Simon R. Carding

Inflammatory bowel disease: cause and immunobiology.

Crohns disease and ulcerative colitis are idiopathic inflammatory bowel disorders. In this paper, we discuss how environmental factors (eg, geography, cigarette smoking, sanitation and hygiene), infectious microbes, ethnic origin, genetic susceptibility, and a dysregulated immune system can result in mucosal inflammation. After describing the symbiotic interaction of the commensal microbiota with the host, oral tolerance, epithelial barrier function, antigen recognition, and immunoregulation by the innate and adaptive immune system, we examine the initiating and perpetuating events of mucosal inflammation. We pay special attention to pattern-recognition receptors, such as toll-like receptors and nucleotide-binding-oligomerisation-domains (NOD), NOD-like receptors and their mutual interaction on epithelial cells and antigen-presenting cells. We also discuss the important role of dendritic cells in directing tolerance and immunity by modulation of subpopulations of effector T cells, regulatory T cells, Th17 cells, natural killer T cells, natural killer cells, and monocyte-macrophages in mucosal inflammation. Implications for novel therapies, which are discussed in detail in the second paper in this Series, are covered briefly.

γδ T cells: functional plasticity and heterogeneity

γδ T cells remain an enigma. They are capable of generating more unique antigen receptors than αβ T cells and B cells combined, yet their repertoire of antigen receptors is dominated by specific subsets that recognize a limited number of antigens. A variety of sometimes conflicting effector functions have been ascribed to them, yet their biological function(s) remains unclear. On the basis of studies of γδ T cells in infectious and autoimmune diseases, we argue that γδ T cells perform different functions according to their tissue distribution, antigen-receptor structure and local microenvironment; we also discuss how and at what stage of the immune response they become activated.

The relationship of IL-4- and IFNγ-producing T cells studied by lineage ablation of IL-4-producing cells

Subsets of CD4 T cells are defined by the cytokines that they produce; these cytokines determine the effector function of these cells. Cloned CD4 T cells fall into two subsets, producing either interferon-gamma (IFN gamma) or interleukin-4 (IL-4) in combination with other cytokines, and are called Th1 and Th2 cells, respectively. The lineage relationship between naive T cells and effector Th1- and Th2-type cells is unclear. We generated transgenic mice in which IL-4-producing cells express herpes simplex virus 1 thymidine kinase and are eliminated by ganciclovir (GANC). Activation of transgenic T cells in the presence of GANC eliminates IL-4 and IFN gamma production, showing that IL-4- and IFN gamma-producing cells express or have expressed IL-4. These results show that effector cells producing either IL-4 or IFN gamma have a common precursor, which expresses the IL-4 gene.

CD4+ T Cells: Specificity and Function

The majority of T lymphocytes, and those best-characterized at the present time, recognize foreign antigens as peptide fragments associated with self major histocompalibility complex (MHC)-encoded proteins. The reason for this preoccupation of T cells with MHC-encoded cell surface molecules is not presently understood, and controversy surrounds many aspects of MHC restriction in T cell specificity. Both T cells and MHC molecules can be subdivided into two classes. MHC class I molecules are found on the surface of most somatic cells, while MHC class II molecules are expressed selectively on the surfaces of cells involved in immune responses, such as B cells and macrophages. T cells can be subdivided by the cell surface expression of the CD4 and CD8 markers: CD4 T cells recognize foreign protein antigen fragments associated with self class II MHC molecules, while CDS T cells recognize foreign protein antigen fragments associated with self class I MHC molecules. T cells also respond to polymorphic differences in MHC molecules. Recognition of allogeneic or non-self MHC molecules largely follows the pattern of foreign antigen recognition, with CD4 T cells predominantly recognizing class II MHC polymorphisms, and CD8 T cells largely recognizing class I MHC polymorphisms. These responses to non-self MHC molecules involve a very high proportion of normal T cells, it being estimated that 1-10% of T cells will respond to a particular non-self MHC molecule. This surprising commitment of T cells to recognition of MHC molecules not normally encountered requires a biologically satisfying explanation. These fmdings raise several questions: First, why is expression of CD4 associated with recognition of foreign antigen in association with self class II MHC molecules? Second, how can one account for the high frequency of alloreactive T cells? And third, why

Dysbiosis of the gut microbiota in disease

There is growing evidence that dysbiosis of the gut microbiota is associated with the pathogenesis of both intestinal and extra-intestinal disorders. Intestinal disorders include inflammatory bowel disease, irritable bowel syndrome (IBS), and coeliac disease, while extra-intestinal disorders include allergy, asthma, metabolic syndrome, cardiovascular disease, and obesity. In many of these conditions, the mechanisms leading to disease development involves the pivotal mutualistic relationship between the colonic microbiota, their metabolic products, and the host immune system. The establishment of a ‘healthy’ relationship early in life appears to be critical to maintaining intestinal homeostasis. Whilst we do not yet have a clear understanding of what constitutes a ‘healthy’ colonic microbiota, a picture is emerging from many recent studies identifying particular bacterial species associated with a healthy microbiota. In particular, the bacterial species residing within the mucus layer of the colon, either through direct contact with host cells, or through indirect communication via bacterial metabolites, may influence whether host cellular homeostasis is maintained or whether inflammatory mechanisms are triggered. In addition to inflammation, there is some evidence that perturbations in the gut microbiota is involved with the development of colorectal cancer. In this case, dysbiosis may not be the most important factor, rather the products of interaction between diet and the microbiome. High-protein diets are thought to result in the production of carcinogenic metabolites from the colonic microbiota that may result in the induction of neoplasia in the colonic epithelium. Ever more sensitive metabolomics methodologies reveal a suite of small molecules produced in the microbiome which mimic or act as neurosignallers or neurotransmitters. Coupled with evidence that probiotic interventions may alter psychological endpoints in both humans and in rodent models, these data suggest that CNS-related co-morbidities frequently associated with GI disease may originate in the intestine as a result of microbial dysbiosis. This review outlines the current evidence showing the extent to which the gut microbiota contributes to the development of disease. Based on evidence to date, we can assess the potential to positively modulate the composition of the colonic microbiota and ameliorate disease activity through bacterial intervention.

Cytokines in T-cell development

The thymus provides a unique environment for the development of T cells, supporting both precursor cell proliferation and differentiation. The control of these processes is unknown but they may be mediated by cytokines, or other soluble factors, or by interactions with specific elements of the thymic stroma. Here, Simon Carding, Adrian Hayday and Kim Bottomly describe cellular, immunochemical and molecular studies of the production and action of cytokines within the human and mouse thymus and demonstrate their essential role in T-cell development.

Colonic dendritic cells, intestinal inflammation, and T cell-mediated bone destruction are modulated by recombinant osteoprotegerin.

Autoimmune associated bone disease and intestinal inflammation are closely linked with deregulation and hyperactivation of autoreactive CD4 T cells. How these T cells are activated and mediate disease is not clear. Here we show that in the Interleukin 2-deficient mouse model of autoimmunity spontaneous osteopenia and colitis are caused by increased production of the ligand for receptor activator of NFkappaB (RANKL). RANKL acting via its receptor, receptor activator of NFkappaB (RANK), increases bone turnover and promotes intestinal dendritic cell (DC) survival in vivo. Modulation of RANKL-RANK interactions with exogenous recombinant osteoprotegerin (Fc-OPG) reverses skeletal abnormalities and reduces colitis by decreasing colonic DC numbers. This study identifies a common causal link between bone disease and intestinal inflammation and establishes the importance of DC in mediating colonic inflammation in vivo.

The importance of gd T cells in the resolution of pathogen‐induced inflammatory immune responses

Summary: The aim of our research is to determine the biological function of gd T lymphocytes and in particular the role they play in microbial immunity. Although evidence of gd T‐cell activation and expansion has been obtained from numerous infectious diseases, how they contribute to pathogen‐induced immune responses is still not clear. Based upon extensive studies of gd T‐cell involvement in the immune response to viral and bacterial pathogens in both mice and humans, we have uncovered evidence of their direct involvement in terminating host immune responses to infection and preventing chronic disease. We have identified an interaction between peripheral gd T cells and a population of activated, pro‐inflammatory macrophages elicited by infection that occurs late in the course of infection during or after pathogen clearance. As a result of this interaction, activated gd T cells acquire cytotoxic activity and kill the stimulatory macrophages, leading us to propose a model for gd T‐cell–macrophage interactions that contributes to macrophage homeostasis, the resolution of inflammatory immune responses, and prevention of chronic inflammatory disease.

Crohn disease: A current perspective on genetics, autophagy and immunity

Crohn disease (CD) is a chronic and debilitating inflammatory condition of the gastrointestinal tract.1 Prevalence in western populations is 100–150/100,000 and somewhat higher in Ashkenazi Jews. Peak incidence is in early adult life, although any age can be affected and a majority of affected individuals progress to relapsing and chronic disease. Medical treatments rely significantly on empirical corticosteroid therapy and immunosuppression, and intestinal resectional surgery is frequently required. Thus, 80% of patients with CD come to surgery for refractory disease or complications. It is hoped that an improved understanding of pathogenic mechanisms, for example by studying the genetic basis of CD and other forms of inflammatory bowel diseases (IBD), will lead to improved therapies and possibly preventative strategies in individuals identified as being at risk.

Gamma/delta T lymphocytes in viral Infections

T lymphocyte progenitors differentiate into two distinct T cell lineages. Although the αβ and γδ T cell lineages resemble each other phenotypically and functionally, there are some striking differences. Some γδ T cells recognize, similarly to αβ T cells, peptides presented by major histocompatibility complex (MHC) proteins or MHC‐like molecules. However, there are γδ T cells that recognize MHC molecules in a fundamentally different manner in comparison with αβ T cells. Also in contrast to aLβ T lymphocytes, many γδ T cells are capable of recognizing nonpeptide antigens. Most responses of γδ T cells appear to be directed against microbial pathogenic agents including bacteria, parasites, and viruses. In particular, the potent cytotoxic responses of γδ T cells against cells infected with, for example, herpesviruses or lentiviruses may be essential for the overall antiviral defense of vertebrates. The analysis of antiviral immunosurveillance by γδ T cells is crucial for understanding the unique biological role of this lymphocyte subset.