Intestinal macrophages: unique effector cells of the innate immune system

Mechanisms of disease: protease functions in intestinal mucosal pathobiology

Of all our organ systems, the gastrointestinal tract contains the highest levels of endogenous and exogenous proteases (also known as proteinases and peptidases); however, our understanding of their functions and interactions within the gastrointestinal tract is restricted largely to nutrient digestion. The gut epithelium is a sensor of the luminal environment, not only controlling digestive, absorptive and secretory functions, but also relaying information to the mucosal immune, vascular and nervous systems. These functions involve a complex array of cell types that elaborate growth factors, cytokines and extracellular matrix (ECM) proteins, the activity and availability of which are regulated by proteases. Proteolytic activity must be tightly regulated in the face of diverse environmental challenges, because unrestrained or excessive proteolysis leads to pathological gastrointestinal conditions. Moreover, enteric microbes and parasites can hijack proteolytic pathways through 'pathogen host mimicry'. Understanding how the protease balance is maintained and regulated in the intestinal epithelial cell microenvironment and how proteases contribute to physiological and pathological outcomes will undoubtedly contribute to the identification of new potential therapeutic targets for gastrointestinal diseases.

Enteroendocrine cells express functional Toll-like receptors

Intestinal epithelial cells (IECs) provide a physical and immunological barrier against enteric microbial flora. Toll-like receptors (TLRs), through interactions with conserved microbial patterns, activate inflammatory gene expression in cells of the innate immune system. Previous studies of the expression and function of TLRs in IECs have reported varying results. Therefore, TLR expression was characterized in human and murine intestinal sections, and TLR function was tested in an IEC line. TLR1, TLR2, and TLR4 are coexpressed on a subpopulation of human and murine IECs that reside predominantly in the intestinal crypt and belong to the enteroendocrine lineage. An enteroendocrine cell (EEC) line demonstrated a similar expression pattern of TLRs as primary cells. The murine EEC line STC-1 was activated with specific TLR ligands: LPS or synthetic bacterial lipoprotein. In STC-1 cells stimulated with bacterial ligands, NF-kappaB and MAPK activation was demonstrated. Furthermore, the expression of TNF and macrophage inhibitory protein-2 were induced. Additionally, bacterial ligands induced the expression of the anti-inflammatory gene transforming growth factor-beta. LPS triggered a calcium flux in STC-1 cells, resulting in a rapid increase in CCK secretion. Finally, conditioned media from STC-1 cells inhibited the production of nitric oxide and IL-12 p40 by activated macrophages. In conclusion, human and murine IECs that express TLRs belong to the enteroendocrine lineage. Using a murine EEC model, a broad range of functional effects of TLR activation was demonstrated. This study suggests a potential role for EECs in innate immune responses.

Immunité muqueuse et vaccination

Mucosae constitute the major entry for most microbial pathogens but also innocuous antigens derived from ingested food, airborne matter or commensal bacteria. A large and highly specialized innate and adaptative mucosal immune system protects the mucosal surfaces and the body interior from potential injuries from the environment. The mucosal immune system has developed a variety of immune mechanisms to discriminate between non-pathogenic and pathogenic invaders. It is able to maintain tolerance against the plethora of environmental antigens and to induce potent protective immunity to avoid mucosal colonisation and organism invasion by dangerous microbial pathogens. Mucosal immunisation with appropriate antigens and immunostimulatory molecules may induce potent protective immunity against harmful pathogens. Alternatively, mucosally-induced tolerance against auto-antigens or allergens may be generated by mucosal administration of these antigens alone or with immunomodulators potentiating regulatory responses. Here, we review the properties of the mucosal immune system and briefly discuss the advances in the development of mucosal vaccines for protection against infections and for the treatment of inflammatory disorders such as autoimmune diseases or type I allergies.

Adaptations of intestinal macrophages to an antigen-rich environment

Intestinal macrophages, preferentially located in the subepithelial lamina propria, represent in humans the largest pool of tissue macrophages. To comply with their main task, i.e. the efficient removal of microbes and particulate matter that might have gained access to the mucosa from the intestinal lumen while maintaining local tissue homeostasis, several phenotypic and functional adaptations evolved. Most notably, microbe-associated molecular pattern (MAMP) receptors, including the lipopolysaccharide receptors CD14 and TLR4, but also the Fc receptors for IgA and IgG are absent on most intestinal Mø. Here we review recent findings on the phenotypic and functional adaptations of intestinal Mø and their implications for the pathogenesis of inflammatory bowel diseases.

Hematopoietic prostaglandin D synthase suppresses intestinal adenomas in ApcMin/+ mice

Aspirin and other nonsteroidal anti-inflammatory drugs prevent some cases of colon cancer by inhibiting prostaglandin (PG) synthesis. PGE(2) promotes colon neoplasia, as shown by knockout mouse studies on enzymes and receptors in the PG cascade. A few experiments 20 to 30 years ago suggested that PGD(2) may suppress tumors, but a role for biosynthetic enzymes for PGD(2) in tumor development has not been studied. We report here that disruption of the gene for hematopoietic PGD synthase in Apc(Min/+) mice led to approximately 50% more intestinal adenomas compared with controls. Tumor size was not affected. By immunohistochemistry, we detected hematopoietic PGD synthase mainly in macrophages and monocytes of the gut mucosa. The mean number of tumors did not increase with knockout of the gene for the lipocalin type of the enzyme, which is not produced in the intestine. On the other hand, Apc(Min/+) mice with transgenic human hematopoietic PGD synthase tended to have 80% fewer intestinal adenomas. The transgene produced high mRNA levels (375-fold over endogenous). There was a suggestion of higher urinary excretion of 11beta-PGF(2alpha) and a lower excretion of a PGE(2) metabolite in transgenic mice, but differences (30-40%) were not statistically significant. The results support an interpretation that hematopoietic PGD synthase controls an inhibitory effect on intestinal tumors. Further studies will be needed to prove possible mechanisms, such as routing of PG production away from protumorigenic PGE(2) or inhibition of the nuclear factor-kappaB cascade by PGD(2) metabolites.

Suppression of experimental colitis by intestinal mononuclear phagocytes

The contribution of innate immunity to inflammatory bowel disease (IBD) remains an area of intense interest. Macrophages (MØ) and dendritic cells (DC) are considered important factors in regulating the onset of IBD. The goal of this study was to determine if intestinal mononuclear phagocytes (iMNP) serve a pathological or protective role in dextran sulfate sodium (DSS)-induced colitis in mice. Using a conditional MØ/DC depletion transgenic mouse line--MØ Fas-induced apoptosis--to systemically deplete iMNP, DSS colitis histopathology was shown to be more severe in MØ/DC-depleted compared with MØ/DC-intact mice. Similarly, localized iMNP depletion by clodronate-encapsulated liposomes into C57BL/6, BALB/c, and CB.17/SCID mice also increased DSS colitis severity, as indicated by increased histopathology, weight loss, rectal bleeding, decreased stool consistency, and colon length compared with MØ/DC-intact, DSS-treated mice. Histology revealed that iMNP depletion during DSS treatment led to increased neutrophilic inflammation, increased epithelial injury, and enhanced mucin depletion from Goblet cells. iMNP depletion did not further elevate DSS-induced expression of TNF-alpha and IFN-gamma mRNA but significantly increased expression of CXCL1 chemokine mRNA. Myeloperoxidase activity was increased in colons of MØ/DC-depleted, DSS-treated mice, compared with DSS alone, coincident with increased neutrophil infiltration in diseased colons. Neutrophil depletion combined with MØ/DC depletion prevented the increase in DSS colitis severity compared with MØ/DC depletion alone. This study demonstrates that iMNP can serve a protective role during development of acute colitis and that protection is associated with MØ/DC-mediated down-regulation of neutrophil infiltration.

The early transcriptional response of pig small intestinal mucosa to invasion by Salmonella enterica serovar typhimurium DT104

Salmonella enterica serovar typhimurium (S. typhimurium) species are a leading cause of human invasive gastroenteritis. There is increasing in vitro evidence about Salmonella interaction with isolated cells or cell lines (macrophages, and enterocytes) on the molecular level, however, very little is known about in vivo interactions during actual invasion. We investigated the early interaction of S. typhimurium with intact small intestinal mucosa, in a pig model. Intestinal segments were infected with or without S. typhimurium DT104, and perfused. Whole mucosal gene expression was analyzed by cDNA array on 0, 2, 4, and 8h post-infection. Invasion resulted in the upregulation of only eight transcripts in jejunal mucosa, among those the proinflammatory IL-8 (at 4h only), and the antiinflammatory STAT3 (at 4 and 8h). The limited number of differentially expressed genes found here in vivo compared to in vitro is most likely due to the presence of multiple, heterogenous cell interactions in intact mucosa. Furthermore, it is concluded that S. typhimurium evades strong host responses by downregulating the local inflammatory response.

A unique dendritic cell subset accumulates in the celiac lesion and efficiently activates gluten-reactive T cells

BACKGROUND & AIMS

Celiac disease is a chronic inflammation of the duodenal mucosa driven by gluten-reactive T cells restricted by the disease-associated HLA-DQ2 molecule. The mechanisms that regulate the activation of mucosal T cells are, however, understood poorly. The aim of this study was to identify the antigen-presenting cells that are responsible for the activation of gluten-reactive T cells in the celiac lesion.

METHODS

Intestinal biopsy specimens obtained from untreated and treated celiac patients and normal controls were either snap-frozen directly or incubated for 24 hours with or without gluten peptides. Cryosections were subjected to multicolor immunofluorescence applying monoclonal antibodies to a range of antigen-presenting cell markers. Macrophages and dendritic cells were isolated from enzymatically digested small intestinal biopsies of untreated patients and incubated with gluten-reactive T-cell clones to measure their antigen-presenting capacity.

RESULTS

HLA-DQ2+ cells in the normal duodenal mucosa consisted of 2 distinct cell populations: about 80% were CD68+ DC-lysosome intercellular adhesion molecule-3-grabbing nonintegrin+ macrophages and 20% were CD11c+ dendritic cells. Importantly, the CD11c+ dendritic cells accumulated in the celiac lesion and revealed an activated phenotype expressing CD86 and DC-specific-associated membrane protein. Moreover, when isolated from challenged biopsy specimens, the CD11c+ dendritic cells efficiently activated gluten-reactive T cells.

CONCLUSIONS

Our results suggest that a unique subset of dendritic cells are responsible for local activation of gluten-reactive T cells in the celiac lesion.