Introduction Leishmania are protozoan parasites with a digenetic life cycle involving blood-feeding phlebotomine insect vectors and mammals. Some Leishmania species are also the aetiological agents of major human diseases known as leishmaniases. In insects, Leishmania adopt a flagellated promastigote (Pm) form and initially multiply in the lumen of the abdominal midgut. The resulting metacyclic Pm (MPm), which are infective for mammals, accumulate in the anterior parts of the digestive tract from where they can be injected into the dermis of mammals during a blood meal. Parasites surviving in this environment are rapidly phagocytosed, mainly by resident macrophages (for a review, see Peters and Killick-Kendrick, 1987). Once inside macrophages, MPm slowly differentiate into amastigotes (Am) (Courret et al., 2001) within parasitophorous vacuoles (PVs) displaying the properties of phagolysosomes (Antoine et al., 1998; Courret et al., 2002). Although macrophages are generally considered to be the main or only host cells allowing Leishmania to survive and to multiply, other cells can harbour intact/viable parasites at least transiently and may play important roles in the parasitism or in the control of parasite growth. For example, L. major parasites have been observed in polymorphonuclear leukocytes, fibroblasts and dendritic cells (DCs) from BALB/c and C57BL/6 mice at different stages of infection [(Beil et al., 1992); Y. Belkaid and G. Milon, unpublished data cited by Tacchini-Cottier et al. (Tacchini- Cottier et al., 2000) Bogdan et al., 2000; Moll et al., 1995]. DCs form a family of leukocytes that play critical roles in the innate and adaptive immune systems as, (i) they are specialised in the recognition of pathogens through pattern recognition receptors, a process that can lead to their activation; (ii) they ensure the transport of the antigens they capture from the peripheral tissues to the draining secondary lymphoid organs and (iii) they are apparently the only antigen- 315 In their mammalian hosts, Leishmania are obligate intracellular parasites that mainly reside in macrophages. They are also phagocytosed by dendritic cells (DCs), which play decisive roles in the induction and shaping of T cell- dependent immune responses. Little is known about the role of DCs in the Leishmania life cycle. Here, we examined the ability of mouse bone marrow-derived DCs to serve as hosts for L. amazonensis. Both infective stages of Leishmania (metacyclic promastigotes and amastigotes) could be phagocytosed by DCs, regardless of whether they had previously been experimentally opsonized with either the complement C3 component or specific antibodies. Parasites could survive and even multiply in these cells for at least 72 hours, within parasitophorous vacuoles displaying phagolysosomal characteristics and MHC class II and H-2M molecules. We then studied the degree of maturation reached by infected DCs according to the parasite stage internalised and the type of opsonin used. The cell surface expression of CD24, CD40, CD54, CD80, CD86, OX40L and MHC class II molecules was barely altered following infection with unopsonized promastigotes or amastigotes from nude mice or with C3-coated promastigotes. Even 69 hours post-phagocytosis, a large proportion of infected DCs remained phenotypically immature. In contrast, internalisation of antibody- opsonized promastigotes or amastigotes induced DCs to mature rapidly, as shown by the over-expression of co- stimulatory, adhesion and MHC class II molecules. Thus, in the absence of specific antibodies (e.g. shortly after infecting naive mammals), infected DCs may remain immature or semi-mature, meaning that they are unable to elicit an efficient anti-Leishmania T cell response. Absence of DC maturation or delayed/incomplete DC maturation could thus be beneficial for the parasites, allowing their establishment and amplification before the onset of immune responses. Supplemental data available online Key words: Leishmania, Promastigote, Amastigote, Dendritic cell, Phagocytosis, Parasitophorous vacuole, Maturation Summary Dendritic cells as host cells for the promastigote and amastigote stages of Leishmania amazonensis: the role of opsonins in parasite uptake and dendritic cell maturation Eric Prina 1, *, Sofiane Zaki Abdi 1 , Maï Lebastard 1 , Emmanuelle Perret 2 , Nathalie Winter 3 and Jean-Claude Antoine 1 1 Unité d’Immunophysiologie et Parasitisme Intracellulaire, 2 Centre d’Imagerie Dynamique and 3 Unité de Génétique Mycobactérienne, Institut Pasteur, Paris, France *Author for correspondence (e-mail: eprina@pasteur.fr) Accepted 5 September 2003 Journal of Cell Science 117, 315-325 Published by The Company of Biologists 2004 doi:10.1242/jcs.00860 Research Article