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In the present study, we investigated the in vitro migratory activity of human epidermal Langerhans cells (LC). Freshly isolated LC exhibit very low spontaneous migration. In contrast, a strong migration is recorded 6 h after the isolation. This migration is due to the presence of GM-CSF released by surrounding keratinocytes in vitro. Picomolar concentrations of GM-CSF promote the migration of LC, but nanomolar concentrations are inhibitory. Checker-board experiments indicate that GM-CSF acts as a chemokinetic mediator for LC, Bulk cultured LC exhibit a significant decrease of their spontaneous migration but retain the capacity to respond to GM-CSF only at nanomolar concentrations. In contrast, LC cultured in the presence of picomolar concentrations of exogenous GM-CSF exhibit a spontaneous migratory activity comparable to that of 6 h rested LC but do not respond to GM-CSF. These results suggest that GM-CSF represents an essential factor triggering the egress of LC from their epidermal environment.

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