Constitutive CD8 expression allows inefficient maturation of CD4+ helper T cells in class II major histocompatibility complex mutant mice

TCR-MHC docking orientation: natural selection, or thymic selection?

T cell receptors (TCR) dock on their peptide-major histocompatibility complex (pMHC) targets in a conserved orientation. Since amino acid sidechains are the foundation of specific protein-protein interactions, a simple explanation for the conserved docking orientation is that key amino acids encoded by the TCR and MHC genes have been selected and maintained through evolution in order to preserve TCR/pMHC binding. Expectations that follow from the hypothesis that TCR and MHC evolved to interact are discussed in light of the data that both support and refute them. Finally, an alternative and equally simple explanation for the driving force behind the conserved docking orientation is described.

NKT cells: from totipotency to regenerative medicine

The recent discovery that natural killer T (NKT) cell nuclei are totipotent opens a novel avenue for further understanding NKT cell function in normal and diseased states. The progeny of a cloned mouse harboring the in-frame rearranged Valpha14-Jalpha18 T cell receptor in one allele showed a significant increase in NKT cell number compared with wild-type or littermate control mice that possessed a different TCR. Importantly, NKT cells from such progeny produced both interferon-gamma and interleukin-4, a hallmark of NKT cells. In these progeny, NKT cell development appeared to be instructively, rather than permissively, determined. Using embryonic stem cells prepared via the somatic cell nuclear transfer of NKT nuclei, relatively mature NKT cells were induced under conditions permissible for T cell induction. Furthermore, these NKT cells matured autonomously upon injection into mice, resulting in an antigen-specific adjuvant effect.

Lineage fate and intense debate: myths, models and mechanisms of CD4- versus CD8-lineage choice

Following successful gene rearrangement at alphabeta T-cell receptor (TCR) loci, developing thymocytes express both CD4 and CD8 co-receptors and undergo a life-or-death selection event, which is known as positive selection, to identify cells that express TCRs with potentially useful ligand specificities. Positively selected thymocytes must then differentiate into either CD4(+) helper T cells or CD8(+) cytotoxic T cells, a crucial decision known as CD4/CD8-lineage choice. In this Review, we summarize recent advances in our understanding of the cellular and molecular events involved in lineage-fate decision and discuss them in the context of the major models of CD4/CD8-lineage choice.

CD4-CD8 lineage commitment is regulated by a silencer element at the ThPOK transcription-factor locus

The transcription factor ThPOK is necessary and sufficient to trigger adoption of the CD4 lymphocyte fate. Here we investigate the regulation of ThPOK expression and its subsequent control of CD4+ T cell commitment. Treatment of immature thymocytes with anti-TCR (T cell receptor) showed that TCR signals were important in ThPOK induction and that the CD4+8lo stage was the likely target of the inductive TCR signal. We identified at the ThPOK locus a key distal regulatory element (DRE) that mediated its differential expression in class I- versus II-restricted CD4+8lo thymocytes. The DRE was both necessary for suppression of ThPOK expression in class I-restricted thymocytes and sufficient for its induction in class II-restricted thymocytes. Mutagenesis analysis defined an essential 80bp core DRE sequence and its potential regulatory motifs. We propose a silencer-dependent model of lineage choice, whereby inactivation of the DRE silencer by a strong TCR signal leads to CD4 commitment, whereas continued silencer activity leads to CD8 commitment.

A novel mouse model for invariant NKT cell study

We have generated a novel mouse model harboring the in-frame rearranged TCRValpha specific for invariant NKT (iNKT) cells (Valpha14-Jalpha18) on one allele by crossing the mouse cloned from NKT cells with wild-type mice. This genomic configuration would ensure further rearrangement and expression of TCRValpha14-Jalpha18 under the endogenous promoters and enhancers. Mice harboring such an in-frame rearranged TCRValpha (Valpha14-Jalpha18 mouse) possessed an increase in iNKT cells in the thymus, liver, spleen, and bone marrow. Intriguingly, both Th1- and Th2-type cytokines were produced upon stimulation with alphaGalactosylceramide, an agonist of iNKT cells, and the IgE level in the serum remained unaffected in the Valpha14-Jalpha18 mouse. These features markedly distinguish the nature of iNKT cells present in the Valpha14-Jalpha18 mouse from that of iNKT cells found in the Valpha14-Jalpha18 transgenic mouse. Besides these, the expression of TCRVgammadelta cells remained intact, and the use of the TCRVbeta repertoire in iNKT cells was highly biased to TCRVbeta8 in the Valpha14-Jalpha18 mouse. Furthermore, alphaGalactosylceramide-CD1d dimer-reactive immature iNKT cells expressed less Rag2 as compared with the conventional immature T cells at the positive selection stage. Cell cycle analysis on the thymocytes revealed that no particular subset proliferated more vigorously than the others. Crossing the Valpha14-Jalpha18 mouse with the CD1d knockout mouse revealed a novel population of iNKT cells whose coreceptor expression profile was similar to that assigned to iNKT precursor cells. These mice will be useful for the study on the development of iNKT cells as well as on their functions in the immune system.

Role of the transcription factor Th-POK in CD4:CD8 lineage commitment

The molecular basis of CD4:CD8 lineage commitment, in particular the mechanism by which the precise correlation between lineage choice and T-cell receptor (TCR) specificity toward class I or II major histocompatibility complex is achieved, remains controversial. Both stochastic/selective and instructive models in various forms have been proposed to explain this correlation. The two main experimental approaches previously employed to elucidate this process have focused on the beginning and end of the process, i.e. the influence of TCR signaling and the alternate transcriptional control of the CD4 and CD8 loci during commitment. The recent finding that the transcription factor Th-POK is necessary and sufficient for CD4 commitment has now provided a direct entry point for studying the intracellular pathways that govern lineage commitment. Here, we review data leading to the identification and characterization of this factor and discuss the implications of these studies in the context of current models of lineage commitment.

The role of erk1 and erk2 in multiple stages of T cell development

Activation of extracellular-signal-regulated protein kinase (Erk) is central to growth-factor-receptor-mediated signaling including that originating from the T cell antigen receptor. It integrates cytoplasmic signals to effect changes in transcription associated with differentiation, proliferation, and survival. In this report, we present an analysis of mice with targeted deletions in Erk1 and Erk2 to assess the relationship between Erk activity and cell-cycle progression, thymocyte development, and lineage commitment. These studies show that Erk is selectively retained during beta selection-driven proliferation, and yet Erk1/2 are not required to complete differentiation to CD4+CD8+ preselection stage of development. Erk activity is essential for the process of positive selection, and it differentially affects CD4 and CD8 T cell maturation; yet, diminished expression itself is not sufficient to alter lineage commitment.

Functional and Molecular Analysis of the Double-Positive Stage-Specific CD8 Enhancer E8III during Thymocyte Development

Several developmental stage-, subset-, and lineage-specific Cd8 cis-regulatory regions have been identified. These include the E8(III) enhancer, which directs expression in double-positive (DP) thymocytes, and E8(II), which is active in DP cells and CD8(+) T cells. Using a transgenic reporter expression assay, we identified a 285-bp core fragment of the E8(III) enhancer that retains activity in DP thymocytes. In vitro characterization of the core enhancer revealed five regulatory elements that are required for full enhancer activity, suggesting that multiple factors contribute to the developmental stage-specific activity. Furthermore, deletion of E8(III) in the mouse germline showed that this enhancer is required for nonvariegated expression of CD8 in DP thymocytes when E8(II) is also deleted. These results indicate that E8(III) is one of the cis-elements that contribute to the activation of the Cd8a and Cd8b gene complex during T cell development.

Ikaros null mice display defects in T cell selection and CD4 versus CD8 lineage decisions

Previous evidence suggested that the hemopoietic-specific nuclear factor Ikaros regulates TCR signaling thresholds in mature T cells. In this study, we test the hypothesis that Ikaros also sets TCR signaling thresholds to regulate selection events and CD4 vs CD8 lineage determination in developing thymocytes. Ikaros null mice were crossed to three lines of TCR-transgenic mice, and positive selection, negative selection, and CD4 vs CD8 lineage decisions were analyzed. Mice expressing a polyclonal repertoire or a MHC class II-restricted TCR transgene exhibited enhanced positive selection toward the CD4 lineage. Moreover, in the absence of Ikaros, CD4 development can occur with decreased thresholds of TCR signaling. In addition, CD4 single-positive thymocytes were detected in MHC class I-restricted TCR-transgenic Ikaros null mice. To assess the role of Ikaros in negative selection, we analyzed deletion of T cells induced by conventional Ag or by endogenous superantigen. Surprisingly, negative selection was impaired in Ikaros null thymocytes despite evidence of high levels of TCR signal and no intrinsic defect in apoptosis ex vivo. To our knowledge, these data identify Ikaros as the first nuclear factor that plays a critical role in regulating negative selection as well as CD4 vs CD8 lineage decisions during positive selection.

Biology of T lymphocytes

T cells constitute one arm of the adaptive immune system. The accumulating information on various aspects of T-cell biology shows the intricacies in the regulation of immune responses. How we translate the cellular and molecular details of this regulation into innovation and development of therapies for disease management remains a fundamental, but exciting, challenge.

In a Transgenic Model of Spontaneous Autoimmune Diabetes, Expression of a Protective Class II MHC Molecule Results in Thymic Deletion of Diabetogenic CD8+ T Cells

H-2(d) mice expressing both the influenza virus hemagglutinin (HA) as a transgene-encoded protein on pancreatic islet beta cells (InsHA), as well as the Clone 4 TCR specific for the dominant H-2K(d)-restricted HA epitope, can be protected from the development of spontaneous autoimmune diabetes by expression of the H-2(b) haplotype. Protection occurs due to the deletion of K(d)HA-specific CD8+ T cells. This was unexpected as neither the presence of the InsHA transgene nor H-2(b), individually, resulted in thymic deletion. Further analyses revealed that thymic deletion required both a hybrid MHC class II molecule, Ebeta(b) Ealpha(d), and the K(d) molecule presenting the HA epitope, which together synergize to effect deletion of CD4+CD8+ thymocytes. This surprising example of protection from autoimmunity that maps to a class II MHC molecule, yet effects an alteration in the CD8+ T cell repertoire, suggests that selective events in the thymus represent the integrated strength of signal delivered to each cell through recognition of a variety of different MHC-peptide ligands.

T-cell development and the CD4-CD8 lineage decision

Cell-fate decisions are controlled typically by conserved receptors that interact with co-evolved ligands. Therefore, the lineage-specific differentiation of immature CD4+ CD8+ T cells into CD4+ or CD8+ mature T cells is unusual in that it is regulated by clonally expressed, somatically generated T-cell receptors (TCRs) of unpredictable fine specificity. Yet, each mature T cell generally retains expression of the co-receptor molecule (CD4 or CD8) that has an MHC-binding property that matches that of its TCR. Two models were proposed initially to explain this remarkable outcome--'instruction' of lineage choice by initial signalling events or 'selection' after a stochastic fate decision that limits further development to cells with coordinated TCR and co-receptor specificities. Aspects of both models now appear to be correct; mistake-prone instruction of lineage choice precedes a subsequent selection step that filters out most incorrect decisions.

Combined deletion of CD8 locus cis-regulatory elements affects initiation but not maintenance of CD8 expression

Developmental stage-, subset-, and lineage-specific CD8 enhancers have been identified recently by transgenic reporter analyses. Enhancer E8(II) (CIV-4,5) is active in both immature double-positive thymocytes (DP) and mature CD8 single-positive (SP) thymocytes and T cells, whereas E8(I) (CIII-1,2) directs expression only in mature cells. In mice lacking either E8(I) (CIII-1,2) or E8(II) (CIV-4,5), there was no effect on CD8 expression in DP thymocytes. However, deletion of both enhancers resulted in variegated expression of CD8, with appearance of CD4(+)CD8(-) SP thymocytes expressing surface markers characteristic of DP thymocytes. Consequently, fewer mature CD8(+) T cells developed from the reduced pool of DP cells. These results suggest that the initiation of CD8 expression is mediated by cis-regulatory elements that are distinct from any that may be involved in maintenance of expression.

On the brink of becoming a T cell

Recent studies provide fresh insight into the mechanisms by which precursor cells are committed to and develop within the T-lymphocyte lineage. Precursor/product studies have identified developmental stages between that of the pluripotent hematopoietic stem cell and thymocytes committed to the T lineage. Specific ligands and signaling pathways interacting with the Notch-1 receptor and its ability to influence commitment within the lymphoid lineage have been described. Although the structural features or putative ligands endowing the pre-TCR with constitutive signaling capacity remain elusive, numerous distal mediators of pre-TCR signaling have been identified. It remains for the future to determine what roles they may have in survival, proliferation, lineage commitment and allelic exclusion of TCR genes. Receptor editing and lineage commitment of alphabeta T cells still represent controversial topics that need further study.

Deletion of the CD4 silencer element supports a stochastic mechanism of thymocyte lineage commitment

The mechanism of T cell lineage commitment remains controversial; to examine it we deleted the CD4-silencer element in the germ line of a mouse using a combination of gene targeting and Cre/LoxP-mediated recombination. We found that these mice were unable to extinguish CD4 expression either in immature thymocytes or mature CD8+ cytotoxic T cells (CTLs), which resulted in the development of major histocompatibility complex class II-restricted double-positive CTLs in the periphery. This finding strongly supports a stochastic over an instructive mechanism of coreceptor down-regulation.

A molecular chart of thymocyte positive selection

As a new slant on T lymphocyte repertoire selection, we have examined batteries of TCR sequences in thymi from transgenic mice engineered to exhibit limited, focussed TCR diversity. We have tracked the fate of differentiating thymocytes expressing a set of particular TCR through the positive selection process. Subtly different TCR sequences can promote different maturation pathways and commitment choices. Two distinct routes are followed by CD8-lineage cells interacting with MHC class I molecules, via TCR(hi) CD4(+)CD8(+) or CD4(+)CD8(int) intermediates, while CD4-lineage cells mature exclusively via a CD4(+)CD8(int) stage. The CD8-lineage routes are partially exclusive, indicating that the latter cell type is not always preceded by the former. The distribution of sequences also indicates that CD4 / CD8-lineage commitment is not strictly correlated with the class of MHC molecule engaged, and that some mechanism prevents mismatched intermediates from achieving full maturity.

Activated p56lck directs maturation of both CD4 and CD8 single-positive thymocytes

p56(lck) is a protein tyrosine kinase expressed throughout T cell development. It associates noncovalently with the cytoplasmic domains of the CD4 and CD8 coreceptor molecules and has been implicated in TCR signaling in mature T cells. Its role in early thymocyte differentiation has been demonstrated in vivo, both by targeted gene disruption and by transgene expression. Previously, we showed that expression of a dominant-negative form of p56(lck) in double-positive thymocytes inhibits positive selection. We now demonstrate that expression of constitutively activated p56(lck) (p56(lck)F505) accelerates the transition from the double-positive to the single-positive stage. Importantly, p56(lck)F505 drives survival and lineage commitment of thymocytes in the absence of TCR engagement by appropriate MHC molecules. These results indicate that activation of p56(lck) constitutes an early step in conveying maturational signals after TCR ligation by a positively selecting ligand. Our study provides direct in vivo evidence for the role of p56(lck) in regulating TCR signaling.

T-cell lineage fate: instructed by receptor signals?

Mechanisms of lineage choice represent a challenging problem in developmental biology. Recent studies have shown that different T-cell receptor signals can affect CD4 or CD8 lineage choice. Thus, all the ingredients for instructive mechanisms of lineage fate are in place but other mechanisms cannot be completely ruled out.

Preferential proliferation and differentiation of double-positive thymocytes into CD8(+) single-positive thymocytes in a novel cell culture medium

The identification of factors that regulate the proliferation and differentiation of double-positive (DP) into CD4(+) and CD8(+) single-positive (SP) thymocytes has proven difficult due to the inability of DP thymocytes to proliferate, expand, and differentiate into SP thymocytes in available cell culture media. Here we report on the ability of DP thymocytes to differentiate in a novel conditioned medium, termed XLCM, derived from the supernatant of mitogen activated human cord blood mononuclear cells. During a 5-day culture in XLCM in the absence of thymic stromal cells, DP thymocytes from normal mice and MHC double knockout mice (lack SP thymocytes) proliferate, expand, and differentiate into several (alphabetaTCR(+), NK1.1(+)alphabetaTCR(+), and gammadeltaTCR(+)) subsets of CD4(+) and predominantly CD8(+) SP thymocytes. These studies suggest that the use of XLCM may aid in the characterization of factors that regulate the differentiation of DP thymocytes into CD8(+) SP thymocytes.

Highly efficient selection of CD4 and CD8 lineage thymocytes supports an instructive model of lineage commitment

We undertook a kinetic analysis of the generation of mature T cells in TCR and coreceptor transgenic mice using BrdU labeling. We observed that the selection efficiency of mature CD4-CD8+ and CD4+CD8- thymocytes could be as high as 40% and 90% of CD4+CD8+ precursors, respectively. The surprisingly high efficiency of selection favors an instructional model of lineage commitment and is incompatible with a stochastic model in which the efficiency of selection would be no greater than 100% in both lineages combined.

Lck activity controls CD4/CD8 T cell lineage commitment

Thymocytes carrying MHC class I-restricted TCRs differentiate into CD8 T cells, while those recognizing MHC class II become CD4 T cells. The mechanisms underlying how MHC class recognition, coreceptor expression, and effector function are coordinated are not well understood. Since the tyrosine kinase Lck binds with more affinity to CD4 than CD8, it has been proposed as a candidate to mediate this process. By using transgenic mice with altered Lck activity, we show that thymocytes carrying a class II-restricted TCR develop into functional CD8 T cells when Lck activity is reduced. Conversely, thymocytes carrying a class I-restricted TCR develop into functional CD4 T cells when Lck activity is increased. These results directly show that quantitative differences in the Lck signal control the CD4/CD8 lineage decision.

Commitment to the CD4 Lineage Mediated by Extracellular Signal-Related Kinase Mitogen-Activated Protein Kinase and Lck Signaling

The development of T cells results in a concordance between the specificity of the TCR for MHC class I and class II molecules and the expression of CD8 and CD4 coreceptors. Based on analogy to simple metazoan models of organ development and lineage commitment, we sought to determine whether extracellular signal-related kinase (Erk) mitogen-activated protein (MAP) kinase pathway signaling acts as an inductive signal for the CD4 lineage. Here, we show that, by altering the intracellular signaling involving the Erk/MAP kinase pathway, T cells with specificity for MHC class I can be diverted to express CD4, and, conversely, T cells with specificity for MHC class II can be diverted to express CD8. Furthermore, we find that activation of the src-family tyrosine kinase, p56lck is an upstream mediator of lineage commitment. These results suggest a simple mechanism for lineage commitment in T cell development.

The Role of CD8α′ in the CD4 Versus CD8 Lineage Choice

During thymic development the recognition of MHC proteins by developing thymocytes influences their lineage commitment, such that recognition of class I MHC leads to CD8 T cell development, whereas recognition of class II MHC leads to CD4 T cell development. The coreceptors CD8 and CD4 may contribute to these different outcomes through interactions with class I and class II MHC, respectively, and through interactions with the tyrosine kinase p56lck (Lck) via their cytoplasmic domains. In this paper we provide evidence that an alternatively spliced form of CD8 that cannot interact with Lck (CD8α′) can influence the CD4 vs CD8 lineage decision. Constitutive expression of a CD8 minigene transgene that encodes both CD8α and CD8α′ restores CD8 T cell development in CD8α mutant mice, but fails to permit the development of mismatched CD4 T cells bearing class I-specific TCRs. These results indicate that CD8α′ favors the development of CD8-lineage T cells, perhaps by reducing Lck activity upon class I MHC recognition in the thymus.

Th Cells and Th2 Responses Can Develop in the Absence of MHC Class II-CD4 Interactions

In this paper, we address the question whether CD4 and MHC class II expression are necessary for the development of the T helper lineage during thymocyte maturation and for activation-induced Th2 responses. To bypass the CD4-MHC class II interaction requirements for positive selection and activation, we used mice that are doubly transgenic for CD8 and for the MHC class I-restricted TCR F5. This transgene combination leads to MHC class I-dependent maturation of CD4 lineage cells. Upon activation, these CD4 lineage T cells secrete IL-4 and give help to B cells but show no cytotoxic activity. Remarkably, neither MHC class II nor CD4 expression are necessary for the generation and helper functions of these cells. This suggests that under normal conditions, coreceptor-MHC interactions are necessary to ensure the canonical combinations of coreceptor and function in developing thymocytes, but that they do not determine functional commitment. Our results also imply that expression of the CD4 gene does not influence, but is merely associated with the decision to establish the T helper program. In addition, we show that activation through TCR-MHC class I interactions can induce Th2 responses independently of CD4 and MHC class II expression.

The regulation of CD4 and CD8 coreceptor gene expression during T cell development

The two major subsets of T lymphocytes in the peripheral immune system, the helper and cytotoxic T cells, are defined by their expression of either the CD4 or the CD8 glycoproteins, respectively. Expression of these molecules, which serve as coreceptors by interacting specifically with either MHC class II or class I molecules, also defines discrete stages of T cell development within the thymus. Thus, CD4+ and CD8+ single-positive (SP) thymocytes arise from common progenitor double positive (DP) cells that express both CD4 and CD8, during a process known as positive selection. The molecular mechanisms underlying the developmental choice toward the helper or cytotoxic lineage remain poorly understood. Because regulation of coreceptor gene expression appears to be coupled to the phenotypic choice of the differentiating T cell, it is likely that shared signaling pathways direct CD4 and CD8 transcription and the development of an uncommited DP thymocyte toward either the helper or cytotoxic lineage. Therefore, an understanding of how CD4 and CD8 expression is regulated will not only provide insights into transcriptional control mechanisms in T cells, but may also result in the identification of molecular factors that are involved in lineage choices during T cell development. In this review, we summarize recent progress that has been made toward an understanding of how CD4 and CD8 gene expression is regulated.

Development and function of autospecific dual TCR+ T lymphocytes

Recent studies have challenged the long held concept that each T lymphocyte expresses on its surface only a single, unique alphabetaTCR. Dual TCR+ T cells have been recognized, however, their origin and potential to escape screening for self-reactivity remain obscure. We now report the thymic generation of dual alphabetaTCR+ T cells in the H-2Db/H-Y-specific TCR transgenic (Tg) mouse. Dual TCR+ thymocytes were positively selected less efficiently than single TCR+ thymocytes, although a subset attained maturity. Importantly, when TCR Tg mice were bred onto a negatively selecting background, auto-specific cells survived central deletion and matured as CD4+ dual TCR+ cells. These cells were autoreactive when CD8 expression was restored. The existence of autospecific, dual TCR+ T cells may have implications for the maintenance of self tolerance.

Visualization of CD4/CD8 T cell commitment

A system to innocuously visualize T cell lineage commitment is described. Using a "knock-in" approach, we have generated mice expressing a beta-galactosidase reporter in place of CD4; expression of beta-galactosidase in these animals appears to be an accurate and early indicator of CD4 gene transcription. We have exploited this knock-in line to trace CD4/CD8 lineage commitment in the thymus, avoiding important pitfalls of past experimental approaches. Our results argue in favor of a selective model of thymocyte commitment, demonstrating a fundamentally symmetrical process: engagement of either class of major histocompatibility complex (MHC) molecule by a differentiating CD4(+)CD8(+) cell can give rise to T cell antigen receptor (TCR)hi thymocytes of either lineage. Key findings include (a) direct demonstration of a substantial number of CD4-committed, receptor/coreceptor-mismatched cells in MHC class II- deficient mice, a critical prediction of the selective model; (b) highly efficient rescue of such "mismatched" intermediates by forced expression of CD8 in a TCR transgenic line, and an explanation of why previous experiments of this nature were less successful-a major past criticism of the selective model; (c) direct demonstration of an analogous, though smaller, population of CD8-committed mismatched intermediates in class I-deficient animals. Finally, we found no evidence of a CD4 default pathway.

CD4/CD8 lineage commitment: matching fate with competence

The outcome of positive selection of T lymphocytes is that there is a close match between the lineage adopted by a particular cell (CD4+ or CD8+) and the specificity of the T-cell receptor for the class of Major Histocompatibility Complex molecule recognized. How this match is obtained has been a matter of debate. We review the evidence, from recent and older experiments, that indicates that the process follows a selective logic, rather than an instructive one.

T-cell fate

Recent studies suggest that lineage commitment steps, which occur during T-cell differentiation, follow principles in common with fate specification in simple invertebrates. Here we review T-cell development from the perspective of developmental biology. We present models for alpha beta vs gamma delta and CD4 vs CD8 lineage commitment that are consistent with previously published and newly presented experiments.

Multiple developmental stage-specific enhancers regulate CD8 expression in developing thymocytes and in thymus-independent T cells

We and others have recently identified a CD8 locus enhancer (E8) that directs expression in mature CD8 single-positive thymocytes and peripheral CD8+ T cells and in extrathymically derived intestinal intraepithelial lymphocytes (IEL). In this study, we show that deletion of E8, by homologous recombination results in reduced CD8alphaalpha homodimer expression on IEL. Since CD8 expression on thymus-derived T cells was normal, other enhancers regulate CD8 expression in these cells. By exploiting a transgenic reporter expression assay, we identified three additional enhancers that directed expression in diverse thymocyte subsets and mature T cells but not in CD8alphaalpha+ IEL. The results suggest that CD8alpha expression is primarily regulated by E8, in IEL and by the novel enhancers in the thymus-dependent lineages.

HIV peptide conjugated to heat-killed bacteria promotes antiviral responses in immunodeficient mice

Enhancement of immunity in the setting of HIV infection is difficult owing to loss of functional CD4+ T cells. The MHC class II-deficient mouse (II-/-) environment simulates that of the immunocompromised HIV-infected individual, since these mice have low CD4+ T cell numbers, defective CD4-dependent responses, and are susceptible to opportunistic infection. This strain was used to test whether heat-killed Brucella abortus (BA), covalently conjugated to the V3 peptide of HIV-1 (MN), could elicit anti-HIV responses. V3-BA, but not the T-dependent antigen V3-KLH, induced high levels of IL-12, IFN-gamma, and IL-10 mRNA in both wild-type (WT) and II-/- mice within 24 hr of injection. V3-BA-treated, but not V3-KLH-treated, II-/- mice developed serum IgG and IgA anti-V3 antibodies, with IgG2b and IgG3 as the predominant isotype. Viral neutralization studies, using a syncytium inhibition assay, demonstrated that the antibodies generated by V3-BA in II-/- mice were capable of neutralizing HIV. These experiments demonstrate that a heat-inactivated bacterium such as BA, when used as a carrier, can generate a cytokine environment that results in the production of neutralizing antiviral antibodies in an immunodeficient host. Such strategies could be important in the development of immunotherapies and vaccines for HIV-1 patients.

CD4+ T Cells Mature in the Absence of MHC Class I and Class II Expression in Ly-6A.2 Transgenic Mice

The TCRs expressed on T lymphocytes recognize foreign peptides bound to MHC molecules. This reactivity is the basis of specific immune response to the foreign Ag. How such specificities are generated in the thymus is still being debated. Signals generated through TCR upon interaction with self MHC-peptide complexes are critical for maturation of the CD4+ helper and CD8+ cytotoxic subsets. We have observed maturation of CD4+ but not CD8+ T cells in Ly-6A.2 transgenic MHC null mice. Since there can be no interactions with MHC molecules in these mice, these CD4+ cells must express the T cell repertoire that exists before positive and negative selection. Interestingly, despite an absence of selection by MHC molecules, the CD4+ cells that mature recognize MHC molecules at a frequency as high as in CD4+ cells in normal mice. These results demonstrate that: 1) the germline sequences encoding TCRs are biased toward reactivity to MHC molecules; and 2) CD4+ cells as opposed to CD8+ cells have distinct lineage commitment signals. These results also suggest that signals originating from Ly-6 can promote or substitute for signals generated from TCR that are required for positive selection. Moreover, this animal model offers a system to study T cell development in the thymus that can provide insights into mechanisms of lineage commitment in developing T cells.

Diminished expression of CD40 ligand may contribute to the defective humoral immunity in patients with MHC class II deficiency

Major histocompatibility complex (MHC) class II deficiency (bare lymphocyte syndrome, BLS) is a rare primary immunodeficiency classified as a subgroup of severe combined immunodeficiency. We studied T and B lymphocyte function by examining the CD40 ligand/CD40 system in three BLS patients from two unrelated families. CD40 ligand expression by maximally activated BLS T cells was diminished. This abnormality may represent immunological naïveté rather than a general T cell defect, since expression of activation marker CD69 and proliferative responses to PHA or anti-CD3 were normal, and BLS T cells primed and restimulated in vitro expressed normal amounts of CD40 ligand. BLS B cells proliferated and produced IgE if stimulated with anti-CD40 or soluble CD40 ligand and IL-4. Activation of BLS B cells with soluble CD40 ligand and IL-4 induced normal expression of activation markers, although MHC class II expression remained absent. Depressed antibody titers, lack of amplification and failure to undergo isotype switching in response to immunization with bacteriophage phi x 174 demonstrated defective T cell help. We conclude that BLS B cells are functionally normal if appropriately stimulated, and that the defective humoral immunity observed may be related to diminished expression of CD40 ligand on BLS T cells.

An enhancer that directs lineage-specific expression of CD8 in positively selected thymocytes and mature T cells

Positive selection of CD4+CD8+ T cells to the CD4+CD8- helper and CD4- CD8+ cytotoxic lineages is a multistep process that involves complex regulation of coreceptor gene expression. By analyzing expression of a reporter gene in transgenic mice, we have identified a DNA segment, located between the murine CD8beta and CD8alpha genes, that has enhancer activity restricted to CD8 lineage cells. Remarkably, this enhancer functions in thymocytes undergoing positive selection to the CD4-CD8+ phenotype but not in immature double-positive thymocytes. The enhancer also functions in gut intraepithelial lymphocytes that express CD8alpha but not CD8beta, suggesting that it is specific for CD8alpha expression. The tight correlation between activation of this enhancer and the final step in positive selection has important implications for understanding the mechanism of lineage commitment in thymocytes.

Signals through CD8 or CD4 can induce commitment to the CD4 lineage in the thymus

Differentiation of thymocytes into mature single-positive T cells is an ordered process involving sequential interactions between T cell receptor (TCR), co-receptors (CD4 or CD8) and their appropriate major histocompatibility complex-encoded ligands. Precisely how these receptor/co-receptor engagements determine lineage commitment is still controversial, but recently it has been suggested that quantitative differences in the signal transmitted by co-ligation of CD4 versus CD8 with TCR might provide the discriminating signal. We examine this hypothesis, using bispecific F(ab')2 antibodies to mimic TCR/ co-receptor engagement during thymocyte differentiation. These bispecific antibodies lack Fc and can engage surface molecules without extensive cross-linking or targeting to Fc receptor-bearing cells. We show that TCR/CD3 co-ligation with CD4 induces efficient differentiation of mature CD4 lineage cells, irrespective of their TCR specificity. Interestingly, TCR/CD3 co-ligation with CD8 also induces maturation of CD4 T cells, although less efficiently, but not of CD8 T cells. Thus, although the signals delivered by co-ligation of TCR and CD8 appear weaker than from co-ligation of TCR and CD4, the outcome from either engagement is the same. These data suggest that differences in signal intensity alone do not determine lineage commitment in the thymus, but that distinct signals are required for CD4 and CD8 single-positive cell differentiation.

Positive selection of thymocytes bearing alpha beta T cell receptors

The past year has been marked by the application of new technologies to study thymocyte development. These studies have shown that positive selection affects a surprisingly large proportion of thymocytes, and now are beginning to investigate the affinities/avidities of the reactions involved. The lengthy controversy over how thymocytes decide whether to express CD4 or CD8 has taken a new turn with the discovery that levels of expression of the fate-determining protein Notch may affect the decision.

Lineage commitment in the thymus: only the most differentiated (TCRhibcl-2hi) subset of CD4+CD8+ thymocytes has selectively terminated CD4 or CD8 synthesis

Lineage commitment is a developmental process by which individual CD4+CD8+ (double positive, DP) thymocytes make a decision to differentiate into either CD4+ or CD8+ T cells. However, the molecular event(s) that defines lineage commitment is controversial. We have previously proposed that lineage commitment in DP thymocytes can be molecularly defined as the selective termination of CD4 or CD8 coreceptor synthesis. The present study supports such a molecular definition by showing that termination of either CD4 or CD8 synthesis is a highly regulated event that is only evident within the most differentiated DP subset (CD5hiCD69hiTCRhibcl-2hi). In fact, essentially all cells within this DP subset actively synthesize only one coreceptor molecule. In addition, the present results identify three distinct sub-populations of DP thymocytes that define the developmental progression of the lineage commitment process and demonstrate that lineage commitment is coincident with upregulation of TCR and bcl-2. Thus, this study supports a molecular definition of lineage commitment and uniquely identifies TCRhibcl-2hi DP thymocytes as cells that are already committed to either the CD4 or CD8 T cell lineage.

Unexpectedly complex regulation of CD4/CD8 coreceptor expression supports a revised model for CD4+CD8+ thymocyte differentiation

CD4+ CD8+ TCRlo thymocytes are the precursors of CD4+ and CD8+ mature T cells, whose receptors show specific recognition of peptide-MHC class II and MHC class I complexes, respectively. How T cells emerge from the intrathymic differentiation process with selective expression of either CD8 molecule or CD4 molecule coordinated with the MHC class specificity of the TCR has been the subject of intense examination. Many previous studies of this question have been based on the assumption that extinction of CD4 or CD8 expression by the precursor thymocytes was a steady, uninterrupted process. Here we show that this is an incorrect assumption, with CD4 and CD8 expression undergoing an unexpectedly complex series of expression changes involving down-modulation, kinetically asymmetric up-regulation, and then selective loss. Based on these data, we propose a model for the differentiation pathway of alphabeta TCR thymocytes that explains previous, apparently contradictory findings and establishes useful parameters for future studies at the cellular and gene level.

MHC class II-specific T cells can develop in the CD8 lineage when CD4 is absent

The generation of mature CD4 T cells from CD4+CD8+ precursor thymocytes usually requires corecognition of class II MHC by a TCR and CD4, while the production of mature CD8 T cells requires corecognition of class I MHC by a TCR and CD8. To assess the role of the CD4 coreceptor in development and lineage commitment, we generated CD4-deficient mice expressing a transgenic class II-specific TCR. Surprisingly, in the absence of CD4 a large number of T cells mature, but these cells appear in the CD8 lineage. Thus, when CD4 is present, the majority of immature T cells with this class II-specific TCR choose the CD4 lineage but develop in the CD8 pathway when CD4 is absent. The results indicate that even for TCRs that are not dependent on coreceptor for MHC recognition, the coreceptor can influence the lineage choice. These findings are considered in terms of a quantitative signaling model for CD4/CD8 lineage commitment.

The cytoplasmic domain of CD4 promotes the development of CD4 lineage T cells

Thymocytes must bind major histocompatibility complex (MHC) proteins on thymic epithelial cells in order to mature into either CD8+ cytotoxic T cells or CD4+ helper T cells. Thymic precursors express both CD8 and CD4, and it has been suggested that the intracellular signals generated by CD8 or CD4 binding to class I or II MHC, respectively, might influence the fate of uncommitted cells. Here we test the notion that intracellular signaling by CD4 directs the development of thymocytes to a CD4 lineage. A hybrid protein consisting of the CD8 extracellular and transmembrane domains and the cytoplasmic domain of CD4 (CD884) should bind class I MHC but deliver a CD4 intracellular signal. We find that expression of a hybrid CD884 protein in thymocytes of transgenic mice leads to the development of large numbers of class I MHC-specific, CD4 lineage T cells. We discuss these results in terms of current models for CD4 and CD8 lineage commitment.

The regulation and function of the CD4 coreceptor during T lymphocyte development

The data reviewed in this chapter suggest that the primary developmental function of the CD4 and CD8 coreceptors is to improve the efficacy by which a thymocyte recognizes peptide/MHC. During positive selection, DP thymocytes down-regulate expression of either CD4 or CD8 in response to signals that originate from the TCR/coreceptor complex. Experiments with transgenic and MHC-null mice have shown that coreceptor down-regulation and lineage commitment can occur stochastically in a manner that is independent of TCR specificity for MHC. Nevertheless, the positive selection of a given thymocyte is contingent on sustained expression of the coreceptor that is appropriate for the MHC specificity of its TCR. In most cases, loss of the required coreceptor blocks developmental progression and results in thymocyte apoptosis. CD4 expression is controlled by both positive and negative regulatory sequences embedded in the CD4 gene and it is likely that similar sequences regulate the CD8 gene. The down-regulation of coreceptor expression is coupled to a functional commitment which ensures that mature CD4+ T cells have a helper phenotype and CD8+ T cells have a cytotoxic phenotype. The molecular basis for this coupling and the identity of the switching mechanism which governs coreceptor regulation remain to be determined.

Preferential positive selection of T lymphocytes which express two different TCR alpha chains, an endogenous and a transgenic

A hallmark of positive selection in T-cell receptor (TCR)-transgenic mice is a strong skewing towards the CD4+ or the CD8+ subset, depending on the class II or I restriction of the TCR, respectively. However, previous experiments in TCR transgenic mice specific for an Ig light chain (lambda 2(315)/I-Ed class II molecule did not fit into this scheme because the authors observed an anomalous skewing towards CD8. In this paper the authors show that endogenous TCR alpha chains are expressed on > 90% of CD4+ and CD8+ cells in this particular transgenic strain, even on a selecting H-2d haplotype. Endogenous TCR alpha chains are first detected when double-positive thymocytes down-regulate either CD4 or CD8. Endogenous V alpha seems to influence generation of T-cell subsets because CD4+ and CD8+ cells express different frequencies of endogenous V alpha 2 and V alpha 8. In the absence of endogenous TCR alpha chains in recombination-deficient TCR-transgenic severe combined immunodeficiency (SCID) mice, a strong skewing towards CD4+ T cells is seen, but such mice are severely T-cell deficient. As an explanation for these results, the authors suggest that the transgenic TCR has a too low affinity for efficient positive selection, therefore, TCR alpha gene rearrangements proceed. Endogenous TCR alpha paired with transgenic TCR beta could bind to class I or class II molecules, enhance positive selection and thereby production of CD4+ or CD8+ cells. Most of the 'mismatched' CD8+ cells are lambda 2(315)-specific and I-Ed class II restricted, and may function as idiotype-specific suppressors of B cells. These results may help explain the origin of dual TCR alpha T cells. Furthermore, the authors suggest that T cells 'mismatched' for co-receptor/TCR MHC-specificity may be enriched among dual TCR alpha T cells.

Human CD8 transgene regulation of HLA recognition by murine T cells

A series of human CD8 transgenic (hCD8 Tg) mice with differential expression in the thymus and periphery were produced to investigate CD8 coreceptor regulation of repertoire selection and T cell responses. Expression of hCD8 markedly enhanced responses to both HLA class I molecules and hybrid A2/Kb molecules providing functional evidence for a second interaction site, outside of the alpha 3 domain, which is essential for optimal coreceptor function. Peripheral T cell expression of hCD8 was sufficient to augment responsiveness to HLA class I, as hCD8 Tg mice which lacked thymic expression responded as well as mice expressing hCD8 in the thymus and periphery. Both murine CD8+ and CD4+ T cells expressing hCD8 transgenes exhibited markedly enhanced responses to foreign HLA class I, revealing the ability of T cell receptor repertoires selected on either murine class I or class II to recognize human class I major histocompatibility complex (MHC). In contrast to recognition of foreign class I, thymic expression of hCD8 transgenes was absolutely required to enhance recognition of antigenic peptide restricted by self-HLA class I. Thus, our studies revealed disparate requirements for CD8 coreceptor expression in the thymus for selection of a T cell repertoire responsive to foreign MHC and to antigenic peptides bound to self-MHC, providing a novel demonstration of positive selection that is dependent on human CD8.

Stochastic coreceptor shut-off is restricted to the CD4 lineage maturation pathway

Kinetics of mature T cell generation in the thymus of normal or major histocompatibility complex (MHC) class I- or II-deficient mice were studied by the bromodeoxyuridine pulse labeling method. As previously described, the early activation and final maturation phases were found to be synchronous for the two T cell lineages, but CD4+8- cells were generated faster than CD4-8+ cells in MHC class I- and II-deficient mice, respectively. CD8 downregulation started on day 2 after cell proliferation even in the absence of MHC class II expression. CD8 downregulation thus appears to be stochastic at its beginning. By contrast, CD4 shut-off was found totally instructive, as the generation of CD4lo8+ cells with a high TCR density was not observed in class I-deficient mice. The analysis of the V beta 14 TCR frequencies in CD4/8 subsets in normal and MHC-deficient mice confirmed that CD4 and CD8 generation pathways are not symmetrical. These findings show that commitment towards the CD4+8- or CD4-8+ phenotype is controlled at the CD8lo step for the former and at the CD4+8+ double-positive stage for the latter.

Intermediate steps in positive selection: differentiation of CD4+8int TCRint thymocytes into CD4-8+TCRhi thymocytes

The differentiation potential of putative intermediates between CD4+8+ thymocytes and mature T cells has been examined. Such intermediate populations were sorted, in parallel with CD4+8+ thymocytes, from three types of C57BL/6 mice: major histocompatibility complex (MHC) class II-deficient mice, mice transgenic for an alpha/beta T cell receptor (TCR) restricted by class I MHC and normal mice. The sorted populations were then transferred into the thymus of nonirradiated C57BL/Ka mice differing in Thy 1 allotype, and the progeny of the transferred cells were analyzed 2 d later. Surprisingly, with all three types of donor mice, a major proportion of the CD4+8intTCRint-derived progeny were found to be CD4-8+TCRhi cells, thus delineating a new alternative pathway for development of the CD8 lineage. In contrast, the transfer of CD4int8+TCRint thymocytes produced CD4-8+TCRhi cells but no significant proportion of CD4+8-TCRhi cells, suggesting that there is no equivalent alternative pathway for the CD4 lineage. The results negate some of the evidence for a stochastic/selective model of lineage commitment, and point to an asymmetry in the steps leading to CD4-8+ versus CD4+8- T cells.

Asymmetric signaling requirements for thymocyte commitment to the CD4+ versus CD8+ T cell lineages: a new perspective on thymic commitment and selection

Differentiation of immature CD4+ CD8+ thymocytes into mature CD4+ CD8- and CD4-CD8+ T cells requires that synthesis of one or the other coreceptor molecule be terminated, a process referred to as lineage commitment. The present study has utilized a novel coreceptor reexpression assay to identify lineage commitment in immature thymocytes and has found that the MHC recognition requirements for CD4 commitment and CD8 commitment fundamentally differ from one another. Remarkably, we found that thymocyte commitment to the CD8+ lineage requires MHC class I-dependent instructional signals, whereas thymocyte commitment to the CD4+ lineage is MHC independent and may occur by default. In addition, an unanticipated relationship between lineage commitment and surface phenotype has been identified. These results are incompatible with current concepts and require a new perspective on lineage commitment and positive selection, which we refer to as asymmetric commitment.