The human gut is involved in other functions besides digestion. In fact, the gut houses almost 70 percent of the entire immune system. This large portion is represented by a huge mass of lymphoid tissue known as gut-associated lymphoid tissue (GALT). GALT is located in the small intestine, colon and appendix, and is composed of different types of immune cells. Many of these cells, especially the B and T lymphocytes, play important roles in humoral (involves antibodies) and cell-mediated (involves specialized immune cells) immunity.
For many years, researchers have known that the gut microbiota – a diverse but harmonious community of microbes hosted by the digestive system – directly influences immune function. This has been clearly demonstrated in many animal studies. A healthy gut microbiota always translates to overall wellness and a stronger immune system, while dysbiosis (microbial imbalance) results in a dysfunctional immune system, persistent inflammation and gastric diseases like inflammatory bowel disease (IBD). While the logic behind these associations is clear, the underlying mechanisms are yet to be elucidated.
Recently, researchers have made good progress in delineating the extent of the gut microbiota's involvement in conferring immunity. According to immunologists at the NIH, the gut microbiota "plays a fundamental role in the induction, training and function of the host immune system." The alliance formed by these two ensures that proper defensive responses will be mounted against invading pathogens, while a certain amount of tolerance is maintained for the survival of beneficial microorganisms.
In their latest study, NIH researchers have finally uncovered how the gut microbiota fends off harmful invaders – a process known as colonization resistance – and enhances host immunity in the process. For their experiment, they first isolated microbiota that had previously been exposed to Klebsiella pneumoniae (Kpn), a common gut bacterium that causes severe infections outside of the intestines. After transferring the microbiota to germ-free mice, the researchers then exposed the animals to Kpn and found that the microbiota helped protect the mice from infection.
Further analysis allowed the researchers to pinpoint Deltaproteobacteria, a small class of Proteobacteria found in the guts of various organisms, as the main bacteria involved in preventing the infection. They also discovered that taurine, a multi-functional amino acid naturally produced by the body, plays a pivotal role in the development of colonization resistance as it serves as the trigger for Deltaproteobacteria activity. (Related: Boost brain health with taurine, an amino acid with antidepressant properties.)
According to the researchers, the body produces taurine in response to infection. Simultaneously, the gut microbiota undergoes functional changes so that the number of microbes that can utilize taurine increases. This allows the gut microbiota to efficiently convert taurine into sulfide. Sulfide is a compound that inhibits cellular respiration, so the presence of sulfide limits the ability of pathogens to complete their invasion. When the researchers supplemented mice with taurine, they found that this alone was enough to prime their gut microbiota to fight off foreign invaders.
On the other hand, treatment with the over-the-counter drug bismuth subsalicylate, an inhibitor of hydrogen sulfide production, decreased the resistance of mice against infections. This demonstrates how conventional medications can easily interfere with the body's natural defenses and, as a consequence, reduce a person's immunity. Taken together, the findings suggest that a single mild infection is enough to induce the gut microbiota to develop resistance against a specific pathogen. Furthermore, the results point to the liver and the gallbladder – where bile acids containing taurine are synthesized – being capable of developing long-term immunity against infections.
The study also reveals another important function of taurine, which, apart from forming bile salts, also helps maintain hydration and electrolyte balance, regulates the amount of minerals within cells and supports the functions of the central nervous system.
Learn more about how your immune system works at ImmuneSystem.news.