Malaria: Progress in the Host-Pathogen Arms Race

The 2015 Nobel Prize in Physiology or Medicine was awarded to three scientists for their discoveries on novel therapies against parasitic infections. Youyou Tu from China was honored for isolating artemisinin, the active ingredient from the Chinese herb Artemesia annua, which rapidly kills malaria parasites. Artemisinin combination therapy is the current gold standard treatment for malaria, and it has had a major impact on diminishing the burden of the disease. Malaria parasites are transmitted to humans by female Anopheles mosquitoes, and, after initial replication in the liver, they invade and subsequently destroy red blood cells (RBCs), which triggers the clinical symptoms of the disease. The complex parasite lifecycle in both the human host and the mosquito vector has necessitated a multipronged approach in global efforts to eliminate this remarkably successful pathogen. Preventive measures, including the use of insecticide-treated bed nets and indoor residual spraying with insecticides, have been combined with rapid diagnostic test kits in order to detect malaria as early as possible. These interventions, followed by immediate and appropriate treatment of the patient, have contributed to a dramatic global decline in malaria since 2000 and to achievement of the United Nations’ Millennium Development Goal by 2015.¹ 

Despite this progress, 3.2 billion people remain at risk of contracting malaria. According to the latest World Health Organization report,²  an estimated 214 million new cases and approximately 438,000 deaths were reported in 2015. Sub-Saharan Africa continues to bear the brunt of the disease, accounting for 88 percent of the morbidity and 90 percent of the mortality, mainly in children younger than five years.

The gains that have been achieved are threatened by the development of insecticide resistance in African vector species and by the alarmingly rapid development and spread of drug-resistant parasites, which have rendered several antimalarial drugs useless. Resistance to artemisinin by Plasmodium falciparum, the most deadly of the five species of Plasmodium parasites that infect humans, has recently been detected in five countries in the Greater Mekong region. This threat has accelerated research into the biology of P. falciparum and its interaction with the human host, and has fueled efforts to develop new drugs and a vaccine. Several major advances have been reported this year, and results from four studies are highlighted here.

In a multi-institutional collaboration³  led by Dr. Kasturi Haldar, the specific molecular target of artemisinin in early ring-stage P. falciparum was identified as phosphatidylinositol-3-kinase (PfPI3K). However, PfPI3K polymorphisms were not detected in all resistant strains, and previous genome-wide association studies (GWAS) revealed selective pressure on chromosome 13, in particular on the pfkelch13 gene. The mammalian orthologue is a substrate adaptor for an E3 ubiquitin ligase; the putative substrate binding domain of the parasite PfKelch13 protein has a characteristic kelch propeller domain, and mutations in this region correlated with artemisinin resistance. In an elegant series of biochemical and cell biology experiments, the researchers demonstrated that one of the most prevalent mutations, C580Y, inhibited the binding of PfKelch13 to PfPI3K, which decreased ubiquitination of the enzyme and limited subsequent proteasomal degradation, resulting in increased amounts of enzyme and its lipid product, phosphatidylinositol-3-phosphate (PI3P). It is hypothesized that artemisinin resistance is mediated by PIP3-induced amplification of downstream targets that are currently unknown. This study is important since it provides a tool to screen and monitor clinical parasite isolates for the presence of artemisinin resistance, and it identifies a target that can be exploited for novel drug development.

The invasion of RBCs by P. falciparum is a complex process, and interactions between host receptors and parasite ligands are not fully understood. Genetic manipulation experiments to identify host factors influencing susceptibility to infection have been hampered by the absence of a nucleus in mature erythrocytes. To overcome this challenge, Dr. Elizabeth Egan and colleagues⁴  exploited two recent technological advances of RNA interference and ex vivo production of RBCs to design a forward hematopoietic stem cell-based genetic screen. They identified CD55 or decay-accelerating factor (DAF) as an essential host receptor and verified their laboratory findings by demonstrating that RBCs from a patient lacking CD55 were refractory to invasion by P. falciparum. CD55 is a membrane-bound glycoprotein that regulates the complement system, and these findings suggest that the role of complement in parasite invasion and pathogenesis should be re-examined. The study also identifies CD55 as a potential target for the development of novel antimalaria therapeutics.

P. falciparum is a highly virulent parasite that can cause severe disease, including cerebral malaria. Features that contribute to virulence include binding of infected RBCs to the microvasculature and also to other RBCs to form rosettes. These host-pathogen interactions are poorly understood. Dr. Mats Wahlgren and colleagues⁵  used molecular and cell biology approaches to demonstrate that P. falciparum expressed repetitive interspersed families of polypeptides (RIFINs) on the surface of infected RBCs and formed rosettes by preferentially binding to blood group A erythrocytes. RIFINs also mediated microvasculature sequestration by interacting with A antigens on endothelial cells. These findings indicate that RIFINs play a major role in the pathogenesis of severe malaria and that blood group A predisposes to the development of severe disease, which correlates with previous clinical studies on African children. This study highlights the impact of malaria on the human genome and suggests that RIFINs contributed to the evolutionary pressure that led to the emergence of the protective blood group O and the variable global distribution of human ABO blood groups.

The development of a malaria vaccine has been actively pursued for several decades but has been an elusive and frustrating target. The RTS,S Clinical Trials Partnership published the final results⁶  of an individually randomized, controlled phase III trial on the most advanced malaria vaccine candidate RTS,S/AS01. More than 15,000 participants, including young children (ages 5 to 17 months) and infants (ages 6 to 12 weeks) were enrolled in seven sub-Saharan African countries representing different levels of malaria transmission. The vaccine was administered at months 0, 1, and 2, with a booster at 20 months. The results showed modest protection against clinical P. falciparum malaria throughout 48 months, with a vaccine efficacy of 36.3 percent in young children and 25.9 percent in infants, and illustrated the importance of the booster to sustain the protective effect. Recent recommendations from international expert committees who evaluated the trial are to conduct three to five real-world demonstration pilot studies with 200,000 African children each time over the next few years to assess the impact and feasibility of implementing the four-dose vaccine strategy.

In conclusion, despite the major advances reported in these four studies, they have highlighted our lack of in-depth understanding of the complex interplay between the malaria parasite and its human host. Numerous challenges remain and it will require an enhanced effort not only by researchers, but also by political leaders and funders, to ensure that the current momentum is not lost in the battle against this formidable pathogen.