Date Published: May 16, 2013
Publisher: Public Library of Science
Author(s): Steve M. Taylor, Carla Cerami, Rick M. Fairhurst, Chetan E. Chitnis.
Plasmodium falciparum malaria kills over 500,000 children every year and has been a scourge of humans for millennia. Owing to the co-evolution of humans and P. falciparum parasites, the human genome is imprinted with polymorphisms that not only confer innate resistance to falciparum malaria, but also cause hemoglobinopathies. These genetic traits—including hemoglobin S (HbS), hemoglobin C (HbC), and α-thalassemia—are the most common monogenic human disorders and can confer remarkable degrees of protection from severe, life-threatening falciparum malaria in African children: the risk is reduced 70% by homozygous HbC and 90% by heterozygous HbS (sickle-cell trait). Importantly, this protection is principally present for severe disease and largely absent for P. falciparum infection, suggesting that these hemoglobinopathies specifically neutralize the parasite’s in vivo mechanisms of pathogenesis. These hemoglobin variants thus represent a “natural experiment” to identify the cellular and molecular mechanisms by which P. falciparum produces clinical morbidity, which remain partially obscured due to the complexity of interactions between this parasite and its human host. Multiple lines of evidence support a restriction of parasite growth by various hemoglobinopathies, and recent data suggest this phenomenon may result from host microRNA interference with parasite metabolism. Multiple hemoglobinopathies mitigate the pathogenic potential of parasites by interfering with the export of P. falciparum erythrocyte membrane protein 1 (PfEMP1) to the surface of the host red blood cell. Few studies have investigated their effects upon the activation of the innate and adaptive immune systems, although recent murine studies suggest a role for heme oxygenase-1 in protection. Ultimately, the identification of mechanisms of protection and pathogenesis can inform future therapeutics and preventive measures. Hemoglobinopathies slice the “Gordian knot” of host and parasite interactions to confer malaria protection, and offer a translational model to identify the most critical mechanisms of P. falciparum pathogenesis.
In the 4th century BC, Alexander the Great conquered the known Western world . Prior to his conquests in Asia, he encountered the Gordian knot, a complex knot of bark affixing a mythic ox-cart to a post in the town of Gordium. Alexander—a pupil of Aristotle—set his mind to untangling the knot, but, like others before him, could not find the ends (and thus the means) to do so. Faced with this intractable problem, Alexander sliced through the Gordian knot with a stroke of his sword and freed the cart. As one of history’s greatest military commanders, Alexander subsequently assembled and ruled an empire stretching from the Eastern Mediterranean to the Himalayas while remaining undefeated in battle. These military conquests were presaged by his “Alexandrian solution” to the Gordian knot, demonstrating decisiveness and imagination in the face of a complex and seemingly unsolvable problem.
The red blood cell (RBC) is critical for the propagation of malaria parasites (Figure 1A). After inoculation into a human by a mosquito and a brief, clinically silent incubation in the liver, P. falciparum parasites enter the erythrocytic stage of their life-cycle. It is during this time that parasites sequentially invade and egress from their host RBCs and cause the signs and symptoms of malaria. While developing within the RBC, the parasite traffics proteins to the RBC surface that mediate binding to extracellular host receptors and enable the parasite to sequester in the placenta, brain, and virtually every other organ. The attenuation of malaria by repeated, sub-lethal P. falciparum infections suggests a significant role for adaptive immunity, but the targets of this attenuating immune response remain largely obscure. Though this adaptive immunity can be protective, the development of maladaptive and dysregulated immune responses can also contribute to the pathogenesis of malaria.
Numerous investigations of the invasion and growth of P. falciparum in RBCs containing variant hemoglobins rapidly followed the development of in-vitro cultivation systems by Trager and Jensen, and Haynes et al. in 1976 (Table 2) , . Reductions in RBC invasion have been reported for a variety of hemoglobinopathies including α-thalassemia trait , HbH disease , , HbEE , , HbAE , and the compound heterozygous β-thalassemia/HbE disorder , , ; reductions in the intraerythrocytic growth or maturation of parasites have been reported for HbH disease , , β-thalassemia minor , HbSS , , HbAS , HbCC –, HbEE , HbAE , and HbF –. In addition to these positive findings, conflicting data have been reported from many of these investigations (see Table 2).
Two major pathogenic phenotypes of iRBCs have been described: those that mediate binding of iRBCs to endothelial receptors (“cytoadherence”)  and those that mediate binding of iRBCs to uninfected RBCs (“rosetting”) , . Both adherence phenotypes are conferred by the expression of P. falciparum erythrocyte membrane protein 1 (PfEMP1) –, a family of highly variant proteins that are concentrated in protuberant structures called “knobs” on the iRBC surface. Different PfEMP1 variants mediate the binding of iRBCs to microvascular endothelial cells (via CD36, ICAM-1, etc.) , placental syncytiotrophoblasts (via chondroitin sulfate A) , , and uninfected RBCs (via complement receptor 1, A and B blood group antigens, and heparin sulfate-like antigens) –. Other pathogenic mechanisms that may be associated with disease include the production of cytokines in response to P. falciparum glycosylphosphatidylinositol (PfGPI)  and parasite-derived uric acid , direct hemolysis due to parasite egress from RBCs, and PfEMP1-mediated suppression of inflammatory cytokines (discussed below) .
There is an emerging recognition of the impact of aberrant host responses in the pathogenesis of malaria, particularly severe falciparum malaria (reviewed in –). Studies of adjunctive interventions to modulate this response in humans have not yielded sustained successes , but experiments in murine models continue to demonstrate benefit , and new modalities remain under active investigation , , .
Evidence from field studies supports an association between several hemoglobinopathies, adaptive immunity, and protection from malaria , , though investigations of these relationships are complicated by the absence of reliable correlates of immune protection.
In this review, we have artificially partitioned the evidence for diverse mechanisms of protection, but pathogenic pathways overlap substantially, and it is similarly likely that protective mechanisms in vivo also involve multiple pathways. As noted above, field evidence indicates that hemoglobinopathies do not impair parasite infection but instead attenuate malaria; this pattern suggests that protection from malaria syndromes is not mediated against the pre-erythrocytic stages of the P. falciparum life-cycle, and that hemoglobinopathies may influence the transition from parasite infection to disease.
In the spirit of Alexander, we propose that hemoglobinopathies may be nature’s “Alexandrian solution” to the problem of understanding fundamental aspects of falciparum malaria. This bold slice through the Gordian knot of malaria pathogenesis represents a unique opportunity to isolate and identify the molecular correlates of falciparum malaria pathogenesis in humans in vivo, and to translate these findings into future interventions to prevent, treat, and eliminate this ancient and intractable scourge.