Bacterial lipopolysaccharide, TNF-α, and calcium ionophore under serum-free conditions promote rapid dendritic cell-like differentiation in CD14⁺ monocytes through distinct pathways that activate NF-κB

To facilitate the study of signaling pathways involved in myeloid dendritic cell (DC) differentiation, we have developed a serum-free culture system in which human CD14⁺ peripheral blood monocytes differentiate rapidly in response to bacterial LPS, TNF-α, or calcium ionophore (CI). Within 48-96 h, depending on the inducing agent, the cells acquire many immunophenotypical, morphological, functional, and molecular properties of DC. However, there are significant differences in the signaling pathways used by these agents, because 1) LPS-induced, but not CI-induced, DC differentiation required TNF-α production; and 2) cyclosporin A inhibited differentiation induced by CI, but not that induced by LPS. Nevertheless, all three inducing agents activated members of the NF-κB family of transcription factors, including RelB, suggesting that despite differences in upstream elements, the signaling pathways all involve NF-κB. In this report we also demonstrate and offer an explanation for two observed forms of the RelB protein and show that RelB can be induced in myeloid cells, either directly or indirectly, through a calcium-dependent and cyclosporin A-sensitive pathway.