Current Status of Malaria Vaccinology
In order to assess the current status of malaria vaccinology, one must first take an overview of the whole disease. One must understand the disease and its enormity on a global basis. Malaria is a protozoan disease, of which over 150 million cases are reported annually. In tropical Africa alone, more than 1 million children under the age of fourteen die each year from malaria.
From these figures, it is easy to see that the eradication of this disease is of the utmost importance. The disease is caused by one of four species of Plasmodium: P. falciparium, P. malariae, P. vivax, and P. ovale. Malaria does not only affect humans, but can also infect a variety of hosts ranging from reptiles to monkeys. It is, therefore, necessary to consider all aspects to assess the possibility of a vaccine. The disease has a long and complex life cycle, which creates problems for immunologists. The vector for malaria is the Anopheles mosquito, in which the life cycle of malaria both begins and ends.
The parasitic protozoan enters the bloodstream via the bite of an infected female mosquito. During her feeding, she transmits a small amount of anticoagulant and haploid sporozoites along with saliva. The sporozoites head directly for the hepatic cells of the liver, where they multiply by asexual fission to produce merozoites. These merozoites can now travel one of two paths: they can infect more hepatic liver cells or they can attach to and penetrate erythrocytes. When inside the erythrocytes, the plasmodium enlarges into uninucleated cells called trophozoites. The nucleus of this newly formed cell then divides asexually to produce a schizont, which has 6-24 nuclei. Now the multinucleated schizont then divides to produce mononucleated merozoites. Eventually, the erythrocytes reach lysis, and as a result, the merozoites enter the bloodstream and infect more erythrocytes. This cycle repeats itself every 48-72 hours (depending on the species of Plasmodium involved in the original infection). The sudden release of merozoites, toxins, and erythrocyte debris is what causes the fever and chills associated with malaria. Of course, the disease must be able to transmit itself for survival.
This is done at the erythrocytic stage of the life cycle. Occasionally, merozoites differentiate into macrogametocytes and microgametocytes. This process does not cause lysis, and therefore the erythrocyte remains stable. When the infected host is bitten by a mosquito, the gametocytes can enter its digestive system where they mature into sporozoites. Thus, the life cycle of the Plasmodium is begun again, waiting to infect its next host. At present, people infected with malaria are treated with drugs such as Chloroquine, Amodiaquine, or Mefloquine. These drugs are effective at eradicating the exoerythrocytic stages, but resistance to them is becoming increasingly common.
Therefore, a vaccine looks like the only viable option. The wiping out of the vector, i.e. Anopheles mosquito, would also prove an effective way of stopping disease transmission, but the mosquitoes are also becoming resistant to insecticides. So, again, we must look to a vaccine as a solution. Having read certain attempts at creating a malaria vaccine, several points become clear.
The first is whether the theory of malaria vaccinology is a viable concept. I found the answer to this in an article published in Nature from July 1994 by Christopher Dye and Geoffrey Targett. They used the MMR (Measles, Mumps, and Rubella) vaccine as an example to which they could compare a possible malaria vaccine. Their article said that “simple epidemiological theory states that the critical fraction (p) of all people to be immunized with a combined vaccine (MMR) to ensure eradication of all three pathogens is determined by the infection that spreads most quickly through the population; that is, by the age of one with the largest basic case reproduction number (Ro). In the case of MMR, this is measles with Ro of around 15, which implies that p > 1-1/Ro ? 0.93.” Gupta et al. points out that if a population of malaria parasite consists of a collection of pathogens or strains that have the same properties as common childhood viruses, the vaccine coverage would be determined by the strain with the largest Ro, rather than the Ro of the whole parasite population.
While estimates of the latter have been as high as 100, the former could be much lower. The above shows us that if a vaccine can be made against the strain with the highest Ro, it could provide immunity to all malaria plasmodium. Another problem faced by immunologists is the difficulty in identifying the exact antigens that are targeted by a protective immune response. Isolating the specific antigen is impeded by the fact that several cellular and humoral mechanisms probably play a role in natural immunity to malaria, but as shown later, there may be an answer to the dilemma. While researching current candidate vaccines, I came across some that seemed more viable than others, and I will briefly look at a few of these in this essay.
The first is a study carried out in the Gambia from 1992 to 1995 (taken from The Lancet of April 1995). The subjects were 63 healthy adults and 56 malaria-identified children from an outpatient clinic. Their test was based on the fact that experimental models of malaria have shown that Cytotoxic T Lymphocytes, which kill parasite-infected hepatocytes, can provide complete protective immunity from certain species of Plasmodium in mice. From the tests they carried out in the Gambia, they have provided what they see as indirect evidence that cytotoxic T lymphocytes play a role against P. falciparum in humans. Using a human leukocyte antigen-based approach termed reversed immunogenetics, they previously.
Having identified these, they then went on to identify CTL epitopes for HLA class 1 antigens that are found in most individuals from Caucasian and African populations. Most of these epitopes are in conserved regions of P. falciparum. They also found CTL peptide epitopes in a further two antigens: thrombospondin-related anonymous protein and sporozoite threonine and asparagine-rich protein. This indicated that a subunit vaccine designed to induce a protective CTL response may need to include parts of several parasite antigens.
In the tests they carried out, they found that CTL levels in both children with malaria and in semi-immune adults from an endemic area were low, suggesting that boosting these low levels by immunization may provide substantial or even complete protection against infection and disease. Although these tests were not a huge success, they do show that a CTL-inducing vaccine may be the road to take in looking for an effective malaria vaccine. There is now accumulating evidence that CTL may be protective against malaria, and that levels of these cells are low in naturally infected people. This evidence suggests that malaria may be an attractive target for a new generation of CTL-inducing vaccines. The next candidate vaccine that caught my attention was one which I read about in Vaccine vol. 12, 1994. This was a study of the safety, immunogenicity, and limited efficacy of a recombinant Plasmodium falciparum circumsporozoite vaccine.
The study was carried out in the early 1990s using healthy male Thai rangers between the ages of 18 and 45. The vaccine, named R32 Tox-A, was produced by the Walter Reed Army Institute of Research, SmithKline Pharmaceuticals, and the Swiss Serum and Vaccine Institute, all working together. R32 Tox-A consisted of the recombinantly produced protein R32LR, amino acid sequence [(NANP)15(NVDP)]2 LR, chemically conjugated to Toxin A (detoxified) of Pseudomonas aeruginosa. Each 0.4 mL dose of R32 Tox-A contained 320 mg of the R32 LR-Toxin-A conjugate (molar ratio 6.6:1), absorbed to aluminum hydroxide (0.4% w/v), with merthiolate (0.01%) as a preservative. The Thai test was based on specific humoral immune responses to sporozoites, which are stimulated by natural infection and are directed predominantly against the central repeat region of the major surface molecule, the circumsporozoite (CS) protein.
Monoclonal CS antibodies given prior to sporozoite challenge have achieved passive protection in animals. Immunization with irradiated sporozoites has produced protection associated with the development of high levels of polyclonal CS antibodies, which have been shown to inhibit sporozoite invasion of human hepatoma cells. Despite such encouraging animal and in vitro data, evidence linking protective immunity in humans to levels of CS antibody elicited by natural infection has been inconclusive, possibly because of the short serum half-life of the antibodies. This study involved the volunteering of 199 Thai soldiers.
X percentage of these were vaccinated using R32 Tox-A, prepared in the way previously mentioned. As mentioned before, this was done to evaluate its safety, immunogenicity, and efficacy. This was done in a double-blind manner; all of the 199 volunteers either received R32Tox-A or a control vaccine (tetanus/diphtheria toxoids [10 and 1 Lf units respectively]) at 0, 8, and 16 weeks. Immunization was performed in a malaria non-transmission area. After completion, volunteers were deployed to an endemic border area and monitored closely to allow early detection and treatment of infection.
The vaccine was found to be safe and to elicit an antibody response in all vaccinees. Peak CS antibody (IgG) concentrated in malaria-experienced vaccinees exceeded those in malaria-naive vaccinees (mean 40.6 versus 16.1 mg/ml; p = 0.005) as well as those induced by previous CS protein-derived vaccines and observed in association with natural infections. A log-rank comparison of time to falciparium malaria revealed no differences between vaccinated and non-vaccinated subjects. Secondary analyses revealed that CS antibody levels were lower in vaccinee malaria cases than in non-cases, 3 and 5 months after the third dose of vaccine. Because antibody levels had fallen substantially before peak malaria transmission occurred, the question of whether or not high levels of CS antibody are protective still remains to be seen.
So in the end, we are once again left without conclusive evidence, but we are now even closer to creating the sought-after malaria vaccine. Finally, we reach the last and by far the most promising, prevalent, and controversial candidate vaccine. This I found continually mentioned throughout several scientific magazines. “Science” (Jan 95) and “Vaccine” (95) were two which had no bias reviews, so the following information is taken from these. The vaccine to which I am referring is the SPf66 vaccine.
This vaccine has caused much controversy and raised certain dilemmas. It was invented by a Colombian physician and chemist called Manuel Elkin Patarroyo, and it is the first of its kind. His vaccine could prove to be one of the few effective weapons against malaria, but it has run into a lot of criticism and has split the malaria research community. Some see it as an effective vaccine that has proven itself in various tests, whereas others view it as of marginal significance and say more study needs to be done before a decision can be reached on its widespread use.
Recent trials have shown some promise. One trial carried out by Patarroyo and his group in Colombia during 1990 and 1991 showed that the vaccine cut malaria episodes by over 39% and first episodes by 34%. Another trial, which was completed in 1994 on Tanzanian children, showed that it cut the incidence of first episodes by 31%. It is these results that have caused the rift within research areas. Over the past 20 years, vaccine researchers have concentrated mainly on the early stages of the parasite after it enters the body in an attempt to block infection at the outset (as mentioned earlier). Patarroyo, however, took a more complex approach.
He spent his time designing a vaccine against the more complex blood stage of the parasite, aiming to stop the disease, not just the infection. His decision to try and create synthetic peptides raised much interest. At the time, peptides were thought capable of stimulating only one part of the immune system – the antibody-producing B cells – whereas the prevailing wisdom required T cells as well in order to achieve protective immunity. Sceptics also pounced on the elaborate and painstaking process of elimination that Patarroyo used to find the right peptides.
He took 22 “immunologically interesting” proteins from the malaria parasite, which he identified using antibodies from people immune to malaria, and injected these antigens into monkeys. Eventually, he found four that provided some immunity to malaria. He then sequenced these four antigens and reconstructed dozens of short fragments of them. Again using monkeys (more than a thousand), he tested these peptides individually and in combination until he hit on what he considered to be the jackpot vaccine. However, the WHO considers a 31% success rate to be in the grey area, and so there is still no decision on its use.
In conclusion, it is obvious that malaria is proving to be a difficult disease to establish an effective and cheap vaccine for. Some tests are inconclusive, and others, while they seem to work, do not reach a high enough standard. But having said that, I hope that a viable vaccine will present itself in the near future (with a little help from the scientific world, of course).
