Mobilization of Monocytic Myeloid-Derived Suppressor Cells is Regulated by PTH1R Activation in Bone Marrow Stromal Cells

Abstract Myeloid-derived suppressor cells (MDSC) originate in the bone marrow and suppress anti-tumoral T cells in the tumor microenvironment (TME). Contrary to the critical functions and regulation in the TME, mechanism of MDSC mobilization from the bone marrow remains unclear. We have previously reported that tumor-derived parathyroid hormone-related peptide (PTHrP) levels correlate with MDSC. Because PTHrP is a potent osteoblast-stimulating factor, we hypothesized that PTHrP regulates MDSC in the bone marrow via osteoblasts. Continuous infusion of PTHrP (amino acids 1–34, a receptor-binding fragment) in mice increased monocytic subtype MDSC (M-MDSC), but not granulocytic MDSC (G-MDSC) in the circulation. M-MDSC was mobilized by PTHrP(1–34) or PTH(1–34), both functional ligands of PTH1R, but not by hematopoietic mobilizing agents, AMD-3100 or GM-CSF, suggesting that activation of osteoblasts is essential to M-MDSC mobilization. Subsequently, our in vitro cell binding assays demonstrated that α4β1 integrin and vascular cell adhesion molecule (VCAM)-1, expressed on M-MDSC and osteoblasts, respectively, are key to M-MDSC binding to osteoblasts. Neutralizing antibodies to VCAM-1 or β1 integrin suppressed M-MDSC binding to osteoblasts. PTH1R activation increased VEGF-A and interleukin-6 expression in osteoblast, leading to activation of Src family kinase in M-MDSCs. Indeed, Src inhibitors suppressed PTHrP-induced MDSC release in vitro and in vivo. Furthermore, activation of Src in M-MDSC increased expression of two proteases, ADAM-17 and MMP7, leading to VCAM-1 shedding and subsequent disruption of M-MDSC tethering to osteoblasts. Collectively, tumor-derived PTHrP stimulates osteoblasts via PTH1R, leading to cytokine expression in osteoblasts and Src-dependent expression of ADAM-17 and MMP-7, leading to M-MDSC mobilization.