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Through immune memory, infections have a lasting effect on the host. While memory cells enable accelerated and enhanced responses upon re-challenge with the same pathogen, their impact on susceptibility to unrelated diseases is unclear. We identify a subset of memory T helper 1 (Th1) cells termed innate acting memory T (TIA) cells that originate from a viral infection and produce IFN-γ with innate kinetics upon heterologous challengein vivo. Activation of memory TIAcells is induced in response to IL-12 in combination with IL-18 or IL-33 but is TCR-independent. Rapid IFN-γ production by memory TIAcells is protective in subsequent heterologous challenge with the bacterial pathogenLegionella pneumophila. In contrast, antigen-independent re-activation of CD4+memory TIAcells accelerates disease onset in an autoimmune model of multiple sclerosis. Our findings demonstrate that memory Th1 cells can acquire additional TCR-independent functionality to mount rapid, innate-like responses that modulate susceptibility to heterologous challenges.

T helper cells integrate signals from their microenvironment to acquire distinct specialization programs for efficient clearance of diverse pathogens or for immunotolerance. Ionic signals have recently been demonstrated to affect T cell polarization and function. Sodium chloride (NaCl) was proposed to accumulate in peripheral tissues upon dietary intake and to promote autoimmunity via the Th17 cell axis. Here we demonstrate that high NaCl conditions induced a stable, pathogen-specific, anti-inflammatory Th17 cell fate in human T cells in vitro. The p38/MAPK pathway, involving NFAT5 and SGK1, regulated FoxP3 and interleukin (IL)-17A-expression in high-NaCl conditions. The NaCl-induced acquisition of an anti-inflammatory Th17 cell fate was confirmed in vivo in an experimental autoimmune encephalomyelitis (EAE) mouse model, which demonstrated strongly reduced disease symptoms upon transfer of T cells polarized in high NaCl conditions. However, NaCl was coopted to promote murine and human Th17 cell pathogenicity, if T cell stimulation occurred in a pro-inflammatory and TGF-β-low cytokine microenvironment. Taken together, our findings reveal a context-dependent, dichotomous role for NaCl in shaping Th17 cell pathogenicity. NaCl might therefore prove beneficial for the treatment of chronic inflammatory diseases in combination with cytokine-blocking drugs.

Neuroinflammation after stroke is characterized by the activation of resident microglia and the invasion of circulating leukocytes into the brain. Although lymphocytes infiltrate the brain in small number, they have been consistently demonstrated to be the most potent leukocyte subpopulation contributing to secondary inflammatory brain injury. However, the exact mechanism how this minimal number of lymphocytes can profoundly affect stroke outcome is still largely elusive. Here, using a mouse model for ischemic stroke, we demonstrated that early activation of microglia in response to stroke is differentially regulated by distinct T cell subpopulations. Acute treatment with engineered T cells overexpressing IL-10 administered into the cisterna magna after stroke induces a switch of microglial gene expression to a profile associated with pro-regenerative functions. These findings substantiate the role of T cells in stroke with large impact on the cerebral inflammatory milieu by polarizing the microglial phenotype. Targeting T cell-microglia interactions can have direct translational relevance for further development of immune-targeted therapies for stroke and other neuroinflammatory conditions.SummaryThe crosstalk between brain infiltrating T cells and microglia in response to stroke remains elusive. Benakis et al. report that transcriptional signature of the stroke-associated microglia is reprogrammed by distinct T cell subpopulations. Engineered T cells overexpressing IL-10 administered four hours after stroke reinitiate microglial function inducing a pro-regenerative environment.

The key role of B cells in the pathophysiology of multiple sclerosis (MS) is supported by the presence of oligoclonal bands in the cerebrospinal fluid, by the association of meningeal ectopic B cell follicles with demyelination, axonal loss and reduction of astrocytes, as well as by the high efficacy of B lymphocyte depletion in controlling inflammatory parameters of MS. Here, we use a spontaneous model of experimental autoimmune encephalomyelitis (EAE) to study the clonality of the B cell response targeting myelin oligodendrocyte glycoprotein (MOG). In particular, 94% of SJL/j mice expressing an I-As: MOG92-106 specific transgenic T cell receptor (TCR1640) spontaneously develop a chronic paralytic EAE between the age of 60-500 days. The immune response is triggered by the microbiota in the gut-associated lymphoid tissue, while there is evidence that the maturation of the autoimmune demyelinating response might occur in the cervical lymph nodes owing to local brain drainage. Using MOG-protein-tetramers we tracked the autoantigen-specific B cells and localized their enrichment to the cervical lymph nodes and among the brain immune infiltrate. MOG-specific IgG1 antibodies were detected in the serum of diseased TCR1640 mice and proved pathogenic upon adoptive transfer into disease-prone recipients. The ontogeny of the MOG-specific humoral response preceded disease onset coherent with their contribution to EAE initiation. This humoral response was, however, not sufficient for disease induction as MOG-antibodies could be detected at the age of 69 days in a model with an average age of onset of 197 days. To assess the MOG-specific B cell repertoire we FACS-sorted MOG-tetramer binding cells and clonally expand them in vitro to sequence the paratopes of the IgG heavy chain and kappa light chains. Despite the fragility of clonally expanding MOG-tetramer binding effector B cells, our results indicate the selection of a common CDR-3 clonotype among the Igk light chains derived from both disease-free and diseased TCR1640 mice. Our study demonstrates the pre-clinical mobilization of the MOG-specific B cell response within the brain-draining cervical lymph nodes, and reiterates that MOG antibodies are a poor biomarker of disease onset and progression.

Over the past decade, our understanding of the immune reactivity and, in particular, of autoimmune disorders has witnessed a silent revolution. It has become clear that many, if not all autoimmune diseases entertain an intimate connection to the bacterial gut flora, a cosmos of trillions of different bacteria, forming diverse consortia distributed along the length of the intestinal tube. However, the microbiota’s role seems to be remarkably ambiguous, depending on the particular autoimmune disease. Experimental studies of disease models indicate that, for example, in autoimmune diseases of the brain and the eye microbiota are essentially required to trigger and maintain autoimmunity (1, 2), while in stark contrast, in autoimmune type 1 diabetes (T1D), gut bacteria may protect from disease (3⇓⇓–6). Local interactions between luminal microbes with the surrounding gut-associated lymphoid tissue (GALT) drive the immune system into either direction. In PNAS, Sorini et al. (7) describe an alternative interactive mechanism. They observe that in 2 models of T1D changes in the luminal gut wall precede onset of clinical autoimmune diabetes. These changes are most evident in the mucus layer of the large intestine, where they allow translocation of bacteria from the lumen into extraluminal tissues and where they enable activated self-reactive T cells to home to the pancreatic islets and ultimately destroy the insulin-forming β cells (Fig. 1). Fig. 1. A compromised intestinal barrier triggers activation of diabetogenic T cells in the GALT. An intact intestinal barrier ( Left ) is composed of a thick mucus layer and an epithelial layer with tight junctions (orange). The rare diabetogenic T cells in the underlying lamina propria remain quiescent. In contrast, a compromised intestinal barrier ( Right ) characterized by a thin and discontinuous mucus layer and a breached epithelial barrier allows invasion of microbiota into the lamina propria. This results in … [↵][1]1To whom correspondence may be addressed. Email: hwekerle{at}neuro.mpg.de. [1]: #xref-corresp-1-1

It is known that differentiation of Th17 cells is promoted by activation of STAT3 and inhibited by activation of STAT1. Although both transcription factors are activated by several cytokines, including IL-6, IL-21, and IL-27, each of these cytokines has a very different effect on Th17 differentiation, ranging from strong induction (IL-6) to strong inhibition (IL-27). To determine the molecular basis for these differences, we measured STAT3 and STAT1 activation profiles for IL-6, IL-21, and IL-27, as well as for cytokine pairs over time. We found that the ratio of activated STAT3/activated STAT1 is crucial in determining whether cytokines promote or inhibit Th17 differentiation. IL-6 and IL-21 induced p-STAT3/p-STAT1 ratios > 1, leading to the promotion of Th17 differentiation, whereas IL-27 or IL-6+IL-27 induced p-STAT3/p-STAT1 ratios < 1, resulting in inhibition of Th17 differentiation. Consistent with these findings, we show that IL-27 induces sufficient p-STAT3 to promote Th17 differentiation in the absence of STAT1. Furthermore, IL-27–induced STAT1-deficient T cells were indistinguishable from bona fide highly proinflammatory Th17 cells because they induced severe experimental autoimmune encephalomyelitis upon adoptive transfer. Our results suggest that the ratio of p-STAT3/p-STAT1 induced by a cytokine or cytokine pairs can be used to predict whether they induce a competent Th17-differentiation program.

Podoplanin (PDPN, also known as Gp38) is highly expressed on the surface of lymphatic endothelial cells, where it regulates development of lymphatic vessels. We have recently observed that PDPN is also expressed on effector T cells that infiltrate target tissues during autoimmune inflammation; however, the function of PDPN in T cells is largely unclear. Here, we demonstrated that global deletion of Pdpn results in exaggerated T cell responses and spontaneous experimental autoimmune encephalomyelitis (EAE) in mice with a susceptible genetic background. In contrast, T cell–specific overexpression of PDPN resulted in profound defects in IL-7–mediated T cell expansion and survival. Consequently, these animals exhibited a more rapid resolution of CNS inflammation, characterized by a reduced effector CD4+ T cell population in the CNS. Mice harboring a T cell–specific deletion of Pdpn developed exacerbated EAE, with increased accumulation of effector CD4+ T cells in the CNS. Transcriptional profiling of naturally occurring PDPN+ effector T cells in the CNS revealed increased expression of other inhibitory receptors, such as Pd1 and Tim3, and decreased expression of prosurvival factors, including Il7ra. Together, our data suggest that PDPN functions as an inhibitory molecule on T cells, thereby promoting tissue tolerance by limiting long-term survival and maintenance of CD4+ effector T cells in target organs.

Th17 cells are a subset of CD4+ T helper cells that play an important role in host defense and have been strongly associated with the pathogenesis of autoimmunity. The immense research effort on Th17 cells has benefited in recent years from major breakthroughs in genomic profiling technologies. The picture emerging from these studies has led us away from thinking about T cell differentiation in terms of rigid, separate pathways that give rise to canonical 'types'. Instead, it has encouraged us to understand T cell differentiation and function through a complex network of transcriptional regulators that can lead to different and potentially plastic cell states. This review summarizes some of the lessons we have learned from these studies about the identity, interplay, and function of the factors that are involved in Th17 cell differentiation, effector functions and plasticity. It also highlights some applications, challenges, and limitations of large-scale systematic analyses of T cell function.

A number of autoimmune diseases, including multiple sclerosis, are mediated by self-reactive T cells that have escaped the deletional mechanisms of central tolerance. Usually, these T cells are kept at bay through peripheral tolerance mechanisms including regulation through coinhibitory receptors and suppression by regulatory T cells (Tregs). However, if these mechanisms fail, self-reactive T cells are activated and autoimmune responses ensue. This review outlines how the coinhibitory receptors CTLA-4, PD-1, Tim-3 and TIGIT act at different checkpoints to inhibit autoreactive T cells and suppress the development of CNS autoimmunity. Loss of each of these receptors predisposes to autoimmunity, indicating a non-redundant role in maintaining peripheral tolerance. At the same time, their functional patterns seem to overlap to a large degree. We therefore propose that only the concerted action of a combination of inhibitory receptors is able to maintain peripheral tolerance and prevent autoimmunity.

Th17 cells have been shown to be strong inducers of tissue inflammation and autoimmune diseases. However, not all Th17 cells are pathogenic and increasing data suggests that Th17 cells may come in different flavors. Thus, Th17 cells cannot be described using a narrow schematic, but instead Th17 cells comprise a wide spectrum with a range of effector phenotypes. Here, we review the key factors that generate such diversity, as well as the cytokines and transcription factors that are differentially expressed in pathogenic and nonpathogenic Th17 cells. This new knowledge can be used to identify molecules that make Th17 cells pathogenic and determine how these cells could be targeted to suppress autoimmune diseases.

Th17 cells are a subset of effector CD4 T cells that induce neutrophilic tissue inflammation, particularly in mucosal tissues. While they play a critical role in host defense against extracellular bacteria and fungi, they also have been implicated in the pathogenesis of a number of T cell–mediated autoimmune disorders, including multiple sclerosis, rheumatoid arthritis, and psoriasis. Consistent with their dichotomous roles in promoting host defense and inducing autoimmunity, several subtypes of Th17 cells have been described with distinct effector functions and differing ability to induce autoimmune tissue inflammation. Moreover, Th17 cells are markedly plastic, both in their expression of cytokines typically associated with other effector T cell subsets and in their relationship with regulatory CD4 T cells; and this plasticity has important functional consequences. In this article, we describe the mechanisms that control Th17 cell differentiation, function, and plasticity, as well as highlight Th22 cells, a recently described and closely related CD4 T cell subset.

Upon encounter with antigen and depending on the cytokine milieu, T helper (Th) cells can differentiate into various effector T cell subsets, including the well-defined Th1 and Th2 subsets as well as the more recently discovered Th17, Th9, and follicular T helper (TFH) subsets. Among these, self-reactive Th1 and Th17 cells have been associated with organ-specific autoimmunity, while self-reactive TFH cells have been shown to promote aberrant B cell responses in systemic autoimmune diseases. On the other hand, regulatory T cell subsets including Foxp3+ regulatory T cells (Tregs) and type 1 regulatory T cells (Tr1) can regulate autoimmunity by suppressing effector T cell responses. Here, we give an overview of the different Th cell subsets, their development, and how the interplay between them can promote or inhibit autoimmune tissue inflammation.

Interleukin 17 (IL-17)-producing helper T cells (T(H)17 cells) are often present at the sites of tissue inflammation in autoimmune diseases, which has led to the conclusion that T(H)17 cells are main drivers of autoimmune tissue injury. However, not all T(H)17 cells are pathogenic; in fact, T(H)17 cells generated with transforming growth factor-β1 (TGF-β1) and IL-6 produce IL-17 but do not readily induce autoimmune disease without further exposure to IL-23. Here we found that the production of TGF-β3 by developing T(H)17 cells was dependent on IL-23, which together with IL-6 induced very pathogenic T(H)17 cells. Moreover, TGF-β3-induced T(H)17 cells were functionally and molecularly distinct from TGF-β1-induced T(H)17 cells and had a molecular signature that defined pathogenic effector T(H)17 cells in autoimmune disease.

SummaryEctopic lymphoid follicles are hallmarks of chronic autoimmune inflammatory diseases such as multiple sclerosis (MS), rheumatoid arthritis, Sjögren's syndrome, and myasthenia gravis. However, the effector cells and mechanisms that induce their development are unknown. Here we showed that in experimental autoimmune encephalomyelitis (EAE), the animal model of MS, Th17 cells specifically induced ectopic lymphoid follicles in the central nervous system (CNS). Development of ectopic lymphoid follicles was partly dependent on the cytokine interleukin 17 (IL-17) and on the cell surface molecule Podoplanin (Pdp), which was expressed on Th17 cells, but not on other effector T cell subsets. Pdp was also crucial for the development of secondary lymphoid structures: Pdp-deficient mice lacked peripheral lymph nodes and had a defect in forming normal lymphoid follicles and germinal centers in spleen and lymph node remnants. Thus, Th17 cells are uniquely endowed to induce tissue inflammation, characterized by ectopic lymphoid follicles within the target organ.

Summary To initiate adaptive immunity, dendritic cells (DCs) move from parenchymal tissues to lymphoid organs by migrating along stromal scaffolds that display the glycoprotein podoplanin (PDPN). PDPN is expressed by lymphatic endothelial and fibroblastic reticular cells and promotes blood-lymph separation during development by activating the C-type lectin receptor, CLEC-2, on platelets. Here, we describe a role for CLEC-2 in the morphodynamic behavior and motility of DCs. CLEC-2 deficiency in DCs impaired their entry into lymphatics and trafficking to and within lymph nodes, thereby reducing T cell priming. CLEC-2 engagement of PDPN was necessary for DCs to spread and migrate along stromal surfaces and sufficient to induce membrane protrusions. CLEC-2 activation triggered cell spreading via downregulation of RhoA activity and myosin light-chain phosphorylation and triggered F-actin-rich protrusions via Vav signaling and Rac1 activation. Thus, activation of CLEC-2 by PDPN rearranges the actin cytoskeleton in DCs to promote efficient motility along stromal surfaces., Highlights ► The CLEC-2-PDPN interaction promotes DC migration to LNs ► CLEC-2 activation induces actin-rich protrusions and motility in DCs ► CLEC-2 activation reduces actomyosin contraction and promotes actin polymerization