Naturally occurring immune response against bacteria commonly involved in upper respiratory tract infections: analysis of the antigen-specific salivary IgA levels.

Lyophilized bacterial lysates, which actively stimulate the immune response, are widely used as vaccines or 'biological response modifiers' in subjects with recurrent bacterial respiratory infections. Since vaccines are indicated in the absence or in the presence of a weak constitutive immune response activity, a better knowledge on the 'naturally' occurring antibacterial immune response at the oropharingeal level should be helpful. A study was, therefore, designed to quantify the presence of salivary IgA directed against surface antigens bacteria frequently involved in the pathogenesis of upper respiratory tract infections: Klebsiella pneumoniae (KP), Staphylococcus aureus (SA), Streptococcus pyogenes (SPy), Morraxella catarrhalis (MC), Haemophylus influenzae (HI), and Streptococcus pnumoniae (SPn). In 34 volunteers (21 adults and 13 children), salivary fluid was collected and the presence of microorganism-specific IgA antibodies evaluated by a novel enzyme immuno-assay. In the whole population only 29 and 24% of subjects had IgA directed, respectively, to KP and SA, while the immune-response against other microbes was detectable in a small population ranging from 12 to 15% of all subjects studied. We found higher proportions of individuals with strain specific salivary IgA in the adult than in the pediatric population for all the microorganism evaluated. In addition, in children, the only strain inducing a significant production of specific IgA at oropharingeal level was KP. Interestingly, only ten out of 21 adults and two out 13 children have at least one significantly high antibody titer against one of the bacteria evaluated. Nevertheless, when a group of healthy donors was treated with a polyvalent mechanical bacterial lysate (Ismigen t.), the large majority developed a specific immune-response in the salivary fluid. These results are thus consistent with the good features of the novel enzyme-immunoassay and with a poor frequency of naturally induced specific anti-microbe antibodies in children and in adults despite the presence on recurrent respiratory infections in their clinical history.