Dendritic cells as host cells for the promastigote and amastigote stages ofLeishmania amazonensis: the role of opsonins in parasite uptake and dendritic cell maturation

Abstract

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 costimulatory, 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.

Leishmania amazonensis impairs DC function by inhibiting CD40 expression via A2B adenosine receptor activation

“…Different Leishmania species are known to be able to interfere with DC surface marker expression [22][23][24][25][26]. However, in most of these studies, bulk effects were analysed and no differentiation was made between cells that became infected and cells that, albeit being in contact with the parasite, remained uninfected was made.…”

The amastigote forms of Leishmania are experts at exploiting host cell processes to establish infection and persist

“…Leishmania amastigote entry into dendritic cells, which occurs without induction of maturation, has been characterized as 'silent entry' (Bennett et al, 2001;Brandonisio et al, 2004). Although amastigote entry does not induce dendritic cell maturation, infected dendritic cells can be induced to undergo maturation in response to exogenous stimuli (Prina et al, 2004). This would suggest that the parasite's effects on the cell are mostly localized.…”

“…Apparently, amastigotes avoid the activation of dendritic cells, not by elaborating virulence factors that deactivate host cell processes, but by selective interaction with a surface receptor that does not result in cell activation. Uptake of antibody opsonized amastigotes resulted in the induction of dendritic cell maturation (Prina et al, 2004).…”

Estudio ultraestructural de la fagocitosis de promastigotes y amastigotes de Leishmania mexicana por la línea de células dendríticas FSDC

“…Se empleó la cepa de L. mexicana 000/00/WR-L11 CL802, donada al banco de criopreservación Los amastigotes se extrajeron de la lesión desarrollada en la almohadilla plantar de ratones BALB/c inoculados 9 semanas antes con 1x10 6 promastigotes en 50 µl de solución salina estéril. El tejido obtenido se maceró en un filtro metálico con el émbolo de una jeringa plástica, adicionando lentamente 10 ml de medio de Schneider.…”

“…Si bien la aparición de gran cantidad de vacuolas en las células que no fagocitaron puede tener causas diversas, una posibilidad es que se deba a un estímulo por parte de las células infectadas, como se ha propuesto (6). Sin embargo, esto tendría que demostrarse por métodos de inmunomicroscopía electrónica para este caso.…”