Immunity to Malaria – DEV Immunopaedia

Malaria is a mosquito-borne parasitic infection, caused by an intracellular parasite, the protozoan Plasmodium spp. It affects most tropical areas around the world where mosquitos of the Anopheles genus are located. Around 200 to 400 million people are infected each year, resulting in approximately 400,000 deaths annually. It typically causes morbidity and mortality in infants and young children in sub-Saharan Africa and most of the mortality occurs among children below the age of 5.

Plasmodium species

Malaria can be caused by one of 5 Plasmodium species: P. falciparum, P. vivax, P. ovale, P. malariae, and the zoonotic monkey parasite, P. knowlesi.
P. falciparum is the deadliest malaria parasite and the most prevalent on the African continent. P. vivax is the dominant malaria parasite in most countries outside of sub-Saharan Africa. Globally, Plasmodium falciparum and Plasmodium vivax account for the majority of cases of malaria.
Severe malaria is almost exclusively caused by P. falciparum infection and is associated with severe anaemia or coma and death if untreated.
P. knowlesi can also cause very severe disease. Splenomegaly, severe headache, cerebral ischemia, hepatomegaly, and hemoglobinuria with renal failure, i.e., black water fever, may also be seen in severe malaria.
Recurrent malaria is seen in both P. vivax and P. ovale infections and the disease can relapse months or years after initial infection, with recurrent waves of parasitemia

Malaria life cycle

The malaria life cycle is complex, and the parasite has two hosts – the human and the female Anopheles mosquito, which is also the vector. The stages of the life cycle include the pre-erythrocytic, asexual erythrocytic and sexual erythrocytic stages (Figure 1).
Pre-erythrocytic stage (skin/sporozoite and liver stages)
- Infected mosquito bites human host and injects the plasmodium parasite from the saliva into the host tissues in the form of a sporozoite and infection is initiated.
- The sporozoites then infect the liver cells (hepatocytes) where they develop into merozoites.
Asexual erythrocytic stage (red blood cell stage)
- The merozoites infect red blood cells (erythrocytes) and develop into trophozoites and schizonts containing up to 20 daughter merozoites.
- These merozoites then multiply and rapture the red blood cells and reinfect new erythrocytes.
- The synchronous release of merozoites from erythrocytes causes the cyclic fevers associated with malaria.
Sexual erythrocytic stage
- A subset of developing merozoites differentiate into male and female gametocytes, which, when taken up by a feeding mosquito, give rise to extracellular gametes.
In the mosquito mid-gut, the gametes fuse to form a motile zygote (ookinete), which penetrates the mosquito gut wall and forms an oocyst, within which meiosis takes place and haploid sporozoites develop.

Figure 1: Life Cycle of P. falciparum and monoclonal antibody (mAb) targets. Monoclonal antibodies (mAbs) can act at three primary points in this life cycle: the pre-erythrocytic stage (i.e., before the infection of the hepatocyte), the asexual erythrocytic stage, and the sexual erythrocytic stage (when gametocytes are formed and are eventually taken up by the mosquito). At the pre-erythrocytic stage, mAbs target sporozoites and infected hepatocytes thus preventing infection, clinical illness, and transmission. At the asexual erythrocytic stage, mAbs target merozoites or infected red blood cells (RBCs) thus preventing replication, clinical illness, and transmission. At the sexual erythrocytic stage, mAbs target gametes and ookinetes thus preventing infection of mosquitoes by malaria parasites. [Wells T. and Donini C. 2022. Monoclonal Antibodies for Malaria. N Engl J Med]

Basics of innate and adaptive antimalarial immunity

While multiple plasmodium species can cause malaria, Plasmodium falciparum is responsible for most cases and deaths, particularly in Africa, and has been the major focus of vaccine development.
Both humoral (antibody-mediated) and cell-mediated immunity mechanisms are important contributors for the development of acquired immunity against malaria (Figure 2).

Figure 2: (a) When the sporozoites are injected into the skin the infection is clinically silent and diagnostically inaccessible, and there is no evidence for naturally acquired immunity. In this case, the only known immune effectors that can reduce or block sporozoites in the skin are antibodies. In mouse models, some sporozoites enter draining lymph nodes from the skin, where they are presented by dendritic cells and prime CD8+ T cells. On the other hand, the highly motile sporozoites go to the liver, traverse Kupffer cells, and invade a small number of hepatocytes (b). In humans, the infection continues to be clinically 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; on the innate production of inducible nitric oxide synthase (iNOS) and nitric oxide (NO); and on natural killer (NK) cells, NKT cells, and γδT cells. (c) Approximately one week after hepatocyte invasion, merozoites exit the liver into the bloodstream and begin a 48-h cycle (d) of red blood cell (RBC) invasion, replication, RBC rupture, and merozoite release (e). Clinical symptoms of malaria occur only 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, a process that involves exporting variant surface antigens (VSAs) such as P. falciparum membrane protein 1s (PfEMP1s) to the RBC surface. VSAs act as receptors for a variety of endothelial cell ligands and mediate binding of infected RBCs (iRBCs) to the microvascular endothelium of various organs (f), allowing parasites to avoid splenic clearance. However, the sequestration of iRBCs in the microvasculature 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 iRBCs and the release of merozoites and various parasite 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 antibody. CD4+ cytokine-producing T cells also play a role as do NK, NKT, and γδT cells and macrophages through the production of NO and iNOS. A small number of blood-stage parasites differentiate into sexual gametocytes, which are taken up by mosquitos in blood meals (i). 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, block parasite development, and consequently transmission [Crompton P.D. et al., 2014. Malaria immunity in man and mosquito: insights into unsolved mysteries of a deadly infectious disease. Annu Rev Immunol.]

Malaria Vaccine development

Three types of vaccine strategies have been attempted (Figure 3): pre-erythrocytic vaccines, blood stage vaccines and transmission blocking vaccines.

Ideally what is desired the most from attempts on vaccine development against malaria is a vaccine that can target the first stages of parasitic development i.e. sporozoites that develop during the pre-erythrocytic stage, such that transmission can be prevented altogether.

Figure 2. The life cycle of Plasmodium falciparum and targets for vaccine development. The different life cycle stages of Plasmodium include sporozoites that are injected into the host skin and will infect hepatocytes and develop (pre-erythrocytic stages) into merozoites that infect RBCs and develop into trophozoites and schizonts containing up to 20 daughter merozoites (asexual blood stages). These merozoites can reinfect new erythrocytes, giving rise to a cyclical blood-stage infection with a periodicity of 48 to 72 hours, depending on the Plasmodium species. A yet unknown factor or factors trigger a subset of developing merozoites to differentiate into male and female gametocytes, which, when taken up by a feeding mosquito, give rise to extracellular gametes (sexual transmission stages). In the mosquito mid-gut, the gametes fuse to form a motile zygote (ookinete), which penetrates the mid-gut wall and forms an oocyst, within which meiosis takes place and haploid sporozoites develop. Pre-erythrocytic vaccine candidates target sporozoites in the blood or developing parasites in hepatocytes. Blood-stage vaccines predominantly target merozoites, which are extracellular for a short time after release from the liver or schizont before they invade erythrocytes, but some target the parasite antigen PfEMP1 expressed on the surface of infected erythrocytes. Transmission blocking vaccines induce antibodies against the transmission forms of the Plasmodium parasite, the gametes and ookinetes, to prevent infection of mosquitoes and subsequent transmission of sporozoites to another human host [James G. Beeson et al. Challenges and strategies for developing efficacious and long-lasting malaria vaccines. Sci Transl Med. 2019 Jan 9;11(474)]

Immune responses to malaria vaccine candidates

Malaria vaccine development strategies now seem to be focused on two main types of vaccines – subunit vaccines consisting of well-defined parasite antigens and vaccines based on the use of whole attenuated sporozoites.
- RTS,S/AS01 (also known as Mosquirix) is a recombinant protein-based malaria vaccine endorsed by WHO for use in children. It was engineered using genes from the circumsporozoite protein (CSP) on the sporozoite surface of the falciparum, a viral envelope protein of hepatitis B virus (HBsAg), and a chemical adjuvant (AS01). Infection is prevented by inducing humoral and cellular immunity, with high antibody titers, that block the parasite from infecting the liver.
- Vaccination with attenuated whole sporozoites has also been reported. For instance, limited clinical trials with volunteers from non-endemic areas demonstrated the development of sterilising immunity in all immunized volunteers after 5 immunizations with irradiated sporozoites administered intravenously.
- It was shown that immunized individuals developed strong antibody responses to sporozoites as well as T cell responses.
The progressive development of naturally acquired immunity to the erythrocytic stages of falciparum malaria with age has been recognized for many years.
Some data have suggested that naturally acquired immunity to malaria is primarily dependent on immune responses to parasite-encoded antigens found on the surface of the infected red blood cells rather than on the merozoite.
Antibodies targeting malaria antigens have been isolated from volunteers immunized with whole parasite vaccine, volunteers immunized with subunit vaccine, individuals from endemic areas as well as mice that have been vaccinated with irradiated sporozoites.
The contributions of antibodies to this immunity are due in part to a ground-breaking study showing that passive transfer of IgG from adult Africans long exposed to malaria could drive down erythrocytic malaria parasite levels in children with this disease.
Many blood-stage vaccine candidates have been identified based on studies of mouse malaria parasites, and in some cases protective immunity to these proteins has also been demonstrated in non-human primates.
In 2018, Kisalu and colleagues reported that several human monoclonal antibodies directed against the Plasmodium falciparum (Pf) circumsporozoite protein (PfCSP) were isolated from several subjects immunized with an attenuated Pf whole-sporozoite (SPZ) vaccine (Sanaria PfSPZ Vaccine).
They also showed that the passive transfer of one of these antibodies, monoclonal antibody CIS43, conferred high-level, sterile protection in two different mouse models of malaria infection.
In 2021, Gaudinski and colleagues also showed that administration of the long-acting monoclonal antibody CIS43LS to adults who had never had malaria infection or vaccination prevented malaria after controlled infection.
The demonstration that CIS43 is highly effective for passive prevention of malaria has potential application for use in travellers, military personnel, elimination campaigns and identifies a new and conserved site of vulnerability on PfCSP for next-generation rational vaccine design.