Model multiple antigenic and homopolymeric peptides from non-repetitive sequences of malaria merozoite proteins elicit biologically irrelevant antibodies

Three model peptides containing B-epitopes from conserved, non-repetitive r egions of the merozoite surface antigens, MSA2 and MSA1, and the erythrocyt e binding protein EBP of Plasmodium falciparum were synthesised. The peptid es incorporated GPG spacers and C residues at the N and C termini, and were polymerised by oxidation to form cystine bridges. Multiple copies of essen tially the same peptide sequences were also synthesised on a branching lysy l matrix to form a tetrameric multiple antigen peptide. Rabbits were immuni sed with the polymerised and multiple antigen peptides, in alum followed by Freund's adjuvant, and the antibody responses examined by IFA and ELISA. R eproducible antibody responses were obtained against the MSA1 and EBP but n ot MSA2 peptides. IgG antibody levels detected by ELISA after three injecti ons of antigen in alum, increased significantly after further immunisation in Freund's adjuvant. IgG levels were largely maintained for at least 23 we eks after the final immunisation. IgM antibodies, generally detectable only after immunisation in Freund's adjuvant, were absent 23 weeks later. Antib ody titres against the native protein on fixed parasites, assayed by IFA, w ere three to five orders of magnitude lower than the corresponding ELISA ti tres against the peptides, Antibody-dependent inhibition of P. falciparum g rowth in vitro could not be demonstrated with the immune rabbit sera. The M SA1 and EBP peptides elicited cross-reactive antibodies. The results sugges t that the selected non-repetitive sequences are conformationally constrain ed in the native proteins and only a small proportion of the anti-peptide a ntibodies bind to the native proteins. The significance of the findings for the development of peptide vaccines and the use of peptides in immunoassay s is discussed. (C) 1999 Elsevier Science B.V. All rights reserved.