Malaria vaccine.

ABSTRACT

Recently it has become evident that he same candidate antigen can be shared by several of the parasite stages, and thus the concept of a multistage vaccine is becoming more and more attractive. A TDR Task Force evaluated the promise and stage of development of some 20 existing asexual blood stage candidate antigens and prepared a strategy for their development leading to clinical testing and field trials, Amongst these are merozoite surface protein 1 (MSP-1), Serine Rich Antigen (SERA), Apical Membrane Antigen (AMA-1), and Erythrocyte Binding Antigen (EBA). A field study conducted in Tanzanian children showed that the SPf66 Colombian vaccine was safe, induced antibodies, and reduced the risk of developing clinical malaria by around 30%. This study, confirmed the potential of the vaccine to confer partial protection in areas of high as well as low intensity of transmission. Pfs25 is a leading candidate antigen for a transmission blocking vaccine. It is found in the ookinete stage of the parasite in the mosquito midgut. Gramme amounts of GMP-grade material have been produced and a vaccine based on the Pfs25 antigen formulated with alum should have gone into phase I and II clinical trials in the USA and Africa during 1995. Because the first malaria prototype vaccine to be tried out in people on a large scale has been the polymerized synthetic peptide developed by patarroye on the basis of the SPf66 antigen of P. faliciparum, the results are with much interest. It is still premature to predict the effectiveness of this vaccine globally, but its development will encourage further progress in a fields that has repeatedly been characterized by raised and then dashed drops. These various vaccines are based on the classical approach to vaccination, which is to raise host immunity against the parasite so as to reduce parasite densities or to sterilize an infection. A newer approach is development of antidisease vaccines which aim to alleviate morbidity by suppressing immunopathology in the host. Antidisease vaccines are based on neutralizing parasite components that induce host pathology, leaving the parasite itself directly unaffected. These effects would occur when each type of the disease is considered by it self; however, synergistic effects may be expected when they are used in combination. The rational for vaccines based on any of these stages was that immunization of various hosts with whole parasites of each of these stages has been able to induce protection or total transmission-blocking immunity. Less significant but not to be discounted is the fact that natural malaria infections in humans have been shown to induce immunity against every one of these parasite stages against which vaccines are being developed, an exception to this are those stages that are present only in the mosquito vector with component molecules not presented to the human host, such as exclusively ookinete antigens. For several very apparent reasons a vaccine today is conceived of as subnit as opposed to show1 parasite vaccines, either in the form of a recombinant product or as synthetic peptide constructs. Genes coding for several antigens of P. falciparum and some of P. vivax have been seems to be common to many Plasmodium antigens; this is that they contain tandem repeats of oligopeptide sequences which often code for immunodominant epitopes. Following several decades of research on malaria vaccine development, the field at a glace may present a conflicting picture, with several achievements, and some disappointments and controversies. Issues facing the development of a malaria vaccine are complex. It is not clear how far we may yet be from achieving this goal. The work of the past decades has laid an extensive foundation of ralevant knowledge and technologies, and the goal it self remains as important as ever, will scientists remain committed to this objective?