Immune deviation of 2C transgenic intraepithelial lymphocytes in antigen-bearing hosts

Spontaneous autoimmune gastritis and hypochlorhydria are manifest in the ileitis-prone SAMP1/YitFcs mice

SAMP1/YitFcs mice serve as a model of Crohn's disease, and we have used them to assess gastritis. Gastritis was compared in SAMP1/YitFcs, AKR, and C57BL/6 mice by histology, immunohistochemistry, and flow cytometry. Gastric acid secretion was measured in ligated stomachs, while anti-parietal cell antibodies were assayed by immunofluorescence and enzyme-linked immunosorbent spot assay. SAMP1/YitFcs mice display a corpus-dominant, chronic gastritis with multifocal aggregates of mononuclear cells consisting of T and B lymphocytes. Relatively few aggregates were observed elsewhere in the stomach. The infiltrates in the oxyntic mucosa were associated with the loss of parietal cell mass. AKR mice, the founder strain of the SAMP1/YitFcs, also have gastritis, although they do not develop ileitis. Genetic studies using SAMP1/YitFcs-C57BL/6 congenic mice showed that the genetic regions regulating ileitis had comparable effects on gastritis. The majority of the cells in the aggregates expressed the T cell marker CD3 or the B cell marker B220. Adoptive transfer of SAMP1/YitFcs CD4(+) T helper cells, with or without B cells, into immunodeficient recipients induced a pangastritis and duodenitis. SAMP1/YitFcs and AKR mice manifest hypochlorhydria and anti-parietal cell antibodies. These data suggest that common genetic factors controlling gastroenteric disease in SAMP1/YitFcs mice regulate distinct pathogenic mechanisms causing inflammation in separate sites within the digestive tract.

Selection of self-reactive T cells in the thymus

On the whole, the healthy adaptive immune system is responsive to foreign antigens and tolerant to self. However, many individual lymphocytes have, and even require, substantial self-reactivity for their particular functions in immunity. In this review, we discuss several populations of lymphocytes that are thought to experience agonist stimulation through the T cell receptor during selection: nTreg cells, iNKT cells, nIELs, and nTh17s. We discuss the nature of this self-reactivity, how it compares with conventional T cells, and why it is important for overall immune health. We also outline molecular pathways unique to each lineage and consider possible commonalities to their development and survival.

Peripheral T cells re-enter the thymus and interfere with central tolerance induction

The thymus mainly contains developing thymocytes that undergo thymic selection. In addition, some mature activated peripheral T cells can re-enter the thymus. We demonstrated in this study that adoptively transferred syngeneic Ag-specific T cells can enter the thymus of lymphopenic mice, where they delete thymic dendritic cells and medullary thymic epithelial cells in an Ag-specific fashion, without altering general thymic functions. This induced sustained thymic release of autoreactive self-Ag-specific T cells suggested that adoptively transferred activated T cells can specifically alter the endogenous T cell repertoire by erasing negative selection of their own specificities. Especially in clinical settings in which adoptively transferred T cells cause graft-versus-host disease or graft-versus-leukemia, as well as in adoptive tumor therapies, these findings might be of importance, because the endogenous T cell repertoire might be skewed to contribute to both manifestations.

Intestinal T cells of Dicentrarchus labrax (L.): gene expression and functional studies

Cellular and molecular data have evidenced a gut-associated lymphoid tissue in a variety of teleost species, abundantly containing T cells, whose origin, selection and functions are still unclear. This study reports CD4, CD8-α, MHCI-α, MHCII-β, rag-1 and TCR-β gene transcription along the intestine (anterior, middle and posterior segments) and in the thymus of one year-old Dicentrarchus labrax (L.). Real-time PCR findings depicted a main role of the thymus in T-cell development, but also rag-1 and CD8-α transcripts are detected in the intestine, having significant expression in the posterior segment. In the whole intestine TCR-β and CD8-α exceeded CD4 transcripts. RNA ISH confirmed these data and detailed that mucosal CD8-α+ cells were especially numerous in the epithelium and in aggregates in the lamina propria. Regional differences in T-cell-specific gene expressions are first described in the intestine of a bony fish. High non-specific cytotoxic activity against xenogeneic and allogeneic cells was found in lymphocytes purified from the intestinal mucosa, providing further insight into their local defence roles.

Intestinal T cells: facing the mucosal immune dilemma with synergy and diversity

The epithelium of the gastrointestinal tract, which represents the greatest body surface area exposed to the outside environment, is confronted with a plethora of foreign and potentially harmful antigens. Consequently, the immune system of the gut faces the daunting task of distinguishing harmless dietary proteins and commensal bacteria from potentially dangerous pathogens, and of then responding accordingly. Mucosal T cells play a central role in maintaining barrier function and controlling the delicate balance between immune activation and immune tolerance. This review will focus on the unique features of mucosal T cell subsets that reside in the epithelium and lamina propria of the gut.

The same genomic region conditions clonal deletion and clonal deviation to the CD8alphaalpha and regulatory T cell lineages in NOD versus C57BL/6 mice

Clonal deviation is a mechanism by which immature thymocytes expressing a self-reactive T cell antigen receptor (TCR) are rescued from clonal deletion by adopting an alternative differentiation pathway resistant to apoptosis. Here, we confirm and generalize previous indications that genetic alleles in NOD mice condition ineffective clonal deviation toward the CD8alphaalpha lineage, a peculiar population of TCRalphabeta lymphocytes that electively colonizes the intraepithelial lymphocyte pool in the gut. Thymic selection of CD8alphaalpha cells was very age-dependent, occurring almost exclusively in the postnatal period. Fewer CD8alphaalpha cells were found in the thymus and intraepithelial lymphocytes of BDC2.5 TCR transgenic mice on the NOD than on the C57BL/6 (B6) background; this paucity extended to standard NOD mice, albeit to a lesser extent. CD8alphaalpha cells resided in the BDC2.5 pancreatic infiltrate, and they were more abundant on the B6 than the NOD background, correlating with aggressivity of the lesion. A (B6(g7) x NOD)F(2) intercross in agonist-challenged BDC2.5 fetal thymic organ cultures demonstrated the existence of a major quantitative trait locus on chromosome 3, coincident with an interval associated with resistance to clonal deletion. A replicate linkage confirmed these positions and showed that the same region also controls clonal deviation toward the CD4(+)FoxP3(+) regulatory T cell lineage. That clonal deviation toward the CD8alphaalpha and regulatory T cell pathways share genetic control further highlights the similarities between these two "rescue lineages," consistent with an immunoregulatory role for CD8alphaalpha cells.

Intraepithelial lymphocytes: their shared and divergent immunological behaviors in the small and large intestine

At the front line of the body's immunological defense system, the gastrointestinal tract faces a large number of food-derived antigens, allergens, and nutrients, as well as commensal and pathogenic microorganisms. To maintain intestinal homeostasis, the gut immune system regulates two opposite immunological reactions: immune activation and quiescence. With their versatile immunological features, intraepithelial lymphocytes (IELs) play an important role in this regulation. IELs are mainly composed of T cells, but these T cells are immunologically distinct from peripheral T cells. Not only do IELs differ immunologically from peripheral T cells but they are also comprised of heterogeneous populations showing different phenotypes and immunological functions, as well as trafficking and developmental pathways. Though IELs in the small and large intestine share common features, they have also developed differences as they adjust to the two different environments. This review seeks to shed light on the immunological diversity of small and large intestinal IELs.

Thymic differentiation of TCRαβ+CD8αα+IELs

Intraepithelial lymphocytes (IELs) contain several subsets, but the origin of the T-cell receptor (TCR)alphabeta(+) CD8 alpha alpha(+) IELs has been particularly controversial. Here we provide a synthesis, based on recent work, that attempts to unify the divergent views. The intestine has a primordial function in lymphopoiesis, and precursors with the potential to differentiate into T cells are found both in the epithelium and underlying lamina propria. Moreover, the thymus has been reported to export cells to the intestine that are not fully differentiated. TCR alpha beta(+) CD8 alpha alpha(+) IELs can differentiate in the intestine from each of these sources, but in normal euthymic mice, the thymus appears to be the major source for TCR alpha beta(+) CD8 alpha alpha(+) IELs. This unique IEL subset is a self-reactive population that requires exposure to self-agonists for selection in the thymus, similar to other regulatory T-cell populations. IELs transition through a double-positive (DP) intermediate in the thymus, but they originate from a subset of the DP cells that can be identified by its expression of CD8 alpha alpha homodimers. The agonist-selected cells in the thymus are TCRbeta(+) but CD4 and CD8 double negative. The evidence suggests that reacquired expression of CD8 alpha alpha and downregulation of CD5 occur after thymus export, perhaps in the intestine under the influence of interleukin-15. As a result of agonist exposure, a new gene expression program is activated. Therefore, the increased understanding of the developmental origin of TCR alpha beta(+) CD8 alpha alpha(+) IELs may help us to understand how they participate in immune regulation and protection in the intestine.

IELs: enforcing law and order in the court of the intestinal epithelium

The intestinal intraepithelial lymphocytes (IELs) are mostly T cells dispersed as single cells within the epithelial cell layer that surrounds the intestinal lumen. IELs are, therefore, strategically located at the interface between the antigen-rich outside world and the sterile core of the body. The intestine of higher vertebrates has further evolved to harbor numerous commensal bacteria that carry out important functions for the host, and while defensive immunity can effectively protect against the invasion of pathogens, similar immune reactions against food-derived antigens or harmless colonizing bacteria can result in unnecessary and sometimes damaging immune responses. Probably as a result of this unique dilemma imposed by the gut environment, multiple subsets of IEL have differentiated, which all display characteristics of 'activated yet resting' immune cells. Despite this common feature, IELs are heterogeneous with regard to their phenotype, ontogeny, and function. In this review, we discuss the different subtypes of IELs and highlight the distinct pathways they took that led to their unique differentiation into highly specialized effector memory T cells, which provide the most effective immune protection yet in a strictly regulated fashion to preserve the integrity and vital functions of the intestinal mucosal epithelium.

Regional oral tolerance in transgenic 2C mice

BACKGROUND

Antigen injected subcutaneously (SQ) results in a strong systemic immune response, whereas antigen infused orally or by the portal vein tends to induce tolerance. Nontransgenic C57BL/6J (H-2(b)), or B6, mice and transgenic 2C mice (that express a cytotoxic T cell receptor against major histocompatibility complex Class I L(d)) were used to investigate the regional and systemic responses to oral antigen.

METHODS

B6 (H-2(b)) and 2C (H-2(b)) mice were given either saline or BALB/cByJ (H-2(d), L(d+)) (BALB/c) spleen cells (SCs) (25 x 10(6)) by oral gavage (PO) on day 0. Injection of 10 x 10(6) BALB/c SCs SQ was performed after 7 days, followed by a footpad injection of 10 x 10(6) BALB/c SC on day 14. Specific footpad swelling was measured 24 hours later by means of a micrometer. Mixed lymphocyte culture was performed on splenic and mesenteric lymph node (MLN) lymphocytes. The percentage of splenic and MLN CD4+ and CD8+ T cells was quantitated by fluorescence activated cell sorter. Cytokine messenger RNA (mRNA) and protein was measured by reverse transcriptase-polymerase chain reaction and enzyme-linked immunosorbent assay.

RESULTS

Whereas B6 mice were shown to have specifically decreased responsiveness to Balb/c cells in vivo and in vitro after oral Balb/c, 2C mice did not demonstrate any downregulation in delayed-type hypersensitivity responsiveness or splenic lymphocyte proliferation. However, MLN lymphocytes from 2C mice did demonstrate decreased proliferation against Balb/c after oral Balb/c gavage (P < .01). Subset percentages of naïve B6 (n=8) spleen and MLN lymphocytes were 50% to 52% for CD4+ and 46% to 47% for CD8+ T cells. These percentages were unchanged in B6 mice after PO Balb/c. Subset percentages of naïve 2C (n=6) spleen and MLN lymphocytes were 2% to 5% for CD4+ and 61% to 64% for CD8+ T cells. After PO Balb/c, MLN CD4+ and CD8+ T cells were unchanged, whereas splenic CD8+ T cells decreased to 44% to 48% and CD4+ T cells increased to 32% to 33% (P < .05, P < .05 vs naïve 2C spleen). After PO Balb/c, inflammatory interleukin (IL)-2 and interferon gamma mRNA and protein were increased in 2C spleen cells, whereas they were decreased in 2C MLN. Anti-inflammatory cytokine IL-4 and IL-10 mRNA and protein were increased in 2C MLN after PO Balb/c, whereas they were not detectable in 2C spleen cells.

CONCLUSIONS

Systemic oral tolerance can be induced in B6 mice, whereas only regional mesenteric lymph node tolerance develops in transgenic 2C mice. The inability to induce systemic oral tolerance in 2C mice appears to be due to a significant increase in peripheral (splenic) CD4+ T cells.

Mucosal T lymphocytes--peacekeepers and warriors

Normal immune homeostasis of the intestine requires peaceful coexistence with commensal flora, combined with host defense against pathogens. Perhaps as a result of this unique dilemma, distinct populations of regulatory and effector T lymphocytes are found in the lamina propria and epithelium of the intestine. Here we summarize the properties and functions of these unusual T cells, and describe the molecular and cellular interactions that lead to their development and function. Some mucosal T cells, sometimes called type a, are conventional activated/memory T cells that have received instructions to migrate to the intestine during priming by dendritic cells in the mesenteric lymph node and elsewhere. Others, however, particularly subsets residing permanently in the epithelium, are intestine-specific T cell subpopulations generated by an atypical differentiation pathway.

Inducible lymphoid tissues in the adult gut: recapitulation of a fetal developmental pathway?

The intestinal immune system faces an extraordinary challenge from the large numbers of commensal bacteria and potential pathogens that are restrained by only a single layer of epithelial cells. Here, I discuss evidence that the intestinal immune system develops an extensive network of inducible, reversible lymphoid tissues that contributes to the vital equilibrium between the gut and the bacterial flora. I propose that this network is induced by cryptopatches, which are small clusters of dendritic cells and lymphoid cells that are identical to fetal inducers of lymph-node and Peyer's-patch development.

Characterization of a B220+Lymphoid Cell Subpopulation with Immune Modulatory Functions in Nasal-Associated Lymphoid Tissues

Complex mechanisms operate on mucosal tissues to regulate immune responsiveness and tolerance. When the lymphocyte subpopulations from murine nasal-associated lymphoid tissues (NALT) were characterized, we observed an accumulation of B220(low)CD3(low)CD4(-)CD8(-)CD19(-)c-Kit(+) cells. TCR transgenic mice and athymic mice were used for monitoring T cell lineage and the presence of extrathymic T cell precursors. The majority of cells from NALT exhibited a T cell precursor phenotype (CD4(-)CD8(-)CD19(-)c-Kit(+)). Fas-independent apoptosis was their main mechanism of cell death. We also demonstrated that B220(low)CD4(-)CD8(-)CD19(-) cells from NALT exhibited the potential to down-regulate the activation of mature T cells. However, the innate immunity receptor TLR2 was also highly expressed by this cell subpopulation. Moreover, nasal stimulation with a TLR2/6 agonist resulted in a partial activation of the double-negative cells. These results suggest that the immune responses in NALT may be in part modulated by a cell subpopulation that maintains a tolerogenic milieu by its proapoptotic status and suppressive activity, which can be reverted through stimulation of a TLR signaling cascade.

Starting at the beginning: new perspectives on the biology of mucosal T cells

The gastrointestinal tract is the central organ for uptake of fluids and nutrients, and at the same time it forms the main protective barrier between the sterile environment of the body and the outside world. In mammals, the intestine has further evolved to harbor a vast load of commensal bacteria that have important functions for the host. Discrimination by the host defense system of nonself from self can prevent invasion of pathogens, but equivalent responses to dietary or colonizing bacteria can lead to devastating consequences for the organism. This dilemma imposed by the gut environment has probably contributed significantly to the evolutionary drive that has led to sophisticated mechanisms and diversification of the immune system to allow for protection while maintaining the integrity of the mucosal barrier. The immense expansion and specialization of the immune system is particularly mirrored in the phylogeny, ontogeny, organization, and regulation of the adaptive intraepithelial lymphocytes, or IEL, which are key players in the unique intestinal defense mechanisms that have evolved in mammals.

Resistance to viral infection by intraepithelial lymphocytes in HIV-1 P18-I10-specific T-cell receptor transgenic mice

For the analysis of mucosal immunity to HIV-1, we have recently established a line of transgenic (Tg) mice expressing the TCRalpha and TCRbeta genes of the murine CTL clone RT1 specific for P18-I10 (RGPGRAFVTI), an immunodominant gp160 envelope-derived epitope of IIIB isolate, restricted by the H-2D(d) MHC-I molecule. Here we examine those cells bearing specific TCR among the intraepithelial lymphocytes (IELs), with flow cytometric analysis using H-2D(d)/P18-I10 tetramers. We observed three distinct CD3(+), tetramer positive populations among the IELs: extra-thymic CD8alphabeta(+), alphabetaTCR T-cells; CD8 alphaalpha+, gammadeltaTCR T-cells; and thymus-derived CD8alphabeta+, alphabetaTCR T-cells. Challenge of these Tg mice with P18-I10 encoded by a vaccinia virus vector, either intrarectally (i.r.) or intraperitoneally (i.p.), revealed that the intraepithelial compartment seems to be a major site for prevention of the spread of viral infection. Such immunity appears due to the thymus-derived, CD8alphabeta+ antigen-specific CTLs together with CD8alphaalpha+ gammadelta cells, which regulate virus spread. This model system for studying CTL based immunity at mucosal sites should prove helpful in developing rational approaches for HIV control.

Virus-Induced Activation of Self-Specific TCRαβ CD8αα Intraepithelial Lymphocytes Does Not Abolish Their Self-Tolerance in the Intestine

TCRalphabeta CD8alphaalpha intestinal intraepithelial lymphocytes (IEL) represent an enigmatic subset of T cells, particularly, in regard to their potential functions and the apparent persistence of cells expressing self-specific TCR. We have used mice that are transgenic for the TCRalphabeta specific for the lymphocytic choriomeningitis virus (LCMV)-derived peptide gp33, and TCRalphabeta-transgenic mice that coexpress the gp33 Ag ubiquitously, to analyze the functional properties of TCRalphabeta CD8alphaalpha IEL in the presence, or absence, of their specific MHC-restricted Ag, and to assess the impact of molecular mimicry during a potent LCMV infection on potentially self-reactive TCRalphabeta CD8alphaalpha IEL. In this study, we show that the presence of the specific self-Ag results in reduced expression of IL-2, IFN-gamma, and IL-10 by resident TCRalphabeta CD8alphaalpha IEL while expression of mRNA for TGFbeta is not affected. We further demonstrate that despite their secluded location in the epithelium, TCRalphabeta CD8alphaalpha IEL are activated after infection of the intestinal mucosa with LCMV. Importantly, LCMV-induced activation of self-specific TCRalphabeta CD8alphaalpha IEL does not reverse their tolerance as no cytotoxic activity or up-regulated expression of proinflammatory cytokines is detected and no overt signs of autoimmunity are seen. Taken together, these results are in support of an immunoregulatory role for self-specific TCRalphabeta CD8alphaalpha in the intestinal mucosa and clearly speak against an involvement of this cell subset in inflammatory reactions and tissue destruction.

Thymic Selection and Peripheral Activation of CD8 T Cells by the Same Class I MHC/Peptide Complex

Thymic selection is controlled by the interaction between TCR and MHC/peptide. Strength and quality of the signal determine whether thymocytes are selected or deleted. The factors that contribute to this signal remain poorly defined. Here we show that fetal thymic organ cultures (FTOCs) derived from OT-I transgenic mice (the OT-I TCR is restricted by K(b)-SIINFEKL) on a K(b)D(b-/-) background support positive selection, but only when provided with soluble H-2K(b)-SIINFEKL complexes. Selection of CD8 T cells is independent of the valency of the ligand or its capability to coengage CD8 molecules. Both CD8alphaalpha and CD8alphabeta T cells are selected by H-2K(b)-SIINFEKL, but only CD8alphabeta cells are capable of releasing IFN-gamma in response to the same ligand. The alpha(4)beta(7) integrin is up-regulated on postselection thymocytes from FTOCs. After adoptive transfer, FTOC-derived OT-I CD8 T cells divide in response to the agonist peptide SIINFEKL. These results establish that CD8 T cells responsive to their nominal peptide-Ag can be generated in FTOC supplemented with soluble MHC class I molecules equipped with the same peptide.

Precursors of functional MHC class I- or class II-restricted CD8alphaalpha(+) T cells are positively selected in the thymus by agonist self-peptides

The origin and specificity of alphabeta TCR(+) T cells that express CD8alphaalpha have been controversial issues. Here we provide direct evidence that precursors of functional CD8alphaalpha T cells are positively selected in the thymus in the presence of agonist self-peptides. Like conventional positive selection, this agonist selection process requires functional TCR alpha-CPM, whereas it is independent of CD8beta expression. Furthermore, CD8alphaalpha expression on mature, agonist-selected T cells does not imply selection by MHC class I, and CD8alphaalpha(+) T cells can be either class I or class II restricted. Our data define a distinct agonist-dependent, positive selection process in the thymus, and they suggest a function for CD8alphaalpha distinct from the conventional TCR coreceptor function of CD8alphabeta or CD4.

Positive selection of an MHC class-I restricted TCR in the absence of classical MHC class I molecules

The H-2Ld alloreactive 2C T cell receptor (TCR) is commonly considered as being positively selected on the H-2Kb molecule. Surprisingly, 2C TCR+ CD8+ single-positive T cells emerge in massive numbers in fetal thymic organ culture originating from 2C transgenic, H-2KbD(b-/-) (2C+KbD(b-/-)) but not in fetal thymic organ culture from beta2-microglobulin(-/-) 2C transgenic animals. Mature CD8+ T cells are observed in newborn but not in adult 2C+KbD(b-/-) mice. These CD8+ T cells express the alpha4beta7 integrin, which allows them to populate the intestine, a pattern of migration visualized by intrathymic injection of FITC and subsequent accrual of FITC-labeled lymphocytes in the gut. We conclude that the 2C TCR is reactive not only with H-2Ld and H-2Kb, but also with nonclassical MHC class I products to enable positive selection of 2C+ T cells in the fetal and newborn thymus and to support their maintenance in the intestine.

Selective expansion of intraepithelial lymphocytes expressing the HLA-E-specific natural killer receptor CD94 in celiac disease

BACKGROUND & AIMS

Celiac disease is a gluten-induced enteropathy characterized by the presence of gliadin-specific CD4(+) T cells in the lamina propria and by a prominent intraepithelial T-cell infiltration of unknown mechanism. The aim of this study was to characterize the subset(s) of intraepithelial lymphocytes (IELs) expanding during active celiac disease to provide insights into the mechanisms involved in their expansion.

METHODS

Flow-cytometric analysis of isolated IELs and/or immunohistochemical staining of frozen sections were performed in 51 celiac patients and 50 controls with a panel of monoclonal antibodies against T-cell and natural killer (NK) receptors. In addition, in vitro studies were performed to identify candidate stimuli for NK receptor expression.

RESULTS

In normal intestine, different proportions of IELs, which were mainly T cells, expressed the NK receptors CD94/NKG2, NKR-P1A, KIR2D/3D, NKp46, Pen5, or CD56. During the active phase of celiac disease, the frequency of CD94(+) IELs, which were mostly alphabeta T cells, was conspicuously increased over controls. In contrast, the expression of other NK markers was not modified. Furthermore, expression of CD94 could be selectively induced in vitro by T-cell receptor activation and/or interleukin 15, a cytokine produced by intestinal epithelial cells.

CONCLUSIONS

The gut epithelium favors the development of T cells that express NK receptors. In active celiac disease, there is a specific and selective increase of IELs expressing CD94, the HLA-E-specific NK receptor that may be related to T-cell receptor activation and/or interleukin 15 secretion.

Autospecific gammadelta thymocytes that escape negative selection find sanctuary in the intestine

alphabeta or gammadelta thymocytes whose T-cell receptors (TCRs) recognize endogenously expressed antigens (Ag) are autospecific and, thus, potentially self-reactive. In the thymus, such T cells are eliminated during T-cell development through a process known as negative selection. As a model of negative selection of gammadelta T cells, we have used G8 gammadelta-T cell transgenic mice, which express a gammadelta TCR that recognizes the nonpolymorphic MHC class I TL(b) molecule. Here, we demonstrate that negative selection of autospecific gammadelta T cells is almost complete in the adult thymus but is markedly attenuated in the neonatal thymus. A consequence of this attenuated negative selection is that potentially self-reactive gammadelta thymocytes are allowed to escape negative selection, undergo extrathymic differentiation, and find sanctuary in the intestinal epithelium. Interestingly, the ability of these potentially self-reactive gammadelta T cells to find sanctuary requires both the intestinal epithelial environment and the extrathymic presence of the self-Ag. The implications of these findings on the development and persistence of autoreactive T cells in autoimmune disease are discussed.

High- and low-affinity single-peptide/MHC ligands have distinct effects on the development of mucosal CD8alphaalpha and CD8alphabeta T lymphocytes

In this study, we compared the influence of two peptides on the selection of CD8alphaalpha and CD8alphabeta intraepithelial lymphocytes (IELs) of the intestine, which develop by a unique and partially thymus-independent process. Mice were used in which all T cells carried one transgenic T cell antigen receptor (TCR) (F5), and in which only well defined transgenic peptides were presented by H-2Db. The first peptide, for which the F5 TCR has a high affinity, derives from the influenza virus nucleoprotein (NP68). The second peptide, NP34, is an antagonistic variant of NP68 and is recognized by the F5 TCR with low affinity. To avoid presentation of endogenous peptides or production of T cells carrying alternative TCRs, F5 TCR transgenic mice were generated that were deficient for Tap-1 and Rag-1. In these mice, no CD3(+)CD8(+) cells were found in lymph nodes, spleen, or intestine. Introduction of transgenes encoding either NP34 or NP68 along with an endoplasmic reticulum signal sequence enabled Tap-1-independent expression of each peptide in these mice. Positive selection of F5TCR+CD8(+) thymocytes was not rescued by these transgenic peptides. However, the high-affinity NP68 peptide induced maturation of CD8alphaalpha IEL, whereas the low-affinity NP34 peptide stimulated development of both CD8alphabeta and CD8alphaalpha IEL, but in smaller numbers. When both peptides were present, CD8alphabeta T cells failed to develop and the number of CD8alphaalpha IELs was lower than in mice carrying the NP68 transgene alone. These data demonstrate that single ligands with a high or low affinity for TCR are capable of inducing or inhibiting the maturation of alternative subsets of IELs.

Activation and peripheral expansion of murine T-cell receptor gamma delta intraepithelial lymphocytes

BACKGROUND & AIMS

The intestinal epithelial compartment is populated by CD8(+) alpha beta and gamma delta intraepithelial lymphocytes (IELs), which monitor the integrity of the epithelial barrier. alpha beta IELs are activated by peptide antigens presented by class I major histocompatibility complex (MHC) molecules, but it is unclear how gamma delta IELs are activated.

METHODS

G8 T-cell receptor (TCR) gamma delta transgenic (Tg) mice (specific for the class I MHC alloantigen, T22/10(b)) were crossed to class I MHC-deficient beta2-microglobulin-knockout (beta2m degrees) mice, and Tg+ IELs were examined for relative yields and surface and functional phenotype.

RESULTS

Evidence for class I MHC-induced activation of Tg+ IELs was supported by the detection of 4-fold greater numbers of Tg+ IELs in G8 x beta2m+ mice that proliferated at 15-fold higher levels than IELs from G8 x beta2m degrees mice. However, expression of CD69, production of cytokine (interleukin 2 and interferon gamma), and detection of cytolytic function for IELs in G8 x beta2m degrees mice suggested that class I MHC was not required for gamma delta IEL development or maturation.

CONCLUSIONS

These results suggest that CD8(+) TCR gamma delta IELs do not require class I MHC for development but support the notion that antigens presented by class I MHC molecules are involved in the peripheral expansion and differentiation of this subset.

Natural killer and T cells of innate and adaptive immunity: lymphoid compartments with different requirements for common gamma chain-dependent cytokines

A group of cytokines, including interleukin-2, -4, -7, -9 and -15, are related through the usage of a shared receptor subunit, the common cytokine receptor gamma chain, gamma c. gamma c-dependent cytokines critically affect the development and maintenance of the lymphoid system. This review will highlight our current knowledge on the gamma c-dependent cytokine network and on the non-redundant roles that these cytokines play in the development and homeostasis of T and natural killer cells involved in innate and adaptive immunity.

An opposite pattern of selection of a single T cell antigen receptor in the thymus and among intraepithelial lymphocytes

The differentiation of intestinal intraepithelial lymphocytes (IEL) remains controversial, which may be due in part to the phenotypic complexity of these T cells. We have investigated here the development of IEL in mice on the recombination activating gene (RAG)-2(-/-) background which express a T cell antigen receptor (TCR) transgene specific for an H-Y peptide presented by Db (H-Y/Db x RAG-2(-) mice). In contrast to the thymus, the small intestine in female H-Y/Db x RAG-2(-) mice is severely deficient in the number of IEL; TCR transgene+ CD8alphaalpha and CD8alphabeta are virtually absent. This is similar to the number and phenotype of IEL in transgenic mice that do not express the Db class I molecule, and which therefore fail positive selection. Paradoxically, in male mice, the small intestine contains large numbers of TCR+ IEL that express high levels of CD8alphaalpha homodimers. The IEL isolated from male mice are functional, as they respond upon TCR cross-linking, although they are not autoreactive to stimulator cells from male mice. We hypothesize that the H-Y/Db TCR fails to undergo selection in IEL of female mice due to the reduced avidity of the TCR for major histocompatibility complex peptide in conjunction with the CD8alphaalpha homodimers expressed by many cells in this lineage. By contrast, this reduced TCR/CD8alphaalpha avidity may permit positive rather than negative selection of this TCR in male mice. Therefore, the data presented provide conclusive evidence that a TCR which is positively selected in the thymus will not necessarily be selected in IEL, and furthermore, that the expression of a distinct CD8 isoform by IEL may be a critical determinant of the differential pattern of selection of these T cells.

Increased Threshold for TCR-Mediated Signaling Controls Self Reactivity of Intraepithelial Lymphocytes

To examine the effect of self Ag on activation requirements of TCR-αβ intestinal intraepithelial lymphocytes (IELs), we utilized the 2C transgenic (Tg) mouse model specific for a peptide self Ag presented by class I MHC, H-2Ld. CD8αα and CD4−CD8− IELs from syngeneic (H-2b, self Ag−) and self Ag-bearing (H-2b/d, self Ag+) strains were examined for their ability to respond in vitro to P815 (H-2d) cell lines expressing the endogenous antigenic peptide, p2Ca. Proliferation, cytokine production, and CTL activity were elicited in IEL T cells isolated from self Ag− H-2b mice when stimulated with P815 cells expressing basal levels of self Ag. These responses were enhanced following the addition of exogenous p2Ca peptide and ectopic expression of the costimulatory molecule, B7-1. By comparison, IEL from self Ag-bearing mice failed to respond to basal levels of self Ag presented by P815 cells even in the presence of B7-1-mediated costimulation. However, the addition of increasing amounts of exogenous p2Ca peptide induced a response from the in vivo “tolerized” T cells. These results suggest that exposure to self Ag in vivo increased the threshold of TCR activation of Ag-exposed self-reactive IELs. The dependence of increased signal 1 to activate self-reactive IELs suggests a defect in TCR signaling that may maintain self tolerance in vivo. These data suggest that conditions that overcome signal 1 IEL defects may initiate autoreactive responses in the intestine.

TCR zeta chain in T cell development and selection

Current data suggest that an important function of the multimeric structure of the TCR is to enable the assembly of structurally and functionally different forms of the TCR, the pre-TCR and alphabetaTCR complexes, at different stages in development. Four distinct TCR subunits (the CD3gamma, delta, and epsilon chains and the zeta chain) contain signal transducing motifs; however, the zeta chain is notable for containing three of these elements. These motifs, especially those within the zeta chain, function to amplify signals generated by the TCR, and this property is especially critical during thymocyte selection. The results of several recent experiments argue that positive and negative selection of thymocytes may involve activation of distinct downstream signaling pathways. The outcome of thymocyte selection can also be influenced, however, by quantitative effects such as changes in ligand concentration or direct alteration of the TCR signaling potential. Recent studies pertaining to the kinetics of TCR-ligand interactions may provide insight into how signaling through the TCR can be regulated either quantitatively or qualitatively.

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.