Leishmania donovani Lipophosphoglycan : Modulation of Macrophage and Dendritic Cell Function

Abstract: Leishmania donovani is a blood-borne tropicial parasite, which infects humans through bites by Phlebotomus sandflies. The parasite survives and multiplies inside macrophages in inner organs, and causes the deadly disease visceral leishmaniasis (Kala-Azar).Macrophages and dendritic cells (DC) are professional antigen-presenting cells involved in the initiation of immune responses. Immature DC are present in all tissues where they internalise and process antigen, in response to which they migrate from tissue, into draining lymphoid organs, undergo maturation and present antigens to lymphocytes. Control measures for leishmaniasis include testing of new diagnostics and development of affordable and effective vaccines for humans.Lipophosphoglycan (LPG) is the major surface component of Leishmania donovani promastigotes. LPG comprises a membrane-anchoring lysophosphatidylinositol part and an extracellular chain of disaccharide phosphates. These repetitions are crucial for parasite survival inside macrophages following phagocytosis. LPG has several specific effects on the host cell including inhibition of protein kinase C (PKC) activity, and inhibition of phagosomal maturation, a process requiring depolymerization of periphagosomal F-actin.Confocal microscopy and image analysis were used to follow F-actin dynamics in single macrophages during phagocytosis of L. donovani promastigotes and LPG-coated particles. F-actin did not depolymerize, but instead progressively polymerized around phagosomes with LPG-containing prey. This correlated with reduced translocation of PKCα to the phagosome and blocked phagosomal maturation. LPG also inhibited cortical actin turnover, which could be the underlying cause of the reduced uptake of LPG-containing prey. Extracellular- and intracellular calcium was necessary for phagocytosis, periphagosomal F-actin breakdown and phagosomal maturation in macrophages interacting with unopsonized prey,and for the action of LPG.We also studied F-actin turnover in macrophages overexpressing dominant-negative (DN) PKCα. DN PKCα macrophages showed increased amounts of cortical F-actin, decreased phagocytic capacity, inhibition of periphagosomal F-actin breakdown and defective phagosomal maturation. When DN PKCα macrophages interacted with LPG-containing prey, phagocytosis was almost completely blocked.Moreover, we found that Leishmania promastigotes and particularly LPG inhibit DC maturation and detachment from distinct surfaces. Thus, LPG from Leishmania donovani could directly inhibit DC migration to lymphoid organs, antigen-presentation and development of immunity.