While no associations were demonstrated with the abundance of IgA+ MBCs or BAFF-R expression, we did find that reactivation of EBV infection was associated with greater expression of HLA-DR and markers of a T-bet differentiation pathway amongst IgM+ MBCs, suggesting that EBV infection might influence the initiation of disease-associated B cell abnormalities in CIS/MS patients. This evidence concerns the gene TNFRSF13C and in situ carcinoma.