Early multiple sclerosis activity associated with TBX21+CD21loCXCR3+ B cell expansion resembling EBV-induced phenotypes.

Epstein-Barr virus (EBV) infection precedes multiple sclerosis (MS) onset and plays a poorly understood etiologic role. To investigate possible viral pathogenesis, we analyzed single-cell expression in peripheral B cells from people with early MS collected longitudinally during the Immune Tolerance Network STAyCIS Trial. Expression profiles were compared with single-cell RNA-Seq (scRNA-Seq) from in vitro EBV models, autoimmune disorders, chronic infectious diseases, and healthy controls. Analyses focused on CD19+CD20+CD21loCD11c+T-bet+ atypical B cells (ABCs). ABCs were significantly enriched in early MS PBMCs versus healthy controls by scRNA-Seq and flow cytometry, establishing ABC expansion as a clinical feature. EBV-associated ABC expression, including CXCR3, programmed cell death ligand 1 (PD-L1), and PD-L2, was enriched in early MS; however, direct EBV infection of ABCs was not detected. Early MS ABCs exhibited significantly upregulated inflammatory cytokine mRNAs (CXCL8, IL18, VEGFA). Further, de novo EBV-infected B cells secreted IL-8 and VEGF. MS activity stratification revealed rare, distinctive inflammatory ABCs significantly underrepresented in individuals with no evidence of activity long-term versus people with additional relapsing-remitting MS activity at the primary endpoint. Moreover, CXCR3+ ABCs increased after baseline diagnosis and were significantly enriched in people with disease exacerbation during the study. Thus, ABC expansion and inflammatory responses correlate to early MS activity, possibly as a bystander response to EBV.