page_title: Microbial symbiosis and immunity
section_title: Cancer
hierarchical_section_title: Microbial symbiosis and immunity / Role in disease / Cancer
caption_attribution_description: English: Superresolution image of a group of killer T cells (green and red) surrounding a cancer cell (blue, center). When a killer T cell makes contact with a target cell, the killer cell attaches and spreads over the dangerous target. The killer cell then uses special chemicals housed in vesicles (red) to deliver the killing blow. This event has thus been nicknamed “the kiss of death”. After the target cell is killed, the killer T cells move on to find the next victim.
context_page_description: Long-term close-knit interactions between symbiotic microbes and their host can alter host immune system responses to other microorganisms, including pathogens, and are required to maintain proper homeostasis. The immune system is a host defense system consisting of anatomical physical barriers as well as physiological and cellular responses, which protect the host against harmful microorganisms while limiting host responses to harmless symbionts. Humans are home to 10¹³ to 10¹⁴ bacteria, roughly equivalent to the number of human cells, and while these bacteria can be pathogenic to their host most of them are mutually beneficial to both the host and bacteria.
  The human immune system consists of two main types of immunity: innate and adaptive. The innate immune system is made of non-specific defensive mechanisms against foreign cells inside the host including skin as a physical barrier to entry, activation of the complement cascade to identify foreign bacteria and activate necessary cell responses, and white blood cells that remove foreign substances.
context_section_description: Gut microbes may play a role in cancer development through a variety of mechanisms. Sulfate-reducing bacteria produce hydrogen sulfide, which results in genomic DNA damage. Higher rates of colon cancer are associated with higher amounts of sulfate-reducing bacteria in the gut. Additionally, anaerobic bacteria in the colon transform primary bile acids into secondary bile acid which has been implicated in colorectal carcinogenesis. Gut bacteria metabolites such as short-chain fatty acids (SCFAs), B vitamins and N¹, N¹²-diacetylspermine have also been implicated in suppressing colorectal cancer. Gram-negative bacteria produce lipopolysaccharide (LPS), which binds to TLR-4 and through TGF-β signaling, leads to the expression of growth factors and inflammatory mediators that promote neoplasia.
  Members of a healthy gut microbiome have been shown to increase interferon-γ-producing CD8 T-cells and tumor-infiltrating dendritic cells (TILs) in the intestine. Not only do these CD8 T-cells enhance resistance against intracellular pathogens such as Listeria monocytogenes but they also have been shown to be important in anti-cancer immunity specifically against MC38 adenocarcinoma where they along with the TILs increase MHC I expression.
caption: Killer T-cells surround cancer cell. Photo credit: The National Institutes of Health
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