CURRENT STATUS OF MALARIA VACCINOLOGY

In order to assess the current status of malaria vaccinology one must 
first take an overview of the whole 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 per annum. 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 eradication of this disease is of 
the utmost importance.
The disease is caused by one of four species of Plasmodium These four 
are P. falciparium, P .malariae, P .vivax and P .ovale. Malaria does not 
only effect humans, but can also infect a variety of hosts ranging from 
reptiles to monkeys. It is therefore necessary to look at all the 
aspects in order 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 Anophels 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 go to infect more hepatic liver cells or they can attach to and 
penetrate erytherocytes. When inside the erythrocytes the plasmodium 
enlarges into uninucleated cells called trophozites 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 reaches lysis and as 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 erythrocytes 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 there fore the erythrocyte remains 
stable and when the infected host is bitten by a mosquito the 
gametocytes can enter its digestive system where they mature in to 
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 exoethrocytic stages but resistance to them is becoming 
increasing common. Therefore a vaccine looks like the only viable 
option.
The wiping out of the vector i.e. Anophels mosquito would also prove as 
an effective way of stopping disease transmission but the mosquito are 
also becoming resistant to insecticides and 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 that is the theory of Malaria 
vaccinology 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 immunised 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 case the 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 which 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 is shown later there may be an answer to the 
dilemma.
While researching current candidate vaccines I came across some which 
seemed more viable than others and I will briefly look at a few of these 
in this essay.
The first is one which 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 out patient 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 to be indirect evidence that 
cytotoxic T lymphocytes play a role against P falciparium in humans 
Using a human leucocyte antigen based approach termed reversed 
immunogenetics they previously identified peptide epitopes for CTL in 
liver stage antigen-1 and the circumsporozoite protein of P falciparium 
which is most lethal of the falciparium to infect humans. Having these 
identified 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 epidopes are in conserved regions of P. 
falciparium.
They also found CTL peptide epitopes in a further two antigens 
trombospodin 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, 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 immunisation may provide 
substantial or even complete protection against infection and disease.
Although these test 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 
falciparium circumsporozoite vaccine. The study was carried out in the 
early nineties 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) if 
Pseudomanas aeruginosa. Each 0.4 ml dose of R32 Tox-A contained 320mg of 
the R32 LR-Toxin-A conjugate (molar ratio 6.6:1), absorbed to aluminium 
hydroxide (0.4 % w/v), with merthiolate (0.01 %) as a preservative.
The Thai test was based on specific humoral immune responses to 
sporozoites are stimulated by natural infection and are directly 
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. Immunisation 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 have been inconclusive possibly this is 
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 and 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/diptheria toxiods (10 and 1 Lf units respectively) at 0, 8 and 
16 weeks. Immunisation was performed in a malaria non-transmission area, 
after completion of which 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 elicit an antibody 
response in all vaccinees. Peak CS antibody (IgG) concentrated in 
malaria-experienced vaccinees exceeded those in malaria-naïve vaccinees 
(mean 40.6 versus 16.1 mg ml-1; 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 at the 
end we are once again left without conclusive evidence, but 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 and so the following 
information is taken from these. The vaccine to which I am referring to 
is the SPf66 vaccine. This vaccine has caused much controversy and 
raised certain dilemmas. It was invented by a Colombian physician and 
chemist called Manual Elkin Patarroyo and it is the first of its kind. 
His vaccine could prove to be one the few effective weapons against 
malaria, but 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 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 by Patarroyo 
and his group in Columbia during 1990 and 1991 showed that the vaccine 
cut malaria episodes by over 39 % and first episodes by 34%. Another 
trail 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 - stopping 
the disease not 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 Patarroyo used to find the right peptides. He took 22 
"immunologically interesting" proteins from the malaria parrasite, which 
he identified using antibodies from people immune to malaria, and 
injected these antigens into monkeys and eventually 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. But the WHO a 31% 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 a difficult disease 
to establish an effective and cheap vaccine for in that some tests and 
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).