Essay/Term paper: Current status of malaria vaccinology

Essay, term paper, research paper:  Medicine

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
childhoodviruses, 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).