In this study the effects of different in vitro conditioning with transforming growth factor (TGF) beta 1 on human monocyte-derived DC maturation (hMo-DC) were investigated. hMo-DC differentiated in the presence of physiologically relevant concentrations of TGF beta 1(2 ng/ml) failed to undergo complete maturation despite adequate stimulation with LPS or LPS + IFN gamma. These hMo-DC did not produce IL-12p70 or PGE2, and showed decreased IL-10 and IL-18 production and HLA-DR expression. However, the expression of these molecules, except for IL-12p70, was not significantly affected in hMo-DC differentiated in the presence of lower concentrations of TGF beta 1 (0.2 and 0.02 ng/ml). Exposure of hMo-DC to TGF beta 1 (2 ng/ml) after they had completed differentiation had minimal effects. Thus, the functional response of hMo-DC to LPS or LPS + IFN gamma depended on the stage of hMo-DC differentiation at which cells were first exposed to TGF beta 1 and on the concentration of TGF beta 1. These results suggest that in the in vivo micro-environment, the concentrations and the timing of monocyte exposure to TGF beta 1 may be crucial in the differentiation of DC toward more or less mature phenotypes, and this may have important implications for DC functions. The decrease in T-cell proliferation and a small increase in IL-5 production by T cells co-cultured with hMo-DC that had been treated with TGF beta 1, suggest the possibility that in vivo such DC may provide chronic, but incomplete signals to T cells, and this could be a potential mechanism underlying polarisation of T cells towards anergy. (c) 2007 Elsevier B.V. All rights reserved.