Leishmania spp.: on the interactions they establish with antigen-presenting cells of their mammalian hosts

Identification of macrophages as host cells for the mammalian stage of Leishmania spp. traces back to about 40 years ago, but many questions concerning the ways these parasites establish themselves in these cells, which are endowed with potent innate microbicidal mechanisms, are still unanswered. It is known that microbicidal activities of macrophages can be enhanced or induced by effector T lymphocytes following the presentation of antigens via MHC class I or class II molecules expressed at the macrophage plasma membrane. However, Leishmania spp. have evolved mechanisms to evade or to interfere with antigen presentation processes, allowing parasites to partially resist these T cell-mediated immune responses. Recently, the presence of Leishmania amastigotes within dendritic cells has been reported suggesting that they could also be host cells for these parasites. Dendritic cells have been described as the only cells able to induce the activation of naive T lymphocytes. However, certain Leishmania species infect dendritic cells without inducing their maturation and impair the migration of these cells, which could delay the onset of the adaptive immune responses as both processes are required for naive T cell activation. This review examines how Leishmania spp. interact with these two cell types, macrophages and dendritic cells, and describes some of the strategies used by Leishmania spp. to survive in these inducible or constitutive antigen-presenting cells.