ABSTRACT An epithelial layer, comprising epithelial cells coated with secretory immunoglobulin A (IgA) and mucus, and the intraepithelial lymphocytes (IEL), is the first defense line against pathogenic insults from the alimentary tract. This barrier structure develops until around the weaning stage in the presence of indigenous intestinal bacteria. Now this immunological development can be recapitulated in gnotobiotic mice or rats monoassociated with a kind of indigenous bacteria, segmented filamentous bacteria (SFB). SFB-monoassociation induces the expansion and activation of IEL, particularly TCR-bearing IEL, and the stimulation of IgA secretion within 2-3 weeks. In the development of IEL, it seems that whole-flora association induces preferential increase in thymus-independent CD8αα subset, while SFB-association equally stimulate the expansion of both the CD8αα and CD8αβ subsets. Tight binding of SFB to the villus and Peyer`s patch-covered epithelial cells is supposed to trigger the expansion of IEL and IgA-committed B cells, respectively.
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