Effect of Advanced Oxidation Protein Products (AOPPs) and aging on the osteoclast differentiation of Myeloid-Derived Suppressor Cells (MDSCs) and its preliminary mechanism

To verify the osteoclast differentiation ability of MDSCs from mice of different ages and explore the effect of AOPPs on the osteoclast differentiation of bone marrow MDSCs. Bone marrow cells from C57BL/6 (a.k.a C57) mice of different ages were subjected to flow cytometry, and CD11bLy6CLy6G MDSCs were sorted out. After induction of osteoclast differentiation, these cells were subjected to tartrate-resistant acid phosphatase (TRAP) and F-actin. MDSCs from bone marrows of old mice were injected into the tibial medullary cavity of young mice. One week later, the bone marrows were subjected to histological examination, TRAP, and cell count. MDSCs from bone marrows of old mice were sorted for induction of osteoclast differentiation, intervened with reactive oxygen species (ROS) scavenger, inducible nitric oxide synthase (iNOS) inhibitor, and nitric oxide (NO) scavenger, and then subjected to TRAP. 8-weeks-old C57 mice were injected with the same concentrations of either AOPPs or mouse serum albumin (MSA). Four weeks later, MDSCs from bone marrows were sorted and subjected to induction of osteoclast differentiation, followed by IHC staining and TRAP. MDSCs of 8-weeks-old C57 mice were extracted and subjected to in vitro induction of osteoclast differentiation with different concentrations of AOPPs, followed by TRAP training. The number of MDSCs in the bone marrows of old mice was significantly higher than that in young mice. MDSCs from bone marrows of old mice differentiated into large multinucleated TRAP osteoclasts, which were significantly different from those in the middle-aged and young mice in terms of cell quantity and morphology. The actin rings formed in the differentiated osteoclasts from MDSCs of bone marrows were densely distributed in the whole field of view, which were significantly denser than those in the middle-aged and young mice. After injection of MDSCs of old mice, the number of TRAP  osteoclasts in the tibial medullary cavity of young mice was significantly increased. NO inhibitor can significantly inhibit the osteoclast differentiation capacity of MDSCs from bone marrows of old mice. In vivo treatment with AOPPs significantly increased the proportion of MDSCs in the bone marrow, which is up to 55.2%. After injection of AOPPs in 8-week-old mice and induction of osteoclast differentiation from the MDSCs, the ratios of CD11b and Gr1 cells were significantly higher than that in the control and MSA groups but was not significantly different from that in the 15-month-old mice. Upon in vitro treatment with different concentrations of AOPPs, the MDSCs did not show any sign of osteoclast differentiation. MDSCs can directly undergo osteoclast differentiation, the capacity of which is stronger in MDSCs of bone marrows of old mice; the NO pathway is a potential mechanism underlying this phenomenon. In vivo but not in vitro AOPPs treatment can induce osteoclast differentiation of MDSCs, indicating there might be other factors in the body that can interact with AOPPs to induce osteoclast differentiation of MDSCs.