The steady-state turnover of murine epidermal Langerhans cells

We have investigated the steady-state turnover of murine epidermal Langerha ns cells (LCs) using an X-irradiation model, H-3-thymidine autoradiography and cultured epidermal sheet explants, and by assessing the LC population i n normal mice, The LC density after whole-body irradiation without any cuta neous shielding was not significantly different from that in skin shielded during whole-body irradiation (P > 0.05), indicating that the additional ir radiation to the skin did not contribute to a decrease in LC density. In bo th instances, the LC number gradually decreased in a linear fashion. The re sults indicate that epidermal LCs continuously leave the epidermis and are continually replaced by circulating precursor cells from the bone marrow at a steady rate. Autoradiographic studies after a pulse injection of H-3-thy midine showed a labelling index of 0.013%, indicating that local mitosis is not an important contributor to the maintenance of the epidermal LC popula tion. Although local X-irradiation resulted in temporary reduction of LC de nsity, epidermal sheet explant culture obtained immediately after local X-i rradiation showed no difference in LC density as compared with control unir radiated skin, indicating that the decrease in LC density was not due to si gnificant LC destruction, From these data, we calculated that the half-life of murine LCs in the epidermis is approximately 9 days.