The identification of autoantibodies against aquaporin-4 (AQP4) and myelin oligodendrocyte glycoprotein (MOG) has been essential in distinguishing neuromyelitis optica spectrum disorder (NMOSD) and MOG antibody-associated disease (MOGAD) from classical multiple sclerosis (MS), with important implications for treatment. However, for several patients within this disease spectrum, the target of the autoimmune response remains unknown. Here, we describe the modulator of VRAC current 1 (MLC1), a membrane protein with extracellular epitopes enriched at astrocytic end feet, as an autoantigen. Serum MLC1 antibodies were verified with a cell-based assay, identifying four MLC1 immunoglobulin G (IgG)-positive patients among 297 patients with inflammatory autoimmune diseases of the central nervous system who were screened. All four MLC1 IgG-positive patients exhibited overlapping yet atypical clinical features of MS and NMOSD and tested negative for AQP4 and MOG antibodies. Moreover, treatment with a monoclonal MLC1 antibody induced astrocytopathy in mouse cerebellar slice cultures and in a rat encephalitis model. Thus, MLC1 antibodies identify a subset of patients with an NMOSD-like phenotype, underscoring their potential as a disease marker with pathogenic relevance.

Cerebellar ataxias (CAs) encompass a wide spectrum of genetic and sporadic origins, of which a portion is driven by immune-mediated pathomechanisms. While some patients harbor known autoantibodies, assisting diagnosis and treatment, many remain seronegative. Here, we identify synaptophysin (SYP) as an autoantigen in CA by serum IgG staining on primate tissue, combined with human protein array and cell-based assay (CBA). SYP is abundant in presynaptic vesicles and is transiently exposed on the cell surface. Out of 43 patients with CA with synaptic serum reactivity on cerebellar sections, SYP antibodies were identified by CBA in 2 patients. Exposure of human induced pluripotent stem cell (iPSC)-derived glutamatergic neurons to patient’s IgG with SYP antibodies causes selective SYP accumulation at the presynaptic membrane. Patient’s IgG and SYP monoclonal antibody reduce neuronal populational activity recorded by multi-electrode array. Altogether, we identify SYP as an autoantigen for further stratifying patients with CA. Our functional experiments with SYP antibodies uncover a previously unrecognized mechanism of antibody-mediated synaptic dysfunction.

In pDCs, BTK-inhibition (BTKi) blocks the IFN-α production via TLR-9, but not via TLR-7. Upon TLR-7 stimulation, BTKi enhances the production of IFN-α by blocking the inhibitory BDCA-2 pathway. This might explain partially the failure of BTKi in SLE and is of interest for BTKi trials in multiple sclerosis.

While CD20 was initially characterized as a B cell-specific marker, its expression on memory T cells has expanded our understanding of this molecule's distribution and function. Here, we identify a previously unrecognized CD20-expressing NK cell population and demonstrate its functional significance. CD56+CD20+ NK cells exhibit hallmarks of cellular activation, including elevated NKp46, CD69, and CD137 expression, enhanced proliferative capacity, and increased production of inflammatory cytokines (IFN-γ, GM-CSF, TNF-α, IL-10). Functional analyses revealed enhanced cytotoxicity against K562 targets, correlating with increased expression of cytolytic mediators including granzymes A, B, and K, perforin, FASL, and TRAIL. Single-cell transcriptional profiling demonstrated that MS4A1-expressing NK cells possess a distinct molecular signature characterized by elevated granzyme K expression and memory-like features. These cells preferentially localize to secondary lymphoid organs and accumulate in inflammatory tissues. Notably, CD56+CD20+ NK cells are enriched in multiple inflammatory conditions, including multiple sclerosis, autoimmune hepatitis, hepatitis B infection, hepatocellular carcinoma, and lung cancer. Treatment with rituximab depletes this population, suggesting potential therapeutic implications. Our findings establish CD20+ NK cells as a functionally distinct lymphocyte subset with enhanced effector capabilities and tissue-homing properties, providing new insights into immune regulation in inflammatory diseases.

Ravulizumab (ULTOMIRIS™) an intravenous glycoengineered humanized anti-C5 IgG2/4 monoclonal antibody (mAb), is a new FDA-approved treatment for Aquaporin-4-antibody (AQP4-IgG) positive Neuromyelitis Optica Spectrum Disorder (NMOSD). Based on the importance of intermediate and terminal components of the complement cascade during disease pathogenesis, in the last few years, a mAb targeting the C5 complement factor (anti-C5, eculizumab) has already been in use for treating AQP4-IgG positive NMOSD. Ravulizumab acts similarly to eculizumab, binding specifically to the complement protein C5, thereby blocking the generation of the anaphylatoxin C5a and of C5b, which is crucial for generating the membrane attack complex (C5b-9). Here, we discuss the main findings obtained during the phase 3 clinical trial (CHAMPIONNMOSD [NCT0420126]) for ravulizumab and new developments in anti-complement therapy, with other complement inhibitors for the treatment of autoimmune diseases and paroxysmal nocturnal hemoglobinuria (PNH) (zilucoplan, avacopan and, pegcetacoplan). The approval of the new long-acting anti-C5 mAb adds another therapeutic option to the already existing inhibitors of complement currently in use, increasing therapeutic options for inflammatory and autoimmune diseases.

The mucosal immunity is crucial for restricting SARS-CoV-2 at its entry site. Intramuscularly applied vaccines against SARS-CoV-2 stimulate high levels of neutralizing Abs in serum, but the impact of these intramuscular vaccinations on features of mucosal immunity is less clear. Here, we analyzed kinetic and functional properties of anti-SARS-CoV-2 Abs in the saliva after vaccination with BNT162b2. We analyzed a total of 24 healthy donors longitudinally for up to 16 months. We found that specific IgG appeared in the saliva after the second vaccination, declined thereafter and reappeared after the third vaccination. Adjusting serum and saliva for the same IgG concentration revealed a strong correlation between the reactivity in these two compartments. Reactivity to VoCs correlated strongly as seen by ELISAs against RBD variants and by live-virus neutralizing assays against replication-competent viruses. For further functional analysis, we purified IgG and IgA from serum and saliva. In vaccinated donors we found neutralizing activity towards authentic virus in the IgG, but not in the IgA fraction of the saliva. In contrast, IgA with neutralizing activity appeared in the saliva only after breakthrough infection. In serum, we found neutralizing activity in both the IgA and IgG fractions. Together, we show that intramuscular mRNA vaccination transiently induces a mucosal immunity that is mediated by IgG and thus differs from the mucosal immunity after infection. Waning of specific mucosal IgG might be linked to susceptibility for breakthrough infection.

Myelin oligodendrocyte glycoprotein (MOG)-antibody (Ab)-associated disease (MOGAD) is an inflammatory demyelinating disease of the CNS. Although MOG is encephalitogenic in different mammalian species, the mechanisms by which human MOG-specific Abs contribute to MOGAD are poorly understood. Here, we use a systems-level approach combined with high-dimensional characterization of Ab-associated immune features to deeply profile humoral immune responses in 123 patients with MOGAD. We show that age is a major determinant for MOG-antibody-related immune signatures. Unsupervised clustering additionally identifies two dominant immunological endophenotypes of MOGAD. The pro-inflammatory endophenotype characterized by increased binding affinities for activating Fcγ receptors (FcγRs), capacity to activate innate immune cells, and decreased frequencies of galactosylated and sialylated immunoglobulin G (IgG) glycovariants is associated with clinically active disease. Our data support the concept that FcγR-mediated effector functions control the pathogenicity of MOG-specific IgG and suggest that FcγR-targeting therapies should be explored for their therapeutic potential in MOGAD.

The BAFF-APRIL system is crucial for the pathogenesis of systemic lupus erythematosus (SLE) by promoting B cell survival, differentiation and the maintenance of humoral autoimmunity. Here, we investigated the relationship of BCMA expression on B cell subsets with its ligands BAFF and APRIL, together with soluble BCMA, and with clinical and serologic variables in a cohort of 100 SLE patients (86 under conventional and 14 under belimumab therapy) and 30 healthy controls (HCs) using multicolor flow cytometry and ELISA. We found that BCMA expression in SLE patients was significantly increased on all B cell subsets compared to HCs, with all examined components of the BAFF-APRIL system being upregulated. BCMA expression was significantly increased on switched and unswitched memory B cells compared to naïve B cells, both in HCs and SLE. BCMA expression on B cells correlated with plasmablast frequencies, serum anti-dsDNA antibodies and complement consumption, while soluble BCMA correlated with plasmablast frequency, highlighting its potential as a clinical biomarker. Belimumab treatment significantly reduced BCMA expression on most B cell subsets and soluble TACI and contributed to the inhibition of almost the entire BAFF-APRIL system and restoration of B cell homeostasis. These results provide insights into the complex dysregulation of the BAFF-APRIL system in SLE and highlight the therapeutic potential of targeting its components, particularly BCMA, in addition to its use as a biomarker for disease activity.

Myelin sheaths comprise compacted layers of oligodendroglial membrane wrapped spirally around axons. Each sheath, if imagined unwrapped, has a cytoplasm-filled space at its perimeter, linking it to the oligodendrocyte soma via a short process. By electron microscopy (EM), this space, which we term the ‘ myelinic channel system ’ contains microtubules and membranous organelles, but whether these are remnants of development or serve a function is unknown. Performing live imaging of myelinating oligodendrocytes expressing fluorescent reporters, we found that the myelinic channel system serves microtubule-dependent organelle transport. Further, the intra-myelinic movement of peroxisomes was modulated by neuronal electrical activity in these mixed neural cell cultures. Loss of oligodendroglial Kif21b or CNP in vivo led to apparent stasis of myelin organelles and secondary axon pathology. This suggests that oligodendrocytes require motor transport in the myelinic channel system to maintain axonal integrity. ### Competing Interest Statement The authors have declared no competing interest.

Autoimmune neuropathy associated with antibodies against pan-neurofascin is a new subtype of nodo-paranodopathy. It is relevant because it is associated with high morbidity and mortality. Affected patients often require intensive care unit treatment for several months, and data on the reversibility and long-term prognosis are limited. The pathogenicity including IgG subclass-associated mechanisms has not been unravelled, nor directly compared to anti-neurofascin-155 IgG4-related pathology. Understanding the underlying pathology might have a direct impact on treatment of these severely affected patients. By a multicentre combined prospective and retrospective approach, we provide clinical data of a large cohort of patients with anti-neurofascin-associated neuropathy (n = 18) including longitudinal titre and neurofilament light chain assessment via Ella® and relate clinical data to in vitro pathogenicity studies of anti-neurofascin antibodies. We assessed antibody binding characteristics and the pathogenic effects of anti-pan-neurofascin versus neurofascin-155 antibodies on living myelinating dorsal root ganglia co-cultures. Additionally, we analysed the IgG subclass profile and the complement binding capacity and effector functions considering the effects of intravenous immunoglobulin preparations via enzyme-linked immunosorbent and cell-based assays. In contrast to chronic neurofascin-155 IgG4-associated neuropathy, anti-pan-neurofascin-associated disease presented with a high morbidity and mortality, but as a monophasic and potentially reversible disorder. During follow-up, antibodies were no longer detectable in 8 of 11 patients. Anti-pan-neurofascin had direct access to the nodes of Ranvier in myelinating cultures titre-dependently, most probably inducing this severe phenotype. Antibody preincubation led to impaired paranode formation, destruction of paranodal architecture and alterations on paranodal myelin and sensory neurons in the cultures, with more severe effects than neurofascin-155 antibodies. Besides IgG4, subclass IgG3 was detected and associated with complement binding and cytotoxic effects in vitro. As a possible correlate of axonal damage in vivo, we detected highly increased serum neurofilament light chain levels (sNF-L), correlating to serum C3a. Still, sNF-L was not identified as a marker for poor prognosis, but rather as an intra- and interindividual marker for acuteness, severity and course, with a strong decrease during recovery. Our data provide evidence that anti-pan-neurofascin antibodies directly attack the node and induce severe and acute, but potentially reversible, nodo-paranodal pathology, possibly involving complement-mediated mechanisms. Screening for autoantibodies thus is crucial to identify this subset of patients who benefit from early antibody-depleting therapy. Titre and sNF-L might serve as valuable follow-up parameters. The prospect of a favourable outcome has high relevance for physicians, patients and relatives during months of critical care.

Ublituximab, an intravenous glycoengineered chimeric anti-CD20 IgG1 monoclonal antibody (mAb), is a new FDA-approved treatment for relapsing forms of Multiple Sclerosis (MS). Reassembling the other three anti-CD20 mAbs already in use for MS (rituximab, ocrelizumab and ofatumumab), ublituximab leads to depletion of B cells but spars long-lived plasma cells. Here, we discuss the main findings obtained during the phase 3 clinical trials (ULTIMATE I and II) for ublituximab versus teriflunomide. The current emergence and approval of new anti-CD20 mAbs with different dose regimens, routes of application, glycoengineering and mechanisms of action may contribute to different clinical outcomes.

Demyelinating disorders of the central nervous system (CNS) occur when myelin and oligodendrocytes are damaged or lost. Remyelination and regeneration of oligodendrocytes can be achieved from endogenous oligodendrocyte precursor cells (OPCs) that reside in the adult CNS tissue. Using a cuprizone mouse model of demyelination, we show that infusion of fractalkine (CX3CL1) into the demyelinated murine brain increases de novo oligodendrocyte formation and enhances remyelination in the corpus callosum and cortical gray matter. This is achieved by increased OPC proliferation in the cortical gray matter as well as OPC differentiation and attenuation of microglia/macrophage activation both in corpus callosum and cortical gray matter. Finally, we show that activated OPCs and microglia/macrophages express fractalkine receptor CX3CR1 in vivo, and that in OPC-microglia co-cultures fractalkine increases in vitro oligodendrocyte differentiation by modulating both OPC and microglia biology. Our results demonstrate a novel pro-regenerative role of fractalkine in a demyelinating mouse model.

BACKGROUND AND OBJECTIVES:Antibodies (Abs) against the cytoplasmic protein glutamic acid decarboxylase 65 (GAD65) are detected in patients with neurologic syndromes together referred to as GAD65-Ab spectrum disorders. The response of some of these patients to plasma exchange or immunoglobulins indicates that GAD65-Abs could contribute to disease pathogenesis at least at some stages of disease. However, the involvement of GAD65-reactive B cells in the CNS is incompletely understood.METHODS:We studied 7 patients with high levels of GAD65-Abs and generated monoclonal Abs (mAbs) derived from single cells in the CSF. Sequence characteristics, reactivity to GAD65, and the role of somatic hypermutations of the mAbs were analyzed.RESULTS:Twelve CSF-derived mAbs were generated originating from 3 patients with short disease duration, and 7/12 of these mAbs (58%) were GAD65 reactive in at least 1 detection assay. Four of 12 (33%) were definitely positive in all 3 detection assays. The intrathecal anti-GAD65 response was polyclonal. GAD65-Abs were mostly of the IgG1 subtype and had undergone affinity maturation. Reversion of 2 GAD65-reactive mAbs to their corresponding germline-encoded unmutated common ancestors abolished GAD65 reactivity.DISCUSSION:GAD65-specific B cells are present in the CNS and represent a sizable fraction of CSF B cells early in the disease course. The anti-GAD65 response in the CSF is polyclonal and shows evidence of antigen-driven affinity maturation required for GAD65 recognition. Our data support the hypothesis that the accumulation of GAD65-specific B cells and plasma cells in the CSF is an important feature of early disease stages.

This scientific commentary refers to ‘Temporal lobe epilepsy with GAD antibodies: neurons killed by T cells not by complement membrane attack complex’ by Tröscher et al. (https://doi.org/10.1093/brain/awac404).