In order to assess the current status of malaria vaccinology one must first takean overview of the whole of the whole disease.
One must understand the diseaseand its enormity on a global basis. Malaria is a protozoan disease of which over 150 million cases are reported perannum. In tropical Africa alone more than 1 million children under the age offourteen die each year from Malaria. From these figures it is easy to see thateradication 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 effecthumans, 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 thepossibility of a vaccine. The disease has a long and complex life cycle which creates problems forimmunologists. The vector for Malaria is the Anophels Mosquito in which the lifecycle of Malaria both begins and ends. The parasitic protozoan enters thebloodstream via the bite of an infected female mosquito. During her feeding shetransmits a small amount of anticoagulant and haploid sporozoites along withsaliva. The sporozoites head directly for the hepatic cells of the liver wherethey multiply by asexual fission to produce merozoites.
These merozoites can nowtravel one of two paths. They can go to infect more hepatic liver cells or theycan attach to and penetrate erytherocytes. When inside the erythrocytes theplasmodium enlarges into uninucleated cells called trophozites The nucleus ofthis newly formed cell then divides asexually to produce a schizont, which has6-24 nuclei. Now the multinucleated schizont then divides to produce mononucleated merozoites. Eventually the erythrocytes reaches lysis and as result the merozoites enterthe bloodstream and infect more erythrocytes.
This cycle repeats itself every48-72 hours (depending on the species of plasmodium involved in the originalinfection) The sudden release of merozoites toxins and erythrocytes debris iswhat causes the fever and chills associated with Malaria. Of course the disease must be able to transmit itself for survival. This is doneat the erythrocytic stage of the life cycle. Occasionally merozoitesdifferentiate into macrogametocytes and microgametocytes. This process does notcause lysis and there fore the erythrocyte remains stable and when the infectedhost is bitten by a mosquito the gametocytes can enter its digestive systemwhere they mature in to sporozoites, thus the life cycle of the plasmodium isbegun again waiting to infect its next host. At present people infected with Malaria are treated with drugs such asChloroquine, Amodiaquine or Mefloquine.
These drugs are effective at eradicatingthe 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 aneffective way of stopping disease transmission but the mosquito are alsobecoming resistant to insecticides and so again we must look to a vaccine as asolutionHaving read certain attempts at creating a malaria vaccine several points becomeclear. 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 byChristopher Dye and Geoffrey Targett. They used the MMR (Measles Mumps andRubella) vaccine as an example to which they could compare a possible Malariavaccine Their article said that “simple epidemiological theory states that thecritical fraction (p) of all people to be immunised with a combined vaccine(MMR) to ensure eradication of all three pathogens is determined by theinfection that spreads most quickly through the population; that is by the ageof one with the largest basic case reproduction number Ro. In case the of MMRthis is measles with Ro of around 15 which implies that p> 1-1/Ro0.
93Gupta et al points out that if a population of malaria parasite consists of acollection of pathogens or strains that have the same properties as commonchildhoodviruses, the vaccine coverage would be determined by the strain withthe largest Ro rather than the Ro of the whole parasite population. Whileestimates 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 thehighest Ro it could provide immunity to all malaria plasmodium “Another problem faced by immunologists is the difficulty in identifying theexact antigens which are targeted by a protective immune response. Isolating thespecific antigen is impeded by the fact that several cellular and humoralmechanisms probably play a role in natural immunity to malaria – but as is shownlater there may be an answer to the dilemma.
While researching current candidate vaccines I came across some which seemedmore 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 to1995. (taken from the Lancet of April 1995). The subjects were 63 healthy adultsand 56 malaria identified children from an out patient clinicTheir test was based on the fact that experimental models of malaria have shownthat Cytotoxic T Lymphocytes which kill parasite infected hepatocytes canprovide complete protective immunity from certain species of plasmodium in mice.
From the tests they carried out in the Gambia they have provided, what they seeto be indirect evidence that cytotoxic T lymphocytes play a role against Pfalciparium in humansUsing a human leucocyte antigen based approach termed reversed immunogeneticsthey previously identified peptide epitopes for CTL in liver stage antigen-1 andthe circumsporozoite protein of P falciparium which is most lethal of thefalciparium to infect humans. Having these identified they then went on toidentify CTL epitopes for HLA class 1 antigens that are found in mostindividuals from Caucasian and African populations. Most of these epidopes arein conserved regions of P. falciparium.
They also found CTL peptide epitopes in a further two antigens trombospodinrelated anonymous protein and sporozoite threonine and asparagine rich protein. This indicated that a subunit vaccine designed to induce a protective CTLresponse may need to include parts of several parasite antigens. In the tests they carried out they found, CTL levels in both children withmalaria and in semi-immune adults from an endemic area were low suggesting thatboosting these low levels by immunisation may provide substantial or evencomplete protection against infection and disease. Although these test were not a huge success they do show that a CTL inducingvaccine may be the road to take in looking for an effective malaria vaccine.
There is now accumulating evidence that CTL may be protective against malariaand that levels of these cells are low in naturally infected people. Thisevidence suggests that malaria may be an attractive target for a new generationof CTL inducing vaccines. The next candidate vaccine that caught my attention was one which I read aboutin Vaccine vol 12 1994. This was a study of the safety, immunogenicity andlimited efficacy of a recombinant Plasmodium falciparium circumsporozoitevaccine. The study was carried out in the early nineties using healthy male Thairangers between the ages of 18 and 45. The vaccine named R32 Tox-A was producedby the Walter Reed Army Institute of Research, Smithkline Pharmaceuticals andthe Swiss Serum and Vaccine Institute all working together.
R32 Tox-A consistedof the recombinantly produced protein R32LR, amino acid sequence (NANP)15(NVDP)2 LR, chemically conjugated to Toxin A (detoxified) if Pseudomanasaeruginosa. Each 0. 4 ml dose of R32 Tox-A contained 320mg of the R32 LR-Toxin-Aconjugate (molar ratio 6. 6:1), absorbed to aluminium hydroxide (0. 4 % w/v), withmerthiolate (0. 01 %) as a preservative.
The Thai test was based on specific humoral immune responses to sporozoites arestimulated by natural infection and are directly predominantly against thecentral repeat region of the major surface molecule, the circumsporozoite (CS)protein. Monoclonal CS antibodies given prior to sporozoite challenge haveachieved passive protection in animals. Immunisation with irradiated sporozoiteshas produced protection associated with the development of high levels ofpolyclonal CS antibodies which have been shown to inhibit sporozoite invasion ofhuman hepatoma cells. Despite such encouraging animal and in vitro data,evidence linking protective immunity in humans to levels of CS antibody elicitedby natural infection have been inconclusive possibly this is because of theshort serum half-life of the antibodies. This study involved the volunteering of 199 Thai soldiers.
X percentage ofthese were vaccinated using R32 Tox -A prepared in the way previously mentionedand as mentioned before this was done to evaluate its safety, immunogenicity andefficacy. This was done in a double blind manner all of the 199 volunteerseither received R32Tox-A or a control vaccine (tetanus/diptheria toxiods (10 and1 Lf units respectively) at 0, 8 and 16 weeks. Immunisation was performed in amalaria non-transmission area, after completion of which volunteers weredeployed to an endemic border area and monitored closely to allow earlydetection and treatment of infection. The vaccine was found to be safe andelicit an antibody response in all vaccinees. Peak CS antibody (IgG)concentrated in malaria-experienced vaccinees exceeded those in malaria-navevaccinees (mean 40. 6 versus 16.
1 mg ml-1; p = 0. 005) as well as those inducedby previous CS protein derived vaccines and observed in association with naturalinfections. A log rank comparison of time to falciparium malaria revealed nodifferences between vaccinated and non-vaccinated subjects. Secondary analysesrevealed that CS antibody levels were lower in vaccinee malaria cases than innon-cases, 3 and 5 months after the third dose of vaccine. Because antibodylevels had fallen substantially before peak malaria transmission occurred, thequestion of whether or not high levels of CS antibody are protective stillremains to be seen. So at the end we are once again left without conclusiveevidence, but are now even closer to creating the sought after malaria vaccine.
Finally we reach the last and by far the most promising, prevalent andcontroversial candidate vaccine. This I found continually mentioned throughoutseveral scientific magazines. “Science” (Jan 95) and “Vaccine” (95) were twowhich 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 hascaused much controversy and raised certain dilemmas.
It was invented by aColombian physician and chemist called Manual Elkin Patarroyo and it is thefirst of its kind. His vaccine could prove to be one the few effective weaponsagainst malaria, but has run into a lot of criticism and has split the malariaresearch community. Some see it as an effective vaccine that has proven itselfin various tests whereas others view as of marginal significance and say morestudy 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 hisgroup in Columbia during 1990 and 1991 showed that the vaccine cut malariaepisodes by over 39 % and first episodes by 34%. Another trail which wascompleted in 1994 on Tanzanian children showed that it cut the incidence offirst episodes by 31%. It is these results that have caused the rift withinresearch areas. Over the past 20 years, vaccine researchers have concentrated mainly on theearly stages of the parasite after it enters the body in an attempt to blockinfection at the outset (as mentioned earlier).
Patarroyo however, took a morecomplex approach. He spent his time designing a vaccine against the more complexblood stage of the parasite – stopping the disease not the infection. Hisdecision to try and create synthetic peptides raised much interest. At the timepeptides were thought capable of stimulating only one part of the immune system;the antibody producing B cells whereas the prevailing wisdom required T cells aswell in order to achieve protective immunity.
Sceptics also pounced on the elaborate and painstaking process of eliminationPatarroyo used to find the right peptides. He took 22 “immunologicallyinteresting” proteins from the malaria parrasite, which he identified usingantibodies from people immune to malaria, and injected these antigens intomonkeys and eventually found four that provided some immunity to malaria. Hethen sequenced these four antigens and reconstructed dozens of short fragmentsof them. Again using monkeys (more than a thousand) he tested these peptidesindividually and in combination until he hit on what he considered to be thejackpot vaccine. But the WHO a 31% rate to be in the grey area and so there isstill no decision on its use. In conclusion it is obvious that malaria is proving a difficult disease toestablish an effective and cheap vaccine for in that some tests and inconclusiveand others while they seem to work do not reach a high enough standard.
Buthaving said that I hope that a viable vaccine will present itself in the nearfuture (with a little help from the scientific world of course).Category: Science
