Lactate Activates TGF ‐ β / SNAIL Signaling to Drive M2 Macrophage Polarization and CD8 + T Cell Exhaustion in Breast Cancer

ABSTRACT This study demonstrates that lactate promotes M2‐like macrophage polarization by activating the TGF‐ β /SNAIL signaling axis, thereby weakening CD8 + T cell–mediated antitumor immunity and promoting breast cancer progression. In vitro experiments using bone marrow–derived macrophages (BMDMs) and THP‐1 cells treated with 25 mM lactate revealed a marked increase in M2 markers (CD206, Arg‐1, IL‐10) and a reduction in M1 markers (iNOS, TNF‐ α , IL‐12), confirmed by Western blotting and flow cytometry. RNA‐Seq analysis identified TGF‐ β /SNAIL pathway activation, with increased TGFBR1/2 expression, Smad2/3 phosphorylation, and PI3K/AKT pathway enrichment. Functional studies revealed that lactate‐polarized M2 macrophages impaired CD8 + T cell cytotoxicity (reduced IFN‐γ, GzmB, PRF1; elevated PD‐1, Tim‐3) and disrupted mitochondrial metabolism. In vivo validation using a breast cancer xenograft model showed that lactate treatment increased tumor growth and angiogenesis (VEGF/CD31 + ), while TGF‐ β inhibition (SB431542) reversed these effects. Mechanistically, lactate‐induced TGF‐ β /SNAIL signaling promoted EMT in cancer cells and created an immunosuppressive TME. These findings establish lactate as a critical metabolic regulator that coordinates macrophage polarization and T cell exhaustion through the TGF‐ β /SNAIL axis, highlighting this pathway as a promising therapeutic target for breast cancer immunotherapy.