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 viru
ses, 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-naive 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 com
parison 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).
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