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-nave 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). 
 
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