slan/M-DC8+ cells constitute a distinct subset of dendritic cells in human tonsils

Human blood dendritic cells (DCs) include three main distinct subsets, namely the CD1c(+) and CD141(+) myeloid DCs (mDCs) and the CD303(+) plasmacytoid DCs (pDCs). More recently, a population of slan/M-DC8(+) cells, also known as slanDCs, has been described in blood and detected even in inflamed secondary lymphoid organs and non-lymphoid tissues. Nevertheless, hallmarks of slan/M-DC8(+) cells in tissues are poorly defined. Herein, we report a detailed characterization of the phenotype and function of slan/M-DC8(+) cells present in human tonsils. We found that tonsil slan/M-DC8(+) cells represent a unique DC cell population, distinct from their circulating counterpart and also from all other tonsil DC and monocyte/macrophage subsets. Phenotypically, slan/M-DC8(+) cells in tonsils display a CD11c(+)HLA-DR(+)CD14(+)CD11b(dim/neg)CD16(dim/neg)CX3CR1(dim/neg) marker repertoire, while functionally they exhibit an efficient antigen presentation capacity and a constitutive secretion of TNFa. Notably, such DC phenotype and functions are substantially reproduced by culturing blood slan/M-DC8(+) cells in tonsil-derived conditioned medium (TDCM), further supporting the hypothesis of a full DC-like differentiation program occurring within the tonsil microenvironment. Taken together, our data suggest that blood slan/M-DC8(+) cells are immediate precursors of a previously unrecognized competent DC subset in tonsils, and pave the way for further characterization of slan/M-DC8(+) cells in other tissues.