Dendritic cells and Plasmodium falciparum: studies in vitro and in the human host

Abstract: Malaria is one of the world’s most threatening diseases. About half the world’s population is at risk of infection and the infection claims a million lives each year. A vast majority of the deaths occur in children below the age of 5 in sub-Saharan Africa. Survivors typically acquire immunity only after long time of repeated exposure and immunity is rapidly lost. Immunity is created by the activation of naive T cells and their differentiation into effector cells. The most potent activators of naive T cells are dendritic cells (DCs). The life cycle of DCs is adapted to find and process microbes in order to be able to present their antigens to T cells and thereby activate them. Antigen presentation typically takes place in the lymph nodes and that is why migration to these areas is an essential part of the DC life cycle. Various studies have shown that DC function may be hampered by the malaria parasite or its components.We have investigated activation and migratory capacities of DCs upon in vitro exposure of the malarial pigment hemozoin and Plasmodium falciparum infected red blood cells. Furthermore, we have assessed the activation status of blood DCs in the Fulani, a traditionally nomadic population that respond better to malaria infection and exhibit less clinical symptoms than other ethnicities living under similar conditions, and a neighbouring ethnic group, the Dogon, in Mali.Our results indicate that DCs are semi-activated upon malaria exposure in vitro, including enhanced migratory capacity, partial up-regulation of co-stimulatory markers and no IL-12, which may lead to inappropriate T-cell priming. We also observed that DCs from the Fulani have a higher degree of activation than DCs from the Dogon upon malaria exposure in vivo.We hypothesize that this increased DC activation may be the reason for the relatively increased protection against malaria.Taken together, our findings suggest that improper DC activation may contribute to poor immunity in Malaria.