Difference in the Signals Induced by Commensal or Probiotic Bacteria to the Gut Epithelial and Immune Cells

MALDONADO GALDEANO, CAROLINA; DOGI, CECILIA; PERDIGÓN, GABRIELA

Probiotics : Immunobiotics and Immunogenics.

CRC PRESS

Lugar: Nueva York; Año: 2013; p. 36 - 53

The main function of the immune system is to protect the body against harmful agents or pathogens, therefore, the immune cells are distributed throughout the body mainly at the mucosal sites, places that are continuously exposed to antigens from the external environment. However, one of the unresolved mysteries is how immune cells can distinguish between self and non-self antigens, especially between pathogenic and non-pathogenic microorganisms. Achieving this objective in the complex ecosystem made up of three main components that are permanently in contact and interact with one another in a perfect balance: host cells, nutrients and the commensal microbiota. The intestinal microbiota can be divided into two main groups: the autochthonous or commensal biota, which proliferates in the intestine from the time of birth, becoming stable after weaning. The second group, called the allochthone or non-commensal microbiota, which are temporary and consist of different microorganisms introduced during ingestion of food. Probiotic bacteria are included in the last group. These bacteria are present in many of the fermented products that are consumed daily. The commensal intestinal microbiota plays an important role in maintaining the normal physiology and health of the host (Moreau and Gaboriou-Routhiau 2000). The composition and activity of commensal microorganisms are responsible for three basic functions: (i) metabolic: i.e., food breakdown, short chain fatty acids and vitamin synthesis (Backhed et al. 2004); (ii) barrier: i.e., protection against external and invading pathogens; (iii) interactions with the host and particularly infl uencing the development of the mucosal immune system (Sansonetti 2004). This last point shows once again the connection between the three main components of the intestinal ecosystem. A healthy intestinal tract is characterized by controlled homeostasis due to the balanced interaction between the commensal bacteria and the host mucosal immune system. Changes in this homeostasis, caused by abnormal microbiota, dysregulation of the immune responses or a combination of both, may infl uence the susceptibility of the host to chronic infl amatory conditions of the intestine such as infl ammatory bowel diseases (Tlaskalova- Hogenova et al. 2004; Xavier and Podolsky 2007). In this scenario, the ability of the immune system to recognize between commensal and pathogenic microorganisms is essential to maintain homeostasis and prevent pathologies. It is accepted that the commensal microbiota do not initiate strong immunological responses such as an infl ammatory response; however, commensal bacteria can constantly enter into the gut in small numbers and survive within the intestinal dendritic cells (DCs). The commensal-loaded DCs are restricted to the mucosal immune compartment by the mesenteric lymph node, which ensures that the immune responses to commensal bacteria are induced locally and that these bacteria-loaded DC induce mucosal responses (Macpherson 2004). This allows DCs to selectively induce