(Background)
THE NEED FOR A MALARIA VACCINE
"Malaria is a stubborn disease, slow to kill, quick to incapacitate and hard to cure. All through human history, in times of peace as in times of war, it has taken its steady toll of human life. Against this persistent affliction, many of the best minds in public health and in medicine have, during the past few decades, been forging increasingly effective weapons. Not a year passes without some improvement in techniques or tactics against what has been termed the greatest single threat to human health"
~ R.B. Fosdick, President of the Rockefeller Foundation, 1946
Mortality and Morbidity:
Malaria is one of the world’s most devastating diseases, killing more people than any other parasitic infection. It is estimated that over 3.2 billion people – roughly 40% of the world’s population – live in areas where they are at-risk for malaria (1). There are more than 500 million new clinical cases of malaria each year, 90% of them in Sub Saharan Africa (2). The death toll of malaria is enormous, killing 1.5-2.7 million people per year – one death every 30 seconds (1). Children under age five and pregnant women are particularly susceptible to the devastating consequences of malaria, including permanent physical and mental disabilities, and death.
Malaria, along with tuberculosis and HIV/AIDS, is considered one of the “big three” infectious killers in the world today. The combined death toll of the big three is now well over 6 million people per year (3).
Malaria prevention and control programs in developing countries are breaking down, suffering from shortage of funds, poor infrastructure, and inadequate political and public interest. As public health systems falter, and populations increase in size, malaria morbidity and mortality continue to grow (4).
Historically, vaccines have been extremely effective in reducing the disease burden of diseases such as smallpox, polio, and measles. An effective malaria vaccine has the potential to save thousands or even millions of lives, and to dramatically reduce morbidity associated with this devastating disease.
Drug Resistance:
Malaria parasites are quickly developing resistance to one antimalarial drug after another. Already, chloroquine and sulfadoxine-pyrimethamine resistance has developed in most endemic countries. Resistance to mefloquine is emerging in many areas (1). Drug developers face a constant battle to stay one step ahead of the rapidly evolving parasites.
Meanwhile, the malaria vector – the mosquito – is also developing resistance. Many insecticides that were once useful in repelling mosquitoes are now ineffective in many malaria endemic areas (5).
Parasite and vector resistance are important contributors to the rising disease burden of malaria. The development of an effective malaria vaccine would offer humans a much-needed advantage in this neck-and-neck race between drug developers and parasites.
Economic Costs of Malaria:
“In Africa today, malaria is understood to be both a disease of poverty and a cause of poverty.” (6)
Malaria is a major barrier to economic development. Countries with heavy malaria prevalence consistently show less annual economic growth than countries unburdened by malaria. The disease saps money and resources from the country, constraining opportunities for development and economic progress. It also interferes with the country’s abilities to adequately address other pressing health issues. In some highly endemic African countries, nearly 40% of public health expenditures are devoted to malaria prevention and control (6).
Direct economic costs of malaria include:
Indirect economic costs of malaria include:
Vaccines are often extremely cost-effective means to reduce disease burden. An effective vaccine for malaria would have an enormously positive impact on the economies of many endemic nations.
Biological Barriers to Developing the Vaccine
Complex life cycle:
The vast majority of vaccines target viruses or bacteria. Parasites are considerably more complex, and thus require a more complex vaccine. The life cycle of the malaria parasites is particularly complicated, involving many different developmental stages and invading many different tissues within the human and mosquito. An effective broad-spectrum malaria vaccine will likely have to target multiple stages of the parasitic life cycle (7).
Natural immunity is incomplete:
One of the surest signs that a disease is a good candidate for vaccine development is when humans can develop natural immunity to the disease. In most cases, vaccines are designed to mimic the mechanisms of this natural immunity. In the case of malaria, however, natural immunity is weak and incomplete, and seems to require constant restimulation by the malaria parasite (7).
A person who is repeatedly exposed to a certain strain of malaria over a long period of time can eventually develop a sort of natural immunity to the disease. However, it is not truly immunity that is being established, but rather a less harmful balance between parasite and host (8). Further complicating the matter, this “immunity” also appears to be quite strain-specific. Within a single Plasmodium species, there are many alleles of genes that encode antigens. In population A, the parasites may express one combination of alleles, while in nearby population B the parasites expresses a different combination. The immune response is often specific to the alleles being expressed (1). Thus, a person born and raised in a village in Tanzania could be exposed to the local strain of malaria many times, gradually building natural resistance to the negative effects of the parasite. However, if he were to travel a mere 30 miles to a different village, he could be exposed to a different strain of malaria, rendering his natural resistance ineffective (4).
With no model of complete natural immunity on which to base the vaccine, researchers face major difficulties in developing a vaccine that can offer long-lasting protection and can also protect against multiple strains of the disease.
Antigenic variation of P. falciparum:
P. falciparum, the most virulent species of the malaria parasite, is very successful at evading the immune response. This is largely due to a process known antigenic variation. Red blood cells infected with P. falciparum display antigens on their cell membranes. These P. falciparum-infected membrane protein 1 (PfEMP1) antigens are encoded by many different genes. The parasite is capable of rapidly switching the type of PfEMP1 antigen being displayed on the erythrocytes, thus allowing the parasites to repeatedly evade the antibody response (9).
Antigenic variation means that a vaccine based on a single PfEMP1 antigen would likely be ineffective in preventing malaria. An effective vaccine for P. falciparum must be able to overcome the parasite’s advanced adaptations for evading the immune response.
CURRENT PROGRESS towards a malaria vaccine
Despite the enormous need for a malaria vaccine, there is still no licensed vaccine available. Vaccine researchers continue to struggle to overcome the parasite’s complex life cycle and clever defenses against the human immune system. Progress has been slow, but many scientists are optimistic that an effective malaria vaccine will be licensed within the next decade.
Over 90 candidate malaria vaccines are currently in pre-clinical and clinical trials (7). Most of the candidate vaccines fall into six major categories:
PRE-ERYTHROCYTIC VACCINES
Clinical symptoms of malaria manifest during the erythrocytic cycle of the parasite (10). Therefore, a pre-erythrocytic vaccine is essential to induce sterile immunity (11). That is, this type of vaccine has the potential to elicit an immune response that completely prevents the establishment of an infection within the human host. This in turn could greatly reduce the burden of disease in endemic areas (12). In addition, this class of vaccine is important because it prevents the transmission of the malarial parasite to the mosquito vector by interrupting the life cycle of the parasite prior to gametocyte production (10).
Pre-erythrocytic vaccines may be designed to act at two separate stages during the parasite’s life cycle. A sporozoite-stage vaccine prevents sporozoites from invading hepatocytes, whereas a liver-stage vaccine targets the development of the parasite within hepatocytes (10).
Sporozoite-stage Vaccines:
The rationale for the development of a sporozoite-stage vaccine stems from the finding that immunization with irradiated sporozoites offers some protection to humans. Immunity to infection in human volunteers was found to last many months and was protective against multiple strains of the parasite (11). However, this vaccine proved to be very impractical as it required the use of live mosquitoes; the mosquitoes bit humans in order to release the irradiated sporozoites and vaccinate the volunteer, though vaccination required 1,000 bites or more (12). The success of this vaccination has inspired the development of many other vaccine strategies, each targeting a specific aspect of the sporozoite stage. For example, many preliminary vaccines have been based on the P. falciparum circumsporozoite surface antigen, circumsporozoite protein (CSP), or epitopes from it because the body’s immune response generally targets this protein when mounting an attack on the parasite (11).
Example:
RTS,S is one of the most successful candidates for malaria vaccines thus far (11). This sporozoite-stage vaccine resulted from a collaborative effort between the Walter Reed Army Institute of Research and GlaxoSmithKline Biologicals, Rixensart, Belgium. It is a recombinant fusion protein and incorporates an epitope of CSP. In clinical trials the vaccine has offered partial protection to many of its subjects, however, it still must be improved upon (3). Currently, the RTS,S vaccine is undergoing testing in Sierra Leone, Gabon, Tanzania, Ghana and Senegal (11).
Liver-stage Vaccines:
It is hypothesized that cell mediated immune responses, and CD8+ T cells in particular, play a role in protecting the human host from developing malaria (11). It has been demonstrated in vitro that CD8+ T cells kill hepatocytes that have been invaded by the malarial parasite. However, the importance of this action in immunity against malaria and the mechanism through which the CD8+ cells act is largely unknown. Thus, there is a great need to further study the relevance of T cells to natural malarial immunity (12).
Example:
Liver stage antigen 1 (LSA-1) is a protein found on pre-erythrocytic P. falciparum parasites, and in fact is one of the few proteins known to be expressed by liver-stage parasites (11). LSA-1 is synthesized following the invasion of the hepatocytes by sporozoites. Production increases as the liver cycle continues (13). T cell responses to LSA-1 have been associated with protection in individuals naturally exposed to malaria as well as those immunized with irradiated sporozoites. Even still, the function of the protein is unknown (13). A phase 1/2a clinical trial has been conducted in which LSA-1 was injected into two groups of volunteers, each group receiving a different adjuvant with the injection. While no serious adverse events were reported, the vaccine showed no efficacy. That is, it showed no delay in the onset of infection nor was protection against infection observed (11).
BLOOD-STAGE VACCINES
The asexual blood stage of the Plasmodium parasite, characterized by progressive invasion of erythrocytes by the parasite, is the cause of the classic symptoms and underlying pathology of malaria, including fever, headache, nausea and vomiting, muscle aches, and, in severe cases, unconsciousness and eventual death (14). Thus, unlike pre-erythrocytic vaccines, the goal of blood-stage vaccines is not to prevent infection by inducing sterile immunity, but rather to reduce the parasitic density in the blood after infection. Such a reduction in parasitic load can greatly reduce incidence of morbidity and mortality associated with severe malaria (14).
The protection afforded by an effective blood-stage vaccine could have a major impact on preventing the most severe forms of malaria, such as cerebral malaria. When Plasmodium falciparum merozoites infect red blood cells, the infected erythrocytes display surface proteins that are adhere to the walls of the blood vessels (15). As more and more erythrocytes are infected, large masses of infected red blood cells can actually block blood vessels, causing hemorrhaging in severe cases (15). In cerebral malaria, such hemorrhaging in the brain can lead to coma and death. A vaccine that inhibits the further spread of infective merozoites to new red blood cells can reduce the likelihood of hemorrhaging and other severe pathology, such as is seen in cerebral malaria.
In order to reduce parasitic burden during the blood stage of the Plasmodium life cycle, asexual blood stage vaccines thus aim to prevent the parasite from entering or developing in the red blood cells (16). Because red blood cells do not have major histocompatibility complex antigens, a CD8+ T-cell-mediated cellular immune response and subsequent destruction of infected erythrocytes has been found to be unlikely at this stage (17). Instead, the goal of blood-stage vaccines is to induce an enhanced antibody response, causing increased production of Plasmodium-protective cytokines (17).
The specific targets of blood-stage vaccines vary. The goal of vaccines that target enhancement of antibody production against surface proteins of the merozoite is to prevent the infection of red blood cells by mounting an increased humoral immune response to merozoites circulating in the blood (17). Other blood-stage vaccines target specific ring-infected surface antigens that are displayed on infected erythrocytes. Although destruction of already infected erythrocytes by enhancing a cellular immune response may be less feasible than targeting free-circulating merozoites, promising blood-stage vaccines currently in development include a combination of both categories of antibodies (17).
Example:
Combination B is a blood-stage vaccine that targets merozoite surface proteins 1 and 2 (MSP-1 and MSP-2), which are antigens that are expressed by the merozoites during the asexual blood stage of the Plasmodium life cycle. Both MSP-1 and MSP-2 are essential for mediating the process of merozoite invasion of new red blood cells (17). Thus, MSP-1 and MSP-2 antibodies target viable merozoites in the blood, thus preventing further erythrocytic infection. Combination B also includes antibodies to ring-infected erythrocytic surface antigens (RESA), which are surface proteins displayed on red blood cells that have already been infected. Thus, the vaccine also aims to enhance cellular immunity, resulting in the lysis of infected red blood cells.
TRANSMISSION-BLOCKING VACCINES:
Unlike the other classes of malaria vaccines, the goal of transmission-blocking vaccines is not to prevent infection or associated disease in the human host, but rather to prevent onwards transmission of the parasite through infected vectors (18). Thus, an effective transmission-blocking vaccine would not confer protection from disease upon the individual who receives it, but would prevent that individual from transmitting the infection to malaria vectors. For this reason, transmission-blocking vaccines have been labeled “altruistic” (19).
The specific target of transmission-blocking vaccines is the sexual-stage Plasmodium gametocyte or ookinete. When competent mosquito vectors take a blood meal from a human infected with Plasmodium, the mosquitoes ingest immature gametocytes, which much develop into ookinetes in the mosquito before they are infective to humans. Transmission-blocking vaccines aim to inhibit development of these sexual-stage gametocytes by inducing an immune response to the gametocytes by use of human antibodies (17). The ultimate goal is inhibit development, either of the gametocyte in the human, or of the ookinete in the mosquito, before gametocytes have matured to the infective stage in the mosquito and onwards transmission can occur.
Example:
Transmission-blocking vaccines that use ookinete-blocking antibodies are currently considered extremely promising (18). The targets of one such vaccine for Plasmodium vivax are the ookinete surface proteins Pfs25 and Pfs28. These surface proteins are critical in order for ookinetes to penetrate the gut of the mosquito, where they can develop into the subsequent oocyst stage (17). Human antibodies are used to mount an immune response against these antigens, which inhibit development of Pfs25 and Pfs28-expressing ookinetes, and leaving only ookinetes that lack these two essential proteins. Such ookinetes are much more likely to be destroyed by proteases in the midgut of the mosquito, or to be unable to penetrate the epithelium of the gut, thus preventing further development and transmission to humans (17).
Vaccine for Malaria during Pregnancy
Cell-mediated immunity is suppressed during pregnancy, leaving pregnant women particularly susceptible to malaria infection. Once infected, pregnant women are significantly more likely to die or experience severe morbidity than non-pregnant women. Malaria during pregnancy is associated with negative outcomes for both mother and fetus. Outcomes for the mother include severe anemia, kidney failure, pulmonary edema, and increased susceptibility to cerebral malaria. Outcomes for the fetus include low birth weight, premature birth, and spontaneous abortion (20).
The severe form of malaria experienced by pregnant women is known as placental malaria. In placental malaria, the parasites are able to temporarily escape the immune system by “hiding” inside the placenta. Infected erythrocytes bind to membrane proteins found on the placental endothelium. The mother’s natural humoral response eventually produces antibodies that inhibit the binding of infected erythrocytes to the endothelium (20).
It has been observed that women in their first pregnancy are significantly more likely to develop severe pregnancy malaria than women in subsequent pregnancies. Indeed, with each pregnancy, the risk of infection and the severity of the illness decreases. Researchers believe that repeated exposure and infection during pregnancies results in this build-up of immunity. Women who have had multiple pregnancies have higher levels of antibodies that inhibit infected erythrocyte binding in the placenta (20).
Researchers are hopeful that an effective vaccine for placental malaria could be derived from a compound that disrupts infected erythrocyte binding in the placenta, mimicking the natural resistance enjoyed by women with multiple pregnancies (7). This vaccine could be delivered to women before their first pregnancy, potentially saving thousands of maternal and infant lives.
Example:
P. falciparum erythrocyte membrane protein 1 (PfEMP1) is believed to be the primary molecule responsible for the binding of infected erythrocytes to the placental endothelium. Several leading candidate vaccines for placental malaria target the var gene family encoding PfEMP1 (7).
DNA VACCINES
DNA vaccines are among the most promising candidates for effective malaria vaccines. Sections of DNA are extracted from the parasite’s genome and inserted into a vector (such as a plasmid genome, an attenuated viral genome, or a liposome), which then enters the human host’s cells via endocytosis. Once inside the human cell, the parasite’s DNA fragment is incorporated into the host DNA. Protein synthesis results in the production of cell surface markers (epitopes) that label the host cell as “infected”. A T cell response is initiated, and the immune system generates a population of memory T cells that are sensitive to that particular epitope. Initial tests of candidate DNA vaccines for malaria have stimulated high T cell activation, but poor antibody production (21).
DNA vaccines can be designed to contain multiple DNA segments coding for different epitopes. This critical feature of DNA vaccines can be exploited in the fight against malaria. Firstly, the vaccine can be designed to incorporate epitopes from multiple stages of the complex parasitic life cycle, thus inducing immune responses at several levels of the parasite’s development. Secondly, the vaccine can contain epitopes from several different strains of malaria, thus helping to overcome the problem of strain specificity. Thirdly, the vaccine can contain multiple PfEMP1 antigen characteristics, helping to combat the barrier of antigenic variation. And finally, the vaccine can be engineered to contain epitopes recognized by both B and T cells, thus stimulating both humoral and cell-mediated immunity (22).
REFERENCES
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(14) Graves, P. and H. Gelband. “Vaccines for preventing malaria (blood-stage).” The Cochrane Colllaboration Review. May 2007. http://www.thecochranelibrary.com
(15) “Malaria.” Wikipedia. May 2007. http://en.wikipedia.org/wiki/Malaria
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(17) Komisar, Jack L. “Malaria vaccines.” Frontiers in Bioscience 12 (2007):3928-3955
(18) Hisaeda, et al. “Antibodies to Malaria Vaccine Candidates Pvs25 and Pvs28 Completely Block the Ability of Plasmodium vivax to Infect Mosquitoes.” Infection and Immunity. 68 (2000): 6618-6623.
(19) Carter, Richard and Anthony Stowers. “Current developments in malaria transmission-blocking vaccines.” Expert Opinion on Biological Therapy. 1 (2001): 619-628.
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(21) Seder et al. “DNA Vaccines: Immunology, Application and Optimization.” Annu. Rev. Immunology 18 (2000):927-974.
(22) “Vaccine Strategies.” Brown University, Bio 160 website. http://www.brown.edu/Courses/Bio_160/Projects1999/malaria/vacc.html