Circulating Myeloid-derived Suppressor Cells Facilitate Invasion of Thyroid Cancer Cells by Repressing miR-486-3p

Background Myeloid-derived suppressor cells (MDSCs) have become increasingly recognized as facilitators of tumor development. However, the role of MDSCs in papillary thyroid carcinoma (PTC) progression has not been clearly explored. Objective We aimed to evaluate the levels and function of circulating MDSCs in PTC. Methods The proportion of circulating polymorphonuclear (PMN)-MDSCs and mononuclear-MDSCs from patients with PTC or benign thyroid nodules and healthy controls was measured using flow cytometry. For immunosuppressive activity analysis, sorted circulating MDSCs were cocultured with CD3/CD28-costimulated T lymphocytes and the proliferation of T cells was determined. PTC cell lines (TPC-1 and BC-PAP) were cocultured with PMN-MDSCs, and the effects on cell migration, invasion, proliferation, and apoptosis were evaluated. The differential expressed microribonucleic acids (RNAs) and messenger RNAs and their function were also explored in TPC-1 cells cocultured with or without PMN-MDSCs. Results PMN-MDSCs were increased in peripheral blood mononuclear cells of patients with PTC. Circulating PMN-MDSCs displayed strong T cell suppressive activity. PTC cells demonstrated enhanced invasive capabilities in vitro and in vivo when cocultured with sorted PMN-MDSCs. PMN-MDSCs decreased expression of miR-486-3p and activated nuclear factor kappa B2 (NF-κB2), a direct target of miR-486-3p. Rescue of miR-486-3p diminished the cell migration and invasion induced by PMN-MDSCs. Conclusion Collectively, our work indicates that circulating PMN-MDSCs promote PTC progression. By suppressing miR-486-3p, PMN-MDSCs promote the activity of the NF-κB2 signaling pathway, resulting in accelerated invasion of PTC cells, which may provide new therapeutic strategies for treatment of thyroid cancer.