Cerebral malaria claims more than 1 million lives per year. We report that heme oxygenase-1 (HO-1, encoded by Hmox1) prevents the development of experimental cerebral malaria (ECM). BALB/c mice infected with Plasmodium berghei ANKA upregulated HO-1 expression and activity and did not develop ECM. Deletion of Hmox1 and inhibition of HO activity increased ECM incidence to 83% and 78%, respectively. HO-1 upregulation was lower in infected C57BL/6 compared to BALB/c mice, and all infected C57BL/6 mice developed ECM (100% incidence). Pharmacological induction of HO-1 and exposure to the end-product of HO-1 activity, carbon monoxide (CO), reduced ECM incidence in C57BL/6 mice to 10% and 0%, respectively. Whereas neither HO-1 nor CO affected parasitemia, both prevented blood-brain barrier (BBB) disruption, brain microvasculature congestion and neuroinflammation, including CD8(+) T-cell brain sequestration. These effects were mediated by the binding of CO to hemoglobin, preventing hemoglobin oxidation and the generation of free heme, a molecule that triggers ECM pathogenesis.

Intracellular bacteria and parasites typically invade host cells through the formation of an internalization vacuole around the invading pathogen. Plasmodium sporozoites, the infective stage of the malaria parasite transmitted by mosquitoes, have an alternative mechanism to enter cells. We observed breaching of the plasma membrane of the host cell followed by rapid repair. This mode of entry did not result in the formation of a vacuole around the sporozoite, and was followed by exit of the parasite from the host cell. Sporozoites traversed the cytosol of several cells before invading a hepatocyte by formation of a parasitophorous vacuole, in which they developed into the next infective stage. Sporozoite migration through several cells in the mammalian host appears to be essential for the completion of the life cycle.

Plasmodium sporozoites are deposited in the skin of their vertebrate hosts through the bite of an infected female Anopheles mosquito. Most of these parasites find a blood vessel and travel in the peripheral blood circulation until they reach the liver sinusoids. Once there, the sporozoites cross the sinusoidal wall and migrate through several hepatocytes before they infect a final hepatocyte, with the formation of a parasitophorous vacuole, in which the intrahepatic form of the parasite grows and multiplies. During this period, each sporozoite generates thousands of merozoites. As the development of Plasmodium sporozoites inside hepatocytes is an obligatory step before the onset of disease, understanding the parasite's requirements during this period is crucial for the development of any form of early intervention. This Review summarizes our current knowledge on this stage of the Plasmodium life cycle.

Sequestration of malaria-parasite-infected erythrocytes in the microvasculature of organs is thought to be a significant cause of pathology. Cerebral malaria (CM) is a major complication of Plasmodium falciparum infections, and PfEMP1-mediated sequestration of infected red blood cells has been considered to be the major feature leading to CM-related pathology. We report a system for the real-time in vivo imaging of sequestration using transgenic luciferase-expressing parasites of the rodent malaria parasite Plasmodium berghei. These studies revealed that: (i) as expected, lung tissue is a major site, but, unexpectedly, adipose tissue contributes significantly to sequestration, and (ii) the class II scavenger-receptor CD36 to which PfEMP1 can bind is also the major receptor for P. berghei sequestration, indicating a role for alternative parasite ligands, because orthologues of PfEMP1 are absent from rodent malaria parasites, and, importantly, (iii) cerebral complications still develop in the absence of CD36-mediated sequestration, dissociating parasite sequestration from CM-associated pathology. Realtime in vivo imaging of parasitic processes may be used to evaluate the molecular basis of pathology and develop strategies to prevent pathology.imaging ͉ Plasmodium ͉ P. berghei ͉ luciferase ͉ real-time in vivo imaging I nfected red blood cells (irbc) of many species of malaria parasites adhere to the endothelial cells of the microvasculature of numerous deep tissues (1, 2). Termed sequestration, this characteristic may facilitate parasite multiplication, avoiding removal of the irbc by the spleen (3, 4). In some parasite-host combinations, the process of sequestration is associated with pathogenesis, for example, Plasmodium falciparum in humans (1, 2, 5) and Plasmodium berghei in certain mouse strains (6, 7). Cerebral malaria (CM) is a major complication of P. falciparum infections, and the sequestration of irbc has been considered to be the major feature leading to CM-related pathology. Sequestration may lead to vascular obstruction, local endothelial cell activation, and the release of proinflammatory cytokines, resulting in damage to adjacent tissues (2, 7, 8). In P. falciparum, the class II scavenger receptor CD36 is a major endothelial receptor. CD36 is involved in not only the adherence of irbc (1, 9, 10) through specific domains of the surface variant antigen PfEMP-1 but also in the modulation of innate and adaptive immune responses (11,12). To date, most investigations of the dynamics of irbc-receptor interactions rely on in vitro observations with cultured cells and immobilized receptors (2). Despite the increase in knowledge of the molecules involved in the binding of irbc to endothelial cells, the specific interactions that lead to pathology have yet to be established. Infection with P. berghei in laboratory rodents is a well established model for the investigation of associations among CM, proinflammatory cytokines, and endothelial receptors involved in the sequestration of irbc, leukocytes, and platelet...