Characterization of CD56+ dendritic-like cells: a normal counterpart of blastic plasmacytoid dendritic cell neoplasm?

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare hematological malignancy. Plasmacytoid DCs (pDCs), which are defined as lineage marker (Lin)(-)HLA-DR(+)CD56(-)CD123(+)CD11c(-) cells, are considered to be the normal counterpart of BPDCNs. However, BPDCN can be distinguished from pDCs by uniform expression of CD56. In this study, to identify a normal counterpart of BPDCN, we searched for a Lin(-)HLA-DR(+)CD56(+) population and focused on a minor subpopulation of Lin(-)DR(+)CD56(+)CD123(+)CD11c(-) cells that we designated as pDC-like cells (pDLCs). pDLC constituted 0.03% of peripheral blood mononuclear cells (PBMCs), and the pDLC/pDC ratio was higher in bone marrow cells than in PBMCs. pDLC clearly expressed BDCA2, BDCA4, and myeloid antigens, which are frequently expressed by BPDCN. pDLCs exhibited modest expression of Toll-like receptors and produced less interferon-α after CpG stimulation, but presented very low endocytic ability unlike mDCs. These functional differences were attributed to the expression profile of transcriptional factors. After in vitro culture with Flt3-ligand and GM-CSF, pDLCs expressed CD11c and BDCA1. These data suggested that pDLCs are a distinct subpopulation, with an immunophenotype similar to BPDCNs. Moreover, our results indicate that pDLCs might be immature DCs and might contribute to the immunophenotypical diversity of BPDCNs.