Langerhans cells are bone marrow–derived epidermal dendritic cells that represent the principal hematopoietic barrier to the external environment, but little has been known about their life cycle. In November 4 Nature Immunology, Miriam Merad and colleagues at Stanford University School of Medicine, Stanford, USA show that LCs renew in the skin throughout life under steady-state conditions and are replaced by blood-borne progenitors only after severe inflammatory changes (Nature Immunology, doi:10.1038/ni852, November 4, 2002).

Merad et al. analyzed the recruitment of circulating LC precursors to the skin of congenic parabiotic mice and lethally irradiated mice transplanted with congenic bone marrow. They observed that under steady state conditions and following injuries that were only weakly inflammatory, bone marrow precursors were not recruited to the skin and LCs were maintained by a stable renewable population present in the skin. But after UV light–induced skin inflammation, blood LC precursors expressing...

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