# Expansion of bone marrow adipocytes in obese mice leads to PD-L1-driven bone marrow immunosuppression and osteoclastogenesis

## TL;DR

Obesity increases bone marrow fat, which promotes bone loss through immune signals involving PD-L1 and osteoclast formation.

## Contribution

This study reveals a novel mechanism linking bone marrow adipose expansion to obesity-related bone loss via PD-1/PD-L1 signaling and osteoclastogenesis.

## Key findings

- Obesity increases bone marrow adiposity and Mcp-1 expression, promoting osteoclast formation.
- PD-L1+ myeloid cells stimulate osteoclast precursors through PD-1/PD-L1 signaling.
- Depleting bone marrow adipocytes reduces PD-L1+ cells and prevents obesity-related bone loss.

## Abstract

Bone marrow adipocytes are known to have a critical role within the bone marrow niche. However, our understanding of bone marrow adipose tissue expansion with obesity and the role it plays in immune cell regulation and osteoclastogenesis is limited. Here, we showed the expansion of bone marrow adipocytes promoted osteoclast differentiation and subsequently led to obesity-related trabecular and cortical bone loss through a stimulatory effect of the PD-1/PD-L1 axis. Bone marrow adipocytes isolated from obese mice had increased Mcp-1 expression, a key regulator of osteoclastogenesis and myeloid cell accumulation. With the increase in bone marrow adipose tissue-derived Mcp-1, we found an increase in the number of PD-L1+ myeloid cells. While these cells inhibited activated T-cells, we found evidence of a stimulatory osteoclastogenic effect of PD-L1+ myeloid cells on PD-1-expressing osteoclast precursors. The inhibition of PD-1/PD-L1 signaling during early osteoclastogenesis prevented myeloid cell commitment and resulted in decreased cell fusion, supporting the role of PD-1/PD-L1 signaling in osteoclastogenesis. Using a bone marrow adipocyte depletion mouse model (BMAd-Pparg KO), we demonstrated that obese BMAd-Pparg KO mice had a reduced number of bone marrow PD-L1+ myeloid cells, accompanied by a decrease in PD-1+ osteoclast precursors. The reduction in these precursors resulted in fewer osteoclasts, subsequently leading to improved trabecular bone volume. Since osteoclasts are myeloid cell-derived, these results suggest that bone marrow adipocytes are critical for the commitment and differentiation of myeloid cells into osteoclasts. Targeting bone marrow adipogenesis could ameliorate enhanced osteoclastogenesis and provide a novel approach to treat obesity-related bone loss.

Obesity-induced expansion of BM adipocytes leads to PD-1/PD-L1-driven osteoclastogenesis and subsequent bone loss in obese, HFD-fed (OB-HFD) mice. After 12 weeks on a HFD, OB-HFD mice had a significant increase in BM adiposity and BMAT-derived Mcp-1 expression. The increase in BMAT-specific Mcp-1 expression was coupled with an increase in PD-1+ osteoclast (OC) precursors and PD-L1+ myeloid cells. In the context of obesity, the PD-1/PD-L1 axis has a stimulatory effect that enhances osteoclastogenesis and leads to trabecular and cortical bone loss. By depleting BM adipocytes with obesity, BMAT-derived Mcp-1 expression was decreased, as well as a decrease in PD-1+ OC precursors and PD-L1+ myeloid cells. This prevented obesity-related trabecular bone loss. Overall, this work demonstrated a strong correlation between BMAT expansion and PD-1/PD-L1-driven osteoclastogenesis as a mechanism for obesity-induced bone loss. (This image was created using BioRender).

Obesity-induced expansion of BM adipocytes leads to PD-1/PD-L1-driven osteoclastogenesis and subsequent bone loss in obese, HFD-fed (OB-HFD) mice. After 12 weeks on a HFD, OB-HFD mice had a significant increase in BM adiposity and BMAT-derived Mcp-1 expression. The increase in BMAT-specific Mcp-1 expression was coupled with an increase in PD-1+ osteoclast (OC) precursors and PD-L1+ myeloid cells. In the context of obesity, the PD-1/PD-L1 axis has a stimulatory effect that enhances osteoclastogenesis and leads to trabecular and cortical bone loss. By depleting BM adipocytes with obesity, BMAT-derived Mcp-1 expression was decreased, as well as a decrease in PD-1+ OC precursors and PD-L1+ myeloid cells. This prevented obesity-related trabecular bone loss. Overall, this work demonstrated a strong correlation between BMAT expansion and PD-1/PD-L1-driven osteoclastogenesis as a mechanism for obesity-induced bone loss. (This image was created using BioRender).