Current Status Of Malaria Vaccinology Essay Research

Paper

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-naпve 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).