Superinfection with Plasmodium species increases the risk of fatal disease in individuals with low immunity, yet it is not frequent in young children. Portugal et al. provide a possible explanation for this dichotomy, showing that blood-stage parasitemia in mice inhibits liver-stage development of a second Plasmodium species infection due to iron sequestration. In regions of high rates of malaria transmission, mosquitoes repeatedly transmit liver-tropic Plasmodium sporozoites to individuals who already have blood-stage parasitemia1. This manifests itself in semi-immune children (who have been exposed since birth to Plasmodium infection and as such show low levels of peripheral parasitemia but can still be infected) older than 5 years of age by concurrent carriage of different parasite genotypes at low asymptomatic parasitemias2. Superinfection presents an increased risk of hyperparasitemia and death in less immune individuals but counterintuitively is not frequently observed in the young3,4. Here we show in a mouse model that ongoing blood-stage infections, above a minimum threshold, impair the growth of subsequently inoculated sporozoites such that they become growth arrested in liver hepatocytes and fail to develop into blood-stage parasites. Inhibition of the liver-stage infection is mediated by the host iron regulatory hormone hepcidin5, whose synthesis we found to be stimulated by blood-stage parasites in a density-dependent manner. We mathematically modeled this phenomenon and show how density-dependent protection against liver-stage malaria can shape the epidemiological patterns of age-related risk and the complexity of malaria infections seen in young children. The interaction between these two Plasmodium stages and host iron metabolism has relevance for the global efforts to reduce malaria transmission and for evaluation of iron supplementation programs in malaria-endemic regions.

Malaria remains one of the greatest burdens to global health, causing nearly 500,000 deaths in 2014. When manifesting in the lungs, severe malaria causes acute lung injury/acute respiratory distress syndrome (ALI/ARDS). We have previously shown that a proportion of DBA/2 mice infected with Plasmodium berghei ANKA (PbA) develop ALI/ARDS and that these mice recapitulate various aspects of the human syndrome, such as pulmonary edema, hemorrhaging, pleural effusion and hypoxemia. Herein, we investigated the role of neutrophils in the pathogenesis of malaria-associated ALI/ARDS. Mice developing ALI/ARDS showed greater neutrophil accumulation in the lungs compared with mice that did not develop pulmonary complications. In addition, mice with ALI/ARDS produced more neutrophil-attracting chemokines, myeloperoxidase and reactive oxygen species. We also observed that the parasites Plasmodium falciparum and PbA induced the formation of neutrophil extracellular traps (NETs) ex vivo, which were associated with inflammation and tissue injury. The depletion of neutrophils, treatment with AMD3100 (a CXCR4 antagonist), Pulmozyme (human recombinant DNase) or Sivelestat (inhibitor of neutrophil elastase) decreased the development of malaria-associated ALI/ARDS and significantly increased mouse survival. This study implicates neutrophils and NETs in the genesis of experimentally induced malaria-associated ALI/ARDS and proposes a new therapeutic approach to improve the prognosis of severe malaria. A deeper understanding of the pathogenesis of malaria-associated ALI/ARDS may help in the development of new therapeutic approaches to improve the prognosis of severe cases of malaria. Using the Plasmodium berghei ANKA-infection mouse model of ALI/ARDS, which resembles the human disease in many aspects, this study reports the critical role of neutrophils in the pathogenesis of this syndrome. Mice developing ALI/ARDS showed abundant lung-infiltrating neutrophils in association with the increased production of neutrophil-attracting chemokines, myeloperoxidase and reactive oxygen species. The parasites Plasmodium falciparum and P. berghei ANKA both induced the formation of neutrophil extracellular traps ex vivo. By targeting neutrophils and neutrophil extracellular traps with specific drugs, we succeeded in preventing the development of malaria-associated ALI/ARDS and significantly increased mouse survival.

The spectrum of the clinical presentation and severity of malaria infections is broad, ranging from uncomplicated febrile illness to severe forms of disease such as cerebral malaria (CM), acute lung injury (ALI), acute respiratory distress syndrome (ARDS), pregnancy-associated malaria (PAM) or severe anemia (SA). Rodent models that mimic human CM, PAM and SA syndromes have been established. Here, we show that DBA/2 mice infected with P. berghei ANKA constitute a new model for malaria-associated ALI. Up to 60% of the mice showed dyspnea, airway obstruction and hypoxemia and died between days 7 and 12 post-infection. The most common pathological findings were pleural effusion, pulmonary hemorrhage and edema, consistent with increased lung vessel permeability, while the blood-brain barrier was intact. Malaria-associated ALI correlated with high levels of circulating VEGF, produced de novo in the spleen, and its blockage led to protection of mice from this syndrome. In addition, either splenectomization or administration of the anti-inflammatory molecule carbon monoxide led to a significant reduction in the levels of sera VEGF and to protection from ALI. The similarities between the physiopathological lesions described here and the ones occurring in humans, as well as the demonstration that VEGF is a critical host factor in the onset of malaria-associated ALI in mice, not only offers important mechanistic insights into the processes underlying the pathology related with malaria but may also pave the way for interventional studies. Malaria remains a major source of morbidity and mortality throughout the tropical regions of the world causing up to 1 million deaths every year, mainly in children. Although infection with malaria parasites is common, only 1 to 2% of infections lead to severe life-threatening disease characterized by a range of clinical features including coma, severe anemia, respiratory distress, metabolic acidosis, or multiorgan failure. Animal models of infection are indispensable tools to better understand the dynamic host-parasite interactions that lead to the onset of different infection outcomes. We now show that DBA/2 mice infected with P. berghei ANKA constitute a rodent model for malaria-associated acute lung injury (ALI). Up to 60% of these infected mice develop respiratory problems including dyspnea, airway obstruction and hypoxemia and die soon after. The most common pathological findings were pleural effusion, pulmonary hemorrhage and edema, features common to human malaria patients that show life-threatening respiratory distress. Malaria-associated ALI in this model correlates with high levels of circulating vascular permeability factor, VEGF, and its blockage by different means leads to protection from ALI. The existence of such a model of disease will certainly contribute to a better understanding of malaria-associated pathology and possibly to the design of novel intervention strategies.