B cell-specific protein FCRLA is expressed by plasmacytoid dendritic cells in humans

Background: Fc receptor-like A (FCRLA) is a unique member of a family of Fc receptor like-molecules that lacks a transmembrane region and is an ER-resident protein. In mice and humans, FCRLA has been known as a B cell specific protein. We report here that, in humans, FCRLA is also expressed in a subpopulation of plasmacytoid dendritic cells (pDCs). Methods: Human peripheral blood mononuclear cells (PBMC), splenocytes, and tonsillar cells were stained for lineage markers followed by fixation/saponin permeabilization and intracellular staining for FCRLA, and then analyzed by flow cytometry with CD123 and CD303 used as pDC markers. Results: We conducted an extensive flow cytometric analysis of a rare population of CD19-FCRLA+ cells found for the first time in human lymphoid tissues that we assigned to pDCs as they were lin-/CD123+/CD303+. FCRLA expression in human pDCs was further confirmed by the RT-PCR analysis of cDNA of pDCs isolated from the peripheral blood of a healthy donor. FCRLA-positive pDCs expressed a lower level of HLA-DR than their FCRLA-negative counterparts. Conclusions: FCRLA has long been viewed as a B cell specific protein, and this is the first time its expression has also been shown in human pDCs.