Plasmodium falciparum malaria is a major public health problem, infecting 5–10 percent of the global population and killing two million children annually. Those affected by and dying of malaria may variously suffer single-organ, multi-organ or systemic involvement, including cerebral malaria (CM), renal failure, pulmonary edema, acute respiratory distress, metabolic acidosis, hypoglycemia, coagulopathy, shock, and severe malarial anemia (SMA).Particularly among African children, the three most serious syndromes are CM, metabolic acidosis, and SMA. Despite their profound importance for global public health, the molecular etiology of these syndromes remains obscure.
Nonetheless, disease syndromes and fatalities are understood to result from the intersection of four fundamental processes: (1) the site-specific localization of parasites among target organs through vascular cytoadherence or sequestration; (2) the local and systemic action of bioactive parasite products, such as toxins, on host tissues; (3) the local and systemic production of pro-inflammatory and counter-regulatory cytokines and chemokines by the innate and acquired immune systems in response to parasite products; and (4) the activation, recruitment, and infiltration of inflammatory cells. I have been focusing on the role of innate regulatory mechanisms and a malarial toxin as key determinants of malarial pathogenesis, particularly in rodent models. And I am now extending my work to the role of the innate system and malaria toxin in pathogenesis and clinical immunity in human populations.This requires an integrated approach at the molecular, cellular, whole animal, and population biology levels.
Specifically, I am continuing to investigate the molecular basis of the CM and SMA syndromes using superior animal models, immunological and pathophysiological endpoints, microarray profiling, pure glycosylphosphatidylinositol (GPI), and anti-GPI vaccination during rodent malaria infection. As an example of this work, we recently developed a credible rodent model of SMA. Severe malarial anemia of low parasite burden in rodent models results from accelerated clearance of uninfected erythrocytes. SMA is the most serious pernicious complication of malaria and may contribute to the majority of malarial deaths worldwide. The syndrome is thought to arise in part from increased destruction of uninfected red blood cells (RBCs); however, there is a paucity of experimental evidence concerning this process. In general, the study of SMA in animal models is confounded by susceptibility to excessive parasitemias that induce hemolytic anemia. Hemolysis due to hyperparasitization may not be informative with regard to the etiology of human SMA, which is typically associated with comparatively low parasite burdens. In this study we are describing Plasmodium berghei infections of semi-immune mice and naive rats with low parasite burdens that develop pronounced SMA, similar to that seen in semi-immune monkeys and experimental infections in naive humans. We found that SMA was independent of the level of peak or cumulative parasitemia but was linked temporally to the duration of patent infection. In animals with SMA, the entire blood compartment was turned over in approximately one week. The survival rate of both resident and transfused uninfected RBCs was markedly reduced in anemic animals, but reverted to normal upon transfer of RBCs from anemic donors to naive recipients, suggesting no lasting changes to target RBCs. Anemia was significantly alleviated by depletion of host phagocytic cells and CD4+ T lymphocytes and thus appears to result predominantly from accelerated reticuloendothelial phagocytosis of uninfected RBCs, a process that is under the control of the acquired immune system. We are currently developing similar models of erythropoietic suppression in malarial anemia. Models such as these should provide tractable systems to test prevailing hypotheses of the etiology of these important disease syndromes.
An integral part of the research program is to undertake translational research activities in human biology and field settings. Together with Dr. Ivo Mueller and Dr. John Reeder of the Papua New Guinea Institute of Medical Research (PNGIMR), I am coinvestigator responsible for human immunology in a longitudinal, prospective case-cohort study titled “Intermittent Preventive Therapy During EPI for the Prevention of Malaria and Anemia in Papua New Guinean Infants,” which was recently funded by the Bill and Melinda Gates Foundation in the context of the worldwide IPTi consortium (http://www.IPTi-malaria.org). Starting in January 2006, 1518 children are being followed in a three-year longitudinal cohort case-control study, with regular sampling for sera, blood, and peripheral blood mononuclear cells (PBMCs). We will examine disease associations with serological and cellular immunological parameters. Primary outcome measures will be the impact of IPTi on incidence and prevalence of clinical malaria, incidence of anemia, and Hb levels as a quantitative trait. Field-site capabilities include FACS and quantitative PCR. The study is nested in an ongoing demographic surveillance (DSS) catchment with case detection of 20,000 individuals. Also with Dr. Mueller of PNGIMR, I actively collaborate in two additional funded research projects. In the first, under the aegis of a Merit Award grant to Dr. Chris King (Case Western University) that is funded by the U.S. Veterans Affairs Administration, we are investigating serological and cellular correlates of immunity in a "time-to-reinfection" trial, in which a cohort of semi-immune primary school children have received radical antimalarial treatment and are being followed for a year with regular biweekly bleeds, clinical measurements and questionnaire, and both active and passive case detection. NK cell responses are being matched prospectively with risk of acute morbid episode and Hb levels. In the second project, I collaborate on the Papua New Guinea Institute of Medical Research's grant number NIH RO3-AI63135, undertaking in Papua New Guinea's Wosera region a two-year longitudinal study of 280 one- to four-year-olds, which involves bimonthly bleeds, fortnightly active morbidity surveillance, and comprehensive passive case detection. Plasma and PBMC samples from strictly defined cases and controls, and baseline and ongoing samples will be used to test positive and negative associations of GPI bioactivity, anti-GPI antibody responses, NK cell responses, and innate immune pathways with susceptibility and resistance to disease. These studies will ascertain the role of the toxin and innate responses in disease and the role of antitoxin antibodies and counter-regulatory mechanisms in clinical immunity to malaria. The findings may provide a rational basis for future interventions against key causal pathways in fatal pathogenesis.
Last updated August 2008
