Osteoactivin Mediates the Inhibitory Effects of Myeloid Derived Suppressor Cells

Myeloid derived suppressor cells (MDSC) play an important role in the regulation of immune responses by suppressing the function of antigen presenting cells (APC) and T cells. However, the origin and development of these cells in humans is not well understood. We observed that incubation of peripheral blood monocytes with IL-10 during their differentiation to dendritic cells (DC) 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 resulted in an inhibition of DC induced T cell proliferation in a mixed lymphocyte reaction. 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. 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. 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 the next set of experiments we isolated MDSC with the CD14+ HLA-DR low cell phenotype from buffy coats of healthy donors. We found, that these cells similar to the IL-10-APCs express high levels of osteoactivin that can be further upregulated by the addition of IL-10.

In order to analyze the possible mechanisms and molecules involved in these inhibitory effects we found that IL-10-APC and isolated MDSC expressed higher levels of PD-1, PD-L1, GITR, GITRL and osteoactivin, an immunosuppressive molecule that was shown to inhibit the function of T cells. Inhibition of osteoactivin on MDSC by utilizing blocking antibodies restored the function of DC in co-incubation experiments. Recently, it was shown that ß-glucans can decrease the suppressive function of MDSCs via Dectin-1. In our experiments, activation of MDSC with Dectin-1 ligand curdlan reduced the expression of osteoactivin and PD-L1 thus explaining how ß-glucans reduce the inhibitory effects of MDSC. In contrast, incubation of MDSC with TLR 2,3,4 and 7/8 ligands upregultaed osteoactivin, PD-1 and PD-L1 on the cell surface. Our results demonstrate that osteoactivin is a key molecule that mediates the inhibitory effects of MDSC on DC function.