Role of methyl-CpG-binding domain protein-2 (MBD2) in colonic inflammation
The human GI tract has evolved to simultaneously absorb nutrients and be the frontline in host defence. These seemingly mutually exclusive goals are achieved by a single cell thick epithelial barrier, and a complex resident immune system which lives in symbiosis with the intestinal microflora and is also able to rapidly respond to invading pathogens. An immunological balance is therefore required to permit tolerance to the normal intestinal microflora, but also prevent the dissemination of pathogenic micro-organisms to the rest of the host. Inappropriate immune responses in genetically susceptible individuals are the hallmark of human inflammatory bowel disease (IBD) and are thus targeting effector immune cells and their cytokines remains the mainstay of treatment. However despite vigorous efforts to delineate the genetic contribution to IBD disease susceptibility using large multinational cohorts, the majority of disease heritability remains unknown. Epigenetics describes heritable changes in chromatin that are not conferred by DNA sequence. These incorporate changes to histones, chromatin structure and DNA methylation, which confer changes to gene transcription and thus gene expression and cellular function. Methylbinding proteins (MBD) have the ability to bind to methylated DNA and recruit large chromatin remodeling complexes that underpin a variety of epigenetic modifications. Methyl- CpG-binding domain protein 2 (MBD2) is one such MBD that is required for appropriate innate (dendritic cell) and adaptive (T cell) immune function, though its role has not been investigated in the GI tract. We hypothesized that alterations in chromatin are central to the reprogramming of normal gene expression that occurs in disease states. By defining the phenotype of immune cells in the absence of MBDs we hope to understand the mechanisms of chromatin-dysregulation that lead to immune-mediated diseases such as IBD. We therefore aimed to assess the role of MBD2 in colon immune cells in the steady state and in murine models of GI tract inflammation, thereafter identifying the culprit cell types and genes responsible for any observed changes. We envisaged that investigating heritable, epigenetic changes in gene expression that are inherently more amenable to environmental manipulation than our DNA code, may provide novel insight to a poorly understood mechanism of disease predisposition. In addition identifying the cellular and gene targets of Mbd2 mediated changes to immune homeostasis that may provide exciting and novel approaches to therapeutic modulation of pathological inflammatory responses. In chapter 3 we assessed the expression of Mbd2/MBD2 in the murine/human GI tract. Consistent with existing mouse data, levels of Mbd2 mRNA increased between anatomical divisions of small (duodenum, ileum, terminal ileum) and large intestine (caecum, colon, rectum). In addition MBD2 mRNA was greater in the rectum versus ileum, with active IBD associated with lower rectal MBD2 mRNA compared to quiescent IBD controls. Thus we sought to understand the role of Mbd2 in the colon, where mRNA levels were the highest in the GI tract and where appropriate immune function is central to prevent damaging inflammation. To address these aims required the development of existing methods of cell surface marker expression analysis using flow cytometry techniques to simultaneously identify multiple innate and adaptive immune populations. Using naïve Mbd2 deficient mice (Mbd2-/-) we observed CD11b+ CD103+ DCs were significantly reduced in number in Mbd2 deficiency. To understand the role of Mbd2 in colonic inflammation we employed a mouse model of chemical (DSS) and infectious (T. gondii) colitis comparing Mbd2-/- and littermate controls (WT). Mbd2-/- were extremely sensitive to DSS and T. gondii mediated colonic inflammation, characterized by increased symptom score, weight loss and histological score of tissue inflammation (DSS) and increased antibody specific cytokine responses (T. gondii) in Mbd2 deficient animals. Flow cytometry analysis of colon LP cells in both infectious and chemical colitis revealed significant accumulation of monocytes and neutrophils in Mbd2-/-. Indeed monocytes and neutrophils were the principal myeloid sources of IL-1b and TNF in DSS colitis and the number of IL-1b/TNF+ monocytes/neutrophils was significantly greater in Mbd2-/-. Lastly we employed our colon LP isolation techniques to analyse immune populations in active and quiescent IBD and healthy controls, using endoscopically acquired biopsy samples. Analysis revealed that as in murine colitis, active human IBD is characterized by the accumulation of CD14High monocyte-like cells, with an associated increased ratio of macrophage:monocyte-like cells. In Chapter 4 we sought to understand the cellular sources of Mbd2 that may explain the predisposition of Mbd2-/- to colitis. Firstly we restricted Mbd2 deficiency to haematopoietic cells using grafting Mbd2-/- bone marrow (BM) into lethally irradiated WT mice. These animals treated with DSS displayed increased weight loss, symptom score, neutrophil accumulation and histopathology score compared to mice irradiated and grafted with WT BM. Given the accumulation of monocytes in Mbd2-/- DSS treated mice, and existing literature supporting a pathogenic role in this model, we then investigated the role of Mbd2 in monocyte function. Colon monocytes sorted from Mbd2-/- and WT DSS treated mice displayed similar expression for many pro-inflammatory genes (Il6, Il1a, Il1b, Tnf), but demonstrated significantly dysregulated expression for some others (Regb, Lyz1, Ido1, C4a). To investigate this in a more refined model, we lethally irradiated WT mice and repopulated them with a WT:Mbd2-/- BM mix. This enabled the analysis of WT and Mbd2-/- haematopoietic cells in the same animal. Colon WT and Mbd2-/- monocyte recruitment and cytokine production in DSS treated mixed BM chimeras was equivalent between genotypes suggesting that Mbd2 deficiency in monocytes alone did not explain the increased susceptibility of Mbd2-/- to DSS colitis. We then restricted Mbd2 deficiency to CD11c expressing cells, given the known role for Mbd2 in their function, and for CD11c+ cells in DSS, using a CD11cCreMbd2Fl/Fl system. DSS treated mice with Mbd2 deficient CD11c+ cells demonstrated increased weight loss, symptoms score, histolopathology score, monocyte and neutrophil colon accumulation compared to controls. To further explore the role of Mbd2 in colon CD11c+ cells, macrophage and DCs from DSS treated WT and Mbd2-/- mice were purified and their gene expression analysed. Mbd2-/- versus WT macrophages demonstrated significantly altered expression of both pro- (Il1a, C6, Ido1, Trem2) and antiinflammatory (Tgfbi, Retnla) pathways that we hypothesized was a method for attempted host control of excessive colon damage in Mbd2-/- mice. DC gene expression analysis was hampered by small sample size, but demonstrated a large number of small expression changes, including IL-12/IL-23 (Jak2) and autophagy (Lrrk2) pathways. Lastly levels of costimualtory molecules (CD40/CD80) were increased in Mbd2-/- but not CD11cΔMbd2 colon LP DCs/macrophages suggesting that non-CD11c+ cellular sources of Mbd2 were required to produce increased activation phenotype in these cells. Finally in Chapter 5 we explored the role for Mbd2 in non-haematopoietic cells, namely the colonic epithelium. Here we first developed a novel method for identifying and purifying these cells using flow cytometry. Mbd2 deficient colonic epithelium demonstrated increased expression of activation markers MHC II and LY6A/E in the steady state and in DSS / T. muris mediated colonic inflammation. Indeed FACS purified colon epithelial cells from naive and DSS treated, Mbd2-/- and WT mice revealed conserved dysregulated gene expression independent of inflammation: Both naïve and inflamed Mbd2 deficient epithelium displayed significantly increased expression of genes responsible for antigen processing/presentation (MHC I, MHC II, immunoproteasome) and decreased expression of genes involved in cell-cell adhesion (Cldn1, Cldn4). Lastly we investigated whether the observed differences in Mbd2-/- cell types conferred alterations in the makeup of the intestinal microflora. Interestingly independent of co-housing of Mbd2-/- and WT animals, Mbd2 deficiency consistently predicted the microbial composition, with increased levels of Clostridales and decreased levels of Parabacteroides bacteria. Collectively we have identified CD11c+ cells, monocytes and colon epithelial cells as key cell types for Mbd2 mediated changes in gene expression that affect mucosal immune responses. These data thus identify Mbd2 gene targets within these cell types as exciting new areas for investigation and therapeutic modulation to limit damaging GI tract inflammation.