Malaria immunity in man and mosquito: insights into unsolved mysteries of a deadly infectious disease

Abstract

Malaria is a mosquito-borne disease caused by parasites of the obligate intracellular Apicomplexa phylum the most deadly of which, Plasmodium falciparum, prevails in Africa. Malaria imposes a huge health burden on the world's most vulnerable populations, claiming the lives of nearly one million children and pregnant women each year. Although there is keen interest in eradicating malaria, we do not yet have the necessary tools to meet this challenge, including an effective malaria vaccine and adequate vector control strategies. Here we review what is known about the mechanisms at play in immune resistance to malaria in both the human and mosquito hosts at each step in the parasite's complex life cycle with a view toward developing the tools that will contribute to the prevention of disease and death and, ultimately, to the goal of malaria eradication. In so doing, we hope to inspire immunologists to participate in defeating this devastating disease.

Figure 1. The Plasmodium life cycle

The Plasmodium life cycle in humans includes the asymptomatic liver stage, the blood-stage which causes disease, and the sexual gametocyte blood-stage which infects mosquitoes that transmit the parasite. Infection begins when a female Anopheles mosquito injects saliva that contains sporozoites into the skin and blood as it takes a blood meal (a). At this point the infection is clinically silent and there is no evidence for naturally acquired immunity. However, immunization with attenuated whole sporozoites induces sterilizing immunity and in this case, the only known immune effector that can reduce or block sporozoites in the skin are antibodies. In mouse models sporozoites have been shown to enter draining lymph nodes from the skin where they are presented by DCs and prime CD8⁺ T cells. The highly motile sporozoites migrate to the liver, transverse Kupffer cells and invade a small number of hepatocytes (b). In humans the infection continues to be clinically and immunologically silent at the liver stage and sterilizing immunity is not naturally acquired. However, in humans and in mice immunization with attenuated sporozoites induces sterilizing immunity that appears to rely on adaptive CD8 + and CD4 + T cells, the innate production of iNOS and NOS and on NK, NKT and γδ T cells. Each sporozoites infected hepatocyte gives rise to tens of thousands of asexual parasites called merozoites (c). Approximately one week after hepatocyte invasion merozoites exit the liver into the bloodstream and begin a 48 hr cycle (d) of RBC invasion, replication, RBC rupture, and merozoite release (e). Clinical symptoms of malaria only occur during the blood-stage and can begin as early as three days after the release of merozoites from the liver. Inside RBCs the parasite dramatically remodels the RBC including the export of variant surface antigens (VSAs) such as PfEMP1 to the RBC surface. VSAs act as receptors for a variety of endothelial cell ligands and mediate binding of iRBCs to the microvascular endothelium of various organs (f) allowing parasites to avoid splenic clearance. However, the sequestration of iRBCs in the microvaculature promotes the inflammation and circulatory obstruction associated with clinical syndromes of severe malaria including cerebral malaria with iRBC sequestration in the brain and pregnancy-associated malaria with iRBCs in the placenta (g). VSA-mediated rosetting of iRBCs to uninfected RBCs may also contribute to disease (h). Coincident with the rupture of merozoites iRBC and the release of merozoites and variety of merozoites products are inflammation and the clinical symptoms of malaria. Both adaptive and innate immune responses are readily detected. The key immune effector at this stage is antibodies. CD4⁺ T cell cytokine producing T cells also play a role as do NK, NKT and γδ T cells and macrophages through the production of NO and NOI. A small number of blood-stage parasites differentiate into sexual gametocytes which are taken up by mosquitos in blood meals (i). In the mosquito the gametes fuse, ultimately forming sporozoites that enter the mosquito salivary gland to complete the life cycle (see Fig. 3). In the mosquito innate immune mechanisms serve to control parasite development. Immunization of the vertebrate host with proteins expressed by the parasite in the mosquito host results in the production of antibodies that are taken up by the mosquito with the blood meal and have been shown to block parasite development and consequently block transmission.

Figure 2. The acquisition of immunity to malaria in the context of intense seasonal P. falciparum transmission

In areas of intense P. falciparum transmission immunity to severe life-threatening malaria is generally acquired by the age of five years, whereas children remain susceptible to repeated episodes of febrile malaria into adolescence, eventually acquiring near complete immunity to the symptoms of malaria by adulthood while remaining susceptible to becoming infected with blood-stage parasites. The mechanisms of immunity to severe malaria are unclear but may involve the acquisition of ‘strain-specific’ antibodies that neutralize key P. falciparum variant antigens that drive the pathogenesis of severe disease (e.g. subsets of PfEMP1s that mediate sequestration) and the induction of ‘strain-transcendent’ regulatory mechanisms that control excessive P. falciparum-induced inflammation, both of which may depend on ongoing P. falciparum exposure to be maintained. In young children P. falciparum-specific antibody responses to acute infection are generally short-lived, but with each year of exposure there is a gradual increase in the breadth of antigen specificity and serum levels of P. falciparum-specific IgG that persists in the absence of transmission (i.e. during the dry season in the case of seasonal malaria), only reliably conferring protection against malaria symptoms when an ill-defined threshold is surpassed.

Figure 3. Plasmodium life cycle in the mosquito

Mosquitoes become infected when they ingest gametocytes (a) that transform into mature gametes in the midgut lumen (b). Fertilization takes place, giving rise to a zygote (c) that matures into an ookinete, a motile stage that invades the mosquito midgut (d). Ookinetes transform into oocysts (e) when they reach the midgut basal lamina and begin to divide continuously, generating thousands of sporozoites that are released into the mosquito circulatory system (f), invade the salivary glands (g) and are injected into a new host when the mosquito takes a second blood meal (h). (Insert) When ookinetes invade the midgut, they disrupt the barriers, such as the peritrophic matrix (blue line in inset), that prevent direct contact between the gut micribiota and epithelial cells and inflict damage on the invaded midgut cell. Midgut invasion triggers the release of a hemocyte differentiation factor (HDF) that increases the number of circulating phagocytic cells called granulocytes (green cells). This priming enhances the immune response to subsequent Plasmodium infections.