OPG-producing B Cells and RANKL-expressing T cells define immune signatures predictive of bone metastases in breast cancer

Bone metastases represent a common and devastating complication of breast cancer (BC); however, the immune determinants of skeletal colonization, particularly the role of adaptive lymphocyte subsets, have only recently been elucidated. Here, we identified a non-classical regulatory role of CD19⁺ B cells in the modulation of bone remodeling and BC metastatic progression. Using murine models of metastatic 4T1 and non-metastatic 67NR mammary tumors, we show that 67NR-bearing mice exhibit increased trabecular bone mass, associated with osteoprotegerin (OPG) secretion by CD19⁺IgD⁺IgM⁺CD138⁻ B cells. Functional assays demonstrated that 67NR-primed B cells suppressed bone resorption in vitro and counteracted 4T1-specific T cell-mediated osteolytic activity in vivo. Adoptive transfer of 67NR-primed CD19⁺ B cells into 4T1-bearing immunocompetent or immunodeficient hosts preserved bone architecture, reduced RANKL production, inhibited metastases, and limited tumor growth. These effects are T cell-independent at the effector phase but require T cell licensing for the acquisition of the OPG-producing phenotype. Silencing OPG abrogated this protective function. Mechanistically, B cells must be transferred during the early stages of tumor progression to retain their therapeutic potential. Molecular analyses supported the enrichment of RANKL⁺CD4⁺ T cells in tumors and lymph nodes, whereas OPG⁺ B cells were restricted to tumors. Transcriptomic data from The Cancer Genome Atlas (TCGA) further support the prognostic relevance of these immune phenotypes. In a retrospective human cohort, high RANKL⁺ lymphocyte infiltration in primary tumors correlated with bone metastases, whereas OPG⁺ infiltration predominated in bone metastasis-free cases. These findings uncover a B cell-driven axis that restrains osteolysis and tumor progression.