Migration and differentiation of Langerhans cell precursors.

Epidermal Langerhans cells (LC) are the first sentinels of the skin immune system. To study immigration of human LC precursor cells into the skin, we established a two-compartmental skin model consisting of a dermal matrix and an epidermal sheet of keratinocytes. We tested the individual components of the skin model for their influence on phenotype and function of LC precursors. A time window at day 5/6 of differentiation was determined, during which in vitro generated LC precursors expressed adhesion molecules and chemokine receptors required for transmigration across endothelial cell layers and the dermis towards the epidermis. They expressed L-selectin, integrins, platelet endothelial cell adhesion molecule-1, E-cadherin and CC-chemokine receptor 6 and were thus fitted out for transendothelial migration and immigration into the dermis. In a transwell system, these LC precursors migrated towards the chemokine MIP3alpha, demonstrating functional integrity of chemokine receptor 6. For the in vitro reconstituted skin, keratinocytes were grown on a de-epidermized dermis for one to three weeks and formed an epidermal sheet. We allowed LC precursor cells to migrate into this two-compartmental model from the dermal side and examined the presence of CD1alpha--positive cells. LC precursors migrated through the dermal matrix towards the layer of keratinocytes representing the epidermis and could be identified by immunohistology. Experiments designed to investigate the influence of signals provided by both the skin components and by the LC precursors on LC immigration into the skin are in progress.