alpha-v-beta-8 integrin expression regulation and its role in immune homeostasis in the gut – InteReg

In order to study the mechanisms by which intestinal dendritic cells acquire immunoregulatory functions (one of which is the ability to activate TGF-ß through specific expression of avß8 integrin), I am using both in vitro and in vivo approach to establish how regulation of b8 integrin expression in DCs shapes intestinal T cell responses in the gut. In parallel, I am determining whether b8 expression regulation is conserved in Humans. These clinical studies will establish whether avß8-mediated TGF-ß activation by DCs represents a “universal” mechanism of control of T cell differentiation in physiological and inflammatory settings. Finally, I aim to establish the molecular mechanisms by which DC subsets differentially regulate b8 expression. Together, these results could ultimately lead to new avenues in the development of therapeutics for IBD.

Factors from the intestinal microenvironment have a hierarchical role in regulating DC ß8 expression in vivo
By using both genetic and dietary manipulation in mouse models, we have demonstrated that ß8 expression in intestinal DCs is conditioned by immune and environmental (food, microflora) factors specifically derived from the intestinal microenvironment.

ß8 expression is differentially regulated in distinct DC subsets
To determine whether these factors from the mucosal microenvironment required for ß8 expression in vivo act by direct effects on DCs, we used an in vitro cell culture model we have developed using DC subsets of different origins. We have shown that upon stimulation with factors associated with the intestinal microenvironment, ß8 is rapidly induced to high sustained levels in DC subsets developmentally related to CD103+ intestinal DCs, while in other DC subsets, ß8 is only induced moderately and transiently. Together, these data show that cell lineage plays a critical role in permitting expression of ß8 in response to conditioning factors, and support the concept that the ability of DCs to express ß8 integrin is determined during development.

In Humans, ß8 integrin is also expressed by intestinal DCs
In preliminary studies, we have shown that ß8 integrin is expressed in DCs isolated from intestinal biopsies.

These studies should provide a detailed understanding of how DCs acquire a key immune function, the activation of TGF-ß, through the expression of avß8 integrin. It will provide critical insights on the mechanisms by which TGF-ß signaling in the gut is regulated spatially and temporally by avß8 integrins on DCs. Furthermore, I will investigate whether this function is disrupted in patients with IBD and if ß8 expression can be manipulated in DCs. Together, this will provide new information on how intestinal homeostasis is established and maintained, and will open new areas of research for the development of diagnostic tools and treatments to re-establish immune regulation in IBD.

Results from this project have been presented to several national and international conferences via poster and oral communications.
In addition, a manuscript presenting this work is currently under preparation and is to be submitted in the near future.

In the intestine, the immune system must maintain effective immune surveillance and defense against pathogens, whilst preventing inappropriate inflammatory responses to the multitude of commensal microbes. Breakdown of this balance between response and regulation underlies many forms of Inflammatory Bowel Disease (IBD). Induced regulatory T cells (iTregs) and T helper 17 cells cells (Th17) are important components of intestinal immune responses. Interestingly both iTreg and Th17 cells require the regulatory cytokine transforming growth factor-beta (TGF-beta) for their development and recent studies have emphasized the role of TGF-beta in maintaining gut homeostasis. TGF-beta is ubiquitously expressed but must be activated in order to enable subsequent signaling and functions. The activation of TGF-beta therefore represents a major point of regulation of intestinal immune responses.

We have shown that TGF-beta must be activated by alpha(v) integrins expressed on dendritic cells, before it can signal to T cells, which is critical for generation of iTreg and Th17 cells and maintenance of intestinal immune homeostasis. More recently, I’ve demonstrated that expression of the TGF-beta-activating integrin alpha(v)beta(8) is highly restricted to certain dendritic cell subpopulations. I’ve also shown that expression of beta(8) integrin defines the population of dendritic cells with the preferential ability to induce TGF-beta-dependant T cell responses and thus to control intestinal inflammation. However the mechanisms by which beta(8) integrin expression is regulated in intestinal dendritic cells are unknown.

The specific hypothesis of my proposal is that the ability of dendritic cells to activate TGF-beta and orchestrate intestinal immunity is regulated at the level of beta(8) expression. I propose a model in which a combination of lineage, environment and immune factors shapes intestinal dendritic cells into critical gatekeepers of TGF-beta-mediated immune signalling through regulation of beta(8) integrin expression.

In this project I propose to (i) further understand how beta(8) expression regulates immune response, (ii) decipher the mechanisms by which lineage and environment factors control beta(8) expression program and (iii) determine whether this program can be circumvented in order to modulate immune responses in the gut.

These studies will take advantage of cellular and animal models, as well as clinical studies. Therefore, this project will provide a detailed understanding of how dendritic cells acquire a key immune-regulatory function, the activation of TGF-beta. Furthermore, they will explore whether this is disrupted in patients with IBD and if beta(8) expression can be manipulated to re-establish immune regulation. Altogether results from this project will provide critical insights into how intestinal inflammatory disorders such as ulcerative colitis and Crohn’s disease may arise, and have the potential to lead to new therapeutic approaches for IBD treatment.