B Cell Maturation Antigen deficiency increases inflammatory B-cell functions in multiple sclerosis

Abstract

Abstract B cells have emerged as a therapeutic target for multiple sclerosis (MS). Depleting B cells with anti-CD20 antibodies are effective in treating MS. Yet, TACI-Ig treatment, which blocks BAFF and APRIL (two cytokines important for B cell development and function), paradoxically increased disease activity in MS patients. The stark differences in clinical outcomes with these therapies demonstrate that B cells have both inflammatory and regulatory effects in MS. B cell Maturation Antigen (BCMA), a receptor that binds BAFF and APRIL, plays a prominent role in B cell function. The role BCMA plays in MS remains enigmatic and therefore we explored the impact BCMA deficiency has on the development and progression of the experimental autoimmune encephalomyelitis (EAE), a rodent model of MS. Here, we identified that the expression of BCMA is dysregulated in B cells from MS patients and is important in inhibiting inflammatory B cell responses in EAE. We discovered that BCMA expression is decreased on transitional B cells in MS. In mice we found that EAE induced with MOG35–55 was severe in BCMA−/− mice compared to BCMA+/+. The increased disease was associated with increased CNS inflammation, significantly reduced transitional B cells and a skewing of B-cells toward a mature/inflammatory phenotype. Bone marrow chimeric and in vitro experiments demonstrated that BCMA directly inhibits memory B cell expansion and anti-inflammatory cytokine production by B cells. Strikingly, BCMA impacts the efficacy of B cell depleting therapies, as anti-CD20 antibody treatment attenuated EAE in BCMA-deficient mice but not BCMA-sufficient mice. These data provide novel insights into B-cell dysregulation in MS.