Oncolytic biomineralized bacteria for tumor immunotherapy

Bacteria-based cancer immunotherapy, dated back to Coley’s toxins (inactivated bacteria) in 1893, has recently regained substantial attentions, usually by using attenuated bacteria to transform immune-silent “cold” tumors into immune-inflamed “hot” ones. However, while inactivated bacteria showed limited antitumor efficacy, attenuated live bacteria often possessed significant safety risks. Herein, by biomineralizing growth of manganese dioxide on the surface of paraformaldehyde-fixed gram-negative Salmonella typhimurium (S. typhimurium), we obtained MnO2-coated fixed S. typhimurium (M@F.S), which showed potent immune-stimulating effects via activating multiple pathways including Toll-like receptors (TLRs), cyclic GMP-AMP Synthase (cGAS)-stimulator of interferon genes (STING) and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs). Single intratumoral administration of M@F.S at safe doses resulted in surprisingly strong efficacies in suppressing various types of mouse tumor models and a rabbit cancer model, and the cured mice and rabbits gained immune memory to reject re-challenged tumors. An abscopal antitumor effect was also observed, suggesting systemic antitumor immunity triggered by local injection of M@F.S. The antitumor mechanisms of M@F.S were preliminarily demonstrated to be innate immune activation initiated by multiple signaling pathways, followed by subsequent activation of tumor-specific immune responses, together with the modulation of immunosuppressive tumor microenvironment. We further demonstrated the efficacy of biomineralized bacteria in inhibiting an orthotopic breast tumor model established on tree shrews, an alternative animal model to primates with better clinical relevance. Such oncolytic biomineralized bacteria could be a potent yet safe immunotherapeutic agent for treatment of various solid tumors. 6 mouse tumor tissues (3 mice each in the blank and M@F.S-treated groups) on Day 3 post-treatment.