Lympho-epithelial cross talk in the gut: implications for maintaining and restoring immune homeostasis

Abstract Intestinal immunological environment is shaped by a continuous cross-talk involving three major components: intestinal epithelium, immune system and local microbiota. This work intended to investigate the role of lympho-epithelial interactions in response to food components and microorganisms. As part of this work the role of the immune-derived cytokine interleukin (IL)-12 in development of food allergy has been investigated. It was previously found that in peanut-sensitized mice there was decreased level of IL-12 in the intestine. We have observed that this is accompanied with increase in thymic stromal lymphopoietin (TSLP), a cytokine produced by the intestinal epithelium and indispensable for development of allergy. Oral delivery of recombinant Lactococcus lactis secreting bioactive IL-12 resulted in amelioration of the allergic symptoms and decrease in TSLP, thus suggesting a regulatory interaction between these two cytokines. Further investigation of the mechanism of this cross-regulation revealed that intestinal epithelial cells express incomplete but functional IL-12 receptor. However, mice deficient in IL-12 signalling displayed normal levels of TSLP implying involvement of other factors in the IL-12/TSLP axis. In addition, the availability of mice defective in IL-12 associated pathways prompted us to test the hypothesis that alteration of the cytokine network in the gut may contribute to shape the intestinal microbiota. Thus, by using 16S pyrosequencing we have studied the composition of the gut microbiota in mice deficient for IL-12p40, IL-12Rβ2, and IFN-γ in comparison to WT mice. In parallel, we have monitored the metabolome of the microbiota and the host intestinal tissue. Finally, we have described a novel pathogen-exclusion mechanism mediated by CX3CR1+ cells that migrate into the intestinal lumen upon Salmonella infection. This event is orchestrated by a lympho-epithelial crosstalk involving MyD88-dependant epithelial signal. CX3CR1-dependant migration is vital for pathogen protection in the early stages of infection

CX3CR1+ Cell–Mediated Salmonella Exclusion Protects the Intestinal Mucosa during the Initial Stage of Infection

During Salmonella Typhimurium infection, intestinal CX3CR1(+) cells can either extend transepithelial cellular processes to sample luminal bacteria or, very early after infection, migrate into the intestinal lumen to capture bacteria. However, until now, the biological relevance of the intraluminal migration of CX3CR1(+) cells remained to be determined. We addressed this by using a combination of mouse strains differing in their ability to carry out CX3CR1-mediated sampling and intraluminal migration. We observed that the number of S. Typhimurium traversing the epithelium did not differ between sampling-competent/migration-competent C57BL/6 and sampling-deficient/migration-competent BALB/c mice. In contrast, in sampling-deficient/migration-deficient CX3CR1(-/-) mice the numbers of S. Typhimurium penetrating the epithelium were significantly higher. However, in these mice the number of invading S. Typhimurium was significantly reduced after the adoptive transfer of CX3CR1(+) cells directly into the intestinal lumen, consistent with intraluminal CX3CR1(+) cells preventing S. Typhimurium from infecting the host. This interpretation was also supported by a higher bacterial fecal load in CX3CR1(+/gfp) compared with CX3CR1(gfp/gfp) mice following oral infection. Furthermore, by using real-time in vivo imaging we observed that CX3CR1(+) cells migrated into the lumen moving through paracellular channels within the epithelium. Also, we reported that the absence of CX3CR1-mediated sampling did not affect Ab responses to a noninvasive S. Typhimurium strain that specifically targeted the CX3CR1-mediated entry route. These data showed that the rapidly deployed CX3CR1(+) cell-based mechanism of immune exclusion is a defense mechanism against pathogens that complements the mucous and secretory IgA Ab-mediated system in the protection of intestinal mucosal surface