Proliferative recruitment of intestinal intraepithelial lymphocytes after microbial colonization of germ-free mice

Bacterial translocation (BT) in cirrhosis

Gut flora and bacterial translocation (BT) play an important role in the pathogenesis of the complications of cirrhosis. Research on the pathogenesis of BT and its clinical significance transcends established boundaries between microbiology, cell biology, intestinal pathophysiology, and immunology. This review delineates multiple mechanisms involved in the process of BT, with an emphasis on alterations in intestinal flora and mucosal barrier function, particularly immunological defense mechanisms. Current knowledge on the innate and adaptive immune response that allows a "friendly" communication between bacteria and host is summarized, and alterations occurring in cirrhosis that may facilitate BT are discussed. In addition, definition of a "pathological" BT is proposed together with an analysis of the anatomical site and route of BT. Finally, therapeutic approaches for the prevention of BT in experimental and human cirrhosis are reviewed. Future research in the field of BT in cirrhosis will allow the development of new therapeutic targets in the prevention of infections and other complications of cirrhosis.

Antigen-induced regulatory T cells in autoimmunity

The ultimate goal of any treatment for autoimmune diseases is antigen- and/or site-specific suppression of pathology. Autoaggressive lymphocytes need to be eliminated or controlled to prevent tissue damage and halt the progression of clinical disease. Strong evidence is emerging that the induction of regulatory T (T(Reg)) cells by autoantigens can suppress disease, even if the primary, initiating autoantigens are unknown and if inflammation is progressive. An advantage of these autoreactive T(Reg) cells is their ability to act as bystander suppressors and dampen inflammation in a site-specific manner in response to cognate antigen expressed locally by affected tissues. In this review, we consider the nature and function of such antigen-specific T(Reg) cells, and strategies for their therapeutic induction are discussed.

Development and compartmentalization of the porcine TCR delta repertoire at mucosal and extraintestinal sites: the pig as a model for analyzing the effects of age and microbial factors

gammadelta T cells are an important component of the mucosal immune system. Previously, we have shown that the TCR delta repertoire in human intestine is polyclonal at birth and becomes increasingly restricted with age. In this study, we expand those studies to the pig which allows more extensive experiments including several organs. Tissues from different mucosal sites like the stomach, duodenum, ileum, Peyer's patches, jejunum, and colon, and also extraintestinal sites like the lung, spleen, thymus and mesenteric lymph nodes, were obtained from conventionally reared pigs aged 2 wk to 5.5 years. In addition, tissues were also obtained from 10-wk-old specified pathogen- and germ-free pigs. TCRDV1-DV5 transcripts were amplified by RT-PCR after which complementarity-determining region 3 spectratyping was performed. Individual bands were excised from the gels and directly sequenced. The intestinal TCR delta repertoire showed increasing restriction with age and was highly oligoclonal in the adult 2- to 5.5-year-old pigs. In old pigs, we observed a striking compartmentalization. Different TCR delta repertoires were present between the lungs and the intestinal mucosa but also within different parts of the gastrointestinal tract. However, occasionally we observed identical TCR delta transcripts in the intestine and the lungs and shared clones could be detected also along the entire gastrointestinal tract. Thus, subsets of gammadelta T cells are likely to transport immunological information between different compartments of the immune system. Furthermore, these data support the hypothesis that in each mucosal site, different Ags are responsible for selecting and maintaining the gammadelta TCR over time.

The role of the gut flora in health and disease, and its modification as therapy

The gut flora is a vast interior ecosystem whose nature is only beginning to be unravelled, due to the emergence of sophisticated molecular tools. Techniques such as 16S ribosomal RNA analysis, polymerase chain reaction amplification and the use of DNA microarrays now facilitate rapid identification and characterization of species resistant to conventional culture and possibly unknown species. Life-long cross-talk between the host and the gut flora determines whether health is maintained or disease intervenes. An understanding of these bacteria-bacteria and bacteria-host immune and epithelial cell interactions is likely to lead to a greater insight into disease pathogenesis. Studies of single organism-epithelial interactions have revealed the large range of metabolic processes that gut bacteria may influence. In inflammatory bowel diseases, bacteria drive the inflammatory process, and genetic predisposition to disease identified to date, such as the recently described NOD2/CARD15 gene variants, may relate to altered bacterial recognition. Extra-intestinal disorders, such as atopy and arthritis, may also have an altered gut milieu as their basis. Clinical evidence is emerging that the modification of this internal environment, using either antibiotics or probiotic bacteria, is beneficial in preventing and treating disease. This natural and apparently safe approach holds great appeal.

Evidence for the extrathymic origin of intestinal TCRgammadelta(+) T cells in normal rats and for an impairment of this differentiation pathway in BB rats

The BB rat lyp mutation, one of its diabetes susceptibility genes, is responsible for a 5-fold decrease in the number of peripheral TCRalphabeta(+) T cells. In this study we show that TCRgammadelta(+) T cells are virtually undetectable among splenic T cells and intestinal intraepithelial T lymphocytes (IEL) of BB rats, while they account for 3 and 30% of these two T cell populations, respectively, in normal animals. It has been shown that murine IEL expressing TCRgammadelta develop extrathymically. We determined whether this is the case in rats. Athymic radiation chimeras reconstituted with normal hemopoietic precursors were devoid of donor-derived TCRalphabeta(+) T cells and TCRgammadelta(+) splenocytes but contained a normal number of TCRgammadelta(+) IEL, suggesting that in unmanipulated rats some of the TCRgammadelta(+) IEL may have an extrathymic origin. This was further supported by the observation that RAG1 transcripts are present in IEL of unmanipulated animals. No T cells developed in chimeras reconstituted with BB hemopoietic precursors, demonstrating that the BB rat lyp mutation inhibits both intrathymic and extrathymic development of TCRgammadelta(+) T cells.

Transgenic mouse model of intestine-specific mucosal injury and repair

Most studies of injury and repair to mucosal tissue have used nonspecific mediators to induce injury. Damage to the mucosal epithelium resulting from chemical or radiation treatment associated with cancer therapy may fall into this category of injury. When such treatments are applied, it is generally not possible to predict or control the extent of possible injury. This fact makes analysis of inductive and reparative processes difficult. In addition, the role of the immune system in the etiology and subsequent healing of mucosal tissue following cancer therapy with or without bone marrow transplantation remains unclear. To study tissue- and antigen-specific immune damage of intestinal mucosal tissue, we generated transgenic mice that express a nominal antigen exclusively in intestinal epithelial cells. The transfer of antigen-specific CD8 T cells with concomitant virus infection resulted in the destruction of intestinal epithelial cells and disease. The destructive phase in some cases was followed by complete recovery and tolerance induction. This model will provide a system that can be regulated for analysis of the mediators of mucosa-specific tissue damage and repair.

Structure of the intestinal flora responsible for development of the gut immune system in a rodent model

The intestinal flora comprising indigenous, autochthonous bacteria is constantly present in the alimentary tract of host animals, including humans. The indigenous bacteria greatly affect the structure and functions of the intestinal mucosa. Studies involving gnotobiotic mice or rats have shown that the presence of limited kinds of intestinal bacteria is responsible for the development of the gut immune system, such as secretory IgA, major histocompatibility complex molecules and intraepithelial lymphocytes. Understanding of the structure of the intestinal flora or the organization of the microbial population in the intestine, based on evaluation of the immunological responses, may clarify its functions in the host animal.

Regional variations in the distribution of small intestinal intraepithelial lymphocytes in three inbred strains of mice

The regional variation in the intraepithelial lymphocytes (IELs) in the small intestine was examined in BALB/c male and female mice and C3H/He and C57BL/6 male mice. The small intestines were taken from 11 to 12-week-old mice and divided equally into 3 parts (the proximal, middle and distal parts). IELs were isolated from each part of the intestine and analyzed with flow cytometer. The number of IELs was highest in the proximal part and lowest in the distal part. The distribution of IEL subsets was markedly different between the proximal and the distal parts, and that in the middle part showed the intermediate pattern. The percentage of alphabeta T cells were higher in the distal part. In alphabeta T cell subset, the percentage of CD8alphaalpha T cells was higher in the proximal part, whereas those of CD4 and CD4CD8alphaalpha double positive T cells were higher in the distal part. In gammadelta T cell subset, no regional variations were found. The regional variations in the number and subsets of IELs showed almost the same patterns between male and female BALB/c mice and similar patterns among three strains of mice. This strongly suggests that the regional variations in the small intestinal IELs are common to mouse species.

Speculation on the lineage relationships among CD4(-)8(+) gut-derived T cells and their role(s)

The thymus-independent T lymphopoietic capacity of the murine intestinal mucosa has been established. Cryptopatches have now been identified as the location of the elusive precursors for gut-derived T cells. These cryptopatch cells have been shown to give rise to intestinal T cells expressing either TCRgammadelta or TCRalphabeta. Here we discuss the role of MHC in the development and selection of gut-derived T cells. Through the analysis of iIEL selection in animals expressing a transgenic TCRalphabeta, in the presence or absence of p56(lck), we discuss lineage relationships among CD4(-)8(+) iIEL subsets, and their possible function(s).

Differential roles of segmented filamentous bacteria and clostridia in development of the intestinal immune system

The presence of microflora in the digestive tract promotes the development of the intestinal immune system. In this study, to evaluate the roles of two types of indigenous microbe, segmented filamentous bacteria (SFB) and clostridia, whose habitats are the small and large intestines, respectively, in this immunological development, we analyzed three kinds of gnotobiotic mice contaminated with SFB, clostridia, and both SFB and clostridia, respectively, in comparison with germfree (GF) or conventionalized (Cvd) mice associated with specific-pathogen-free flora. In the small intestine, the number of alpha beta T-cell receptor-bearing intraepithelial lymphocytes (alpha betaIEL) increased in SFB-associated mice (SFB-mice) but not in clostridium-associated mice (Clost-mice). There was no great difference in Vbeta usage among GF mice, Cvd mice, and these gnotobiotic mice, although the association with SFB decreased the proportion of Vbeta6(+) cells in CD8beta- subsets to some extent, compared to that in GF mice. The expression of major histocompatibility complex class II molecules on the epithelial cells was observed in SFB-mice but not in Clost-mice. On the other hand, in the large intestine, the ratio of the number of CD4(-) CD8(+) cells to that of CD4(+) CD8(-) cells in alpha betaIEL increased in Clost-mice but not in SFB-mice. On association with both SFB and clostridia, the numbers and phenotypes of IEL in the small and large intestines changed to become similar to those in Cvd mice. In particular, the ratio of the number of CD8alpha beta+ cells to that of CD8alpha alpha+ cells in alpha betaIEL, unusually elevated in the small intestines of SFB-mice, decreased to the level in Cvd mice on contamination with both SFB and clostridia. The number of immunoglobulin A (IgA)-producing cells in the lamina propria was more elevated in SFB-mice than in Clost-mice, not only in the ileum but also in the colon. The number of IgA-producing cells in the colons of Clost-mice was a little increased compared to that in GF mice. Taken together, SFB and clostridia promoted the development of both IEL and IgA-producing cells in the small intestine and that of only IEL in the large intestine, respectively, suggesting the occurrence of compartmentalization of the immunological responses to the indigenous bacteria between the small and large intestines.

Physiological roles of gammadelta T-cell receptor intraepithelial lymphocytes in cytoproliferation and differentiation of mouse intestinal epithelial cells

In this study we aimed to elucidate the physiological role of gammadelta intraepithelial lymphocytes (IEL) in the mouse intestine. For this purpose, we used T-cell receptor (TCR) Vgamma4/Vdelta5 transgenic mice (KN 6 Tg: BALB/c background, H-2d), and compared the immunological and physiological characteristics of the intestinal tracts of KN 6 Tg and non-transgenic (non-Tg) littermates. In KN 6 Tg littermates, 95% of small intestinal (SI) and large intestinal (LI) IEL expressed gammadelta TCR, and their TCR was replaced by Tg gammadelta TCR. In these mice, class II major histocompatibility complex (MHC) expression was up-regulated in the SI epithelium, compared with the non-Tg littermates, under specific pathogen-free (SPF) conditions. Competitive reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that the mRNAs of the I-Ealpha chain on the SI epithelial cells was higher in KN 6 Tg than in non-Tg littermates. However, in the LI, class II MHC molecules were not expressed in either KN 6 Tg or non-Tg littermates. The epithelial cell mitotic index in the SI, but not in the LI, was higher in KN 6 Tg than in non-Tg littermates under SPF conditions. However, differentiation markers for SI epithelial cells, such as alkaline phosphatase and disaccharidase (lactase, maltase and sucrase) activities, were similar in KN 6 Tg and non-Tg littermates. MHC class II molecule expression on the SI epithelium was absent in germ-free (GF) Tg mice, but was induced under SPF conditions, coinciding with the increase of interferon-gamma (IFN-gamma) mRNA in gammadelta TCR SI-IEL. These findings suggest that gammadelta TCR IEL regulate epithelial cell regeneration and class II MHC expression, but not cell differentiation in the SI. However, these functions were not observed in the gammadelta TCR IEL in the LI. In addition, the activation step in the gammadelta TCR SI-IEL is dependent on the presence of gut microflora.

Diurnal changes in intraepithelial lymphocytes (IELs) in the small intestine of mice

Diurnal changes in small intestinal intraepithelial lymphocytes (IELs) were examined in 8- to 10-week-old BALB/cA male mice. The ratio of T cell subsets expressing CD8 alpha alpha homodimer/CD8 alpha beta heterodimer was found to be higher in the dark period than that in the light period. Increased expression of Thy-1.2 on gamma delta T cells was also observed in the light period. No significant changes were found in other subsets. This is the first report to document diurnal changes in the small intestinal IELs in mice.

Creating and maintaining the gastrointestinal ecosystem: what we know and need to know from gnotobiology

Studying the cross talk between nonpathogenic organisms and their mammalian hosts represents an experimental challenge because these interactions are typically subtle and the microbial societies that associate with mammalian hosts are very complex and dynamic. A large, functionally stable, climax community of microbes is maintained in the murine and human gastrointestinal tracts. This open ecosystem exhibits not only regional differences in the composition of its microbiota but also regional differences in the differentiation programs of its epithelial cells and in the spatial distribution of its component immune cells. A key experimental strategy for determining whether "nonpathogenic" microorganisms actively create their own regional habitats in this ecosystem is to define cellular function in germ-free animals and then evaluate the effects of adding single or several microbial species. This review focuses on how gnotobiotics-the study of germ-free animals-has been and needs to be used to examine how the gastrointestinal ecosystem is created and maintained. Areas discussed include the generation of simplified ecosystems by using genetically manipulatable microbes and hosts to determine whether components of the microbiota actively regulate epithelial differentiation to create niches for themselves and for other organisms; the ways in which gnotobiology can help reveal collaborative interactions among the microbiota, epithelium, and mucosal immune system; and the ways in which gnotobiology is and will be useful for identifying host and microbial factors that define the continuum between nonpathogenic and pathogenic. A series of tests of microbial contributions to several pathologic states, using germ-free and ex-germ-free mice, are proposed.

Expression of the mucosal gamma delta T cell receptor V region repertoire in patients with IgA nephropathy

IgA nephropathy (IgAN) is characterized by the deposition of IgA in the glomerular mesangium and often leads to progressive renal dysfunction and kidney failure. We have previously shown that the mesangial IgA is likely to derive from the bone marrow plasma cells, and suggested that a primary abnormality within the mucosal immune system may underly the pathogenesis of IgAN. This study has analyzed the T cell receptor (TCR) variable (V) region expression by gamma delta T cells in the intestinal mucosa of patients with IgAN. The V gamma and V delta usage of TCR transcripts was determined using a semiquantitative reverse transcriptase-PCR protocol. Primers specific for the four human V gamma and six V delta subfamilies were used each with a constant (C) gamma or C delta specific primer, and the PCR-amplified TCR transcripts were detected by Southern blotting and oligonucleotide hybridization. gamma delta TCR V region expression was determined in gut biopsies and peripheral blood of 11 patients with IgAN, and the TCR V gamma and V delta repertoires were compared to those in gut and peripheral blood of 11 control individuals. gamma delta T cells in normal blood predominantly expressed V gamma 2 (V gamma 9 gene) and V delta 2 gene segments whereas those in normal gut mainly expressed V gamma 3 and V delta 3. In IgAN patients, V delta 2 was also the predominant V delta gene utilized by peripheral blood gamma delta T cells, however, we observed a predominance of V gamma 3 and reduced V gamma 2 usage by these cells. gamma delta T cells in the gut of IgAN patients mainly used V gamma 3 and V delta 1. While the gamma and delta TCR V region repertoires did not differ significantly between the peripheral blood of patients and controls, there were significant differences in V gamma and V delta repertoire expression between IgAN and control gut biopsies. V gamma 3 gene expression was significantly decreased in IgAN gut compared to control gut (P = 0.023). In addition, there was a significant decrease in V delta 3 gene expression in IgAN gut compared to control gut (P = 0.043). These findings indicate that a subpopulation of gamma delta T cells, which represent the majority of gamma delta T cells in normal gut mucosa, are significantly diminished in the gut of patients with IgAN. This suggests that a "hole" in the mucosal gamma delta T cell repertoire may play a fundamental role in contributing to the pathogenesis of IgAN.

The molecular basis of intractable diarrhoea of infancy

The intractable diarrhoeas of infancy present very major problems of clinical management. However, the conceptual importance of these conditions lies in the information that they may provide about normal small-intestinal function in humans: among such infants will be found the human equivalents of the 'knock-out' mice, in which targeted gene disruption allows sometimes unexpected insight into the regulation of intestinal function. The challenge posed by the intractable diarrhoeal syndromes, of working backwards from an apparently common phenotype to probably multiple genotypes, is, however, immense. Very few of these conditions have been described at the genetic level, although the molecular basis of pathogenesis has been better explored in recent years. The two major groups of intractable diarrhoea are due to (1) primary epithelial abnormalities (which usually present within the first few days of life) and (2) immunologically mediated (which generally present after the first few weeks). The high prevalence of autoimmune enteropathy among infantile autoimmune disease, in contrast to adult autoimmunity, is intriguing and may reflect constitutive abnormality of extrathymic lymphocyte maturation. The use of potent immunosuppressive drugs and increasing expertise with parenteral nutrition are improving the outlook of these previously fatal conditions. Viewed globally, however, the pressing problem is to treat effectively the millions of infants who die from severe persistent diarrhoea and wasting, which would certainly not be considered intractable in wealthy countries.

The anatomical basis of intestinal immunity

The lymphoid tissues associated with the intestine are exposed continuously to antigen and are the largest part of the immune system. Many lymphocytes are found in organised tissues such as the Peyer's patches and mesenteric lymph nodes, as well as scattered throughout the lamina propria and epithelium of the mucosa itself. These lymphocyte populations have several unusual characteristics and the intestinal immune system is functionally and anatomically distinct from other, peripheral compartments of the immune system. This review explores the anatomical and molecular basis of these differences, with particular emphasis on the factors which determine how the intestinal lymphoid tissues discriminate between harmful pathogens and antigens which are beneficial, such as food proteins or commensal bacteria. These latter antigens normally provoke immunological tolerance, and inappropriate responses to them are responsible for immunopathologies such as food hypersensitivity and inflammatory bowel disease. We describe how interactions between local immune cells, epithelial tissues and antigen-presenting cells may be critical for the induction of tolerance and the expression of active mucosal immunity. In addition, the possibility that the intestine may act as an extrathymic site for T-cell differentiation is discussed. Finally, we propose that, under physiological conditions, immune responses to food antigens and commensal bacteria are prevented by common regulatory mechanisms, in which transforming growth factor beta plays a critical role.