Hemopoietic precursors and development of dendritic cell populations.

The antigen presenting dendritic cells (DCs) are bone marrow (BM) derived cells. Despite their common functions of antigen-processing and T-lymphocyte activation, DCs are diverse in surface markers, migratory patterns and cytokine output. These differences can determine the fate of the T cells they activate. Several subsets of mature DCs have been described in both mouse and human, but tracing the origin of these specialised DC subsets has not been a trivial task. The original concept that all DCs were of myeloid origin was questioned by several recent studies, which demonstrated that in addition to the DCs derived from conventional myeloid precursors, some DCs could also be efficiently generated from lymphoid-restricted precursors. Moreover, it has been shown that both myeloid-restricted and lymphoid-restricted precursors were able to generate DC subsets with similar surface phenotype. These observations demonstrate the existence of both myeloid- and lymphoid-derived DC lineages and suggest an early developmental flexibility of DC precursors. The downstream points where the DC sub-lineages branch off from the conventional myeloid and lymphoid precursors, and the cytokines and environmental factors required for inducing their specialised functions are yet to be determined.