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Myeloid-Derived Suppressor Cells at the Intersection of Inflammaging and Bone Fragility

ABSTRACT Age-related alteration of the immune system with aging, or immunosenescence, plays a major role in several age-associated conditions, including loss of bone integrity. Studies over the past several years have clearly established the immune system is chronically activated with advanced aging, termed inflammaging, and is characterized by elevated levels of proinflammatory cytokines in response to physiological or environmental cues that essentially result in an arrested immune system that maintains a low-level state of activation. This age-associated inflammation impacts several biological systems including the innate immune system, where aging results in a skewing of the hematopoiesis toward the myeloid lineage, including the expansion of myeloid-derived suppressor cells (MDSCs). This heterogeneous population of myeloid cells classically displays immunosuppressive capacity but they also have the ability to directly differentiate into osteoclasts. This review explores the possibility of inflammaging to be involved in reduction of bone microarchitecture and loss of bone mass/strength through the expansion of MDSCs and the osteoclastogenic capacity and activity.