Reactivity profile of human anti-82-kilodalton rhoptry protein antibodies generated during natural infection with Plasmodium falciparum.

Immunization of monkeys with the 82-kDa rhoptry protein (p82) of Plasmodium falciparum can protect them against a lethal blood stage challenge, and monoclonal antibodies to p82 inhibit parasite growth in vitro. The role that a p82-specific immune response might play in human immunity to the parasite is not known. To determine to what extent humans produce antibodies to p82 following infection with P. falciparum, sera from individuals believed to be hyperimmune, semi-immune, or never infected with the parasite were examined. Portions of the p82 gene were expressed separately as fusion proteins and used on immunoblots to test for antibodies to the recombinant proteins. All but 1 of the 30 immune sera possessed antibodies to p82, while nonimmune sera produced, at best, only a marginal signal to the fusion proteins. The signal intensity produced with the human immune sera depended on the region of p82 being assayed, with the N-terminal 37% of p82 producing stronger signals than more C-terminal parts of p82. This N-terminal domain contains a tandem octapeptide repeat (consensus KSSSPSXT/V) of the structure (repeat)2-Q-T-S-G-S/L-(repeat)3. It is shown here that the sequence of this repetitive motif is conserved among four parasite isolates at both the nucleotide and amino acid levels; the five-residue repeat interruption peptide QTSGS/L separating the two sets of repeats contains the only amino acid substitution (Ser or Leu) detected in this region to date. Despite their conservation of structure, the repeats do not appear to be the only epitope recognized by the human antibodies, since sera which recognize the N-terminal fusion protein containing the repeats also bind a related protein after truncation and removal of the repeats. These results indicate that the structurally conserved p82 molecule contains multiple B-cell epitopes and is likely to be immunogenic in most individuals during natural infections with P. falciparum. These observations are consistent with the idea that antibodies to p82 generated during parasite infection have a role in the development of immunity to the organism.