Antigenic cross-reactivity and sequence homology between Actinobacillus actinomycetemcomitans GroEL protein and human fibronectin

The immunologic cross-reactivity between human fibronectin and Actinobacillus actinomycetemcomitans GroEL was examined. Analyses by SDS-PAGE/Western immunoblotting and ELISA showed that a polyclonal antibody directed against the purified GroEL protein of A. actinomycetemcomitans, but not against the Escherichia coli GroEL, cross-reacts with human fibronectin. No antigenic cross-reactivity was observed between anti-A. actinomycetemcomitans GroEL antibody and type IV collagen, another important constituent of the basement membrane. A comparative analysis of the amino acid sequences of A. actinomycetemcomitans GroEL and human fibronectin revealed eight instances of four-amino acid sequence homology between the two proteins. Six of these tetrapeptide sequences were also shared with E. coli GroEL, suggesting that the remaining two tetrapeptides, GQLI (Glycine-Glutamine-Leucine-Isoleucine) and TGLE (Threonine-Glycine-Leucine-Glutamic acid), may be associated with the epitope that the anti-A. actinomycetemcomitans GroEL antibody specifically recognizes. Reactivity between TGLE, but not GQLI, with anti-A. actinomycetemcomitans GroEL antibody was confirmed by a biospecific interaction analysis using a biosensor technology. Although additional investigations are required, the observed phenomenon may lead to an autoimmune response and thus contribute to tissue destruction during periodontitis.