Cerebral malaria (CM) develops in a small proportion of persons infected with Plasmodium falciparum and accounts for a substantial proportion of the mortality due to this parasite. The actual pathogenic mechanisms are still poorly understood, and in humans investigations of experimental CM are unethical. Using an established Plasmodium berghei-mouse CM model, we have investigated the role of host immune cells at the pathological site, the brain. We report in this study the detailed quantification and characterization of cells, which migrated and sequestered to the brain of mice with CM. We demonstrated that CD8+ αβ T cells, which sequester in the brain at the time when neurological symptoms appear, were responsible for CM mortality. These observations suggest a mechanism which unifies disparate observations in humans.

After mosquito bite, the malaria parasite first infects the liver, where it is thought to be undetected by the host immune system as it develops into the blood-stage pathogen. Maria Mota and her colleagues now report that Plasmodium RNA is detected by hepatocytes, triggering an interferon response that controls the parasite burden in the liver and blood of infected mice. Before they infect red blood cells and cause malaria, Plasmodium parasites undergo an obligate and clinically silent expansion phase in the liver that is supposedly undetected by the host. Here, we demonstrate the engagement of a type I interferon (IFN) response during Plasmodium replication in the liver. We identified Plasmodium RNA as a previously unrecognized pathogen-associated molecular pattern (PAMP) capable of activating a type I IFN response via the cytosolic pattern recognition receptor Mda5. This response, initiated by liver-resident cells through the adaptor molecule for cytosolic RNA sensors, Mavs, and the transcription factors Irf3 and Irf7, is propagated by hepatocytes in an interferon-α/β receptor–dependent manner. This signaling pathway is critical for immune cell–mediated host resistance to liver-stage Plasmodium infection, which we find can be primed with other PAMPs, including hepatitis C virus RNA. Together, our results show that the liver has sensor mechanisms for Plasmodium that mediate a functional antiparasite response driven by type I IFN.

In this study we present the first systematic analysis of the immunity induced by normal Plasmodium yoelii sporozoites in mice. Immunization with sporozoites, which was conducted under chloroquine treatment to minimize the influence of blood stage parasites, induced a strong protection against a subsequent sporozoite and, to a lesser extent, against infected RBC challenges. The protection induced by this immunization protocol proved to be very effective. Induction of this protective immunity depended on the presence of liver stage parasites, as primaquine treatment concurrent with sporozoite immunization abrogated protection. Protection was not found to be mediated by the Abs elicited against pre-erythrocytic and blood stage parasites, as demonstrated by inhibition assays of sporozoite penetration or development in vitro and in vivo assays of sporozoite infectivity or blood stage parasite development. CD4⁺ and CD8⁺ T cells were, however, responsible for the protection through the induction of IFN-γ and NO.

IFN‐γ has been implicated in the pathogenesis of experimental cerebral malaria (ECM). We have used mice lacking the α chain of the IFN‐γ receptor (KO mice) to define its role in the pathogenesis of ECM. Infected KO mice did not develop ECM and showed no leukocyte or parasite sequestration in the brain, and no hemorrhages. The resistance of KO mice to ECM was associated with the absence of any increases of TNF‐α and ICAM‐1 proteins in the brain, which are both essential for ECM. Wild‐type (WT) mice which do not develop ECM, despite increased local production of TNF‐α protein, showed no leukocyte accumulation in the brain and this was correlated with the absence of ICAM‐1 protein from brain microvessels. KO mice infected with 10⁶ parasitized erythrocytes (PE) of Plasmodium berghei ANKA (PbA) did not develop ECM, but they had high parasitemia and died earlier than WT mice which did not develop ECM. However, KO mice did not develop higher parasitemia than WT mice when both groups were infected with a lower dose (5×10⁵ PE) of PbA‐infected red blood cells. This indicates that different doses of PE may trigger different IFN‐γ responses and that there may be a threshold concentration for protection against parasitemia.

Cerebral malaria is the most severe complication of human infection withPlasmodium falciparum. It was shown thatPlasmodium bergheiANKA-induced cerebral malaria was prevented in 100% of mice depleted of CD8⁺T cells 1 day prior to the development of neurological signs. However, the importance of parasites in the brains of these mice was never clearly investigated. Moreover, the relevance of this model to human cerebral malaria has been questioned many times, especially concerning the relative importance of leukocytes versus parasitized erythrocytes sequestered in the brain. Here, we show that mice protected from cerebral malaria by CD8⁺T-cell depletion have significantly fewer parasites in the brain. Treatment of infected mice with an antimalarial drug 15 to 20 h prior to the estimated time of death also protected mice from cerebral malaria without altering the number of CD8⁺T cells in the brain. These mice subsequently developed cerebral malaria with parasitized red blood cells in the brain. Our results clearly demonstrated that sequestration of CD8⁺T cells in the brain is not sufficient for the development of cerebral malaria in C57BL/6 mice but that the concomitant presence of parasitized red blood cells is crucial for the onset of pathology. Importantly, these results also demonstrated that the experimental cerebral malaria model shares many features with human pathology and might be a relevant model to study its pathogenesis.