A Probiotic Approach Targeting Hypersialylation Revives CD8+ T-cell Immunity and Boosts Photodynamic Therapy Efficacy in Solid Tumors

Excess terminal sialic acids on tumor glycans engage inhibitory Siglec receptors and mask peptide and glycan epitopes, including major histocompatibility complex class I (MHC-I), thereby reducing antigen visibility and dampening cytotoxic T-cell surveillance. In this study, we showed that a compact, lectin-free Salmonella typhimurium sialidase (Sia) efficiently desialylated melanoma and colorectal carcinoma cells, abolished Siglec-E binding, and spared MHC-I from lysosomal degradation. Functionally, Sia treatment enhanced dendritic cell phagocytosis and tumor antigen presentation, expanded IFNγ+/granzyme B+ CD8+ T cells, and slowed tumor growth in a CD8+ T cell-dependent manner without systemic toxicity. Engineering the probiotic Escherichia coli Nissle 1917 to coexpress the photosensitizer KillerRed (KR) and Sia (ProKR/Sia) enabled localized targeting. Following intratumoral delivery, ProKR/Sia selectively colonized hypoxic tumor cores while sparing healthy tissues; no off-target desialylation or organ colonization was detected. Brief white-light irradiation lysed the bacteria, synchronously releasing Sia to remodel tumor glycans and activating KR to generate reactive oxygen species that drive immunogenic cell death. In syngeneic murine tumor models, ProKR/Sia photodynamic therapy (PDT) outperformed either monotherapy and induced robust cellular immunity with expansion of cytotoxic CD8+ and CD4+ T cells and multiple innate responses, including natural killer cell activation and M1-like macrophage polarization. Furthermore, ProKR/Sia PDT established antigen-specific memory that rejected rechallenge. These data define a spatially confined, antigen-agnostic strategy that restores MHC-I visibility and activates antitumor immunity in immunologically "cold" tumors, leading to a durable response. Rational combinations with immunotherapy, deep-tissue illumination, and programmable biosafety switches may further overcome tumor antigen heterogeneity and broaden clinical applicability. SIGNIFICANCE: Probiotic sialidase-KillerRed photodynamic therapy overcomes hypersialylation-mediated immunosuppression by restoring antigen visibility and igniting durable antitumor T-cell responses, offering an antigen agnostic platform for treating solid tumors.