Relationships Between The Intestinal Immune System And Intestinal Microbiota

The intestinal microbiota has marked influences on the intestinal and peripheral host's immunity. In some cases, the effects are produced by the whole intestinal microbiota, whereas in other cases only one predominant bacterium is capable of producing a certain immunostimulatory effect that is as effective as that of the whole microbiota. Moreover, the post-natal period seems to play a crucial role in the cross talk between the intestinal microbiota and the development of some important immunoregulatory processes, especially those involved in the suppressive responses.

Most of the data come from original experimental animal models of germ-free (GF) mice and gnotobiotic mice, i.e., GF mice colonized with known bacteria. The role of intestinal microbiota in humans has largely been extrapolated from studies conducted on probiotic bacteria, mainly Bifidobacterium and Lactobacillus strains, and from epidemiological studies.

The intestinal microbiota acts on the three lines of defense of IIS. Recently, very interesting papers have been published on the role of intestinal bacteria on natural defenses, which are more or less related to innate defenses, especially on epithelium, which belong to the IIS. Thus intestinal microbiota should act on: intestinal permeability (55), production of fucosylated glycoconjugates (56), glycosylation of the intestinal cell layer which is involved in resistance or susceptibility to intestinal infections by the presence or absence of appropriately glycosylated receptors (57) and, expression of angiogenins, especially angiogenin 4 which may have microbiocidal properties (58). These results and others, showing that the intestinal microbiota influence the gene expression in epithelial cells (59), give new insights in the wonderful cross talk existing between bacteria and epithelium.

The intestinal microbiota also interacts with the other lines of defense, innate and acquired immunities. These effects can be of particular importance during the early postnatal life that is a period of high risk for intestinal disorders due to enteric pathogens and/or food hypersensitivities. During the neonatal period, mammalian species exhibit some degree of reduced immunocompetence that could be attributed to a functional immaturity in cells involved in immune intestinal responses. It could be also attributed to the lack of bacterial stimulation given by the intestinal microbiota which is absent during the fetal life. After birth, a well-balanced bacterial colonization will "educate" the IIS in a good manner allowing immunoregulatory mechanisms governing IIS functions to operate rapidly.

As already mentioned in the introduction, the activation, modulation and regulation of the IIS are the main effects exerted by the intestinal microbiota. Gnotobiotic animal models are useful in analyzing such effects of intestinal microbiota on IIS activities.

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