Studies of the antigenic structure of two cross-reacting proteins, pertussis and cholera toxins, using synthetic peptides.

Peptide fragments of pertussis toxin subunit 1 (PT-S1) have been synthesized in order to investigate their antigenic and immunogenic activity, and to evaluate their possible use as components of a new vaccine. Two peptides (sequence 73-82, EAERAGRGTG and sequence 107-116, YVDTYGDNAG) were selected for their predictable exposure on the surface of the molecule, and a third (8-18, YRYDSRPPEDV) for its homology with the sequence 6-16 of cholera toxin subunit A (CT-A 6-16) (YRADSRPPDEI). Antipeptide polyclonal antibodies produced in rabbits, were tested in different immunoassays for their ability to interact with toxin proteins. All of them proved interactive with recombinant PT-S1 (rPT-S1); CT-A interact not only, as expected, with anti 8-18 antibodies, due to the high homology between the two toxins in this region, but also, unexpectedly, with anti 107-116 antibodies, in spite of the lack of homology of this peptide with the entire CT. We also found a direct cross-reactivity between the two toxins: anti PT and anti rPT-S1 antibodies interacted with CT-A, whereas anti CT antibodies did not recognize PT. Antipertussis antibodies also recognized the peptide 8-18, which therefore represents at least a part of an antigenic determinant of the toxin, while no interaction could be evidenced between anti-cholera antibodies and any of the peptides, thus demonstrating important differences in the antigenic structures of the two toxins. None of the antipeptide antibodies examined showed protective activity against the toxins in a Chinese hamster ovary (CHO) cell test.