Hip Arthritis Reshapes the Bone Marrow, but Its Immune Brakes Keep Working

Osteoarthritis has long been dismissed as simple wear and tear, a mechanical consequence of decades of load-bearing on aging joints. But a growing body of research is dismantling that view, and a new study published in GeroScience adds a striking dimension to the rethink: the disease appears to leave its fingerprint not in the joint alone, but deep inside the bone marrow itself. Researchers in Padua, Italy, found that elderly patients undergoing hip replacement for osteoarthritis carried a significantly altered immune landscape in their proximal femoral bone marrow compared with patients of similar advanced age operated on for accidental femoral fractures. The finding positions the bone marrow as an active immunological compartment in osteoarthritis, one that may help sustain the chronic, low-grade inflammation that drives joint degeneration.

The team, led by immunologist Susanna Mandruzzato of the Veneto Institute of Oncology and the University of Padua, focused on a family of cells known as myeloid-derived suppressor cells, or MDSCs. These immature cells of the myeloid lineage, which includes precursors of granulocytes and monocytes, are best known from cancer research, where they accumulate in tumors and suppress the T cell responses that would otherwise attack malignant tissue. But MDSCs are not exclusive to cancer. They expand during infection, sepsis, and chronic inflammatory diseases, and recent work has implicated them in bone pathologies ranging from osteoporosis to inflammatory arthritis, where they can even act as dysfunctional osteoclast progenitors that contribute directly to bone destruction.

What made the question urgent for osteoarthritis is the disease’s peculiar inflammatory profile. Hip osteoarthritis is not an autoimmune condition like rheumatoid arthritis, yet it features a persistent, smoldering inflammation marked by elevated levels of cytokines such as IL-1β, TNF-α, and IL-6 in the joint environment. This inflammatory milieu promotes synovial activation, disrupts subchondral bone remodeling, enhances osteoclast formation, and accelerates cartilage breakdown through catabolic enzymes like metalloproteinases. In parallel, aging itself drives a phenomenon called inflammaging, a chronic low-grade inflammatory state that biases hematopoietic stem cells in the bone marrow toward producing myeloid cells at the expense of lymphoid lineages. The Padua team asked whether these two forces, aging and disease-associated inflammation, converge in the bone marrow of osteoarthritis patients.

To find out, the researchers collected fresh bone marrow aspirates from 13 patients undergoing total hip arthroplasty for osteoarthritis and 13 patients undergoing surgery for proximal femoral fracture, all recruited at the University-Hospital of Padova. The samples were drawn from the medullary canal of the femur during the earliest stage of surgical preparation, before any broaching of the canal, ensuring that the cellular composition reflected the living marrow environment. Patients with active malignancies, autoimmune diseases, infections, or hematological disorders were excluded. The two cohorts were not identical: the fracture patients were, on average, older than the osteoarthritis group, a difference the authors acknowledge as a potential confounder, but it makes the direction of their key finding all the more remarkable.

Using multiparametric flow cytometry, the team quantified the myeloid and lymphoid compartments in each sample. Despite being younger on average, the osteoarthritis patients showed a significantly higher proportion of myeloid cells in their bone marrow than the fracture patients, a difference that reached statistical significance at p = 0.0036. Lymphoid cell frequencies, by contrast, did not differ between the groups. This selective myeloid enrichment could not be explained by advanced age alone, suggesting that the chronic inflammation associated with hip osteoarthritis actively amplifies the age-related skewing of hematopoiesis toward the myeloid lineage. In other words, the disease may be superimposing its own inflammatory signature on top of the normal immunological remodeling of aging.

The researchers then drilled down into the maturation stages of these myeloid cells, using the surface markers CD11b and CD16 to distinguish granulocytic precursors at different points along their developmental path. Here a specific subset stood out. Cells with a CD11b-positive, CD16-low phenotype, which correspond to metamyelocytes, an intermediate stage of granulocyte maturation, were significantly enriched in the osteoarthritis samples, both when analyzed directly ex vivo and after the cells were cultured for four days with the growth factors G-CSF and GM-CSF to drive myeloid expansion. After cytokine stimulation, this metamyelocyte-like fraction reached a median of 31.5 percent in the osteoarthritis cultures versus 20.3 percent in the fracture cultures, a persistent difference indicating that the osteoarthritic marrow environment favors the production of these intermediate myeloid cells even under standardized laboratory conditions.

The functional heart of the study, however, lay in the suppressive capacity of these cells. In earlier work, the same group had established that the immunosuppressive power of bone marrow-derived MDSCs resides almost entirely in the most immature fraction, a CD11b-negative or low, CD16-negative population with promyelocyte-like features. In the new experiments, the researchers co-cultured these myeloid fractions with T cells from healthy donors that had been labeled with a fluorescent proliferation dye and activated with anti-CD3 and anti-CD28 antibodies. The result was unambiguous: only the immature fraction significantly inhibited T cell proliferation, in both osteoarthritis and fracture samples, while the more differentiated CD11b-positive fractions and unsorted bulk cultures had little or no effect. Suppression was accompanied by a marked downregulation of CD3 expression on the T cell surface, a well-established indicator of impaired T cell receptor signaling.

Crucially, the degree of suppression did not differ between the two patient groups. Even though the osteoarthritic marrow was more myeloid-skewed and richer in intermediate metamyelocyte-like cells, the core immunoregulatory machinery of the immature MDSC fraction remained intact and functionally equivalent to that of the fracture donors. This extends the team’s earlier observation that MDSC suppressive function is preserved during aging, from comparisons between pediatric and elderly donors, into the specific context of chronic osteoarthritis-associated inflammation. Morphological analysis by May-Grünwald-Giemsa staining of sorted populations confirmed the phenotypic story: the suppressive CD11b-low fraction showed uniform promyelocyte-like morphology with prominent nucleoli, while the more mature fractions contained myelocytes, metamyelocytes, band cells, and even macrophage-like cells with cytoplasmic vacuolation.

The authors are careful to frame these findings as an association rather than proof of a disease-specific mechanism. The fracture cohort, they note, is not a healthy control group; femoral fracture triggers its own acute, time-limited inflammatory response, and no ethically defensible route exists for harvesting bone marrow from truly healthy elderly volunteers. Differences in age and sex distribution between the cohorts, along with the modest sample size, particularly for the correlation analyses, further limit statistical power. The study also did not assess whether the osteoarthritic MDSCs can differentiate into osteoclasts, a question of obvious importance given evidence from breast cancer and inflammatory arthritis models that MDSCs can drive pathological bone resorption while simultaneously suppressing T cells. Whether the same dual function operates in hip osteoarthritis remains an open question for future work.

Even with those caveats, the study reframes osteoarthritis as a systemic immunological condition rather than a purely local joint disease. If the bone marrow of osteoarthritis patients is chronically pumping out expanded myeloid populations, those cells could feed the synovial inflammation, cartilage degradation, and aberrant subchondral bone remodeling that characterize disease progression, while their preserved suppressive activity simultaneously holds immune activation in check. The authors suggest this balance between heightened myelopoiesis and conserved immunoregulation may define the distinctive low-grade, persistent inflammatory state of osteoarthritis, setting it apart from overt autoimmune arthritis. It also raises a therapeutic possibility worth taking seriously: modulating myeloid responses, rather than broadly suppressing the immune system, might one day offer a way to slow joint degeneration in the elderly without compromising host defense. For a disease affecting hundreds of millions of people worldwide and currently treatable only with pain management and, ultimately, joint replacement, that would be a genuinely transformative shift in perspective.

Subject of Research: Bone marrow myeloid cell remodeling and myeloid-derived suppressor cell function in elderly patients with hip osteoarthritis

Article Title: Bone marrow myeloid skewing and preserved immunoregulation in hip osteoarthritis of elderly patients

Article References: Tushe, A., Binatti, E., Belluzzi, E., Pozzuoli, A., Francescato, S., Buldini, B., Biz, C., Ruggieri, P., & Mandruzzato, S. (2026). Bone marrow myeloid skewing and preserved immunoregulation in hip osteoarthritis of elderly patients. GeroScience. https://doi.org/10.1007/s11357-026-02578-0

Image Credits: AI Generated

DOI: 10.1007/s11357-026-02578-0

Keywords: hip osteoarthritis, bone marrow, myeloid-derived suppressor cells, inflammaging, myelopoiesis, immunosenescence, T cell suppression, osteoimmunology, flow cytometry, granulopoiesis, chronic inflammation, GeroScience