Bone marrow myeloid skewing and preserved immunoregulation in hip osteoarthritis of elderly patients.

Osteoarthritis (OA) is increasingly recognized as a disease involving low-grade inflammation and immune remodeling, yet the contribution of the bone marrow (BM) myeloid compartment to hip OA remains poorly defined. Myeloid-derived suppressor cells (MDSCs) are immature myeloid cells with immunoregulatory activity and potential roles in inflammatory bone disease. Here, we investigated ex vivo BM myeloid composition and the functional properties of BM-MDSCs in elderly patients with hip OA compared with patients undergoing surgery for proximal femoral fracture. BM aspirates from both cohorts were analyzed by flow cytometry and cultured with G-CSF and GM-CSF to induce BM-MDSCs. Despite the lower mean age of the hip osteoarthritis cohort, these patients showed a significantly higher proportion of BM myeloid cells than fracture patients, whereas lymphoid cell frequencies were not significantly different. Analysis of myeloid maturation stages revealed enrichment of CD11b⁺/CD16low metamyelocyte-like cells in hip OA samples both ex vivo and after cytokine-driven expansion. Functional assays demonstrated that inhibition of activated T cell proliferation was confined to the most immature myeloid fraction, which displayed promyelocyte-like features. This suppressive activity was preserved in both hip OA and fracture cohorts, with no significant difference between groups. These findings indicate that hip OA is associated with BM myeloid skewing and selective enrichment of myeloid subsets reassembling metamyelocytes-like cells, while maintaining conserved immunoregulatory function. The BM may represent an active immunological compartment linked to the balance between chronic inflammation and immune regulation in hip OA.