<i>Mycobacterium smegmatis</i> enhances shikonin-induced immunogenic cell death—an efficient <i>in situ</i> tumor vaccine strategy

Tumor vaccines are a promising avenue in cancer immunotherapy. Despite the progress in targeting specific immune epitopes, tumor cells lacking them can evade treatment. Here, we aimed to construct an efficient <italic>in situ</italic> tumor vaccine Vac-SM, utilizing shikonin (SKN) to induce immunogenic cell death (ICD) and <italic>Mycobacterium smegmatis</italic> (<italic>M. smegmatis</italic>) as an immune adjuvant to enhance <italic>in situ</italic> tumor vaccine efficacy. SKN demonstrated a dose-dependent and time-dependent cytotoxic effect on the tumor cell line as seen using the CCK-8 assay and induced ICD in tumor cells by detecting the expression of relevant indicators respectively. Compared to that in the control groups, <italic>in situ</italic> Vac-SM injection in mouse subcutaneous metastatic tumors significantly inhibited tumor growth and distant tumor growth and improved survival rates. <italic>M. smegmatis</italic> effectively induced bone marrow-derived dendritic cells (DC) maturation and activation and <italic>in vivo</italic> tumor-draining lymph nodes showed increased maturation of DC and a higher proportion of effector memory T-cell subsets with Vac-SM treatment, based on flow cytometry analysis results.Collectively, Vac-SM vaccine effectively induces ICD, improves antigen presentation by DC, activates a specific systemic antitumor T-cell immune response, exhibits favorable safety profile, and holds promise for clinical translation for local tumor immunotherapy.