Key Points• One cell type can develop from multiple pathways.• Cells that have developed from different routes perform similar functions, but can be told apart by the molecules they once expressed. IntroductionPlasmacytoid dendritic cells (pDCs) are a subset of DCs that circulate through the blood and peripheral tissues. 1,2 After activation, pDCs develop dendritic processes, up-regulate expression of major histocompatibility (MHC) class II molecules, and become APCs. Further, on activation, they secrete type 1 IFN and are therefore also known as IFN-producing cells. Despite their classification as DCs, pDCs have many of the attributes of B cells, including the expression of several surface receptors, use of similar antigen presentation machinery, and similar morphology in the unstimulated state. [3][4][5] The developmental origin of pDCs has long been unclear. Early studies showed that both common myeloid progenitors (CMPs) and common lymphoid progenitors (CLPs) had the potential to produce pDCs after transfer into irradiated mice, 6,7 but it was unclear whether these potentials were expressed in steady state. Similar potential has been found in CLPs and CMPs from human cord blood. 8 Later it was shown that some pDCs in the BM and spleen of unmanipulated mice expressed recombination activating gene 1 (RAG1) 9 and possessed D-J rearranged genes at the IgH locus. 9,10 Such gene rearrangement events have been considered as indicators of a lymphoid developmental history. A recent study described a cell-intrinsic requirement for IL-7 signaling in the development of a subset of both splenic cDCs and pDCs, and concluded that some DCs were of lymphoid origin. 11 These results suggested that the pDCs found in normal mice had 2 different origins: myeloid and lymphoid progenitors.Conversely, in a subsequent study, IgH gene rearrangements were found in pDCs derived after transfer from CMPs and CLPs. 12 The investigators concluded that the IgH gene rearrangements were not necessarily markers of a lymphoid developmental history, but rather an accidental by-product of the similarity in transcriptional programs between B cells and pDCs. In addition, a precursor restricted to production of pDCs and conventional DCs (cDCs), termed a common dendritic cell precursor (CDP) or pro-DC, has been isolated from BM. 13,14 The CDP is downstream of the CMPs 13 and so is considered a myeloid lineage cell. As a result of these findings, only the myeloid origin of pDCs tends to be considered at present.To investigate in detail the pathway of pDC differentiation, in the present study, we used a BM culture system driven by fms-like tyrosine kinase 3 ligand (FL). [15][16][17] We have previously demonstrated that this system models the pathway of steady-state spleen DC development, including production of pDCs and cDCs from a CDP intermediate. 14 We now combine RAG1 gene expression and IgH gene rearrangement analysis with the isolation of distinct intermediate precursors to demonstrate the existence of separate myeloid and lymphoid pathways, bot...