Margaret M. Mentink-Kane

Arginase-1–Expressing Macrophages Suppress Th2 Cytokine–Driven Inflammation and Fibrosis

Macrophage-specific expression of Arginase-1 is commonly believed to promote inflammation, fibrosis, and wound healing by enhancing L-proline, polyamine, and Th2 cytokine production. Here, however, we show that macrophage-specific Arg1 functions as an inhibitor of inflammation and fibrosis following infection with the Th2-inducing pathogen Schistosoma mansoni. Although susceptibility to infection was not affected by the conditional deletion of Arg1 in macrophages, Arg1 −/flox ;LysMcre mice died at an accelerated rate. The mortality was not due to acute Th1/NOS2-mediated hepatotoxicity or endotoxemia. Instead, granulomatous inflammation, liver fibrosis, and portal hypertension increased in infected Arg1 −/flox ;LysMcre mice. Similar findings were obtained with Arg1 flox/flox ;Tie2cre mice, which delete Arg1 in all macrophage populations. Production of Th2 cytokines increased in the infected Arg1 −/flox ;LysMcre mice, and unlike alternatively activated wild-type macrophages, Arg1 −/flox ;LysMcre macrophages failed to inhibit T cell proliferation in vitro, providing an underlying mechanism for the exacerbated Th2 pathology. The suppressive activity of Arg1-expressing macrophages was independent of IL-10 and TGF-β1. However, when exogenous L-arginine was provided, T cell proliferation was restored, suggesting that Arg1-expressing macrophages deplete arginine, which is required to sustain CD4+ T cell responses. These data identify Arg1 as the essential suppressive mediator of alternatively activated macrophages (AAM) and demonstrate that Arg1-expressing macrophages function as suppressors rather than inducers of Th2-dependent inflammation and fibrosis.

The Pathogenesis of Schistosomiasis Is Controlled by Cooperating IL-10-Producing Innate Effector and Regulatory T Cells

IL-10 reduces immunopathology in many persistent infections, yet the contribution of IL-10 from distinct cellular sources remains poorly defined. We generated IL-10/recombination-activating gene (RAG)2-deficient mice and dissected the role of T cell- and non-T cell-derived IL-10 in schistosomiasis by performing adoptive transfers. In this study, we show that IL-10 is generated by both the innate and adaptive immune response following infection, with both sources regulating the development of type-2 immunity, immune-mediated pathology, and survival of the infected host. Importantly, most of the CD4+ T cell-produced IL-10 was confined to a subset of T cells expressing CD25. These cells were isolated from egg-induced granulomas and exhibited potent suppressive activity in vitro. Nevertheless, when naive, naturally occurring CD4+CD25+ cells were depleted in adoptive transfers, recipient IL-10/RAG2-deficient animals were more susceptible than RAG2-deficient mice, confirming an additional host-protective role for non-T cell-derived IL-10. Thus, innate effectors and regulatory T cells producing IL-10 cooperate to reduce morbidity and prolong survival in schistosomiasis.

Immunopathology of schistosomiasis.

Waterborne parasitic diseases plague tropical regions of the world with the development of water resources often increasing transmission. Skin‐penetrating cercariae (infectious stages of schistosome parasites) mature within their mammalian host, form sexual pairs and produce several hundred eggs per day. Many eggs are swept within the circulation and in the case of Schistosoma mansoni and S. japonicum, become lodged within hepatic sinusoids, invoking a fibrotic granulomatous response. Animal studies have identified a moderate type 1 helper (Th1) response to parasite antigens; however, a robust Th2 response to egg‐derived antigens dominates and propagates fibrogenesis within the liver. Elegant T helper cell polarization studies have highlighted that critical control of Th1, Th2 and interleukin (IL)‐17‐secreting lymphocytes is necessary to prevent severe liver pathology. Alternatively activated macrophages develop in the Th2 milieu and upregulate Fizz1, Ym‐1 and Arg‐1. The possible contribution of macrophages to fibrogenesis and their role in immune regulation are discussed. Within the liver, natural (CD4+CD25+ Forkhead box protein 3 (Foxp3)+) and inducible (CD4+Foxp3–) Tregs are recruited, providing an essential regulatory arm to stabilize the immune response and limit immunopathology. This review ties together current thinking of how the granulomatous response develops, causing much of the associated immunopathology, with extensive discussions on how regulatory cells and cytokine decoy receptors serve to limit the extent of immune‐mediated pathology during schistosomiasis.

Immunopathogenesis of schistosomiasis

Summary:  In schistosomiasis mansoni, the chronic egg‐induced granulomatous response in the liver and intestines may eventually cause extensive tissue scarring and development of portal hypertension. Indeed, much of the morbidity and mortality associated with this disease is directly attributable to the deposition of connective tissue elements in affected tissues. Elucidating the mechanisms that regulate the severity of schistosomiasis has been a major research objective over the past several years. Research conducted with DNA microarrays as well as investigations with a variety of gene knock‐out mice have been particularly helpful in achieving this goal. A notable accomplishment in the past few years was the identification of interleukin‐13 (IL‐13) and the IL‐13 receptor complex as central regulators of disease progression in schistosomiasis. Liver fibrogenesis is severely decreased in infected IL‐13‐deficient mice as well as in wildtype animals treated with IL‐13 antagonists. In contrast, IL‐13 effector function increases dramatically in IL‐13 receptor α2 (IL‐13Rα2)‐deficient mice. These mice develop severe hepatic fibrosis, fail to downregulate granuloma formation in the chronic phase of S. mansoni infection, and succumb to the disease at an accelerated rate; thus, identifying the ‘decoy’ IL‐13 receptor as a critical life sustaining ‘off’ switch for tissue damaging egg‐induced inflammation.

Regulation and Function of the Interleukin 13 Receptor α 2 During a T Helper Cell Type 2–dominant Immune Response

Highly polarized type 2 cytokine responses can be harmful and even lethal to the host if they are too vigorous or persist too long. Therefore, it is important to elucidate the mechanisms that down-regulate these reactions. Interleukin (IL)-13 has emerged as a central mediator of T helper cell (Th)2-dominant immune responses, exhibiting a diverse array of functional activities including regulation of airway hyperreactivity, resistance to nematode parasites, and tissue remodeling and fibrosis. Here, we show that IL-13 receptor (R)α2 is a critical down-regulatory factor of IL-13–mediated tissue fibrosis induced by the parasitic helminth Schistosoma mansoni. IL-13Rα2 expression was induced after the onset of the fibrotic response, IL-10, IL-13, and Stat6 dependent, and inhibited by the Th1-inducing adjuvant IL-12. Strikingly, schistosome-infected C57BL/6 and BALB/c IL-13Rα2–deficient mice showed a marked exacerbation in hepatic fibrosis, despite displaying no change in granuloma size, tissue eosinophilia, or mastocytosis. Fibrosis increased despite the fact that IL-13 levels decreased significantly in the liver and serum. Importantly, pathology was prevented when IL-13Rα2–deficient mice were treated with a soluble IL-13Rα2-Fc construct, formally demonstrating that their exacerbated fibrotic response was due to heightened IL-13 activity. Together, these studies illustrate the central role played by the IL-13Rα2 in the down-regulation of a chronic and pathogenic Th2-mediated immune response.

Global gene expression profiles during acute pathogen-induced pulmonary inflammation reveal divergent roles for Th1 and Th2 responses in tissue repair

T helper 1 responses are typically proinflammatory, while Th2 responses have been considered regulatory. Interestingly, Th2 responses characterize a number of pulmonary diseases, many of which terminate in tissue remodeling and fibrosis. We developed a mouse model using Schistosoma mansoni eggs and cytokine-deficient mice to induce highly polarized Th1- or Th2-type inflammation in the lung. In this study, we examined the pathology and cytokine profiles in Th1- and Th2-polarized environments and used oligonucleotide microarray analysis to decipher the genes responsible for these effects. We further elaborated on the results using IL-10- and IL-13-deficient mice because these cytokines are believed to be the central regulators of Th2-associated pathology. We found that the Th1-polarized mice developed small granulomas with less fibrosis while expressing genes characteristic of tissue damage. Th2-polarized mice, in contrast, formed large granulomas with massive collagen deposition and up-regulated genes associated with wound healing, specifically, arginase, collagens, matrix metalloproteinases (MMPs), and tissue inhibitors of MMP. In addition, several members of the chitinase-like family were up-regulated in the lung following egg challenge. We also developed a method of defining the net collagen deposition using the expression profiles of several collagen, MMP, and tissue inhibitors of MMP genes. We found that Th1-polarized mice did not elaborate collagens or MMPs and therefore did not have a significant capacity for repair in this model. Thus, Th1-mediated inflammation is characterized by tissue damage, while Th2 directs wound healing and fibrosis.

Retnla (Relmα/Fizz1) Suppresses Helminth-Induced Th2-Type Immunity

Retnla (Resistin-like molecule alpha/FIZZ1) is induced during Th2 cytokine immune responses. However, the role of Retnla in Th2-type immunity is unknown. Here, using Retnla−/− mice and three distinct helminth models, we show that Retnla functions as a negative regulator of Th2 responses. Pulmonary granuloma formation induced by the eggs of the helminth parasite Schistosoma mansoni is dependent on IL-4 and IL-13 and associated with marked increases in Retnla expression. We found that both primary and secondary pulmonary granuloma formation were exacerbated in the absence of Retlna. The number of granuloma-associated eosinophils and serum IgE titers were also enhanced. Moreover, when chronically infected with S. mansoni cercariae, Retnla−/− mice displayed significant increases in granulomatous inflammation in the liver and the development of fibrosis and progression to hepatosplenic disease was markedly augmented. Finally, Retnla−/− mice infected with the gastrointestinal (GI) parasite Nippostrongylus brasiliensis had intensified lung pathology to migrating larvae, reduced fecundity, and accelerated expulsion of adult worms from the intestine, suggesting Th2 immunity was enhanced. When their immune responses were compared, helminth infected Retnla−/− mice developed stronger Th2 responses, which could be reversed by exogenous rRelmα treatment. Studies with several cytokine knockout mice showed that expression of Retnla was dependent on IL-4 and IL-13 and inhibited by IFN-γ, while tissue localization and cell isolation experiments indicated that eosinophils and epithelial cells were the primary producers of Retnla in the liver and lung, respectively. Thus, the Th2-inducible gene Retnla suppresses resistance to GI nematode infection, pulmonary granulomatous inflammation, and fibrosis by negatively regulating Th2-dependent responses.

Unique functions of the type II interleukin 4 receptor identified in mice lacking the interleukin 13 receptor α1 chain

The interleukin 4 receptor (IL-4R) is a central mediator of T helper type 2 (TH2)–mediated disease and associates with either the common γ-chain to form the type I IL-4R or with the IL-13R α1 chain (IL-13Rα1) to form the type II IL-4R. Here we used Il13ra1−/− mice to characterize the distinct functions of type I and type II IL-4 receptors in vivo. In contrast to Il4ra−/− mice, which have weak TH2 responses, Il13ra1−/− mice had exacerbated TH2 responses. Il13ra1−/− mice showed much less mortality after infection with Schistosoma mansoni and much more susceptibility to Nippostrongylus brasiliensis. IL-13Rα1 was essential for allergen-induced airway hyperreactivity and mucus hypersecretion but not for fibroblast or alternative macrophage activation. Thus, type I and II IL-4 receptors exert distinct effects on immune responses.

Opposing roles for IL-13 and IL-13 receptor α2 in health and disease

Summary:  Interleukin (IL)‐13 is a key inducer of several type‐2 cytokine‐dependent pathologies. It regulates inflammation, mucus production, tissue remodeling, and fibrosis. Consequently, it has become an important therapeutic target for a number of debilitating illnesses, including asthma, idiopathic pulmonary fibrosis, ulcerative colitis, as well as several other diseases in which IL‐13 is believed to be overproduced. In the murine model of schistosomiasis, IL‐13 has emerged as a central mediator of chronic infection‐induced liver pathology. Although IL‐4, IL‐5, IL‐10, and IL‐13 each regulate distinct aspects of the granulomatous inflammatory response, IL‐13 was identified as the primary mediator of liver fibrosis. Thus, elucidating the mechanisms that regulate the production and function of IL‐13 has become an intensive area of research. IL‐13 signaling is mediated by the type‐2 IL‐4 receptor, which consists of the IL‐4Rα and IL‐13Rα1 chains. However, another IL‐13‐binding chain, IL‐13Rα2, appears to strongly inhibit the activity of IL‐13. Animals deficient in IL‐13Rα2 fail to downmodulate granuloma formation in the chronic phase of infection. They also develop severe IL‐13‐dependent fibrosis and portal hypertension and quickly succumb to the infection. Here, we summarize findings from the schistosomiasis model, which illustrate opposing activities for IL‐13 and IL‐13Rα2 in health and disease.

The TNF-family cytokine TL1A drives IL-13-dependent small intestinal inflammation

The tumor necrosis factor (TNF)-family cytokine TL1A (TNFSF15) costimulates T cells through its receptor DR3 (TNFRSF25) and is required for autoimmune pathology driven by diverse T-cell subsets. TL1A has been linked to human inflammatory bowel disease (IBD), but its pathogenic role is not known. We generated transgenic mice that constitutively express TL1A in T cells or dendritic cells. These mice spontaneously develop IL-13-dependent inflammatory small bowel pathology that strikingly resembles the intestinal response to nematode infections. These changes were dependent on the presence of a polyclonal T-cell receptor (TCR) repertoire, suggesting that they are driven by components in the intestinal flora. Forkhead box P3 (FoxP3)-positive regulatory T cells (Tregs) were present in increased numbers despite the fact that TL1A suppresses the generation of inducible Tregs. Finally, blocking TL1A–DR3 interactions abrogates 2,4,6 trinitrobenzenesulfonic acid (TNBS) colitis, indicating that these interactions influence other causes of intestinal inflammation as well. These results establish a novel link between TL1A and interleukin 13 (IL-13) responses that results in small intestinal inflammation, and also establish that TL1A–DR3 interactions are necessary and sufficient for T cell-dependent IBD.