Toll-like receptor-positive cells and recognition of pathogens: how human myeloid dendritic cells respond to in vitro infection with Leishmania infantum

Dendritic cells (DCs), instructed by the priming signals from microbial factors, can produce interleukin (IL)-12p70 and promote T helper (Th)1 proliferation and interferon (IFN)-gamma production. This event seems to be critical for the containment of infections caused by intracellular pathogens, even including Leishmania infection. In the present in vitro study we have investigated: 1) phagocytic capacities and IL-12 production by human monocyte-derived DCs and macrophages (MØs), infected with Leishmania infantum promastigotes; 2) IFN-gamma production by human CD4+ T cells co-incubated with DCs or macrophages pulsed with live promastigotes. Monocyte-derived myeloid DCs and MØs from healthy donors were infected with live metacyclic Leishmania infantum (MON-1) promastigotes, previously opsonized with 5% autologous serum, at 1:4 cell/parasite ratio. Percentage and index of phagocytosis were calculated after 2, 24 and 48 h of incubation. IL-12 production was evaluated by an ELISA in supernatants from 48 h Leishmania-infected or lipopolysaccharides (LPS)-stimulated DCs and MØs, also in the presence of phytohemagglutinin-activated or inactivated CD4+ T cells. For IFN-gamma production, CD4+ T cells were repeatedly stimulated with DCs or MØs, pulsed with live Leishmania promastigotes or activated with LPS. The number of IFN-gamma-secreting cells was evaluated by an ELISpot assay. Results showed that MØs have a higher phagocytic capacity towards L. infantum promastigotes than DCs. Moreover, unlike MØs, Leishmania-infected DCs were able to release IL-12p70; this production significantly increased in the presence of activated CD4+ T cells. Finally, DCs pulsed with live parasites and added to autologous CD4+ T cells induced a higher number of IFN-gamma-secreting cells than MØs, thus indicating their ability to polarize Th cells toward the Th1 subset. These data indicate that DCs are able to promote protective Th1 immune responses in our experimental model of Leishmania infantum infection, thus representing the grounds for initiating immunoterapeutic and vaccinal strategies.