Novel putative protective antigens of Plasmodium falciparum

In an attempt to identify putative host-protective antigens of the human malarial parasite P. falciparum, a differential immunoscreen of a cDNA expression library of the parasite was performed using malaria immune and patient sera. Eight expression clones were identified which showed no reactivity with acute patient sera but reacted extensively with malaria immune sera. Southern Blot analysis of five of these genes showed that these were coded by unique single genes, and sequence of the cDNA inserts showed that these were as yet unidentified genes of P. falciparum. The clone which was reactive to the largest number of immune sera, has been shown to be the ribosomal phosphoprotein, PO, of P. falciparum. The presence of multiple transcripts and a differential expression of the transcripts of PO in different erythrocytic substages of the parasite was observed. An extensive characterisation using antibodies to the PO protein has been performed. The results show that in addition to its role in the ribosomal assembly, the PO protein is also localized on the parasite surface, and plays an important role in the red blood cell invasion. Most of the other differential cDNA clones were found to be rare transcripts, and the expression domains were not very immunogenic. PCR analysis of three of these genes demonstrated that these are conserved throughout the Plasmodium species. These protein domains, therefore, constitute potential protective target epitopes of the malarial parasite.