Tumour-targeted therapies and immunotherapy have raised hope for curing many malignant cancers. Live tumour-targeting bacteria are a distinctive option for tumour therapy. Bacterial vectors can be reprogrammed following simple genetic rules or sophisticated synthetic bioengineering principles to produce and deliver antitumour agents based on clinical needs. Attenuated Salmonella typhimurium, Clostridium novyi, Bifidobacterium and Listeria strains have been tested in animal models and have shown preferential targeting of solid tumours, and several of these strains have advanced to clinical trials[16–20]. Various therapeutic payloads delivered by these tumour-targeting bacteria have since been developed [21–23]. However, these strains are all pathogenic bacteria, and systemic toxicity limits their clinical use. Although researchers have been focused on attenuating the virulence of these bacteria, there are many challenges.
Escherichia coli Nissle 1917 (EcN) is known to be avirulent and consumed as the probiotic preparation Mutaflor, which is used for the treatment of various intestinal disorders; EcN successfully colonizes the human gut and can survive and proliferate in both hypoxic and oxygenated environments[24, 25]. Previous studies have demonstrated that EcN has excellent performance in preferentially localizing to tumours when administered systematically or orally to different tumour-bearing mouse models[7, 26]. EcN has also been engineered to deliver various antitumour agents and displays significant tumour-suppressive effects[26, 27],.
IL-2 is a monomeric secreted glycoprotein with a molecular weight of 15 kDa that exerts a wide spectrum of effects on the immune system and plays crucial roles in regulating both immune activation and homeostasis. It was approved for use in clinical cancer immunotherapy several years ago. However, high-dose IL-2 administration can result in severe systemic toxicity, such as malaise, fever, anasarca, jaundice, renal dysfunction and capillary leak syndrome, in many patients[30, 31]. Researchers have attempted to diminish the adverse effects of systemically administered IL-2 by altering the dose, schedule and route of administration[32–36]. Nevertheless, the toxic effects of IL-2 therapy persist to various degrees. These shortcomings have made it necessary to create a better method of using IL-2 for tumour therapy. Localized administration delivered by tumour-targeting bacteria is an appropriate option. In an attempt to establish a local delivery system for IL-2 that may diminish or prevent side effects, Denial A and colleagues used attenuated Salmonella typhimurium to produce the human IL-2 protein and significantly reduced the hepatic metastasis of colon cancer through gavage feeding of their engineered bacteria to model mice[38, 39].
Based on the current understanding of the tumour microenvironment and recombinant DNA technology, in this study, we addressed three major questions. First, engineered EcN expressing IL-2 can target CT26 in model mice. Second, IL-2 can enhance immune responses in the tumour microenvironment. Last, improved immune responses can disturb tumour tissues and suppress tumour growth. To achieve sustained high levels of IL-2 in the tumour microenvironment while avoiding systemic toxicity, we utilized the oxygen-dependent promoter of the haemoglobin gene (vhb) of Vitreoscilla and the pelB leader sequence to facilitate IL-2 expression in the hypoxic tumour region. These engineered bacteria containing IL-2 are similar to a vaccine against tumours, and this vaccination strategy does not require knowledge of tumour antigens. Therefore, this approach may have advantages over nonimmunogenic approaches.
Taken together, our data demonstrate that the tumour-targeted bacteria EcN can express soluble hIL-2, localize in solid tumours and elicit local immune responses that induce tumour suppression while avoiding systemic toxicity. This live vector system is relatively inexpensive and does not require vast laboratory resources to produce antitumour reagents. However, the clinical development of live bacteria as therapeutic agents faces substantial hurdles mainly because of potential infection-associated toxicities, especially when administered systematically. Major efforts should be made to develop proper administration routes that can minimize systemic toxicities. Employing an oral route of administration in a syngeneic, xenograft CT26 colon cancer mouse model and exploring the cellular and immunological mechanisms of how EcN(hIL-2) facilitates the inhibition of tumours are the subjects of our ongoing research.