Studies have demonstrated that developing haematopoietic cells express TLRs7,25 and MG-132 price therefore would be expected to be sensitive to stimulation with their ligands. Our experiments indicate that the presence of TLR4 or TLR9 ligands (LPS and CpG ODN, respectively) during the generation of BMDCs in the presence of GM-CSF inhibits the differentiation of cells with the phenotype of BMDCs. This is in agreement with other studies which show that LPS or CpG ODN inhibit in vitro differentiation of DCs.26–28 Bartz et al.28 demonstrated that the generation of myeloid DCs from murine bone marrow was impaired by stimulation

with LPS or CpG ODN. The cells generated exhibited reduced expression of CD11c and MHCII and a reduced ability to activate T lymphocytes. In humans, LPS stimulation has been shown to influence both early and late monocyte differentiation by blocking their ability to differentiate

into DCs in vitro.25 The addition of LPS to cultures of monocytes containing GM-CSF and IL-4 see more reduced the cell yields, altered the morphology and phenotype of the cells generated, and compromised their capacity to present antigen.27,28 We did not explore the antigen-presentation function of the cells generated, but their phenotype, CD11clo/MHCIIlo, suggests a reduced antigen-presentation capacity because of the crucial role of MHCII in this process. Taken together, these findings confirm the inhibitory effects of LPS and CpG ODN stimulation during DC generation. Our experiments indicate that TLR stimulation during the development of BMDCs in vitro inhibited the differentiation of CD11c+/MHCII+ cells while simultaneously enhancing the production of CD11clo/MHCIIlo cells. Experiments with knockout mice revealed that TLR4 (data not shown) and MyD88 were required to generate both of these effects. TLR4 and MyD88

have been shown to be expressed by developing haematopoietic cells,5 and this study demonstrated that MyD88-dependent signalling promoted myeloid lineage differentiation from HSC-enriched cultures stimulated Dichloromethane dehalogenase with LPS in serum-free, stromal cell-free conditions. The differentiation potential of lymphoid progenitors has also been shown to be influenced by TLR9 ligation in a MyD88-dependent manner,29 and CpG ODN-induced inhibition of BMDC production is known to require TLR9.28 Although signalling via TLRs on granulocyte and macrophage progenitors has been shown to obviate the need for growth and differentiation factors to direct the differentiation of haematopoietic cells in vitro7 it was likely that the effects we observed were mediated by cytokines produced in response to TLR stimulation. This suggestion is supported by several reports which indicate that cytokines provide differentiation cues for developing haematopoietic cells.30–33 Tumour necrosis factors have been shown to inhibit haematopoiesis in vitro.