Application of exoantigens of Babesia and Plasmodium in vaccine development

The University of Illinois malaria vaccine programme uses culture-derived soluble exoantigens of Plasmodium falciparum and the squirrel monkey as an experimental model. Exoantigens are soluble polypeptides naturally released into the blood plasma of animals infected with Babesia or Plasmodium species, or into the supernatant medium of in vitro cultures of these organisms. Immunization with soluble B. bovis and B. bigemina exoantigens prepared from culture supernatant fluids protected cattle against homologous and heterologous challenge. Similarly, vaccination of squirrel monkeys with supernatant fluids from P. falciparum cultures containing exoantigen induced protective immunity against acute clinical malaria. Susceptible monkeys have been vaccinated with an aluminium hydroxide-fortified antigenic fraction partially purified from supernatants of P. falciparum strains Indochina I and Genève/SGE-1; this conferred significant clinical protection against needle challenge with the homologous Indochina I strain, and a moderate degree of immunity to the heterologous strain. Following sequential purification by high performance liquid chromatography, the N-terminal amino acid sequences of P. falciparum 100 kDa, 83 kDa and 70 kDa exoantigens were determined. A 29 amino acid peptide constructed from the N-terminal sequence of the P. falciparum (Genève strain) 83 kDa exoantigen has been synthesized. When coupled to a carrier protein, the peptide was immunogenic in rabbits, mice and squirrel monkeys, inducing antibodies which were trophozoite-specific, reactive to native parasite proteins in a two-site enzyme immunoassay (EIA) and in Western blots, and which inhibited P. falciparum growth in vitro. Using this synthetic peptide, EIAs are being developed for the detection of antibodies to P. falciparum blood-phase parasites in individuals living in malaria-endemic areas of Africa, Asia and South America.