IMMUNOGENICITY OF THE NONREPETITIVE REGIONS OF THE CIRCUMSPOROZOITE PROTEIN OF PLASMODIUM-KNOWLESI

The circumsporozoite antigen (CS) of the simian malarial parasite Plas modium knowlesi consists of tandemly repeated immunodominant peptide u nits that are Variable and may play a role in evading the immune syste m. To study the immunogenicity of this antigen in the absence of the i mmunodominant repeats, the entire nonrepetitive region of the antigen was expressed in Escherichia Coli as two fusion proteins with glutathi one-S-transferase (GST) representing the amino terminal (GST-CSN) and the carboxy terminal domains (GST-CSC) of the CS antigen. The immunoge nicity of these fusion proteins was studied in rabbits and different s trains of mice. Antibody raised against both the CSN and CSC domains i n both rabbits and every strain of mice recognized the native protein, as detected by immunofluorescence assay (IFA) using P. knowlesi sporo zoites. A positive IFA reaction was also obtained with P. vivax sporoz oites using antisera raised against the CSC domain. High titer antiser a were raised in rabbits against both the domains, whereas mice showed comparatively low titers. On Western blots, mice showed specific resp onse against the CSC domain. In both rabbits and mice, significant tit ers of antibodies were raised against region II, which has been shown to be the putative sporozoite binding site for hepatocytes in the case of P. falciparum.