Why are vaccines important?
Along with tuberculosis and HIV/AIDS, malaria is thought to be one of the ‘big three' infectious killers in the world today, affecting over 200 million and killing more than 3 million people each year. Pregnant women and children under the age of five are mainly at risk to the harmful consequences of malaria. In developing countries the malaria prevention and control strategies are breaking down as a result of shortage in funding, poor infrastructure, and insufficient public and political interest.
Despite ambitious goals by control organizations such as those of the Roll Back Malaria Initiative to halve malaria deaths by 2010, malaria mortality and mortality rates have actually risen halfway through the program and continue to increase as the public health systems weaken and populations increase in size. It is clear that the measures currently available to control malaria, or the ways in which they are being used, are not working. Hence, the failure of existing methods for malaria control has generated interest in several new approaches, which include effective and affordable antimalarial drugs, the development of genetically modified mosquitoes (GMMs) and renewed efforts to find a vaccine. These approaches should either reduce population sizes or replace existing populations with vectors which would not be capable of transmitting malaria. An effective malaria vaccine would have the potential to save millions of lives, and to significantly reduce morbidity rates associated with this disease.
Malaria is a re-emerging disease and it has recently returned to regions (such as Sri Armenia and North Korea) where it was believed to have been eradicated. The incidence rate of malaria has also increased in endemic areas such as South America. This situation may have arisen due the increasing resistance of the parasite to general antimalarials. As previously mentioned, chloroquine and sulfadoxine-pyrimethamine resistance has already developed in most endemic countries, and resistance to mefloquine is now emerging in many areas. In addition, many insecticides that were once useful in repelling mosquitoes are now ineffective in many malaria endemic areas because the malaria vector is developing resistance to insecticides. Another factor for the re-emergence of malaria is the migration of refugee populations from non-endemic areas into malaria endemic regions. Also, because of environmental changes (e.g. atmospheric carbon accumulation) malaria transmission are now more prevalent. This situation was made worse by weak national control strategies, planning problems, unsuccessful use of resources, and poor health-care facilities. This remains a major challenge for drug developers as parasite and vector resistance are important contributors to the rising disease burden of malaria. The development of an effective malaria vaccine would provide individuals with a much-needed advantage in this close battle between malaria parasites and drug developers.
“In Africa today, malaria is understood to be both a disease of poverty and a cause of poverty.” (WHO, 2007)
Malaria is a major barrier to economic development. Endemic countries with widespread malaria consistently show low economic growth every year compared to those countries free from the disease. Malaria soaks money and resources from the country, limiting any prospect for development and economic progress. About 40% of public health expenditures are provided for the prevention and control of malaria in some endemic areas of Africa. For these reasons, vaccines are considered to be an extremely cost-effective strategy for reducing the disease burden. The development of an effective malaria vaccine would have an extremely positive influence on the economic development of many endemic countries.
An ideal malaria vaccine would offer cross-species immunity, induce permanent sterilising immunity, be protective in children and compatible with the expanded programme on immunisation (EPI) so that it can be administered as part of the routine immunisation programme.
Why has a malaria vaccine not been developed yet?
Even though the need for a malaria vaccine is enormous, there is still no licensed vaccine available. The development of a harmless and effective vaccine would be the easiest method of controlling malaria, but even after decades of search malaria vaccine is still elusive. This is because vaccine developers continue to struggle with the parasite's complex life cycle, with its allelic diversity and antigenic variations, which makes the development and implementation of an effective malaria control problematic. The two major groups of potential recipients of malaria vaccines that researchers are aiming are short-term, non-immune travellers to a malaria-endemic area (Type 1 malaria vaccine) and long-term residents of a malaria-endemic region (Type 2 malaria vaccine).
Complex life cycle: Most vaccines target viruses or bacteria. However, parasites have a more complicated structure and life cycle than bacteria and viruses. The complex life cycle of the malaria parasites includes many distinct developmental stages and invading many different tissues within the human and mosquito. This is a problematic factor in the development of vaccine and also increases the number of potential targets for a vaccine. Hence, an effective wide-ranging malaria vaccine is more likely to target multiple stages of the parasitic life cycle.
Partial natural immunity: A vaccine is effective when humans can develop natural immunity to the disease. Generally, vaccines are designed to mimic the mechanisms of this natural immunity. However, in the case of malaria, natural immunity is weak and incomplete, and needs continuous stimulation by the parasite. Frequently being exposed to a certain strain of malaria for a long period of time can finally develop a kind of naturally acquired immunity within the person to malaria. This is done to establish a harmless balance between parasites and host. The situation is further complicated because this ‘immunity' seems to be fairly strain-specific. A single Plasmodium species contains many alleles of genes that encode antigens. For instance in one population, the parasites may express one combination of alleles, whereas in another close population the parasites may express a different combination. The immune response is often specific to the alleles being expressed. Hence, an individual born and bought up in a Tanzanian community, where he is repeatedly being exposed to the local strain of malaria could slowly build natural resistance to the negative effects of the parasite. Consequently, if he moves a long distance to a different community, he might be exposed to a different strain of malaria that would cause his natural resistance to be ineffective. There is currently no model of complete natural immunity on which to base the vaccine. Thus researchers are facing major problems in producing a vaccine that can provide long-lasting protection against the multiple strains of the disease.
Antigenic variation: P. falciparum (most virulent species of the malaria parasite) is very successful at evading the immune response due to a process known antigenic variation. P. falciparum infected red blood cells display antigens, known as P. falciparum-infected membrane protein 1 (PfEMP1), on the surface of their cell membranes. These antigens are encoded by many different genes and the parasite is able to rapidly switch the type of PfEMP1 antigen being expressed on the erythrocytes, and so the parasites are able to continuously escape the antibody response. Because of this antigenic variation, a vaccine based on a single PfEMP1 antigen would be less likely to prevent malaria. Hence, a vaccine against P. falciparum must be developed which will conquer the parasite's sophisticated adaptations for escaping the immune response.
In summary, the following areas of vaccine research are challenging to overcome:
* vaccine production difficulties which include not being able to grow the parasite in large amounts
* evaluation problems
* the process by which Parasites evade host's immune response
* Complications of carrying out clinical and field trials
* Parasite mutations
* Antigenic variations
* Multiple antigens which is specific to species and stage
POSSIBLE TARGETS FOR MALARIA VACCINES
There are three distinct stages of the parasite's life cycle which are potential targets for both subunit and whole-organism vaccines: a) pre-erythrocytic stage, b) asexual erythrocytic or blood stage and c) sexual or gametocyte stage. More than 90 candidate malaria vaccines are currently in pre-clinical and clinical trials. Most of the candidate vaccines fall into following categories:
* Pre-erythrocytic vaccines (sporozoite and liver stage)
* Blood stage vaccines
* Transmission blocking vaccines
* Anti-disease vaccines
The goal of these vaccines is to provide protection against malaria infection and preferably offer sterilizing immunity for the non-immune person. After inoculation, the first stage in the parasite's life cycle is a fairly short pre-erythrocytic phase. If any sporozoites are able to evade the host's immune system then the clinical symptoms of malaria appear during the blood stage of the parasite. For this reason, a pre-erythrocytic vaccine is important for inducing sterile immunity. A vaccine at this stage must be able to stimulate an immune response that completely prevents the infection from developing within the human host. Additionally, pre-erythrocytic vaccine has the potential to prevent the transmission of the parasite to the mosquito vector by interrupting the parasite's life cycle before gametocyte production can begin.
Pre-erythrocytic vaccines may be designed to act at two separate stages during the parasite's life cycle. A sporozoite-stage vaccine can prevent sporozoites from invading hepatocytes, while a liver-stage vaccine targets the parasite's development within hepatocytes.
The basic aim of the vaccine is to generate humoral immune response which will neutralize the sporozite and prevent it from invading the hepatocytes. Studies have found that that immunization with irradiated sporozoites can present some protection to humans. Immunity in human volunteers lasted many months and was protective against multiple strains of the parasite. But this vaccine was not very practical since it required the use of live mosquitoes which bit humans more than 1,000 times in order to release the irradiated sporozoites and vaccinate the volunteer. After this vaccination achievement many other vaccine strategies were developed, each targeting a specific aspect of the sporozoite stage. For example, many pre-erythcroytic vaccines candidates have focused largely on the P. falciparum sporozoite main surface antigen, circumsporozoite protein (CSP) or epitopes from it because the host's immune response usually targets CSP when attacking the parasite.
An effective pre-erythrocytic vaccine will either kill the sporozoite before they invade hepatocytes or destroy it once they are inside the hepatocyte. This will give rise to a disease preventing liver stage vaccine, which will result in sterile immunity. When sporozoites reach the immune-protected intracellular shelter of a hepatocyte, cell mediated immunity is initiated to control the infection. CD8+ T cells may play a major role in inducing immune responses and protecting the human host from developing malaria. It has been established in vitro that CD8+ T cells kill hepatocytes that have been invaded by the malarial parasite. Small peptides or antigens may be processed and displayed with MHC class I molecule, for recognition by CD8+ T cells, resulting in a cell-mediated response that would kill infected hepatocyes.
Blood stage vaccines
These vaccines are aimed to mainly protect against malaria disease, and not against infection. The specific targets of blood-stage vaccines can vary, they could either destroy the merozoites in the short time before they invade red blood cells or target malarial antigens expressed on erythrocyte surface by invading parasites. These vaccines will suppress the continuous growth of dividing merozoites, thereby reducing the disease.
1. Merozoites transform into trophozoites inside the red blood cells, which grow and then divide finally to give dozens of merozoites. These merozoites then reinvade the new red blood cells after the infected cells burst. The surface proteins of these merozoites can be the vaccine target by enhancing antibody production against these surface proteins which can prevent infection of red blood cells by stimulating increased humoral immune response to merozoites circulating in the blood. However, this approach is made difficult due to the lack of Major histocompatibility (MHC) molecules expressed on the surface of erythrocyte.
2. Instead of MHC antigens, potential blood-stage vaccines can target specific ring-infected surface antigens that are expressed on the surface of infected erythrocytes. There is much less possibility of damaging already infected erythrocytes by increasing a cellular immune response than by targeting circulating merozoites, but effective blood-stage vaccines that are currently in development include a combination of both categories of antibodies. Thus the goal of blood-stage vaccines is to reduce the parasitic density in the blood after infection.
Hence, protection offered by these vaccines will be both antibody dependent and cell mediated immunity. An effective blood-stage vaccine can offer protection by preventing the most severe forms of malaria, such as cerebral malaria. When P. falciparum merozoites infect erythrocytes, the infected erythrocytes express surface proteins that adhere to blood vessels walls. The number of erythrocytes being infected increase and these infected erythrocytes can block blood vessels, leading to hemorrhages in severe cases. Such brain haemorrhages can result in coma and death. A vaccine that attempts to block the further spread of infective merozoites to new red blood cells and inhibits the process of erythrocyte adherence to blood vessel walls can be proven therapeutic and reduce the likelihood of hemorrhaging and further deterioration.
Thus these blood stage vaccines can prevent the parasite from entering or developing in the erythrocytes. Since MHC molecules are not expressed on the erythrocyte's surface, there is less likelihood of a CD8+ T-cell-mediated cellular immune response and consequent damage of infected erythrocytes at this stage. Instead, blood-stage vaccines may stimulate an increased antibody response resulting in a rise in the production of Plasmodium-protective cytokines.
Transmission blocking vaccine
These vaccines can potentially target against the sexual-stage Plasmodium gametocyte or ookinete. Unlike other classes of malaria vaccines, the aim of transmission-blocking vaccines is to prevent onwards transmission of the parasite through infected vectors. These transmission-blocking vaccines are termed altruistics, because they mediate their action within the mosquito and would not offer any protective benefits to the vaccinated individual, but would prevent that individual from transmitting the infection to malaria vectors, i.e. prevent the next person from being beaten by that mosquito.
When mosquitoes take a blood meal from an infected human, they ingest immature gametocytes, which develop into ookinetes within the mosquito's intestinal wall before they are infective to humans. Transmission-blocking vaccines can potentially inhibit the development of these gametocytes by inducing an immune response to the surface antigens of gametocytes by the use of human antibodies that the mosquito takes up when it bites, thereby neutralizing the sexual stages
Thus the vaccine aims to block the development of the gametocyte in the human or the ookinete in the mosquito, before onwards transmission can take place. This type of vaccine is potentially very important as it can be used to eliminate the parasite from low malaria incidence regions or to prevent the development and spread of vaccine-resistant parasites.
During the asexual blood stage when the schizont ruptures, a number of malarial toxins are released which can lead to malaria pathogenesis. Studies have shown the glycosylphosphatidyl inositol (GPI) anchor, which binds several of the Plasmodium antigens to the membrane, are highly toxic in mouse models. Administeration of GPI into mice formed features of severe malarial infection, e.g. hypoglycaemia and severe anaemia. The damaging effects of malarial GPI are associated with its ability to stimulate a pro-inflammatory response through cytokines such as TNF-alpha. Hence, malaria toxins represent another possible target for anti-disease vaccines, where these vaccines may induce immune response by the production of human antibodies which neutralize harmful soluble parasite toxins.
P. falciparum is the only species associated with cytoadhesion, the binding of infected erythrocytes to vascular endothelium. This process is involved in the pathogenesis, virulence and survival of P. falciparum, and is induced by several adhesins that are encoded by the parasite, PfEMP1 family members. PfEMP1 binds to CD36 receptor on endothelial cells and are the only surface antigens on infected erythrocytes that present the parasite with its ability to adhere and remain sequestered. Damage caused by cytoadhesion is most evident in the central nervous system or in kidneys and parasitic sequestration in brain microcapillaries leads to neurological deficits or even coma. Studies have shown that naturally acquired antibodies against PfEMP1 during malarial infection seem to be protective. Thus these molecules are another potential target for anti-disease vaccines, where the antibodies act against surface antigens on the infected erythrocytes and may agglutinate the erythrocytes and prevent cytoadherence by inhibiting receptor-ligand interactions (CD-36 receptor), thus preventing any deadly outcome.
Other potential vaccines
Malaria vaccine during pregnancy
Pregnant women are more susceptible to malaria infection because cell-mediated immunity is suppressed during pregnancy. Placental malaria is the severe form of malaria which can be experienced by pregnant women, where the parasites can briefly evade the immune system by hiding inside the placenta. The infected erythrocytes bind to membrane proteins displayed on the placental endothelium. Finally, the pregnant woman's natural humoral response generates antibodies that block the binding between infected erythrocytes and the endothelium. Studies have shown that in their first pregnancy, women have more possibility of developing severe pregnancy malaria than women in subsequent pregnancies. In fact, the risk of infection and the severity of the illness seem to decrease with each pregnancy. It is thus considered that repeated exposure and infection during pregnancies may cause this build-up of immunity. There are increased level of antibodies in women who have had multiple pregnancies (miltigevardi), that prevent the infected erythrocyte from binding in the placenta.
Vaccine researchers are optimistic that an effective vaccine for placental malaria derived from a compound, such as PfEMP1, could inhibit infected erythrocyte binding in the placenta, copying the natural resistance experienced by women with multiple pregnancies. If given to women before their first pregnancy, this vaccine would have the potential to save thousands of pregnant women and infants.
DNA vaccine technology offers the best possibility for an effective malaria vaccine. DNA sections are extracted from the parasite's genome and inserted into a vector (e.g. a plasmid genome, an attenuated viral genome, or a liposome), which then enters the human host's cells via endocytosis. The parasite's DNA fragment is then integrated into the host DNA and protein synthesis leads to the production of cell surface markers (epitopes) that label the host cell as ‘infected'. T cell response is initiated and the immune system stimulates a number of memory T cells that respond to that particular epitope. DNA vaccines can be designed to contain multiple DNA segments coding for different epitopes. This vital feature of DNA vaccines can be utilised in the following ways: 1) the vaccine can be designed to incorporate epitopes from various stages of the complex parasitic life cycle, thus inducing immunity at several stages of the parasite's development. 2) The vaccine can contain epitopes from several different strains of malaria, thus reducing the limitations of strain specificity. 3) The vaccine can contain numerous PfEMP1 antigen features to fight against the antigenic variation barrier. 4) The vaccine can be designed to include epitopes that can be recognized by both B and T cells, hence inducing both humoral and cell-mediated immune response.