Monocyte-derived dendritic cells formed at the infection site control the induction of protective T helper 1 responses against Leishmania.

Infection-induced inflammatory reactions involve a strong increase in dendritic cells (DCs) at the infection site and draining lymph nodes (dLNs). Whether inflammatory DCs are recruited to these locations or differentiate locally, and what their functional relevance is, remain unclear. Here we showed that during Leishmania infection, monocytes were recruited to the dermis and differentiated into "dermal monocyte-derived DCs," which subsequently migrated into the dLNs. In addition, monocyte recruitment to the dLNs resulted in the differentiation into "LN monocyte-derived DCs." Analysis of the kinetics of monocyte differentiation into DCs, susceptibility to infection, IL-12 production, and L. major-specific T cell stimulation potential suggest that dermal monocyte-derived DCs controlled the induction of protective T helper 1 responses against Leishmania. Thus, the demonstration of monocyte differentiation potential into DCs during in vivo infection and of local DC differentiation in inflammatory foci suggests that de novo formed monocyte-derived DCs are essential in T cell immunity against pathogens.