Generation and functional characterization of MDSC-like cells

Myeloid derived suppressor cells (MDSC) are critical in regulating immune responses by suppressing antigen presenting cells (APC) and T cells. We previously observed that incubation of peripheral blood monocytes with IL-10 during their differentiation to dendritic cells (moDC) results in the generation of an APC population with a CD14+HLA-DRlow phenotype (IL-10-APC) with reduced stimulatory capacity similar to human MDSC. Here, we show that co-incubation of IL-10-APC and moDC caused a reduction of DC-induced T cell proliferation, of the expression of maturation markers and of secreted cytokines and chemokines. Furthermore, addition of IL-10-APC increased the immunosuppressive molecule osteoactivin and its corresponding receptor syndecan-4 on moDC. Using transcriptome analysis, we could identify a set of molecules and pathways mediating the immunosuppressive effects of IL-10-APCs. In addition, we found that IL-10-APC as well as human isolated MDSC, expressed higher levels of programmed death (PD)-1, PD-ligand-1, glucocorticoid-induced-tumor-necrosis-factor-receptor-related-protein (GITR) and GITR-ligand. Inhibition of osteoactivin, syndecan-4, PD-1 or PD-L1 on MDSC using blocking antibodies restored the stimulatory capacity of DC in co-incubation experiments. Our results demonstrate that osteoactivin/syndecan-4 and PD-/PD-L1 are key molecules that are profoundly involved in the inhibitory effects of MDSC on DC function and might be promising tools for an application in the clinics. Isolated human monocytes from peripheral blood were differentiated to monocyte-derived dendritic cells (moDC) in presence or absence of interleukin-10 (IL-10). Additional cell maturation by application of LPS leads to a total number of 4 conditions with 3 samples each.