Delta-like ligand 4 mediated myeloid-derived suppressor cell metabolic reprogramming promotes neoadjuvant therapy resistance in titin-inactivated triple-negative breast cancer

Triple-negative breast cancer (TNBC) is characterized by complex genomic background and treatment resistance. We first defined "Titin (TTN) inactivation", a state of TTN expression deficiency or mutation, affecting tumor progression. However, how TTN inactivation regulates immune escape and affects therapeutic resistance remains unclear. Using whole-exome sequencing, single-cell transcriptome sequencing, and spatial transcriptome sequencing, we screened the clinical features of TNBC patients with TTN inactivation who accepted neoadjuvant therapy. Meanwhile, we used CRISPR-Cas9 technology to construct various mutant TNBC cell lines. Lentiviral vector carrying TTN and delta-like ligand 4 (DLL4) was validated in vivo to verify potential mechanism. Myeloid-derived suppressor cells (MDSCs) metabolic function was measured using glycolysis-related molecular experiments. Immunotherapeutic agents against TNBC with TTN inactivation were explored in the orthotopic MCT4^fl/fl genetically modified mouse model. DLL4-regulated ecological niche was established in TNBC with TTN inactivation. Mechanistically, TTN deficiency and mutation led to DLL4 secretion in TNBC. DLL4 enhanced MCT4-mediated glycolysis via MDSCs-derived NOTCH2 signaling pathway, driving the malignant function and lactate acid excretion of MDSCs. DLL4-derived MDSCs promoted stemness-mediated drug resistance by inducing histone lactate modification in TNBC, suppressing the anti-tumor activities of CD8⁺T cells. Blocking the DLL4-MCT4 axis stimulated anti-tumor immunity and synergized with anti-PD-1, improving response rates for first-line neoadjuvant therapy in TNBC. Our study revealed intrinsic mechanism by which TTN regulates the tumor immune microenvironment and provided a potential target for immunotherapy in TNBC with TTN inactivation.