These studies show that B cells play critical positive and negative regulatory roles in EAE immunopathogenesis. Consequently, B cell depletion had 2 opposing effects on disease. Early B cell depletion (day –7) in mice with otherwise normal immune systems exacerbated not only the early and peak phase of EAE induction but also the recovery phase of disease (Figure ). Moreover, the adoptive transfer of IL-10–producing regulatory CD1dhiCD5+ B10 cells, but not other B cells, normalized EAE pathogenesis (Figure ). Thereby, B cell depletion and removal of the CD1dhiCD5+ regulatory B10 cell subset before EAE induction (day –7; Figure ) appeared to induce the influx or expansion of encephalitogenic T cells within the CNS (Figure and Figure B), which significantly exacerbated disease symptoms (Figures and ). That B cell depletion enhanced EAE severity in the absence of MOG-specific autoantibodies (Figure ) also argues that B cells and their antibody products are not required for EAE induction or regulation. Therefore, we propose that increased EAE severity following total B cell depletion before disease induction results from depletion of the B10 cell subset (Figure F).
B cell depletion during the course of EAE development (day 14) dramatically reduced disease symptoms (Figures and ), impaired MOG-specific T cell expansion (Figure , B and C), and significantly inhibited the influx or expansion of encephalitogenic T cells within the CNS and draining lymph nodes (Figures and ). Thereby, B cells are essential for generating optimal pathogenic CD4+
T cell responses following MOG immunizations, while B10 cells appeared to exert their antiinflammatory effects early, but not late, during the course of EAE (Figure ). B cells could also serve as antigen-presenting cells to prime MOG-specific T cells (Figure A), as shown in previous studies (13
). However, it is equally possible that B cells are providing costimulation, an appropriate microenvironment, and/or cytokines that amplify T cell activation. Thus, the reciprocal positive and negative regulatory roles for B cells are likely to overlap during the course of disease. In support of this, B cell depletion at day 7 before disease symptom onset resulted in a normal course of EAE pathogenesis (Figure ). Thereby, the balance of at least 2 opposing B cell functions shapes the normal course of EAE immunopathogenesis. This balance is likely to also be reflected in other T cell–mediated autoimmune diseases. However, B cell depletion early, but not late, significantly attenuates the course of collagen-induced arthritis in DBA-1 mice (37
), diabetes in NOD mice (38
), Sjogren-like disease in Id3-deficient mice (39
), and systemic sclerosis-like disease in tight-skin mice (40
). These beneficial effects are likely to depend on reduced CD4+
T cell activation, pathogenic B cell depletion, and reductions in autoantibody production (34
). Thereby, B cell contributions during autoimmunity can be divided into distinct pathogenic and regulatory activities.
The current findings resolve previously unexplained contradictions between studies showing the importance of B cells in EAE. As in the current studies, B cell IL-10 production has been previously shown to suppress EAE severity (10
). IL-10–produced by B cells can also down-regulate other autoimmune and inflammatory diseases such as collagen-induced arthritis, inflammatory bowel disease, and contact hypersensitivity (17
). By contrast, mice genetically deficient for B cells appear to develop EAE normally but fail to resolve the disease (7
). However, immune system development and T cell priming are abnormal in mice lacking B cells since birth (43
). Thereby, an absence of regulatory B10 cells in combination with abnormal T cell activation in congenitally B cell–deficient mice may explain normal EAE induction. Regardless, the lack of disease resolution in both models suggests that regulatory B cells may be critical during disease induction and for resolving disease.
EAE pathogenesis is modulated by counter-regulatory B cell subsets and therefore illustrates complexities that must be considered in developing new B cell depletion therapies. However, both pathogenic and regulatory B cell activities may often overlap. For example, B cell depletion 7 days after MOG immunization did not alter the course of EAE (Figure ). Moreover, the importance of B cells as antigen-presenting or costimulatory cells during disease induction may be negated by EAE induction by use of a potent adjuvant during high-dose MOG immunizations, while B cell–mediated T cell activation may be more important later in the course of disease as antigen concentrations decrease as recently described for other autoantigens and protein antigens (34
). Thereby, dendritic cells and other antigen-presenting cells may play a more critical role in antigen presentation or CD4+
T cell activation during disease initiation, while B cells are maximally important for autoreactive T cell maintenance during EAE development rather than early EAE initiation. Under these conditions, B cell depletion may only reduce disease during EAE progression, while B10 cell function may be most obvious during disease induction. The current studies also suggest the possibility that the selective depletion of mature B cells while sparing IL-10–producing B10 cells may offer a potent therapeutic approach for treating patients with MS and other autoimmune or inflammatory diseases. However, antigen-specific B10 cells may be required since B10 cells from naive mice were without effect unless they were transferred before MOG immunization (Figure , C–F). Our current studies are focused on the identification of pathways that regulate B10 cell activation, expansion, and function, which will allow this potent B cell subset to be manipulated for therapeutic benefit.
B cell depletion after the onset of EAE symptoms ameliorated disease progression (Figure ), making this strategy applicable for treating human MS after disease onset. However, adverse disease following B cell depletion before EAE induction in the current study also suggests that B cell depletion may promote the occurrence of MS in some undiagnosed cases. Nonetheless, B cell depletion during EAE development reduced EAE severity both clinically and histologically (Figures and ) and was accompanied by significantly reduced autoantibody levels (Figure ). Reduced autoantibody production may be clinically important since plasma exchange can reduce clinical disease activity in a subset of MS patients (46
). CD20 mAb treatment depletes memory cells in mice but does not deplete long-lived plasma cells (32
). Thus, CD20+
B cell depletion may be most beneficial when carried out before the long-lived plasma cell pool is established. Similarly, B cell depletion significantly attenuates early foreign- and autoantigen-specific CD4+
T cell proliferation in vivo (34
). Also, B cell depletion early in the course of autoimmune mouse models has maximal benefit, as it is often not possible to reverse T cell expansion or disease progression once inflammatory disease is initiated (37
). Thereby, B cell depletion shortly after diagnosis of autoimmune disease may offer the most optimal strategy for disease management.