Development of CD8alpha/alpha and CD8alpha/beta T cells in major histocompatibility complex class I-deficient mice

Commensal bacteria maintain a Qa-1b-restricted unconventional CD8+ T population in gut epithelium

Intestinal intraepithelial lymphocytes (IELs) are characterized by an unusual phenotype and developmental pathway, yet their specific ligands and functions remain largely unknown. Here by analysis of QFL T cells, a population of CD8+ T cells critical for monitoring the MHC I antigen processing pathway, we established that unconventional Qa-1b-restricted CD8+ T cells are abundant in intestinal epithelium. We found that QFL T cells showed a Qa-1b-dependent unconventional phenotype in the spleen and small intestine of naïve wild-type mice. The splenic QFL T cells showed innate-like functionality exemplified by rapid response to cytokines or antigens, while the gut population was refractory to stimuli. Microbiota was required for the maintenance, but not the initial gut homing of QFL T cells. Moreover, monocolonization with Pediococcus pentosaceus, which expresses a peptide that cross-activated QFL T cells, was sufficient to maintain QFL T cells in the intestine. Thus, microbiota is critical for shaping the Qa-1b-restricted IEL landscape.

Commensal Bacteria Maintain a Qa-1 b -restricted Unconventional CD8 + T Population in Gut Epithelium

Intestinal intraepithelial lymphocytes (IELs) are characterized by an unusual phenotype and developmental pathway, yet their specific ligands and functions remain largely unknown. Here by analysis of QFL T cells, a population of CD8 + T cells critical for monitoring the MHC I antigen processing pathway, we established that unconventional Qa-1 b -restricted CD8 + T cells are abundant in intestinal epithelium. We found that QFL T cells showed a Qa-1 b -dependent unconventional phenotype in the spleen and small intestine of naïve wild-type mice. The splenic QFL T cells showed innate-like functionality exemplified by rapid response to cytokines or antigen, while the gut population was refractory to stimuli. Microbiota was required for the maintenance, but not the initial gut homing of QFL T cells. Moreover, monocolonization with Pediococcus pentosaceus, which expresses a peptide that cross-activated QFL T cells, was sufficient to maintain QFL T cells in the intestine. Thus, microbiota is critical for shaping the Qa-1 b -restricted IEL landscape.

Characterization of Bovine Intraepithelial T Lymphocytes in the Gut

Intraepithelial T lymphocytes (T-IELs), which constitute over 50% of the total T lymphocytes in the animal, patrol the mucosal epithelial lining to defend against pathogen invasion while maintaining gut homeostasis. In addition to expressing T cell markers such as CD4 and CD8, T-IELs display T cell receptors (TCR), including either TCRαβ or TCRγδ. Both humans and mice share similar T-IEL subsets: TCRγδ+, TCRαβ+CD8αα+, TCRαβ+CD4+, and TCRαβ+CD8αβ+. Among these subsets, human T-IELs are predominantly TCRαβ+ (over 80%), whereas those in mice are mostly TCRγδ+ (~60%). Of note, the majority of the TCRγδ+ subset expresses CD8αα in both species. Although T-IELs have been extensively studied in humans and mice, their profiles in cattle have not been well examined. Our study is the first to characterize bovine T-IELs using flow cytometry, where we identified several distinct features. The percentage of TCRγδ+ was comparable to that of TCRαβ+ T-IELs (both ~50% of CD3+), and the majority of bovine TCRγδ+ T-IELs did not express CD8 (CD8-) (above 60%). Furthermore, about 20% of TCRαβ+ T-IELs were CD4+CD8αβ+, and the remaining TCRαβ+ T-IELs were evenly distributed between CD4+ and CD8αβ+ (~40% of TCRαβ+ T-IELs each) with no TCRαβ+CD8αα+ identified. Despite these unique properties, bovine T-IELs, similar to those in humans and mice, expressed a high level of CD69, an activation and tissue-retention marker, and a low level of CD62L, a lymphoid adhesion marker. Moreover, bovine T-IELs produced low levels of inflammatory cytokines such as IFNγ and IL17A, and secreted small amounts of the immune regulatory cytokine TGFβ1. Hence, bovine T-IELs' composition largely differs from that of human and mouse, with the dominance of the CD8- population among TCRγδ+ T-IELs, the substantial presence of TCRαβ+CD4+CD8αβ+ cells, and the absence of TCRαβ+CD8αα+ T-IELs. These results provide the groundwork for conducting future studies to examine how bovine T-IELs respond to intestinal pathogens and maintain the integrity of the gut epithelial barrier in animals.

Diverse developmental pathways of intestinal intraepithelial lymphocytes

The intestinal epithelial barrier is patrolled by resident intraepithelial lymphocytes (IELs) that are involved in host defence against pathogens, wound repair and homeostatic interactions with the epithelium, microbiota and nutrients. Intestinal IELs are one of the largest populations of lymphocytes in the body and comprise several distinct subsets, the identity and lineage relationships of which have long remained elusive. Here, we review advances in unravelling the complexity of intestinal IEL populations, which comprise conventional αβ T cell receptor (TCRαβ)⁺ subsets, unconventional TCRαβ⁺ and TCRγδ⁺ subsets, group 1 innate lymphoid cells (ILC1s) and ILC1-like cells. Although these intestinal IEL lineages have partially overlapping effector programmes and recognition properties, they have strikingly different developmental pathways. We suggest that evolutionary pressure has driven the recurrent generation of cytolytic effector lymphocytes to protect the intestinal epithelial layer, but they may also precipitate intestinal inflammatory disorders, such as coeliac disease.

Human intraepithelial lymphocytes

The location of intraepithelial lymphocytes (IEL) between epithelial cells, their effector memory, cytolytic and inflammatory phenotype positions them to kill infected epithelial cells and protect the intestine against pathogens. Human TCRαβ+CD8αβ+ IEL have the dual capacity to recognize modified self via natural killer (NK) receptors (autoreactivity) as well as foreign antigen via the T cell receptor (TCR), which is accomplished in mouse by two cell subsets, the naturally occurring TCRαβ+CD8αα+ and adaptively induced TCRαβ+CD8αβ+ IEL subsets, respectively. The private/oligoclonal nature of the TCR repertoire of both human and mouse IEL suggests local environmental factors dictate the specificity of IEL responses. The line between sensing of foreign antigens and autoreactivity is blurred for IEL in celiac disease, where recognition of stress ligands by induced activating NK receptors in conjunction with inflammatory signals such as IL-15 can result in low-affinity TCR/non-cognate antigen and NK receptor/stress ligand interactions triggering destruction of intestinal epithelial cells.

P, Emiliano J. R, Amaro R. G, Briones S. L. High fat diet induces alterations to intraepithelial lymphocyte and cytokine mRNA in the small intestine of C57BL/6 mice

The aim of this work was to investigate the possible effect of high fat diet (HFD) induced obesity on iIEL subsets and their cytokine mRNA levels in C57BL/6 mice.

Creating class I MHC-null pigs using guide RNA and the Cas9 endonuclease

Pigs are emerging as important large animal models for biomedical research, and they may represent a source of organs for xenotransplantation. The MHC is pivotal to the function of the immune system in health and disease, and it is particularly important in infection and transplant rejection. Pigs deficient in class I MHC could serve as important reagents to study viral immunity as well as allograft and xenograft rejection. In this study, we report the creation and characterization of class I MHC knockout pigs using the Cas9 nuclease and guide RNAs. Pig fetal fibroblasts were genetically engineered using Cas9 and guide RNAs, and class I MHC(-) cells were then used as nuclear donors for somatic cell nuclear transfer. We produced three piglets devoid of all cell surface class I proteins. Although these animals have reduced levels of CD4(-)CD8(+) T cells in peripheral blood, the pigs appear healthy and are developing normally. These pigs are a promising reagent for immunological research.

Elevated T cell receptor signaling identifies a thymic precursor to the TCRαβ(+)CD4(-)CD8β(-) intraepithelial lymphocyte lineage

The origin and developmental pathway of intestinal T cell receptor αβ(+) CD4(-)CD8β(-) intraepithelial lymphocytes (unconventional iIELs), a major population of innate-like resident cytolytic T cells, have remained elusive. By cloning and expressing several TCRs isolated from unconventional iIELs, we identified immature CD4(lo)CD8(lo)(DP(lo))CD69(hi)PD-1(hi) thymocytes as the earliest postsignaling precursors for these cells. Although these precursors displayed multiple signs of elevated TCR signaling, a sizeable fraction of them escaped deletion to selectively engage in unconventional iIEL differentiation. Conversely, TCRs cloned from DP(lo)CD69(hi)PD-1(hi) thymocytes, a population enriched in autoreactive thymocytes, selectively gave rise to unconventional iIELs upon transgenic expression. Thus, the unconventional iIEL precursor overlaps with the DP(lo) population undergoing negative selection, indicating that, concomitant with the downregulation of both CD4 and CD8 coreceptors, a balance between apoptosis and survival signals results in outcomes as divergent as clonal deletion and differentiation to the unconventional iIEL lineage.

Cytotoxic and proinflammatory CD8+ T lymphocytes promote development of vulnerable atherosclerotic plaques in apoE-deficient mice

BACKGROUND

Heart attacks and strokes, leading causes of deaths globally, arise from thrombotic occlusion of ruptured vulnerable atherosclerotic plaques characterized by abundant apoptosis, large necrotic cores derived from inefficient apoptotic cell clearance, thin fibrous caps, and focal inflammation. The genesis of apoptosis and necrotic cores in these vulnerable atherosclerotic plaques remains unknown. Cytotoxic CD8(+) T lymphocytes represent up to 50% of leukocytes in advanced human plaques and dominate early immune responses in mouse lesions, yet their role in atherosclerosis also remains unresolved.

METHODS AND RESULTS

CD8(+) T-lymphocyte depletion by CD8α or CD8β monoclonal antibody in apolipoprotein E-deficient mice fed a high-fat diet ameliorated atherosclerosis by reducing lipid and macrophage accumulation, apoptosis, necrotic cores, and monocyte chemoattractant protein 1, interleukin 1β, interferon γ, and vascular cell adhesion molecule 1. Transfer of CD8(+) T cells into lymphocyte-deficient, apolipoprotein E-deficient mice partially reconstituted CD8(+) T cells in lymphoid compartments and was associated with CD8(+) T-cell infiltration in lesions, increased lipid and macrophage accumulation, apoptotic cells, necrotic cores, and interleukin 1β in atherosclerotic lesions. Transfer of CD8(+) T cells deficient in perforin, granzyme B, or tumor necrosis factor α but not interferon γ failed to increase atherosclerotic lesions despite partial reconstitution in the lymphoid system and the presence in atherosclerotic lesions. Macrophages, smooth muscle cells, and endothelial cells were identified as apoptotic targets.

CONCLUSIONS

We conclude that CD8(+) T lymphocytes promote the development of vulnerable atherosclerotic plaques by perforin- and granzyme B-mediated apoptosis of macrophages, smooth muscle cells, and endothelial cells that, in turn, leads to necrotic core formation and further augments inflammation by tumor necrosis factor α secretion.

Effective RNAi-mediated β2-microglobulin loss of function by transgenesis in Xenopus laevis

To impair MHC class I (class I) function in vivo in the amphibian Xenopus, we developed an effective reverse genetic loss of function approach by combining I-SceI meganuclease-mediated transgenesis with RNAi technology. We generated transgenic outbred X. laevis and isogenetic laevis/gilli cloned lines with stably silenced expression of β2-microglobulin (b2m) critical for class I function. Transgenic F₁ frogs exhibited decreased surface class I expression on erythrocytes and lymphocytes, decreased frequency of peripheral CD8 T cells and impaired CD8 T cell-mediated skin allograft rejection. Additionally, b2m knockdown increased susceptibility to viral infection of F₀ transgenic larvae. This loss of function strategy offers new avenues for studying ontogeny of immunity and other developmental processes in Xenopus.

In situ differentiation of CD8αα Τ cells from CD4 T cells in peripheral lymphoid tissues

Mutually exclusive cell fate determination of CD4 helper or CD8 killer T cells occurs in the thymus. These T-cell subsets are not believed to redirect other lineages. Here we showed that retinoic acid and transforming growth factor-β1 promoted the differentiation of CD8αα T cells from CD4 T cells in a Runx3-dependent manner. These cells were inferred to belong to immunoregulatory populations because subpopulations of CD8αα+TCRαβ T cells are known to suppress activated T cells, and mice with Runx3(-/-) T cells showed defects during recovery from experimental allergic encephalomyelitis. Our results demonstrate that CD4 T cells play fundamental roles in controlling immune reactions through promotion and attenuation. We accordingly anticipate that clarifying the mechanisms underlying this process will provide insights leading to autoimmune and immunodeficiency disease therapies.

Intraepithelial lymphocytes in celiac disease immunopathology

Celiac disease is a T cell-mediated immune disorder induced by dietary gluten that is characterized by the development of an inflammatory anti-gluten CD4 T cell response, anti-gluten antibodies, and autoantibodies against tissue transglutaminase 2 and the activation of intraepithelial lymphocytes (IELs) leading to the destruction of the intestinal epithelium. Intraepithelial lymphocytes represent a heterogeneous population of T cells composed mainly of cytotoxic CD8 T cells residing within the epithelial layer, whose main role is to maintain the integrity of the epithelium by eliminating infected cells and promoting epithelial repair. Dysregulated activation of IELs is a hallmark of CD and is critically involved in epithelial cell destruction and the subsequent development of villous atrophy. In this review, we compare and contrast the phenotype and function of human and mouse small intestinal IELs under physiological conditions. Furthermore, we discuss how conditions of epithelial distress associated with overexpression of IL-15 and non-classical MHC class I molecules induce cytotoxic IELs to become licensed killer cells that upregulate activating NKG2D and CD94/NKG2C natural killer receptors, acquiring lymphokine killer activity. Pathways leading to dysregulated IEL activation could eventually be targeted to prevent villous atrophy and treat patients who respond poorly to gluten-free diet.

Small intestinal intraepithelial lymphocytes expressing CD8 and T cell receptor γδ are involved in bacterial clearance during Salmonella enterica serovar Typhimurium infection

The intestinal immune system is crucial for the maintenance of mucosal homeostasis and has evolved under the dual pressure of protecting the host from pathogenic infection and coexisting with the dense and diverse commensal organisms in the lumen. Intestinal intraepithelial lymphocytes (iIELs) are the first element of the host T cell compartment available to respond to oral infection by pathogens. This study demonstrated that oral infection by Salmonella enterica serovar Typhimurium promoted the expansion of iIELs, particularly CD8(+) TCRγδ(+) IELs, enhanced expression of NKG2D on iIELs, increased expression of MULT1, and decreased expression of Qa-1 by intestinal epithelial cells (IECs), leading to activation of, particularly, CD8(+) TCRγδ(+) iIELs and cytolytic activity against S. Typhimurium-infected IECs. Blockade of NKG2D recognition or depletion of TCRγδ(+) cells using a depleting monoclonal antibody significantly attenuated the clearance of S. Typhimurium in the intestine and other tissues. This study suggests that iIELs, particularly CD8(+) TCRγδ(+) iIELs, play important roles in the detection of pathogenic bacteria and eradication of infected epithelial cells and, thus, provide protection against invading pathogens. These data further our understanding of the mechanisms by which the immune system of the intestinal mucosa discriminates between pathogenic and commensal organisms.

Global expression profiling of peripheral Qa-1-restricted CD8αα+TCRαβ+ regulatory T cells reveals innate-like features: implications for immune-regulatory repertoire

Among peripheral regulatory T cells, CD8(+) T cells also play an important role in the maintenance of immune homeostasis. A subset of CD8(+) Treg that express αβ T cell receptor (TCR) and CD8αα homodimers can recognize TCR-derived peptides in the context of the class Ib MHC molecule Qa-1. To gain a better understanding of the nature and phenotype of CD8αα(+)TCRαβ+ Treg, a global gene expression profiling using microarray, real-time quantitative polymerase chain reaction, and flow-cytometric analysis was performed using functional Treg clones and lines. The study findings show that CD8(+) Treg shared gene profile expressed by innate-like lymphocytes, including murine intraepithelial lymphocytes and thymic CD8αα(+)TCRαβ+ T-cell populations. In addition, this subset displays differential expression of several key regulatory molecules, including CD200. CD8αα(+) Treg expressed higher levels of a number of natural killer cell-related receptors and molecules belonging to the TNF superfamily. Collectively, peripheral class Ib-reactive CD8αα(+)TCRαβ+ T cells represent a unique regulatory population different from class Ia major histocompatibility complex-restricted conventional T cells. These studies have important implications for the regulatory mechanisms mediated by the CD8(+) Treg population in general.

Inhibitory receptors specific for MHC class I educate murine NK cells but not CD8αα intestinal intraepithelial T lymphocytes

The engagement of inhibitory receptors specific for major histocompatibility complex class I (MHC-I) molecules educates natural killer (NK) cells, meaning the improvement of the response of activation receptors to subsequent stimulation. It is not known whether inhibitory MHC-I receptors educate only NK cells or whether they improve the responsiveness of all cell types, which express them. To address this issue, we analyzed the expression of inhibitory MHC-I receptors on intestinal intraepithelial lymphocytes (iIELs) and show that T-cell receptor (TCR)-αβ CD8αα iIELs express multiple inhibitory receptors specific for MHC-I molecules, including CD94/NKG2A, Ly49A, and Ly49G2. However, the presence of MHC-I ligand for these receptors did not improve the response of iIELs to activation via the TCR. The absence of iIEL education by MHC-I receptors was not related to a lack of inhibitory function of these receptors in iIELs and a failure of these receptors to couple to the TCR. Thus, unlike NK cells, iIELs do not undergo an MHC-I-guided education process. These data suggest that education is an NK cell-specific function of inhibitory MHC-I receptors.

CCR2-dependent intraepithelial lymphocytes mediate inflammatory gut pathology during Toxoplasma gondii infection

Mice of the C57BL/6 strain develop acute ileal inflammation after infection with the protozoan parasite Toxoplasma gondii. This pathology resembles many key features of human Crohn's disease, including a Th1 cytokine profile with high levels of interferon gamma (IFN-gamma), interleukin 12 (IL)-12, and tumor necrosis factor alpha (TNF)-alpha, presence of pathogenic CD4(+) T cells, and infiltration of gut flora into inflammed tissue. Using CCR2(-/-) mice, we identify a role for this chemokine receptor in the pathogenesis of inflammatory pathology during T. gondii infection. Lack of chemokine (C-C motif) receptor 2 (CCR2) was associated with low levels of CD103(+) T lymphocytes in the intraepithelial compartment, Peyer's patch, and lamina propria relative to wild-type animals. Adoptive transfer of wild-type, but not IFN-gamma(-/-), intraepithelial T lymphocytes converted CCR2 knockout mice from a resistant to susceptible phenotype with respect to parasite-triggered inflammatory gut pathology. These results for the first time show a role for intraepithelial T lymphocytes in pathogenesis of ileitis triggered by a microbial pathogen.

Revival of CD8+ Treg-mediated suppression

Despite their first recognition almost 40 years ago, CD8(+) 'suppressor' T cells remain poorly characterized. Recent studies of these lymphocytes, now popularly referred to as regulatory CD8(+) T cells (CD8(+) Tregs), have helped clarify their important role in the regulation of autoimmune disease. Here, we review progress related to the identification, phenotype and function of CD8(+) Tregs. We also focus on a newly described subset, CD8alphaalpha(+)TCRalphabeta(+) Tregs, which in mice recognize a T-cell receptor-derived peptide in the context of the class Ib major histocompatibility complex molecule Qa-1. These Tregs target only activated T cells and complement the suppression provided by CD4(+)Foxp3(+) Tregs. Investigations leading to the detailed identification, expansion, maintenance and function of CD8alphaalpha(+) Tregs should result in new therapeutic strategies for human inflammatory diseases.

Recognition of double-stranded RNA by TLR3 induces severe small intestinal injury in mice

The role of TLRs on intestinal epithelial cells (IECs) is controversial, and the mechanisms by which TLRs influence mucosal homeostasis are obscure. In this study, we report that genomic dsRNA from rotavirus, and its synthetic analog polyinosinic-polycytidylic acid (poly(I:C)), induce severe mucosal injury in the small intestine. Upon engaging TLR3 on IECs, dsRNA triggers IECs to secrete IL-15, which functions to increase the percentage of CD3+NK1.1+ intestinal intraepithelial lymphocytes (IELs) and enhances the cytotoxicity of IELs. Moreover, The CD3+NK1.1+ IELs are proved as CD8alphaalpha+ IELs. These results provide direct evidence that abnormal TLR3 signaling contributes to breaking down mucosal homeostasis and the first evidence of pathogenic effects mediated by CD8alphaalpha+ IELs. The data also suggest that genomic dsRNA may be involved in the pathogenesis of acute rotavirus gastroenteritis.

Mouse TCRalphabeta+CD8alphaalpha intraepithelial lymphocytes express genes that down-regulate their antigen reactivity and suppress immune responses

Mouse small intestine intraepithelial lymphocytes (IEL) that express alphabetaTCR and CD8alphaalpha homodimers are an enigmatic T cell subset, as their specificity and in vivo function remain to be defined. To gain insight into the nature of these cells, we performed global gene expression profiling using microarray analysis combined with real-time quantitative PCR and flow cytometry. Using these methods, TCRalphabeta(+)CD8alphaalpha IEL were compared with their TCRalphabeta(+)CD8beta(+) and TCRgammadelta(+) counterparts. Interestingly, TCRalphabeta(+)CD8alphaalpha IEL were found to preferentially express genes that would be expected to down-modulate their reactivity. They have a unique expression pattern of members of the Ly49 family of NK receptors and tend to express inhibitory receptors, along with some activating receptors. The signaling machinery of both TCRalphabeta(+)CD8alphaalpha and TCRgammadelta(+) IEL is constructed differently than other IEL and peripheral T cells, as evidenced by their low-level expression of the linker for activation of T cells and high expression of the non-T cell activation linker, which suppresses T cell activation. The TCRalphabeta(+)CD8alphaalpha IEL subset also has increased expression of genes that could be involved in immune regulation, including TGF-beta(3) and lymphocyte activation gene-3. Collectively, these data underscore the fact that, while TCRalphabeta(+)CD8alphaalpha IEL resemble TCRgammadelta(+) IEL, they are a unique population of cells with regulated Ag reactivity that could have regulatory function.

Human CD8+intraepithelial lymphocytes: a unique model to study the regulation of effector cytotoxic T lymphocytes in tissue

The epithelium of the human small intestine contains a large population of intraepithelial cytolytic alphabeta T-cell receptor (TCR) CD8 alpha beta T lymphocytes (IE-CTLs), whose main role is to sustain epithelial integrity by rapidly eliminating infected and damaged cells. In mouse, the recognition of inducible/modified self-molecules, i.e. non-classical major histocompatibility complex (MHC) class I molecules, is mediated by the TCR and natural killer receptors (NKRs) co-expressed on the cell surface of a non-conventional autoreactive CD8 alpha alpha alpha beta TCR cell subset. In contrast, in humans, the recognition of non-classical MHC class I molecules induced by stress and inflammation on intestinal epithelial cells (IECs) is principally mediated by NKRs expressed on conventional CD8 alpha beta alpha beta TCR cells. By sensing microenvironmental signals of inflammation and stress through NKRs, IE-CTLs fine tune their TCR activation threshold. Furthermore, IE-CTLs under particular conditions, involving interleukin-15 upregulation, acquire the capacity to kill distressed intestinal epithelial cells in an antigen non-specific manner. Adaptive IE-CTLs appear hence to have autoreactive properties and modulate their immune response based on innate signals, reflecting the fitness of the tissue.

The extrathymic T-cell differentiation in the murine gut

The gut epithelial border is in continuous contact with exogenous antigens and harbors a distinctive and very abundant CD8 alpha alpha intraepithelial T-lymphocyte effector population. We describe here the characteristics of these cells that distinguish them from all other T-cell types in the body as well as their functions in local protection. We also describe how these cells differentiate from local precursors present in the gut cryptopatches (CPs) following a pathway of T-cell differentiation unique to the gut wall. Finally, we describe the origin of the precursors of CD8 alpha alpha T cells, which come from the bone marrow in athymic mice but are first imprinted in the thymus in euthymic mice. Indeed, CD3(-)CD4(-)CD8(-) T-cell-committed precursors can leave the thymus before T-cell receptor rearrangements and then colonize the gut CPs, proceeding with their differentiation within the gut wall.

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.

Generation of gut-homing T cells and their localization to the small intestinal mucosa

The intestinal mucosa represents the largest body surface toward the external environment and harbors numerous T lymphocytes that take up resident within the intestinal epithelium or in the underlying lamina propria (LP). The intraepithelial lymphocytes include subsets of 'unconventional' T cells with unclear ontogeny and reactivity that localize to this site independently of antigen-specific activation in secondary lymphoid organs. In contrast, the majority of the 'conventional' gut T cells are recruited into the intestinal mucosa subsequent to their activation in intestinal inductive sites, including Peyer's patches (PPs) and mesenteric lymph nodes (MLNs). T cells homing to the small intestine express a distinct pattern of homing molecules, allowing them to interact with and transmigrate across intestinal postcapillary endothelium. At least some of these homing molecules, including the integrin alpha(4)beta(7) and the chemokine receptor CCR9, are induced on T cells during their activation in PPs or MLNs. Mucosal dendritic cells (DCs) play a key role in this process, but not all intestinal DCs possess the ability to confer a gut-homing capacity to T cells. Instead, functionally specialized CD103(+) DCs derived from the small intestinal LP appear to selectively regulate T-cell homing to the small intestine.

Regulation of allergic airway inflammation by class I-restricted allergen presentation and CD8 T-cell infiltration

BACKGROUND

CD8 T cells are known to respond to exogenous antigens through cross-presentation. The importance of the CD8 cell response in the lung after inhalation of allergen and its effects on asthmatic inflammation are less clear.

OBJECTIVE

We sought to determine the dynamics, nature, and immunoregulatory activities of the class I CD8 T-cell response to inhaled allergen.

METHODS

We studied a murine model of respiratory allergen sensitization, adoptive transfer of transgenic T cells, and flow cytometric analysis of lung infiltrates.

RESULTS

Class I-restricted CD8 T cells responded rapidly to inhaled allergen and dominated the acute infiltration of T cells into the lung after secondary exposure. CD8 cells in the lung expressed a type 1 phenotype and suppressed the systemic IgE response to subsequent immunization. Dendritic cells purified from conducting airways or lung tissue were highly efficient at cross-presentation of antigen into the class I pathway after intranasal challenge. Adoptive transfer of transgenic antigen-specific CD8, but not CD4, cells resulted in increased IL-12 levels and reduced IL-13 and IL-5 levels in bronchoalveolar lavage fluid, coupled with substantially reduced airway eosinophilia after repeated allergen inhalation, a process mimicked by intranasal administration of IL-12 and inhibited by anti-IL-12 antibody.

CONCLUSION

The data suggest that CD8 cells specific for inhaled allergens are generated in draining lymph nodes but suppress allergic airway inflammation through induction of IL-12 in the lung during interaction with respiratory dendritic cells.

CLINICAL IMPLICATIONS

Novel peptide immunotherapeutics targeting the class I-restricted CD8 T-cell response to allergen represent a promising strategy for extrinsic asthma.

Significance of unconventional peripheral CD4+CD8dim T cell subsets

Routine T cells phenotyping occasionally reveals a CD4+CD8dim T cell subset with an apparently homogeneous dot plot. The aim of this study was to elucidate their immunological significance from analysis of 31 healthy donors, 21 elderly and 220 immune deficient patients. CD4+CD8dim T cells expressed reduced levels of CD8 (11-17,000 compared to 96-128,000 mol/cell on CD8+ T Cells). CD4 was expressed at the same level as on CD4+ T cells. The occurrence of raised CD4+CD8dim T cells (> 20 cells/muL) was similar in kidney transplant recipients (28.4%) and healthy donors (26%). It was somewhat lower in HIV+ patients (19.7%) possibly due to virally induced CD4+ T lymphopenia. However, an age effect is possible because the occurrence was raised (33.3%) in 70 volunteers (chi2 test NS). On the other hand, the size of the CD4+CD8dim subset was not correlated with age. CD4+CD8dim T cells did not express the activation markers CD69 (n = 220) or CD25 (n = 10) and expressed the homodimeric (alphaalpha) isoform of CD8, suggesting they are related to mucosal immunity (MALT). We selected 29 patients with unambiguous dot plots. In 26 of them one predominant TCR Vbeta clonotype was expressed on 18 to 94% of CD4+CD8dim T cells and never on more than 10% of conventional T cells. The predominant clonotypes were Vbeta8 (n = 5), Vbeta2 (n = 4), Vbeta13.1 and Vbeta 21 (n = 3 each). Whether this reveals a chronic stimulation or an emerging lymphoproliferative disorder must be elucidated. We propose to name this entity: "Oligoclonal Clonopathy of Undetermined Significance (OCUS)."

TCR gamma delta intraepithelial lymphocytes are required for self-tolerance

Neonatal thymectomy (NTX) impairs T cell regulation and leads to organ-specific autoimmune disease in susceptible mouse strains. In the NOD mouse model of spontaneous type 1 diabetes, we observed that NTX dramatically accelerated autoimmune pancreatic beta cell destruction and diabetes. NTX had only a minor effect in NOD mice protected from diabetes by transgenic expression of the beta cell autoantigen proinsulin in APCs, inferring that accelerated diabetes after NTX is largely due to failure to regulate proinsulin-specific T cells. NTX markedly impaired the development of intraepithelial lymphocytes (IEL), the number of which was already reduced in euthymic NOD mice compared with control strains. IEL purified from euthymic NOD mice, specifically CD8alphaalpha TCRgammadelta IEL, when transferred into NTX-NOD mice, trafficked to the small intestinal epithelium and prevented diabetes. Transfer of prototypic CD4+CD25+ regulatory T cells also prevented diabetes in NTX-NOD mice; however, the induction of these cells by oral insulin in euthymic mice depended on the integrity of TCRgammadelta IEL. We conclude that TCRgammadelta IEL at the mucosal interface between self and nonself play a key role in maintaining peripheral tolerance both physiologically and during oral tolerance induction.

Effective control of chronic gamma-herpesvirus infection by unconventional MHC Class Ia-independent CD8 T cells

Control of virus infection is mediated in part by major histocompatibility complex (MHC) Class Ia presentation of viral peptides to conventional CD8 T cells. Although important, the absolute requirement for MHC Class Ia–dependent CD8 T cells for control of chronic virus infection has not been formally demonstrated. We show here that mice lacking MHC Class Ia molecules (K^b−/−xD^b−/− mice) effectively control chronic γ-herpesvirus 68 (γHV68) infection via a robust expansion of β₂-microglobulin (β₂-m)-dependent, but CD1d-independent, unconventional CD8 T cells. These unconventional CD8 T cells expressed: (1) CD8αβ and CD3, (2) cell surface molecules associated with conventional effector/memory CD8 T cells, (3) TCRαβ with a significant Vβ4, Vβ3, and Vβ10 bias, and (4) the key effector cytokine interferon-γ (IFNγ). Unconventional CD8 T cells utilized a diverse TCR repertoire, and CDR3 analysis suggests that some of that repertoire may be utilized even in the presence of conventional CD8 T cells. This is the first demonstration to our knowledge that β₂-m–dependent, but Class Ia–independent, unconventional CD8 T cells can efficiently control chronic virus infection, implicating a role for β₂-n–dependent non-classical MHC molecules in control of chronic viral infection. We speculate that similar unconventional CD8 T cells may be able to control of other chronic viral infections, especially when viruses evade immunity by inhibiting generation of Class Ia–restricted T cells.

In this paper the authors identify a β2-microglobulin–dependent but major histocompatibility complex (MHC) Class Ia– and CD1-independent class of CD8 T cells that effectively control chronic γ-herpesvirus infection in mice. The important point that should be of general interest to the readers of PLoS Pathogens is that an effective CD8 T cell response develops during chronic infection of mice lacking MHC Class Ia molecules. Enormous efforts have gone into characterizing the role of conventional CD8 T cells that recognize viral peptides together with MHC Class Ia molecules during chronic viral infection, and many vaccine approaches focus solely on this response. This paper shows that additional types of CD8 T cells can operate during chronic infection, and that indeed, conventional MHC Class Ia–restricted T cells may be dispensable for control of chronic herpesvirus infection. The authors believe this is a fundamentally important point because it raises the question of whether unconventional CD8 T cells are important for control of other chronic viral infections such as infection with HIV, Hepatitis C virus, Hepatitis B virus, or human herpesviruses.

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.

Infection-induced expansion of a MHC Class Ib-dependent intestinal intraepithelial gammadelta T cell subset

Salmonella species invade the host via the intestinal epithelium. Hence, intestinal intraepithelial lymphocytes (iIELs) are potentially the first element of the immune system to encounter Salmonella during infection. In this study, we demonstrate, in a mouse model, the expansion of a CD8alphabeta(+)CD94(-)TCRgammadelta(+) T cell subset within the iIEL population in response to oral infection with virulent or avirulent Salmonella. This population can be detected 3 days following infection, represents up to 15% of the TCRgammadelta(+) iIELs, and is dependent on the MHC class Ib molecule T23 (Qa-1). Qa-1 is expressed by intestinal epithelial cells and thus accessible for iIEL recognition. Such cells may play a role in the early immune response to Salmonella.

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.

The CD8 isoform CD8αα is not a functional homologue of the TCR co-receptor CD8αβ

Although structurally similar, CD8alphabeta and CD8alphaalpha have notably diverted with regard to function. Whereas CD8alphabeta functions as a T-cell receptor (TCR) co-receptor on MHC-class-I-restricted thymocytes and mature T cells, CD8alphaalpha is unable to support conventional positive selection, and can be expressed on T cells independent of the MHC restriction of their TCR. CD8alphaalpha induction is consistent with antigenic stimulation through the TCR, and recent developments have now shown that CD8alphaalpha induced on agonist-triggered immature thymocytes, antigenic-stimulated conventional CD8alphabeta T cells and mucosal T cells mediates the specific modulation of TCR activation signals to facilitate their survival and differentiation into various specialized T-cell subsets.

Immunoregulation in the tissues by gammadelta T cells

For a T-cell subset to be classified as immunoregulatory, it might reasonably be predicted that in its absence, animals would experience pathological immune dysregulation. Moreover, reconstitution of the subset should restore normal immune regulation. So far, these criteria have been satisfied by only a few of the candidate regulatory T-cell subsets, but among them is the intraepithelial gammadelta T-cell receptor (TCR)+ subset of mouse skin. In this article, we look at immunoregulatory gammadelta T cells, and the growing evidence for tissue-associated immunoregulation mediated by both gammadelta T cells and alphabeta T cells.

Extrathymic T cell lymphopoiesis: ontogeny and contribution to gut intraepithelial lymphocytes in athymic and euthymic mice

In the absence of thymopoiesis, T lymphocytes are nevertheless present, mainly in the gut epithelium. Ontogeny of the extrathymic pathway and the extent of its involvement in euthymic mice are controversial. These questions have been addressed by assessing the expression of recombinase activating gene (RAG) through the use of green fluorescent protein RAG2 transgenic mouse models. In athymic mice, T lymphopoiesis occurs mainly in the mesenteric lymph node and less in the Peyer's patches. Ontogenic steps of this lymphopoiesis resemble those of thymopoiesis, but with an apparent bias toward gamma delta T cell production and with a paucity of oligoclonal alpha beta T cells possibly resulting from a deficit in positive selection. Whether in athymic or euthymic mice, neither T intraepithelial lymphocytes (IEL) nor cryptopatch cells (reported to contain precursors of IEL) displayed fluorescence indicating recent RAG protein synthesis. Newly made T cells migrate from the mesenteric node into the thoracic duct lymph to reach the gut mucosa. In euthymic mice, this extrathymic pathway is totally repressed, except in conditions of severe lymphocytic depletion. Thus, in normal animals, all gut T IEL, including CD8 alpha alpha(+) cells, are of thymic origin, CD8 alpha alpha(+) TCR alpha beta(+) IEL being the likely progeny of double negative NK1-1(-) thymocytes, which show polyclonal V alpha and V beta repertoires.

MHC specificity of iIELs

Intestinal intra-epithelial lymphocytes (iIELs) are a major lymphocyte population, reside in close proximity to the intestinal lumen and are conserved throughout vertebrate evolution. iIELs consist of several unique T-cell phenotypes and express both non-rearranged innate immune receptors and rearranged adaptive immune receptors. The ligands for the innate immune receptors on iIELs, such as NKG2D (natural killer-cell receptor), often bind to non-classical MHC class I molecules, such as the human MHC class I-related molecules MICA or MICB. These ligands costimulate T-cell receptor (TCR)-mediated signaling. In most cases, the MHC molecules that bind to the TCR are still unknown. However, recent efforts to understand the MHC molecules that are involved in the development of and antigen recognition by iIELs have revealed several important results. Here, we focus systematically on recent developments in innate immunity and in TCR recognition of different subtypes of iIELs by various MHC molecules.

Total parenteral nutrition-associated changes in mouse intestinal intraepithelial lymphocytes

Intraepithelial lymphocytes (IEL) play a major role in mucosal defense mechanisms against intraluminal foreign antigens. To address the role luminal nutrients have on the phenotype and function of the IEL, we administered total parenteral nutrition (TPN) to mice, with the absence of enteral intake. We hypothesized that administration of TPN would result in changes in the phenotype and function of the IEL. For this, we utilized a mouse model of TPN. A significant decline in the CD4+ IEL population occurred with TPN. Additionally, the CD8+,CD44+ IEL subset showed a 65% decline (P < 0.05), and the CD4+,CD44+ subset declined by 55% with TPN (P < 0.05). The CD8alphabeta+ population (a marker of thymic-dependence) also declined by 92% (P < 0.01) with TPN. IEL in the TPN group showed a significantly lower degree of in vitro proliferation. In conclusion, the IEL showed significant phenotypic changes with TPN including the loss of the thymic-derived population. Functionally, the IEL showed a significant decline in proliferation. Such changes demonstrate the important role luminal nutrients have on IEL phenotype and function.

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.

Intraepithelial lymphocytes: exploring the Third Way in immunology

Locally resident intraepithelial lymphocytes (IELs) are primarily T cells with potent cytolytic and immunoregulatory capacities, which they use to sustain epithelial integrity. Here, we consider that most IEL compartments comprise a variable mixture of two cell types: T cells primed to conventional antigen in the systemic compartment and T cells with ill-defined reactivities and origins, whose properties seem to place them mid-way between the adaptive and innate immune responses. We review the capacity of IELs to limit the dissemination of infectious pathogens and malignant cells and to control the infiltration of epithelial surfaces by systemic cells. An improved characterization of IELs would seem essential if we are to understand how immune responses and immunopathologies develop at body surfaces.

Murine female reproductive tract intraepithelial lymphocytes display selection characteristics distinct from both peripheral and other mucosal T cells

Despite immense effort, the development of vaccines effective at mucosal sites has proceeded at a faltering pace. Efforts concentrating on humoral immunity but neglecting cellular immunity may be misdirected by ignoring many viral mucosal pathogens. Improved understanding of the development and maintenance of lymphocytes populating the reproductive tract (rtIELs) may inform advances in vaccination strategies for sexually transmitted diseases. Recent studies highlight tissue-specific differences in the development of mucosal immunity and suggest that the local milieu may play a role in selection, maintenance and function of resident lymphocytes. Here, we describe MHC class I and thymus dependence of subpopulations of rtIELs. TCRalphabeta+ CD8alphabeta+ T cells in the periphery, intestine, and female reproductive tract are all developmentally dependent on classical class I MHC and the thymus. TCRalphabeta+ CD8alphaalpha+ are absent from the periphery and the rtIELs, but are present and classical MHC class I-independent, in the intestine. In contrast to intestinal TCRgammadelta+ cells, TCRgammadelta+ rtIELs are CD8 negative and thymus dependent. In contrast to peripheral TCRgammadelta+ cells, murine TCRgammadelta+ rtIELs express not a diverse array of Vdelta genes, but rather, a canonical Vdelta1. In summary, lymphocytes isolated from the murine female reproductive tract have characteristics distinct from both peripheral T cells and those found at other mucosal sites. Therefore, for the purpose of vaccination strategies, the female reproductive tract should be regarded neither as peripheral nor mucosal, but rather as a tissue with distinctive immunological characteristics.

MHC class I allele dosage alters CD8 expression by intestinal intraepithelial lymphocytes

The development of TCR alphabeta(+), CD8alphabeta(+) intestinal intraepithelial lymphocytes (IEL) is dependent on MHC class I molecules expressed in the thymus, while some CD8alphaalpha(+) IEL may arise independently of MHC class I. We examined the influence of MHC I allele dosage on the development CD8(+) T cells in RAG 2(-/-) mice expressing the H-2D(b)-restricted transgenic TCR specific for the male, Smcy-derived H-Y Ag (H-Y TCR). IEL in male mice heterozygous for the restricting (H-2D(b)) and nonrestricting (H-2D(d)) MHC class I alleles (MHC F(1)) were composed of a mixture of CD8alphabeta(+) and CD8alphaalpha(+) T cells, while T cells in the spleen were mostly CD8alphabeta(+). This was unlike IEL in male mice homozygous for H-2D(b), which had predominantly CD8alphaalpha(+) IEL and few mostly CD8(-) T cells in the spleen. Our results demonstrate that deletion of CD8alphabeta(+) cells in H-Y TCR male mice is dependent on two copies of H-2D(b), whereas the generation of CD8alphaalpha(+) IEL requires only one copy. The existence of CD8alphabeta(+) and CD8alphaalpha(+) IEL in MHC F(1) mice suggests that their generation is not mutually exclusive in cells with identical TCR. Furthermore, our data imply that the level of the restricting MHC class I allele determines a threshold for conventional CD8alphabeta(+) T cell selection in the thymus of H-Y TCR-transgenic mice, whereas the development of CD8alphaalpha(+) IEL is dependent on, but less sensitive to, this MHC class I allele.

Contribution of double-negative thymic precursors to CD8alpha alpha (+) intraepithelial lymphocytes of the gut in mice bearing TCR transgenes

Using male and female RAG(-/-) mutant mice expressing TCR transgenes specific for MHC class I- or II-presented HY peptides, we performed quantitative and phenotypic comparisons between the TCR(+) lymphocytes present in the lymphoid organs and the gut mucosa in euthymic versus athymic (nude) animals. These comparisons suggest that only a minority of the TCR(+) CD8alpha alpha (+) intraepithelial lymphocytes (IEL) of the transgenic euthymic mice originate from hematopoietic precursors acquiring a TCR in the gut wall, while a majority of these CD8alpha alpha(+) IEL appear to be of thymic origin (as were all TCR(+) CD8alpha beta (+) or CD4(+) in any location); these last cells are released from the thymus as double-negative thymocytes, which are at a more immature stage (CD44(+)CD25(+)) in female mice than in males (CD44(-)). In view of previous observations that in non-transgenic athymic mice the CD8alpha alpha (+) TCR(+) IEL populations are also markedly reduced quantitatively, the possibility of a thymic contribution to these ontogenically peculiar populations may also exist in normal mice. At which stage of differentiation such precursors might leave the thymus of normal adult mice remains to be explored.

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.

Qa-2-dependent selection of CD8alpha/alpha T cell receptor alpha/beta(+) cells in murine intestinal intraepithelial lymphocytes

Murine intestinal intraepithelial lymphocytes (iIELs) are made up of a heterogeneous mix of T cells with unique phenotypes. Whereas CD8(+) T cells in peripheral lymphoid organs use CD8alpha/beta and are selected on MHC class Ia molecules, a majority of iIELs use CD8alpha/alpha. Here, we report that the presence of CD8alpha/alpha TCR-alpha/beta cells in iIELs is independent of classical MHC class I molecules K(b) and D(b), as illustrated by their presence in K(b)/D(b) double-knockout mice and in mice lacking a nonclassical MHC class I molecule, CD1d. Most strikingly, their presence is decreased by approximately 70% in mice lacking transporter associated with antigen processing (TAP). The TAP-dependent nonclassical MHC class I molecule Qa-2 is strongly implicated in the presence of these cells, as inferred from the low numbers of CD8alpha/alpha TCR-alpha/beta T cells in mice deficient in Qa-2 genes. Second, a Qa-2-transgenic mouse made in a Qa-2(-) strain showed an increase in the numbers of CD8alpha/alpha cells among its iIELs. Thus, the presence of CD8alpha/alpha TCR-alpha/beta cells in iIELs is mainly dependent on the nonclassical MHC class I molecule Qa-2.

Cutting edge: an essential role for Notch-1 in the development of both thymus-independent and -dependent T cells in the gut

Whereas most T cells arise in the thymus, a distinct lineage of extrathymically derived T cells is present in the gut mucosa. The developmental origin of extrathymic T cells is poorly understood. We show here that Notch-1, a transmembrane receptor involved in T cell fate specification of bipotential T/B precursors in the thymus, is absolutely required for the development of extrathymic (as well as thymus-derived) mature T cells in the intestinal epithelium. In the absence of Notch-1, CD117(+) T cell precursors are relatively more abundant in the gut than the thymus, whereas immature B cells accumulate in the thymus but not the gut. Collectively, these data demonstrate that Notch-1 is essential for both thymic and extrathymic T cell fate specification and further suggest that bipotential T/B precursors that do not receive a Notch-1 signal adopt a B cell fate in the thymus but become developmentally arrested in the gut.

Structure of the intestinal flora responsible for development of the gut immune system in a rodent model

The intestinal flora comprising indigenous, autochthonous bacteria is constantly present in the alimentary tract of host animals, including humans. The indigenous bacteria greatly affect the structure and functions of the intestinal mucosa. Studies involving gnotobiotic mice or rats have shown that the presence of limited kinds of intestinal bacteria is responsible for the development of the gut immune system, such as secretory IgA, major histocompatibility complex molecules and intraepithelial lymphocytes. Understanding of the structure of the intestinal flora or the organization of the microbial population in the intestine, based on evaluation of the immunological responses, may clarify its functions in the host animal.

Host immune response to intracellular bacteria: A role for MHC-linked class-Ib antigen-presenting molecules

MHC-linked class-Ib molecules are a subfamily of class-I molecules that display limited genetic polymorphism. At one time these molecules were considered to have an enigmatic function. However, recent studies have shown that MHC-linked class-Ib molecules can function as antigen presentation structures that bind bacteria-derived epitopes for recognition by CD8+ effector T cells. This role for class-Ib molecules has been demonstrated across broad classes of intracellular bacteria including Listeria moncytogenes, Salmonella typhimurium, and Mycobacterium tuberculosis. Additionally, evidence is emerging that MHC-linked class-Ib molecules also serve an integral role as recognition elements for NK cells as well as several TCR alpha/beta and TCR gamma/delta T-cell subsets. Thus, MHC-linked class-Ib molecules contribute to the host immune response by serving as antigen presentation molecules and recognition ligands in both the innate and adaptive immune response to infection. In this review, we will attempt to summarize the work that supports a role for MHC-linked class-Ib molecules in the host response to infection with intracellular bacteria.

Functional natural killer T cells in experimental mouse strains, including NK1.1- strains

Natural killer T (NKT) cells are a newly discovered subset of lymphocytes. It appears that this subset has potential as important regulators of immune responses. But because there are relatively few NKT cells in lymphoid organs and because of technical difficulties in detecting NKT cells in most mouse strains, the roles of NKT cells have not been fully identified and little attention has been paid to the roles of NKT cells in immunological experiments in which NK1.1- strains were used. To examine the existence of functional NKT cells in various strains of experimental mice, including NK1.1- strains, we utilized alpha-galactosylceramide (KRN7000) which is thought to react specifically with NKT cells. Indeed, we could confirm that early cytokine (IL-4 and IFN-gamma) secretion at 2 h after the injection of KRN7000 was dependent on NKT cells. With this in vivo system, we have successfully detected the presence of functional NKT cells in various mouse strains, including AKR/N, BALB/c, C3H/HeJ, C3H/HeN, C57BL/6, C.B-17, CBA/N, NC, NOD, SJL, W/Wv, aly/aly and aly/+. Notable increases of serum IL-4 were detected in W/Wv and aly/+ strains, and defective response of IFN-gamma in SJL mice and that of IL-4 in NOD mice were observed. This is the first report to show the functional significance of NKT cells in cytokine secretion in various mouse strains in response to a ligand for the T cell receptor of NKT cells.