Langerhans cells (LC) are Ia+ leukocytes that account for less than 2% of the cells in murine epidermal isolates. We purified LC by cell sorting to study their capacity to stimulate antigen-specific responses from unprimed and sensitized T cells. Sorting was performed after 12 or 72 h of epidermal culture, since our earlier work had indicated that LC became immunologically active during that time interval. At 12 and 72 h, the LC were uniformly and equally rich in the Ia glycoproteins that are recognized by helper T cells. At both time points, LC were comparable in their capacity to stimulate sensitized helper T lymphocytes, and would cluster the T cells in an antigen-dependent fashion at 4 degrees C. However, 12-h LC did not sensitize T cells, as indicated by their inactivity in stimulating the primary MLR or antibody response, and they were unable to cluster T cells in an antigen-independent fashion at 37 degrees C. The latter properties were acquired during 72 h of culture. As a result, the function of 72-h LC fully resembled that of lymphoid dendritic cells. We propose that the maturation of stimulatory function within the dendritic cell lineage represents an important control point in the induction phase of cell-mediated immunity.

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