Immunoglobulin (Ig) class-switch DNA recombination (CSR) is thought to be highly dependent upon engagement of CD40 on B cells by CD40 ligand on T cells. We show here that dendritic cells up-regulate BLyS and APRIL upon exposure to interferon-alpha, interferon-gamma or CD40 ligand. In the presence of interleukin 10 (IL-10) or transforming growth factor-beta, BLyS and APRIL induce CSR from C(mu) to C(gamma) and/or C(alpha) genes in B cells, whereas CSR to C(epsilon) requires IL-4. Secretion of class-switched antibodies requires additional stimulation by B cell antigen receptor engagement and IL-15. By eliciting CD40-independent Ig class switching and plasmacytoid differentiation, BLyS and APRIL critically link the innate and adaptive immune responses.