoimmunity.

In multiple sclerosis (MS), there is a growing interest in inhibiting the pro-inflammatory effects of granulocyte-macrophage colony-stimulating factor (GM-CSF). We sought to evaluate the therapeutic potential and underlying mechanisms of GM-CSF receptor alpha (Rα) blockade in animal models of MS. We show that GM-CSF signaling inhibition at peak of chronic experimental autoimmune encephalomyelitis (EAE) results in amelioration of disease progression. Similarly, GM-CSF Rα blockade in relapsing-remitting (RR)-EAE model prevented disease relapses and inhibited T cell responses specific for both the inducing and spread myelin peptides, while reducing activation of mDCs and inflammatory monocytes. In situ immunostaining of lesions from human secondary progressive MS (SPMS), but not primary progressive MS patients shows extensive recruitment of GM-CSF Rα+ myeloid cells. Collectively, this study reveals a pivotal role of GM-CSF in disease relapses and the benefit of GM-CSF Rα blockade as a potential novel therapeutic approach for treatment of RRMS and SPMS.

The TNF-family receptor DR3 is essential for diverse T cell-mediated inflammatory diseases.

DR3 (TRAMP, LARD, WSL-1, TNFRSF25) is a death-domain-containing tumor necrosis factor (TNF)-family receptor primarily expressed on T cells. TL1A, the TNF-family ligand for DR3, can costimulate T cells, but the physiological function of TL1A-DR3 interactions in immune responses is not known. Using DR3-deficient mice, we identified DR3 as the receptor responsible for TL1A-induced T cell costimulation and dendritic cells as the likely source for TL1A during T cell activation. Despite its role in costimulation, DR3 was not required for in vivo T cell priming, for polarization into T helper 1 (Th1), Th2, or Th17 effector cell subtypes, or for effective control of infection with Toxoplasma gondii. Instead, DR3 expression was required on T cells for immunopathology, local T cell accumulation, and cytokine production in Experimental Autoimmune Encephalomyelitis (EAE) and allergic lung inflammation, disease models that depend on distinct effector T cell subsets. DR3 could be an attractive therapeutic target for T cell-mediated autoimmune and allergic diseases.

Generation of pathogenic T(H)17 cells in the absence of TGF-ß signalling.

CD4(+) T-helper cells that selectively produce interleukin (IL)-17 (T(H)17), are critical for host defence and autoimmunity. Although crucial for T(H)17 cells in vivo, IL-23 has been thought to be incapable of driving initial differentiation. Rather, IL-6 and transforming growth factor (TGF)-ß1 have been proposed to be the factors responsible for initiating specification. Here we show that T(H)17 differentiation can occur in the absence of TGF-ß signalling. Neither IL-6 nor IL-23 alone efficiently generated T(H)17 cells; however, these cytokines in combination with IL-1ß effectively induced IL-17 production in naive precursors, independently of TGF-ß. Epigenetic modification of the Il17a, Il17f and Rorc promoters proceeded without TGF-ß1, allowing the generation of cells that co-expressed RORγt (encoded by Rorc) and T-bet. T-bet(+)RORγt(+) T(H)17 cells are generated in vivo during experimental allergic encephalomyelitis, and adoptively transferred T(H)17 cells generated with IL-23 without TGF-ß1 were pathogenic in this disease model. These data indicate an alternative mode for T(H)17 differentiation. Consistent with genetic data linking IL23R with autoimmunity, our findings re-emphasize the importance of IL-23 and therefore may have therapeutic implications.

Polyclonal Treg cells modulate T effector cell trafficking.

In this study, we have analyzed the in vivo dynamics of the interaction between polyclonal Foxp3(+) Treg cells, effector T (Teff) cells, and DCs in order to further our understanding of the mechanisms of Treg cell-mediated suppression. Cotransfer of polyclonal activated Treg cells into healthy mice attenuated the induction of EAE. Suppression of disease strongly correlated with a reduced number of Teff cells in the spinal cord, but not with Treg cell-mediated inhibition of Th1/Th17 differentiation. Cotransfer of Treg cells with TCR-Tg Teff cells followed by immunization by multiple routes resulted in an enhanced number of Teff cells in the lymph nodes draining the site of immunization without an inhibition of Teff-cell differentiation. Fewer Teff cells could be detected in the blood in the presence of Treg cells and fewer T cells could access a site of antigen exposure in a modified delayed-type hypersensitivity assay. Teff cells recovered from LNs in the presence of Treg cells expressed decreased levels of CXCR4, syndecan, and the sphingosine phosphate receptor, S1P1 (sphingosine 1-phosphate receptor 1). Thus, polyclonal Treg cells influence Teff-cell responses by targeting trafficking pathways, thus allowing immunity to develop in lymphoid organs, but limiting the number of potentially auto-aggressive cells that are allowed to enter the tissues.

Engagement of TLR2 does not reverse the suppressor function of mouse regulatory T cells, but promotes their survival.

TLRs are a class of conserved pattern recognition receptors that are used by cells of the innate immune system. Recent studies have demonstrated the expression of TLRs on both human and mouse T cells raising the possibility that TLRs play a direct role in adaptive immunity. TLR2 is activated primarily by bacterial wall components including peptidoglycan and lipoproteins. Several studies have shown that mouse regulatory T (Treg) cells express TLR2 and claimed that engagement of TLR2 by synthetic ligands reversed their suppressive function. In contrary, enhancement of Treg function was observed following engagement of TLR2 on human Treg. We have reexamined the expression and function of TLR2 on mouse Treg purified from Foxp3-GFP knock-in mice. TLR2 ligation by TLR2 agonist, the synthetic bacterial lipoprotein Pam3CSK4, enhanced the proliferative responses of both conventional T cells and Treg in response to TLR stimulation in the absence of APC. Treatment of Foxp3+ Treg with Pam3CSK4 did not alter their suppressive function in vitro or in vivo and did not reduce their level of Foxp3 expression. An additional effect of TLR2 stimulation of Treg was induction of Bcl-x(L) resulting in enhanced survival in vitro. Treatment of mice with the TLR2 agonist enhanced the Ag-driven proliferation of Treg in vivo, but did not abolish their ability to suppress the development of experimental autoimmune encephalomyelitis. Development of methods to selectively stimulate TLR2 on Treg may lead to a novel approaches for the treatment of autoimmune diseases.

Polyclonal Treg cells enhance the activity of a mucosal adjuvant.

The efficacy of vaccines can be greatly improved by adjuvants that enhance and modify the magnitude and the duration of the immune response. Several approaches to design rational adjuvants are based on the suppression of regulatory T-cell (Treg) function. Here, we evaluated whether removal or addition of Treg at the time of vaccination with tetanus toxoid and the mucosal adjuvant cholera toxin (CT), would affect immune responses. We found that depletion/inactivation of CD4(+)CD25(+) Treg, either by treatment of BALB/c mice with anti-CD25 monoclonal antibodies or by adoptive transfer of CD4(+)CD25(-) T lymphocytes depleted of CD4(+)CD25(+) Treg into nu/nu mice, impaired antibody production after mucosal immunization in the presence of CT. Conversely, transfer of polyclonal, but not Ag-specific, CD4(+)CD25(+)Foxp3(+) Treg to normal BALB/c mice enhanced CT-induced antibody responses. An increased titer of both immunoglobulin IgG1 and IgG2a antibody subclasses was found, however, the ratio between IgG1/IgG2a with or without polyclonal Treg was comparable, suggesting that polyclonal Treg influence the magnitude, but not the quality of the immune response. Recipients of polyclonal Treg that had been immunized with CT had an increased number of Ag-specific CD4(+) T cells with an activated phenotype (CD44(hi)) in the draining lymph nodes. This accumulation of Ag-specific CD4(+) T lymphocytes could favour the germinal centre formation and may promote T-dependent B-cell responses. Overall, our study indicates that Foxp3(+) Treg can not only function as suppressor cells but also as helper T cells, depending on the type of immune response being evaluated and the microenvironment in which the response is generated.

Th1, Th2, and Th17 effector T cell-induced autoimmune gastritis differs in pathological pattern and in susceptibility to suppression by regulatory T cells.

Th cells can be subdivided into IFN-gamma-secreting Th1, IL-4/IL-5-secreting Th2, and IL-17-secreting Th17 cells. We have evaluated the capacity of fully differentiated Th1, Th2, and Th17 cells derived from a mouse bearing a transgenic TCR specific for the gastric parietal cell antigen, H(+)K(+)-ATPase, to induce autoimmune gastritis after transfer to immunodeficient recipients. We have also determined the susceptibility of the disease induced by each of the effector T cell types to suppression by polyclonal regulatory T cells (Treg) in vivo. Each type of effector cell induced autoimmune gastritis with distinct histological patterns. Th17 cells induced the most destructive disease with cellular infiltrates composed primarily of eosinophils accompanied by high levels of serum IgE. Polyclonal Treg could suppress the capacity of Th1 cells, could moderately suppress Th2 cells, but could suppress Th17-induced disease only at early time points. The major effect of the Treg was to inhibit the expansion of the effector T cells. However, effector cells isolated from protected animals were not anergic and were fully competent to proliferate and produce effector cytokines ex vivo. The strong inhibitory effect of polyclonal Treg on the capacity of some types of differentiated effector cells to induce disease provides an experimental basis for the clinical use of polyclonal Treg in the treatment of autoimmune disease in humans.

Antisecretory factor expression is regulated by inflammatory mediators and influences the severity of experimental autoimmune encephalomyelitis.

Antisecretory factor (ASF) was originally identified as a potent inhibitor of intestinal fluid secretion induced by a number of enterotoxins. In addition to its involvement in intestinal fluid secretion, ASF modulates the proliferation of memory/effector T cells and is expressed by cells of the immune system. This report describes the role of ASF in modulating immune responses and assesses the regulation of ASF during an in vivo immunological reaction. ASF expression was redistributed during adoptively transferred experimental autoimmune encephalomyelitis (EAE), and in response to other inflammatory stimuli. Administration of the anti-ASF antibody TLD-1A8A increased the clinical severity and duration of the disease. Consistent with these findings, addition of TLD-1A8A to T cell proliferation assays resulted in up-regulation of the proinflammatory cytokines IL-18 and IL-6 and in down-regulation of IL-10. Furthermore, we identified cytokines that regulated the expression of ASF at both the mRNA and protein level. ASF, therefore, appears to play a previously unappreciated and potentially important role in the regulation of immune responses.

Down-regulation of Gfi-1 expression by TGF-beta is important for differentiation of Th17 and CD103+ inducible regulatory T cells.

Growth factor independent 1 (Gfi-1), a transcriptional repressor, is transiently induced during T cell activation. Interleukin (IL) 4 further induces Gfi-1, resulting in optimal Th2 cell expansion. We report a second important function of Gfi-1 in CD4 T cells: prevention of alternative differentiation by Th2 cells, and inhibition of differentiation of naive CD4 T cells to either Th17 or inducible regulatory T (iTreg) cells. In Gfi1(-/-) Th2 cells, the Rorc, Il23r, and Cd103 loci showed histone 3 lysine 4 trimethylation modifications that were lacking in wild-type Th2 cells, implying that Gfi-1 is critical for epigenetic regulation of Th17 and iTreg cell-related genes in Th2 cells. Enforced Gfi-1 expression inhibited IL-17 production and iTreg cell differentiation. Furthermore, a key inducer of both Th17 and iTreg cell differentiation, transforming growth factor beta, repressed Gfi-1 expression, implying a reciprocal negative regulation of CD4 T cell fate determination. Chromatin immunoprecipitation showed direct binding of the Gfi-1-lysine-specific demethylase 1 repressive complex to the intergenic region of Il17a/Il17f loci and to intron 1 of Cd103. T cell-specific Gfi1 conditional knockout mice displayed a striking delay in the onset of experimental allergic encephalitis correlated with a dramatic increase of Foxp3(+)CD103(+) CD4 T cells. Thus, Gfi-1 plays a critical role both in enhancing Th2 cell expansion and in repressing induction of Th17 and CD103(+) iTreg cells.

The critical contribution of TGF-beta to the induction of Foxp3 expression and regulatory T cell function.

Stimulation of mouse CD4+ T cells in the presence of TGF-beta results in the expression of Foxp3 and induction of Treg function. Stimulation of human CD4+ T cells under similar conditions results in the expression of Foxp3, but the cells lack regulatory cell function. TGF-beta expressed on the surface of Treg also induces Foxp3 expression and Treg function in responder cells. Both of these mechanisms may play a role in vivo in the induction of antigen-specific extra-thymic Treg.

Role of TGF-Beta in the induction of Foxp3 expression and T regulatory cell function.

INTRODUCTION: A number of studies have suggested that transforming growth factor beta (TGF-beta) plays a critical role in immune suppression mediated by Foxp3(+) regulatory T cells. TGF-beta in concert with interleukin 2 is a potent induction factor for the differentiation of Foxp3(+) Treg from naive precursors. Polyclonal TGF-beta-induced Treg (iTreg) are capable of preventing the autoimmune syndrome that develops in scurfy mice that lack Foxp3(+) Treg. Antigen-specific iTreg can be used to both prevent and treat autoimmune gastritis that is induced by transfer of naive or primed autoantigen-specific T cells. TGF-beta complexed with latency-associated peptide is expressed on the surface of activated thymus-derived Treg. Coculture of activated Treg with naive responder T cells results in the de novo generation of fully functional Foxp3(+) T cells in a contact-dependent and TGF-beta-dependent manner. CONCLUSIONS AND SPECULATIONS: Generation of functional Foxp3(+) T cells via this pathway may represent a mechanism by which Treg maintain tolerance and expand their repertoire.

Retinoic acid inhibits Th17 polarization and enhances FoxP3 expression through a Stat-3/Stat-5 independent signaling pathway.

CD4(+) helper T (Th) cells play a crucial role in the delicate balance between host defense and autoimmune disease. Two important populations of helper T cells are the proinflammatory, interleukin-17 (IL-17)-producing (Th17) cells and the anti-inflammatory forkhead box P3-positive (FoxP3(+)) T regulatory (Treg) cells. Here we show that all-trans retinoic acid (ATRA) and other agonists of the retinoic acid receptor alpha (RARalpha) inhibit the formation of Th17 cells and promote FoxP3 expression. Conversely, inhibition of retinoic acid signaling constrains transforming growth factor beta (TGF-beta1) induction of FoxP3. The effect of ATRA is mediated independently of IL-2, signal transducer and activator of transcription 5 (Stat5) and Stat3, representing a novel mechanism for the induction of FoxP3 in CD4 T cells. As previous studies have shown that vitamin A derivatives are protective in animal models of autoimmune disease, the current data suggest a previously unrecognized role for RARalpha in the regulation of CD4(+) T-cell differentiation and provide a mechanism for the anti-inflammatory effects of retinoic acid.

CD4+ FoxP3+ regulatory T cells confer infectious tolerance in a TGF-beta-dependent manner.

CD4(+)FoxP3(+) regulatory T (T reg) cells comprise a separate lineage of T cells that are essential for maintaining immunological tolerance to self. The molecular mechanism(s) by which T reg cells mediate their suppressive effects remains poorly understood. One molecule that has been extensively studied in T reg cell suppression is transforming growth factor (TGF)-beta, but its importance remains controversial. We found that TGF-beta complexed to latency-associated peptide (LAP) is expressed on the cell surface of activated but not resting T reg cells. T reg cell LAP-TGF-beta plays an important role in the suppression of the proliferation of activated T cells, but it is not required for the suppression of naive T cell activation. More importantly, T reg cell-derived TGF-beta could generate de novo CD4(+)FoxP3(+) T cells in vitro from naive precursors in a cell contact-dependent, antigen-presenting cell-independent and alpha(V) integrin-independent manner. The newly induced CD4(+)FoxP3(+) T cells are suppressive both in vitro and in vivo. Transfer of activated antigen-specific T reg cells with naive antigen-specific responder T cells to normal recipients, followed by immunization, also results in induction of FoxP3 expression in the responder cells. T reg cell-mediated generation of functional CD4(+)FoxP3(+) cells via this TGF-beta-dependent pathway may represent a major mechanism as to how T reg cells maintain tolerance and expand their suppressive abilities.

Autoantigen-specific TGFbeta-induced Foxp3+ regulatory T cells prevent autoimmunity by inhibiting dendritic cells from activating autoreactive T cells.

Several strategies are being designed to test the therapeutic potential of Ag-specific regulatory T cells to prevent or treat autoimmune diseases. In this study, we demonstrate that naive CD4+ Foxp3- T cells specific for a naturally expressed autoantigen (H+/K+ ATPase) can be converted to Foxp3+ T regulatory cells (Tregs) when stimulated in presence of TGFbeta. TGFbeta-induced Tregs (iTregs) have all the characteristics of naturally generated regulatory T cells in vitro, and more importantly, are effective at preventing organ-specific autoimmunity in a murine model of autoimmune gastritis. H+/K+ ATPase specific iTregs were able to inhibit the initial priming and proliferation of autoreactive T cells, and appear to do so by acting on H+/K+ ATPase presenting dendritic cells (DC). DC exposed to iTregs in vivo were reduced in their ability to stimulate proliferation and cytokine production by H+/K+ ATPase specific T cells. iTregs specifically reduced CD80 and CD86 expression on the surface of H+/K+ ATPase presenting DC in vitro. These studies reveal the therapeutic potential of Ag specific iTregs to prevent autoimmunity, and provide a mechanism by which this population of regulatory T cells, and perhaps others, mediate their suppressive effects in vivo.

Cutting Edge: IL-2 is essential for TGF-beta-mediated induction of Foxp3+ T regulatory cells.

TGF-beta is a pluripotent cytokine that is capable of inducing the expression of Foxp3 in naive T lymphocytes. TGF-beta-induced cells are phenotypically similar to thymic-derived regulatory T cells in that they are anergic and suppressive. We have examined the cytokine and costimulatory molecule requirements for TGF-beta-mediated induction and maintenance of Foxp3 by CD4(+)Foxp3(-) cells. IL-2 plays a non-redundant role in TGF-beta-induced Foxp3 expression. Other common gamma-chain-utilizing cytokines were unable to induce Foxp3 expression in IL-2-deficient T cells. The role of CD28 in the induction of Foxp3 was solely related to its capacity to enhance the endogenous production of IL-2. Foxp3 expression was stable in vitro and in vivo in the absence of IL-2. As TGF-beta-induced T regulatory cells can be easily grown in vitro, they may prove useful for the treatment of autoimmune diseases, for the prevention of graft rejection, and graft versus host disease.

Interleukin-2 signaling via STAT5 constrains T helper 17 cell generation.

Recent work has identified a new subset of effector T cells that produces interleukin (IL)-17 known as T helper 17 (Th17) cells, which is involved in the pathophysiology of inflammatory diseases and is thought to be developmentally related to regulatory T (Treg) cells. Because of its importance for Treg cells, we examined the role of IL-2 in Th17 generation and demonstrate that a previously unrecognized aspect of IL-2 function is to constrain IL-17 production. Genetic deletion or antibody blockade of IL-2 promoted differentiation of the Th17 cell subset. Whereas STAT3 appeared to be a key positive regulator of RORgammat and IL-17 expression, absence of IL-2 or disruption of its signaling by deletion of the transcription factor STAT5 resulted in enhanced Th17 cell development. We conclude that in addition to the promotion of activation-induced cell death of lymphocytes and the generation of Treg cells, inhibition of Th17 polarization appears to be an important function of IL-2.

An experimental model of autoimmune pancreatitis in the rat.

Autoimmune pancreatitis (AIP), a recently defined disease of unknown etiology, is characterized by inflammatory infiltrates in the pancreas with conspicuous involvement of the ducts. The disease clinically manifests in humans as epigastric pain, weight loss, and jaundice. This report describes the development of a novel animal model of this disease in the rat, which we have termed experimental autoimmune pancreatitis. Adoptive transfer of amylase-specific CD4(+) T cells was able to confer pancreatitis to naive syngeneic recipient animals. No treatments before the adoptive transfer of T cells were necessary for disease to ensue, and the severity of disease was proportional to the number of T cells administered. The pancreatic lesions of rats with experimental autoimmune pancreatitis were characterized histologically as overwhelmingly lymphocytic with occasional plasma cells, neutrophils, and mast cells. Acinar tissue destruction and ductular inflammation were common features, with less frequent involvement of larger ducts. Immunohistochemical analysis revealed the presence of CD4(+) T cells in large numbers as well as CD8(+) T cells, macrophages, and dendritic cells. Expression of MHC I and MHC II also increased at the site of the lesion. Clinically, the disease manifested as either failure to gain weight at a rate concomitant with control animals or as outright weight loss. Thus, administration of activated CD4(+) T cells specific for the pancreatic enzyme amylase can induce pancreatitis in the rat in a manner that is reminiscent of human AIP.

Distribution and immunoregulatory properties of antisecretory factor.

Inflammatory processes and the mechanisms by which they are initiated and controlled within the central nervous system (CNS) involve a vast array of cell types and molecules. One cell type believed to be involved in the control of inflammation in the CNS is the microglial cell. The TLD antibodies are a panel of monoclonal antibodies reactive to rat microglial antigens. One antibody from this panel, clone TLD-1A8A, is specific for antisecretory factor (ASF). ASF is a previously identified protein characterized as a potent inhibitor of enterotoxin-induced intestinal fluid secretion. Our results extend the function of this molecule to include the regulation of immune reactions. Administration of TLD-1A8A to T-cell proliferation or mixed leukocyte response assays resulted in increased proliferation of T cells. Flow cytometric analysis indicates that ASF is expressed on macrophages, B cells and dendritic cells, but not on T cells or granulocytes. Immunohistochemical analysis indicates that ASF is expressed by macrophages and cells of dendritic morphology in the spleen, thymus, lymph nodes, Peyer's patch, and in the perivascular area in the CNS. Furthermore, Western blot analysis indicates that ASF is expressed in many tissues including all secondary lymphoid organs. These data suggest that ASF may have a previously unsuspected role in regulating the immune system.