Current Status of Malaria Vaccinologyannon
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-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).
