Recombinant cholera toxin B subunit and gene fusion proteins for oral vaccination

Abstract

The B subunit portion of cholera toxin (CTB) is a safe and effective oral immunizing agent in humans, affording protection against both cholera and diarrhoea caused by enterotoxigenic Escherichia coli producing heat-labile toxin (LT) (Clemens et. al., 1986; 1988). CTB may also be used as a carrier of various “foreign” antigens suitable for oral administration. To facilitate large-scale production of CTB for vaccine development purposes, we have constructed recombinant overexpression systems for CTB proteins in which the CTB gene is under the control of strong foreign (non-cholera) promoters and in which it is also possible to fuse oligonucleotides to the CTB gene and thereby achieve overexpression of hybrid proteins (Sanchez and Holmgren, 1989; Sanchez et al., 1988). We here expand these findings by describing overexpression of CTB by a constitutive tacP promoter as well as by the T7 RNA-polymerase promoter, and also by describing gene fusions leading to overexpression of several hybrid proteins between heat-stable E. coli enterotoxin (STa)-related peptides to either the amino or carboxy ends of CTB. Each of the hybrid proteins, when tested as immunogens in rabbits, stimulated significant anti-STa as well as anti-CTB antibody formation, although the anti-STa antibody levels attained (c.a. 1–15 μg/ml specific anti-STa immunoglobulin) were too low to give more than partial neutralization of STa intestinal challenge in baby mice. The hybrid proteins also had a nearnative conformation, as apparent from their oligomeric nature and their strong reactivity with both a neutralizing antibody against the B subunit and a neutralizing monoclonal antibody (mAb) against STa. However, only hybrid protein presenting the STa peptide with a free carboxy end was able to also react with another available STa mAb. Our results suggest that even minor modifications of a given antigenic region may lead to complete epitope hiding and/or to its lack of antibody reactivity. Alternate positioning of such peptides in the carboxy end of the CTB protein was found to assist in antibody recognition and is proposed as a means to help exposure of some foreign epitopes by CTB fusion proteins. The results may be of significance for the development of recombinant oral vaccines based on gene fusions to CTB or to the closely related B subunit of LT.