Effects of tropism and virulence of Leishmania parasites on cytokine production by infected human monocytes

The nature of early interactions between Leishmania and macrophages which determine the outcome of infection can be related directly to parasite biological properties. Here we compared the capacity of L. major (Lm) strains, reported to be high (LmHV) and low virulent and (LmLV) in the mouse model and L. infantum (Li) strains, dermotropic (LiD) and viscerotropic (LiV), to infect and modulate cytokine production in human peripheral blood derived monocytes. Monocytes were infected with metacyclic promastigotes for 24, 48 and 72 h. Parasite burden was significantly higher in Lm‐ than in Li‐infected monocytes. LmHV and LiD induced a significantly higher parasite burden than LmLV and LiV respectively. Cytokine production was evaluated in monocytes infected for 24 h. Contrary to interleukin (IL)‐12p70, monocyte chemotactic protein‐1 and transforming growth factor‐β production was increased significantly in infected monocytes with no differences between strains. Lm isolates induced significantly higher quantities of tumour necrosis factor (TNF)‐α than Li isolates. Low levels of IL‐10 were induced by all Leishmania strains and, interestingly, co‐stimulation with lipopolysaccharide (LPS) was accompanied by a dramatic increase in IL‐10 production by infected monocytes. In conclusion, Lm isolates displaying different levels of virulence in mice exhibited significant differences in parasite burden but similar abilities to modulate cytokine production in human monocytes. Li strains showed weaker infectivity and TNF‐α inducing‐capacity compared with Lm strains. The dramatic increase of IL‐10 production in infected monocytes co‐stimulated by LPS may play a role in disease progression considering the presence of LPS during bacterial superinfections observed during human leishmaniasis.