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Intestinal dendritic cells and gut T-cell homing in inflammatory bowel diseases

Periodic Report Summary - GUT DCS IN IBD (Intestinal dendritic cells and gut T-cell homing in inflammatory bowel diseases)

DC from different tissues had different phenotype and stimulatory capacity for T-cells. Those differences included variations not only in their own tissue-homing profile but also their capacity to prime responding T-cells with a differential tissue-homing profile. DC from tissues exposed to high antigen doses (like intestinal or epidermal DC) had a lower stimulatory capacity compared to their counterparts from other tissues (blood or dermis, respectively) providing therefore a mechanisms helping to maintain immune homeostasis in tissues which are exposed to a high antigen load. When DC were cultured 'in vitro' in the absence of other factors in time they lost the expression of their homing markers. Thus, monocyte-derived DC (which had been cultured in vitro for at least five days in the presence of IL-4 and GM-CSF) did not express any of the gut and skin homing markers studied. Many authors have recently suggested that the tissue of residence conditions the phenotype and function of DC providing an explanation as to why DC lose their homing profile when they are cultured 'in vitro' in the absence of a physiological microenvironment. We wondered therefore if we could induce a homeostatic gut-like' phenotype in human blood enriched DC. To that end, blood DC were cultured 'in vitro' in the presence of a colonic microenvironment from healthy controls.

Following conditioning with an intestinal microenvironment, human blood DC acquired a homeostatic 'gut-like' phenotype with all the associated phenotypic and functional characteristics of resident intestinal DC including a lower stimulatory capacity for T-cells as well as a gut-homing priming capacity on T-cells that they stimulated. On the contrary, if human blood enriched DC were conditioned with an IBD microenvironment they failed to acquire a homeostatic phenotype and became more stimulatory for T-cells. Interestingly, T-cells stimulated by IBD-conditioned DC failed to acquire a gut-homing restricted phenotype and on the contrary acquired an aberrant skin-homing profile providing a molecular explanation for the development of extra-intestinal manifestation (usually skin-associated) in some IBD patients.

We next aimed to identify which factors in healthy controls mediate the acquisition of a homeostatic gut-like phenotype by DC and which presumably are lost or masked in IBD patients. We have identified that both retinoic acid (RA) and TGFbeta have complementary roles in those processes. Intestinal RA controls the acquisition of a gut-homing profile both by DC and T-cells that they stimulate, although it did not have an effect on their stimulatory capacity. Intestinal TGFbeta on the contrary induced a lower stimulatory phenotype on conditioned DC although it did not participate in the mechanisms which control the gut-homing properties. Finally, if human blood enriched DC were pre-pulsed with RA prior to conditioning with an IBD microenvironment acquisition of a higher stimulatory phenotype as well as the capacity to generate aberrant skin-homing T-cells was abrogated; these finding provide new insights into the development of potential new tissue-specific therapies to prevent the inflammatory activities in the gut in IBD patients.

Finally, we have identified that the intestinal microbiota have a direct and essential role in controlling intestinal homeostasis through their capacity to modulate the phenotype of intestinal DC towards a regulatory or homeostatic phenotype. That unexpected finding is currently under study with some of our collaborators.

Our results have revealed that both TGFb and retinoic acid have a central role in humans controlling intestinal immune homeostasis. That evidence may help development of new tissue-specific therapies in IBD patients based in immuno-nutrition and / or development of DC vaccines.