Bone loss related to obesity is partly caused by changes inside the bone marrow fat compartment that reshape immune signaling and increase osteoclast formation, according to researchers at the MaineHealth Institute for Research. In a study published in Bone Research, the team found that expansion of bone marrow adipose tissue in obese people changes the marrow environment toward immunosuppression through PD-L1 signaling, which in turn promotes bone-resorbing osteoclast activity that reduces bone volume.
“We discovered that bone marrow fat is not simply a passive tissue but actively reshapes immune signaling in ways that accelerate bone loss in obesity,” said senior author Clifford J. Rosen, MD, senior scientist at the MaineHealth Institute for Research.
The team noted that obesity influences bone health not just due to a higher body weight but also by altering the bone marrow environment. The increase in bone marrow fat promotes immunosuppressive PD-L1 signaling, which enhances osteoclast formation and accelerates bone loss.
The study identified a pathway in which bone marrow adipocytes increase expression of MCP-1, a signaling molecule that recruits myeloid immune cells. These recruited cells shift toward a PD-L1–expressing phenotype, with PD-L1 interacting with PD-1 receptors, which are found not only on T cells but also on osteoclast precursors. In immune biology, PD-1/PD-L1 signaling is typically known for suppressing T-cell activation and promoting immune braking. This new study shows that this same form of suppressive signaling also directly enhances osteoclast differentiation.
According to the study results, as PD-L1+ myeloid cells accumulate, they suppress T-cell activity in bone marrow, creating an immunosuppressive environment. At the same time, PD-L1 engagement with PD-1 on osteoclast precursors promotes their maturation into active osteoclasts, which break down bone tissue, increase resorption and reduce bone density.
To learn more about this mechanism, the investigators used diet-induced obese mouse models, co-culture systems, and genetic depletion approaches. An important model in this work were mice lacking bone marrow adipocytes, which allowed the researchers to isolate the role of marrow fat. The team also blocked PD-1/PD-L1 signaling during early osteoclast formation in vitro. In both cases, osteoclast differentiation decreased and bone structure improved. The mice lacking bone marrow adipocytes showed fewer PD-L1+ myeloid cells, fewer PD-1+ osteoclast precursors, and higher trabecular bone volume even under high-fat diet conditions.
Earlier research has shown a link between obesity and bone loss, but studies reported trabecular bone loss without cortical effects, while others found no significant bone changes under diet-induced obesity. The MaineHealth team noted that these earlier studies often focused on shifts in osteoblast activity as opposed to their approach which identified a pro-osteoclastic mechanism driven by immune signaling.
In addition, the Maine Health finding also added to evidence that has established that obesity is associated with impaired immune responses, including reduced vaccine effectiveness and altered macrophage activity. In this study, the marrow environment in obese mice resembled features seen in tumor-associated immune suppression, where PD-L1 expression is elevated and immune activity is dampened. The researchers wrote that “the increase in PD-L1 expression seen in OB-HFD mice is related to the increase in Mcp-1 in part because previous cancer research has suggested the recruitment of myeloid cells via Mcp-1 creates an immunosuppressive tumor microenvironment.”
The findings suggest potential strategies for preserving bone bones in obese people by targeting bone marrow adiposity or the PD-1/PD-L1 pathway. Because PD-1/PD-L1 inhibitors are already used in oncology, there is a compelling case for repurposing or adapting immune checkpoint modulation therapies already approved for cancer treatment for bone disorders linked to metabolic disease. The authors also noted another strategy could be to reduce the amount of bone marrow fat itself to restore immune balance and limit osteoclast-driven bone loss.
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