Cancer vaccines, aimed at evolving the human immune system to eliminate tumor cells, have long been explored as a method of cancer treatment with significant clinical potential. Traditional delivery systems face significant challenges in directly targeting tumor cells and delivering adequate amounts of antigen due to the hostile tumor microenvironment. Emerging evidence suggests that certain bacteria naturally home in on tumors and modulate antitumor immunity, making bacterial vectors a promising vehicle for precision cancer vaccines. Live bacterial vehicles offer several advantages, including tumor colonization, precise drug delivery, and immune stimulation, making them a compelling option for cancer immunotherapy. In this review, we explore the mechanisms of action behind living bacteria-based vaccines, recent progress in popular bacterial chassis, and strategies for specific payload delivery and biocontainment to ensure safety. These approaches will lay the foundation for developing an affordable, widely applicable cancer vaccine delivery system. This review also discusses the challenges and future opportunities in harnessing bacterial-based vaccines for enhanced therapeutic outcomes in cancer treatment.

Wilms' tumor 1 (WT1) is aberrantly overexpressed across hematologic and solid malignancies and ranks among top-priority antigens for T cell receptor (TCR)-based immunotherapy. Among WT1 epitopes, the HLA-A*02:01-restricted nonamer RMFPNAPYL (WT1₁₂₆-₁₃₄; RL9) has advanced clinically, including with the naturally selected C4 TCR. Despite encouraging activity in early trials, the molecular basis of C4 recognition and its tolerance to vaccine-motivated anchor modifications remain unclear. Here we report high-resolution crystal structures of C4 bound to HLA-A*02:01 presenting either the native RL9 peptide or the R1Y variant (YMFPNAPYL; YL9). C4 docks diagonally with a peptide-centric footprint biased toward central and C-terminal positions, contrasting with TCR-mimic antibodies that depend on N-terminal R1 contacts. Although R1Y preserves the peptide backbone, p1Y reorients upon TCR engagement, eliminating the E94 (CDRα3)-p1R salt bridges, remodeling the electrostatic landscape, and weakening interfacial hydrogen-bond/salt-bridge networks. Differential scanning fluorimetry showed that YL9-HLA-A*02:01 is more thermally stable than RL9-HLA-A*02:01 (T_m = 48.9 ± 0.25°C versus 54.0 ± 0.21°C). Surface plasmon resonance revealed micromolar binding of C4 to RL9-HLA-A*02:01 (K_D = 13.8 ± 1.53 μM) but weak binding to YL9-HLA-A*02:01 (K_D >150 μM). Together, these structural and biophysical data clarify the recognition logic of a clinically validated WT1-specific TCR reveal that an HLA-stabilizing R1Y modification can reduce recognition by C4, even while improving pHLA stability. These findings do not exclude the ability of YL9 to elicit other cross-reactive TCR clonotypes, but highlight the need to evaluate both HLA stabilization and TCR-recognition fidelity during epitope selection and structure-informed TCR engineering in WT1-targeted immunotherapy.