Epithelial tissues of various organs contain immature Langerhans cell (LC)-type dendritic cells, which play key roles in immunity. LCs reside for long time periods at an immature stage in epithelia before migrating to T-cell-rich areas of regional lymph nodes to become mature interdigitating dendritic cells (DCs). LCs express the epithelial adhesion molecule E-cadherin and undergo homophilic E-cadherin adhesion with surrounding epithelial cells. Using a defined serum-free differentiation model of human CD34(+) hematopoietic progenitor cells, it was demonstrated that LCs generated in vitro in the presence of transforming growth factor beta1 (TGF-beta1) express high levels of E-cadherin and form large homotypic cell clusters. Homotypic LC clustering can be inhibited by the addition of anti-E- cadherin monoclonal antibodies (mAbs). Loss of E-cadherin adhesion of LCs by mechanical cluster disaggregation correlates with the rapid up-regulation of CD86, neo-expression of CD83, and diminished CD1a cell surface expression by LCs-specific phenotypic features of mature DCs. Antibody ligation of E-cadherin on the surfaces of immature LCs after mechanical cluster disruption strongly reduces the percentages of mature DCs. The addition of mAbs to the adhesion molecules LFA-1 or CD31 to parallel cultures similarly inhibits homotypic LC cluster formation, but, in contrast to anti-E-cadherin, these mAbs fail to inhibit DC maturation. Thus, E-cadherin engagement on immature LCs specifically inhibits the acquisition of mature DC features. E-cadherin-mediated LC maturation suppression may represent a constitutive active epithelial mechanism that prevents the uncontrolled maturation of immature LCs. (Blood. 2000;96:4276-4284)