Inhibitory activity against sporozoites induced by antibodies directed against nonrepetitive regions of the circumsporozoite protein of Plasmodium vivax.

In previous studies, Saimiri sciureus boliviensis monkeys have been immunized with four recombinant proteins reproducing part of the circumsporozoite (CS) protein of Plasmodium vivax sporozoites (NS1(81) V20, rPvCS-1, rPvCS-2, rPv-CS-3), or with irradiated sporozoites of P. vivax Salvador I strain. To analyze the antibody response elicited against epitopes located outside the immunodominant repeat region of the CS protein, serum samples from these animals were tested for their ability to inhibit the in vitro development of liver stages of P. vivax VK247 strain, characterized from the other strains only by a specific repeat region on the CS protein. Results indicated that there is at least one protective B-cell epitope outside the repeat region of the CS protein of P. vivax sporozoites, and that this epitope can be expressed by irradiated sporozoites, rPvCS-1 and -3, but not by rPvCS-1 or NS1(81)V20. Therefore, we designed peptides from the amino acid sequences present both in rPvCS-2 and -3, but not included in the recombinant proteins rPvCS-1 and NS1(81)V20. Anti-peptide antibodies had no activity on the development of sporozoites of P. vivax Salvador I strain, into schizonts in primary culture of Saimiri monkey hepatocytes. In addition, antisporozoite antibodies did not react with any of the peptides. These results suggest that the in vitro inhibition observed in this study could depend upon the conformation of the CS protein. This study also demonstrates that antibody response to unnatural linear epitopes can be induced by immunization with recombinant proteins.