LCs are the first example of a dendritic cell type that is maintained locally. Merad and colleagues examined LCs in mice that had received a bone marrow transplant. Although LCs in the bloodstream were replaced by cells from the donor, LCs in the skin remained of host origin. “Langerhans were thought to be constantly reproduced by the bone marrow,” says Merad. “But LCs in skin are replacing themselves with a local store of precursors.” Thus, precursor pools in the skin are probably set during embryogenesis.
Despite the local source, LCs were also recruited from the blood, but only when the need was great. Both minor and severe injuries depleted LCs in the skin, but bone marrow–derived LC precursors were recruited from the blood only after severe injuries, such as exposure to UV light. Recruitment required the chemokine receptor CCR2 and secretion of its ligands by the injured skin.
The results fuel the argument that bone marrow–derived stem cells may provide a source of repair cells that can be rapidly mobilized after injury. Muscle mesenchymal cells and microglia brain cells likewise maintain themselves via local stores and might recruit new stem cells from the blood only upon injury or inflammation, perhaps to minimize outside influences on sensitive tissues. ▪