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An important comorbidity of chronic inflammation is anemia, which may be related to dysregulated activity of hematopoietic stem and progenitor cells (HSPCs) in the bone marrow (BM). Among HSPCs, we found that the receptor for IL-33, ST2, is expressed preferentially and highly on erythroid progenitors. Induction of inflammatory spondyloarthritis in mice increased IL-33 in BM plasma, and IL-33 was required for inflammation-dependent suppression of erythropoiesis in BM. Conversely, administration of IL-33 in healthy mice suppressed erythropoiesis, decreased hemoglobin expression, and caused anemia. Using purified erythroid progenitors in vitro, we show that IL-33 directly inhibited terminal maturation. This effect was dependent on NF-κB activation and associated with altered signaling events downstream of the erythropoietin receptor. Accordingly, IL-33 also suppressed erythropoietin-accelerated erythropoiesis in vivo. These results reveal a role for IL-33 in pathogenesis of anemia during inflammatory disease and define a new target for its treatment., Competing Interests: Disclosures: F. Powrie reported personal fees from GSK, grants from Janssen, personal fees from Genentech, and personal fees from Kintai Therapeutics outside the submitted work. No other disclosures were reported., (© 2020 Swann et al.)

Dysregulated hematopoiesis occurs in several chronic inflammatory diseases, but it remains unclear how hematopoietic stem cells (HSCs) in the bone marrow (BM) sense peripheral inflammation and contribute to tissue damage in arthritis. Here, we show the HSC gene expression program is biased toward myelopoiesis and differentiation skewed toward granulocyte-monocyte progenitors (GMP) during joint and intestinal inflammation in experimental spondyloarthritis (SpA). GM-CSF-receptor is increased on HSCs and multipotent progenitors, favoring a striking increase in myelopoiesis at the earliest hematopoietic stages. GMP accumulate in the BM in SpA and, unexpectedly, at extramedullary sites: in the inflamed joints and spleen. Furthermore, we show that GM-CSF promotes extramedullary myelopoiesis, tissue-toxic neutrophil accumulation in target organs, and GM-CSF prophylactic or therapeutic blockade substantially decreases SpA severity. Surprisingly, besides CD4 + T cells and innate lymphoid cells, mast cells are a source of GM-CSF in this model, and its pathogenic production is promoted by the alarmin IL-33.

Innate lymphoid cells (ILCs) contribute to host defence and tissue repair but can induce immunopathology. Recent work has revealed tissue-specific roles for ILCs; however, the question of how a small population has large effects on immune homeostasis remains unclear. We identify two mechanisms that ILC3s utilise to exert their effects within intestinal tissue. ILC-driven colitis depends on production of granulocyte macrophage-colony stimulating factor (GM-CSF), which recruits and maintains intestinal inflammatory monocytes. ILCs present in the intestine also enter and exit cryptopatches in a highly dynamic process. During colitis, ILC3s mobilize from cryptopatches, a process that can be inhibited by blocking GM-CSF, and mobilization precedes inflammatory foci elsewhere in the tissue. Together these data identify the IL-23R/GM-CSF axis within ILC3 as a key control point in the accumulation of innate effector cells in the intestine and in the spatio-temporal dynamics of ILCs in the intestinal inflammatory response.

The role of intestinal eosinophils in immune homeostasis is enigmatic and the molecular signals that drive them from protective to tissue damaging are unknown. Most commonly associated with Th2 cell-mediated diseases, we describe a role for eosinophils as crucial effectors of the interleukin-23 (IL-23)-granulocyte macrophage colony-stimulating factor (GM-CSF) axis in colitis. Chronic intestinal inflammation was characterized by increased bone marrow eosinopoiesis and accumulation of activated intestinal eosinophils. IL-5 blockade or eosinophil depletion ameliorated colitis, implicating eosinophils in disease pathogenesis. GM-CSF was a potent activator of eosinophil effector functions and intestinal accumulation, and GM-CSF blockade inhibited chronic colitis. By contrast neutrophil accumulation was GM-CSF independent and dispensable for colitis. In addition to TNF secretion, release of eosinophil peroxidase promoted colitis identifying direct tissue-toxic mechanisms. Thus, eosinophils are key perpetrators of chronic inflammation and tissue damage in IL-23-mediated immune diseases and it suggests the GM-CSF-eosinophil axis as an attractive therapeutic target., (Copyright © 2015 The Authors. Published by Elsevier Inc. All rights reserved.)

FOXP3(+) regulatory T cells (Treg cells) are abundant in the intestine, where they prevent dysregulated inflammatory responses to self and environmental stimuli. It is now appreciated that Treg cells acquire tissue-specific adaptations that facilitate their survival and function; however, key host factors controlling the Treg response in the intestine are poorly understood. The interleukin (IL)-1 family member IL-33 is constitutively expressed in epithelial cells at barrier sites, where it functions as an endogenous danger signal, or alarmin, in response to tissue damage. Recent studies in humans have described high levels of IL-33 in inflamed lesions of inflammatory bowel disease patients, suggesting a role for this cytokine in disease pathogenesis. In the intestine, both protective and pathological roles for IL-33 have been described in murine models of acute colitis, but its contribution to chronic inflammation remains ill defined. Here we show in mice that the IL-33 receptor ST2 is preferentially expressed on colonic Treg cells, where it promotes Treg function and adaptation to the inflammatory environment. IL-33 signalling in T cells stimulates Treg responses in several ways. First, it enhances transforming growth factor (TGF)-β1-mediated differentiation of Treg cells and, second, it provides a necessary signal for Treg-cell accumulation and maintenance in inflamed tissues. Strikingly, IL-23, a key pro-inflammatory cytokine in the pathogenesis of inflammatory bowel disease, restrained Treg responses through inhibition of IL-33 responsiveness. These results demonstrate a hitherto unrecognized link between an endogenous mediator of tissue damage and a major anti-inflammatory pathway, and suggest that the balance between IL-33 and IL-23 may be a key controller of intestinal immune responses.

The immune response in the gastrointestinal tract is a tightly controlled balance between effector and regulatory cell responses. Here, we have investigated the role of OX40 in influencing the balance between conventional T cells and Foxp3+ regulatory T (T reg) cells. Under steady-state conditions, OX40 was required by T reg cells for their accumulation in the colon, but not peripheral lymphoid organs. Strikingly, under inflammatory conditions OX40 played an essential role in T reg cell-mediated suppression of colitis. OX40(-/-) T reg cells showed reduced accumulation in the colon and peripheral lymphoid organs, resulting in their inability to keep pace with the effector response. In the absence of OX40 signaling, T reg cells underwent enhanced activation-induced cell death, indicating that OX40 delivers an important survival signal to T reg cells after activation. As OX40 also promoted the colitogenic Th1 response, its expression on T reg cells may be required for effective competition with OX40-dependent effector responses. These results newly identify a key role for OX40 in the homeostasis of intestinal Foxp3+ T reg cells and in suppression of colitis. These fi ndings should be taken into account when considering OX40 blockade for treatment of IBD.

Invariant natural killer T (iNKT) cells promote immune responses to various pathogens, but exactly how iNKT cells control antiviral responses is unclear. Here, we showed that iNKT cells induced tissue-specific antiviral effects in mice infected by lymphocytic choriomeningitis virus (LCMV). Indeed, iNKT cells inhibited viral replication in the pancreas and liver but not in the spleen. In the pancreas, iNKT cells expressed the OX40 molecule and promoted type I interferon (IFN) production by plasmacytoid dendritic cells (pDCs) through OX40-OX40 ligand interaction. Subsequently, this iNKT cell-pDC cooperation attenuated the antiviral adaptive immune response in the pancreas but not in the spleen. The dampening of pancreatic anti-LCMV CD8(+) T cell response prevented tissue damage in transgenic mice expressing LCMV protein in islet beta cells. Thus, this study identifies pDCs as an essential partner of iNKT cells for mounting an efficient, nondeleterious antiviral response in peripheral tissue.

Invariant NKT (iNKT) cells can prevent diabetes by inhibiting the differentiation of anti-islet T cells. We recently showed that neither iNKT cell protection against diabetes nor iNKT cell inhibition of T cell differentiation in vitro requires cytokines such as IL-4, IL-10, IL-13, and TGF-beta. In contrast, cell-cell contacts were required for iNKT cell inhibition of T cell differentiation in vitro. The present study was designed to determine whether the CD1d molecule is involved in the inhibitory function of iNKT cells. Experiments were performed in vitro and in vivo, using cells lacking CD1d expression. The in vivo experiments used CD1d-deficient mice that were either reconstituted with iNKT cells or expressed a CD1d transgene exclusively in the thymus. Both mouse models had functional iNKT cells in the periphery, even though CD1d was not expressed in peripheral tissues. Surprisingly, both in vitro inhibition of T cell differentiation by iNKT cells and mouse protection against diabetes by iNKT cells were CD1d-independent. These results reveal that iNKT cells can exert critical immunoregulatory effects in the absence of CD1d recognition and that different molecular interactions are involved in iNKT cell functions.

Type 1 diabetes is an autoimmune disease due to the destruction of insulin-producing pancreatic beta cells. Natural Killer T (NKT) cells are a T-cell subset that links the innate and adaptive immune systems. NKT cells play a key regulatory role in type 1 diabetes. The absence of NKT cells correlates with exacerbation of type 1 diabetes, whereas an increased frequency and/or activation of NKT cells prevents beta-cell autoimmunity. Various mechanisms are involved in the protective effect of NKT cells. The goal is now to translate knowledge gained from mouse models into human therapeutics.

Invariant NKT (iNKT) cells have been implicated in the regulation of autoimmune diseases. In several models of type 1 diabetes, increasing the number of iNKT cells prevents the development of disease. Because CD8 T cells play a crucial role in the pathogenesis of diabetes, we have investigated the influence of iNKT cells on diabetogenic CD8 T cells. In the present study, type 1 diabetes was induced by the transfer of CD8 T cells specific for the influenza virus hemagglutinin into recipient mice expressing the hemagglutinin Ag specifically in their beta pancreatic cells. In contrast to previous reports, high frequency of iNKT cells promoted severe insulitis and exacerbated diabetes. Analysis of diabetogenic CD8 T cells showed that iNKT cells enhance their activation, their expansion, and their differentiation into effector cells producing IFN-gamma. This first analysis of the influence of iNKT cells on diabetogenic CD8 T cells reveals that iNKT cells not only fail to regulate but in fact exacerbate the development of diabetes. Thus, iNKT cells can induce opposing effects dependent on the model of type 1 diabetes that is being studied. This prodiabetogenic capacity of iNKT cells should be taken into consideration when developing therapeutic approaches based on iNKT cell manipulation.

NKT cells are potent regulatory T cells that prevent the development of several autoimmune diseases. Analysis of NKT cell regulatory function in the NOD mouse has revealed that NKT cells inhibit the development of type 1 diabetes by impairing the differentiation of anti-islet T cells into Th1 effector cells. In the present study, we have performed in vitro and in vivo experiments to determine the respective role of cytokines and cell contacts in the blockade of T cell differentiation by NKT cells. These experiments reveal that cytokines such as IL-4, IL-10, IL-13, and TGF-beta, that have been involved in other functions of NKT cells, play only a minor role if any in the blockade of T cell differentiation by NKT cells. Diabetes is still prevented by NKT cells in the absence of functional IL-4, IL-10, IL-13, and TGF-beta. In contrast, we show for the first time that cell contacts are crucial for the immunoregulatory function of NKT cells.