Interleukin-15 production at the early stage after oral infection with Listeria monocytogenes in mice

Interleukin 15 in murine models of colitis

Inflammatory bowel diseases (IBDs) are characterized by abnormal, non-antigen specific chronic inflammation of unknown etiology. Genome-wide association studies show that many IBD genetic susceptibility loci map to immune function genes and compelling evidence indicate that environmental factors play a critical role in IBD pathogenesis. Clinical and experimental evidence implicate the pro-inflammatory cytokine IL-15 in the pathogenesis of IBD. IL-15 and IL-15α expression is increased in the inflamed mucosa of IBD patients. IL-15 contributes to the maintenance of different cell subsets in the intestinal mucosa. However, very few studies have addressed the role of IL-15 in pre-clinical models of colitis. In this study, we use three well-characterized models of experimental colitis to determine the contribution of IL-15 to pathological intestinal inflammation.

IL-15 and RANKL Play a Synergistically Important Role in Osteoclastogenesis

Interleukin-15 (IL-15), a cytokine secreted by several cell types, has important physiological roles in the activity, proliferation, and viability of immune cells. It has both chemoattractant and proinflammatory properties, and may promote bone destruction. A previous study has shown that IL-15 alone exerts no effect on osteoclastogenesis. Therefore, the current study addressed the synergistic effect of IL-15 on osteoclast formation using RAW264.7 (RAW) cells by co-stimulation with receptor activator of nuclear factor (NF)-κB ligand (RANKL) that has a major role in osteoclastogenesis involving the pathogenesis of rheumatoid arthritis and periodontal disease. Co-stimulation of RAW cells by IL-15 and RANKL significantly increased the gene expression of osteoclast differentiation and osteoclastogenesis markers compared with stimulation by RANKL or IL-15 independently as evaluated by tartrate-resistant acid phosphate-positive cell numbers, the fusion index, a pit formation assay with Alizarin red staining (calcification estimation), and quantitative polymerase chain reaction. Phosphorylation of extracellular signal-regulated kinase (ERK), c-jun N-terminal kinase, p38 mitogen-activated protein kinase, and NF-κB was significantly increased by RANKL and IL-15 (P < 0.05) compared with RANKL alone. In addition, these differentiation activities induced by RANKL and IL-15 were comparatively suppressed by inhibition of ERK, suggesting that this synergistic effect on osteoclastogenesis is mainly mediated by ERK. Taken together, our results demonstrate that IL-15 and RANKL induce osteoclastogenesis synergistically, and IL-15 might play a novel and major role in destructive inflammatory bone diseases. J. Cell. Biochem. 118: 739-747, 2017. © 2016 Wiley Periodicals, Inc.

Effect of IL-15 and natural killer cells on osteoclasts and osteoblasts in a mouse coculture

This study analyzes the effect of interleukin-15 (IL-15) on osteoclast formation using a coculture of mouse osteoblasts and bone marrow cells (BMCs) stimulated with prostaglandin E2 (PGE2), which both have important role in rheumatoid arthritis (RA) and periodontal disease (PD). BMCs isolate lacking T (BM(T-)) or NK (BM(NK-)) cells, BMCs with no cells removed (BM(T+NK+)), purified NK cells, and purified T cells were each cocultured with osteoblasts in the presence or absence of PGE2 and/or IL-15. The number of both osteoclasts and osteoblasts was decreased by IL-15 in a dose-dependent manner in BM(T+NK+), BM(T-). However, the reductions were improved in BM(NK-). The expression of caspase3 in osteoblasts cocultured with NK cells was increased in a dose-dependent manner by IL-15. IL-15 stimulates apoptosis of osteoblasts via activation of NK cells. Since osteoblasts have an important role in bone formation, IL-15 may be an inflammatory bone destructive factor in RA and PD.

Soluble interleukin-15 complexes are generated in vivo by type I interferon dependent and independent pathways

Interleukin (IL)-15 associates with IL-15Rα on the cell surface where it can be cleaved into soluble cytokine/receptor complexes that have the potential to stimulate CD8 T cells and NK cells. Unfortunately, little is known about the in vivo production of soluble IL-15Rα/IL-15 complexes (sIL-15 complexes), particularly regarding the circumstances that induce them and the mechanisms responsible. The main objective of this study was to elucidate the signals leading to the generation of sIL-15 complexes. In this study, we show that sIL-15 complexes are increased in the serum of mice in response to Interferon (IFN)-α. In bone marrow derived dendritic cells (BMDC), IFN-α increased the activity of ADAM17, a metalloproteinase implicated in cleaving IL-15 complexes from the cell surface. Moreover, knocking out ADAM17 in BMDCs prevented the ability of IFN-α to induce sIL-15 complexes demonstrating ADAM17 as a critical protease mediating cleavage of IL-15 complexes in response to type I IFNs. Type I IFN signaling was required for generating sIL-15 complexes as in vivo induction of sIL-15 complexes by Poly I:C stimulation or total body irradiation (TBI) was impaired in IFNAR-/- mice. Interestingly, serum sIL-15 complexes were also induced in mice infected with Vesicular stomatitis virus (VSV) or mice treated with agonistic CD40 antibodies; however, sIL-15 complexes were still induced in IFNAR-/- mice after VSV infection or CD40 stimulation indicating pathways other than type I IFNs induce sIL-15 complexes. Overall, this study has shown that type I IFNs, VSV infection, and CD40 stimulation induce sIL-15 complexes suggesting the generation of sIL-15 complexes is a common event associated with immune activation. These findings reveal an unrealized mechanism for enhanced immune responses occurring during infection, vaccination, inflammation, and autoimmunity.

CXCR3 expression defines a novel subset of innate CD8+ T cells that enhance immunity against bacterial infection and cancer upon stimulation with IL-15

Innate CD8(+) T cells are a heterogeneous population with developmental pathways distinct from conventional CD8(+) T cells. However, their biology, classification, and functions remain incompletely understood. We recently demonstrated the existence of a novel population of chemokine (C-X-C motif) receptor 3 (CXCR3)-positive innate CD8(+) T cells. Here, we investigated the functional properties of this subset and identified effector molecules and pathways which mediate their function. Adoptive transfer of IL-15 activated CXCR3(+) innate CD8(+) T cells conferred increased protection against Listeria monocytogenes infection in susceptible IFN-γ(-/-) mice compared with similarly activated CXCR3(-) subset. This was associated with enhanced proliferation and IFN-γ production in CXCR3(+) cells. Further, CXCR3(+) innate cells showed enhanced cytotoxicity against a tumor cell line in vitro. In depth analysis of the CXCR3(+) subset showed increased gene expression of Ccl5, Klrc1, CtsW, GP49a, IL-2Rβ, Atp5e, and Ly6c but reduced IFN-γR2 and Art2b. Ingenuity pathway analysis revealed an up-regulation of genes associated with T-cell activation, proliferation, cytotoxicity, and translational initiation in CXCR3(+) populations. Our results demonstrate that CXCR3 expression in innate CD8(+) T cells defines a subset with enhanced cytotoxic potential and protective antibacterial immune functions. Immunotherapeutic approaches against infectious disease and cancer could utilize CXCR3(+) innate CD8(+) T-cell populations as novel clinical intervention strategies.

Irradiation with a low-level diode laser induces the developmental endothelial locus-1 gene and reduces proinflammatory cytokines in epithelial cells

We demonstrated previously that low-level diode laser irradiation with an indocyanine green-loaded nanosphere coated with chitosan (ICG-Nano/c) had an antimicrobial effect, and thus could be used for periodontal antimicrobial photodynamic therapy (aPDT). Since little is known about the effects of aPDT on periodontal tissue, we here investigated the effect of low-level laser irradiation, with and without ICG-Nano/c, on cultured epithelial cells. Human oral epithelial cells were irradiated in a repeated pulse mode (duty cycle, 10 %; pulse width, 100 ms; peak power output, 5 W). The expression of the developmental endothelial locus 1 (Del-1), interleukin-6 (IL-6), IL-8, and the intercellular adhesion molecule-1 (ICAM-1) were evaluated in Ca9-22 cells stimulated by laser irradiation and Escherichia coli-derived lipopolysaccharide (LPS). A wound healing assay was carried out on SCC-25 cells irradiated by diode laser with or without ICG-Nano/c. The mRNA expression of Del-1, which is known to have anti-inflammatory activity, was significantly upregulated by laser irradiation (p < 0.01). Concurrently, LPS-induced IL-6 and IL-8 expression was significantly suppressed in the LPS + laser group (p < 0.01). ICAM-1 expression was significantly higher in the LPS + laser group than in the LPS only or control groups. Finally, compared with the control, the migration of epithelial cells was significantly increased by diode laser irradiation with or without ICG-Nano/c. These results suggest that, in addition to its antimicrobial effect, low-level diode laser irradiation, with or without ICG-Nano/c, can suppress excessive inflammatory responses via a mechanism involving Del-1, and assists in wound healing.

Aggregatibacter actinomycetemcomitans lipopolysaccharide stimulated epithelial cells produce interleukin-15 that regulates T cell activation

OBJECTIVE

Oral epithelial cells act not only as mechanical barriers but also as immunological barriers by producing various mediators such as cytokines. Since, in periodontal disease, limited information is available regarding the role of oral epithelial cell-derived cytokines on T cell activation, we investigated the responses of human T cells (Jurkat cell) to cytokines in KB cells (an oral epithelial cell line) that had been stimulated with Aggregatibacter actinomycetemcomitans lipopolysaccharide (LPS).

DESIGN

To evaluate T cell activation in response to the culture supernatant of KB cells, we examined cell proliferation and interferon gamma (IFN-γ) production, which is closely related to periodontal disease, in Jurkat cells. Culture supernatant of LPS-stimulated KB cells enhanced cell proliferation and IFN-γ production in Jurkat cells. To determine the active component within the culture supernatant, the production of epithelial cell-derived cytokines, interleukin-12 (IL-12), IL-15 and IL-18, in LPS-stimulated KB cells was analysed.

RESULTS

IL-15, but not IL-18, was significantly increased in the culture supernatant of LPS-stimulated KB cells. Moreover, additional anti-IL-15 neutralizing antibody abolished culture supernatant-induced IFN-γ expression in Jurkat cells.

CONCLUSION

These results suggest that periodontal pathogens induce the production of IL-15 from epithelial cells, and leading the activation of T cells in periodontal lesions.

Early gene response of human brain microvascular endothelial cells to Listeria monocytogenes infection

The gene expression of human brain microvascular endothelial cells (HBMEC) in response to 4 h of infection by Listeria monocytogenes was analyzed. Four hours after infection, the expression of 456 genes of HBMEC had changed (p < 0.05). We noted that many active genes were involved in the formyl-methionyl-leucyl-phenylalanine pathway in infected HBMEC. In the upregulated genes, mRNA levels of interleukin-8 and interleukin-15 in infected cells increased according to microarray and real-time reverse transcription - PCR analyses. Since both cytokines are regarded as potent chemotactic factors, the results suggest that HBMEC are capable of recruiting cells of innate and adaptive immune responses during early L. monocytogenes infection.

Azithromycin may inhibit interleukin-8 through suppression of Rac1 and a nuclear factor-kappa B pathway in KB cells stimulated with lipopolysaccharide

BACKGROUND

Recent studies have shown that the 15-member macrolide antibiotic azithromycin (AZM) not only has antibacterial activity, but also results in the role of immunomodulator. Interleukin (IL)-8 is an important inflammatory mediator in periodontal disease. However, there have been no reports on the effects of AZM on IL-8 production from human oral epithelium. Therefore, we investigated the effects of AZM on IL-8 production in an oral epithelial cell line.

METHODS

KB cells were stimulated by Escherichia coli or Aggregatibacter actinomycetemcomitans (previously Actinobacillus actinomycetemcomitans) lipopolysaccharide (LPS) with or without AZM. IL-8 mRNA and protein expression and production in response to LPS were analyzed by quantitative polymerase chain reaction, flow cytometry, and enzyme-linked immunosorbent assay. The activation of nuclear factor-kappa B (NF-κB) and Rac1, which is important for IL-8 expression, was analyzed by enzyme-linked immunosorbent assay and Western blotting, respectively.

RESULTS

IL-8 mRNA expression, IL-8 production, and NF-κB activation in LPS-stimulated KB cells were inhibited by the addition of AZM. LPS-induced Rac1 activation was also suppressed by AZM.

CONCLUSIONS

This study suggests that AZM inhibits LPS-induced IL-8 production in an oral epithelial cell line, in part caused by the suppression of Rac1 and NF-κB activation. The use of AZM might provide possible benefits in periodontal therapy, with respect to both its antibacterial action and apparent anti-inflammatory effect.

Cell-specific interleukin-15 and interleukin-15 receptor subunit expression and regulation in pneumococcal pneumonia--comparison to chlamydial lung infection

Interleukin (IL)-15 has critical impact on the homeostasis and activation of natural killer cells, natural killer T cells, gammadeltaT cells, and CD8(+)T cells, and contributes to antimicrobial defenses particularly at mucosal sites. The respiratory tract comprises a large mucosal surface and harbors significant amounts of lymphocytes, however the expression pattern of IL-15 in the lung and its role in local immune responses are largely unknown. We therefore analyzed the differential expression of IL-15 and the IL-15 receptor (IL-15R) complex in the lungs of mice and demonstrated substantial constitutive expression in bronchial and alveolar epithelial cells, alveolar macrophages, and vascular smooth muscle cells, implicating contribution to pulmonary immune cell homeostasis already under normal conditions. The induction of pneumococcal pneumonia but not the infection with Chlamydophila pneumoniae evoked a significant up-regulation of IL-15 on alveolar macrophages and bronchial epithelial cells, with the latter presenting de-novo expression of IL-15 on their basolateral surface and additional up-regulation of IL-15Ralpha. Moreover, transcriptome analysis as well as semi-quantitative PCR indicated at least partial transcriptional regulation in mice lungs. In conclusion IL-15 is suggested being of functional importance in the pulmonary immune response against pneumococcal pneumonia.

Interleukin-15 enhances cytotoxicity, receptor expression, and expansion of neonatal natural killer cells in long-term culture

Newborn infants have a higher susceptibility to various pathogens due to developmental defects in their host defense system, including deficient natural killer (NK) cell function. In this study, the effects of interleukin-15 (IL-15) on neonatal NK cells was examined for up to 12 weeks in culture. The cytotoxicity of fresh neonatal mononuclear cells (MNC) as assayed by K562 cell killing is initially much less than that of adult MNC but increases more than eightfold after 2 weeks of culture with IL-15 to a level equivalent to that of adult cells. This high level of cytotoxicity was maintained for up to 12 weeks. In antibody-dependent cellular cytotoxicity (ADCC) assays using CEM cells coated with human immunodeficiency virus gp120 antigen, IL-15 greatly increased ADCC lysis by MNC from cord blood. IL-15 increased expression of the CD16+ CD56+ NK markers of cord MNC fivefold after 5 weeks of incubation. Cultures of neonatal MNC with IL-15 for up to 10 weeks resulted in a unique population of CD3- CD8+ CD56+ cells (more than 60%), which are not present in fresh cord MNC. These results show that IL-15 can stimulate neonatal NK cells and sustain their function for several weeks, which has implications for the clinical use of IL-15.

T-cell activation in the curious world of the intestinal intraepithelial lymphocyte

In conventional terms, when T cells encounter appropriate stimuli, they are induced to undergo molecular and physical changes that confer upon them a state of activation. Once initiated, activation generally results in a state of full T-cell responsiveness in an all-or-none manner. Uniquely, however, the intestinal intraepithelial lymphocytes (IELs) bear features that are decidedly different from those of T cells located throughout other immunological compartments in that they exhibit some but not all properties of activated T cells, yet they can be induced to move further into activation provided appropriate costimulatory signals have been received. IEL costimulatory molecules some of which are constitutively expressed, whereas others are upregulated following T-cell receptor (TCR)/CD3 stimulation appear to hold the key to determining the nature and magnitude of the activational process. A system of activation such as this in the intestine would be expected to have great immunological protective value for the host because it would provide an untrammeled process of T-cell activation at a barrier site where the level of antigen exposure is consistently high. Clearly, however, mechanisms must be in place to insure that the IEL activation process is not inadvertently breached. These and other issues central to the operational workings of the intestinal immune system are elaborated in this article, and a model is presented in which IEL activation can be viewed as a layered, three-stage activational process.

Dendritic cell-derived IL-2 production is regulated by IL-15 in humans and in mice

Dendritic cells (DCs) are involved in the initiation and regulation of innate and adaptive immune responses. Several molecular mechanisms regulate these diverse DC functions, and we have previously reported that mouse dendritic cells (mDCs) can produce interleukin-2 (IL-2) in vitro and in vivo, in response to microbial activation and T-cell-mediated stimuli. This property is shared by different DC subtypes, including Langerhans cells. Here we show that, on appropriate stimulation, human DCs, both plasmacytoid and myeloid subtypes, also express IL-2. Interestingly, the production of IL-2 by myeloid DCs is induced by T-cell-mediated stimuli and depends on the presence of IL-15. The key role of this cytokine in regulating IL-2 production was also confirmed in the mouse system. In particular, we could show that DCs from IL-15-deficient mice were strongly impaired in the ability to produce IL-2 after interactions with different microbial stimuli. Our results indicate that DC-produced IL-2 is tightly coregulated with the expression of IL-15.

Development of TCR.ALPHA..BETA. CD8.ALPHA..ALPHA. Intestinal Intraepithelial Lymphocytes Is Promoted by Interleukin-15-Producing Epithelial Cells Constitutively Stimulated by Gram-Negative Bacteria via TLR4

The microbes present in the intestine have a strong influence on the development and maturation of lymphoid organs. The cross-talk mechanisms between intestinal intraepithelial lymphocytes (i-IEL) and noninvasive microbes are still poorly understood. The influence of microbes and lipopolysaccharides on the development of i-IEL, especially the TCR alpha beta(+) CD8 alpha alpha subset, was investigated using the different TLR4-mutant mouse strains C3H/HeJ, BALB/lps(d), and C57BL/10ScCr. Intestinal epithelial cells (i-EC) from TLR4-mutant strains did not express interleukin (IL)-15 mRNA, while IL-15 mRNA expression in i-EC from the corresponding wild-type, C3H/He, BALB/c, and C57BL/10ScSn mice was detected. The development of TCR alpha beta(+) CD8 alpha alpha cells in i-IEL significantly decreased in TLR4-mutant mice compared with the corresponding wild-type mice, while other T cell subsets in i-IEL showed similar percentages in the TLR4-mutant and wild-type mice. Adult thymectomized (ATx-) and lethally irradiated C3H/HeJ mice reconstituted with T cell-depleted bone marrow cells from C3H/He mice showed a significantly lower percentage of TCR alpha beta CD8 alpha alpha i-IEL than ATx-C3H/He mice after transfer of C3H/HeJ BM cells. The percentage of TCR alpha beta CD8 alpha alpha i-IEL and IL-15 mRNA expression in i-EC from BALB/lps(d) mice did not increase during Salmonella typhimurium infection but was significantly enhanced during Listeria monocytogenes infection. Our findings suggest that LPS induces IL-15 production by i-EC, resulting in the development of TCR alpha beta CD8 alpha alpha i-IEL.

Most murine CD8+ intestinal intraepithelial lymphocytes are partially but not fully activated T cells

Murine small intestine intraepithelial lymphocytes (IELs) bear properties of both activated and nonactivated T cells, although the significance of that dichotomy remains unclear. In this study, we show that although IELs express CD69 in situ and ex vivo, and have cytotoxic activity ex vivo, most CD8(+) IELs from normal mice are phenotypically similar to naive T cells in that they are CD45RB(high), CD44(low/int), and lack or have low levels of expression of CD25, Ly-6C, OX40, Fas ligand (FasL), and intracellular IFN-gamma synthesis. Unlike CD8(+) lymph node cells, IELs express high levels of the FasL gene, but do not express surface FasL until after CD3-mediated stimulation has occurred. Additionally, anti-CD3 stimulation of IELs in the presence of actinomycin-D did not inhibit FasL expression, suggesting that regulation FasL expression on IELs is controlled at least partially at the posttranscriptional level. Following CD3-mediated stimulation, IELs synthesize and secrete IFN-gamma more rapidly and to greater levels than CD8(+) lymph node cells, and they acquire the phenotype of fully activated effector cells as seen by an up-regulation of CD44, Ly-6C, OX40, FasL, and CD25 with the kinetics of memory T cells, with down-regulation of CD45RB expression. These findings indicate that contrary to previous interpretations, most small intestine IELs are not fully activated T cells, but rather that they are semiactivated T cells ready to shift to a fully activated state once a CD3-mediated signal has been received. These data also imply that under appropriate conditions it is possible for T cells to be sustained in a state of partial activation.

CD8 alpha-deficient mice are highly susceptible to 5-fluorouracil-induced lethality

Intestinal intraepithelial lymphocytes (i-IEL) expressing CD8 alpha are located in the intestine and may confer protection against invasion of intestinal microflora. We found that mice rendered deficient in CD8 alpha molecules by homologous recombination were susceptible to 5-fluorouracil (5-FU)-induced lethality accompanied by translocation of members of the enterobacteria. The number of i-IEL was greatly reduced on day 6 after 5-FU administration in both CD8 alpha(+/-) mice and CD8 alpha(-/-) mice, whereas the recovery of the level of i-IEL thereafter was significantly impaired in CD8 alpha(-/-) mice compared with that in CD8 alpha(+/-) mice. The ability of i-IEL to produce gamma interferon in response to immobilized T-cell receptor (TCR) alpha beta or TCR gamma delta monoclonal antibodies was significantly lower in CD8 alpha(-/-) mice than in CD8 alpha(+/-) mice. Transfer of CD8(+) i-IEL conferred significant protection against 5-FU-induced lethality in CD8 alpha(-/-) mice. The results suggest that CD8(+) i-IEL play an important role in protection against 5-FU-induced lethality with translocation of Enterobacteriaceae.

Differences in interleukin-12 and -15 production by dendritic cells at the early stage of Listeria monocytogenes infection between BALB/c and C57 BL/6 mice

The mechanisms responsible for the resistance of C57BL/6 mice and for the susceptibility of BALB/c mice to infection with Listeria monocytogenes were studied by comparing early IL-12 and IL-15 production by dendritic cells (DC) after infection with L. monocytogenes. Splenic DC expressing CD11b(low) and CD11c(+) obtained from C57BL/6 mice at 3 and 6 h after L. monocytogenes infection expressed higher levels of IL-12 p40 mRNA and IL-12 p40 protein than did those from BALB/c mice. Concurrently, a larger amount of IFN-gamma was produced by the splenic T cells from C57BL/6 mice in response to immobilized anti-TCRalphabeta mAb than by those from BALB/c mice, while the splenic T cells from BALB/c mice produced a higher level of IL-4 upon TCR alphabeta stimulation than did those of C57BL/6 mice. IL-15 mRNA and intracellular IL-15 protein were detected more abundantly in the DC from C57BL/6 mice than in those from BALB/c mice on day 3 after infection. CD3(+) IL2Rbeta (+) cells in the spleen were increased in C57BL/6 mice but not in BALB/c mice at the early stage after infection. Furthermore, IL-12Rbeta2 gene expression was up-regulated in T cells from C57BL/6 mice but not in those from BALB/c mice at the early stage after listerial infection. These results suggest that the difference in early production of IL-12 and IL-15 by DC may at least partly underlie the difference in susceptibility to L. monocytogenes between C57BL/6 and BALB/c mice.

Immune response to orally consumed antigens and probiotic bacteria

The gut mucosal system must fulfil conflicting roles in suppressing immune responses against orally fed antigens (tolerance) while still retaining the ability to respond to potential enteric pathogens. It must also, to a large degree, not mount an immune response against commensal enteric bacteria and the administration of large numbers of probiotic bacteria formulated as dietary supplements in food products. Contrary to this dogma, it has been found that feeding ovalbumin as a marker antigen, in association with selected probiotic bacteria, appears to prime for an intestinal immune response that is further augmented by skin vaccination. Skin immunization is known to stimulate a strong innate, humoral and cellular immune response. Such dominant immunogenic signals appear to override tolerogenic signals engendered by oral feeding of antigen. High-dose antigen feeding stimulated a strong Th2-dependent antibody response to skin vaccination but completely suppressed cytotoxic T cell responses. This was true even when ovalbumin was administered in conjunction with various selected probiotic bacteria. However, while yeast appeared to be better at priming for an enhanced humoral response, Lactobacillus fermentum and Staphylococcus carnosus were more effective in enhancing the postvaccinal lymphoproliferative response against ovalbumin.

The interaction of intestinal epithelial cells and intraepithelial lymphocytes in host defense

Intestinal intraepithelial lymphocytes (i-IEL) are located at the basolateral surfaces of intestinal epithelial cells (i-EC) and play important roles in the homeostasis of intestinal microenvironment. i-IEL comprise unique T cell populations including CD4-CD8alphaalpha+ T cells expressing T cell receptor (TCR)alphabeta or TCRgammadelta and CD4+ CD8alphaalpha+ T cells expressing TCR alphabeta. We show here that CD4+ CD8alphaalpha+ i-IEL belongs to Th1 type T cells capable of responding to self-MHC class I on i-EC and that a significant fraction of i-IEL expressed Fas ligand (Fas-L) and induced apoptosis in the i-EC via Fas-dependent pathway. i-IEL may recognize and eliminate the effete i-EC for homeostatic regulation of intestinal epithelia. The interaction of i-EC and i-IEL through E-cadherin/alphaEbeta7 integrin is important for homing and maintenance of i-IEL in intestine. Listeria monocytogenes are also known to interact with E-cadherin on i-EC and invade into the epithelial cells. Invasion of L. monocytogenes into i-EC activated NFkappa-B and subsequently up-regulated the expression of IL-15 gene, which has a NFkappa-B binding site at the promoter region. i-IEL, especially gammadelta T cells, were significantly activated to produce Th1 type cytokines at the early stage after oral infection with L. monocytogenes in mice and rats. The activation of i-IEL coincided with a peak response of IL-15 production by i-EC after infection. Taken together, mutual interaction of i-IEL and i-EC may be important not only for homeostatic regulation but also host defense against microbial infection in intestine.