Malaria vaccine

Malaria vaccines are an area of intensive research. However, there is no effective vaccine that has been introduced into clinical practice.

The global burden of P. falciparum malaria increased through the 1990s due to drug-resistant parasites and insecticide-resistant mosquitoes; this is illustrated by re-emergence of the disease in areas that had been previously malaria-free. The first decade of the 21st century has seen reduction. Though the reasons are not entirely clear, improving socioeconomic indices, deployment of artemisinin-combination drugs and insecticide-treated bednets are all likely to have contributed. There has been a major scaling-up in distribution of malaria control measures particularly since the advent of The Global Fund to Fight AIDS, Tuberculosis and Malaria. It is unclear what the future will hold for disease burden trends. If political will and funding is maintained, the disease burden could drop further; if as in the past funding lapses or clinically significant resistance develops to the main antimalarial drugs and insecticides used then the disease burden may rise again. Early evidence of resistance to artemisinins, the most important class of antimalarials, is now confirmed, having manifested as delayed parasite clearance times in the western region of Cambodia on the border with Thailand. This is also the region where resistance to earlier antimalarial drugs emerged and then subsequently spread throughout much of the world in the case of chloroquine. The Bill and Melinda Gates Foundation has launched a call for the aim of the malaria community to shift from sustained control to eradication. It is agreed that eradication is not possible with current tools and that research and development of new drugs, diagnostics, insecticides and a cost-effective deployable vaccine will be needed to facilitate eradication. There has been a great increase in funding for such research in the 21st century.

Vaccines are often the most cost-effective tools for public health. They have historically contributed to a reduction in the spread and burden of infectious diseases and have played the major part in previous elimination campaigns for smallpox and the ongoing polio and measles initiatives. Yet no effective vaccine for malaria has so far been developed. Despite this, researchers remain hopeful. Optimism is justified for several reasons, the first of these being that individuals who are exposed to the parasite in endemic countries develop acquired immunity against disease and death. Such immunity does not however prevent malaria infection; immune individuals often harbour asymptomatic parasites in their blood. Additionally, research shows that if immunoglobulin is taken from immune adults, purified and then given to individuals that have no protective immunity, some protection can be gained.^{[citation needed]} In addition to this, clinical and animal studies have shown that experimental vaccination has some degree of success when using attenuated sporozites and using the RTS,S/AS01 malaria vaccine candidate.^{[citation needed]}

Agents under development

A completely effective vaccine is not yet available for malaria, although several vaccines are under development. SPf66 was tested extensively in endemic areas in the 1990s, but clinical trials showed it to be insufficiently effective.^[1] Other vaccine candidates, targeting the blood-stage of the parasite's life cycle, have also been insufficient on their own.^[2] Several potential vaccines targeting the pre-erythrocytic stage are being developed, with RTS,S showing the most promising results so far.^[3]

RTS,S/AS01 (commercial name: Mosquirix)^[4], which started Pivotal Phase III evaluation in May 2009 and is designed not for travellers but for children resident in malaria-endemic areas who suffer the burden of disease and death related to malaria. The RTS,S vaccine was engineered using genes from the outer protein of Plasmodium falciparum malaria parasite and a portion of a hepatitis B virus plus a chemical adjuvant to boost the immune system response. It is being developed by PATH and GlaxoSmithKline (GSK) with support from the Bill and Melinda Gates Foundation. In October 2011 the group reported first findings from the Phase III trial of RTS,S indicating that 46% of 15,460 inoculated infants and children were protected for 15 months.^[5][6][7]

Considerations for vaccine development

The task of developing a preventative vaccine for malaria is a complex process. There are a number of considerations to be made concerning what strategy a potential vaccine should adopt.

The diversity of the parasite

P. falciparum has demonstrated the capability, through the development of multiple drug-resistance parasites, of evolutionary change. The Plasmodium species has a very high rate of replication, much higher than that actually needed to ensure transmission in the parasite’s life cycle. This enables pharmaceutical treatments that are effective at reducing the reproduction rate, but not halting it, to exert a high selection pressure, thus favoring the development of resistance. The process of evolutionary change is one of the key considerations necessary when considering potential vaccine candidates. The development of resistance could cause a significant reduction in efficacy of any potential vaccine thus rendering useless a carefully developed and effective treatment.

Choosing to address the symptom or the source

There are two main types of immune response than could be elicited by the parasite. These are anti-parasitic immunity and anti-toxic immunity.

• "Anti-parasitic immunity" addresses the source; it consists of an antibody response (humoral immunity) and a cell-mediated immune response. Ideally a vaccine would enable the development of anti-plasmodial antibodies in addition to generating an elevated cell-mediated response. Potential antigens against which a vaccine could be targeted will be discussed in greater depth later. Antibodies are part of the specific immune response. They exert their effect by activating the complement cascade, stimulating phagocytic cells into endocytosis through adhesion to an external surface of the antigenic substances, thus ‘marking’ it as offensive. Humoral or cell-mediated immunity consists of many interlinking mechanisms that essentially aim to prevent infection entering the body (through external barriers or hostile internal environments) and then kill any micro-organisms or foreign particles that succeed in penetration. The cell-mediated component consists of many white blood cells (such as monocytes, neutrophils, macrophages, lymphocytes, basophils, mast cells, natural killer cells, and eosinophils) that target foreign bodies by a variety of different mechanisms. In the case of malaria both systems would be targeted to attempt to increase the potential response generated, thus ensuring the maximum chance of preventing disease.

• "Anti-toxic immunity" addresses the symptoms; it refers to the suppression of the immune response associated with the production of factors that either induce symptoms or reduce the effect that any toxic by-products (of micro-organism presence) have on the development of disease. For example, it has been shown that Tumor necrosis factor-alpha has a central role in generating the symptoms experienced in severe P. falciparum malaria. Thus a therapeutic vaccine could target the production of TNF-a, preventing respiratory distress and cerebral symptoms. This approach has serious limitations as it would not reduce the parasitic load; rather it only reduces the associated pathology. As a result, there are substantial difficulties in evaluating efficacy in human trials.

Taking this information into consideration an ideal vaccine candidate would attempt to generate a more substantial cell-mediated and antibody response on parasite presentation. This would have the benefit of increasing the rate of parasite clearance, thus reducing the experienced symptoms and providing a level of consistent future immunity against the parasite.

Potential targets of a vaccine

By their very nature, parasites are more complex organisms than bacteria and viruses, with more complicated structures and life cycles. This presents problems in vaccine development but also increases the number of potential targets for a vaccine. These have been summarised into the life cycle stage and the antibodies that could potentially elicit an immune response.

The life cycle of the malaria parasite is particularly complex, presenting initial developmental problems. Despite the huge number of vaccines available at the current time, there are none that target parasitic infections. The distinct developmental stages involved in the life cycle present numerous opportunities for targeting antigens, thus potentially eliciting an immune response. Theoretically, each developmental stage could have a vaccine developed specifically to target the parasite. The initial stage in the life cycle, following inoculation, is a relatively short "pre-erythrocytic" or "hepatic" phase. A vaccine at this stage must have the ability to protect against sporozoites invading and possibly inhibiting the development of parasites in the hepatocytes (through inducing cytotoxic T-lymphocytes that can destroy the infected liver cells). However, if any sporozoites evaded the immune system they would then have the potential to be symptomatic and cause the clinical disease.

The second phase of the life cycle is the "erythrocytic" or blood phase. A vaccine here could prevent merozoite multiplication or the invasion of red blood cells. This approach is complicated by the lack of MHC molecule expression on the surface of erythrocytes. Instead, malarial antigens are expressed, and it is this towards which the antibodies could potentially be directed. Another approach would be to attempt to block the process of erythrocyte adherence to blood vessel walls. It is thought that this process is accountable for much of the clinical syndrome associated with malarial infection; therefore a vaccine given during this stage would be therapeutic and hence administered during clinical episodes to prevent further deterioration. The last phase of the life cycle that has the potential to be targeted by a vaccine is the "sexual stage". This would not give any protective benefits to the individual inoculated but would prevent further transmission of the parasite by preventing the gametocytes from producing multiple sporozoites in the gut wall of the mosquito. It therefore would be used as part of a policy directed at eliminating the parasite from areas of low prevalence or to prevent the development and spread of vaccine-resistant parasites. This type of transmission-blocking vaccine is potentially very important. The evolution of resistance in the malaria parasite occurs very quickly, potentially making any vaccine redundant within a few generations. This approach to the prevention of spread is therefore essential.

Any vaccine produced would ideally have the ability to be of therapeutic value as well as preventing further transmission and is likely to consist of a combination of antigens from different phases of the parasite’s development.

Sporozoite

• Abs that block hepatocyte invasion
• Abs that kill the sporozoite via complement fixation or opsonization

Infected hepatocyte

• CTL mediated lysis
• CD4+ help for the activation and differentiation of CTL
• Localized cytokine release by T cells or APCs
• ADCC or C' mediated lysis,this CD4+ is useful in phagocytic cell to bind the MHC 11

Asexual erythrocytic

• Localized cytokine release that directly kills infected erythrocyte or intracellular parasite
• Abs that agglutinate the merozoites before schizont rupture
• Abs that block merozoite invasion of RBCs
• Abs that kill iRBC via opsonization or phagocytotic mechanisms
• Abs engulfed with the merozoite at time of invasion which kill intraerythrocytic parasite
• Abs which agglutinate iRBCs and prevent cytoadherence by blocking receptor-ligand interactions (CD-36 is such a receptor)
• Abs which neutralize harmful soluble parasite toxins

Sexual erythrocytic

• Cytokines which kill gametocytes within the iRBC
• Abs that kill gametocytes within iRBC via C'
• Abs that interfere with fertilization
• Abs that inhibit transformation of the zygote into the ookinete
• Abs that block the egress of the ookinete from the mosquito midgut (Doolan and Hoffman)

When selecting the most suitable vaccine target the following considerations are made:

a) How accessible is the antigen to the immune system?

b) How susceptible is the antigen to evolutionary change?

c) How critical is the antigen to parasitic biological functions?

d) How likely is a protective response in animal models?

e) Does the antigen contain epitopes that are recognisable by HLA allele superfamilies?

f) How compatible is the antigen with other potential antigens?

Mix of antigenic components

Increasing the potential immunity generated against Plasmodia can be achieved by attempting to target multiple phases in the life cycle. This is additionally beneficial in reducing the possibility of resistant parasites developing. The use of multiple-parasite antigens can therefore have a synergistic or additive effect.

One of the most successful vaccine candidates currently in clinical trials consists of recombinant antigenic proteins to the circumsporozoite protein.^[8] (This is discussed in more detail below.)

Vaccine delivery system

The selection of an appropriate system is fundamental in all vaccine development, but especially so in the case of malaria. A vaccine targeting several antigens may require delivery to different areas and by different means in order to elicit an effective response. Some adjuvants can direct the vaccine to the specifically targeted cell type—e.g. the use of Hepatitis B virus in the RTS,S vaccine to target infected hepatocytes—but in other cases, particularly when using combined antigenic vaccines, this approach is very complex. Some methods that have been attempted include the use of two vaccines, one directed at generating a blood response and the other a liver-stage response. These two vaccines could then be injected into two different sites, thus enabling the use of a more specific and potentially efficacious delivery system.

To increase, accelerate or modify the development of an immune response to a vaccine candidate it is often necessary to combine the antigenic substance to be delivered with an adjuvant or specialised delivery system. These terms are often used interchangeably in relation to vaccine development; however in most cases a distinction can be made. An adjuvant is typically thought of as a substance used in combination with the antigen to produce a more substantial and robust immune response than that elicited by the antigen alone. This is achieved through three mechanisms: by affecting the antigen delivery and presentation, by inducing the production of immunomodulatory cytokines, and by affecting the antigen presenting cells (APC). Adjuvants can consist of many different materials, from cell microparticles to other particulated delivery systems (e.g. liposomes).

Adjuvants are crucial in affecting the specificity and isotype of the necessary antibodies. They are thought to be able to potentiate the link between the innate and adaptive immune responses. Due to the diverse nature of substances that can potentially have this effect on the immune system, it is difficult to classify adjuvants into specific groups. In most circumstances they consist of easily identifiable components of micro-organisms that are recognised by the innate immune system cells. The role of delivery systems is primarily to direct the chosen adjuvant and antigen into target cells to attempt to increase the efficacy of the vaccine further, therefore acting synergistically with the adjuvant. There is increasing concern that the use of very potent adjuvants could precipitate autoimmune responses, making it imperative that the vaccine is focused on the target cells only. Specific delivery systems can reduce this risk by limiting the potential toxicity and systemic distribution of newly developed adjuvants. Studies into the efficacy of malaria vaccines developed to date have illustrated that the presence of an adjuvant is key in determining any protection gained against malaria. A large number of natural and synthetic adjuvants have been identified throughout the history of vaccine development. Options identified thus far for use combined with a malaria vaccine include mycobacterial cell walls, liposomes, monophosphoryl lipid A and squalene.

Vaccines developed up to now

^[when?]

The epidemiology of malaria varies enormously across the globe, and has led to the belief that it may be necessary to adopt very different vaccine development strategies to target the different populations. A Type 1 vaccine is suggested for those exposed mostly to P.falciparum malaria in sub-Saharan Africa, with the primary objective to reduce the number of severe malaria cases and deaths in infants and children exposed to high transmission rates. The Type 2 vaccine could be thought of as a ‘travellers’ vaccine’, aiming to prevent all cases of clinical symptoms in individuals with no previous exposure. This is another major public health problem, with malaria presenting as one of the most substantial threats to travellers’ health. Problems with the current available pharmaceutical therapies include costs, availability, adverse effects and contraindications, inconvenience and compliance many of which would be reduced or eliminated entirely if an effective (greater than 85-90%) vaccine was developed.

There are many antigens present throughout the parasite life cycle that potentially could act as targets for the vaccine. More than 30 of these are currently being researched by teams all over the world in the hope of identifying a combination that can elicit immunity in the inoculated individual. Some of the approaches involve surface expression of the antigen, inhibitory effects of specific antibodies on the life cycle and the protective effects through immunization or passive transfer of antibodies between an immune and a non-immune host. The majority of research into malarial vaccines has focused on the Plasmodium falciparum strain due to the high mortality caused by the parasite and the ease of a carrying out in vitro/in vivo studies. The earliest vaccines attempted to use the parasitic circumsporozoite (CS) protein. This is the most dominant surface antigen of the initial pre-erythrocytic phase. However, problems were encountered due to low efficacy, reactogenicity and low immunogenicity.

The CSP was a vaccine developed that initially appeared promising enough to undergo trials. It is also based on the circumsporozoite protein, but additionally has the recombinant (Asn-Ala-Pro15Asn-Val-Asp-Pro)2-Leu-Arg(R32LR) protein covalently bound to a purified Pseudomonas aeruginosa toxin (A9). However at an early stage a complete lack of protective immunity was demonstrated in those inoculated. The study group used in Kenya had an 82% incidence of parasitaemia whilst the control group only had an 89% incidence. The vaccine intended to cause an increased T-lymphocyte response in those exposed, this was also not observed.

The NYVAC-Pf7 multistage vaccine attempted to use different technology, incorporating seven P.falciparum antigenic genes. These came from a variety of stages during the life cycle. CSP and sporozoite surface protein 2 (called PfSSP2) were derived from the sporozoite phase. The liver stage antigen 1 (LSA1), three from the erythrocytic stage (merozoite surface protein 1, serine repeat antigen and AMA-1) and one sexual stage antigen (the 25-kDa Pfs25) were included. This was first investigated using Rhesus monkeys and produced encouraging results: 4 out of the 7 antigens produced specific antibody responses (CSP, PfSSP2, MSP1 and PFs25). Later trials in humans, despite demonstrating cellular immune responses in over 90% of the subjects had very poor antibody responses. Despite this following administration of the vaccine some candidates had complete protection when challenged with P.falciparum. This result has warranted ongoing trials.

In 1995 a field trial involving [NANP]19-5.1 proved to be very successful. Out of 194 children vaccinated none developed symptomatic malaria in the 12 week follow up period and only 8 failed to have higher levels of antibody present. The vaccine consists of the schizont export protein (5.1) and 19 repeats of the sporozoite surface protein [NANP]. Limitations of the technology exist as it contains only 20% peptide and has low levels of immunogenicity. It also does not contain any immunodominant T-cell epitopes.

RTS,S is the most recently developed recombinant vaccine. It consists of the P. falciparum circumsporozoite protein from the pre-erythrocytic stage. The CSP antigen causes the production of antibodies capable of preventing the invasion of hepatocytes and additionally elicits a cellular response enabling the destruction of infected hepatocytes. The CSP vaccine presented problems in trials due to its poor immunogenicity. The RTS,S attempted to avoid these by fusing the protein with a surface antigen from Hepatitis B, hence creating a more potent and immunogenic vaccine. When tested in trials an emulsion of oil in water and the added adjuvants of monophosphoryl A and QS21 (SBAS2), the vaccine gave 7 out of 8 volunteers challenged with P. falciparum protective immunity.

Vaccine development strategies for the future

The development of a vaccine of therapeutic and protective benefit against the malaria parasite requires a novel approach as to date there are no vaccines available that effectively target a parasitic infection. The focus so far has been predominately on the use of sub-unit vaccines. The use of live, inactivated or attenuated whole parasites is not feasible and therefore antigenic particles, or subunits, from the parasite are isolated and tested for immunogenicity i.e. the ability to elicit an immune response. The majority of subunits tested have been discussed above and are frequently combined with adjuvants and specialised delivery systems to increase the very variable level of immune response. The most recent advances in the field of sub-unit vaccine development include the use of DNA vaccination. This approach involves removing sections of DNA from the parasitic genome and inserting the sequences into a vector, examples including plasmid genomes, attenuated DNA viral genomes, liposomes or proteoliposes, and other carrier complex molecules. When inoculated the plasmid or attenuated virus is endocytosed into a host cell, the DNA sequence is then incorporated into the host DNA and replicated by protein synthesis. The proteins then produced are expressed on the cell surface membrane of the ‘infected’ cell. These bind to the HLA molecules, priming T cells and therefore creating a population of memory T cells specific to the inoculated DNA sub-unit. This technique has been shown to produce a high rate of T cell response but poor level of antibody production. The efficacy of DNA vaccines can be assessed using an ELISPOT assay. The development of this method of testing for immune responses is extremely beneficial when examining the potential efficacy of a vaccine candidate and is hoped to enable critical analysis of the mechanisms that provide ‘partial’ protection, thus facilitating a greater understanding of vaccine technology. This approach of potentially allowing the modification of vaccine candidates to improve development techniques and further scientific understanding is known as ‘iterative development’. The advantage of DNA vaccines over classical attenuated vaccines are numerous and include being able to mimic MHC class 1 CD8+ T cell specific responses that potentially could reduce some of the safety concerns associated with vaccine therapy and additionally provide a substantial reduction in production cost and due to the nature of DNA vaccines, increased ease of storage.

References

1. ^ Graves P, Gelband H (2006). "Vaccines for preventing malaria (SPf66)". Cochrane Database Syst Rev (2): CD005966. doi:10.1002/14651858.CD005966. PMID 16625647. 
2. ^ Graves P, Gelband H (2006). "Vaccines for preventing malaria (blood-stage)". Cochrane Database Syst Rev (4): CD006199. doi:10.1002/14651858.CD006199. PMID 17054281. 
3. ^ Graves P, Gelband H (2006). "Vaccines for preventing malaria (pre-erythrocytic)". Cochrane Database Syst Rev (4): CD006198. doi:10.1002/14651858.CD006198. PMID 17054280. 
4. ^ Commercial name of RTS,S
5. ^ "First Results of Phase 3 Trial of RTS,S/AS01 Malaria Vaccine in African Children". New England Journal of Medicine. 2011. doi:10.1056/NEJMoa1102287. PMID 22007715.  edit
6. ^ Stein, R. Experimental malaria vaccine protects many children, study shows. Washington Post 18 October 2011.
7. ^ 46% protection using Mosquirix vaccine
8. ^ Plassmeyer ML, Reiter K, Shimp RL, et al. (July 2009). "Structure of the Plasmodium falciparum Circumsporozoite Protein, a Leading Malaria Vaccine Candidate". J. Biol. Chem. 284 (39): 26951–63. doi:10.1074/jbc.M109.013706. PMC 2785382. PMID 19633296. http://www.jbc.org/cgi/pmidlookup?view=long&pmid=19633296. 

Bibliography

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External links

• Malaria vaccine
• National institute of health
• Malaria vaccines UK
• Gates foundation
• Brown University
• Cure for Neglected Diseases: Funding
• Identification of 11 potential malaria vaccine candidates using Bioinformatics