Osteoclast-derived apoptotic bodies inhibit naive CD8 T cell activation via Siglec15, promoting breast cancer secondary metastasis.

The bone microenvironment promotes cancer cell proliferation and dissemination. During periodic bone remodeling, osteoclasts undergo apoptosis, producing large numbers of apoptotic bodies (ABs). However, the biological role of osteoclast-derived ABs, which are residents of the bone-tumor niche, remains largely unknown. Here, we discover that AB-null MRL/lpr mice show resistance to breast cancer cell implantation, with more CD8 T cell infiltrations and a higher survival rate. We uncover that the membranous Siglec15 on osteoclast-derived ABs binds with sialylated Toll-like receptor 2 (TLR2) and blocks downstream co-stimulatory signaling, leading to the inhibition of naive CD8 T cell activation. In addition, our study shows that treatment with Siglec15 neutralizing antibodies significantly reduces the incidence of secondary metastases and improves the survival rate of mice with advanced breast cancer bone metastasis. Our findings reveal the immunosuppressive function of osteoclast-derived ABs in the bone-tumor niche and demonstrate the potential of Siglec15 as a common target for anti-resorption and immunotherapy.