Regulatory plasma cells.

B cells can regulate immunity negatively or positively. Identifying the B cell subsets mediating these antagonistic activities, and the molecular mechanisms governing their differentiation, might enable the development of novel approaches to target B cells therapeutically. The suppressive functions of B cells are primarily mediated through their production of interleukin (IL)-10 and IL-35. Recent studies have shown that distinct sets of IgM(+)CD19(+)CD138(hi) plasma cells were the major B cell subsets producing these cytokines in a regulatory manner in vivo during autoimmune and infectious diseases. This review summarizes current knowledge on these 'regulatory plasma cells', and discusses the emerging data showing that the mechanisms involved in their generation partly overlap with those controlling the differentiation of 'effector regulatory T cells'.