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Background Antigen presentation is a critical barrier to effective immunity in triple-negative breast cancers. We devised a triplet regimen that combines pegylated liposomal doxorubicin (Doxil) to release tumor antigens, recombinant Flt3 ligand (Flt3L) to expand dendritic-cell (DC) precursors, and a CD40 agonist antibody (CD40a) to license DCs. Here we dissect the cellular and molecular mechanisms by which this regimen amplifies DC activation and drives anti-tumor immune responses.Methods EO771, AT3, or 4T1 tumors were implanted orthotopically in female mice and treatment started after tumors reached 50 mm 3. Triplet therapy consisted of Doxil IV on day 1, Flt3L IP on days 1–5, and CD40a IP on days 11, 14, and 17. Immune dependencies were probed by depleting CD8 T cells, CD4 T cells, cDC1 cells, or regulatory T cells (Tregs), and by neutralizing interleukin-12 (IL-12), CXCL9, or neutralizing CXCR3. Tumors and draining lymph nodes (TDLNs) were analyzed by single-cell RNA sequencing. Chemotaxis assays exposed Tregs to conditioned medium with or without CCL22.Results Triplet therapy slowed tumor growth and extended median survival versus Doxil alone. Single-cell profiling resolved five DC subsets - cDC1, cDC2, and three populations of mature DCs enriched in regulatory features (mregDC 1–3). mregDC subsets expressed the highest Cd40, Ccr7, and Il12b transcript levels among DC lineages. At the same time, mregDCs co-expressed inhibitory mediators including Ccl22 and Cd274 (PD-L1). Medium containing CCL22 attracted significantly more Tregs than CCL22-depleted medium. Flt3L increased intra-tumoral cDC1, cDC2, and mregDC frequencies in tumor and TDLN. CD40a reduced DC counts within tumors, consistent with CCR7-mediated migration, and up-regulated Cd80, Cd86, β2M, and Cxcl9 while increasing Il12b in the mregDC 1 subset. Tumor control was lost when CD8 and CD4 T cells were depleted together or when cDC1 cells were removed. Neutralizing IL-12b abolished efficacy, whereas neutralizing CXCL9 or CXCR3 had minimal impact. Combining Treg depletion with the triplet led to tumor eradication.Conclusions Flt3L, CD40 agonism, and doxorubicin cooperate to expand and activate DCs, unleashing IL-12-dependent CD4 and CD8 T-cell immunity that restrains triple-negative breast tumors. mregDCs, enriched for CD40, deliver IL-12-driven stimulation yet likely recruit Tregs through CCL22. Targeting the CCL22-Treg axis may yield additional therapeutic gains.