Immunobiology of Intestinal Eosinophils - A Dogma in the Changing?

Infiltration of eosinophils into the intestinal mucosa is a typical hallmark of antiparasite immune responses and inflammatory disorders of the intestinal tract, and eosinophils are thought to contribute to these processes by release of their cytotoxic granule content. However, utilizing novel tools to study eosinophils, it has been recognized that eosinophils are constitutively present in the gastrointestinal tract. In addition, as the dogmatic antiparasite function of eosinophils has proven difficult to document experimentally, it has become increasingly clear that eosinophils are likely to have a more complex role than previously appreciated. Thus, the prevailing dogma of eosinophils merely as antiparasitic effector cells is changing. Instead, it has been suggested that eosinophils can contribute also to several other processes in the intestinal mucosa, e.g. local tissue homeostasis and adaptive immune responses. This review describes the current knowledge regarding the characteristics and functions of intestinal eosinophils, and the regulation of eosinophil trafficking to the intestinal mucosa during the steady state and inflammation. Finally, potential additional and new roles of intestinal eosinophils in the intestinal mucosal immune system are discussed

Acute infection with the intestinal parasite Trichuris muris has long-term consequences on mucosal mast cell homeostasis and epithelial integrity

A hallmark of parasite infection is the accumulation of innate immune cells, notably granulocytes and mast cells, at the site of infection. While this is typically viewed as a transient response, with the tissue returning to steady state once the infection is cleared, we found that mast cells accumulated in the large-intestinal epithelium following infection with the nematode Trichuris muris and persisted at this site for several months after worm expulsion. Mast cell accumulation in the epithelium was associated with the induction of type-2 immunity and appeared to be driven by increased maturation of local progenitors in the intestinal lamina propria. Furthermore, we also detected increased local and systemic levels of the mucosal mast cell protease MCPt-1, which correlated highly with the persistent epithelial mast cell population. Finally, the mast cells appeared to have striking consequences on epithelial barrier integrity, by regulation of gut permeability long after worm expulsion. These findings highlight the importance of mast cells not only in the early phases of infection but also at later stages, which has functional implications on the mucosal tissue

Chemokine Receptor CCR9: Studies on the Generation and Localization of Gut Tropic Lymphocytes

Lymphocytes are important mediators of adaptive immune responses. Naive T lymphocytes circulate in the blood and lymph between different secondary lymphoid organs in search for their specific antigen. Upon activation, lymphocytes change their expression of adhesion molecules and chemokine receptors and become able to enter non-lymphoid tissues where they take part in fighting infections. Selective migration of lymphocytes is regulated by interactions between distinct surface expressed adhesion molecules and chemokine receptors on lymphocyte subsets and their specific ligands expressed on blood vessel endothelial cells and within tissues. The chemokine CCL25 is selectively and constitutively expressed by human small intestinal epithelial cells and its receptor, CCR9 expressed on small intestinal lymphocytes and a peripheral blood memory T cell population co-expressing markers associated with gut tropism. Together these observations suggest an important role for CCL25 and CCR9 in small intestinal immunity. In this thesis, we have investigated the mechanism involved in the generation and localization of CD8ab gut tropic T cells. We have found that; i) CCL25 and CCR9 play an important role in the localization of CD8ab T cells to the small intestinal epithelium; ii) gut tropic CCR9 a4b7 CD8ab T cells are selectively generated in mesenteric lymph nodes (MLN) by MLN dendritic cells (DCs); iii) MLN DC-induced CCR9 and a4b7 expression requires a soluble factor generated in MLN DC:T cell co-cultures in an antigen dose dependent manner

CCL25/CCR9 promotes the induction and function of CD103 on intestinal intraepithelial lymphocytes.

The integrin CD103 and the chemokine receptor CCR9 are co-expressed on small intestinal CD8+ intraepithelial lymphocytes (IEL), naïve murine CD8+ T cells and by a small population of effector/memory CD8+ T cells, indicating a potential role for CCR9 in regulating CD103 expression and function. Here, we demonstrate that CD103, in contrast to CCR9, is down-regulated on CD8+ T cells following their activation in mesenteric lymph nodes and that effector CD8+ T cells upon initial entry into the small intestinal epithelium are CCR9+CD103-. CD103 was rapidly induced on wild-type CD8+ T cells subsequent to their entry into the small intestinal epithelium, however, CCR9-/- CD8+ T cells exhibited a significant delay in CD103 induction at this site. In addition, the CCR9 ligand, CCL25, that is constitutively expressed in the small intestinal epithelium, induced transient, dose-dependent and pertussis toxin-sensitive CD103-mediated adhesion of CD8+ small intestinal IEL to a murine E-cadherin human Fc (mEFc) fusion protein. Together, these results demonstrate a role for CCR9/CCL25 in promoting the induction and function of CD103 on CD8+ IEL and suggest that this chemokine receptor/chemokine pair may function to regulate lymphocyte-epithelial interactions in the small intestinal mucosa

Differential homing mechanisms regulate regionalized effector CD8 alpha beta(+) T cell accumulation within the small intestine

The CC chemokine receptor (CCR)9 is expressed on the majority of small intestinal, but few colonic, T cells, whereas its ligand CCL25 is constitutively expressed by small intestinal epithelial cells. As such, CCR9/CCL25 have been proposed to play a central role in regulating small intestinal but not colonic immune responses and thus to organize regionalized immunity within the intestinal mucosa. Here, we demonstrate that CCL25 is expressed at reduced levels by epithelial cells in the distal compared with proximal small intestine, which correlated with less efficient CCR9-dependent effector CD8 alpha ss(+) T cell entry into the ileal epithelium. In vitro-generated alpha(4)ss(+)(7) effector CD8 alpha ss(+) T cell entry into the lamina propria was less dependent on CCR9 than entry into the epithelium along the entire length of the small intestine and in particular in the ileum. CCR9-independent alpha(4)ss(+)(7) effector CD8 alpha ss(+) T cell entry was pertussis toxin-sensitive, suggesting a role for additional Gal-linked G protein-coupled receptors. Finally, in vivo-primed effector CD8 alpha ss(+) T cells displayed regionalized differences in their entry to the small intestinal epithelium with enhanced CCR9-independent entry to the ileum. These results highlight a hitherto underappreciated compartmentalization of immune responses within the small intestine and have direct implications for targeting strategies aimed at regulating T cell localization to the small intestinal mucosa

Functional characterization of the CCL25 promoter in small intestinal epithelial cells suggests a regulatory role for caudal-related homeobox (Cdx) transcription factors

The chemokine CCL25 is selectively and constitutively expressed in the small intestinal epithelium and plays an important role in mediating lymphocyte recruitment to this site. In this study, we demonstrate that CCL25 expression in murine small intestinal epithelial cells is independent of signaling through the lymphotoxin 0 receptor and is not enhanced by inflammatory stimuli, pathways involved in driving the expression of most other chemokines. We define a transcriptional start site in the CCL25 gene and a region -141 to -5 proximal of exon 1 that is required for minimal promoter activity in the small intestinal epithelial cell lines, MODE-K and mICc12. These cell lines expressed far less CCL25 mRNA than freshly isolated small intestinal epithelial cells indicating that they are missing important factors driving CCL25 expression. The CCL25 promoter contained putative binding sites for,the intestinal epithelial-associated Caudal-related homeobox (Cdx) transcription factors Cdx-1 and Cdx-2, and small intestinal epithelial cells but not MODE-K and mICc12 cells expressed Cdx-1 and Cdx-2. EMSA analysis demonstrated that Cdx proteins were present in nuclear extracts from freshly isolated small intestinal epithelial cells but not in MODE-K or mICcl2 cells, and bound to putative Cdx sites within the CCL25 promoter. Finally, cotransfection of MODE-K cells with Cdx transcription factors significantly increased CCL25 promoter activity as well as endogenous CCL25 mRNA levels. Together these results demonstrate a unique pattern of regulation for CCL25 and suggest a role for Cdx proteins in regulating CCL25 transcription

Distinct DC subsets regulate adaptive Th1 and 2 responses during Trichuris muris infection

Low- and high-dose infections with the murine large intestinal nematode Trichuris muris are associated with induction of adaptive Th1 and Th2 responses, respectively, in mesenteric lymph nodes (MLN). Classical dendritic cells (cDC) accumulate in the large intestinal mucosa and MLN upon T. muris infection, yet their role in driving adaptive responses to infection remains largely unknown. We performed low- and high-dose T. muris infections of mice deficient in defined cDC subsets to investigate their role in induction of adaptive immune responses. Mice lacking IRF4-dependent cDC failed to clear a high-dose infection and displayed impaired Th2 responses. Conversely, mice lacking IRF8-dependent cDC cleared a low-dose infection and displayed an impaired Th1 response while increased production of Th2 cytokines. Finally, mice lacking both IRF4- and IRF8-dependent cDC were able to generate a Th2 response and clear a low-dose infection. Collectively, these results suggest that IRF4- and IRF8-dependent cDC act antagonistically during T. muris infection, and demonstrate that intestinal Th2 responses can be generated towards T. muris in the absence of IRF4-dependent cDC

Eosinophils may play regionally disparate roles in influencing IgA+ plasma cell numbers during large and small intestinal inflammation

Background: Eosinophils are innate immune cells present in the intestine during steady state conditions. An intestinal eosinophilia is a hallmark of many infections and an accumulation of eosinophils is also observed in the intestine during inflammatory disorders. Classically the function of eosinophils has been associated with tissue destruction, due to the release of cytotoxic granule contents. However, recent evidence has demonstrated that the eosinophil plays a more diverse role in the immune system than previously acknowledged, including shaping adaptive immune responses and providing plasma cell survival factors during the steady state. Importantly, it is known that there are regional differences in the underlying immunology of the small and large intestine, but whether there are differences in context of the intestinal eosinophil in the steady state or inflammation is not known. Results: Our data demonstrates that there are fewer IgA+ plasma cells in the small intestine of eosinophil-deficient ΔdblGATA-1 mice compared to eosinophil-sufficient wild-type mice, with the difference becoming significant post-infection with Toxoplasma gondii. Remarkably, and in complete contrast, the absence of eosinophils in the inflamed large intestine does not impact on IgA+ cell numbers during steady state, and is associated with a significant increase in IgA+ cells post-infection with Trichuris muris compared to wild-type mice. Thus, the intestinal eosinophil appears to be less important in sustaining the IgA+ cell pool in the large intestine compared to the small intestine, and in fact, our data suggests eosinophils play an inhibitory role. The dichotomy in the influence of the eosinophil over small and large intestinal IgA+ cells did not depend on differences in plasma cell growth factors, recruitment potential or proliferation within the different regions of the gastrointestinal tract (GIT). Conclusions: We demonstrate for the first time that there are regional differences in the requirement of eosinophils for maintaining IgA+ cells between the large and small intestine, which are more pronounced during inflammation. This is an important step towards further delineation of the enigmatic functions of gut-resident eosinophils