Immunomodulatory potential of gut microbiome-derived short-chain fatty acids (SCFAs).

Intestinal microbiota is an element of the bacterial ecosystem in all mammalian organisms. These microorganisms play a very important part in the development, functioning, and modulation of the immune system from the moment of birth. In recent years, owing to the use of modern sequencing techniques, the microbiome composition in healthy people has been identified based on bacterial 16S rRNA analysis. Currently, more and more attention is being given to the influence of microorganisms on the host's cellular metabolism. Analysis of microbial metabolites, among them short-chain fatty acids (SCFAs), and disruption of intestinal microbiota homeostasis in terms of their effects on molecular regulatory mechanisms of immune reactions will surely improve the understanding of the etiology of many common diseases. SCFAs, mainly butyrate, propionate, and acetate, occur in specific amounts, and their proportions can change, depending on the diet, age and diseases. The levels of SCFAs are substantially influenced by the ratio of commensal intestinal bacteria, the disturbance of which (dysbiosis) can lead to a disproportion between the SCFAs produced. SCFAs are regarded as mediators in the communication between the intestinal microbiome and the immune system. The signal they produce is transferred, among others, in immune cells via free fatty acid receptors (FFARs), which belong to the family of G protein-coupled receptors (GPCRs). It has been also confirmed that SCFAs inhibit the activity of histone deacetylase (HDAC) - an enzyme involved in post-translational modifications, namely the process of deacetylation and, what is new, the process of histone crotonylation. These properties of SCFAs have an effect on their immunomodulatory potential i.e. maintaining the anti/pro-inflammatory balance. SCFAs act not only locally in the intestines colonized by commensal bacteria, but also influence the intestinal immune cells, and modulate immune response by multi-protein inflammasome complexes. SCFAs have been confirmed to contribute to the maintenance of the immune homeostasis of the urinary system (kidneys), respiratory system (lungs), central nervous system, and the sight organ.