Obesity alters bone health not only through increased body weight but also through remodeling of the bone marrow environment. The researchers show that expansion of bone marrow fat promotes immunosuppressive PD-L1 signaling, which enhances osteoclast formation and accelerates bone loss. By reducing bone marrow fat in mice, they reduced immune suppression and improved bone structure. These findings reveal a novel mechanism linking metabolism, immunity, and skeletal health, offering potential therapeutic targets for obesity-related bone disorders.
Bone health has traditionally been thought to benefit from higher body weight, with increased mechanical loading thought to strengthen bones. However, recent research challenges this idea, showing that obesity can negatively affect skeletal integrity. A key factor gaining attention is bone marrow adipose tissue, a specialized fat depot in bone that plays an active role in metabolic and immune regulation. Despite its importance, how this fat contributes to bone loss in obesity has remained unclear.
To address this challenge, a team of researchers led by Dr. Clifford J. Rosen, MD, Senior Scientist and Dr. Sergey Ryzhov, PhD, Researcher, both at the Center for Molecular Medicine, MaineHealth Institute for Research, Scarborough, ME, USA, investigated how bone marrow fat affects bone function and immune system reset. Using diet-induced obese mouse models, cell co-culture systems and genetic depletion models, the team examined the interactions between bone marrow adipocytes, immune cells and osteoclast precursors. Their findings were published on March 20, 2026, in volume 14 of the journal Bone research.
The researchers found that obesity leads to a rapid and sustained expansion of bone marrow fat. This expansion changed the molecular profile of adipocytes, increasing the production of signaling molecules such as MCP-1which recruits and remodels myeloid immune cells. As a result, there was a marked increase in myeloid cells expressing PD-L1 within the bone marrow. These cells suppressed T-cell activity, creating an immunosuppressive microenvironment that disrupted the normal immune balance. Importantly, these PD-L1+ The cells not only suppressed immune responses but also directly affected osteoclast growth.
At the same time, this altered immune signaling had a direct impact on bone remodeling. The study revealed that PD-L1-expressing myeloid cells interact with PD-1 receptors on osteoclast precursors, promoting their differentiation into mature osteoclasts. This procedure significantly increased bone resorption, leading to decreased trabecular and cortical bone volume. Specifically, blocking the PD-1/PD-L1 pathway during the early stages of osteoclast formation reduced both the number and activity of these bone-resorbing cells, highlighting its critical role in osteoclastogenesis. Dr. Rosen explained, “We discovered that bone marrow fat is not just a passive tissue, but actively reshapes immune signaling in ways that accelerate bone loss in obesity.”
To further confirm these findings, the researchers used a genetically engineered mouse model that lacked bone marrow fat cells. These mice showed reduced levels MCP-1less PD-L1+ cells of the immune system and a significant decrease in osteoclast precursors. Importantly, this led to improved bone structure and reduced bone resorption, even under obese conditions. These results indicate that bone marrow fat plays a central role in immune suppression and bone degradation.
Dr. Rizoff added, “This immune checkpoint pathway, known to regulate T-cell responses, also directly leads to osteoclast formation, revealing an entirely new relationship between immunity and skeletal health.”
Beyond the mechanistic insights, the study highlights important implications for human health. In the short term, it suggests new strategies to protect bone health in people with obesity by targeting bone marrow fat or immune checkpoint pathways. It may also provide insight into why obesity is associated with reduced immune responses, such as reduced vaccine effectiveness and increased risk of infection.
In the long term, these findings could influence therapeutic approaches in many areas. Since PD-1/PD-L1 inhibitors are already used in cancer treatment, this research suggests possible future exploration of repurposing such therapies to treat bone loss and metabolic disorders. It may also encourage collaborations between immunologists, endocrinologists and bone researchers to explore integrated treatment strategies.
Ultimately, this study redefines the role of bone marrow fat as a key regulator of immune and skeletal health. By revealing how it drives immunosuppression and osteoclast activity, the research provides the basis for developing innovative therapies aimed at reducing obesity-related bone loss and improving overall health outcomes.
