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Novel insights from basic and translational studies are reshaping concepts of the immunopathogenesis of multiple sclerosis and understanding of the different inflammatory responses throughout the disease course. Previously, the cellular immunology of relapsing multiple sclerosis was considered to be principally T-cell driven; however, this process is now understood to involve multiple cell types and their functionally distinct subsets. Particularly, relapsing multiple sclerosis appears to involve imbalanced interactions between T cells, myeloid cells, B cells, and their effector and regulatory subpopulations. The major contributors to such imbalances differ across patients. Several emerging techniques enable comprehensive immune cell profiling at the single-cell level, revealing substantial functional heterogeneity and plasticity that could influence disease state and response to treatment. Findings from clinical trials with agents that successfully limit new multiple sclerosis disease activity and trials of agents that inadvertently exacerbate CNS inflammation have helped to elucidate disease mechanisms, better define the relevant modes of action of current immune therapies, and pave the way for new therapeutic strategies.

The clinical features, diagnostic challenges, neuroimaging appearance, therapeutic options, and pathobiological research progress in childhood—and adolescent—onset multiple sclerosis have been informed by many new insights in the past 7 years. National programmes in several countries, collaborative research efforts, and an established international paediatric multiple sclerosis study group have contributed to revised clinical diagnostic definitions, identified clinical features of multiple sclerosis that differ by age of onset, and made recommendations regarding the treatment of paediatric multiple sclerosis. The relative risks conveyed by genetic and environmental factors to paediatric multiple sclerosis have been the subject of several large cohort studies. MRI features have been characterised in terms of qualitative descriptions of lesion distribution and applicability of MRI aspects to multiple sclerosis diagnostic criteria, and quantitative studies have assessed total lesion burden and the effect of the disease on global and regional brain volume. Humoral-based and cell-based assays have identified antibodies against myelin, potassium-channel proteins, and T-cell profiles that support an adult-like T-cell repertoire and cellular reactivity against myelin in paediatric patients with multiple sclerosis. Finally, the safety and efficacy of standard first-line therapies in paediatric multiple sclerosis populations are now appreciated in more detail, and consensus views on the future conduct and feasibility of phase 3 trials for new drugs have been proposed.

Objective To identify gut microbiome features associated with MRI lesion burden in persons with pediatric‐onset multiple sclerosis (symptom onset <18 years). Methods A cross‐sectional study involving the Canadian Paediatric Demyelinating Disease Network study participants. Gut microbiome features (alpha diversity, phylum‐ and genus‐level taxa) were derived using 16S rRNA sequencing from stool samples. T1‐ and T2‐weighted lesion volumes were measured on brain MRI obtained within 6 months of stool sample procurement. Associations between the gut microbiota and MRI metrics (cube‐root‐transformed) were assessed using standard and Lasso regression models. Results Thirty‐four participants were included; mean ages at symptom onset and MRI were 15.1 and 19.0 years, respectively, and 79% were female. The T1‐ and T2‐weighted lesion volumes were not significantly associated with alpha diversity (age and sex‐adjusted p > 0.08). At the phylum level, high Tenericutes (relative abundance) was associated with higher T1 and T2 volumes (β coefficient = 0.25, 0.37) and high Firmicutes, Patescibacteria or Actinobacteria with lower lesion volumes (β coefficient = −0.30 to −0.07). At the genus level, high Ruminiclostridium, whereas low Coprococcus 3 and low Erysipelatoclostridium were associated with higher lesion volumes. Interpretation Our study characterized the gut microbiota features associated with MRI lesion burden in pediatric‐onset MS, shedding light onto possible pathophysiological mechanisms.

To investigate the serologic response, predictors of response, and clinical outcomes associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination and infection in ozanimod-treated participants with relapsing multiple sclerosis (RMS) from DAYBREAK. DAYBREAK (ClinicalTrials.gov-NCT02576717), an open-label extension study of oral ozanimod 0.92 mg, enrolled participants aged 18-55 years with RMS who completed phase 1-3 ozanimod trials. Participants who were fully vaccinated against SARS-CoV-2 with mRNA or non-mRNA vaccines, were unvaccinated, and/or had COVID-19-related adverse events (AEs, with or without vaccination) and postvaccination serum samples were included (n = 288). Spike receptor binding domain (RBD) antibody levels (seroconversion: ≥0.8 U/mL) and serologic evidence of SARS-CoV-2 infection (nucleocapsid IgG: ≥1 U/mL) were assessed (Roche Elecsys/Cobas e411 platform). In fully vaccinated participants (n = 148), spike RBD antibody seroconversion occurred in 90% (n = 98/109) of those without serologic evidence of prior SARS-CoV-2 exposure (100% [n = 80/80] seroconversion after mRNA vaccination) and in 100% (n = 39/39) of participants with serologic evidence of viral exposure. mRNA vaccination predicted higher spike RBD antibody levels, whereas absolute lymphocyte count (ALC), age, body mass index, and sex did not. COVID-19-related AEs were reported in 10% (n = 15/148) of fully vaccinated participants-all were nonserious and not severe; all participants recovered. Most ozanimod-treated participants with RMS mounted a serologic response to SARS-CoV-2 vaccination and infection, regardless of participant characteristics or ALC levels. In this analysis, all COVID-19-related AEs post-full vaccination in participants taking ozanimod were nonserious and not severe.

Objective The relationship between multiple sclerosis and the gut microbiome has been supported by animal models in which commensal microbes are required for the development of experimental autoimmune encephalomyelitis. However, observational study findings in humans have only occasionally converged when comparing multiple sclerosis cases and controls which may in part reflect confounding by comorbidities and disease duration. The study of microbiome in pediatric‐onset multiple sclerosis offers unique opportunities as it is closer to biological disease onset and minimizes confounding by comorbidities and environmental exposures. Methods A multicenter case–control study in which 35 pediatric‐onset multiple sclerosis cases were 1:1 matched to healthy controls on age, sex, self‐reported race, ethnicity, and recruiting site. Linear mixed effects models, weighted correlation network analyses, and PICRUSt2 were used to identify microbial co‐occurrence networks and for predicting functional abundances based on marker gene sequences. Results Two microbial co‐occurrence networks (one reaching significance after adjustment for multiple comparisons; q < 0.2) were identified, suggesting interdependent bacterial taxa that exhibited association with disease status. Both networks indicated a potentially protective effect of higher relative abundance of bacteria observed in these clusters. Functional predictions from the significant network suggested a contribution of short‐chain fatty acid producers through anaerobic fermentation pathways in healthy controls. Consistent family‐level findings from an independent Canadian‐US study (19 case/control pairs) included Ruminococaccaeae and Lachnospiraceae (p < 0.05). Macronutrient intake was not significantly different between cases and controls, minimizing the potential for dietary confounding. Interpretation Our results suggest that short‐chain fatty acid producers may be important contributors to multiple sclerosis onset.

There is growing recognition that B cell contributions to normal immune responses extend well beyond their potential to become antibody-producing cells, including roles at the innate–adaptive interface and their potential to modulate the responses of other immune cells such as T cells and myeloid cells. These B cell functions can have both pathogenic and protective effects in the context of central nervous system (CNS) inflammation. Here, we review recent advances in the field of multiple sclerosis (MS), which has traditionally been viewed as primarily a T cell–mediated disease, and we consider antibody-dependent and, particularly, emerging antibody-independent functions of B cells that may be relevant in both the peripheral and CNS disease compartments. In this Focus Review, Bar-Or and colleagues discuss the latest evidence that B cells play an important antibody-independent role in multiple sclerosis and the prospects this holds for therapeutic intervention.

Purpose/Introduction The anti‐JC virus (JCV) antibody index is used to stratify the risk of progressive multifocal leukoencephalopathy (PML) in multiple sclerosis (MS), particularly with natalizumab therapy. B‐cell‐depleting therapies may alter antibody levels and potentially confound the estimation of PML risk. This study evaluated the effect of ocrelizumab on anti‐JCV antibody indices during the first 2 years of treatment. Methods: We conducted a retrospective cohort study of 553 MS patients who initiated ocrelizumab between 2017 and 2019. Anti‐JCV antibody indices were measured using the STRATIFY JCV assay at baseline and approximately every 6 months prior to subsequent infusions. Linear mixed‐effects models were used to assess longitudinal changes in log‐transformed JCV indices, adjusting for age, sex, and ethnicity. Secondary analyses evaluated serum B‐cell counts and immunoglobulin levels. Sensitivity analyses addressed missing data. Results/Finding There was no significant change in anti‐JCV antibody index over time following ocrelizumab initiation (mean percent change per infusion cycle −0.049%, 95% CI: −0.449 to 0.351; p = 0.81). Results were consistent across sensitivity analyses and did not differ by age or sex. In contrast, peripheral B‐cell counts declined significantly after treatment initiation and remained suppressed. Serum IgG, IgM, and IgA levels decreased modestly but significantly over time. JCV seroconversion occurred in 4.1% of initially seronegative patients, while seroreversion occurred in 6.6% of initially seropositive patients. Conclusion: Anti‐JCV antibody indices remain stable during the first 2 years of ocrelizumab treatment despite marked B‐cell depletion and modest reductions in serum immunoglobulins. These findings suggest that existing PML risk stratification tools based on anti‐JCV antibody indices remain applicable during early ocrelizumab therapy. Synopsis: This study of 553 MS patients found that anti‐JC virus (JCV) antibody indices remain stable during the first 2 years of ocrelizumab treatment. While peripheral B‐cells were significantly depleted and serum immunoglobulins (IgG, IgM, IgA) showed modest declines, the JCV antibody index did not change significantly. These results suggest that existing PML risk stratification tools based on JCV indices remain reliable during early ocrelizumab.

Objective To examine the gut microbiota in individuals with and without pediatric‐onset multiple sclerosis (MS). Methods We compared stool‐derived microbiota of Canadian Pediatric Demyelinating Disease Network study participants ≤21 years old, with MS (disease‐modifying drug [DMD] exposed and naïve) or monophasic acquired demyelinating syndrome [monoADS] (symptom onset <18 years), and unaffected controls. All were ≥30 days without antibiotics or corticosteroids. V4 region 16S RNA gene‐derived amplicon sequence variants (Illumina MiSeq) were assessed using negative binomial regression and network analyses; rate ratios were age‐ and sex‐adjusted (aRR). Results Thirty‐two MS, 41 monoADS (symptom onset [mean] = 14.0 and 6.9 years) and 36 control participants were included; 75%/56%/58% were female, with mean ages at stool sample = 16.5/13.8/15.1 years, respectively. Nine MS cases (28%) were DMD‐naïve. Although microbiota diversity (alpha, beta) did not differ between participants (p > 0.1), taxa‐level and gut community networks did. MS (vs. monoADS) exhibited > fourfold higher relative abundance of the superphylum Patescibacteria (aRR = 4.2;95%CI:1.6–11.2, p = 0.004, Q = 0.01), and lower abundances of short‐chain fatty acid (SCFA)‐producing Lachnospiraceae (Anaerosporobacter) and Ruminococcaceae (p, Q < 0.05). DMD‐naïve MS cases were depleted for Clostridiales vadin‐BB60 (unnamed species) versus either DMD‐exposed, controls (p, Q < 0.01), or monoADS (p = 0.001, Q = 0.06) and exhibited altered community connectedness (p < 10−9 Kruskal–Wallis), with SCFA‐producing taxa underrepresented. Consistent taxa‐level findings from an independent US Network of Pediatric MS Centers case/control (n = 51/42) cohort included >eightfold higher abundance for Candidatus Stoquefichus and Tyzzerella (aRR = 8.8–12.8, p < 0.05) in MS cases and 72%–80% lower abundance of SCFA‐producing Ruminococcaceae‐NK4A214 (aRR = 0.38–0.2, p ≤ 0.01). Interpretation Gut microbiota community structure, function and connectivity, and not just individual taxa, are of likely importance in MS.

Throughout the course of multiple sclerosis, gradually progressive neurologic impairment can occur, which has been called disability accrual. Current disease-modifying therapies for multiple sclerosis have limited effects on disability accrual unrelated to relapses, which is thought to be partially caused by chronic, nonresolving neuroinflammation within the central nervous system. Tolebrutinib is an oral, brain-penetrant Bruton's tyrosine kinase inhibitor that targets myeloid cells (including microglia) and B cells in both the periphery and central nervous system. There are no approved treatments for nonrelapsing secondary progressive multiple sclerosis. In a phase 3, double-blind, placebo-controlled, event-driven trial, we randomly assigned participants with nonrelapsing secondary progressive multiple sclerosis, in a 2:1 ratio, to receive tolebrutinib (60 mg once daily) or matching placebo. The primary end point was confirmed disability progression that was sustained for at least 6 months, assessed in a time-to-event analysis. A total of 1131 participants underwent randomization: 754 were assigned to receive tolebrutinib and 377 to receive placebo. The median follow-up was 133 weeks. A smaller percentage of participants in the tolebrutinib group than in the placebo group had confirmed disability progression sustained for at least 6 months (22.6% vs. 30.7%; hazard ratio, 0.69; 95% confidence interval, 0.55 to 0.88; P = 0.003). Serious adverse events occurred in 15.0% of the participants in the tolebrutinib group and in 10.4% of those in the placebo group. A total of 4.0% of the participants in the tolebrutinib group and 1.6% of those in the placebo group had increases in alanine aminotransferase levels to more than 3 times the upper limit of the normal range. In participants with nonrelapsing secondary progressive multiple sclerosis, the risk of disability progression was lower among those who received treatment with tolebrutinib than among those who received placebo. (Funded by Sanofi; HERCULES ClinicalTrials.gov number, NCT04411641.).

Oral treatment options for disease-modifying therapy in relapsing multiple sclerosis have substantially increased over the past decade with four approved oral compounds now available: fingolimod, dimethyl fumarate, teriflunomide, and cladribine. Although these immunomodulating therapies are all orally administered, and thus convenient for patients, they have different modes of action. These distinct mechanisms of action allow better adaption of treatments according to individual comorbidities and offer different mechanisms of treatment such as inhibition of immune cell trafficking versus immune cell depletion, thereby substantially expanding the available treatment options. New sphingosine-1-phosphate receptor (S1PR) modulators with more specific S1PR target profiles and potentially better safety profiles compared with fingolimod were tested in patients with relapsing multiple sclerosis. For example, siponimod, which targets S1PR1 and S1PR5, was approved in March, 2019, by the US Food and Drug Administration for the treatment of relapsing multiple sclerosis including active secondary progressive multiple sclerosis. Ozanimod, another S1P receptor modulator in the approval stage that also targets S1PR1 and S1PR5, reduced relapse rates and MRI activity in two phase 3 trials of patients with relapsing multiple sclerosis. Blocking of matrix metalloproteinases or tyrosine kinases are novel modes of action in the treatment of relapsing multiple sclerosis, which are exhibited by minocycline and evobrutinib, respectively. Minocycline reduced conversion to multiple sclerosis in patients with a clinically isolated syndrome. Evobrutinib reduced MRI activity in a phase 2 trial, and a phase 3 trial is underway, in patients with relapsing multiple sclerosis. Diroximel fumarate is metabolised to monomethyl fumarate, the active metabolite of dimethyl fumarate, reduces circulating lymphocytes and modifies the activation profile of monocytes, and is being tested in this disease with the aim to improve gastrointestinal tolerability. The oral immunomodulator laquinimod did not reach the primary endpoint of reduction in confirmed disability progression in a phase 3 trial of patients with relapsing multiple sclerosis. In a phase 2 trial of patients with primary progressive multiple sclerosis, laquinimod also did not reach the primary endpoint of a reduction in brain volume loss, as a consequence the development of this drug will probably not be continued in multiple sclerosis. Several new oral compounds are in late-stage clinical development. With new modes of action introduced to the treatment of multiple sclerosis, the question of how to select and sequence different treatments in individual patients arises. Balancing risks with the expected efficacy of disease-modifying therapies will still be key for treatment selection. However, risks as well as efficacy can change when moving from the controlled clinical trial setting to clinical practice. Because some oral treatments, such as cladribine, have long-lasting effects on the immune system, the cumulative effects of sequential monotherapies can resemble the effects of a concurrent combination therapy. This treatment scheme might lead to higher efficacy but also to new safety concerns. These sequential treatments were largely excluded in phase 2 and 3 trials; therefore, monitoring both short-term and long-term effects of sequential disease-modifying therapies in phase 4 studies, cohort studies, and registries will be necessary.