Follicular regulatory T cells promote experimental autoimmune encephalomyelitis by supporting B cell egress from germinal centers

Follicular regulatory T cells (Tfr cells) constitute a subset of regulatory T cells pivotal to the immune response in germinal centers (GCs) that inhibit autoantibody production. Their role, however, remains ill-defined in autoimmune diseases like multiple sclerosis (MS) and its murine model, experimental autoimmune encephalomyelitis (EAE), which are neuroinflammatory diseases driven by T and B cells. Here, we quantified peripheral blood immune subpopulations in two cohorts of patients with MS and found higher circulating Tfr cell frequencies in patients in relapse compared with patients in remission. To examine the functional role of Tfr cells in autoimmune neuroinflammation, we used EAE mouse models and showed that Foxp3cre/creBcl6fl/fl Tfr cell-deficient mice developed milder EAE than wild-type (WT) mice. Flow cytometry analysis demonstrated that the reduction of encephalomyelitis in Tfr cell-deficient mice was associated with fewer B cells infiltrating the central nervous system. Coculture experiments showed that B cells isolated from brains of WT mice at the peak of the disease fostered pro-inflammatory cytokine production by myelin oligodendrocyte glycoprotein-specific T cells. We furthermore showed that sphingosine-1-phosphate receptor 2 (S1PR2) expression in GC B cells was up-regulated in Tfr cell-deficient mice. Treatment with an S1PR2 receptor antagonist abrogated the improved EAE clinical scores in Tfr cell-deficient mice, and this loss of protection was associated with increased B cell infiltration into the brain and increased pro-inflammatory cytokine production by encephalitogenic T cells. These findings demonstrate that Tfr cells contribute to autoimmune encephalomyelitis and suggest that their blood frequency reflects MS activity.