Characterization of newly defined protective antigens of Plasmodium berghei XAT.

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Malaria is a major cause of chronic ill health, sometimes of death, in the tropics, particularly in childhood. Despite considerable effort and expense, a generally available and highly effective malaria vaccine is unlikely in the near future. One of the reasons that we have not get the effective vaccine yet would be the lack of basic information regarding host-parasite relationships between malaria parasite and human beings. Marine malaria models would be the best for accumulating such information, Plasmodium berghei XAT is an irradiation induced attenuated variant of the lethal strain, P. berghei NK65. Previously, we established hybridomas producing protective monoclonal antibodies (mAb) against P. berghei XAT infection. In the present study, we established the two-sited assay system for the detection of B 1D6 Ag, that is the target antigen of protective monoclonal antibodies, in plasma of infected mice. Passive transfer experiments revealed that administration of the protective mAb suppressed the parasitemia in BALB/c mice up to 1 to 1000^<th> level although in C57BL/6 mice up to one to 50^<th> level, suggesting that the effect of protective antigens could be affected by the genetic background of hosts. The protective antigens were purified by affinity chromatography and the peptides were analyzed by LC-MS/MS. The amino acid sequences of peptides fragment of the protective antigens were estimated after de novo sequencing. Interestingly, sequences with high homology were found in a hypothetical protein of P. falciparum (3D7 strain) but not in marine malaria parasites. The study of characteristics of the protective antigens is now ongoing. To know the mechanisms by which the antigens recognized by protective mAbs are involved in the protection would provide an important information for the development of a blood-stage malarial vaccine.