Malaria vaccines
According to WHO estimates 4 billion people are at the risk of developing malaria and about 500 million cases occur each year resulting in one million death each year mainly of children of 5 years age and pregnant women. In addition, development of resistance against drugs by the species of Plasmodium, and insecticides by mosquitoes have been reported. Therefore, threat of malaria disease is still increasing for humans. Therefore, for control of malaria use of vaccines and vector control programmes would be successful. Much work is going on at Indian Institute of Immunology (New Delhi) and ICGEB on development of malaria vaccine by using modern methodologies. All kind of vaccine development through recombinant antigens, synthetic peptides and direct use of DNA are being attempted. All these attempts indicate that development of malaria vaccine is largely complex process. However, progress towards the development of a malaria vaccine has been slow due to several reasons, one of which has been the lack of in vitro correlates and the suitable animal models for malaria vaccine trials. Plasmodium-vhesus monkey is one of the models for malaria vaccine development.
Malaria vaccines are being developed at three distinct developmental stages of the parasite : (i) pre-erythrocytic stage (to eliminate infection by blocking the sporozoites from entering hepatocytes or by destroying the infected hepatocytes), (ii) blood stage of parasite (to prevent disease or reduce parasitic load), and (iii) sexual stage parasite (to limit transmission of disease).
In India 60-70% malaria is due to P. vivex which do not kill host but results discomfort and morbidity. It is more prevalent throughout Asia; but less is known about immune response of host against P. vivex as it resisted all attempts of culturing the parasite. P. cynomolgi is a simian malaria which is closely related to P. vivex in taxonomy and morphology. Hence it is regarded as a good model to study P. vivex infection as both share a similar clinical course of infection. At present vaccines are being developed at ICGEB against all stages of life cycle of the parasite but it is believed that an asexual blood stage vaccine is most likely to have the greatest impact on the disease.
(i) Expression of vaccine target antigens. The most successful vaccines have been based on attenuated or killed pathogens. Malaria vaccine is limited to well defined molecules which can induce protective immune responses and easily produced by recombinant DNA technology in various systems. Three important vaccine target antigens such as thrombospondin related adhesive protein (TRAP), apical membrane antigen (AMA) and erythrocyte binding protein (EBP) from P. cynomolgi have been cloned and sequenced (Table 5.1). To evaluate their vaccine potential, these antigens have been expressed in E.coli using pQE expression system. Each of these proteins was expressed at high levels as insoluble inclusion bodies. Protocols for the large scale production of the correctly folded PcTRAP have been developed. TRAP has a multidomain structure and localized on the cell surface of P. falciparum sporozoites.
(ii) Animal trials of malaria vaccines. The rhesus monkeys were immunized with recombinant PcTRAP, parasite lysate or adjuvant to study the protective efficacy. The antigens were delivered intramuscularly in three doses (500mg each) on 0, 42, and 62 days. Blood from immunized and unimmunized monkeys was collected on days 1, 14, 29, 52 and 70. High antibody titers (8-16 * 105 and above) were detected against PcTRAP and parasite lysate as measured by ELISA technique. Then immunized monkeys after injection with P. cynomolgi sporozoites (3 * 104) were protected from malaria (ICGEB, 1998). |
