To determine the potential contribution of innate immune responses to the early proinflammatory cytokine response to Plasmodium falciparum malaria, we have examined the kinetics and cellular sources of IFN-γ production in response to human PBMC activation by intact, infected RBC (iRBC) or freeze-thaw lysates of P. falciparum schizonts. Infected erythrocytes induce a more rapid and intense IFN-γ response from malaria-naive PBMC than do P. falciparum schizont lysates correlating with rapid iRBC activation of the CD3−CD56+ NK cell population to produce IFN-γ. IFN-γ+ NK cells are detectable within 6 h of coculture with iRBC, their numbers peaking at 24 h in most donors. There is marked heterogeneity between donors in magnitude of the NK-IFN-γ response that does not correlate with mitogen- or cytokine-induced NK activation or prior malaria exposure. The NK cell-mediated IFN-γ response is highly IL-12 dependent and appears to be partially IL-18 dependent. Exogenous rIL-12 or rIL-18 did not augment NK cell IFN-γ responses, indicating that production of IL-12 and IL-18 is not the limiting factor explaining differences in NK cell reactivity between donors or between live and dead parasites. These data indicate that NK cells may represent an important early source of IFN-γ, a cytokine that has been implicated in induction of various antiparasitic effector mechanisms. The heterogeneity of this early IFN-γ response between donors suggests a variation in their ability to mount a rapid proinflammatory cytokine response to malaria infection that may, in turn, influence their innate susceptibility to malaria infection, malaria-related morbidity, or death from malaria.

SUMMARYThroughout history malaria has proved to be a significant threat to human health. Between 300 and 500 million clinical cases occur each year worldwide, approximately 2 million of which are fatal, primarily in children. The vast majority of malaria-related deaths are due to infection with Plasmodium falciparum ; P. vivax causes severe febrile illness but is rarely fatal. Following repeated exposure to infection, people living in malaria endemic areas gradually acquire mechanisms to limit the inflammatory response to the parasite that causes the acute febrile symptoms (clinical immunity) as well as mechanisms to kill parasites or inhibit parasite replication (antiparasite immunity). Children, who have yet to develop protective immune mechanisms are thus at greater risk of clinical malaria, severe disease and death than adults. However, two epidemiological observations indicate that this is, perhaps, an oversimplified model. Firstly, cerebral malaria -a common manifestation of severe malaria -typically occurs in children who have already acquired a significant degree of antimalarial immunity, as evidenced by lower mean parasite densities and resistance to severe anaemia. One potential explanation is that cerebral malaria is, in part, an immune-mediated disease in which immunological priming occurs during first infection, eventually leading to immunopathology on re-infection. Secondly, among travelers from nonendemic areas, severe malaria is more common -and death rates are higher -in adults than in children. If severe malaria is an immune-mediated disease, what might be priming the immune system of adults from nonendemic areas to cause immunopathology during their first malaria infection, and how do adults from endemic areas avoid severe immunopathology? In this review we consider the role of innate and adaptive immune responses in terms of (i) protection from clinical malaria (ii) their potential role in immunopathology and (iii) the subsequent development of clinical immunity. We conclude by proposing a model of antimalarial immunity which integrates both the immunological and epidemiological data collected to date. Keywords parasitic-protozoa human malaria immunoregulation ANTI-MALARIAL EFFECTOR MECHANISMSAs many of the surface antigens of malaria parasites and the parasite proteins inserted into the plasma membrane of the infected red blood cell are polymorphic or exhibit clonal antigenic variation, it has been proposed that one may need to develop a diverse repertoire of antibodies capable of blocking parasite invasion and tissue adhesion in order to attain effective antiparasite immunity [1]. For example, P. falciparum erythrocyte membrane protein 1 (PfEMP1), an antigen involved in parasite sequestration [2] and possibly pathogenesis of cerebral malaria (CM) [3], is encoded by a family of var genes which undergo frequent nonhomologous recombination leading to heterologous expression of antigenic variants by different parasites. Infection with a parasite variant that is not recognized by the existing an...

Human NK cells are the earliest source of the protective cytokine IFN-γ when PBMC from nonimmune donors are exposed to Plasmodium falciparum-infected RBC (iRBC) in vitro. In this study, we show that human NK cells form stable conjugates with iRBC but not with uninfected RBC and that induction of IFN-γ synthesis is dependent on direct contact between the NK cell and the iRBC. NK cells respond to iRBC only in the presence of a source of IL-12/IL-18 and the subset of NK cells that preferentially respond to iRBC express high levels of the lectin-like receptor CD94/NKG2A. There is heterogeneity between donors in their ability to respond to iRBC. DNA analysis has revealed considerable heterogeneity of killer Ig-like receptor (KIR) genotype among the donor population and has identified 21 new KIR allelic variants in the donors of African and Asian descent. Importantly, we find evidence for significant associations between KIR genotype and NK responsiveness to iRBC. This emphasizes the need for large-scale population-based studies to address associations between KIR genotype and susceptibility to malaria.

Immature dendritic cells (DCs) capture exogenous Ags in the periphery for eventual processing in endolysosomes. Upon maturation by TLR agonists, DCs deliver peptide-loaded class II MHC molecules from these compartments to the cell surface via long tubular structures (endolysosomal tubules). The nature and rules that govern the movement of these DC compartments are unknown. In this study, we demonstrate that the tubules contain multiple proteins including the class II MHC molecules and LAMP1, a lysosomal resident protein, as well as CD63 and CD82, members of the tetraspanin family. Endolysosomal tubules can be stained with acidotropic dyes, indicating that they are extensions of lysosomes. However, the proper trafficking of class II MHC molecules themselves is not necessary for endolysosomal tubule formation. DCs lacking MyD88 can also form endolysosomal tubules, demonstrating that MyD88-dependent TLR activation is not necessary for the formation of this compartment. Endolysosomal tubules in DCs exhibit dynamic and saltatory movement, including bidirectional travel. Measured velocities are consistent with motor-based movement along microtubules. Indeed, nocodazole causes the collapse of endolysosomal tubules. In addition to its association with microtubules, endolysosomal tubules follow the plus ends of microtubules as visualized in primary DCs expressing end binding protein 1 (EB1)-enhanced GFP.