Bone marrow B cell collapse promotes bone metastasis in breast cancer

Metastasis remains the primary cause of cancer-related deaths and is characterized by complex reprogramming of systemic processes. Emerging evidence indicates that extraosseous tumors can rewire bone marrow physiology and disrupt hematopoiesis, thereby compromising effective systemic immune responses. However, how tumor-induced immune alterations in bone marrow contribute to skeletal metastasis remains poorly defined. Here, using immunocompetent mouse models of mammary tumor bone metastasis, we show that mammary cancer cells precondition the bone marrow niche prior to metastatic colonization, driving early remodeling of the microenvironment and depleting bone marrow lymphoid populations. Specifically, cancer cells induce a dramatic B cell reduction, the most abundant lymphoid subset in bone marrow, resulting from dysregulated cell cycle gene expression in pre-B cells, along with impaired B-cell proliferation and differentiation. These findings are further validated in breast cancer bone metastasis patients, who exhibit significant bone marrow B-cell loss alongside disrupted molecular and developmental programs. A causal role for B cells in restraining skeletal metastasis is supported by the finding that experimental B-cell depletion significantly increases both incidence and severity of bone metastasis. Mechanistically, we find that B-cell loss is driven by systemic elevation of G-CSF. Accordingly, pharmacological neutralization of G-CSF significantly reduces both B-cell depletion and bone metastasis susceptibility. Collectively, our data reveal that breast cancer cells can distantly hijack B-cell developmental trajectories, promoting skeletal metastasis. This work identifies B cells and G-CSF as potential therapeutic targets in bone metastasis and highlights the importance of targeting early bone marrow immune dysregulation to prevent or limit skeletal metastasis.