Plasmacytoid monocytes/T cells: a dendritic cell lineage?

Plasmacytoid monocytes/T cells were first described in 1958, yet their origin and function have remained enigmatic. Recently a series of publications brought these cells to the forefront of immunological research. Indeed, plasmacytoid monocytes/T-cells contain natural type-I interferon producing cells and can differentiate in vitro into dendritic cells (DC). It has been proposed that plasmacytoid monocytes/T-cells represent a distinct lineage of cells whose fate it is to differentiate into dendritic cells. Herein we will review recent advances in our understanding of plasmacytoid monocytes/T cells and highlight arguments in favor or against this lineage hypothesis. We propose that plasmacytoid monocytes/T cells represent a composite group of both myeloid and lymphoid early-committed cells that are characterized by their ability to differentiate in vitro into DC.