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.

Visualizing the activation of encephalitogenic T cells in the ileal lamina propria by in vivo two-photon imaging

Autoreactive encephalitogenic T cells exist in the healthy immune repertoire but need a trigger to induce CNS inflammation. The underlying mechanisms remain elusive, whereby microbiota were shown to be involved in the manifestation of CNS autoimmunity. Here, we used intravital imaging to explore how microbiota affect the T cells as trigger of CNS inflammation. Encephalitogenic CD4⁺ T cells transduced with the calcium-sensing protein Twitch-2B showed calcium signaling with higher frequency than polyclonal T cells in the small intestinal lamina propria (LP) but not in Peyer's patches. Interestingly, nonencephalitogenic T cells specific for OVA and LCMV also showed calcium signaling in the LP, indicating a general stimulating effect of microbiota. The observed calcium signaling was microbiota and MHC class II dependent as it was significantly reduced in germfree animals and after administration of anti-MHC class II antibody, respectively. As a consequence of T cell stimulation in the small intestine, the encephalitogenic T cells start expressing Th17-axis genes. Finally, we show the migration of CD4⁺ T cells from the small intestine into the CNS. In summary, our direct in vivo visualization revealed that microbiota induced T cell activation in the LP, which directed T cells to adopt a Th17-like phenotype as a trigger of CNS inflammation.

Broader Epstein-Barr virus-specific T cell receptor repertoire in patients with multiple sclerosis

Epstein-Barr virus (EBV) infection precedes multiple sclerosis (MS) pathology and cross-reactive antibodies might link EBV infection to CNS autoimmunity. As an altered anti-EBV T cell reaction was suggested in MS, we queried peripheral blood T cell receptor β chain (TCRβ) repertoires of 1,395 MS patients, 887 controls, and 35 monozygotic, MS-discordant twin pairs for multimer-confirmed, viral antigen-specific TCRβ sequences. We detected more MHC-I-restricted EBV-specific TCRβ sequences in MS patients. Differences in genetics or upbringing could be excluded by validation in monozygotic twin pairs discordant for MS. Anti-VLA-4 treatment amplified this observation, while interferon β- or anti-CD20 treatment did not modulate EBV-specific T cell occurrence. In healthy individuals, EBV-specific CD8+ T cells were of an effector-memory phenotype in peripheral blood and cerebrospinal fluid. In MS patients, cerebrospinal fluid also contained EBV-specific central-memory CD8+ T cells, suggesting recent priming. Therefore, MS is not only preceded by EBV infection, but also associated with broader EBV-specific TCR repertoires, consistent with an ongoing anti-EBV immune reaction in MS.

Gut microbiome of multiple sclerosis patients and paired household healthy controls reveal associations with disease risk and course

Changes in gut microbiota have been associated with several diseases. Here, the International Multiple Sclerosis Microbiome Study (iMSMS) studied the gut microbiome of 576 MS patients (36% untreated) and genetically unrelated household healthy controls (1,152 total subjects). We observed a significantly increased proportion of Akkermansia muciniphila, Ruthenibacterium lactatiformans, Hungatella hathewayi, and Eisenbergiella tayi and decreased Faecalibacterium prausnitzii and Blautia species. The phytate degradation pathway was over-represented in untreated MS, while pyruvate-producing carbohydrate metabolism pathways were significantly reduced. Microbiome composition, function, and derived metabolites also differed in response to disease-modifying treatments. The therapeutic activity of interferon-β may in part be associated with upregulation of short-chain fatty acid transporters. Distinct microbial networks were observed in untreated MS and healthy controls. These results strongly support specific gut microbiome associations with MS risk, course and progression, and functional changes in response to treatment.

The Genetic Background of Mice Influences the Effects of Cigarette Smoke on Onset and Severity of Experimental Autoimmune Encephalomyelitis

Multiple sclerosis (MS) is the most common inflammatory disorder of the central nervous system (CNS) in young adults leading to severe disability. Besides genetic traits, environmental factors contribute to MS pathogenesis. Cigarette smoking increases the risk of MS in an HLA-dependent fashion, but the underlying mechanisms remain unknown. Here, we explored the effect of cigarette smoke exposure on spontaneous and induced models of experimental autoimmune encephalomyelitis (EAE) by evaluating clinical disease and, when relevant, blood leukocytes and histopathology. In the relapsing-remitting (RR) transgenic model in SJL/J mice, we observed very low incidence in both smoke-exposed and control groups. In the optico-spinal encephalomyelitis (OSE) double transgenic model in C57BL/6 mice, the early onset of EAE prevented a meaningful evaluation of the effects of cigarette smoke. In EAE models induced by immunization, daily exposure to cigarette smoke caused a delayed onset of EAE followed by a protracted disease course in SJL/J mice. In contrast, cigarette smoke exposure ameliorated the EAE clinical score in C57BL/6J mice. Our exploratory studies therefore show that genetic background influences the effects of cigarette smoke on autoimmune neuroinflammation. Importantly, our findings expose the challenge of identifying an animal model for studying the influence of cigarette smoke in MS.

B cells in multiple sclerosis

B lymphocytes have essential roles in the autoimmune pathogenesis of multiple sclerosis (MS). They regulate the autoimmune response and participate in the development of the CNS lesions. This review discusses nature and functions of B cells in MS, and retraces the recruitment of brain-autoimmune B cells from the B cell repertoire. Multiple sclerosis is commonly considered as an autoimmune demyelinating disease, where myelin-reactive T cells enter the CNS from outside, and drive the inflammatory changes that ultimately create the degenerative MS plaque. Most therapeutic strategies focus on eliminating or mitigating these pathogenic T cells. Less consideration has been devoted to the role of autoimmune B cells in the autoimmune pathogenesis. Indeed, this role is now supported by a number of convergent lines of evidence, which are briefly outlined in a first part of this overview. A second part describes experimental studies in transgenic mouse models of brain autoimmunity, EAE, which relate to possible functions of autoimmune B cells and to their recruitment from the regular B cell repertoire.

Nature, nurture, and microbes: The development of multiple sclerosis

This paper argues that multiple sclerosis (MS) is the result of an autoimmune attack against components of the central nervous system (CNS). The effector cells involved in the pathogenic process are CNS-autoreactive T cells present in the healthy immune system in a resting state. Upon activation, these cells cross the blood-brain barrier and attack the CNS target tissue. Recent evidence indicates that autoimmune activation may happen in the intestine, following an interaction of bacterial components of the gut flora with local CNS autoreactive T cells. The consequences of this concept are discussed.

The gut-brain connection: triggering of brain autoimmune disease by commensal gut bacteria

In a transgenic model of spontaneous experimental autoimmune encephalomyelitis, autoimmune attack against the CNS requires the presence of an intact commensal gut flora. Extending this observation to human autoimmune disease, such as multiple sclerosis, we postulate that the pathogenic reaction requires the coincidence of at least three factors: a permissive genetic disposition, a pro-inflammatory intestinal microbial profile, and the accumulation of autoreactive T cells in the gut-associated lymphatic tissue. This concept may offer new approaches to diagnostic markers and non-invasive therapies.

Environmental modifiable risk factors for multiple sclerosis: Report from the 2016 ECTRIMS focused workshop

Multiple sclerosis (MS) is an inflammatory and neurodegenerative demyelinating disease of the central nervous system (CNS), most likely autoimmune in origin, usually beginning in early adulthood. The aetiology of the disease is not well understood; it is viewed currently as a multifactorial disease which results from complex interactions between genetic predisposition and environmental factors, of which a few are potentially modifiable. Improving our understanding of these factors can lead to new and more effective approaches to patient counselling and, possibly, prevention and management of the disease. The 2016 focused workshop of the European Committee for Treatment and Research in Multiple Sclerosis (ECTRIMS) addressed the topic of environmental, modifiable risk factors for MS, gathering experts from around the world, to collate experimental and clinical research into environmental factors that have been associated with the disease onset and, in a few cases, disease activity and progression. A number of factors, including infections, vitamin D deficiency, diet and lifestyle factors, stress and comorbidities, were discussed. The meeting provided a forum to analyse available evidence, to identify inconsistencies and gaps in current knowledge and to suggest avenues for future research.

[Multiple sclerosis and microbiota. From genome to metagenome?]

The individual risk of contracting multiple sclerosis (MS) is determined by genetic predisposition as well as environmental factors. In monozygotic twins the concordance rate for MS is approximately 30 % indicating that environmental factors are even more important than genetic factors. Observations in a T-cell receptor-transgenic, spontaneous mouse model strongly point to an important contribution of the individual gut microbiome (microbiota). Mice maintained in a germ-free environment are completely protected from experimental autoimmune encephalomyelitis (EAE) in this model, whereas mice that are kept under normal conditions spontaneously develop a relapsing-remitting central nervous system (CNS) disease which is astoundingly similar to human MS. It appears that the autoimmune reaction against CNS tissue is "remotely controlled" by the gut microbiota. This may be explained by the facts that the microbiota influences the gut-associated lymphoid tissue (GALT) and, vice versa, the GALT regulates systemic immunity. The precise role of the microbiota in MS remains to be clarified. New methods of DNA sequencing and bioinformatics allow the analysis of very complex bacterial metagenomes. If individual microbial risk profiles can be identified this would provide completely new perspectives for the prophylaxis and therapy of MS.

Vaccination and narcolepsy: Immune link found?

In some children, vaccination against H1N1 influenza spurred production of antibodies to brain receptors linked to the sleep disorder narcolepsy (Ahmed et al., this issue).

The search for the target antigens of multiple sclerosis, part 2: CD8+ T cells, B cells, and antibodies in the focus of reverse-translational research

Interest in CD8+ T cells and B cells was initially inspired by observations in multiple sclerosis rather than in animal models: CD8+ T cells predominate in multiple sclerosis lesions, oligoclonal immunoglobulin bands in CSF have long been recognised as diagnostic and prognostic markers, and anti-B-cell therapies showed considerable efficacy in multiple sclerosis. Taking a reverse-translational approach, findings from human T-cell receptor (TCR) and B-cell receptor (BCR) repertoire studies provided strong evidence for antigen-driven clonal expansion in the brain and CSF. New methods allow the reconstruction of human TCRs and antibodies from tissue-infiltrating immune cells, which can be used for the unbiased screening of antigen libraries. Myelin oligodendrocyte glycoprotein (MOG) has received renewed attention as an antibody target in childhood multiple sclerosis and in a small subgroup of adult patients with multiple sclerosis. Furthermore, there is growing evidence that a separate condition in adults exists, tentatively called MOG-antibody-associated encephalomyelitis, which has clinical features that overlap with neuromyelitis optica spectrum disorder and multiple sclerosis. Although CD8+ T cells and B cells are thought to have a pathogenic role in some subgroups of patients, their target antigens have yet to be identified.

Nature plus nurture: the triggering of multiple sclerosis

Recent clinical and experimental studies indicate that multiple sclerosis develops as consequence of a failed interplay between genetic ("nature") and environmental ("nurture") factors. A large number of risk genes favour an autoimmune response against the body's own brain matter. New experimental data indicate that the actual trigger of this attack is however provided by an interaction of brain-specific immune cells with components of the regular commensal gut flora, the intestinal microbiota. This concept opens the way for new therapeutic approaches involving modulation of the microbiota by dietary or antibiotic regimens.

αβ T-cell receptors from multiple sclerosis brain lesions show MAIT cell-related features

Objectives:

To characterize phenotypes of T cells that accumulated in multiple sclerosis (MS) lesions, to compare the lesional T-cell receptor (TCR) repertoire of T-cell subsets to peripheral blood, and to identify paired α and β chains from single CD8⁺ T cells from an index patient who we followed for 18 years.

Methods:

We combined immunohistochemistry, laser microdissection, and single-cell multiplex PCR to characterize T-cell subtypes and identify paired TCRα and TCRβ chains from individual brain-infiltrating T cells in frozen brain sections. The lesional and peripheral TCR repertoires were analyzed by pyrosequencing.

Results:

We found that a TCR Vβ1⁺ T-cell population that was strikingly expanded in active brain lesions at clinical onset comprises several subclones expressing distinct yet closely related Vα7.2⁺ α chains, including a canonical Vα7.2-Jα33 chain of mucosal-associated invariant T (MAIT) cells. Three other α chains bear striking similarities in their antigen-recognizing, hypervariable complementarity determining region 3. Longitudinal repertoire studies revealed that the TCR chains that were massively expanded in brain at onset persisted for several years in blood or CSF but subsequently disappeared except for the canonical Vα7.2⁺ MAIT cell and a few other TCR sequences that were still detectable in blood after 18 years.

Conclusions:

Our observation that a massively expanded TCR Vβ1-Jβ2.3 chain paired with distinct yet closely related canonical or atypical MAIT cell–related α chains strongly points to an antigen-driven process in early active MS brain lesions.

The autoimmune concept of multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory and demyelinating disease of the central nervous system (CNS). With growing evidence for environmental and genetic factors, MS is now accepted as an autoimmune disease. This complex disease seems to implicate various cell types in both innate and adaptive compartments. Here, we discuss recent advances in the immunological field of MS research.

Transcript profiling of different types of multiple sclerosis lesions yields FGF1 as a promoter of remyelination

Chronic demyelination is a pathological hallmark of multiple sclerosis (MS). Only a minority of MS lesions remyelinates completely. Enhancing remyelination is, therefore, a major aim of future MS therapies. Here we took a novel approach to identify factors that may inhibit or support endogenous remyelination in MS. We dissected remyelinated, demyelinated active, and demyelinated inactive white matter MS lesions, and compared transcript levels of myelination and inflammation-related genes using quantitative PCR on customized TaqMan Low Density Arrays. In remyelinated lesions, fibroblast growth factor (FGF) 1 was the most abundant of all analyzed myelination-regulating factors, showed a trend towards higher expression as compared to demyelinated lesions and was significantly higher than in control white matter. Two MS tissue blocks comprised lesions with adjacent de- and remyelinated areas and FGF1 expression was higher in the remyelinated rim compared to the demyelinated lesion core. In functional experiments, FGF1 accelerated developmental myelination in dissociated mixed cultures and promoted remyelination in slice cultures, whereas it decelerated differentiation of purified primary oligodendrocytes, suggesting that promotion of remyelination by FGF1 is based on an indirect mechanism. The analysis of human astrocyte responses to FGF1 by genome wide expression profiling showed that FGF1 induced the expression of the chemokine CXCL8 and leukemia inhibitory factor, two factors implicated in recruitment of oligodendrocytes and promotion of remyelination. Together, this study presents a transcript profiling of remyelinated MS lesions and identified FGF1 as a promoter of remyelination. Modulation of FGF family members might improve myelin repair in MS.

The immunoregulator soluble TACI is released by ADAM10 and reflects B cell activation in autoimmunity

BAFF and a proliferation-inducing ligand (APRIL), which control B cell homeostasis, are therapeutic targets in autoimmune diseases. TACI-Fc (atacicept), a soluble fusion protein containing the extracellular domain of the BAFF-APRIL receptor TACI, was applied in clinical trials. However, disease activity in multiple sclerosis unexpectedly increased, whereas in systemic lupus erythematosus, atacicept was beneficial. In this study, we show that an endogenous soluble TACI (sTACI) exists in vivo. TACI proteolysis involved shedding by a disintegrin and metalloproteinase 10 releasing sTACI from activated B cells. The membrane-bound stub was subsequently cleaved by γ-secretase reducing ligand-independent signaling of the remaining C-terminal fragment. The shed ectodomain assembled ligand independently in a homotypic way. It functioned as a decoy receptor inhibiting BAFF- and APRIL-mediated B cell survival and NF-κB activation. We determined sTACI levels in autoimmune diseases with established hyperactivation of the BAFF-APRIL system. sTACI levels were elevated both in the cerebrospinal fluid of the brain-restricted autoimmune disease multiple sclerosis correlating with intrathecal IgG production, as well as in the serum of the systemic autoimmune disease systemic lupus erythematosus correlating with disease activity. Together, we show that TACI is sequentially processed by a disintegrin and metalloproteinase 10 and γ-secretase. The released sTACI is an immunoregulator that shares decoy functions with atacicept. It reflects systemic and compartmentalized B cell accumulation and activation.

From classic to spontaneous and humanized models of multiple sclerosis: impact on understanding pathogenesis and drug development

Multiple sclerosis (MS), a demyelinating disease of the central nervous system (CNS), presents as a complex disease with variable clinical and pathological manifestations, involving different pathogenic pathways. Animal models, particularly experimental autoimmune encephalomyelitis (EAE), have been key to deciphering the pathophysiology of MS, although no single model can recapitulate the complexity and diversity of MS, or can, to date, integrate the diverse pathogenic pathways. Since the first EAE model was introduced decades ago, multiple classic (induced), spontaneous, and humanized EAE models have been developed, each recapitulating particular aspects of MS pathogenesis. The advances in technologies of genetic ablation and transgenesis in mice of C57BL/6J background and the development of myelin-oligodendrocyte glycoprotein (MOG)-induced EAE in C57BL/6J mice yielded several spontaneous and humanized EAE models, and resulted in a plethora of EAE models in which the role of specific genes or cell populations could be precisely interrogated, towards modeling specific pathways of MS pathogenesis/regulation in MS. Collectively, the numerous studies on the different EAE models contributed immensely to our basic understanding of cellular and molecular pathways in MS pathogenesis as well as to the development of therapeutic agents: several drugs available today as disease modifying treatments were developed from direct studies on EAE models, and many others were tested or validated in EAE. In this review, we discuss the contribution of major classic, spontaneous, and humanized EAE models to our understanding of MS pathophysiology and to insights leading to devising current and future therapies for this disease.

T cell-activation in neuromyelitis optica lesions plays a role in their formation

Background

Neuromyelitis optica (NMO) is an inflammatory demyelinating disease of the central nervous system (CNS), which is characterized by the presence of pathogenic serum autoantibodies against aquaporin 4 (AQP4) in the vast majority of patients. The contribution of T cells to the formation of astrocyte destructive lesions is currently unclear. However, active human NMO lesions contain CD4⁺ T-lymphocytes expressing the activation marker Ox40, and the expression is more profound compared to that seen in MS lesions of comparable activity. Therefore, we analyzed the role of T-cell activation within the CNS in the initiation of NMO lesions in an experimental model of co-transfer of different encephalitogenic T-cells and human AQP4 antibody containing NMO immunoglobulin (NMO IgG). We further studied the expression of the T-cell activation marker Ox40 in NMO and multiple sclerosis lesions in different stages of activity.

Results

All encephalitogenic T-cell lines used in our experiments induced brain inflammation with a comparable extent of blood brain barrier damage, allowing human NMO IgG to penetrate into the brain and spinal cord tissue. However, astrocyte destructive NMO lesions were only seen with T-cells, which showed signs of activation in the lesions. T-cell activation was reflected by the expression of the activation marker Ox40 and pronounced production of γ-IFN, which was able to increase the production of complement proteins and of the Fc gamma III receptor (Fcgr3) and decreased production of complement inhibitory protein Factor H in microglia.

Conclusions

Our data indicate that local activation of T-cells provide an inflammatory environment in the CNS, which allows AQP4 auto-antibodies to induce astrocyte destructive NMO-like lesions.

MHC class II-dependent B cell APC function is required for induction of CNS autoimmunity independent of myelin-specific antibodies

Antigen presentation, but not antibody secretion, by B cells drives CNS autoimmunity induced by immunization with human MOG.

Whether B cells serve as antigen-presenting cells (APCs) for activation of pathogenic T cells in the multiple sclerosis model experimental autoimmune encephalomyelitis (EAE) is unclear. To evaluate their role as APCs, we engineered mice selectively deficient in MHC II on B cells (B–MHC II^−/−), and to distinguish this function from antibody production, we created transgenic (Tg) mice that express the myelin oligodendrocyte glycoprotein (MOG)–specific B cell receptor (BCR; IgH^MOG-mem) but cannot secrete antibodies. B–MHC II^−/− mice were resistant to EAE induced by recombinant human MOG (rhMOG), a T cell– and B cell–dependent autoantigen, and exhibited diminished Th1 and Th17 responses, suggesting a role for B cell APC function. In comparison, selective B cell IL-6 deficiency reduced EAE susceptibility and Th17 responses alone. Administration of MOG-specific antibodies only partially restored EAE susceptibility in B–MHC II^−/− mice. In the absence of antibodies, IgH^MOG-mem mice, but not mice expressing a BCR of irrelevant specificity, were fully susceptible to acute rhMOG-induced EAE, also demonstrating the importance of BCR specificity. Spontaneous opticospinal EAE and meningeal follicle–like structures were observed in IgH^MOG-mem mice crossed with MOG-specific TCR Tg mice. Thus, B cells provide a critical cellular function in pathogenesis of central nervous system autoimmunity independent of their humoral involvement, findings which may be relevant to B cell–targeted therapies.

Remote control-triggering of brain autoimmune disease in the gut

Converging evidence indicates that multiple sclerosis, an inflammatory demyelinating disease of the central nervous system, is caused by brain-specific, self-reactive T lymphocytes. These are normal components embedded in the human immune system throughout healthy life. Only upon activation in the periphery, the T cells assume properties that enable them to break through the vascular blood-brain barrier and to invade the brain white matter. While activation has been traditionally associated with microbial infections, recently, studies of animal models revealed a critical role of the commensal gut flora as a key triggering factor. These findings may pave the way to new strategies to treat MS and other human autoimmune diseases, and commend a reevaluation of dietary approaches.

Distinction and temporal stability of conformational epitopes on myelin oligodendrocyte glycoprotein recognized by patients with different inflammatory central nervous system diseases

Autoantibodies targeting conformationally intact myelin oligodendrocyte glycoprotein (MOG) are found in different inflammatory diseases of the CNS, but their antigenic epitopes have not been mapped. We expressed mutants of MOG on human HeLa cells and analyzed sera from 111 patients (104 children, 7 adults) who recognized cell-bound human MOG, but had different diseases, including acute disseminated encephalomyelitis (ADEM), one episode of transverse myelitis or optic neuritis, multiple sclerosis (MS), anti-aquaporin-4 (AQP4)-negative neuromyelitis optica (NMO), and chronic relapsing inflammatory optic neuritis (CRION). We obtained insight into the recognition of epitopes in 98 patients. All epitopes identified were located at loops connecting the β strands of MOG. The most frequently recognized MOG epitope was revealed by the P42S mutation positioned in the CC'-loop. Overall, we distinguished seven epitope patterns, including the one mainly recognized by mouse mAbs. In half of the patients, the anti-MOG response was directed to a single epitope. The epitope specificity was not linked to certain disease entities. Longitudinal analysis of 11 patients for up to 5 y indicated constant epitope recognition without evidence for intramolecular epitope spreading. Patients who rapidly lost their anti-MOG IgG still generated a long-lasting IgG response to vaccines, indicating that their loss of anti-MOG reactivity did not reflect a general lack of capacity for long-standing IgG responses. The majority of human anti-MOG Abs did not recognize rodent MOG, which has implications for animal studies. Our findings might assist in future detection of potential mimotopes and pave the way to Ag-specific depletion.

Real-time in vivo analysis of T cell migration and activation in the periphery and in the central nervous system using a genetically encoded calcium indicator (P4103)

Interaction of T cells with cells of the surrounding milieu may result in activation of one or both partners. In particular, T cell activation following recognition of antigen presented by antigen presenting cells (APCs) triggers a swift rise of intracellular calcium levels within the T cell. Synthetic calcium indicators have been successfully used to monitor the activation status of T cells in vitro. However, these chemical dyes do not persist intracellularly for a sustained period which creates a major challenge for calcium imaging in the living animal. We now present a newly developed fluorescence resonance energy transfer (FRET)-based, genetically encoded calcium indicator (GECI) expressed in (auto)antigen-specific CD4+ T cells to study T cell activation in vivo at real time. Using two-photon microscopy, we explored the responses of retrovirally transduced, GECI-expressing T cells to antigen. In lymph nodes, the administration of exogenous antigen caused an almost immediate arrest of T cells around APCs concomitant with an instant rise of cytosolic calcium levels. In contrast, encephalitogenic T cells entering the leptomeningeal space, one main portal into the central nervous system parenchyma during experimental autoimmune encephalomyelitis, showed elevated intracellular calcium levels while still meandering through the space. This approach enables us for the first time to follow the migration and activation patterns of T cells in vivo during the course of the disease.

2-photon imaging of phagocyte-mediated T cell activation in the CNS

Autoreactive T cells can infiltrate the CNS to cause disorders such as multiple sclerosis. In order to visualize T cell activation in the CNS, we introduced a truncated fluorescent derivative of nuclear factor of activated T cells (NFAT) as a real-time T cell activation indicator. In experimental autoimmune encephalomyelitis, a rat model of multiple sclerosis, we tracked T cells interacting with structures of the vascular blood-brain barrier (BBB). 2-photon imaging documented the cytoplasmic-nuclear translocation of fluorescent NFAT, indicative of calcium-dependent activation of the T cells in the perivascular space, but not within the vascular lumen. The activation was related to contacts with the local antigen-presenting phagocytes and was noted only in T cells with a high pathogenic potential. T cell activation implied the presentation of an autoantigen, as the weakly pathogenic T cells, which remained silent in the untreated hosts, were activated upon instillation of exogenous autoantigen. Activation did not cogently signal long-lasting arrest, as individual T cells were able to sequentially contact fresh APCs. We propose that the presentation of local autoantigen by BBB-associated APCs provides stimuli that guide autoimmune T cells to the CNS destination, enabling them to attack the target tissue.

Brain autoimmunity: the CD8 question(s)

In this issue of Immunity, Na et al. (2012) show that the purging of central nervous system (CNS)-specific CD8(+) T cell repertoire requires direct contact with antigen expressing oligodendrocytes and inflammation tips the balance toward autoimmunity.

Cumulative autoimmunity: T cell clones recognizing several self-epitopes exhibit enhanced pathogenicity

T cell receptor (TCR) recognition is intrinsically polyspecific. In the field of autoimmunity, recognition of both self- and microbial peptides by a single TCR has led to the concept of molecular mimicry. However, findings made by our group and others clearly demonstrate that a given TCR can also recognize multiple distinct self-peptides. Based on experimental data we argue that recognition of several self-peptides increases the pathogenicity of an autoreactive T cell; a property we refer to as “cumulative autoimmunity.” The mechanisms of such increased pathogenicity, and the implications of cumulative autoimmunity regarding the pathophysiology of T cell-mediated autoimmune diseases will be discussed.

Myelin-specific B cell receptor transgenic mice reveal critical role for B cell accessory function independent of antibodies in pathogenesis of CNS autoimmune disease (148.2)

Human myelin oligodendrocyte glycoprotein (MOG) induces experimental autoimmune encephalomyelitis (EAE) in a B cell-dependent manner, and is associated with MOG-specific antibodies (Abs). Whether B cells serve as antigen presenting cells (APC) for activation of encephalitogenic MOG-specific T cells in EAE induced by MOG is unknown. To distinguish the role of APC function by MOG-specific B cells from MOG-specific Abs, we created MOG-specific B cell receptor (BCR) transgenic (Tg) mice containing B cells that express membrane MOG-specific Ig only (MOGmIg), but cannot secrete Abs. EAE was compared in MOGmIg mice, MOG-specific BCR knock-in (Th) mice that can secrete all isotypes, in B cell-deficient and wild-type (WT) mice. B cell-deficient mice were resistant, confirming the requirement for B cells in human MOG-induced EAE. MOG induced EAE in MOG BCR knock-in mice, WT mice and MOGmIg Tg mice, indicating that B cells provide an accessory (i.e. APC) function, independent of Ab secretion in MOG-induced EAE. To further evaluate B cell function, mixed bone marrow (bm) chimera mice were created that contained B cells that were selectively MHC II-deficient. Mixed bm chimera mice that contained MHC II+ B cells were susceptible to MOG-induced EAE, whereas mice containing MHC II- B cells were resistant, despite MHC II expression on other APC. Our results conclusively demonstrate that independent of their humoral role, B cells provide a critical cellular function in EAE pathogenesis.

Related B cell clones that populate the CSF and CNS of patients with multiple sclerosis produce CSF immunoglobulin

We investigated the overlap shared between the immunoglobulin (Ig) proteome of the cerebrospinal fluid (CSF) and the B cell Ig-transcriptome of CSF and the central nervous system (CNS) tissue of three patients with multiple sclerosis. We determined the IgG-proteomes of CSF by mass spectrometry, and compared them to the IgG-transcriptomes from CSF and brain lesions, which were analyzed by cDNA cloning. Characteristic peptides that were identified in the CSF-proteome could also be detected in the transcriptomes of both, brain lesions and CSF, providing evidence for a strong overlap of the IgG repertoires in brain lesions and in the CSF.

B cells in spontaneous autoimmune diseases of the central nervous system

B cells and their secreted products participate in the intricate network of pathogenic and regulatory immune responses. In human autoimmune diseases like rheumatoid arthritis, systemic lupus erythematosus and type 1 diabetes, a role for B cells and antibodies is well established. However, in multiple sclerosis (MS), despite the presence of autoantibodies, B cells were less considered as a major participant of autoimmune processes, until recently. Several lines of evidence now indicate a more active role for B cells in disease pathogenesis. In this review, we discuss the diverse roles of B cells in autoimmune diseases with particular focus on multiple sclerosis and its animal model, experimental autoimmune encephalomyelitis (EAE) as well as the recently generated spontaneous EAE mouse models.

Functional and pathogenic differences of Th1 and Th17 cells in experimental autoimmune encephalomyelitis

Background

There is consensus that experimental autoimmune encephalomyelitis (EAE) can be mediated by myelin specific T cells of Th1 as well as of Th17 phenotype, but the contribution of either subset to the pathogenic process has remained controversial. In this report, we compare functional differences and pathogenic potential of “monoclonal” T cell lines that recognize myelin oligodendrocyte glycoprotein (MOG) with the same transgenic TCR but are distinguished by an IFN-γ producing Th1-like and IL-17 producing Th17-like cytokine signature.

Methods and Findings

CD4⁺ T cell lines were derived from the transgenic mouse strain 2D2, which expresses a TCR recognizing MOG peptide 35–55 in the context of I-A^b. Adoptive transfer of Th1 cells into lymphopenic (Rag2^−/−) recipients, predominantly induced “classic” paralytic EAE, whereas Th17 cells mediated “atypical” ataxic EAE in approximately 50% of the recipient animals. Combination of Th1 and Th17 cells potentiated the encephalitogenicity inducing classical EAE exclusively. Th1 and Th17 mediated EAE lesions differed in their composition but not in their localization within the CNS. While Th1 lesions contained IFN-γ, but no IL-17 producing T cells, the T cells in Th17 lesions showed plasticity, substantially converting to IFN-γ producing Th1-like cells. Th1 and Th17 cells differed drastically by their lytic potential. Th1 but not Th17 cells lysed autoantigen presenting astrocytes and fibroblasts in vitro in a contact-dependent manner. In contrast, Th17 cells acquired cytotoxic potential only after antigenic stimulation and conversion to IFN-γ producing Th1 phenotype.

Conclusions

Our data demonstrate that both Th1 and Th17 lineages possess the ability to induce CNS autoimmunity but can function with complementary as well as differential pathogenic mechanisms. We propose that Th17-like cells producing IL-17 are required for the generation of atypical EAE whereas IFN-γ producing Th1 cells induce classical EAE.