A peptide derived from a B cell epitope of Plasmodium falciparum rhoptry associated protein 2 specifically raises antibodies to rhoptry associated protein 1

Mice immunised with a recombinant form of malarial antigen rhoptry-associated protein 2 (RAP2) produce antisera which recognise the native protein by indirect immunofluorescence and immunoblotting. Purified IgG components of the antisera partially inhibit erythrocyte invasion in vitro. This response was obtained only if the recombinant immunogen was presented to the mice in the presence of reducing and denaturing agents. An 8-mer epitope in RAP2 was recognised by antibodies in three of the antisera: E25TEFSKLY32. Immunisation with this octapeptide raised antibodies which strongly recognised reduced RAP2 in seven out of eight mice. However, this antisera either failed to recognise (five out of eight mice), or only weakly (three out of eight mice) recognised nonreduced RAP2. Examination of disulphide bonds in native RAP2 showed that the 4 cysteines of RAP2 form two disulphide bridges: Cys24-Cys88, and Cys277-Cys376. One of these (Cys24-Cys88) is adjacent to the octapeptide in the native protein. Surprisingly, seven out of eight mice immunised with the octapeptide also raised antibodies against native rhoptry-associated protein 1 (RAP1). The raising of antibodies which recognise RAP1 was induced specifically by the RAP2 octapeptide rather than the carrier protein used for immunisation. The epitope in RAP1 recognised by the antibodies was identified and shown not to be the result of any shared contiguous homologous sequence between the two proteins, but to shared homologous amino acids in critical positions within the epitope. Purified IgG components from the antisera of mice immunised with the octapeptide gave partial inhibition of erythrocyte invasion in vitro.