Plasmacytoid dendritic cells (pDCs) are an immune subset specialized in the production of Type I Interferons (IFNs). Conventional dendritic cells (cDCs) originate mostly from a common dendritic cell progenitor (CDP), whereas pDCs have been shown to develop from both CDPs as well as common lymphoid progenitors (CLPs). In contrast to the current literature, we here show that pDCs mostly differentiate from an IL-7R expressing lymphoid progenitor. IL-7R+ progenitors can be subdivided into three distinct subsets based on the expression of SiglecH and Ly6D: double negative (DN), Ly6D+ single positive (SP) and double positive (DP) progenitors. Each of these subsets identifies a specific developmental stage along the pDC lineage, where commitment by IL-7R+ progenitors is achieved upon expression of Ly6D and SiglecH (DP pre-pDCs). Further, RNA sequencing analysis of IL-7R+ lymphoid progenitor subsets revealed the transcriptional landscape of pDC development along the lymphoid branch, where high expression of the transcription factor IRF8 marks pDC commitment and anticipates the increase of TCF4 levels. The transcriptional signature of DP pre-pDCs correlates with the lineage potential assessed in vitro, in which DP pre-pDCs are fully committed to the pDC lineage. Moreover, single cell RNA sequencing on bone marrow and splenic pDCs revealed pDC heterogeneity in both tissues and further supported the dual origin of pDC from myeloid and lymphoid precursors. While all pDCs have the potential to secrete Type I IFNs and have high expression levels of pDC-specific transcript, only myeloid-derived pDCs share with cDCs the capacity to process and present antigen, suggesting that functional specification is directly linked to developmental origin
