Silent infection of bone marrow-derived dendritic cells by Leishmania mexicana amastigotes

Resolution of infection by Leishmania sp. is critically dependent on activa tion of CD4(+) T helper cells. Naive CD4(+) T helper cells are primed by de ndritic cells which have responded to an activation signal in the periphery . However, the role of Leishmania-infected dendritic cells in the activatio n of an anti-leishmania immune response has not been comprehensively addres sed. Using the highly controlled model system of bone marrow-derived dendri tic cell infection by Leishmania mexicana cultured in vitro, we show that u ptake of L. mexicana parasites does not result in activation of immature de ndritic cells or secretion of IL-12. Incubation with L. mexicana promastigo tes results in the activation of a small percentage of dendritic cells whic h do not. appear to contain whole parasites. Activation of dendritic cells is not suppressed by infection, since infected cells can be fully activated on addition of activating stimuli. Therefore, uptake of intact Leishmania mexicana parasites is not sufficient to activate dendritic cells in vitro. We propose that these data provide a basis for interpreting the interaction s between dendritic cells and all Leishmania sp.