Dendritic cells (DCs) are the most potent antigen-presenting cells (APCs) in priming naive T cells. Using an in vitro priming system, we show that DCs incubated with Ixodes ricinus tick saliva initiate the T(h)2 differentiation of CD4(+)T cells. As determined with reverse transcription-PCR, the expression of IL-4 mRNA by these cells is higher than IFN-gamma mRNA. Early endogenous production of IL-4 is thought to be important during the in vitro interaction of saliva-pulsed DCs with CD4(+)T cells. Its neutralization with specific mAbs inhibits the development of IL-4-secreting T(h)2 cells. Moreover, differentiated T(h)2 cells proliferate only when saliva-pulsed DCs and IL-1beta are added together early in the primary culture. As demonstrated by FACS analysis, the treatment in vitro of saliva-pulsed DCs by IL-1beta enhanced the expression of B7 and mainly CD40 co-stimulatory molecules, which provide sufficient signals to stimulate sensitized CD4(+)T cell proliferation. On the other hand, DCs treated with tick saliva only up-regulated mostly B7-2 co-stimulator expression and this was associated with differentiation of naive CD4(+)T cells into T(h)2 type of cells. The in vitro priming system is suitable to investigate the major elements implicated in the anti-tick immune response such as naive CD4(+)T cells, whole DCs population and tick saliva, and it can provide the possibility to delimit further the saliva molecules, the DC subsets and the type of host cells involved in the T(h)2 polarization. Corresponding in vivo experiments involving subcutaneous injection of tick saliva-pulsed DCs into BALB/c mice also elicited a T(h)2 immune response. Ex vivo cultures of draining lymph node T cells stimulated with tick saliva produced higher IL-4 : IFN-gamma ratios compared with controls, confirming the relevance obtained in the in vitro priming model. These experiments demonstrate the importance of tick saliva in priming DCs to initiate a T(h)2-biased immune response in vitro and in vivo.