REGULATION OF INTESTINAL EPITHELIAL BARRIER FUNCTION BY TGF-BETA-1 - EVIDENCE FOR ITS ROLE IN ABROGATING THE EFFECT OF A T-CELL CYTOKINE

Maintenance of the integrity of the single-cell-thick intestinal epith elium as an in vivo barrier between environmental Ags and mucosal immu nocytes is pivotal for health. The T cell cytokine IFN-gamma consisten tly disrupts this epithelial barrier in vitro, but the substances in m ucosa that may be responsible for sustaining or enhancing barrier func tion have not been clearly identified. Therefore, we characterized the effect on the epithelial barrier of TGF-beta 1 and three prominent ne uropeptides (VIP, substance P, somatostatin) by using a model system i n which barrier function of a mature polar human colonic epithelial (T 84) cell monolayer is reflected in 1) the electrical potential differe nce across the apical to basolateral surface of each cell, 2) the tran smonolayer permeability to macromolecules such as horseradish peroxida se, and 3) lactate dehydrogenase release into the medium indicating ep ithelial cell cytolysis. Whereas T84 monolayers exposed to TGF-beta 1 alone demonstrated a modest increase in electrical resistance and barr ier integrity, TGF-beta 1 showed a striking ability to reduce the capa city of IFN-gamma to disrupt epithelial barrier function. Characteriza tion studies demonstrated that this TGF-beta 1 effect was prolonged (e .g., days) after a single exposure, progressive over the dose range 0. 1 to 2.5 ng/ml, reversible with increased concentrations of IFN-gamma, and more pronounced when TGF-beta 1 exposure was to basolateral rathe r than to apical epithelial membranes. Macromolecular (horseradish per oxidase) penetration of epithelium was not simultaneously altered by T GF-beta 1 and epithelial cellular injury was minimal as gauged by lact ate dehydrogenase release. Additional studies using a human pathogen d emonstrated that TGF-beta 1 delayed and decreased the barrier disrupti on caused by exposure to Cryptosporidium parvum. TGF-beta 1 may be the first of a new class of cytokines that maintains and/or enhances barr ier function of human enterocytes, in part by countering the effect of a T cell cytokine.