[Frontier Science Popularization] Anti-tumor weapon after genetic engineering-bacterial anti-tumor-related genetic engineering bacteria and characteristic (III)

[Frontier Science Popularization] Anti-tumor weapon after genetic engineering-bacterial anti-tumor-related genetic engineering bacteria and characteristic (III)

microbial therapy for malignant tumors has a long history. The first report describing the link between infection and tumor tissue was recorded by Egyptian doctors in 2600 BC. In 1866, German doctors recorded the course of treatment of patients with neck tumors infected with Streptococcus pyogenes [1]. Over the next few decades, Dr. William B.Coley, a surgeon at New York Hospital, focused on developing "Coley toxins" (a heat-killed variety of anti-tumor cocktails of Streptococcus pyogenes and Serratia mucus), using live or inactivated bacteria to trigger a targeted immune attack on tumors, which laid the foundation for bacterial therapies (Figure 1).
At present, the treatment of cancer is still focused on surgery, chemotherapy, radiotherapy and immunotherapy, but due to the microenvironment of tumor itself, such as hypoxia and microvascular disorder, the curative effect of various treatments is limited. The advantage of bacterial therapy is that it can use the bacteria's own movement and tumor targeting to go deep into the tumor. Many studies have shown that bacteria can be used as drug carriers and vaccines to treat tumors through genetic modification.
So over the past decade, there has been renewed interest in developing bacterial therapies, including the use of bacteria such as Clostridium, Listeria, Bifidobacterium and Salmonella. Compared with other bacteria, the facultative anaerobes Salmonella have higher motility and accumulation in tumors, and have special advantages in targeting tumors. They have a strong affinity for antigen-presenting cells, a feature associated with the induction and activation mechanisms of the innate immune response and the adaptive anti-tumor immune response.

Figure 1 The development of bacterial therapy for cancer

1. Principles of Bacterial Therapy
Bacterial therapy is based on the accumulation and proliferation of bacteria within the tumor to initiate an anti-tumor immune response. Bacteria can be further programmed by simple genetic manipulations or sophisticated bioengineering techniques to produce and deliver anti-tumor drugs according to clinical needs. Bacteria-targeted tumor therapies can be used as monotherapy or in combination with other anti-tumor therapies to achieve better clinical outcomes. Commonly used bacteria include Clostridium, Bifidobacterium, Listeria, Escherichia coli and Salmonella. Indications for treatment include melanoma, solid tumors, cervical cancer, pancreatic cancer, colorectal cancer, etc.
Researchers have engineered bacteria to improve the effectiveness of cancer therapy. The main strategies include (1) specific expression of anti-tumor drugs or surface antibody genes, (2) expression of cytokines, and (3) expression of anti-vascular factors. The targeting of bacteria can not only target molecular targets such as tumor proteins and antigens, but also target the unique pathological changes of tumors at the tissue level. Recombinant DNA technology can be used to modify engineering bacteria to increase their anti-tumor effects. For example: RNA interference, pro-drug cleavage, cytotoxins, cytokines, expression of anti-tumor drugs, gene transfer, expression of anti-tumor specific antigens and antibodies.
Due to the rapid growth of solid tumor tissue and the uneven distribution of internal blood vessels, the tumor mostly belongs to the hypoxic metabolic environment, and the bacteria itself has a strong selectivity to hypoxic tumor tissue, and can accumulate and multiply in tumor tissue. At present, it is believed that bacteria can escape from the blood circulation to the tumor tissue through passive and active mechanisms to achieve targeted drug delivery. Bacteria as a carrier, through the drug-loaded treatment of tumor is fundamentally able to target the tumor through a unique mechanism to achieve local precision drug delivery, at the same time, because bacteria can survive in the tumor microenvironment, can produce protein drugs, improve local drug dose.
The bacteria in the tumor then induce tumor regression through several different mechanisms. Different bacteria have different tumor suppression mechanisms. Figure 2 shows the main genetic modifications made by several bacteria to treat tumors and the pathogen-associated molecular patterns (PAMPs) and pattern recognition receptors (PRRs) that can be detected. Figure 3 shows several mechanisms that induce tumor regression. For example, Salmonella directly kills tumor cells by inducing tumor cell apoptosis or autophagy through a variety of mechanisms, including the production of toxins or deprivation of tumor cells. Listeria can directly kill tumor cells by activating nicotinamide adenine dinucleotide phosphate oxidase and increasing intracellular calcium levels, both of which lead to higher intracellular reactive oxygen species (ROS) levels. Listeria can also affect tumors by directly or indirectly entering tumor cells through immunosuppressive effects on MDSCs. Clostridium infection secretes a variety of bacterial toxins (such as hemolysin and phospholipase), which can disrupt cell membrane structure or interfere with intracellular functions [2].

Figure 2 Major genetic modifications and detectable PAMPs and PRRs[3]

Figure 2 Major genetic modifications and detectable PAMPs and PRRs[3]

With the deepening of people's understanding of tumor occurrence and development, tumor treatment strategies will tend to be more accurate and comprehensive use of diversified therapies. Bacterial anti-tumor therapy can be used alone or in combination with multiple therapies. The progress of genetic engineering technology and synthetic biology has brought new infinite possibilities for bacterial anti-tumor therapy. In the future, bacteria are likely to be regarded as the "perfect" anti-tumor agent for the treatment of many types of tumors.

2. Bacterial anti-tumor related genetic engineering bacteria and their characteristics
Bacterial medicines based on genetic engineering have properties that cannot be achieved with traditional intervention therapies. It has attracted considerable attention in the treatment of tumors. As the "factory" of a variety of anti-tumor drugs, attenuated bacteria can specifically target tumor tissues and actively penetrate tumor tissues, and play an important role in tumor colonization.
Bacteria such as Clostridium, Bifidobacterium, Salmonella, Escherichia coli and Listeria monocytogenes have been observed to live in tumors. Some of these wild-type or genetically modified strains have been developed to treat tumors, and Mycobacterium bovis is the first bacterium approved to treat tumors. Among them, Salmonella is the most studied candidate bacteria, and the research and application of Salmonella for anti-tumor therapy may be a new direction for bacterial anti-tumor therapy to make a breakthrough as soon as possible.

Figure 4 Relationship between bacterial species and the number of related articles in bacterial therapy for cancer, 1960-2010 [4]

Note: The above main contents are taken from the research report of Yadu.

References

1. Kienle, G.S., Fever in Cancer Treatment: Coley's Therapy and Epidemiologic Observations. Glob Adv Health Med, 2012. 1(1): p. 92-100.
2. Duong, M.T., et al., Bacteria-cancer interactions: bacteria-based cancer therapy. Exp Mol Med, 2019. 51(12): p. 1-15.
3. Rius-Rocabert, S., et al., Oncolytic bacteria: past, present and future. FEMS Microbiol Lett, 2019. 366(12).
4. Forbes, N.S., Engineering the perfect (bacterial) cancer therapy. Nat Rev Cancer, 2010. 10(11): p. 785-94.