Significance of different J chain profiles in human tissues: generation of IgA and IgM with binding site for secretory component is related to the J chain expressing capacity of the total local immunocyte population, including IgG and IgD producing cells, and depends on the clinical state of the tissue.

Most IgA producing cells in normal intestinal and nasal mucosa synthesize dimers or larger polymers as evidenced by 90% cytoplasmic affinity for secretory component (SC) in vitro and almost 100% J chain positivity. The comparable median figures for normal exocrine glands (salivary, lacrimal, lactating mammary) were 84% and 92%, respectively. Conversely, IgA immunocytes in the subepithelial areas of palatine tonsils and in other extraglandular tissues, such as inflamed gingiva and intestinal submucosa, showed only 16-28% SC binding capacity and 18-51% J chain positivity. Similarly decreasing J chain expression, from glandular to extraglandular sites, was revealed not only for IgM immunocytes but also for those producing IgD or IgG, particularly the latter. This observation indicated more extensive overall clonal maturation in tissues without glandular elements since J chain expression seems to be a feature of relatively early memory B cells. The results were supported by studies in patients with selective IgA deficiency. Inflammatory disease caused significantly reduced SC binding capacity of IgA cells, both in intestinal mucosa and tonsils; this change was paralleled by decreased J chain expression, not only for mucosal and tonsillar IgA cells but also for mucosal IgG cells, suggesting local appearance of more mature clones. The resulting change to production of monomeric IgA may adversely affect secretory immunity and thus contribute to perpetuation of chronic inflammatory disease.