Evidence for direct interaction between mast cells and Leishmania parasites

When stimulated through IgE‐ (or IgG‐) immune complexes with parasite antigens, mast cells can release several cytokines, including IL‐4, IL‐6, IL‐10, IL‐12, Interferon‐γ (IFN‐γ) and tumour necrosis factor‐α (TNF‐α) that may influence the host response to Leishmania major in modulating lesion size and persistence during experimental infection in the mouse. Moreover, recent data demonstrated that mast cells are able to be antibody‐independently activated by direct contact with bacteria, making them important elements in innate immunity. Given these data, we asked whether cell‐parasite contact could directly induce mast cell mediator release and whether mast cells could be infected by L. major or L. infantum parasites. In this study, we showed that a pure homogeneous population of mouse bone marrow derived mast cells (BMMC) in contact with living L. major or L. infantum promastigotes, but not with attenuated parasites or soluble parasite antigens, released preformed mediators such as β‐hexosaminidase and the preformed pool of TNF‐α within minutes. Furthermore, direct cell‐parasite contact induced TNF‐α synthesis by mast cells within hours. Moreover, we demonstrated by in vitro co‐culture experiments that metacyclic L. major or L. infantum promastigotes are directly infective for a significant proportion of BMMC and are transformed into intracellular amastigotes. Taken together, these data suggest that mast cell can participate in the first line of defence, i.e. innate immunity, during local cutaneous infection with Leishmania parasites.