Obesity alters bone health not only through weight gain but also through remodeling of the bone marrow environment. Researchers have shown that increased bone marrow fat promotes immunosuppressive PD-L1 signaling, which promotes osteoclast formation and promotes bone loss. Reducing bone marrow fat in mice reduced immunosuppression and improved bone structure. These findings reveal new mechanisms linking metabolic, immune, and skeletal health and provide potential therapeutic targets for obesity-related bone diseases.
Bone health is traditionally thought to benefit from increased body weight, and increased mechanical loading is thought to strengthen bones. However, recent research questions this concept and shows that obesity can negatively impact skeletal integrity. One important element that has received attention is bone marrow adipose tissue. Bone marrow adipose tissue is a specialized fat depot within bone that plays an active role in metabolic and immune regulation. Despite its importance, how this fat contributes to bone loss in obesity remains unclear.
To address this challenge, a research team led by senior scientist Clifford J. Rosen, Ph.D., and investigator Sergey Ryzov, Ph.D., both of the Maine Health Research Center for Molecular Medicine in Scarborough, Maine, investigated how bone marrow fat influences immune function and bone remodeling in obesity. The research team used a mouse model of diet-induced obesity, a cell co-culture system, and a gene depletion model to examine interactions between bone marrow adipocytes, immune cells, and osteoclast precursor cells. Their findings were published in Volume 14 of the journal on March 20, 2026. Bone research.
Researchers have found that obesity leads to rapid and sustained expansion of bone marrow fat. This expansion changed the molecular profile of adipocytes, leading to increased production of signaling molecules such as: MCP-1recruits and repopulates myeloid immune cells. As a result, there was a significant increase in myeloid cells expressing PD-L1 within the bone marrow. These cells suppressed T cell activity and created an immunosuppressive microenvironment that disrupted the normal immune balance. Importantly, these PD-L1+ cells not only suppress the immune response but also directly influence osteoclast development.
At the same time, this change in immune signaling had a direct impact on bone remodeling. This study revealed that bone marrow cells expressing PD-L1 interact with PD-1 receptors on osteoclast precursor cells, promoting their differentiation into mature osteoclasts. This process significantly increased bone resorption and decreased trabecular and cortical bone volume. Notably, blocking the PD-1/PD-L1 pathway during the early stages of osteoclastogenesis reduced both the number and activity of these bone-resorbing cells, highlighting their important role in osteoclastogenesis. Dr. Rosen explained:We discovered that bone marrow fat is not just a passive tissue, but actively reshapes immune signaling in a way that promotes bone loss in obesity.. ”
To further confirm these findings, the researchers used a genetically modified mouse model lacking bone marrow adipocytes. These mice are MCP-1PD-L1+ immune cells are reduced and osteoclast precursor cells are significantly reduced. Importantly, this improved bone structure and reduced bone resorption even in the obese state. These results indicate that bone marrow fat plays a central role in promoting both immunosuppression and bone degradation.
Dr. Rizov added:This immune checkpoint pathway, known for regulating T-cell responses, also directly promotes osteoclast formation, revealing an entirely new relationship between immunity and skeletal health. ”
Beyond mechanistic insights, this study highlights important implications for human health. In the short term, we propose a new strategy to protect bone health in obese patients by targeting bone marrow adipose or immune checkpoint pathways. It may also provide insight into why obesity is associated with reduced immune responses, including reduced vaccine efficacy and increased risk of infection.
In the long term, these findings may impact treatment approaches across multiple disciplines. As PD-1/PD-L1 inhibitors are already used in cancer treatment, this study suggests the possibility of future exploration of repurposing such therapies to address bone loss and metabolic disorders. It also has the potential to foster collaboration between immunologists, endocrinologists, and bone researchers to explore integrative treatment strategies.
Ultimately, this study redefines the role of bone marrow fat as a key regulator of immune and skeletal health. By revealing how immunosuppression and osteoclast activity are promoted, this study provides the foundation for developing innovative treatments aimed at reducing obesity-related bone loss and improving overall health.
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Reference magazines:
SN, Costa, Others. (2026). Bone marrow adipocyte expansion in obese mice leads to PD-L1-mediated bone marrow immunosuppression and osteoclastogenesis. Bone research. DOI: 10.1038/s41413-026-00509-5. https://www.nature.com/articles/s41413-026-00509-5
