Separation of plasmacytoid dendritic cells from B-cell-biased lymphoid progenitor (BLP) and pre-pro B cells using PDCA-1

B-cell-biased lymphoid progenitors (BLPs) and Pre-pro B cells lie at a critical juncture between B cell specification and commitment. However, both of these populations are heterogenous, which hampers investigation into the molecular changes that occur as lymphoid progenitors commit to the B cell lineage. Here, we demonstrate that there are PDCA-1⁺Siglec H⁺ plasmacytoid dendritic cells (pDCs) that co-purify with BLPs and Pre-pro B cells, which express little or no CD11c or Ly6C. Removal of PDCA-1⁺ pDCs separates B cell progenitors that express high levels of a Rag1-GFP reporter from Rag1-GFPow/neg pDCs within the BLP and Pre-pro B populations. Analysis of Flt3-ligand knockout and IL-7Ra knockout mice revealed that there is a block in B cell development at the all-lymphoid progenitor (ALP) stage, as the majority of cells within the BLP or Pre-pro B gates were PDCA-1⁺ pDCs. Thus, removal of PDCA-1⁺ pDCs is critical for analysis of BLP and Pre-pro B cell populations. Analysis of B cell potential within the B220⁺CD19~ fraction demonstrated that AA4.1⁺Ly6D⁺PDCA-1~ Pre-pro B cells gave rise to CD19⁺ B cells at high frequency, while PDCA-1⁺ pDCs in this fraction did not. Interestingly, the presence of PDCA-1⁺ pDCs within CLPs may help to explain the conflicting results regarding the origin of these cells.