Human B cell responses to TLR ligands are differentially modulated by myeloid and plasmacytoid dendritic cells

Selected TLR ligands are under evaluation as vaccine adjuvants and are known to activate dendritic cells (DCs) and B cells to affect vaccine-induced Ab responses. However, the relative contribution of the two main human DC subsets, myeloid (MDCs) and plasmacytoid (PDCs), in supporting B cell responses to TLR ligands remains poorly understood. We found that PDCs but not MDCs markedly enhanced B cell proliferation in response to TLR7/8-L, an imidazoquinoline derivative, and to a lesser extent to TLR9 ligands (CpG ODN classes A, B, and C). PDCs strongly enhanced TLR7/8-L-induced proliferation of both memory and naive B cells but were only able to support memory cells to differentiate to CD27(high) plasmablasts. In response to TLR7/8 stimulation, PDCs mediated the up-regulation of transcription factors B lymphocyte-induced maturation protein 1 and X-box binding protein 1 and enhanced differentiation of B cells into IgM-, IgG-, and IgA-producing cells. Type I IFN produced to high levels by PDCs was the principal mediator of the effects on TLR7/8 stimulation. Although MDCs expressed higher levels of the known B cell growth factors IL-6, IL-10, and B cell-activating factor in response to TLR7/8 stimulation, they were unable to enhance B cell responses in this system. These data help decipher the different roles of PDCs and MDCs for modulating human B cell responses and can contribute to selection of specific TLR ligands as vaccine adjuvants.