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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 effectiv e
ateradicating 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. 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.
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 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-na‹
ve 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).
Word Count: 2,223
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