Splenic dendritic cells pulsed with <i>Ixodes ricinus</i> tick saliva prime naive CD4⁺T to induce Th₂ cell differentiation <i>in vitro</i> and <i>in vivo</i>

Dendritic cells (DCs) are the most potent antigen-presenting cells (APCs) in priming naive T cells. Using an <i>in vitro</i> priming system, we show that DCs incubated with <i>Ixodes ricinus</i> tick saliva initiate the T_h2 differentiation of CD4⁺T cells. As determined with reverse transcription–PCR, the expression of IL-4 mRNA by these cells is higher than IFN-γ mRNA. Early endogenous production of IL-4 is thought to be important during the <i>in vitro</i> interaction of saliva-pulsed DCs with CD4⁺T cells. Its neutralization with specific mAbs inhibits the development of IL-4-secreting T_h2 cells. Moreover, differentiated T_h2 cells proliferate only when saliva-pulsed DCs and IL-1ß are added together early in the primary culture. As demonstrated by FACS analysis, the treatment in vitro of saliva-pulsed DCs by IL-1ß 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_h2 type of cells. The <i>in vitro</i> 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_h2 polarization. Corresponding <i>in vivo</i> experiments involving subcutaneous injection of tick saliva-pulsed DCs into BALB/c mice also elicited a T_h2 immune response. <i>Ex vivo</i> cultures of draining lymph node T cells stimulated with tick saliva produced higher IL-4 : IFN-γ 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_h2-biased immune response in vitro and <i>in vivo</i>.