Redundant and unique regulation of activated mouse B lymphocytes by IL-4 and IL-21.

IL-21 distinctively regulates B cell growth and death, and it redundantly functions with IL-4 for IgG production. B cells likely encounter IL-4 and IL-21 in vivo, as both are secreted by activated T cells. Therefore, the action of both these cytokines was investigated during activation of B cells. IL-21 or the combination of IL-4 and IL-21 inhibited proliferation by purified mouse B cells to LPS or CpG DNA, whereas these cytokines enhanced proliferation after engaging the BCR or CD40. Although B cell subsets expressed somewhat varied levels of the IL-21 receptor, LPS-stimulated follicular and marginal B cell subsets were also dominantly susceptible to IL-21-induced growth arrest and cell death. After activation of B cells with CD40 and LPS, IL-4 and IL-21 distinctively regulated the expression of CD23, CD44, and CD138, and they cooperatively promoted IgG1 class-switching and synthesis. These findings support a model in which the presence of IL-4 and IL-21 inhibits B cells activated by polyclonal innate signals, and they promote B cell expansion and differentiation during T cell-dependent antibody responses, although the individual responses to IL-4 and IL-21 do not always overlap.