Probiotics and the Gut Immune System: Indirect Regulation

Abstract

The gastrointestinal tract (GIT) represents the largest interface between the human organism and the external environment. In the lumen and upper part of the mucus layer, this organ hosts an enormous number of microorganisms whose composition affects the functions of the epithelial barrier and the gut immune system. Consequentially, the microorganisms in the GIT influence the health status of the organism. Probiotics are living microorganisms which, in specific conditions, confer a health benefit to the host. Among others, probiotics have immunomodulatory properties that usually act directly by (a) increasing the activity of macrophages or natural killer cells, (b) modulating the secretion of immunoglobulins or cytokines, or indirectly by (c) enhancing the gut epithelial barrier, (d) altering the mucus secretion, and (e) competitive exclusion of other (pathogenic) bacteria. This review focuses on specific bacteria strains with indirect immunomodulatory properties. Particularly, we describe here the mechanisms through which specific probiotics enhance the gut epithelial barrier and modulate mucus production. Moreover, we describe the antimicrobial properties of specific bacteria strains. Recent data suggest that multiple pathologies are associated with an unbalanced gut microflora (dysbiosis). Although the cause-effect relationship between pathology and gut microflora is not yet well established, consumption of specific probiotics may represent a powerful tool to re-establish gut homeostasis and promote gut health.

Keywords: Gastrointestinal tract; Health status; Immune system; Microbiota; Probiotic.

Fig. 1

Schematic view of the intestinal barrier and main cellular players. The intestinal tract is presented on the upper right. The enlargement represents a schematic view of the intestinal barrier (mucus layer, intestinal epithelium, and bacterial ecosystems). All different cell types are reported below the enlargement. The red arrows highlight the interactions that specific bacteria strains establish with the intestinal barrier. Outlined here are the ① modulation of the tight junction (TJ) proteins, ② modulation of the mucus secretion, and ③ relationship established between different bacterial populations of the gut ecosystem (antimicrobial properties)

Fig. 2

Schematic view of the gut epithelial tight junctions (TJs). Schematic and simplified view of the GIT epithelium. The dashed rectangle is enlarged below and indicates more details of the TJ structure. The proteins constituting the TJ are indicated. Abbreviations: F-actin (filamentous-actin), ZO (zonula occludens) 1-2-3, JAM (junctional adhesion molecule). Other proteins constituting the TJ and not represented in the figure include the following: myosin II (part of the cellular cytoskeleton) and tricellulin (at junction between three cells)