Langerhans cells (LC)' are leukocytes that are distributed in the suprabasal region of squamous epithelia, particularly skin. LC undergo marked changes when epidermal cell suspensions are prepared by a standard trypsinization protocol and placed in culture for 1-3 d. Several cell surface markers (Fc receptors, the F4/80 antigen, membrane ATPase, and nonspecific esterase) decrease substantially (1, 2), while others (class I and II products of the MHC) increase (3). When accessory function for primary T-dependent immune responses is quantitated, the activity of the LC population increases some 10-30-fold (1, 4). Therefore LC seem to mature immunologically in bulk epidermal culture and acquire most of the features of lymphoid dendritic cells.Because LC maturation leads to the development of accessory function for primary immune responses, this process may contribute to the sensitization phase of cell-mediated immunity . Here, we asked whether maturation is autonomous, or ifexogenous cells and factors are required . We developed a panning technique to enrich the trace LC population from freshly dissociated mouse epidermis. When this was done, the viability and function of cultured LC was dependent upon factors that were found in the medium of keratinocyte cultures, as well as stimulated macrophages and T cells . We will describe these experiments and the identification of granulocyte/macrophage colony-stimulating factor (GM-CSF) as the principal if not exclusive mediator for the production of functioning LC.
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