Targeted Enrichment of Engineered Bacteria on Triple‐Negative Breast Cancer Turns Immunologically Cold Tumors Hot

Abstract Tumor immunotherapy has garnered significant attention, however, several notable challenges remain to be addressed, including: 1) enhancing active targeting, 2) effectively shielding immune checkpoints, and 3) inducing the transformation of “cold” tumor into “hot”. In this study, an “all‐in‐one” extracellular anaerobic bacterial nanocomposite system is constructed. Escherichia coli Nissle 1917 (EcN) is enveloped with polydopamine via self‐polymerization ( ^PDA EcN), subsequently conjugated with a chitosan oligosaccharide (COS) nanoparticle immunostimulant backpack, denoted as ^PDA EcN/COS. The PDA coating is capable of concealing EcN polysaccharides, masking the immunogenic bacterial surface antigens, and inducing a mild photothermal therapy (PTT) effect. Moreover, ^PDA EcN/COS exhibits targeted accumulation in hypoxic regions of solid tumors and demonstrates pronounced enrichment on tumor cell surfaces, attributed to the bacterial hypoxic region targeting ability and adhesive properties of PDA, physically obstructing the immune checkpoint. Simultaneously, both EcN and COS markedly enhanced the transformation of M2 macrophages into the M1 phenotype, whereas mild PTT‐induced immunogenic cell death (ICD) further mitigated the immunosuppressive nature of the hypoxic tumor microenvironments (TMEs). This integrated therapeutic approach eradicated tumors without eliciting metastasis or discernible side effects following a single injection and laser irradiation in a murine 4T1 cancer model. Ultimately, the immunostimulatory capacity of ^PDA EcN/COS is considered to hold significant potential for developing novel and efficacious therapies for immunologically “cold” triple‐negative breast cancer (TNBC).