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Malaria

There are over 100 million cases of malaria worldwide each year and at least a million deaths (Whitty 2002). Malaria is prevalent in tropical zones and its effects are the most severe in developing countries where effective anti-malarial drugs are not accessible or affordable for the majority of the people at risk, including millions of children. The objective of this website is to provide a biological background on Plasmodium infection and its interaction with the human immune system.

Life Cycle of Plasmodium Species

Figure 1. Plasmodium species life cycle in human (A & B) and mosquito (C) hosts . Image courtesy of DPDx, CDC's Web site for parasitology identification

Plasmodium falciparum and Plasmodium vivax, the most common agents of malaria in humans are intracellular parasites. Plasmodium species are transmitted between human hosts by female mosquitos of the genus Anopheles. (Numbered stages correspond to figure 1.) The mosquitos inject sporozoites contained in their saliva into the subcutaneous tissue or directly into the bloodstream of a human (1). The sporozoites travel to the liver and invade hepatocytes (2). Specialized receptors on the surface of the sporozoites are recognized by hepatocyte surface molecules. The thrombospondin-related adhesive protein (TRAP) on the sporozoite binds to heparin sulfate proteoglycans on hepatocytes and initiates entry (Miller et.al 2002). Inside the hepatocyte, the sporozoites undergoes asexual reproduction (schizogony) to create a schizont (3). After 1-2 weeks the host cell will rupture releasing tens of thousands of merozoites which enter the bloodstream and invade erythrocytes (4 & 5). Upon invasion of an erythrocyte, the Plasmodium species enter the trophozoite stages, first altering the cell cytoplasm to form a ring. The trophozoite then undergoes fission to form <32 merozoites (Wakelin 1996). The red blood cell (RBC) then bursts, releasing the merozoites which will infect new RBCs (6). Merozoites which infect new RBCs may also form male or female gametocytes to be taken up by a female Anopheles mosquito (7 & 8). Once ingested, the gametocytes will be fertilized in the gut of the mosquito forming a motile zygote which takes residence in the stomach lining and forms an oocyst (9-11). When the oocyst bursts many sporozoites will be released and eventually move to the salivary gland (12).

Figure 2. Plasmodium destroying two red blood cells. Taken from http://marenostrum.org. permission requested

Plasmodium Species and the Immune System

Plasmodium toxins released during growth and reproduction or through the rupture of infected RBCs can lead to overproduction of cytokines, eliciting a systematic immune response. . Plasmodium species grow by digesting hemaglobin in the cytoplasm of the RBC and the byproduct, heme, is converted to a toxic pigment. When multiple RBCs rupture simultaneously there is a massive release of toxins, leading to overproduction of cytokines such as tumor necrosis factor a (TNF a), interferon gamma (IFN g), and interleukin-1 (IL-1). Periodic high levels of TNF a responding to the cycle of RBC rupture are responsible for the high fever associated with malarial infection.

Evasion of the Immune System

Antigenic Variation

Upon entering the RBCs, Plasmodium species present antigens on the surface of these cells. One such molecule, Plasmodium falciparum erythrocyte membrane protein 1 (PtEMP1) is a large molecule of 200,000 to 400,000 kDa and is highly polymorphic and clonally variable (Noguiera 2001) PtEMP1 is encoded by the var gene family composed of two exons separated by 1-kb introns (Chen 2000). Polymorphisms arise from variations in the sequence of short tandem repeats and also from point mutations (Bolad & Berzins 2000). By altering its surface antigens, the parasite can avoid neutralization by pre-existing antibodies.

Sequestration

Plasmodium falciparum and other plasmodium species modify the surface of infected RBCs so that they can adhere to the vascular endothelium. Binding of infected cells to the endothelium removes them from peripheral circulation which protects the parasite from destruction in the spleen. The parasite utilizes several host adhesion molecules to achieve sequestration. ICAM-1 mediates RBC rolling on the surface of the endothelium, whereas strong binding occurs with the endothelial receptor CD36 (Chen 2000). Sequestration can occur in the heart, lungs, brain, liver, kidney, and subcutaneous tissues and is often associated with severe malaria (Miller 2002). By sequestering infected RBCs in the deep postvenous capillaries, the parasite is able to effectively evade the immune system. The parasite may also benefit from the low oxygen environment (Nogueira 2001, Miller 2002).

Cytokines released in response to malarial toxins can also directly contribute to immune evasion through sequestration. IFN <FONT FACE="Symbol">g</FONT> and TNF <FONT FACE="Symbol">a</FONT> upregulate the expression of adhesion molecules such as ICAM-1 on host cells and enhance sequestration of infected RBCs.

Natural Immunity to Malaria

Some individuals possess a natural resistance to malarial infection based largely on genetic characteristics which have developed due to selective pressure. Plasmodium may be prevented from entering host cells which lack the appropriate surface receptors. For example P. vivax infection is limited to reticulocytes and RBCs that are positive for the Duffy blood group protein. In West Africa, where most of the population is Duffy negative P. vivax is no longer a prominent disease causing agent (Miller 2002, Wakelin 1998). Individuals heterozygous for the gene that causes sickle-cell anemia are less likely to develop a sustained malarial infection. When infected sickled cells are sequestered and subject to low levels of oxygen, they will leak potassium, causing the resident merozoites to die (Wakelin 1998).

People living in malaria endemic areas often acquire immunity to the parasite after prolonged low level exposure. This form of acquired immunity wanes if not boosted by frequent reinfection with the same Plasmodium species. Babies and young children do not possess this acquired immunity and are the most susceptible to severe malaria (Miller 2002).

Malaria Vaccines

Pre-erythrocytic stage

The only way to completely prevent the clinical manifestations of malaria is to target the sporozoite or liver-stage parasite thereby preventing merozoite production and release. Because the sporozoites only circulate in the the blood for about 45 minutes before entering hepatocytes, it is necessary to develop a vaccine that elicits a sustained high antibody concentration in the blood (Wakelin 1998). The sporozoite surface is covered by circumsporozoite protein (CSP) which is characterized by 45 repeats of the amino acid sequence asparagine-alanine-asparagine-proline [NANP] which elicit a humoral immune response (Wakelin 1998). Clinical trial success of SPf66 malaria vaccine against P falciparum,which incorporates a CSP protein fused to a hepatitis surface antigen, has been limited because the vaccine fails to protect against heterologous strains (Guerin 2002). Recently DNA vaccines have proved effective in mice by following injection with a DNA plasmid containing P. yoelli with injection of a modified vaccinia. Human trials for similar DNA vaccines began in 1999 (Whitty 2002).

Plasmodium species can also be targeted once they have entered hepatocytes. While residing in hepatocytes, the liver-stage antigen 1 and 2 (LSA-1 and 2) are synthesized and expressed on the surface of the developing merozoite. These proteins can be transported to the hepatocyte cell surface and displayed in MHC I, eliciting a CD8 T cell response. LSA-1 has a relatively conserved sequence within the plasmodium genus and could be an important antigen in vaccine development (Guerin 2002)

Erythrocytic stage

Vaccines against later stages of the Plasmodium life cycle could target extracellular merozoites, prevent entry of merozoites into RBCs or could create a cell-mediated response against infected RBCs. The merozoite surface protein 1 (MSP1) has been identified as a possible component of a vaccine against extracellular merozoites. MSP1 shows little polymorphism within different P. falciparum individuals and it has been shown to elicit both a T and B cell response (Moore 2002). Less focus has been placed on development of erythrocytic stage vaccines partly because the commercial potential is low. Erythrocytic stage vaccines would target people living in endemic areas most of which are developing countries (Whitty 2002).

Malaria Chemoprophylaxis and Treatment

Travelers to endemic areas have no acquired immunity to malaria and should protect themselves against infection by preventing mosquito bites and by taking prescribed chemoprophylactic drugs which prevent Plasmodium species from proliferating during the erythrocytic stage of infection. Due to the emergence of widespread chloroquine resistant as well multi-drug resistant Plasmodium, new drugs and novel combinations of drugs are being used to fight the parasite (Jong & Nothdurft 2001, Whitty 2002).

Table 1. Drugs recommended by the Center for Disease Control and the World Health Organization for use as malaria chemoprophylaxis and treatment (Jong & Nothdurft 2001, Medline Plus Drug Information, Roche Pharmaceuticals Product Information)

References

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