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

Destined for the intestine: thymic selection of TCRαβ CD8αα intestinal intraepithelial lymphocytes

The immune system is composed of a variety of different T-cell lineages distributed through both secondary lymphoid tissue and non-lymphoid tissue. The intestinal epithelium is a critical barrier surface that contains numerous intraepithelial lymphocytes that aid in maintaining homeostasis at that barrier. This review focuses on T-cell receptor αβ (TCRαβ) CD8αα intraepithelial lymphocytes, and how recent advances in the field clarify how this unique T-cell subset is selected, matures, and functions in the intestines. We consider how the available evidence reveals a story of ontogeny starting from agonist selection of T cells in the thymus and finishing through the specific signaling environment of the intestinal epithelium. We conclude with how this story raises further key questions about the development of different ontogenic waves of TCRαβ CD8αα IEL and their importance for intestinal epithelial homeostasis.

Increased TCR signal strength in DN thymocytes promotes development of gut TCRαβ(+)CD8αα(+) intraepithelial lymphocytes

CD4⁽⁺⁾CD8⁽⁺⁾ “double positive” (DP) thymocytes differentiate into diverse αβ T cell sub-types using mechanistically distinct programs. For example, conventional αβ T cells develop from DP cells after partial-agonist T cell receptor (TCR) interactions with self-peptide/MHC, whereas unconventional αβ T cells, such as TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ intraepithelial lymphocytes (IELs), require full-agonist TCR interactions. Despite this, DP cells appear homogeneous, and it remains unclear how distinct TCR signalling instructs distinct developmental outcomes. Moreover, whether TCR signals at earlier stages of development, for example in CD4⁽⁻⁾CD8⁽⁻⁾ double negative (DN) cells, impact on later fate decisions is presently unknown. Here, we assess four strains of mice that display altered TCR signal strength in DN cells, which correlates with altered generation of unconventional TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ IELs. FVB/n mice (compared to C57BL/6 animals) and mice with altered preTCRα (pTα) expression, both displayed weaker TCR signalling in DN cells, an inefficient DN-to-DP transition, and reduced contribution of TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ IELs to gut epithelium. Conversely, TCRαβ⁽⁺⁾CD8αα⁽⁺⁾ IEL development was favoured in mice with increased TCR signal strength in DN cells. Collectively, these data suggest TCR signal strength in DN cells directly impacts on subsequent DP cell differentiation, fundamentally altering the potential of thymocyte progenitors to adopt conventional versus unconventional T cell fates.

Optimal T-cell receptor affinity for inducing autoimmunity

T-cell receptor affinity for self-antigen has an important role in establishing self-tolerance. Three transgenic mouse strains expressing antigens of variable affinity for the OVA transgenic-I T-cell receptor were generated to address how TCR affinity affects the efficiency of negative selection, the ability to prime an autoimmune response, and the elimination of the relevant target cell. Mice expressing antigens with an affinity just above the negative selection threshold exhibited the highest risk of developing experimental autoimmune diabetes. The data demonstrate that close to the affinity threshold for negative selection, sufficient numbers of self-reactive T cells escape deletion and create an increased risk for the development of autoimmunity.

Location, location, location: tissue-specific regulation of immune responses

Discovery of DCs and PRRs has contributed immensely to our understanding of induction of innate and adaptive immune responses. Activation of PRRs leads to secretion of inflammatory cytokines that regulate priming and differentiation of antigen-specific T and B lymphocytes. Pathogens enter the body via different routes, and although the same set of PRRs is likely to be activated, it is becoming clear that the route of immune challenge determines the nature of outcome of adaptive immunity. In addition to the signaling events initiated following innate-immune receptor activation, the cells of the immune system are influenced by the microenvironments in which they reside, and this has a direct impact on the resulting immune response. Specifically, immune responses could be influenced by specialized DCs, specific factors secreted by stromal cells, and also, by commensal microbiota present in certain organs. Following microbial detection, the complex interactions among DCs, stromal cells, and tissue-specific factors influence outcome of immune responses. In this review, we summarize recent findings on the phenotypic heterogeneity of innate and adaptive immune cells and how tissue-specific factors in the systemic and mucosal immune system influence the outcome of adaptive-immune responses.

Clonal deletion and the fate of autoreactive thymocytes that survive negative selection

Clonal deletion of autoreactive thymocytes is important for self-tolerance, but the intra-thymic signals that induce clonal deletion have not been clearly identified. We now report that clonal deletion during negative selection requires CD28 costimulation of autoreactive thymocytes at the CD4⁺CD8^lo intermediate stage of differentiation. Autoreactive thymocytes were prevented from undergoing clonal deletion by either absent CD28 costimulation or transgenic over-expression of the anti-apoptotic factors Bcl-2 or Mcl-1, with surviving thymocytes differentiating into anergic T cell receptor αβ⁺ double negative thymocytes that preferentially migrated to the intestine where they re-expressed CD8α and were sequestered as CD8αα intraepithelial lymphocytes (IELs). This study identifies CD28 costimulation as the intrathymic signal required for clonal deletion and identifies CD8αα IELs as the developmental fate of autoreactive thymocytes that survive negative selection.

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.

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.

Endogenous galectin-1 enforces class I-restricted TCR functional fate decisions in thymocytes

During thymocyte development, the T-cell receptor (TCR) can discriminate major histocompatibility complex (MHC)/peptide ligands over a narrow range of affinities and translate subtle differences into functional fate decisions. How small differences in TCR input are translated into absolute differences in functional output is unclear. We examined the effects of galectin-1 ablation in the context of class-I-restricted thymocyte development. Galectin-1 expression opposed TCR partial agonist-driven positive selection, but promoted TCR agonist-driven negative selection of conventional CD8(+) T cells. Galectin-1 expression also promoted TCR agonist-driven CD8alphaalpha intestinal intraepithelial lymphocytes (IEL) development. Recombinant galectin-1 enhanced TCR binding to agonist/MHC complexes and promoted a negative-selection-signaling signature, reflected in intensified rapid and transient extracellular signal-regulated kinase (ERK) activation. In contrast, galectin-1 expression antagonized ERK activity in thymocytes undergoing positive selection. We propose that galectin-1 aids in discriminating TCR-directed fate decisions by promoting TCR binding to agonist/MHC complexes and enforcing agonist-driven signals, while opposing partial-agonist signals. In this way, galectin-1 widens the distinction between TCR-directed functional fate cues.

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.

Anti-TCR antibody treatment activates a novel population of nonintestinal CD8 alpha alpha+ TCR alpha beta+ regulatory T cells and prevents experimental autoimmune encephalomyelitis

CD8alphaalpha+CD4-TCRalphabeta+ T cells are a special lineage of T cells found predominantly within the intestine as intraepithelial lymphocytes and have been shown to be involved in the maintenance of immune homeostasis. Although these cells are independent of classical MHC class I (class Ia) molecules, their origin and function in peripheral lymphoid tissues are unknown. We have recently identified a novel subset of nonintestinal CD8alphaalpha+CD4-TCRalphabeta+ regulatory T cells (CD8alphaalpha Tregs) that recognize a TCR peptide from the conserved CDR2 region of the TCR Vbeta8.2-chain in the context of a class Ib molecule, Qa-1a, and control- activated Vbeta8.2+ T cells mediating experimental autoimmune encephalomyelitis. Using flow cytometry, spectratyping, and real-time PCR analysis of T cell clones and short-term lines, we have determined the TCR repertoire of the CD8alphaalpha regulatory T cells (Tregs) and found that they predominantly use the TCR Vbeta6 gene segment. In vivo injection of anti-TCR Vbeta6 mAb results in activation of the CD8alphaalpha Tregs, inhibition of the Th1-like pathogenic response to the immunizing Ag, and protection from experimental autoimmune encephalomyelitis. These data suggest that activation of the CD8alphaalpha Tregs present in peripheral lymphoid organs other than the gut can be exploited for the control of T cell-mediated autoimmune diseases.

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.

Identification of pre- and postselection TCRalphabeta+ intraepithelial lymphocyte precursors in the thymus

The immune system preserves and makes use of autoreactive lymphocytes with specialized functions. Here we showed that one of these populations, CD8alphaalpha(+)TCRalphabeta(+) intestinal intraepithelial lymphocytes (IELs), arose from a unique subset of double-positive thymocytes. This subset of cells was precommitted to preferentially give rise to CD8alphaalpha(+)TCRalphabeta(+) IELs, but they required exposure to self-agonist peptides. The agonist-selected TCRalphabeta(+) thymocytes are CD4 and CD8 double-negative, and their final maturation, including the induction of CD8alphaalpha expression, appeared to occur only after thymus export in the IL-15-rich environment of the gut. These developmental steps, including precommitment of immature thymocytes, TCR-mediated agonist selection, and postthymic differentiation promoted by cytokines, define a unique pathway for the generation of CD8alphaalpha(+)TCRalphabeta(+) IEL.

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)."

Self-antigen maintains the innate antibacterial function of self-specific CD8 T cells in vivo

Self-specific CD8 T cells, which are selected by high-affinity interactions with self-Ags, develop into a lineage distinct from conventional CD8 T cells. We have previously shown that these self-specific cells acquire phenotypic and functional similarities to cells of the innate immune system including the expression of functional receptors associated with NK cells. In this study, we show that these self-specific cells have the ability to produce large amounts of IFN-gamma in response to infection with Listeria monocytogenes in a bystander fashion. The rapid production of IFN-gamma is associated with a dramatic reduction in the number of viable bacteria at the peak of infection. Self-specific CD8 T cells provide only marginal innate protection in the absence of self-Ag; however, the presence of self-Ag dramatically increases their protective ability. Exposure to self-Ag is necessary for the maintenance of the memory phenotype and responsiveness to inflammatory cytokines such as IL-15. Significantly, self-specific CD8 T cells are also more efficient in the production of IFN-gamma and TNF-alpha, thus providing more cytokine-dependent protection against bacterial infection when compared with NK cells. These findings illustrate that self-reactive CD8 T cells can play an important innate function in the early defense against bacterial infection.

A shared gene-expression signature in innate-like lymphocytes

Innate and adaptive immunities are the two major arms of the immune system, which rely on distinct cell types. These cells can be distinguished not only by the source of diversity for non-self recognition, of germline or somatic origin, but also by their localization and the pattern and rates of response after encounter of antigenic triggers. In addition, subsets of lymphocytes exist whose receptors require rearrangement but result in semi-invariant structures with a high degree of self-specificity. We hypothesized that these innate-like lymphocytes might share a common gene transcription signature that relates them to classic members of the innate immune system. This relationship was first observed in agonist-induced CD8alphaalpha T cells in fetal/neonatal thymus. We then asked whether this notion could be extended to other innate-like lymphocytes, by comparison of gene expression profiles of innate-like lymphocytes and closely paired adaptive system counterparts (NKT versus CD4T, CD8alphaalphaT versus CD8alphabetaT, and B1 versus B2). A statistically significant 'innate signature' indeed was distilled. Particularly intriguing was the high representation of interferon-inducible guanosine triphophatases crucial for resistance against intracellular pathogens and of small G proteins involved in intracellular vacuole maturation and trafficking. Overall, this combined expression pattern can be designated as an innate signature among lymphocytes.

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.

The timing of TCR alpha expression critically influences T cell development and selection

Sequential rearrangement of the T cell receptor for antigen (TCR) beta and alpha chains is a hallmark of thymocyte development. This temporal control is lost in TCR transgenics because the alpha chain is expressed prematurely at the CD4- CD8- double negative (DN) stage. To test the importance of this, we expressed the HY alpha chain at the physiological CD4+ CD8+ double positive (DP) stage. The reduced DP and increased DN cellularity typically seen in TCR transgenics was not observed when the alpha chain was expressed at the appropriate stage. Surprisingly, antigen-driven selection events were also altered. In male mice, thymocyte deletion now occurred at the single positive or medullary stage. In addition, no expansion of CD8 alpha alpha intestinal intraepithelial lymphocytes (IELs) was observed, despite the fact that HY transgenics have been used to model IEL development. Collectively, these data establish the importance of proper timing of TCR expression in thymic development and selection and emphasize the need to use models that most accurately reflect the physiologic process.

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.

The fourth way? Harnessing aggressive tendencies in the thymus

During late stages of thymic development, T cells must chose between different fates, dictated by their TCR specificity. Typically, this is thought of as a choice between three alternatives (being positive selection for useful T cells vs negative selection or neglect for harmful or useless T cells). However, there is growing evidence for a fourth alternative, in which T cells are positively selected by agonist ligands, which would normally be expected to induce T cell deletion. In this review, we will discuss where and when agonist selection is induced and whether this should be considered as a novel form of thymic selection or as an alternative differentiation state for Ag-exposed T cells.

Regulatory T cells: peace keepers in the gut

The mucosa-associated lymphoid tissue (MALT) has the task of protecting the host from pathogens while maintaining the integrity of the gut. Immune responses are tightly regulated such that there is tolerance of nonpathogenic bacteria as well as dietary antigens present in the intestinal lumen. The failure to control these responses leads to a disruption in tolerance, which has been proposed as one mechanism involved in the development of inflammatory bowel disease (IBD). Different mechanisms are involved in the control of immune responses in the intestinal tract, including active suppression by regulatory T cells. Distinct subsets of regulatory T cells coexist in the intestinal mucosa, which is a fertile environment for their growth. Most of these are defined by their phenotype and/or their ability to produce regulatory cytokines such as interleukin-10 and transforming growth factor-beta A lack of activation and/or expansion of regulatory cells could play a role in the uncontrolled inflammation seen in IBD. Regulatory T cells may be activated by cytokines, and their inductive phase may be antigen-driven. There are limited data relating to the true surface interactions regulating the activation of these cells. Most of the CD4 regulatory T cells (Tr1, Th3, and CD4 CD25+) are thought to interact with dendritic cells. Subsets of regulatory T cells (such as CD8 TrE cells) may recognize antigens presented by intestinal epithelial cells. A better understanding of the mechanisms by which these regulatory T cells are expanded and/or activated in the intestinal mucosa may provide clues as how to use them as a novel therapeutic tool in the treatment of patients with IBD.

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.

Self-reactivity in thymic double-positive cells commits cells to a CD8αα lineage with characteristics of innate immune cells

Thymocytes displaying self-reactive T cell receptors usually undergo negative selection in the thymus. Here we demonstrate that agonist peptides can promote positive selection of immature double-positive thymocytes into distinct lineages, varying with the agonist concentration and the animal's age. Microarray gene expression analyses showed broad transcriptional alterations in a set of transcripts associated with the innate immune system, as well as silencing of CD8 beta expression. The resulting CD8 alpha alpha T cells showed a rapid effector cytokine response. Hence, T cells displaying self-reactive receptors can have the gene expression profile and phenotypic characteristics of innate immune cells.

CD8alphaalpha-mediated survival and differentiation of CD8 memory T cell precursors

Memory T cells are long-lived antigen-experienced T cells that are generally accepted to be direct descendants of proliferating primary effector cells. However, the factors that permit selective survival of these T cells are not well established. We show that homodimeric alpha chains of the CD8 molecule (CD8alphaalpha) are transiently induced on a selected subset of CD8alphabeta+ T cells upon antigenic stimulation. These CD8alphaalpha molecules promote the survival and differentiation of activated lymphocytes into memory CD8 T cells. Thus, memory precursors can be identified among primary effector cells and are selected for survival and differentiation by CD8alphaalpha.

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.

Positive and negative selection of T cells

A functional immune system requires the selection of T lymphocytes expressing receptors that are major histocompatibility complex restricted but tolerant to self-antigens. This selection occurs predominantly in the thymus, where lymphocyte precursors first assemble a surface receptor. In this review we summarize the current state of the field regarding the natural ligands and molecular factors required for positive and negative selection and discuss a model for how these disparate outcomes can be signaled via the same receptor. We also discuss emerging data on the selection of regulatory T cells. Such cells require a high-affinity interaction with self-antigens, yet differentiate into regulatory cells instead of being eliminated.

Human small-intestinal epithelium contains functional natural killer lymphocytes

BACKGROUND & AIMS

CD3(-) non-T lymphocytes constitute the second most abundant lymphoid subset in the human small-bowel epithelium, and these CD3(-) intraepithelial lymphocytes are virtually absent in active celiac disease. Phenotypically, they resemble natural killer cells and have been termed natural killer-like intraepithelial lymphocytes. Because of the limited availability of appropriate human samples, functional studies have not yet been reported, and it is not yet clear whether these are true natural killer cells.

METHODS

We used magnetic bead-based purification and flow cytometry to study several aspects of normal human small-bowel natural killer-like intraepithelial lymphocytes: intracellular cytokine content (basally and after activation); ability to lyse natural killer-sensitive K562 target cells; and expression of perforins, Fas ligand, and other functional markers.

RESULTS

CD3(-) intraepithelial lymphocytes cultured in interleukin-2 showed a higher lymphokine-activated killer activity than CD3(+) intraepithelial lymphocytes (48%-83% lysis exerted by CD3(-) intraepithelial lymphocytes at an effector-target cell ratio of 2:1 vs. 8%-18% by CD3(+) intraepithelial lymphocytes). Perforin content correlated with this lytic potential (75% +/- 4% in CD3(-) vs. 5% +/- 4% in CD3(+) intraepithelial lymphocytes). Both CD3(-) and CD3(+) cells displayed a type I cytokine profile (interferon-gamma > tumor necrosis factor-alpha > interleukin-2; undetectable interleukin-4 and interleukin-10). In addition to their activated phenotype, subsets of natural killer-like intraepithelial lymphocytes expressed CD8alphaalpha and intracellular CD3epsilon chain, showing the existence of heterogeneity within this cell lineage.

CONCLUSIONS

This is the first demonstration of functional natural killer cells within the human gut epithelium. These cells might play an important role in innate mucosal immunity (host defense and tumor surveillance) and tolerance.

A unique subset of self-specific intraintestinal T cells maintains gut integrity

Lymphocytes residing in the intestinal epithelium are exclusively T cells and account for one of the largest collection of T cells in the organism. However, their function remains obscure. We and others have shown that the development of intestinal intraepithelial T cells is compromised in mutant mice prone to chronic intestinal inflammation. These results led us to directly assess their role in regulating the development of colitis secondary to transfer of primary splenic TCRalphabeta(+)CD4(+)CD45RB(hi) T cells into severe combined immunodeficiency (SCID) mice. Here we demonstrate that prior reconstitution of SCID recipients with intraintestinal TCRalphabeta(+)CD4(-)CD8alpha(+)beta(-) T cells prevents disease, and does so in an interleukin (IL)-10-dependent fashion. In contrast, reconstitution with either TCRgammadelta(+) or TCRalphabeta(+)CD4(-) CD8alpha(+)beta(+) intestinal T cells did not prevent colitis. TCRalphabeta(+)CD4(-)8alpha(+)beta(-) T cells are unique to the intestinal epithelium of both rodents and humans. Previous repertoire analyses of TCRalphabeta(+)CD4(-)CD8alpha(+)beta(-) T cells revealed a high proportion of cells expressing high affinity, self-specific TCR within this subset. We demonstrate that monoclonal, self specific TCRalphabeta(+)CD4(-)CD8alpha(+)beta(-) cells derived from TCR transgenic mice also prevent the onset of colitis. Thus, intestinal TCRalphabeta(+)CD4(-)CD8alpha(+)beta(-) T cells, selected based on their self-reactivity, maintain gut integrity in a IL-10-dependent fashion.

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.

Evidence for the extrathymic origin of intestinal TCRgammadelta(+) T cells in normal rats and for an impairment of this differentiation pathway in BB rats

The BB rat lyp mutation, one of its diabetes susceptibility genes, is responsible for a 5-fold decrease in the number of peripheral TCRalphabeta(+) T cells. In this study we show that TCRgammadelta(+) T cells are virtually undetectable among splenic T cells and intestinal intraepithelial T lymphocytes (IEL) of BB rats, while they account for 3 and 30% of these two T cell populations, respectively, in normal animals. It has been shown that murine IEL expressing TCRgammadelta develop extrathymically. We determined whether this is the case in rats. Athymic radiation chimeras reconstituted with normal hemopoietic precursors were devoid of donor-derived TCRalphabeta(+) T cells and TCRgammadelta(+) splenocytes but contained a normal number of TCRgammadelta(+) IEL, suggesting that in unmanipulated rats some of the TCRgammadelta(+) IEL may have an extrathymic origin. This was further supported by the observation that RAG1 transcripts are present in IEL of unmanipulated animals. No T cells developed in chimeras reconstituted with BB hemopoietic precursors, demonstrating that the BB rat lyp mutation inhibits both intrathymic and extrathymic development of TCRgammadelta(+) T cells.

T cell responses modulated through interaction between CD8alphaalpha and the nonclassical MHC class I molecule, TL

The thymus leukemia antigen (TL) is a nonclassical class I molecule, expressed abundantly on intestinal epithelial cells. We show that, in contrast to other major histocompatibility complex (MHC) class I molecules that bind CD8alphabeta, TL preferentially binds the homotypic form of CD8alpha (CD8alphaalpha). Thus, TL tetramers react specifically to CD8alphaalpha-expressing cells, including most intestinal intraepithelial lymphocytes. Compared with CD8alphabeta, which recognizes the same MHC as the T cell receptor (TCR) and thus acts as a TCR coreceptor, high-affinity binding of CD8alphaalpha to TL modifies responses mediated by TCR recognition of antigen presented by distinct MHC molecules. These findings define a novel mechanism of lymphocyte regulation through CD8alphaalpha and MHC class I.

Identification and characterization of lymphoid precursors in the murine intestinal epithelium

Although in vivo evidence supports a role for the murine intestinal epithelium in the extrathymic generation of certain intraepithelial T lymphocytes (IEL), no intraepithelial cells with in vitro lymphoid progenitor potential have yet been demonstrated. Using reaggregate fetal thymic organ culture techniques, we show that a subset of CD3(-) cells isolated from the intestinal epithelium of young mice is capable of generating T cells (alpha beta and gamma delta) and NK1.1(+) cells in vitro. A novel IEL subset bearing a low level of CD45 was identified and found to comprise cells expressing highly immature lymphoid markers including CD34, c-kit, CD122, CD127 and high levels of CD16 and CD44. This subset represents 20-30% of intraepithelial CD45(+) cells from 4-week-old wild-type and nude mouse strains and contains cells with in vitro T cell differentiation capacity. The identification of such an early pluripotent precursor phenotype within the intestinal epithelium implies that the potential for T cell generation exists at this site, and suggests that extrathymic T cell generation may occur within the epithelium itself.

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.

Reevaluation of the origin of CD44(high) "memory phenotype" CD8 T cells: comparison between memory CD8 T cells and thymus-independent CD8 T cells

CD44(high)CD8 T cells in naive mice, which increase with age, and are often referred to as memory CD8 T cells. However, since thymus-independent CD8 T cells have also been shown to be CD44(high), the origin of the CD44(high)CD8 T cells in naive mice remains unclear. In this study, we compared the characteristics of memory CD8 T cells and thymus-independent CD8 T cells in TCR transgenic mice to clarify the origin of the CD44(high)CD8 T cells in naive normal mice. The memory and thymus-independent CD8 T cells showed differences in surface molecules, spontaneous cell death, cytokine production, and response to IL-2R binding of cytokines. Importantly, the "memory phenotype" CD8 T cells in naive normal mice showed similar characteristics to the thymus-independent CD8 T cells, but differed greatly from "true" memory CD8 T cells in the TCR transgenic mice. Therefore, we conclude that a significant part of the CD44(high) memory phenotype CD8 T cells in naive normal mice represents thymus-independent CD8 T cells, which may participate in age-related changes in immune responses.

Mucosal immune network in the gut for the control of infectious diseases

The common mucosal immune system (CMIS) consists of an integrated cross-communication pathway of lymphoid tissues made up of inductive and effector sites for host protection against pathogenic microorganisms. Major effector molecules of the CMIS include IgA antibodies and cytokines, chemokines and their corresponding receptors. Secretory IgA (S-IgA), the major immunoglobulin, is induced by gut-associated lymphoreticular tissue (GALT)-derived B cells with the help of Th1- and Th2-type CD4(+) T lymphocytes. Cytotoxic T lymphocytes (CTLs) in the mucosal epithelium, a subpopulation of intraepithelial lymphocytes (IELs), also help maintain the mucosal barrier. The CMIS is unique in that it can provide both positive and negative signals for the induction and regulation of immune responses in both the mucosal and systemic compartments after oral or nasal antigen exposure. Prevention of infection through mucosal surfaces can be achieved by the CMIS through connections between inductive (e.g. GALT) and effector tissues. When vaccine antigens are enterically administered together with mucosal adjuvants [e.g. cholera toxin (CT), heat-labile toxin produced by Escherichia coli (LT) and IL-12], antigen-specific Th1/Th2 and IgA B cell responses are induced simultaneously in the mucosal effector compartment. Since these antigen-specific immune responses are not generated by oral vaccine without mucosal adjuvant, safe and effective adjuvants for the induction of antigen-specific S-IgA and CTL responses are essential for the development of mucosal vaccines for protection against infectious diseases. Finally, recent findings suggest the presence of a CMIS-independent IgA induction pathway, which also must be considered in the development of mucosal vaccines.

In search of the origin of the thymus: the thymus and GALT may be evolutionarily related

The thymus is the major primary immune tissue for the production of functional T lymphocytes in vertebrates. However, its evolutionary origin is unknown. It has recently been shown that the generation of local T cells also occurs in gut-associated lymphoid tissues (GALT). This suggests that the thymus and GALT have similar functions and that they might be evolutionarily related. We discuss the possibility that the thymus may have evolved from mucosa-associated lymphoid tissues (MALT) located in the gill region in early vertebrates. Various facts supporting this proposal are summarized.

Unusual cytotoxic activities of thymus-independent, self-antigen-specific CD8(+) T cells

We compared the cytotoxic activities of thymus-dependent and thymus-independent CD8(+) T cells. Thymus-dependent CD8(+) T cells, which are foreign antigen specific, acquired cytotoxic activity to tumor cells with a basal dose of the antigen peptides and to hybridoma cells expressing anti-TCR mAb only after differentiation into effector cytotoxic T lymphocytes (CTL). In contrast, thymus-independent CD8(+) T cells, which have been shown to be self-antigen specific, never showed cytotoxic activity to the target cells with a basal dose of the self-antigen peptide, while they could lyse hybridoma cells expressing anti-TCR mAb even without prior antigenic stimulation. Furthermore, the ex vivo cytotoxic activity of thymus-independent CD8(+) T cells was also observed against the target cells with high doses of the antigen peptides, which were not lysed by freshly isolated thymus-dependent CD8(+) T cells. Thus it is revealed that thymus-independent, self-antigen-specific CD8(+) T cells already acquire mature CTL functions in situ but have an increased threshold of TCR-mediated signaling for activation. These differences in cytotoxic activities between thymus-dependent and thymus-independent CD8(+) T cells suggest distinct roles of the two subsets of CD8(+) T cells in vivo.

Gut cryptopatches: direct evidence of extrathymic anatomical sites for intestinal T lymphopoiesis

Athymic cytokine receptor gamma chain mutant mice that lack the thymus, Peyer's patches, cryptopatches (CP), and intestinal T cells were reconstituted with wild-type bone marrow cells. Bone marrow-derived TCR(-) intraepithelial lymphocytes (IEL) first appeared within villous epithelia of small intestine overlying the regenerated CP, and these TCR(-) IEL subsequently emerged throughout the epithelia. Thereafter, TCR(+) IEL increased to a comparable number to that in athymic mice and consisted of TCRgammadelta and TCRalphabeta IEL. In gut-associated lymphoid tissues of wild-type mice, only CP harbored a large population of c-kit(high)IL-7R(+)CD44(+)Thy-1(+/-)CD4(+/-)CD25(low/-)alpha(E) beta(7)(-)Lin(-) (Lin, lineage markers) lymphocytes that included cells expressing germline but not rearranged TCRgamma and TCRbeta gene transcripts. These findings provide direct evidence that gut CP develop progenitor T cells for extrathymic IEL descendants.

Intestinal inflammation observed in IL-2R/IL-2 mutant mice is associated with impaired intestinal T lymphopoiesis

BACKGROUND & AIMS

Although interleukin (IL)-2(-/-) and IL-2Ralpha(-/-) mice develop inflammatory bowel disease, IL-2Rbeta(-/-) animals are apparently free of gut pathology. Intraintestinal T lymphopoiesis is reported to be impaired in IL-2Rbeta(-/-) mice; we have determined whether this characteristic correlated with the apparent resistance of this mutant strain to intestinal inflammation. This led us to reassess intraintestinal T lymphopoiesis in these 3 mutant strains.

METHODS

Intestinal histology and intraintestinal T lymphopoiesis were analyzed in unmanipulated mutant mice and in athymic and euthymic radiation chimeras reconstituted with bone marrow derived from IL-2(-/-), IL-2Ralpha(-/-), and IL-2Rbeta(-/-) donors.

RESULTS

Intraintestinal T lymphopoiesis was ablated in the 3 mutant strains and was associated with cryptopatch abnormalities. The intestinal mucosa of mice reconstituted with lymphocytes from IL-2Rbeta(-/-) mice exhibited lesions of both the small and large bowel similar to those observed in the early stages of human gluten enteropathy and acute ulcerative colitis, respectively. Analysis of euthymic and athymic bone marrow radiation chimeras indicated that T cells located in the intestinal mucosa of unmanipulated IL-2(-/-), IL-2Ralpha(-/-), and IL-2Rbeta(-/-) mice are of thymic origin.

CONCLUSIONS

Null mutations at IL-2/IL-2Ralpha and beta loci differentially affect intraintestinal and intrathymic T lymphopoiesis. These conditions are associated with lesions of intestinal inflammation that are mediated by thymus-derived T cells.

Intestinal intraepithelial lymphocytes exert potent protective cytotoxic activity during an acute virus infection

After systemic infection of mice with 104 PFU of lymphocytic choriomeningitis virus (LCMV), infected cells are detected simultaneously in various organs, including spleen and intestinal mucosa. Most notably, virus-infected cells are also present among CD11c+ dendritic cells in the subepithelial area of the small intestinal mucosa. Some of these virus-infected cells are in close spatial association with intestinal intraepithelial lymphocytes (IEL). Therefore, we compared virus-specific cytotoxic activity of CD8 splenocytes with that of IEL subsets. While ex vivo isolated TCRalphabeta+CD8alphaalpha+ IEL exert only minimal virus-specific cytotoxicity, maximum specific killing mediated by TCRalphabeta+CD8alphabeta+ IEL on day 8 postinfection exceeds maximum cytotoxic activity observed with CD8 splenocytes when assessed in vitro. Maximum cytotoxic activity of IEL is preceded by peak perforin and granzyme B mRNA expression in IEL around day 6 postinfection, suggesting a recent activation in situ. The antivirus cytotoxicity of in vivo primed IEL is further demonstrated by the protection from virus production in the spleen of mice infected with LCMV 10 h before adoptive cell transfer. These data indicate a potent priming of LCMV-specific IEL in situ after systemic LCMV infection and suggest that cytotoxic IEL markedly contribute to the elimination of virus-infected cells in the intestinal mucosa.

TCR-independent activation of extrathymically developed, self antigen-specific T cells by IL-2/IL-15

Naive intrathymically developed T cells, which express foreign Ag-specific TCR, do not express IL-2R. After antigenic stimulation, they express high affinity IL-2R, which enables IL-2 to be used as an autocrine growth factor. On the contrary, extrathymically developed T cells, which express self Ag-specific TCR but are unresponsive to antigenic stimulation, spontaneously express low affinity IL-2R. In this study, we compared the responses of these two subsets of T cells to IL-2R stimulation and examined the influences of TCR-mediated signaling on the responses. IL-2 or IL-15 augmented the proliferative response of Ag-stimulated, intrathymically developed T cells. On the other hand, extrathymically developed T cells proliferated in response to IL-2 or IL-15, independently of Ag stimulation. Furthermore, both IL-2 and IL-15 induced IFN-gamma production of these T cells, which is strikingly augmented by the presence of IL-12. These results revealed functional differences between intrathymically developed, foreign Ag-specific T cells and extrathymically developed, self Ag-specific T cells. The latter can be activated by some inflammatory cytokines, in an Ag-independent manner, similar to NK cells.

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.

Cutting Edge: TCRαβ+ CD8αα+ T Cells Are Found in Intestinal Intraepithelial Lymphocytes of Mice That Lack Classical MHC Class I Molecules

TCRαβ+ intestinal intraepithelial lymphocytes (IEL) can express either the typical CD8αβ heterodimer or an unusual CD8αα homodimer. Both types of CD8+ IEL require class I molecules for their differentiation, since they are absent in β2m−/− mice. To gain insight into the role of class I molecules in forming TCRαβ+ CD8+ IEL populations, we have analyzed the IEL in mice deficient for either TAP, β2m, CD1, or K and D. We find that K−/−D−/− mice have TCRαβ+ CD8αα+ IEL, although they are deficient for TCRαβ+ CD8αβ+ cells. This indicates that at least some TCRαβ+ CD8αα+ IEL require only nonclassical class I molecules for their development. Surprisingly, the TCRαβ+ CD8αα+ IEL are significantly increased in K−/−D−/− mice, suggesting a complex interaction between CD8+ IEL and class I molecules that might include direct or indirect negative regulation by K and D, as well as positive effects mediated by nonclassical class I molecules.