Keeping things calm in the intestinal mucosa.

Introduction

Dysregulated immunity in the intestine can result in the establishment of Inflammatory Bowel Disease (i.e. Crohn’s disease and Ulcerative Colitis).  This, in turn, can increase an individual’s cancer risk.

Elucidating the homeostatic mechanisms that govern mucosal immune responses and prevent inappropriate inflammatory responses can be key in understanding what can go wrong in IBD.

In this paper, the authors show that IFN-γ signalling in intestinal epithelial cells and the expression of extracellular ATPases by intraepithelial T cells act togther to regulate inflammatory responses in the bowel to pathogenic bacteria.  Pertubation of these key control mechanisms is shown to promote colitis.

Main Points

• Citrobacter rodentium infection results in acute colitis, characterised by colon shortening, crypt hyperplasia and infiltration of inflammatory cells into the mucosa.
• Pathological changes occur after 6 days and peak at 12 days, with subsequent disease resolution from 21 days post infection.
• Disease time-course corresponds with STAT 1 phosphorylation and expression of IRF1.
• ifngr^ΔIEC mice have intestinal epithelial cell-specific deletion of IFN-γ receptor 1.
• ifngr^ΔIEC mice show more severe colitis following infection with rodentium, associated with increased numbers of (newly recruited) intra-epithelial (IE) CD4⁺ T cells expressing IFN-γ, IL-17 and GM-CSF.
• Colitis in ifngr^ΔIEC mice is ameliorated by expression of IRF-1, T cell depletion or GM-CSF neutralisation.
• Infection is associated with the release of intracellular ATP, resulting in NLRP3 activation in macrophages, via P2X7 receptor signalling and activation of IL-1β by caspase 1. ifngr^ΔIEC mice show increased extracellular ATP in response to C rodentium.
• Exogenous ATPase administration, inflammasome inhibition and IL-1R antagonism reduce levels of IL-1β and GM-CSF and the number of IE CD4⁺ T cells in colon samples from ifngr^ΔIEC Depletion of macrophages has a similar effect.
• In mice with normal IFN-γ signalling, administration of non-hyrolysable ATP results in more severe colitis
• Antigen presentation by macrophages drives colitis development in ifngr^ΔIEC Specific deletion of MCHII in macrophages but not cDC2 prevents colitis-associated large bowel shortening and reduces the number of GM-CSF⁺ IE CD4⁺ T cells.
• Antigen presentation by intestinal epithelial cells suppresses rodentium infection-mediated pathology. This is enabled by upregulation of both MHCI and MHCII in response to IFN-γ.
• Epithelial cell antigen presentation upregulates extracellular (EC) ATPase expression by CD4+ and CD8αβ⁺ IE T cells.
• Specific deletion of either MHCI or MHCII in the epithelium disrupts ATPase up-regulation in CD8αβ⁺ and CD4⁺ IE T cells. ifngr^ΔIEC mice also demonstrate reduced EC-ATPase expression following infection.
• The importance of IFN-γ signalling in modulating colitis is also seen in the DSS-induced acute colitis model, with ifngr^ΔIEC mice showing increased susceptibility to colitis, associated with increased EC-ATP and more GM-CSF-producing CD4+ IE T cells, with reductions in the expression of MHCI and MHC II by epithelial cells and ATPases by IE T cells. Like the rodentium model, colitis development is ameliorated by a range of interventions including NLRP3 inhibition, exogenous ATPase administration and neutralisation of GM-CSF.
• Furthermore, IFN-γ signalling in the epithelium limits the effect of chronic colitis in promoting tumorigenesis, in the DSS/Azoxymethane model of colorectal carcinogenesis.

Concluding Remarks

IFN-γ secretion by macrophages promotes antigen presentation by intestinal epithelial cells, which is key in dampening and resolving inflammatory responses by intra-epithelial CD4⁺ T cells to antigens presented by macrophages.  Disruption of this communication between macrophages, the  epithelium and the T cells increases the chances of developing severe acute and chronic inflammation, with the latter leading to an increased risk of cancer.

Epistem Services

Epistem offer a range of models that allow researchers to look at the importance of different cell types for the development and maintenance of inflammatory bowel disease.  Models of both acute and chronic colitis are available. Key cell populations can be identified by immuno-labelling techniques and can be administered to control disease, or isolated for phenotyping and/or short-term culture and functional assays.

A range of analytical platforms are available to support this work, including histology and quantitative image analysis, flow cytometry, luminex-based multiplex analysis of cytokines and chemokines and

gene expression analysis