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; however, the immune determinants of skeletal colonization, particularly the role of adaptive lymphocyte subsets, have only recently been elucidated. In this study, we identified a nonclassical regulatory role of CD19+ B cells in the modulation of bone remodeling and breast cancer metastatic progression. Using murine models of metastatic 4T1 and nonmetastatic 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 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. SIGNIFICANCE: This study identifies opposing RANKL+ T cell and OPG-producing B cell immune phenotypes that shape bone metastasis risk in breast cancer. By revealing how these adaptive lymphocyte subsets influence osteolysis and skeletal colonization, our findings define prognostic immune signatures with potential utility for early risk stratification and clinical decision-making.