Distinct contributions of TNF and LT cytokines to the development of dendritic cells in vitro and their recruitment in vivo

TNF/LTα/LTβ (tumor necrosis factor/lymphotoxin-α/lymphotoxin-β) triple knockout (KO) mice show a significant reduction of dendritic cell (DC) number in the spleen, presumably due to defective recruitment and/or production. To distinguish between these possibilities, DCs were generated from bone marrow (BM) cultures prepared from wild-type (wt) and mutant mice in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). The yield of CD11c⁺ major histocompatibility complex (MHC) class II⁺ DCs generated from TNF/LTα/LTβ^-/- BM culture was significantly reduced compared with wt BM culture. In order to further dissect the individual pathways responsible for defective DC properties observed in TNF/LTα/LTβ^-/- mice, the panel of TNF/LT ligand and receptor single KO mice were used. The production of DCs from BM culture was significantly reduced in TNF^-/- and TNF receptor (TNFR) p55^-/- mice, but normal in LTα^-/-, LTβ^-/-, LTβR^-/- mice. Recombinant TNF (rTNF) exogenously added to TNF/LTα/LTβ^-/- BM cultures could reverse this defect, and blocking antibodies showed partial effect on BM cultures of wt mice. Conversely, numbers of mature DCs in spleen were significantly decreased in LTα^-/-, LTβ^-/-, LTβR^-/- mice, but not in TNF^-/- and TNFRp55^-/- mice. These results reveal 2 distinct contributions of TNF/LT cytokines. First, TNF acting through TNF receptor is involved in the development/maturation of DCs in BM progenitor cultures, but this function appears to be redundant in vivo. Second, the microenvironment in peripheral lymphoid organs associated with LTα/LTβ-LTβR signaling and chemokine production is critical for recruitment efficiency of DCs, and this pathway is indispensable.