The Ins and Outs of Central Nervous System Inflammation-Lessons Learned from Multiple Sclerosis.

Multiple sclerosis (MS) is a chronic disease that is characterized by the inappropriate invasion of lymphocytes and monocytes into the central nervous system (CNS), where they orchestrate the demyelination of axons, leading to physical and cognitive disability. There are many reasons immunologists should be interested in MS. Aside from the fact that there is still significant unmet need for patients living with the progressive form of the disease, MS is a case study for how immune cells cross CNS barriers and subsequently interact with specialized tissue parenchymal cells. In this review, we describe the types of immune cells that infiltrate the CNS and then describe interactions between immune cells and glial cells in different types of lesions. Lastly, we provide evidence for CNS-compartmentalized immune cells and speculate on how this impacts disease progression for MS patients.

Cellular architecture of evolving neuroinflammatory lesions and multiple sclerosis pathology.

Multiple sclerosis (MS) is a neurological disease characterized by multifocal lesions and smoldering pathology. Although single-cell analyses provided insights into cytopathology, evolving cellular processes underlying MS remain poorly understood. We investigated the cellular dynamics of MS by modeling temporal and regional rates of disease progression in mouse experimental autoimmune encephalomyelitis (EAE). By performing single-cell spatial expression profiling using in situ sequencing (ISS), we annotated disease neighborhoods and found centrifugal evolution of active lesions. We demonstrated that disease-associated (DA)-glia arise independently of lesions and are dynamically induced and resolved over the disease course. Single-cell spatial mapping of human archival MS spinal cords confirmed the differential distribution of homeostatic and DA-glia, enabled deconvolution of active and inactive lesions into sub-compartments, and identified new lesion areas. By establishing a spatial resource of mouse and human MS neuropathology at a single-cell resolution, our study unveils the intricate cellular dynamics underlying MS.

Niche-Selective Inhibition of Pathogenic Th17 Cells by Targeting Metabolic Redundancy.

Targeting glycolysis has been considered therapeutically intractable owing to its essential housekeeping role. However, the context-dependent requirement for individual glycolytic steps has not been fully explored. We show that CRISPR-mediated targeting of glycolysis in T cells in mice results in global loss of Th17 cells, whereas deficiency of the glycolytic enzyme glucose phosphate isomerase (Gpi1) selectively eliminates inflammatory encephalitogenic and colitogenic Th17 cells, without substantially affecting homeostatic microbiota-specific Th17 cells. In homeostatic Th17 cells, partial blockade of glycolysis upon Gpi1 inactivation was compensated by pentose phosphate pathway flux and increased mitochondrial respiration. In contrast, inflammatory Th17 cells experience a hypoxic microenvironment known to limit mitochondrial respiration, which is incompatible with loss of Gpi1. Our study suggests that inhibiting glycolysis by targeting Gpi1 could be an effective therapeutic strategy with minimum toxicity for Th17-mediated autoimmune diseases, and, more generally, that metabolic redundancies can be exploited for selective targeting of disease processes.

Serum Amyloid A Proteins Induce Pathogenic Th17 Cells and Promote Inflammatory Disease.

Lymphoid cells that produce interleukin (IL)-17 cytokines protect barrier tissues from pathogenic microbes but are also prominent effectors of inflammation and autoimmune disease. T helper 17 (Th17) cells, defined by RORγt-dependent production of IL-17A and IL-17F, exert homeostatic functions in the gut upon microbiota-directed differentiation from naive CD4+ T cells. In the non-pathogenic setting, their cytokine production is regulated by serum amyloid A proteins (SAA1 and SAA2) secreted by adjacent intestinal epithelial cells. However, Th17 cell behaviors vary markedly according to their environment. Here, we show that SAAs additionally direct a pathogenic pro-inflammatory Th17 cell differentiation program, acting directly on T cells in collaboration with STAT3-activating cytokines. Using loss- and gain-of-function mouse models, we show that SAA1, SAA2, and SAA3 have distinct systemic and local functions in promoting Th17-mediated inflammatory diseases. These studies suggest that T cell signaling pathways modulated by the SAAs may be attractive targets for anti-inflammatory therapies.

Recirculating Intestinal IgA-Producing Cells Regulate Neuroinflammation via IL-10.

Plasma cells (PC) are found in the CNS of multiple sclerosis (MS) patients, yet their source and role in MS remains unclear. We find that some PC in the CNS of mice with experimental autoimmune encephalomyelitis (EAE) originate in the gut and produce immunoglobulin A (IgA). Moreover, we show that IgA+ PC are dramatically reduced in the gut during EAE, and likewise, a reduction in IgA-bound fecal bacteria is seen in MS patients during disease relapse. Removal of plasmablast (PB) plus PC resulted in exacerbated EAE that was normalized by the introduction of gut-derived IgA+ PC. Furthermore, mice with an over-abundance of IgA+ PB and/or PC were specifically resistant to the effector stage of EAE, and expression of interleukin (IL)-10 by PB plus PC was necessary and sufficient to confer resistance. Our data show that IgA+ PB and/or PC mobilized from the gut play an unexpected role in suppressing neuroinflammation.

Transcription factor RORα enforces stability of the Th17 cell effector program by binding to a Rorc cis-regulatory element.

T helper 17 (Th17) cells regulate mucosal barrier defenses but also promote multiple autoinflammatory diseases. Although many molecular determinants of Th17 cell differentiation have been elucidated, the transcriptional programs that sustain Th17 cells in vivo remain obscure. The transcription factor RORγt is critical for Th17 cell differentiation; however, it is not clear whether the closely related RORα, which is co-expressed in Th17 cells, has a distinct role. Here, we demonstrated that although dispensable for Th17 cell differentiation, RORα was necessary for optimal Th17 responses in peripheral tissues. The absence of RORα in T cells led to reductions in both RORγt expression and effector function among Th17 cells. Cooperative binding of RORα and RORγt to a previously unidentified Rorc cis-regulatory element was essential for Th17 lineage maintenance in vivo. These data point to a non-redundant role of RORα in Th17 lineage maintenance via reinforcement of the RORγt transcriptional program.

The TGF-ß superfamily cytokine Activin-A is induced during autoimmune neuroinflammation and drives pathogenic Th17 cell differentiation.

Th17 cells are known to exert pathogenic and non-pathogenic functions. Although the cytokine transforming growth factor ß1 (TGF-ß1) is instrumental for Th17 cell differentiation, it is dispensable for generation of pathogenic Th17 cells. Here, we examined the T cell-intrinsic role of Activin-A, a TGF-ß superfamily member closely related to TGF-ß1, in pathogenic Th17 cell differentiation. Activin-A expression was increased in individuals with relapsing-remitting multiple sclerosis and in mice with experimental autoimmune encephalomyelitis. Stimulation with interleukin-6 and Activin-A induced a molecular program that mirrored that of pathogenic Th17 cells and was inhibited by blocking Activin-A signaling. Genetic disruption of Activin-A and its receptor ALK4 in T cells impaired pathogenic Th17 cell differentiation in vitro and in vivo. Mechanistically, extracellular-signal-regulated kinase (ERK) phosphorylation, which was essential for pathogenic Th17 cell differentiation, was suppressed by TGF-ß1-ALK5 but not Activin-A-ALK4 signaling. Thus, Activin-A drives pathogenic Th17 cell differentiation, implicating the Activin-A-ALK4-ERK axis as a therapeutic target for Th17 cell-related diseases.

Insulin-Like Growth Factors Are Key Regulators of T Helper 17 Regulatory T Cell Balance in Autoimmunity.

Appropriate balance of T helper 17 (Th17) and regulatory T (Treg) cells maintains immune tolerance and host defense. Disruption of Th17-Treg cell balance is implicated in a number of immune-mediated diseases, many of which display dysregulation of the insulin-like growth factor (IGF) system. Here, we show that, among effector T cell subsets, Th17 and Treg cells selectively expressed multiple components of the IGF system. Signaling through IGF receptor (IGF1R) activated the protein kinase B-mammalian target of rapamycin (AKT-mTOR) pathway, increased aerobic glycolysis, favored Th17 cell differentiation over that of Treg cells, and promoted a heightened pro-inflammatory gene expression signature. Group 3 innate lymphoid cells (ILC3s), but not ILC1s or ILC2s, were similarly responsive to IGF signaling. Mice with deficiency of IGF1R targeted to T cells failed to fully develop disease in the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis. Thus, the IGF system represents a previously unappreciated pathway by which type 3 immunity is modulated and immune-mediated pathogenesis controlled.

Fate-Mapping of GM-CSF Expression Identifies a Discrete Subset of Inflammation-Driving T Helper Cells Regulated by Cytokines IL-23 and IL-1ß.

Pathogenic lymphocytes initiate the development of chronic inflammatory diseases. The cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) (encoded by Csf2) is a key communicator between pathogenic lymphocytes and tissue-invading inflammatory phagocytes. However, the molecular properties of GM-CSF-producing cells and the mode of Csf2 regulation in vivo remain unclear. To systematically study and manipulate GM-CSF+ cells and their progeny in vivo, we generated a fate-map and reporter of GM-CSF expression mouse strain (FROG). We mapped the phenotypic and functional profile of auto-aggressive T helper (Th) cells during neuroinflammation and identified the signature and pathogenic memory of a discrete encephalitogenic Th subset. These cells required interleukin-23 receptor (IL-23R) and IL-1R but not IL-6R signaling for their maintenance and pathogenicity. Specific ablation of this subset interrupted the inflammatory cascade, despite the unperturbed tissue accumulation of other Th subsets (e.g., Th1 and Th17), highlighting that GM-CSF expression not only marks pathogenic Th cells, but that this subset mediates immunopathology and tissue destruction.

High Glucose Intake Exacerbates Autoimmunity through Reactive-Oxygen-Species-Mediated TGF-ß Cytokine Activation.

Diet has been suggested to be a potential environmental risk factor for the increasing incidence of autoimmune diseases, yet the underlying mechanisms remain elusive. Here, we show that high glucose intake exacerbated autoimmunity in mouse models of colitis and experimental autoimmune encephalomyelitis (EAE). We elucidated that high amounts of glucose specifically promoted T helper-17 (Th17) cell differentiation by activating transforming growth factor-ß (TGF-ß) from its latent form through upregulation of reactive oxygen species (ROS) in T cells. We further determined that mitochondrial ROS (mtROS) are key for high glucose-induced TGF-ß activation and Th17 cell generation. We have thus revealed a previously unrecognized mechanism underlying the adverse effects of high glucose intake in the pathogenesis of autoimmunity and inflammation.

Multiple sclerosis and the intestine: Chasing the microbial offender.

Multiple sclerosis (MS) affects more than 2.8 million people worldwide but the distribution is not even. Although over 200 gene variants have been associated with susceptibility, studies of genetically identical monozygotic twin pairs suggest that the genetic make-up is responsible for only about 20%-30% of the risk to develop disease, while the rest is contributed by milieu factors. Recently, a new, unexpected player has entered the ranks of MS-triggering or facilitating elements: the human gut microbiota. In this review, we summarize the present knowledge of microbial effects on formation of a pathogenic autoreactive immune response targeting the distant central nervous system and delineate the approaches, both in people with MS and in MS animal models, which have led to this concept. Finally, we propose that a tight combination of investigations of human patients with studies of suitable animal models is the best strategy to functionally characterize disease-associated microbiota and thereby contribute to deciphering pathogenesis of a complex human disease.

Mononuclear phagocytes in autoimmune neuroinflammation.

A healthy mammalian central nervous system (CNS) harbors a diverse population of leukocytes including members of the mononuclear phagocyte system (MPS). Exerting their specific functions, CNS tissue-resident macrophages as well as associated monocytes and dendritic cells (DCs) maintain CNS homeostasis. Under neuroinflammatory conditions, leukocytes from the systemic immune compartment invade the CNS. This review focuses on the newly discovered roles of the MPS in autoimmune neuroinflammation elicited by encephalitogenic T cells. We propose that CNS-associated DCs act as gatekeepers and antigen-presenting cells that guide the adaptive immune response while bone marrow (BM)-derived monocytes contribute to immunopathology and tissue damage. By contrast, CNS-resident macrophages primarily support tissue function and promote the repair and maintenance of CNS functions.

The Innate Immune Sensor NLRC3 Acts as a Rheostat that Fine-Tunes T Cell Responses in Infection and Autoimmunity.

Appropriate immune responses require a fine balance between immune activation and attenuation. NLRC3, a non-inflammasome-forming member of the NLR innate immune receptor family, attenuates inflammation in myeloid cells and proliferation in epithelial cells. T lymphocytes express the highest amounts of Nlrc3 transcript where its physiologic relevance is unknown. We show that NLRC3 attenuated interferon-γ and TNF expression by CD4+ T cells and reduced T helper 1 (Th1) and Th17 cell proliferation. Nlrc3-/- mice exhibited increased and prolonged CD4+ T cell responses to lymphocytic choriomeningitis virus infection and worsened experimental autoimmune encephalomyelitis (EAE). These functions of NLRC3 were executed in a T-cell-intrinsic fashion: NLRC3 reduced K63-linked ubiquitination of TNF-receptor-associated factor 6 (TRAF6) to limit NF-κB activation, lowered phosphorylation of eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1), and diminished glycolysis and oxidative phosphorylation. This study reveals an unappreciated role for NLRC3 in attenuating CD4+ T cell signaling and metabolism.

Iron Drives T Helper Cell Pathogenicity by Promoting RNA-Binding Protein PCBP1-Mediated Proinflammatory Cytokine Production.

Iron deposition is frequently observed in human autoinflammatory diseases, but its functional significance is largely unknown. Here we showed that iron promoted proinflammatory cytokine expression in T cells, including GM-CSF and IL-2, via regulating the stability of an RNA-binding protein PCBP1. Iron depletion or Pcbp1 deficiency in T cells inhibited GM-CSF production by attenuating Csf2 3' untranslated region (UTR) activity and messenger RNA stability. Pcbp1 deficiency or iron uptake blockade in autoreactive T cells abolished their capacity to induce experimental autoimmune encephalomyelitis, an animal model for multiple sclerosis. Mechanistically, intracellular iron protected PCBP1 protein from caspase-mediated proteolysis, and PCBP1 promoted messenger RNA stability of Csf2 and Il2 by recognizing UC-rich elements in the 3' UTRs. Our study suggests that iron accumulation can precipitate autoimmune diseases by promoting proinflammatory cytokine production. RNA-binding protein-mediated iron sensing may represent a simple yet effective means to adjust the inflammatory response to tissue homeostatic alterations.

Fas Promotes T Helper 17 Cell Differentiation and Inhibits T Helper 1 Cell Development by Binding and Sequestering Transcription Factor STAT1.

The death receptor Fas removes activated lymphocytes through apoptosis. Previous transcriptional profiling predicted that Fas positively regulates interleukin-17 (IL-17)-producing T helper 17 (Th17) cells. Here, we demonstrate that Fas promoted the generation and stability of Th17 cells and prevented their differentiation into Th1 cells. Mice with T-cell- and Th17-cell-specific deletion of Fas were protected from induced autoimmunity, and Th17 cell differentiation and stability were impaired. Fas-deficient Th17 cells instead developed a Th1-cell-like transcriptional profile, which a new algorithm predicted to depend on STAT1. Experimentally, Fas indeed bound and sequestered STAT1, and Fas deficiency enhanced IL-6-induced STAT1 activation and nuclear translocation, whereas deficiency of STAT1 reversed the transcriptional changes induced by Fas deficiency. Thus, our computational and experimental approach identified Fas as a regulator of the Th17-to-Th1 cell balance by controlling the availability of opposing STAT1 and STAT3 to have a direct impact on autoimmunity.

RAS P21 Protein Activator 3 (RASA3) Specifically Promotes Pathogenic T Helper 17 Cell Generation by Repressing T-Helper-2-Cell-Biased Programs.

Pathogenic Th17 (pTh17) cells drive inflammation and immune-pathology, but whether pTh17 cells are a Th17 cell subset whose generation is under specific molecular control remains unaddressed. We found that Ras p21 protein activator 3 (RASA3) was highly expressed by pTh17 cells relative to non-pTh17 cells and was required specifically for pTh17 generation in vitro and in vivo. Mice conditionally deficient for Rasa3 in T cells showed less pathology during experimental autoimmune encephalomyelitis. Rasa3-deficient T cells acquired a Th2 cell-biased program that dominantly trans-suppressed pTh17 cell generation via interleukin 4 production. The Th2 cell bias of Rasa3-deficient T cells was due to aberrantly elevated transcription factor IRF4 expression. RASA3 promoted proteasome-mediated IRF4 protein degradation by facilitating interaction of IRF4 with E3-ubiquitin ligase Cbl-b. Therefore, a RASA3-IRF4-Cbl-b pathway specifically directs pTh17 cell generation by balancing reciprocal Th17-Th2 cell programs. These findings indicate that a distinct molecular program directs pTh17 cell generation and reveals targets for treating pTh17 cell-related pathology and diseases.

An IL-23-STAT4 pathway is required for the proinflammatory function of classical dendritic cells during CNS inflammation.

Although many cytokine pathways are important for dendritic cell (DC) development, it is less clear what cytokine signals promote the function of mature dendritic cells. The signal transducer and activator of transcription 4 (STAT4) promotes protective immunity and autoimmunity downstream of proinflammatory cytokines including IL-12 and IL-23. In experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), Stat4-/- mice are resistant to the development of inflammation and paralysis. To define whether STAT4 is required for intrinsic signaling in mature DC function, we used conditional mutant mice in the EAE model. Deficiency of STAT4 in CD11c-expressing cells resulted in decreased T cell priming and inflammation in the central nervous system. EAE susceptibility was recovered following adoptive transfer of wild-type bone marrow-derived DCs to mice with STAT4-deficient DCs, but not adoptive transfer of STAT4- or IL-23R-deficient DCs. Single-cell RNA-sequencing (RNA-seq) identified STAT4-dependent genes in DC subsets that paralleled a signature in MS patient DCs. Together, these data define an IL-23-STAT4 pathway in DCs that is key to DC function during inflammatory disease.

Electrical stimulation of the vagus nerve ameliorates inflammation and disease activity in a rat EAE model of multiple sclerosis.

Multiple sclerosis (MS) is a demyelinating central nervous system (CNS) disorder that is associated with functional impairment and accruing disability. There are multiple U.S. Food and Drug Administration (FDA)-approved drugs that effectively dampen inflammation and slow disability progression. However, these agents do not work well for all patients and are associated with side effects that may limit their use. The vagus nerve (VN) provides a direct communication conduit between the CNS and the periphery, and modulation of the inflammatory reflex via electrical stimulation of the VN (VNS) shows efficacy in ameliorating pathology in several CNS and autoimmune disorders. We therefore investigated the impact of VNS in a rat experimental autoimmune encephalomyelitis (EAE) model of MS. In this study, VNS-mediated neuroimmune modulation is demonstrated to effectively decrease EAE disease severity and duration, infiltration of neutrophils and pathogenic lymphocytes, myelin damage, blood-brain barrier disruption, fibrinogen deposition, and proinflammatory microglial activation. VNS modulates expression of genes that are implicated in MS pathogenesis, as well as those encoding myelin proteins and transcription factors regulating new myelin synthesis. Together, these data indicate that neuroimmune modulation via VNS may be a promising approach to treat MS, that not only ameliorates symptoms but potentially also promotes myelin repair (remyelination).

IL-11 induces NLRP3 inflammasome activation in monocytes and inflammatory cell migration to the central nervous system.

The objective of this study is to examine IL-11-induced mechanisms of inflammatory cell migration to the central nervous system (CNS). We report that IL-11 is produced at highest frequency by myeloid cells among the peripheral blood mononuclear cell (PBMC) subsets. Patients with relapsing-remitting multiple sclerosis (RRMS) have an increased frequency of IL-11+ monocytes, IL-11+ and IL-11R+ CD4+ lymphocytes, and IL-11R+ neutrophils in comparison to matched healthy controls. IL-11+ and granulocyte-macrophage colony-stimulating factor (GM-CSF)+ monocytes, CD4+ lymphocytes, and neutrophils accumulate in the cerebrospinal fluid (CSF). The effect of IL-11 in-vitro stimulation, examined using single-cell RNA sequencing, revealed the highest number of differentially expressed genes in classical monocytes, including up-regulated NFKB1, NLRP3, and IL1B. All CD4+ cell subsets had increased expression of S100A8/9 alarmin genes involved in NLRP3 inflammasome activation. In IL-11R+-sorted cells from the CSF, classical and intermediate monocytes significantly up-regulated the expression of multiple NLRP3 inflammasome-related genes, including complement, IL18, and migratory genes (VEGFA/B) in comparison to blood-derived cells. Therapeutic targeting of this pathway with αIL-11 mAb in mice with RR experimental autoimmune encephalomyelitis (EAE) decreased clinical scores, CNS inflammatory infiltrates, and demyelination. αIL-11 mAb treatment decreased the numbers of NFκBp65+, NLRP3+, and IL-1ß+ monocytes in the CNS of mice with EAE. The results suggest that IL-11/IL-11R signaling in monocytes represents a therapeutic target in RRMS.

A B cell-driven EAE mouse model reveals the impact of B cell-derived cytokines on CNS autoimmunity.

In multiple sclerosis (MS), pathogenic T cell responses are known to be important drivers of autoimmune inflammation. However, increasing evidence suggests an additional role for B cells, which may contribute to pathogenesis via antigen presentation and production of proinflammatory cytokines. However, these B cell effector functions are not featured well in classical experimental autoimmune encephalomyelitis (EAE) mouse models. Here, we compared properties of myelin oligodendrocyte glycoprotein (MOG)-specific and polyclonal B cells and developed an adjuvant-free cotransfer EAE mouse model, where highly activated, MOG-specific induced germinal center B cells provide the critical stimulus for disease development. We could show that high levels of MOG-specific immunoglobulin G (IgGs) are not required for EAE development, suggesting that antigen presentation and activation of cognate T cells by B cells may be important for pathogenesis. As our model allows for B cell manipulation prior to transfer, we found that overexpression of the proinflammatory cytokine interleukin (IL)-6 by MOG-specific B cells leads to an accelerated EAE onset accompanied by activation/expansion of the myeloid compartment rather than a changed T cell response. Accordingly, knocking out IL-6 or tumor necrosis factor α in MOG-specific B cells via CRISPR-Cas9 did not affect activation of pathogenic T cells. In summary, we generated a tool to dissect pathogenic B cell effector function in EAE development, which should improve our understanding of pathogenic processes in MS.