An unusual lineage of alpha/beta T cells that contains autoreactive cells

CD8 T cell tolerance results from eviction of immature autoreactive cells from the thymus

CD8 T cell tolerance is thought to result from clonal deletion of autoreactive thymocytes before they differentiate into mature CD8 T cells in the thymus. However, we report that, in mice, CD8 T cell tolerance instead results from premature thymic eviction of immature autoreactive CD8 thymocytes into the periphery, where they differentiate into self-tolerant mature CD8 T cells. Premature thymic eviction is triggered by T cell receptor (TCR)-driven down-regulation of the transcriptional repressor Gfi1, which induces expression of sphingosine-1-phosphate receptor-1 (S1P1) on negatively selected immature CD8 thymocytes. Thus, premature thymic eviction is the basis for CD8 T cell tolerance and is the mechanism responsible for the appearance in the periphery of mature CD8 T cells bearing autoreactive TCRs that are absent from the thymus.

MHC I tetramer staining tends to overestimate the number of functionally relevant self-reactive CD8 T cells in the preimmune repertoire

Previous studies that used peptide-MHC (pMHC) tetramers (tet) to identify self-specific T cells have questioned the effectiveness of thymic-negative selection. Here, we used pMHCI tet to enumerate CD8 T cells specific for the immunodominant gp33 epitope of lymphocytic choriomeningitis virus glycoprotein (GP) in mice transgenically engineered to express high levels of GP as a self-antigen in the thymus. In GP-transgenic mice (GP+ ), monoclonal P14 TCR+ CD8 T cells that express a GP-specific TCR could not be detected by gp33/Db -tet staining, indicative of their complete intrathymic deletion. By contrast, in the same GP+ mice, substantial numbers of polyclonal CD8 T cells identifiable by gp33/Db -tet were present. The gp33-tet staining profiles of polyclonal T cells from GP+ and GP-negative (GP- ) mice were overlapping, but mean fluorescence intensities were ∼15% lower in cells from GP+ mice. Remarkably, the gp33-tet+ T cells in GP+ mice failed to clonally expand after lymphocytic choriomeningitis virus infection, whereas those of GP- mice did so. In Nur77GFP -reporter mice, dose-dependent responses to gp33 peptide-induced TCR stimulation revealed that gp33-tet+ T cells with high ligand sensitivity are lacking in GP+ mice. Hence, pMHCI tet staining identifies self-specific CD8 T cells but tends to overestimate the number of truly self-reactive cells.

Generation of higher affinity T cell receptors by antigen-driven differentiation of progenitor T cells in vitro

Many promising targets for T-cell-based cancer immunotherapies are self-antigens. During thymic selection, T cells bearing T cell receptors (TCRs) with high affinity for self-antigen are eliminated. The affinity of the remaining low-avidity TCRs can be improved to increase their antitumor efficacy, but conventional saturation mutagenesis approaches are labor intensive, and the resulting TCRs may be cross-reactive. Here we describe the in vitro maturation and selection of mouse and human T cells on antigen-expressing feeder cells to develop higher-affinity TCRs. The approach takes advantage of natural Tcrb gene rearrangement to generate diversity in the length and composition of CDR3β. In vitro differentiation of progenitors transduced with a known Tcra gene in the presence of antigen drives differentiation of cells with a distinct agonist-selected phenotype. We purified these cells to generate TCRβ chain libraries pre-enriched for target antigen specificity. Several TCRβ chains paired with a transgenic TCRα chain to produce a TCR with higher affinity than the parental TCR for target antigen, without evidence of cross-reactivity.

αβT cell receptors expressed by CD4(-)CD8αβ(-) intraepithelial T cells drive their fate into a unique lineage with unusual MHC reactivities

Coreceptor CD4 and CD8αβ double-negative (DN) TCRαβ(+) intraepithelial T cells, although numerous, have been greatly overlooked and their contribution to the immune response is not known. Here we used T cell receptor (TCR) sequencing of single cells combined with retrogenic expression of TCRs to study the fate and the major histocompatibility complex (MHC) restriction of DN TCRαβ(+) intraepithelial T cells. The data show that commitment of thymic precursors to the DN TCRαβ(+) lineage is imprinted by their TCR specificity. Moreover, the TCRs they express display a diverse and unusual pattern of MHC restriction that is nonoverlapping with that of CD4(+) or CD8αβ(+) T cells, indicating that they sense antigens that are not recognized by the conventional T cell subsets. The new insights indicate that DN TCRαβ(+) T cells form a third lineage of TCRαβ T lymphocytes expressing a variable TCR repertoire, which serve nonredundant immune functions.

Significant involvement of nuclear factor-κB-inducing kinase in proper differentiation of αβ and γδ T cells

Nuclear factor-κB-inducing kinase (NIK) is known to play a critical role in maintaining proper immune function. This is exemplified in the spontaneous mutant mouse lacking functional NIK, alymphoplasia (aly), which is simultaneously immune-compromised and autoimmune-prone. To investigate the role of NIK in αβ T-cell repertoire formation, we analysed T-cell development in aly/aly mice bearing a transgenic T-cell receptor (TCR). Although there were no apparent abnormalities in the mature αβ T cells of non-transgenic aly/aly mice, the maturation efficiency of idiotype(high+) T cells in the TCR-transgenic mice was lower in aly/aly mice compared with those found in aly/+ mice, suggesting that the mature αβ T-cell repertoire could be altered by the absence of functional NIK. In one strain of TCR-transgenic aly/aly mice with a negatively selecting H-2 background, the proportion of CD8(low+) idiotype(high+) cells, which are thought to potentially represent the γδ lineage of T cells, was markedly decreased. When the γδ T cells in non-transgenic aly/aly mice were investigated, the proportion of γδ T cells in the peripheral organs of aly/aly mice was found to be one-half to one-fifth of those in aly/+ mice. Analyses of bone marrow chimera mice indicated that NIK in host cells, rather than in donor cells was important for generating a normal number of peripheral γδ T cells. Collectively, these results suggest that NIK could be involved in thymic positive selection of some αβ T cells and that NIK in non-haematopoietic cells is important for the optimal development and/or maintenance of γδ T cells.

Re-adapting T cells for cancer therapy: from mouse models to clinical trials

Adoptive T-cell therapy involves the isolation, expansion, and reinfusion of T lymphocytes with a defined specificity and function as a means to eradicate cancer. Our research has focused on specifying the requirements for tumor eradication with antigen-specific T cells and T cells transduced to express a defined T-cell receptor (TCR) in mouse models and then translating these strategies to clinical trials. Our design of T-cell-based therapy for cancer has reflected efforts to identify the obstacles that limit sustained effector T-cell activity in mice and humans, design approaches to enhance T-cell persistence, develop methods to increase TCR affinity/T-cell functional avidity, and pursue strategies to overcome tolerance and immunosuppression. With the advent of genetic engineering, a highly functional population of T cells can now be rapidly generated and tailored for the targeted malignancy. Preclinical studies in faithful and informative mouse models, in concert with knowledge gained from analyses of successes and limitations in clinical trials, are shaping how we continue to develop, refine, and broaden the applicability of this approach for cancer therapy.

Liver immunology

The liver is the largest organ in the body and is generally regarded by nonimmunologists as having little or no lymphoid function. However, such is far from accurate. This review highlights the importance of the liver as a lymphoid organ. Firstly, we discuss experimental data surrounding the role of liver as a lymphoid organ. The liver facilitates tolerance rather than immunoreactivity, which protects the host from antigenic overload of dietary components and drugs derived from the gut and it is instrumental to fetal immune tolerance. Loss of liver tolerance leads to autoaggressive phenomena, which if not controlled by regulatory lymphoid populations, may lead to the induction of autoimmune liver diseases. Liver-related lymphoid subpopulations also act as critical antigen-presenting cells. The study of the immunological properties of liver and delineation of the microenvironment of the intrahepatic milieu in normal and diseased livers provides a platform to understand the hierarchy of a series of detrimental events that lead to immune-mediated destruction of the liver and the rejection of liver allografts. The majority of emphasis within this review will be on the normal mononuclear cell composition of the liver. However, within this context, we will discuss selected, but not all, immune-mediated liver disease and attempt to place these data in the context of human autoimmunity.

Unusual selection and peripheral homeostasis for immunoregulatory CD4(-) CD8(-) T cells

Immunoregulatory CD4(-)  CD8(-) (double-negative; DN) T cells exhibit a unique antigen-specific mode of suppression, yet the ontogeny of DN T cells remains enigmatic. We have recently shown that 3A9 T-cell receptor (TCR) transgenic mice bear a high proportion of immunoregulatory 3A9 DN T cells, facilitating their study. The 3A9 TCR is positively selected on the H2(k) MHC haplotype, is negatively selected in mice bearing the cognate antigen, namely hen egg lysozyme, and there is absence of positive selection on the H2(b) MHC haplotype. Herein, we take advantage of this well-defined 3A9 TCR transgenic model to assess the thymic differentiation of DN T cells and its impact on determining the proportion of these cells in secondary lymphoid organs. We find that the proportion of DN T cells in the thymus is not dictated by the nature of the MHC-selecting haplotype. By defining DN T-cell differentiation in 3A9 TCR transgenic CD47-deficient mice as well as in mice bearing the NOD.H2(k) genetic background, we further demonstrate that the proportion of 3A9 DN T cells in the spleen is independent of the MHC selecting haplotype. Together, our findings suggest that immunoregulatory DN T cells are subject to rules distinct from those imposed upon CD4 T cells.

Role of Hedgehog signalling at the transition from double-positive to single-positive thymocyte

In the thymus, developing T cells receive signals that determine lineage choice, specificity, MHC restriction and tolerance to self-antigen. One way in which thymocytes receive instruction is by secretion of Sonic hedgehog (Shh) from thymic epithelial cells. We have previously shown that Hedgehog (Hh) signalling in the thymus decreases the CD4:CD8 single-positive (SP) thymocyte ratio. Here, we present data indicating that double-positive (DP) thymocytes are Hh-responsive and that thymocyte-intrinsic Hh signalling plays a role in modulating the production of CD4(+) (SP4), CD8(+) (SP8) and unconventional T-cell subsets. Repression of physiological Hh signalling in thymocytes altered the proportions of DP and SP4 cells. Thymocyte-intrinsic Hh-dependent transcription also attenuated both the production of mature SP4 and SP8 cells, and the establishment of peripheral T-cell compartments in TCR-transgenic mice. Additionally, stimulation or withdrawal of Hh signals in the WT foetal thymus impaired or enhanced upregulation of the CD4 lineage-specific transcription factor Gata3 respectively. These data together suggest that Hh signalling may play a role in influencing the later stages of thymocyte development.

CD3+CD4-CD8- (double negative) T cells: saviours or villains of the immune response?

Recent studies have shown that T cells are not just the latecomers in inflammation but might also play a key role in the early phase of this response. In this context, a number of T cell subsets including NKT cells, mucosal-associated invariant T cells and γ/δ T cells have been shown, together with classical innate immune cells, to contribute significantly to the development and establishment of acute and chronic inflammatory diseases. In this commentary we will focus our attention on a somewhat neglected class of T cells called CD3(+)CD4(-)CD8(-) double negative T cells and on their role in inflammation and autoimmunity. We will summarize the most recent views on their origin at the thymic and peripheral levels as well as their tissue localization in immune and non-lymphoid organs. We will then outline their potential pathogenic role in autoimmunity as well as their homeostatic role in suppressing excessive immune responses deleterious to the host. Finally, we will discuss the potential therapeutic benefits or disadvantages of targeting CD3(+)CD4(-)CD8(-) double negative T cells for the treatment of autoimmune disease. We hope that this overview will shed some light on the function of these immune cells and attract the interest of investigators aiming at the design of novel therapeutic approaches for the treatment of autoimmune and inflammatory conditions.

Self-tolerance checkpoints in CD4 T cells specific for a peptide derived from the B cell antigen receptor

Linked recognition of Ag by B and T lymphocytes is ensured in part by a state of tolerance acquired by CD4 T cells to germline-encoded sequences within the B cell Ag receptor (BCR). We sought to determine how such tolerance is attained when a peptide from the BCR variable (V) region is expressed by small numbers of B cells as it is in the physiological state. Mixed bone marrow (BM) chimeras were generated using donor BM from mice with B cells that expressed a transgene (Tg)-encoded κ L chain and BM from TCR Tg mice in which the CD4 T cells (CA30) were specific for a Vκ peptide encoded by the κTg. In chimeras where few B cells express the κTg, many CA30 cells were deleted in the thymus. However, a substantial fraction survived to the CD4 single-positive stage. Among single-positive CA30 thymocytes, few reached maturity and migrated to the periphery. Maturation was strongly associated with, and likely promoted by, expression of an endogenous TCR α-chain. CD4(+) CA30 cells that reached peripheral lymphoid tissues were Ag-experienced and anergic, and some developed into regulatory cells. These findings reveal several checkpoints and mechanisms that enforce a state of self-tolerance in developing T cells specific for BCR V region sequences, thus ensuring that T cell help to B cells occurs through linked recognition of foreign Ag.

Analysis of the inflammatory response in HY-TCR transgenic mice highlights the pathogenic potential of CD4- CD8- T cells

Transgenic mice expressing a rearranged T cell receptor (TCR)-αβ prematurely at the double-negative stage develop an abnormal population of peripheral T cells that lack CD4 and CD8 expression and are hyper-reactive to anti-TCR antibody stimulation. One such example is the HY-TCR transgenic mice. These mice express a TCR transgenic specific for the HY antigen that is expressed in male but not in female mice. As a result, male mice have an abnormal population of HY(+)/CD4(-)8(-) or HY(+)/CD4(-)CD8(low) T cells that are much lower in female mice. In this study, we investigated the potential patho/physiological function of these cells in vivo using a model of delayed-type hypersensitivity (DTH) reaction: the λ-carrageenan-induced paw edema. Interestingly, while both male and female HY-TCR mice develop a classical biphasic inflammatory response to λ-carrageenan, the degree of inflammation in the former was much higher than that in the latter. This was accompanied by a selective expansion of HY(+)/CD4(-)8(-) and HY(+)/CD4(-)CD8(low) T cells in male mice and by a markedly increased production of typical DTH cytokines compared with cells from female mice. These results were specific since analysis of the inflammatory response of HY-TCR transgenic mice subjected to zymosan-induced peritonitis showed no differences between male and female mice. Together, these findings provide novel evidence for the pathological role of self-reactive CD4(-)CD8(-) T cells, previously described in several autoimmune strains and recently identified in patients suffering from autoimmune diseases such as systemic lupus erythematosus.

Checkpoints in lymphocyte development and autoimmune disease

Antigen receptor-controlled checkpoints in B lymphocyte development are crucial for the prevention of autoimmune diseases such as systemic lupus erythematosus. Checkpoints at the stage of pre-B cell receptor (pre-BCR) and BCR expression can eliminate certain autoreactive BCRs either by deletion of or anergy induction in cells expressing autoreactive BCRs or by receptor editing. For T cells, the picture is more complex because there are regulatory T (T(reg)) cells that mediate dominant tolerance, which differs from the recessive tolerance mediated by deletion and anergy. Negative selection of thymocytes may be as essential as T(reg) cell generation in preventing autoimmune diseases such as type 1 diabetes, but supporting evidence is scarce. Here we discuss several scenarios in which failures at developmental checkpoints result in autoimmunity.

Peptide-specific, TCR-alpha-driven, coreceptor-independent negative selection in TCR alpha-chain transgenic mice

As thymocytes differentiate, Ag sensitivity declines, with immature CD4-CD8- double-negative (DN) cells being most susceptible to TCR signaling events. We show that expression of alphabetaTCR from the DN3 stage lowers the threshold for activation, allowing recognition of MHC peptides independently of the TCR beta-chain and without either T cell coreceptor. The MHC class I-restricted C6 TCR recognizes the Y-chromosome-derived Ag HYK(k)Smcy. Positive selection in C6 alphabetaTCR females is skewed to the CD8 compartment, whereas transgenic male mice exhibit early clonal deletion of thymocytes. We investigated the effect of the HYK(k)Smcy complex on developing thymocytes expressing the C6 TCR alpha-chain on a TCR-alpha(-/-) background. On the original selecting haplotype, the skew to the CD8 lineage is preserved. This is MHC dependent, as the normal bias to the CD4 subset is seen on an H2b background. In male H2k C6 alpha-only mice, the presence of the HYK(k)Smcy complex leads to a substantial deletion of thymocytes from the DN subset. This phenotype is replicated in H2k C6 alpha-only female mice expressing an Smcy transgene. Deletion is not dependent on the beta variable segment of the C6 TCR or on a restricted TCR-beta repertoire. In contrast, binding of HYK(k)Smcy and Ag-specific activation of mature CD8+ T cells is strictly dependent on the original C6 beta-chain. These data demonstrate that, in comparison with mature T cells, alphabetaTCR+ immature thymocytes can recognize and transduce signals in response to specific MHC-peptide complexes with relaxed binding requirements.

Functionally mature CD4 and CD8 TCRalphabeta cells are generated in OP9-DL1 cultures from human CD34+ hematopoietic cells

Human CD34(+) hematopoietic precursor cells cultured on delta-like ligand 1 expressing OP9 (OP9-DL1) stromal cells differentiate to T lineage cells. The nature of the T cells generated in these cultures has not been studied in detail. Since these cultures do not contain thymic epithelial cells which are the main cell type mediating positive selection in vivo, generation of conventional helper CD4(+) and cytotoxic CD8(+) TCRalphabeta cells is not expected. Phenotypically mature CD27(+)CD1(-) TCRgammadelta as well as TCRalphabeta cells were generated in OP9-DL1 cultures. CD8 and few mature CD4 single-positive TCRalphabeta cells were observed. Mature CD8 single-positive cells consisted of two subpopulations: one expressing mainly CD8alphabeta and one expressing CD8alphaalpha dimers. TCRalphabeta CD8alphaalpha and TCRgammadelta cells both expressed the IL2Rbeta receptor constitutively and proliferated on IL-15, a characteristic of unconventional T cells. CD8alphabeta(+) and CD4(+) TCRalphabeta cells were unresponsive to IL-15, but could be expanded upon TCR stimulation as mature CD8alphabeta(+) and CD4(+) T cells. These T cells had the characteristics of conventional T cells: CD4(+) cells expressed ThPOK, CD40L, and high levels of IL-2 and IL-4; CD8(+) cells expressed Eomes, Runx3, and high levels of granzyme, perforin, and IFN-gamma. Induction of murine or human MHC class I expression on OP9-DL1 cells had no influence on the differentiation of mature CD8(+) cells. Similarly, the presence of dendritic cells was not required for the generation of mature CD4(+) or CD8(+) T cells. These data suggest that positive selection of these cells is induced by interaction between T precursor cells.

IL-15 is critical for the maintenance and innate functions of self-specific CD8(+) T cells

IL-15 is a pleiotropic cytokine involved in host defense as well as autoimmunity. IL-15-deficient mice show a decrease of memory phenotype (MP) CD8(+) T cells, which develop naturally in naïve mice and whose origin is unclear. It has been shown that self-specific CD8(+) T cells developed in male H-Y antigen-specific TCR transgenic mice share many similarities with naturally occurring MP CD8(+) T cells in normal mice. In this study, we found that H-Y antigen-specific CD8(+) T cells in male but not female mice decreased when they were crossed with IL-15-deficient mice, mainly due to impaired peripheral maintenance. The self-specific TCR transgenic CD8(+) T cells developed in IL-15-deficient mice showed altered surface phenotypes and reduced effector functions ex vivo. Bystander activation of the self-specific CD8(+) T cells was induced in vivo during infection with Listeria monocytogenes, in which proliferation but not IFN-gamma production was IL-15-dependent. These results indicated important roles for IL-15 in the maintenance and functions of self-specific CD8(+) T cells, which may be included in the naturally occurring MP CD8(+) T-cell population in naïve normal mice and participate in innate host defense responses.

Mechanisms of self-nonself discrimination and possible clinical relevance

This review discusses different mechanisms that result in immunological tolerance, such as intrathymic deletion of immature T cells, intrathymic and extrathymic generation of regulatory T cells, effector mechanisms of regulatory T cells as well as molecular pathways involved in extrathymic generation of regulatory T cells in vivo and in vitro. These molecular mechanisms should enable investigators to develop clinical protocols aiming at the specific prevention of unwanted immune responses, thereby replacing indiscriminate immunosuppression that often has fatal consequences.

STAT6 deletion converts the Th2 inflammatory pathology afflicting Lat(Y136F) mice into a lymphoproliferative disorder involving Th1 and CD8 effector T cells

Mutant mice in which tyrosine 136 of linker for activation of T cells (LAT) was replaced with a phenylalanine (Lat(Y136F) mice) develop a lymphoproliferative disorder involving polyclonal CD4 effector T cells that produce massive amounts of IL-4 and trigger severe Th2 inflammation. Naive CD4 T cells can themselves produce IL-4 and thereby initiate a self-reinforcing positive regulatory loop that involves the STAT6 transcription factor and leads to Th2 polarization. We determined the functional outcome that results when Lat(Y136F) T cells differentiate in the absence of such STAT6-dependent regulatory loop. The lack of STAT6 had no effect on the timing and magnitude of the lymphoproliferative disorder. However, in Lat(Y136F) mice deprived of STAT6, the expanding CD4 T cell population was dominated by Th1 effector cells that triggered B cell proliferation, elevated IgG2a and IgG2b levels as well as the production of autoantibodies. In contrast to Lat(Y136F) mice that showed no CD8 T cell expansion, the CD8 T cells present in Lat(Y136F) mice deprived of STAT6 massively expanded and acquired effector potential. Therefore, the lack of STAT6 is sufficient to convert the Th2 lymphoproliferative disorder that characterizes Lat(Y136F) mice into a lymphoproliferative disorder that is dominated by Th1 and CD8 effector T cells. The possibility to dispose of a pair of mice that differs by a single gene and develops in the absence of deliberate immunization large numbers of Th cells with almost reciprocal polarization should facilitate the identification of genes involved in the control of normal and pathological Th cell differentiation.

SIT and TRIM determine T cell fate in the thymus

Thymic selection is a tightly regulated developmental process essential for establishing central tolerance. The intensity of TCR-mediated signaling is a key factor for determining cell fate in the thymus. It is widely accepted that low-intensity signals result in positive selection, whereas high-intensity signals induce negative selection. Transmembrane adaptor proteins have been demonstrated to be important regulators of T cell activation. However, little is known about their role during T cell development. Herein, we show that SIT (SHP2 Src homology domain containing tyrosine phosphatase 2-interacting transmembrane adaptor protein) and TRIM (TCR-interacting molecule), two structurally related transmembrane adaptors, cooperatively regulate TCR signaling potential, thereby influencing the outcome of thymic selection. Indeed, loss of both SIT and TRIM resulted in the up-regulation of CD5, CD69, and TCRbeta, strong MAPK activation, and, consequently, enhanced positive selection. Moreover, by crossing SIT/TRIM double-deficient mice onto transgenic mice bearing TCRs with different avidity/affinity, we found profound alterations in T cell development. Indeed, in female HY TCR transgenic mice, positive selection was completely converted into negative selection resulting in small thymi devoided of double-positive thymocytes. More strikingly, in a nonselecting background, SIT/TRIM double-deficient single-positive T cells developed, were functional, and populated the periphery. In summary, we demonstrated that SIT and TRIM regulate cell fate of developing thymocytes, thus identifying them as essential regulators of central tolerance.

Identification of two major types of age-associated CD8 clonal expansions with highly divergent properties

CD8 memory T cells are tightly regulated in young, healthy individuals but are often perturbed in aged animals by the appearance of large CD8 T cell clones. These clones are associated with impaired immunity in the aged. The molecular basis of this phenomenon remains unclear. Here, it is shown that the issue is confused by the fact that the clones are heterogeneous. Some clones bear high, and others, low levels of integrin alpha(4) (itgalpha4). These subtypes differ by multiple criteria. They appear in mice of different ages, concentrate in different tissues, and have different stabilities in vivo and responses to stimulation in vitro. itgalpha4(high), but not itgalpha4(low), CD8 clonal expansions have several characteristics consistent with a chronically stimulated phenotype. These properties include lowered levels of CD8, decreased expression of some cytokine receptors, and elevated expression of various inhibitory receptors, including the programmed death-1 (PD1) receptor and the killer cell lectin-like receptor G1 (KLRG1). The characteristics of itgalpha4(high) clonal expansions suggest that they may arise from age-dependent alterations in antigen expression and tolerance. These data redefine CD8 clonal expansions into at least two distinct entities and indicate that there are multiple mechanisms that drive age-related alterations of CD8 T cell homeostasis.

Lineage diversion of T cell receptor transgenic thymocytes revealed by lineage fate mapping

Background

The binding of the T cell receptor (TCR) to major histocompatibility complex (MHC) molecules in the thymus determines fates of TCRαβ lymphocytes that subsequently home to secondary lymphoid tissue. TCR transgenic models have been used to study thymic selection and lineage commitment. Most TCR transgenic mice express the rearranged TCRαβ prematurely at the double negative stage and abnormal TCRαβ populations of T cells that are not easily detected in non-transgenic mice have been found in secondary lymphoid tissue of TCR transgenic mice.

Methodology and Principal Findings

To determine developmental pathways of TCR-transgenic thymocytes, we used Cre-LoxP-mediated fate mapping and show here that premature expression of a transgenic TCRαβ diverts some developing thymocytes to a developmental pathway which resembles that of gamma delta cells. We found that most peripheral T cells with the HY-TCR in male mice have bypassed the RORγt-positive CD4⁺8⁺ (double positive, DP) stage to accumulate either as CD4⁻8⁻ (double negative, DN) or as CD8α⁺ T cells in lymph nodes or gut epithelium. Likewise, DN TCRαβ cells in lymphoid tissue of female mice were not derived from DP thymocytes.

Conclusion

The results further support the hypothesis that the premature expression of the TCRαβ can divert DN thymocytes into gamma delta lineage cells.

Dose-dependent modulation of CD8 and functional avidity as a result of peptide encounter

The generation of an optimal CD8(+) cytotoxic T lymphocyte (CTL) response is critical for the clearance of many intracellular pathogens. Previous studies suggest that one contributor to an optimal immune response is the presence of CD8(+) cells exhibiting high functional avidity. In this regard, CD8 expression has been shown to contribute to peptide sensitivity. Here, we investigated the ability of naive splenocytes to modulate CD8 expression according to the concentration of stimulatory peptide antigen. Our results showed that the level of CD8 expressed was inversely correlated with the amount of peptide used for the primary stimulation, with higher concentrations of antigen resulting in lower expression of both CD8alpha and CD8beta. Importantly the ensuing CD8(low) and CD8(high) CTL populations were not the result of the selective outgrowth of naive CD8(+) T-cell subpopulations expressing distinct levels of CD8. Subsequent encounter with peptide antigen resulted in continued modulation of both the absolute level and the isoform of CD8 expressed and in the functional avidity of the responding cells. We propose that CD8 cell surface expression is not a static property, but can be modulated to 'fine tune' the sensitivity of responding CTL to a defined concentration of antigen.

TCR and Notch synergize in αβ versus γδ lineage choice

At two checkpoints, T cell development is controlled by T cell receptor (TCR) signaling, which determines survival and lineage commitment. At the first of these checkpoints, signaling by the pre-TCR, the gammadeltaTCR or the alphabetaTCR has a major but nonexclusive impact on whether cells will become CD4-CD8- gammadelta or CD4+CD8+ alphabeta lineage cells. Pre-TCR signals synergize with moderate Notch signals to generate alphabeta lineage cells. Relatively strong signals by the gammadeltaTCR (or early expressed alphabetaTCR) in the absence of Notch signaling are sufficient to yield gammadelta lineage cells. However, relatively weak signals of the latter two receptors combined with strong Notch signaling result in the formation of alphabeta lineage cells that generate a diverse alphabetaTCR repertoire in pre-TCR-deficient mice. It remains to be determined whether TCR and/or Notch signals instruct or confirm predetermined lineage fate.

Negative selection of the T-cell repertoire: where and when does it occur?

Because of the use of somewhat artificial models for the elucidation of negative selection [superantigen, T-cell receptor (TCR) transgenic mice], there is still considerable uncertainty at what stages of T-cell development negative selection can occur and whether it becomes manifest as developmental arrest, lineage diversion, or induction of apoptotic cell death. Here, experimental evidence is reviewed that excludes developmental arrest and lineage diversion as the sole mechanisms of negative selection. The data emphasize that both CD4+ CD8+ double-positive cortical as well as semi-mature, single-positive, medullary thymocytes are targets of deletion in experimental models employing superantigen and TCR transgenic mice with premature as well as 'timely' onset of TCR expression.

T-cell avidity and tuning: the flexible connection between tolerance and autoimmunity

Thymic T-cell selection mechanisms generate a cross-reactive, self-MHC restricted peripheral T-cell pool. Affinity and avidity are of profound influence on this selection and the generation of immunity. Autoreactive T cells can escape thymic deletion by lowering their avidity and retain this "tuned" state in the periphery. Upon activation, tuned T cells can cause autoimmunity, while immunotherapeutic strategies may be hampered by existing T-cell tolerance. The regulation of T-cell avidity and tuning therefore determines the balance between tolerance and autoimmunity and should be taken into account in the design of therapeutic strategies aimed at T-cell reactivity.

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.

The E delta enhancer controls the generation of CD4- CD8- alphabetaTCR-expressing T cells that can give rise to different lineages of alphabeta T cells

It is well established that the pre-T cell receptor for antigen (TCR) is responsible for efficient expansion and differentiation of thymocytes with productive TCRbeta rearrangements. However, Ptcra- as well as Tcra-targeting experiments have suggested that the early expression of Tcra in CD4- CD8- cells can partially rescue the development of alphabeta CD4+ CD8+ cells in Ptcra-deficient mice. In this study, we show that the TCR E delta but not E alpha enhancer function is required for the cell surface expression of alphabetaTCR on immature CD4- CD8- T cell precursors, which play a crucial role in promoting alphabeta T cell development in the absence of pre-TCR. Thus, alphabetaTCR expression by CD4- CD8- thymocytes not only represents a transgenic artifact but occurs under physiological conditions.

Suppressor of cytokine signaling 1 stringently regulates distinct functions of IL-7 and IL-15 in vivo during T lymphocyte development and homeostasis

SOCS1(-/-) mice accumulate within the thymus and periphery CD8(+) lymphocytes that express memory cell markers and display heightened in vitro responses to common gamma-chain cytokines. To investigate whether dysregulated homeostasis of T lymphocytes and acquisition of memory phenotype by CD8(+) cells in SOCS1(-/-) mice were mediated by IL-7 and/or IL-15 in vivo, we have generated SOCS1(-/-)IL-7(-/-), SOCS1(-/-)IL-15(-/-) and SOCS1(-/-)IL-7(-/-)IL-15(-/-) mice. We observed that in mice lacking SOCS1, either IL-7 or IL-15 skewed thymocyte development toward CD8 lineage, whereas IL-15 is the principal mediator of dysregulated homeostasis in the periphery. Homeostatic proliferation of SOCS1(-/-) CD8(+) lymphocytes in Rag1(-/-), Rag1(-/-)IL-7(-/-), Rag1(-/-)IL-15(-/-), and Rag1(-/-)IL-7(-/-)IL-15(-/-) mice showed that SOCS1 deficiency did not overcome the requirement for IL-7 and IL-15 to sustain homeostatic expansion. Differential expression of memory phenotype markers CD44, CD122, and Ly6C by SOCS1(-/-)IL-15(-/-) CD8(+) lymphocytes suggest that multiple signals contributed to the memory cell differentiation program. To address whether increased IL-15 responsiveness of SOCS1(-/-) CD8(+) lymphocytes required prior TCR sensitization, we generated SOCS1(-/-) H-Y TCR transgenic (Tg) mice. Using female SOCS1(-/-) H-Y TCR(tg) mice in Rag1(+/+) and Rag1(-/-) backgrounds, we show that acquisition of the memory phenotype by SOCS1-deficient CD8(+) lymphocytes did not require prior antigenic stimulation, but required the presence of activated T cells. SOCS1 deficiency accelerated the maturation of CD8 single-positive thymocytes expressing Tg TCR, but did not compromise negative selection in HY-TCR(tg) males. Our findings illustrate distinct functions for IL-7 and IL-15 in T lymphocyte development and homeostasis, and stringent regulation of these processes by SOCS1.

Mechanisms imposing the Vbeta bias of Valpha14 natural killer T cells and consequences for microbial glycolipid recognition

Mouse and human natural killer T (NKT) cells recognize a restricted set of glycosphingolipids presented by CD1d molecules, including self iGb3 and microbial α-glycuronosylceramides. The importance of the canonical Vα14-Jα18 TCR α chain for antigen recognition by NKT cells is well recognized, but the mechanisms underlying the Vβ8, Vβ7, and Vβ2 bias in mouse have not been explored. To study the influences of thymic selection and the constraints of pairing with Vα14-Jα18, we have created a population of mature T cells expressing Vα14-Jα18 TCR α chain in CD1d-deficient mice and studied its recognition properties in vitro and in vivo. Transgenic cells expressed a diverse Vβ repertoire but their recognition of endogenous ligands and synthetic iGb3 was restricted to the same biased Vβ repertoire as expressed in natural NKT cells. In contrast, α-GalCer, a synthetic homologue of microbial α-glycuronosylceramides, was recognized by a broader set of Vβ chains, including the biased NKT set but also Vβ6, Vβ9, Vβ10, and Vβ14. These surprising findings demonstrate that, whereas Vβ8, Vβ7, and Vβ2 represent the optimal solution for recognition of endogenous ligand, many Vβ chains that are potentially useful for the recognition of foreign lipids fail to be selected in the NKT cell repertoire.

Expansion and long-range differentiation of the NKT cell lineage in mice expressing CD1d exclusively on cortical thymocytes

Unlike conventional major histocompatibility complex–restricted T cells, Vα14-Jα18 NKT cell lineage precursors engage in cognate interactions with CD1d-expressing bone marrow–derived cells that are both necessary and sufficient for their thymic selection and differentiation, but the nature and sequence of these interactions remain partially understood. After positive selection mediated by CD1d-expressing cortical thymocytes, the mature NKT cell lineage undergoes a series of changes suggesting antigen priming by a professional antigen-presenting cell, including extensive cell division, acquisition of a memory phenotype, the ability to produce interleukin-4 and interferon-γ, and the expression of a panoply of NK receptors. By using a combined transgenic and chimeric approach to restrict CD1d expression to cortical thymocytes and to prevent expression on other hematopoietic cell types such as dendritic cells, macrophages, or B cells, we found that, to a large extent, expansion and differentiation events could be imparted by a single-cognate interaction with CD1d-expressing cortical thymocytes. These surprising findings suggest that, unlike thymic epithelial cells, cortical thymocytes can provide unexpected, cell type–specific signals leading to lineage expansion and NKT cell differentiation.

Signaling for NKT cell development: the SAP-FynT connection

New studies demonstrate a critical role for the adaptor protein SAP (SLAM-associated protein) during NKT cell development. By connecting homotypic SLAM family receptor interactions with the FynT Src kinase, SAP may integrate a set of long-standing yet seemingly disparate observations characterizing NKT cell development. In fact, SAP-dependent signaling may underlie the development of multiple unconventional T cell lineages whose thymic selection relies on homotypic interactions between hematopoietic cells.

Peripheral "CD8 tuning" dynamically modulates the size and responsiveness of an antigen-specific T cell pool in vivo

In this study, we suggest that CD8 levels on T cells are not static, but can change and, as a result, modulate CD8(+) T cell responses. We describe three models of CD8 modulation using novel weak-agonist (K1A) and super-agonist (C2A) altered peptide ligands of the HY smcy peptide. First, we used peripheral nonresponsive CD8(low) T cells produced after peripheral HY-D(b) MHC class I tetramer stimulation of female HY TCR transgenic and wild-type mice. Second, we used genetically lowered CD8(int) T cells from heterozygote CD8(+/0) mice. Finally, we used pre-existing nonresponsive CD8(low) T cells from male HY TCR transgenic mice. In CD8(low) and CD8(high) mice, presence of a lower level of CD8 greatly decreased the avidity of the peptide-MHC for HY TCR as reflected by avidity (K(D)) and dissociation constant (T(1/2)) measurements. All three models demonstrated that lowering CD8 levels resulted in the requirement for a higher avidity peptide-MHC interaction with the TCR to respond equivalently to unmanipulated CD8(high) T cells of the same specificity. Additionally, direct injections of wild-type HY-D(b) and C2A-D(b) tetramers into female HY TCR or female B6 mice induced a high frequency of peripheral nonresponsive CD8(low) T cells, yet C2A-D(b) was superior in inducing a primed CD8(+)CD44(+) memory population. The ability to dynamically modulate the size and responsiveness of an Ag-specific T cell pool by "CD8 tuning" of the T cell during the early phases of an immune response has important implications for the balance of responsiveness, memory, and tolerance.

Self-reactive memory-phenotype CD8 T cells exhibit both MHC-restricted and non-MHC-restricted cytotoxicity: a role for the T-cell receptor and natural killer cell receptors

We have recently shown that interleukin-2 (IL-2)-activated CD8(+)CD44(hi) cells from normal mice express both adaptive and innate immune system receptors and specifically kill syngeneic tumor cells, particularly those that express NKG2D ligands. Here we show that CD8+ T cells from antigen-expressing H-Y T-cell receptor (TCR) transgenic mice also exhibit characteristics of both T cells and natural killer (NK) cells. Interaction with cognate self-antigen was required for the optimal expansion of these cells in peripheral lymphoid tissues. Although these cells possess a higher activation threshold relative to naive T cells, they can be activated by cytokine alone in vitro. They also undergo bystander proliferation in response to a bacterial infection in vivo. Interestingly, upon activation, the cells express the NKG2D receptor as well as the DNAX activation protein 12 (DAP12) adaptor protein. We provide evidence that NKG2D can act additively with the TCR in the killing of target cells, and it can also function as a directly activating receptor in non-major histocompatibility complex (MHC)-restricted killing of target cells. These properties of CD8+ T cells from H-Y TCR transgenic mice are remarkably similar to CD8(+)CD44(hi) cells that are found in normal mice. The H-Y TCR transgenic mice provide a well-defined system for characterizing the developmental biology and function of these cells.

A role for TCR affinity in regulating naive T cell homeostasis

Homeostatic signals that control the overall size and composition of the naive T cell pool have recently been identified to arise from contact with self-MHC/peptide ligands and a cytokine, IL-7. IL-7 presumably serves as a survival factor to keep a finite number of naive cells alive by preventing the onset of apoptosis, but how TCR signaling from contact with self-MHC/peptide ligands regulates homeostasis is unknown. To address this issue, murine polyclonal and TCR-transgenic CD8+ cells expressing TCR with different affinities for self-MHC/peptide ligands, as depicted by the CD5 expression level, were analyzed for their ability to respond to and compete for homeostatic factors under normal and lymphopenic conditions. The results suggest that the strength of the TCR affinity determines the relative "fitness" of naive T cells to compete for factors that support cell survival and homeostatic proliferation.

CD1d-expressing dendritic cells but not thymic epithelial cells can mediate negative selection of NKT cells

Natural killer T (NKT) cells are a unique immunoregulatory T cell population that is positively selected by CD1d-expressing thymocytes. Previous studies have shown that NKT cells exhibit autoreactivity, which raises the question of whether they are subject to negative selection. Here, we report that the addition of agonist glycolipid alpha-galactosylceramide (alpha-GalCer) to a fetal thymic organ culture (FTOC) induces a dose-dependent disappearance of NKT cells, suggesting that NKT cells are susceptible to negative selection. Overexpression of CD1d in transgenic (Tg) mice results in reduced numbers of NKT cells, and the residual NKT cells in CD1d-Tg mice exhibit both an altered Vbeta usage and a reduced sensitivity to antigen. Furthermore, bone marrow (BM) chimeras between Tg and WT mice reveal that CD1d-expressing BM-derived dendritic cells, but not thymic epithelial cells, mediate the efficient negative selection of NKT cells. Thus, our data suggest that NKT cells developmentally undergo negative selection when engaged by high-avidity antigen or abundant self-antigen.

Thymic selection revisited: how essential is it?

Intrathymic T cell development represents one of the best studied paradigms of mammalian development. Lymphoid committed precursors enter the thymus and the Notch1 receptor plays an essential role in committing them to the T cell lineages. The pre-T cell receptor (TCR), as an autonomous cell signaling receptor, commits cells to the alphabeta lineage while its rival, the gammadeltaTCR, is involved in generating the gammadelta lineage of T cells. Positive and negative selection of immature alphabetaTCR-expressing cells are essential mechanisms for generating mature T cells, committing them to the CD4 and CD8 lineages and avoiding autoimmunity. Additional lineages of alphabetaT cells, such as the natural killer T cell lineage and the CD25+ regulatory T cell lineage, are formed when the alphabetaTCR encounters specific ligands in suitable microenvironments. Thus, positive selection and receptor-instructed lineage commitment represent a hallmark of the thymus. Ectopically expressed organ-specific antigens contribute to thymic self-nonself discrimination, which represents an essential feature for the evolutionary fitness of mammalian species.

Self-specific MHC class II-restricted CD4-CD8- T cells that escape deletion and lack regulatory activity

In the presence of the I-Ealpha protein, transgenic (Tg) mice expressing the 1H3.1 alphabeta TCR that is specific for the Ealpha52-68:I-A(b) complex display drastic intrathymic deletion. Although peripheral T cells from these mice remained unresponsive to the Ealpha52-68:I-A(b) complex, they contained a subpopulation able to specifically react to this complex in the presence of exogenous IL-2, indicating that some 1H3.1 alphabeta TCR Tg T cells have escaped clonal deletion and efficiently populated the periphery. IL-2-dependent, Ealpha52-68:I-A(b) complex-responsive T cells were CD4-CD8- and expressed the 1H3.1 alphabeta TCR. Such T cells could develop intrathymically, did not show sign of regulatory/suppressor activity, displayed a typical naive phenotype, and seemed to persist in vivo over time. CD4-CD8- TCR Tg T cells were also detected when the surface density of the deleting ligand was increased on MHC class II+ cells. In addition, the development of CD4-CD8- 1H3.1 alphabeta TCR Tg T cells could be supported by I-A(b) molecules. These observations indicate that CD4 surface expression neither specifies, nor is required for, the thymic export of mature thymocytes expressing a MHC class II-restricted alphabeta TCR. The data also show that, although the avidity of the interaction involved in intrathymic deletion is significantly lower than that involved in mature T cell activation, its range can be large enough to be influenced by the presence or absence of coreceptors. Finally, the margin created by the absence of CD4 coreceptor was substantial because it could accommodate various amounts of the deleting ligand on thymic stromal cells.

Generation of functional antigen-specific T cells in defined genetic backgrounds by retrovirus-mediated expression of TCR cDNAs in hematopoietic precursor cells

We have developed an alternative to transgenesis for producing antigen-specific T cells in vivo. In this system, clonal naive T cells with defined antigen specificity are generated by retrovirus-mediated expression of T cell antigen receptor cDNAs in RAG1-deficient murine hematopoietic precursor cells. These T cells can be stimulated to proliferate and produce cytokines by exposure to antigen in vitro, and they become activated and expand in vivo after immunization. IL-2-deficient T cells generated by this technique show decreased proliferation and cytokine production, both of which can be rescued by exogenous addition of this growth factor. Thus, retrovirus-mediated expression of T cell antigen receptor cDNAs in hematopoietic precursor cells permits the rapid and efficient analysis of the life history of antigen-specific T cells in different genetic backgrounds and may allow for the long-term production of antigen-specific T cells with different functional properties for prophylactic and therapeutic purposes.

The apoptotic protease-activating factor 1-mediated pathway of apoptosis is dispensable for negative selection of thymocytes

Negative selection is a process to delete potentially autoreactive clones in developing thymocytes. Programmed cell death or apoptosis is thought to play an important role in this selection process. In this study, we investigated the role of apoptotic protease-activating factor 1 (Apaf1), a mammalian homologue of CED-4, in programmed cell death during the negative selection in thymus. There was no developmental abnormality in thymocytes from newborn Apaf1(-/-) mice in terms of CD4 and CD8 expression pattern and thymocyte number. Clonal deletion by endogenous male H-Y Ag of Apaf1-deficient thymocytes with transgenic expression of H-Y Ag-specific TCRs (H-Y Tg/Apaf1(-/-) thymocytes) was normally observed in lethally irradiated wild-type mice reconstituted with fetal liver-derived hemopoietic stem cells. Clonal deletion induced in vitro by a bacterial superantigen was also normal in fetal thymic organ culture. Thus, Apaf1-mediated pathway of apoptosis is dispensable for the negative selection of thymocytes. However, H-Y Tg/Apaf1(-/-) thymocytes showed partial resistance to H-Y peptide-induced deletion in vitro as compared with H-Y Tg/Apaf1(+/-) thymocytes, implicating the Apaf1-mediated apoptotic pathway in the negative selection in a certain situation. In addition, the peptide-induced deletion was still observed in H-Y Tg/Apaf1(-/-) thymocytes in the presence of a broad spectrum caspase inhibitor, z-VAD-fmk, suggesting the presence of caspase-independent cell death pathway playing roles during the negative selection. We assume that mechanisms for the negative selection are composed of several cell death pathways to avoid failure of elimination of autoreactive clones.

T cell-mediated antigen presentation: a potential mechanism of infectious tolerance

Differentiation of the T cell repertoire and the physiology of T cell-mediated antigen presentation are reviewed in relation to mechanisms of self-tolerance. Recent research has indicated that T cell development is a continual process that optimizes partial recognition of self as a homeostatic set-point. Specific T cell antigen recognition of partial agonists is intrinsically linked to expression of class II MHC glycoproteins on T cells. Even ligands that act as TCR antagonists in IL-2 production assays have sufficient agonistic strength to induce expression of class II MHC glycoproteins on T cells. Thus, the intrinsic self-reactivity of the T cell repertoire may promote T-APC activity in vivo and may explain why thymic and peripheral T cells express low but significant levels of class II MHC glycoproteins. T-APC activity induces extensive apoptosis among responder T cells, causes desensitization among surviving responders, and has been implicated in the adoptive transfer of tolerance in the Lewis rat model of experimental autoimmune encephalomyelitis. Overall, these findings support a relationship between the partial recognition of self MHC ligands, expression of class II MHC glycoproteins on mature peripheral T cells, tolerogenic T cell-mediated antigen presentation, and desensitization of pathogenic self-reactive T cells.

TCR/self-antigen interactions drive double-negative T cell peripheral expansion and differentiation into suppressor cells

Mature CD4-CD8- alphabeta+ T cells (DNTC) in the periphery of TCR transgenic mice are resistant to clonal deletion in cognate Ag-expressing (Ag+) mice. Previously, we have characterized DNTC populations bearing the alloreactive 2C TCR in Ag-free (Ag-) and Ag+ mice. Despite appearing functionally anergic when challenged with cognate Ag in vitro, Ag-experienced DNTC exhibit markers of activation/memory, a lowered threshold of activation, ex vivo cytolytic activity, and the ability to rapidly secrete IFN-gamma. Remarkably, these memory-like DNTC also possess potent immunoregulatory properties, competing effectively for bystander-produced IL-2 and suppressing autoreactive CD8+ T cell proliferation via a Fas/FasL-dependent cytolytic mechanism. The fact that DNTC recovered from Ag+ mice possess markers and attributes characteristic of naive CD8+ T cells that have undergone homeostasis-induced proliferation suggested that they may be derived from a similar peripheral expansion process. Naive DNTC adoptively transferred into Ag-bearing hosts rapidly acquire markers and functional attributes of DNTC that have continually developed in the presence of Ag. Thus, the peripheral selection and maintenance of such autoreactive cells may serve to negatively regulate potential autoimmune T cell responses.

Antigen-induced T cell death is regulated by CD4 expression

Activation induced cell death (AICD) is a major physiologic pathway that regulates T cell homeostasis. In CD4 T cells, AICD is mediated mainly through Fas/FasL interactions. Although TCR occupancy triggers AICD, the contribution of its tightly associated CD4 coreceptor to the process that leads to AICD is not known. Here we show that CD4 molecule plays an essential regulatory role of TCR dependent AICD. Loss of CD4 rendered activated 5kc T cell hybridoma resistant to AICD. The resistance of CD4 negative 5kc T cells to AICD was due to selective inhibition of FasL expression and it could be reversed by addition of recombinant FasL. Furthermore, a direct functional link between CD4 and FasL was demonstrated by induction of FasL upon CD4 crosslinking in a TCR independent fashion. The importance of CD4 interaction with MHC/peptide complex in mediating AICD was also evident in normal T cells that could survive chronic stimulation with anti-CD3 but died after short period of proliferation after stimulation with MHC/peptide. Thus it appears that AICD is controlled by the CD4 molecule via regulation of FasL expression. These findings have important implications for our understanding of mechanisms of peripheral tolerance as well as pathogenesis of autoimmune diseases.

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.

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.

Ligand density determines the efficiency of negative selection in the thymus

To study the influence of antigen density on the efficiency of negative selection in the thymus, MHC class I (H-2K(b), K(b)) transgenic mice were generated, which expressed a K(b) transgene under the control of its natural promoter at 33% (K(b-lo)) or 150% (K(b-hi)) the surface density of Kb in C57BL/6 (B6, H-2(b)) mice. These mice were crossed to anti-K(b) T-cell receptor (Des-TCR) transgenic mice. In Des-TCRxK(b-hi) double transgenic mice, Des-TCR bearing T cells were completely eliminated during thymocyte maturation. In contrast, in Des-TCRxK(b-lo) double transgenic mice, two populations of Des-TCR T cells were evident, which either expressed the Des-TCR at intermediate density in the absence of CD8 (Des-TCR(int)CD8(-)) or expressed both the Des-TCR and CD8 at low density (Des-TCRloCD8lo). In the thymus of both types of double transgenic mice, no Des-TCR(+)CD4(+)CD8(+) thymocytes were detected, suggesting that deletion of Des-TCR cells occurred before the CD4(+)CD8(+) stage. Because only very few Des-TCR(+) thymocytes were found in Des-TCRxK(b-hi) transgenic mice, deletion of these T cells apparently occurred upon expression of the Des-TCR. By contrast, Des-TCRxK(b-lo) transgenic mice showed distinct populations of Des-TCR(int)CD4-8- and Des-TCR(lo)CD8(lo) thymocytes, suggesting that expression of the CD8 coreceptor was required to allow negative selection to proceed. Functional analyses showed that sublethally irradiated Des-TCRxK(b-lo) double transgenic mice were protected from lethal graft-versus-host disease by injected Des-TCR lymph node cells.

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.