During the blood stages of malaria, several hundred parasite-encoded proteins are exported beyond the double-membrane barrier that separates the parasite from the host cell cytosol. These proteins have a variety of roles that are essential to virulence or parasite growth. There is keen interest in understanding how proteins are exported and whether common machineries are involved in trafficking the different classes of exported proteins. One potential trafficking machine is a protein complex known as the Plasmodium translocon of exported proteins (PTEX). Although PTEX has been linked to the export of one class of exported proteins, there has been no direct evidence for its role and scope in protein translocation. Here we show, through the generation of two parasite lines defective for essential PTEX components (HSP101 or PTEX150), and analysis of a line lacking the non-essential component TRX2 (ref. 12), greatly reduced trafficking of all classes of exported proteins beyond the double membrane barrier enveloping the parasite. This includes proteins containing the PEXEL motif (RxLxE/Q/D) and PEXEL-negative exported proteins (PNEPs). Moreover, the export of proteins destined for expression on the infected erythrocyte surface, including the major virulence factor PfEMP1 in Plasmodium falciparum, was significantly reduced in PTEX knockdown parasites. PTEX function was also essential for blood-stage growth, because even a modest knockdown of PTEX components had a strong effect on the parasite's capacity to complete the erythrocytic cycle both in vitro and in vivo. Hence, as the only known nexus for protein export in Plasmodium parasites, and an essential enzymic machine, PTEX is a prime drug target.

Although CD8 ؉ T cells do not contribute to protection against the blood stage of Plasmodium infection, there is mounting evidence that they are principal mediators of murine experimental cerebral malaria (ECM). At present, there is no direct evidence that the CD8 ؉ T cells mediating ECM are parasite-specific or, for that matter, whether parasite-specific CD8 ؉ T cells are generated in response to blood-stage infection. To resolve this and to define the cellular requirements for such priming, we generated transgenic P. berghei parasites expressing model T cell epitopes. This approach was necessary as MHC class I-restricted antigens to blood-stage infection have not been defined. Here, we show that blood-stage infection leads to parasite-specific CD8 ؉ and CD4 ؉ T cell responses. Furthermore, we show that P. berghei-expressed antigens are cross-presented by the CD8␣ ؉ subset of dendritic cells (DC), and that this induces pathogen-specific cytotoxic T lymphocytes (CTL) capable of lysing cells presenting antigens expressed by bloodstage parasites. Finally, using three different experimental approaches, we provide evidence that CTL specific for parasiteexpressed antigens contribute to ECM. dendritic cells ͉ malaria ͉ antigen presentation ͉ cytotoxic T lymphocyte ͉ cerebral malaria

NK cells are cytotoxic lymphocytes that also secrete regulatory cytokines and can therefore influence adaptive immune responses. NK cell function is largely controlled by genes present in a genomic region named the NK complex. It has been shown that the NK complex is a genetic determinant of murine cerebral malaria pathogenesis mediated by Plasmodium berghei ANKA. In this study, we show that NK cells are required for cerebral malaria disease induction and the control of parasitemia. NK cells were found infiltrating brains of cerebral malaria-affected mice. NK cell depletion resulted in inhibition of T cell recruitment to the brain of P. berghei-infected animals. NK cell-depleted mice displayed down-regulation of CXCR3 expression and a significant reduction of T cells migrating in response to IFN-γ-inducible protein 10, indicating that this chemokine pathway plays an essential role in leukocyte trafficking leading to cerebral disease and fatalities.

Plasmodium falciparum malaria causes 660 million clinical cases with over 2 million deaths each year. Acquired host immunity limits the clinical impact of malaria infection and provides protection against parasite replication. Experimental evidence indicates that cell-mediated immune responses also result in detrimental inflammation and contribute to severe disease induction. In both humans and mice, the spleen is a crucial organ involved in blood stage malaria clearance, while organ-specific disease appears to be associated with sequestration of parasitized erythrocytes in vascular beds and subsequent recruitment of inflammatory leukocytes. Using a rodent model of cerebral malaria, we have previously found that the majority of T lymphocytes in intravascular infiltrates of cerebral malaria-affected mice express the chemokine receptor CXCR3. Here we investigated the effect of IP-10 blockade in the development of experimental cerebral malaria and the induction of splenic anti-parasite immunity. We found that specific neutralization of IP-10 over the course of infection and genetic deletion of this chemokine in knockout mice reduces cerebral intravascular inflammation and is sufficient to protect P. berghei ANKA-infected mice from fatality. Furthermore, our results demonstrate that lack of IP-10 during infection significantly reduces peripheral parasitemia. The increased resistance to infection observed in the absence of IP-10-mediated cell trafficking was associated with retention and subsequent expansion of parasite-specific T cells in spleens of infected animals, which appears to be advantageous for the control of parasite burden. Thus, our results demonstrate that modulating homing of cellular immune responses to malaria is critical for reaching a balance between protective immunity and immunopathogenesis.