Epithelial Langerhans cells (LC) represent immature dendritic cells that require TGF-beta 1 stimulation for their development. Little is known about the mechanisms regulating LC generation from their precursor cells. We demonstrate here that LC development from human CD34+ hemopoietic progenitor cells in response to TGF-beta 1 costimulation (basic cytokine combination GM-CSF plus TNF-alpha, stem cell factor, and Flt3 ligand) is associated with pronounced cell cluster formation of developing LC precursor cells. This cell-clustering phenomenon requires hemopoietic progenitor cell differentiation, since it is first seen on day 4 after culture initiation of CD34+ cells. Cell cluster formation morphologically indicates progenitor cell development along the LC pathway, because parallel cultures set up in the absence of exogenous TGF-beta 1 fail to form cell clusters and predominantly give rise to monocyte, but not LC, development (CD1a-, lysozyme+, CD14+). TGF-beta 1 costimulation of CD34+ cells induces neoexpression of the homophilic adhesion molecule E-cadherin in the absence of the E-cadherin heteroligand CD103. Addition of anti-E-cadherin mAb or mAbs to any of the constitutively expressed adhesion molecule (CD99, CD31, LFA-1, or CD18) to TGF-beta 1-supplemented progenitor cell cultures inhibits LC precursor cell cluster formation, and this effect is, with the exception of anti-E-cadherin mAb, associated with inhibition of LC generation. Addition of anti-E-cadherin mAb to the culture allows cell cluster-independent generation of LC from CD34+ cells. Thus, functional E-cadherin expression and homotypic cell cluster formation represent a regular response of LC precursor cells to TGF-beta 1 stimulation, and cytoadhesive interactions may modulate LC differentiation from hemopoietic progenitor cells.