Summary B cells regulate immune responses by producing antigen-specific antibody1. However, specific B cell subsets can also negatively regulate immune responses, validating the existence of regulatory B cells2–4. Human and mouse regulatory B cells (B10 cells) with the ability to express the inhibitory cytokine IL-10 have been identified2–5. Although rare, B10 cells are potent negative regulators of antigen-specific inflammation and T cell-dependent autoimmune diseases in mice5–7. How B10 cell IL-10 production and regulation of antigen-specific immune responses are controlled in vivo without inducing systemic immunosuppression are unknown. Using a mouse model for multiple sclerosis, we show here that B10 cell maturation into functional IL-10-secreting effector cells that inhibit in vivo autoimmune disease requires IL-21 and CD40-dependent cognate interactions with T cells. Moreover, the ex vivo provision of CD40 and IL-21 receptor signals can drive B10 cell development and expansion by four-million-fold and generate B10 effector cells producing IL-10 that dramatically inhibit disease symptoms when transferred into mice with established autoimmune disease. Thereby, the ex vivo expansion and reinfusion of autologous B10 cells may provide a novel and effective in vivo treatment for severe autoimmune diseases that are resistant to current therapies.

Malignant B cells responding to external stimuli are likely to gain a growth advantage in vivo. These cells may therefore maintain surface CD19 expression to amplify transmembrane signals and promote their expansion and survival. To determine whether CD19 expression influences this process, Eμ-Myc transgenic (c-MycTg) mice that develop aggressive and lethal B cell lymphomas were made CD19 deficient (c-MycTgCD19−/−). Compared with c-MycTg and c-MycTgCD19+/− littermates, the median life span of c-MycTgCD19−/− mice was prolonged by 81–83% (p < 0.0001). c-MycTgCD19−/− mice also lived 42% longer than c-MycTg littermates following lymphoma detection (p < 0.01). Tumor cells in c-MycTg and c-MycTgCD19−/− mice were B lineage derived, had a similar phenotype with a large blastlike appearance, invaded multiple lymphoid tissues, and were lethal when adoptively transferred into normal recipient mice. Importantly, reduced lymphomagenesis in c-MycTgCD19−/− mice was not due to reductions in early B cell numbers prior to disease onset. In mechanistic studies, constitutive c-Myc expression enhanced CD19 expression and phosphorylation on active sites. Reciprocally, CD19 expression in c-MycTg B cells enhanced c-Myc phosphorylation at regulatory sites, sustained higher c-Myc protein levels, and maintained a balance of cyclin D2 expression over that of cyclin D3. These findings define a new and novel c-Myc:CD19 regulatory loop that positively influences B cell transformation and lymphoma progression.