Epitopes Shared Among Pioneer Oral Flora and S. mutans GbpB

Infants mount immune responses to antigens of commensal flora early in life. Components of the pioneer flora share function, and therefore sequence, with similar components of later colonizing flora. Objective: We asked the question, could secretory antibody to pioneer antigenic epitopes cross-react with similar epitopes of later-colonizing flora, potentially modulating their entry into the oral biofilm? Methods: Using glucan binding protein B (GbpB) as a prototype antigen of the late-colonizing Streptococcus mutans, we explored bacterial genomes of pioneer oral streptococci in silico for sequence, function, and MHC Class II binding similarities with S. mutans GbpB. Culture supernatants of pioneer flora were probed with antibody reagents to GbpB in Western blots to detect cross-reactive components which were then subjected to MALDI-TOF and EMBOSS. Results: In silico analyses revealed two proteins (Streptococcus sanguinis peptidoglycan hydrolase and Streptococcus mitis CHAP domain family) with significant sequence homology (>60% in N and C-terminal regions) to GbpB, similar putative functions, and several shared MHC Class II binding peptides. Both S. mitis and S. sanguinis components reacted with rat and chicken anti-GbpB reagents. MALDI-TOF and EMBOSS analyses of a cross-reacting 45 kDa S. mitis component identified an S. mitis CHAP domain family protein; released tryptic peptides had a high correspondence with S. mutans GbpB. Conclusions: These studies provide the basis for an hypothesis that immune responses to pioneer oral streptococci may recognize epitopes also associated with subsequently encountered flora, potentially influencing their colonization. Previously we reported that salivary IgA antibody reactive with GbpB was associated with delayed S. mutans infection of young children. Supporting the present hypothesis is the observation that salivary IgA antibody of these children also frequently reacted with an approximately 45 kDa S. mitis component, a reaction not usually observed in children whose salivary IgA was unreactive with S. mutans GbpB. NIH:TW-06324,DE-06153; FAPESP:02/07156-1,04/07435-8