Reprogramming pulmonary B cells by ANXA1 silencing halts lung metastatic niche formation in breast cancer.

Lung metastasis remains a determinant of poor prognosis and survival in breast cancer and is understood to depend on a permissive pulmonary immune niche rather than tumor cell traits alone. Here, we developed a host-directed RNA interference strategy to modulate this niche by reprogramming pulmonary B cells for breast cancer lung metastasis treatment. IF7C peptide-decorated cationic liposomes were constructed, which preferentially accumulated in the lung, and were internalized by pulmonary B cells, enabling selective silencing of annexin A1 (ANXA1). In tumor-conditioned primary B cells, ANXA1 knockdown reshaped the transcriptional landscape and shifted cytokine output away from an immunosuppressive profile characterized by IL-10, TGF-β, and IL-35. Functionally, ANXA1-silenced B cells lost their capacity to drive CD4⁺ T cells toward Foxp3⁺ regulatory differentiation and instead promoted Th1 features, while concurrently relieving suppression of CD8⁺ T-cell proliferation. In two postoperative syngeneic breast cancer models, perioperative administration achieved ANXA1 silencing in pulmonary B cells, reduced lung Treg accumulation, enhanced CD8⁺ T-cell infiltration and effector activity, and suppressed metastatic outgrowth with favorable systemic safety. These findings identify pulmonary B cells as an actionable regulator of the lung metastatic niche and establish perioperative, B-cell-focused ANXA1 silencing as a practical approach to prevent postoperative lung metastatic recurrence.