The robust and rapid clinical effect of depleting anti-CD20 monoclonal antibodies (mAb) in multiple sclerosis (MS) demonstrates a critical pathogenic contribution of B cells. The clinical effect of anti-CD20 mAb has been replicated in a relevant preclinical MS model, experimental autoimmune encephalomyelitis (EAE) in marmoset monkeys (Callithrix jacchus). By contrast, treatment with mAbs against two essential cytokines in B cell activation growth and survival, i.e. BlyS/BAFF and APRIL, was only partially effective. All three mAbs induced depletion of CD20+ B cells from the circulation, albeit with different kinetics and based on distinct mechanisms of action. In the current study we analyzed whether the different clinical effect of anti-CD20 mAb or the anti-BLyS and anti-APRIL mAbs is due to different depletion of B cells infected with the EBV of marmosets, CalHV3. Employing a novel PCR-based assay, half of the colony of group-housed marmosets was tested positive for CalHV3 DNA in secondary lymphoid organs. The same prevalence was observed in placebo-treated monkeys. In marmosets treated with anti-CD20 mAb the load of CalHV3 DNA in lymphoid organs was substantially reduced, while this was not observed in the monkeys treated with anti-BLyS or anti-APRIL mAbs. To examine the pathogenic role of virus-transformed B cells, we infused EBV-transformed B lymphoblastic cell (BLC) lines presenting the immunodominant MOG34-56 peptide. We observed in the recipients of MOG34-56 pulsed BLC, but not in their fraternal siblings infused with non-pulsed BLC, activation of anti-MOG34-56 T cells and meningeal inflammation. Collectively, the data show that among CD20+ B cells, the herpesvirus-transformed subset has a particularly important pathogenic role in the marmoset EAE model.