Salivary secretory immunoglobulin A (S-IgA) is important for the maintenance of oral health in humans. However, understanding of how salivary IgA antibodies fulfil their biological role is seriously lacking, and the overall objective of this proposal is therefore to investigate the functions of human IgA antibodies with respect to the oral environment. In addition, many significant oral bacteria, including some associated with plaque formation or with periodontal diseases, produce IgA proteases and glycosidases, which modify the structure of IgA. These enzymes are not only important with respect to oral health problems, but can be exploited to elucidate functional properties of IgA antibodies. Furthermore, the mouth is an interface where salivary S-IgA antibodies confront the systemic compartment of the immune system, represented by other antibody isotypes, complement components, and cellular elements (lymphoid and phagocytic cells, especially neutrophils), which are present in the gingivae and enter the mouth through the gingival crevice. Elucidation of these interactions will be important for understanding the immunopathogenesis of tooth and periodontal diseases. Human IgA antibodies to various antigens, of IgA1 and IgA2 subclasses, and of different molecular form (monomeric, polymeric, or secretory) will be purified from polyclonal and monoclonal sources, chromatographically and avoiding denaturing procedures. Human IgG and IgM antibodies will also be purified for comparative purposes. The functional properties of these various IgA antibodies will be studied with respect to: (i) complement activation, especially by the alternative pathway; (ii) inhibition of complement activation (largely by the classical pathway) induced by IgG (or IgM) antibodies; (iii) opsonization for phagocytosis by neutrophils and other phagocytic cells, without or with the involvement of complement; (iv) interference with phagocytosis induced by IgG (or IgM) antibodies, without or with the involvement of complement. The effects of cleavage of IgA1 and S-IgA1 by bacterial IgA1 proteases, and of removal of carbohydrate residues from IgA and S-IgA antibodies by glycosidases produced by oral streptococci, on these functional properties of IgA will be investigated. Enzyme-linked immunosorbent assays will be used to assay and characterize the antibodies, and to assess complement activation by the fixation of the C3 component. Fluorescent microspheres will be used to prepare phagocytosable particles with a defined antigenic surface, so that phagocytosis mediated by IgA or other antibodies can be assayed quantitatively by fluorescence in a flow cytometer. It is anticipated that the information gained from these studies on the biological functions of human IgA antibodies will be relevant also to other physiological and pathological situations.
