Composition of TCR-CD3 complex in human intestinal intraepithelial lymphocytes: lack of Fc epsilon RI gamma chain

Diversity in recognition and function of human γδ T cells

As interest increases in harnessing the potential power of tissue-resident cells for human health and disease, γδ T cells have been thrust into the limelight due to their prevalence in peripheral tissues, their sentinel-like phenotypes, and their unique antigen recognition capabilities. This review focuses primarily on human γδ T cells, highlighting their distinctive characteristics including antigen recognition, function, and development, with an emphasis on where they differ from their αβ T cell comparators, as well as from γδ T cell populations in the mouse. We review the antigens that have been identified thus far to regulate members of the human Vδ1 population and discuss what players are involved in transducing phosphoantigen-mediated signals to human Vγ9Vδ2 T cells. We also briefly review distinguishing features of these cells in terms of TCR signaling, use of coreceptor and costimulatory molecules and their development. These cells have great potential to be harnessed in a clinical setting, but caution must be taken to understand their unique capabilities and how they differ from the populations to which they are commonly compared.

Staphylococcal enterotoxin B induces potent cytotoxic activity by intraepithelial lymphocytes

In food poisoning, Staphylococcus aureus secretes staphylococcal enterotoxin B (SEB), a superantigen that causes intense T-cell proliferation and cytotoxicity. The effects of SEB on lytic activity by human intestinal intraepithelial lymphocytes (IEL) were investigated. Jejunal IEL, from morbidly obese individuals undergoing gastric bypass operations, were tested for SEB-induced cytotoxicity against C1R B-lymphoblastoid cells, HT-29 adenocarcinoma cells, or CD1d-transfected cells using the 51Cr-release assay. Fas and Fas ligand expression were detected by immunofluorescence and flow cytometry and soluble ligand by enzyme-linked immunosorbent assay (ELISA). In the presence of SEB, IEL became potently cytotoxic against C1R cells and interferon-gamma (IFN-gamma)-precultured HT-29 cells, causing 55+/-10% and 31+/-6% lysis, respectively, greater than that by phytohaemagglutinin (PHA)-, interleukin-2 (IL-2)-, or anti-T-cell receptor (TCR)-activated IEL. SEB-stimulated peripheral blood (PB) CD8+ T cells lysed similar numbers of C1R cells but fewer HT-29 cells (53+/-13% and 8+/-5%, respectively). IEL killing of C1R cells involved interaction of major histocompatibility complex (MHC) class II with TCR, CD2 with CD58, and CD11a with CD54, and was perforin mediated. SEB-induced IEL lysis of HT-29 cells, in contrast, was caused by an unknown target cell structure, not MHC class II or CD1d, and resulted from a combination of perforin and Fas-mediated events. The potent cytotoxic activities of IEL promoted by SEB utilize two different mechanisms, depending on the surface receptors expressed by the target cells.

Review article: Intestinal epithelia and barrier functions

The mucosal epithelia of the digestive tract acts as a selective barrier, permeable to ions, small molecules and macromolecules. These epithelial cells aid the digestion of food and absorption of nutrients. They contribute to the protection against pathogens and undergo continuous cell renewal which facilitates the elimination of damaged cells. Both innate and adaptive defence mechanisms protect the gastrointestinal-mucosal surfaces against pathogens. Interaction of microorganisms with epithelial cells triggers a host response by activating specific transcription factors which control the expression of chemokines and cytokines. This host response is characterized by the recruitment of macrophages and neutrophils at the site of infection. Disruption of epithelial signalling pathways that recruit migratory immune cells results in a chronic inflammatory response. The adaptive defence mechanism relies on the collaboration of epithelial cells (resident sampling system) with antigen-presenting and lymphoid cells (migratory sampling system); in order to obtain samples of foreign antigen, these samples must be transported across the barriers without affecting the integrity of the barrier. These sampling systems are regulated by both environmental and host factors. Fates of the antigen may differ depending on the way in which they cross the epithelial barrier, i.e. via interaction with motile dendritic cells or epithelial M cells in the follicle-associated epithelium.

Human intestinal epithelial cell lines produce factor(s) that inhibit CD3-mediated T-lymphocyte proliferation

Peripheral blood T lymphocytes (PBT) proliferate more to anti-CD3 stimulation than to anti-CD2 stimulation. On the other hand, fresh, but not cultivated, intestinal intraepithelial lymphocytes (iIEL) exhibit a lower response to CD3 stimulation in comparison to CD2. The goal of this study was to show that the anti-CD3 T-cell response depends on the microenvironment and is independent of the origin of the lymphocytes. Cultured T-cell lines were stimulated with either an anti-CD3 mAb or an anti-CD2 mAb. Either conditioned supernatant from intestinal epithelial cell (IEC) lines or non- conditioned medium (negative control) was added. After 2 days cytokine production and proliferation were measured. Conditioned supernatant decreased the proliferative response of small and large bowel iIEL compared to controls (P = 0.04). In the same experiments, the cytokine production was non-significantly decreased. Immortalized iIEL, that are not regularly stimulated by their CD3 pathway, showed a similar decrease in proliferation (P < 0.001) and cytokine production (P = 0.01) when incubated with conditioned supernatant. Similar results were also obtained with a non-immortalized and an immortalized PBT line (P < 0.001). In a small bowel iIEL cell line, that exhibited a significant response to anti-CD2 stimulation, the proliferative response to anti-CD2 stimulation was preserved. Active conditioned supernatant could be generated from three independent IEC lines and a liver derived epithelial cell line, but not from a non-epithelial control cell line or two extraintestinal epithelial cell lines. We conclude that supernatants of cultured IEC contain soluble factor(s) that cause cultured iIEL and extraintestinal lymphocytes to behave like fresh iIEL. These results, therefore, support and extend the studies of others which suggest that the intestinal microenvironment mucosalizes lymphocytes.

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.

The intestinal epithelial cell: immunological aspects

IECs likely play an important role in immunological defense mechanism. Apart from being a passive barrier against luminal bacteria, IECs secrete protective and microbiocidal products such as ITF, complement components and cryptdins into the lumen. Moreover, IECs produce secretory component that is essential for the transport of IgA from the lamina propria into the lumen. IECs also have regulatory functions. They express adhesion molecules important in the homing of T cells and other leukocytes, and likely modulate T cell functions in a paracrine way. Furthermore, IECs secrete cytokines, either constitutively or after bacterial challenge, and they express cytokine receptors. Lastly, IECs may play an important role as non-professional antigen-presenting cells by expressing classical MHC class I and class II and nonclassical MHC class I molecules on the cell surface. This aspect is particularly intriguing in that IECs also express a FcR that may have a function in luminal antigen sampling.