Biogenesis of Leishmania major-harboring vacuoles in murine dendritic cells

In mammalian hosts, Leishmania sp. parasites are obligatory intracellular organisms that invade macrophages and dendritic cells (DC), where they reside in endocytic organelles termed parasitophorous vacuoles (PV). Most of the present knowledge of the characteristics of PV harboring Leishmania sp. is derived from studies with infected macrophages. Since DC play a key role in host resistance to leishmaniasis, there is a need to understand the properties and biogenesis of PV in Leishmania sp.-infected DC. Therefore, we determined the acquisition of endosomal and lysosomal molecules by Leishmania major-containing compartments in DC at different maturation stages, using fluorescence labeling and confocal microscopy. The results show that newly formed phagosomes in DC rapidly develop into late endosomal compartments. However, the small GTPase Rab7, which regulates late fusion processes, was found only in PV of mature bone marrow-derived DC (BMDC); it was absent in immature BMDC, suggesting an arrest of their PV biogenesis at the stage of late endosomes. Indeed, fusion assays with endocytic tracers demonstrated that the fusion activity of L. major-harboring PV toward lysosomes is higher in mature BMDC than in immature BMDC. The inhibition of PV-lysosome fusion in DC is dependent upon the viability and life cycle stage of the parasite, because live promastigotes blocked the fusion almost completely, whereas killed organisms and amastigotes induced a considerable level of fusion activity. The differences in the fusion competences of immature and mature DC may be relevant for their distinct functional activities in the uptake, transport, and presentation of parasite antigens.