BAFF and APRIL signaling through B cell maturation antigen affects regulatory transitional B cells in multiple sclerosis.

Abstract Multiple sclerosis (MS) is a debilitating neuro-autoimmune disease that can be stratified into relapsing remitting (RR) MS, secondary progressive (SP) MS, and primary progressive (PP) MS. B cells can have both inflammatory and regulatory functions in MS. Transitional B cells are considered to have regulatory functions in MS. BAFF and APRIL, two cytokines, signals through the receptors BAFFR, TACI and BCMA and are thought to drive the regulatory function of transitional B cells. Here, we assessed the frequencies and expression of BAFFR, TACI and BCMA on the surface of regulatory transitional B cells in the blood of RRMS, PPMS and healthy individuals along with soluble versions of these receptors in the plasma. We found that transitional B cells were decreased in both RRMS and PPMS patients. The surface expression of BCMA was significantly lower on transitional B cells from RRMS patients but not PPMS patients compared to healthy individuals with no change in the expression of TACI and BAFFR. In addition, we found elevated levels of soluble BCMA in blood of PPMS patients but not RRMS patients compared to healthy and no difference in soluble BAFFR and TACI. Our data suggests that reduced BCMA signaling is associated with reduced numbers of transitional B cells in RRMS and PPMS. In RRMS, expression of BCMA is reduced on regulatory transitional B cells. In PPMS patients, there are higher levels of soluble BCMA which could act as a blocker of APRIL signaling through BCMA on the surface of transitional B cells. Altogether, our data suggests that reduced BCMA signaling in transitional B cells is contributing to the MS disease and the dysregulation of the BCMA signaling in RRMS and PPMS may be through a different mechanism. This study was supported by grants from the NMSS (RG-1602-07722), the NIH (R01AI137047 and R01EY027346) awarded to R.C. Axtell.