Myeloid derived suppressor cells (MDSC) play an important role in the regulation of immune responses by suppressing the function of antigen presenting cells and T cells. In humans several populations of MDSC with different phenotypes were characterized due to their distinct immunosuppressive function. However, the origin and development of these cells is not well understood. We observed that incubation of peripheral blood monocytes with IL-10 during their differentiation to dendritic cells (DC) in the presence of GM-CSF and IL-4 results in the generation of an APC population with dramatically reduced stimulatory capacity and a CD14+ and HLA-DR low phenotype (IL-10-APC) similar to the recently described human MDSC subpopulation. In coincubation experiments we found that the addition of these cells to immature or LPS activated DC generated from peripheral blood monocytes resulted in a cell number dependent inhibition of their stimulatory capacity in the mixed lymphocyte reaction assay. Furthermore, these IL-10-APCs reduced the expression of CD1a and costimulatory molecules on DC as well as their activation by LPS characterized by diminished expression of maturation markers including CD83, CD80, CD86 or CD40. IL-10-APC almost completely abolished the secretion of cytokines and chemokines by mature and immature DC involved in T cell stimulation and migration such as TNF-a, IL-6, MIP-1a or Rantes. These effects were not due to induction of IL-10 production and were not observed when purified CD14+ monocytes were used as a control in the experiments. In order to analyze the possible mechanisms and molecules involved in these inhibitory effects we found that IL-10-APC expressed higher levels of PD-1, GITR, GITRL and osteoactivin, an immunosuppressive molecule that was shown to inhibit the function of T cells. The effects mediated by these molecules were further confirmed by utilizing blocking antibodies. Interestingly, addition of IL-10-APC to mature or immature DC induced an increased expression of osteoactivin and its corresponding receptor syndecan 4 on DC thus demonstrating that osteoactivin mediates its effects by upregulating its own receptor. In the next set of experiments we isolated the CD14+ HLA-DR low cell population from buffy coats of healthy donors. We found, that these cells similar to the IL-10-APCs express high levels of osteoactivin and syndecan-4 that can be further upregulated by the addition of IL-10.
Our results demonstrate that osteoactivin mediates its inhibitory effects by induction of its cognate receptor syndecan 4. In addition, cells with the phenotype and function of MDSC can differentiate from human peripheral blood monocytes in the presence of GM-CSF, IL-4 and IL-10. This immunosuppressive cell population can easily be generated in vitro and might be applied for the treatment of patients with GVHD or autoimmune disorders.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.