The Immunological Paradox of Mastication

Could the process of chewing be both good and bad for your oral health? It all depends on the balance of inflammatory factors, and the unique biological signaling that takes place at the barrier layer of the oral cavity. Research published in the journal Immunity has revealed that mastication stimulates the release of a specific kind of T helper cell associated with barrier membranes, Th17, in the oral mucosa. Th17 is a white blood cell primarily defined by its secretion of interleukin-17 (IL17), a pro-inflammatory signaling molecule that recruits other white blood cells (monoctyes and neutrophils) to fight invading microorganisms.

In the skin and gut, the body's Th17 response is modulated by commensal bacteria, among other factors, helping to selectively fight pathogenic organisms and promote the growth of beneficial bacteria. There is preliminary evidence that the risk of autoimmune diseases is influenced by the types and relative concentrations of these bacteria, especially in the gut. Commensal bacteria communicate with the enteric nervous system through receptors like TLR-9, an immune receptor that communicates directly with nerve fibers feeding back to the vagus nerve, which is involved in the regulation of everything from body-wide inflammation to the physiological stress response.

However, the researchers found a method by which Th17 cells are recruited unique to the oral cavity. Unlike other biological barriers, the physical process of chewing can stimulate an immune response in the oral tissues. To investigate the phenomenon, newly-weaned mice were separated into two groups and fed on either a soft food diet or hard food diet. After 24 weeks, the researchers prepared an immunohistochemistry analysis to determine the levels of Th17, and found that they were significantly higher in mice fed exclusively on hard food. To confirm that the stimulation, and not diet was responsible for this effect, a third cohort of mice was fed on soft food, but had their oral cavity rubbed with a sterile cotton swab every day of the trial. These mice also experienced the rise in Th17 levels seen in the mice who were fed the hard food diet.

So what does a higher Th17 level actually mean in practical terms? The effect is twofold: greater immune system activity to fight infection, and resulting inflammation. While the immunological activity does indeed assist in repelling pathogens, the inflammatory component can have negative consequences for periodontal health. For example, in both the soft food group and hard food group, some of the mice developed periodontitis. However, the amount of periodontal bone loss was higher in the hard food group. The researchers theorize that this is due to the proinflammatory effect of the Th17 cells, as inflammation and periodontitis are closely associated. While mastication is not an inherently harmful process, the results highlight the possibility that excessive stimulation may exacerbate existing periodontitis.

This research continues to reveal the complexities of the oral barrier membrane and further inform our approaches to more comprehensive treatment plans for maintaining oral health in the face of periodontal disease. Could a soft food diet paired with improved hygiene be recommended to help treat periodontitis? Only more research can tell.

Source:
Dutzan, N., Abusleme, L., Bridgeman, H., Greenwell-Wild, T., Zangerle-Murray, T., Fife, M. E., ... Moutsopoulos, N. M. (2017). On-going mechanical damage from Mastication drives Homeostatic th17 cell responses at the oral barrier. Immunity, 46(1), 133–147. doi:10.1016/j.immuni.2016.12.010

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