Role of CD8beta domains in CD8 coreceptor function: importance for MHC I binding, signaling, and positive selection of CD8+ T cells in the thymus

The zinc finger protein cKrox directs CD4 lineage differentiation during intrathymic T cell positive selection

The genetic programs directing CD4 or CD8 T cell differentiation in the thymus remain poorly understood. While analyzing gene expression during intrathymic T cell selection, we found that Zfp67, encoding the zinc finger transcription factor cKrox, was upregulated during the differentiation of CD4(+) but not CD8(+) T cells. Expression of a cKrox transgene impaired CD8 T cell development and caused major histocompatibility complex class I-restricted thymocytes to differentiate into CD4(+) T cells with helper properties rather than into cytotoxic CD8(+) T cells, as normally found. CD4 lineage differentiation mediated by cKrox required its N-terminal BTB (bric-a-brac, tramtrack, broad complex) domain. These findings identify cKrox as a chief CD4 differentiation factor during positive selection.

Flt3-ligand, IL-4, GM-CSF, and adherence-mediated isolation of murine lung dendritic cells: assessment of isolation technique on phenotype and function

Lung dendritic cells (DCs) are difficult to study due to their limited quantities and the complexities required for isolation. Although many procedures have been used to overcome this challenge, the effects of isolation techniques on lung DCs have not been reported. The current study shows that freshly isolated DCs (CD11c+) have limited ability to induce proliferation in allogeneic T cells, and are immature as indicated by low cell surface expression of costimulatory molecules compared with liver or splenic DCs. DCs isolated after overnight culture or from mice treated with Flt3L are phenotypically mature and potent stimulators of allogeneic T cells. DCs could not be propagated from lung mononuclear cells in response to IL-4 and GM-CSF. Contrary to data reported for nonpulmonary DCs, expression of CCR6 was decreased on mature lung DCs, and only a subset of mature DCs expressed higher levels of CCR7. Absence of CD8alpha expression indicates that freshly isolated DCs are myeloid-type, whereas mature DCs induced by overnight culture are both "lymphoid" (CD8alpha+) and "myeloid" (CD8alpha-). DCs from mice genetically deficient in CD8alpha expression were strong simulators of allogeneic T cells which was consistent with data showing that CD8alpha- DCs from CD8alpha-sufficient mice are better APCs compared with CD8alpha+ DCs from the same mice. These data show that freshly isolated lung DCs are phenotypically and functionally distinct, and that the isolation technique alters the biology of these cells. Therefore, lung DC phenotype and function must be interpreted relative to the technique used for isolation.

IL-15/IL-15Ralpha-mediated avidity maturation of memory CD8+ T cells

T cell avidity is critical to viral clearance, but mechanisms of CD8(+) T cell avidity maturation are poorly understood. Here, we find that IL-15 mediates two mechanisms of avidity maturation. (i) By selection at the population level, IL-15 promotes greater survival of high- compared with low-avidity cytotoxic T lymphocytes (CTLs). High-avidity CTLs express higher levels of IL-15Ralpha and persist longer by homeostatic proliferation. (ii) At the individual cell level, IL-15 induces higher levels of surface coreceptor CD8alphabeta, increasing functional avidity. IL-15 during priming selects or induces higher-avidity CTLs. Conversely, high-avidity CTLs are diminished in IL-15Ralpha knockout mice. These results provide an explanation of CD8+ T cell avidity maturation and may contribute to the design of novel vaccines.

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

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

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

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

Interplay between TCR affinity and necessity of coreceptor ligation: high-affinity peptide-MHC/TCR interaction overcomes lack of CD8 engagement

CD8 engagement is believed to be a critical event in the activation of naive T cells. In this communication, we address the effects of peptide-MHC (pMHC)/TCR affinity on the necessity of CD8 engagement in T cell activation of primary naive cells. Using two peptides with different measured avidities for the same pMHC-TCR complex, we compared biochemical affinity of pMHC/TCR and the cell surface binding avidity of pMHC/TCR with and without CD8 engagement. We compared early signaling events and later functional activity of naive T cells in the same manner. Although early signaling events are altered, we find that high-affinity pMHC/TCR interactions can overcome the need for CD8 engagement for proliferation and CTL function. An integrated signal over time allows T cell activation with a high-affinity ligand in the absence of CD8 engagement.

Molecular interactions mediating T cell antigen recognition

Over the past decade, key protein interactions contributing to T cell antigen recognition have been characterized in molecular detail. These have included interactions involving the T cell antigen receptor (TCR) itself, its coreceptors CD4 and CD8, the accessory molecule CD2, and the costimulatory receptors CD28 and CTLA-4. A clear view is emerging of how these molecules interact with their ligands at the cell-cell interface. Structural and binding studies have confirmed that the proteins span small but comparable distances and that, overall, they interact very weakly. However, there have been important surprises as well: that TCR interactions with peptide-MHC are topologically constrained and characterized by considerable conformational flexibility at the binding interface; that coreceptors engage peptide-MHC with extraordinarily fast kinetics and at angles apparently precluding direct interactions with the TCR bound to the same peptide-MHC; that the structural mechanisms allowing recognition by costimulatory and accessory molecules to be weak and yet specific are very heterogeneous; and that because of differences in both binding affinity and stoichiometry, there is enormous variation in the stability of the various costimulatory receptor/ligand complexes. These studies provide the necessary framework for exploring how these molecular interactions initiate T cell activation.

Unraveling a revealing paradox: Why major histocompatibility complex I-signaled thymocytes "paradoxically" appear as CD4+8lo transitional cells during positive selection of CD8+ T cells

The mechanism by which T cell receptor specificity determines the outcome of the CD4/CD8 lineage decision in the thymus is not known. An important clue is the fact that major histocompatibility complex (MHC)-I-signaled thymocytes paradoxically appear as CD4+8lo transitional cells during their differentiation into CD8+ T cells. Lineage commitment is generally thought to occur at the CD4+8+ (double positive) stage of differentiation and to result in silencing of the opposite coreceptor gene. From this perspective, the appearance of MHC-I-signaled thymocytes as CD4+8lo cells would be due to effects on CD8 surface protein expression, not CD8 gene expression. But contrary to this perspective, this study demonstrates that MHC-I-signaled thymocytes appear as CD4+8lo cells because of transient down-regulation of CD8 gene expression, not because of changes in CD8 surface protein expression or distribution. This study also demonstrates that initial cessation of CD8 gene expression in MHC-I-signaled thymocytes is not necessarily indicative of commitment to the CD4+ T cell lineage, as such thymocytes retain the potential to differentiate into CD8+ T cells. These results challenge classical concepts of lineage commitment but fulfill predictions of the kinetic signaling model.

Sialic acid capping of CD8beta core 1-O-glycans controls thymocyte-major histocompatibility complex class I interaction

Bidentate interaction of a T-cell receptor and CD8alphabeta heterodimer with a peptide-MHCI complex is required for the generation of cytotoxic T-lymphocytes. During thymic development, the modification of CD8beta glycans influences major histocompatibility complex class I binding to T-cell precursors called thymocytes. ES mass spectrometry (MS) and tandem MS/MS analysis were used to identify the changes occurring in the CD8beta-glycopeptides during T-cell development. Several threonine residues proximal to the CD8beta Ig headpiece are glycosylated with core-type 1 O-glycans. Non-sialylated glycoforms are present in immature thymocytes but are virtually absent in mature thymocytes. These results suggest how sialylation in a discrete segment of the CD8beta stalk by ST3Gal-1 sialyltransferase creates a molecular developmental switch that affects ligand binding.

Restricting Zap70 expression to CD4+CD8+ thymocytes reveals a T cell receptor-dependent proofreading mechanism controlling the completion of positive selection

Although T cell receptor (TCR) signals are essential for intrathymic T cell-positive selection, it remains controversial whether they only serve to initiate this process, or whether they are required throughout to promote thymocyte differentiation and survival. To address this issue, we have devised a novel approach to interfere with thymocyte TCR signaling in a developmental stage-specific manner in vivo. We have reconstituted mice deficient for Zap70, a tyrosine kinase required for TCR signaling and normally expressed throughout T cell development, with a Zap70 transgene driven by the adenosine deaminase (ADA) gene enhancer, which is active in CD4(+)CD8(+) thymocytes but inactive in CD4(+) or CD8(+) single-positive (SP) thymocytes. In such mice, termination of Zap70 expression impaired TCR signal transduction and arrested thymocyte development after the initiation, but before the completion, of positive selection. Arrested thymocytes had terminated Rag gene expression and up-regulated TCR and Bcl-2 expression, but failed to differentiate into mature CD4 or CD8 SP thymocytes, to be rescued from death by neglect or to sustain interleukin 7R alpha expression. These observations identify a TCR-dependent proofreading mechanism that verifies thymocyte TCR specificity and differentiation choices before the completion of positive selection.

Quantitative analysis of the contribution of TCR/pepMHC affinity and CD8 to T cell activation

The relative roles of CD8, TCR:pepMHC affinity, and TCR:pepMHC dissociation rate in T cell activation have remained controversial. To determine the relationships among these factors, we used T cells transfected with normal and in vitro engineered alphabeta TCRs, in the presence or absence of CD8. The TCRs exhibited a wide range of affinities (K(D) values of 80 microM to 5 nM). T cells with the highest affinity TCRs were efficiently stimulated by peptide, with or without CD8. In contrast, CD8 was required for T cells that expressed TCRs with affinities typical of syngeneic reactions (K(D) values above approximately 3 microM). The results suggest that virtually all normal syngeneic interactions require CD8, which enhances peptide sensitivity by one million-fold or more.

High affinity xenoreactive TCR:MHC interaction recruits CD8 in absence of binding to MHC

The TCR from a xenoreactive murine cytotoxic T lymphocyte clone, AHIII 12.2, recognizes murine H-2D(b) complexed with peptide p1058 (FAPGFFPYL) as well as human HLA-A2.1 complexed with human self-peptide p1049 (ALWGFFPVL). To understand more about T cell biology and cross-reactivity, the ectodomains of the AHIII 12.2 TCR have been produced in E. coli as inclusion bodies and the protein folded to its native conformation. Flow cytometric and surface plasmon resonance analyses indicate that human p1049/A2 has a significantly greater affinity for the murine AHIII 12.2 TCR than does murine p1058/D(b). Yet, T cell binding and cytolytic activity are independent of CD8 when stimulated with human p1049/A2 as demonstrated with anti-CD8 Abs that block CD8 association with MHC. Even in the absence of direct CD8 binding, stimulation of AHIII 12.2 T cells with "CD8-independent" p1049/A2 produces p56(lck) activation and calcium flux. Confocal fluorescence microscopy and fluorescence resonance energy transfer flow cytometry demonstrate CD8 is recruited to the site of TCR:peptide MHC binding. Taken together, these results indicate that there exists another mechanism for recruitment of CD8 during high affinity TCR:peptide MHC engagement.

CD8alpha alpha memory effector T cells descend directly from clonally expanded CD8alpha +beta high TCRalpha beta T cells in vivo

Whereas most peripheral CD8(+) alphabeta T cells highly express CD8alphabeta heterodimer in healthy individuals, there is an increase of CD8alpha(+)beta(low) or CD8alphaalpha alphabeta T cells in HIV infection or Wiskott-Aldrich syndrome and after bone marrow transplantation. The significance of these uncommon cell populations is not well understood. There has been some question as to whether these subsets and CD8alpha(+)beta(high) cells belong to different ontogenic lineages or whether a fraction of CD8alpha(+)beta(high) cells have down-regulated CD8beta chain. Here we assessed clonality of CD8alphaalpha and CD8alpha(+)beta(low) alphabeta T cells as well as their phenotypic and functional characteristics. Deduced from surface antigens, cytotoxic granule constituents, and cytokine production, CD8alpha(+)beta(low) cells are exclusively composed of effector memory cells. CD8alphaalpha cells comprise effector memory cells and terminally differentiated CD45RO(-)CCR7(-) memory cells. T-cell receptor (TCR) Vbeta complementarity-determining region 3 (CDR3) spectratyping analysis and subsequent sequencing of CDR3 cDNA clones revealed polyclonality of CD8alpha(+)beta(high) cells and oligoclonality of CD8alpha(+)beta(low) and CD8alphaalpha cells. Importantly, some expanded clones within CD8alphaalpha cells were also identified within CD8alpha(+)beta(high) and CD8alpha(+)beta(low) subpopulations. Furthermore, signal-joint TCR rearrangement excision circles concentration was reduced with the loss of CD8beta expression. These results indicated that some specific CD8alpha(+)beta(high) alphabeta T cells expand clonally, differentiate, and simultaneously down-regulate CD8beta chain possibly by an antigen-driven mechanism. Provided that antigenic stimulation directly influences the emergence of CD8alphaalpha alphabeta T cells, these cells, which have been previously regarded as of extrathymic origin, may present new insights into the mechanisms of autoimmune diseases and immunodeficiencies, and also serve as a useful biomarker to evaluate the disease activities.

Mouse MHC class I tetramers that are unable to bind to CD8 reveal the need for CD8 engagement in order to activate naive CD8 T cells

Although the role of CD8 as a supplier of lck is unchallenged, its role in contributing to the formation of a stable complex between class I molecules and the TCR, as well as its role as an adhesion molecule, is less clear. To address the role of CD8/MHC-I interactions, we generated tetramers composed of H2-K(b) molecules with mutations in the alpha 3 domain of H2-K(b) that abolish CD8 binding. We show that the ability of tetramers to stain and activate CD8 T cells is strongly dependent on binding of CD8 to the same class I molecule engaged by the TCR. We characterize a mutation in the alpha 3 domain that results in H2-K(b) molecules capable of staining specific CD8 T cells with little ensuing activation. Although CD8 to some extent serves an adhesive function, this contribution is modest and does not substitute for lack of binding of CD8 to the class I molecule engaged by the TCR. We show that CD8 and the TCR associate in a process independent of binding of CD8 to class I. Our data support the notion that CD8 is required to form a stable complex between class I and the TCR.

Peptidic termini play a significant role in TCR recognition

TCR recognition of class I MHC is dependent on the composition of the antigenic peptide and the MHC. Single amino acid substitutions in either the MHC or the peptide may dramatically alter recognition. While the major interactions between TCR and the peptide/MHC complex appear to be focused on the complementarity-determining region (CDR)3, it is also clear from the cocrystal structure of class I MHC and TCR that the amino and carboxyl ends of the peptide may play a role through interactions with the CDR1. In this work we show that gp33 variants substituted at the peptidic termini at the putative CDR1 contact regions show improved recognition in B6 mice. The rank order of recognition is different using the P14 transgenic T cells, suggesting that one reason for improved recognition is a change in the TCR repertoire that recognizes the peptide. However, the affinity of the TCR by some of the peptide/MHC complex with increased recognition is improved, as shown by increased tetramer binding to P14 T cells. These substitutions at the termini of the peptide-binding cleft cause localized conformational changes as seen by changes in mAb binding and crystallographic structures. The different peptide structures also show different conformations in the center of the peptide, but these are shown to be energetically similar and thus most likely have no significance with respect to TCR recognition. Therefore, small conformational changes, localized to the CDR1 contact regions, may play a significant role in TCR recognition.

A role for the alpha-chain connecting peptide motif in mediating TCR-CD8 cooperation

To generate peripheral T cells that are both self-MHC restricted and self-MHC tolerant, thymocytes are subjected to positive and negative selection. How the TCR discriminates between positive and negative selection ligands is not well understood, although there is substantial evidence that the CD4 and CD8 coreceptors play an important role in this cell fate decision. We have previously identified an evolutionarily conserved motif in the TCR, the alpha-chain connecting peptide motif (alpha-CPM), which allows the TCR to deliver positive selection signals. Thymocytes expressing alpha-CPM-deficient receptors do not undergo positive selection, whereas their negative selection is not impaired. In this work we studied the ligand binding and receptor function of alpha-CPM-deficient TCRs by generating T cell hybridomas expressing wild-type or alpha-CPM-deficient forms of the T1 TCR. This K(d)-restricted TCR is specific for a photoreactive derivative of the Plasmodium berghei circumsporozoite peptide(252-260) IASA-YIPSAEK(ABA)I and is therefore amenable to TCR photoaffinity labeling. The experiments presented in this work show that alpha-CPM-deficient TCRs fail to cooperate with CD8 to enhance ligand binding and functional responses.

The T-cell receptor signalosome: a dynamic structure with expanding complexity

Signal transduction in T cells is a dynamic process involving a large number of membrane and cytosolic proteins. The TCR macromolecular complex (signalosome) is initiated by receptor occupancy and becomes more elaborate over time. This review describes how 'vertical' displacement mechanisms and lateral coalescence of lipid-raft-associated scaffold proteins combine to form distinct signalosomes, which control signal specificity.

T-cell differentiation: MHC class I's sweet tooth lost on maturity

The addition of sialic acid to O-linked glycans of the T-cell co-receptor CD8 is regulated during thymocyte differentiation. Two recent papers have shown that this glycosylation changes the avidity of the interaction between CD8 and MHC class I proteins, potentially altering signalling thresholds in thymocyte differentiation.

Improved detection of melanoma antigen-specific T cells expressing low or high levels of CD8 by HLA-A2 tetramers presenting a Melan-A/Mart-1 peptide analogue

MHC class I tetramers containing peptide epitopes are sensitive tools for detecting antigen-specific CD8(+) T-cell responses. We demonstrate here that binding of HLA-A2 tetramers to CD8(+) T cells specific for the melanoma-associated antigen Melan-A/MART-1 can be fine-tuned by altering either the bound peptide epitope or residues in the alpha 3 domain of HLA-A2, which is important for CD8 binding. Antigen-specific T cells expressing high levels of CD8 could be detected using HLA-A2 tetramers containing the peptide AAGIGILTV, an epitope which is naturally processed and presented from Melan-A/MART-1. In contrast, low CD8-expressing, antigen-specific T cells could be detected efficiently only by using a mutated HLA-A2 tetramer with an altered CD8 binding site or, less efficiently, using the wild-type HLA-A2 tetramer loaded with the peptide analogue ELAGIGILTV, which is superior in stimulating antigen-specific T-cell responses. Our results suggest ways to optimize the identification and expansion of antigen-specific T cells with different requirements for the costimulatory CD8 molecule in facilitating T-cell receptor binding to peptide variants.

CD8 binding to MHC class I molecules is influenced by T cell maturation and glycosylation

CD8 serves both as an adhesion molecule for class I MHC molecules and as a coreceptor with the TCR for T cell activation. Here we study the developmental regulation of CD8-mediated binding to noncognate peptide/MHC ligands (i.e., those not bound by the TCR). We show that CD8's ability to bind soluble class I MHC tetramers and to mediate T cell adhesion under shear flow conditions diminishes as double-positive thymocytes mature into CD8(+) T cells. Furthermore, we provide evidence that this decreased CD8 binding results from increased T cell sialylation upon T cell maturation. These data suggest that CD8's ability to interact with class I MHC is not fixed and is developmentally regulated through the T cell's glycosylation state.

CD8beta endows CD8 with efficient coreceptor function by coupling T cell receptor/CD3 to raft-associated CD8/p56(lck) complexes

The extraordinary sensitivity of CD8+ T cells to recognize antigen impinges to a large extent on the coreceptor CD8. While several studies have shown that the CD8beta chain endows CD8 with efficient coreceptor function, the molecular basis for this is enigmatic. Here we report that cell-associated CD8alphabeta, but not CD8alphaalpha or soluble CD8alphabeta, substantially increases the avidity of T cell receptor (TCR)-ligand binding. To elucidate how the cytoplasmic and transmembrane portions of CD8beta endow CD8 with efficient coreceptor function, we examined T1.4 T cell hybridomas transfected with various CD8beta constructs. T1.4 hybridomas recognize a photoreactive Plasmodium berghei circumsporozoite (PbCS) peptide derivative (PbCS (4-azidobezoic acid [ABA])) in the context of H-2K(d), and permit assessment of TCR-ligand binding by TCR photoaffinity labeling. We find that the cytoplasmic portion of CD8beta, mainly due to its palmitoylation, mediates partitioning of CD8 in lipid rafts, where it efficiently associates with p56(lck). In addition, the cytoplasmic portion of CD8beta mediates constitutive association of CD8 with TCR/CD3. The resulting TCR-CD8 adducts exhibit high affinity for major histocompatibility complex (MHC)-peptide. Importantly, because CD8alphabeta partitions in rafts, its interaction with TCR/CD3 promotes raft association of TCR/CD3. Engagement of these TCR/CD3-CD8/lck adducts by multimeric MHC-peptide induces activation of p56(lck) in rafts, which in turn phosphorylates CD3 and initiates T cell activation.

Developmentally regulated glycosylation of the CD8alphabeta coreceptor stalk modulates ligand binding

The functional consequences of glycan structural changes associated with cellular differentiation are ill defined. Herein, we investigate the role of glycan adducts to the O-glycosylated polypeptide stalk tethering the CD8alphabeta coreceptor to the thymocyte surface. We show that immature CD4(+)CD8(+) double-positive thymocytes bind MHCI tetramers more avidly than mature CD8 single-positive thymocytes, and that this differential binding is governed by developmentally programmed O-glycan modification controlled by the ST3Gal-I sialyltransferase. ST3Gal-I induction and attendant core 1 sialic acid addition to CD8beta on mature thymocytes decreases CD8alphabeta-MHCI avidity by altering CD8alphabeta domain-domain association and/or orientation. Hence, glycans on the CD8beta stalk appear to modulate the ability of the distal binding surface of the dimeric CD8 globular head domains to clamp MHCI.

Manipulation of avidity to improve effectiveness of adoptively transferred CD8(+) T cells for melanoma immunotherapy in human MHC class I-transgenic mice

The adoptive transfer of tumor-reactive CD8(+) T cells into tumor-bearing hosts provides an attractive alternative to vaccination-based active immunotherapy of melanoma. The development of techniques that result in the preferential expansion of tumor-reactive T cells is therefore of great importance. In this study, we report the generation of HLA-A*0201-restricted CD8(+) T cell populations that recognize either tyrosinase(369-376) or gp100(209-217) from tolerant human class I MHC-transgenic mice by using single amino acid-substituted variant peptides. Low peptide concentration or restimulation with the parent peptide was used to enhance the functional avidity, defined by stimulation of IFN-gamma accumulation, and cross-reactivity of the resulting T cell populations. We found a direct correlation between the ability of a T cell population to respond in vitro to low concentrations of the precise peptide expressed on the tumor and its ability to delay the outgrowth of B16 melanoma after adoptive transfer. Surprisingly, we found that some T cells that exhibited high functional avidity and were effective in controlling tumor outgrowth exhibited low structural avidity, as judged by MHC-tetramer staining. Our results establish strategies for the development and selection of CD8(+) T cell populations that persist despite peripheral tolerance, and that can control melanoma outgrowth. Furthermore, they support the use of human MHC class I-transgenic mice as a preclinical model for developing effective immunotherapies that can be rapidly extended into therapeutic settings.

Differential modulation of CD8beta by rat gammadelta and alphabeta T cells after activation

Major histocompatibility complex (MHC) class I-restricted alphabeta T cells express the CD8alphabeta heterodimer, which acts as a MHC class I-specific co-receptor. Rats are so far the only species with frequent expression of the CD8alphabeta by MHC-unrestricted gammadelta T cells. This study compares CD8alphabeta expression by splenic rat alphabeta and gammadelta T cells and reveals a lineage-specific difference in the control of CD8beta expression. After activation in vitro, many gammadelta T cells, but not alphabeta T cells, persistently down-modulate the expression of CD8beta, but not CD8alpha, at the RNA level. Down-regulation occurred after stimulation with T-cell receptor (TCR)-specific monoclonal antibody (mAb) and interleukin-2 (IL-2) or CD28-mediated costimulation, and after activation with phorbol 12-myristate 13-acetate (PMA) and ionomycin. Functional differences between modulating and non-modulating cells were not found with respect to interferon-gamma (IFN-gamma) production and cytolytic activity. The modulation could be indicative for a fundamental difference between alphabeta and gammadelta T cells and also limits the use of CD8beta as a stable marker of gammadelta T-cell subsets. Possibly, CD8beta modulation provides a mechanism to escape over-stimulation by (auto-)antigens by increasing the threshold of TCR-mediated activation in gammadelta T cells.

Developmental and functional analyses of CD8(+) NK1.1(+) T cells in class-I-restricted TCR transgenic mice

Using a class-I-restricted T cell receptor (TCR) transgenic mice (Tgm), 2C (Valpha3.1/Vbeta 8.2, specific for L(d) + LSPFPFDL), the development and cytokine production of tg-TCR(+) NKT cells were analyzed. We found that CD8(+) or double negative (DN) NKT cells constituted a major population of NKT cells in the H-2(b/b) 2C Tgm (positive selecting background) or the H-2(b/d) 2C Tgm (negative selecting background), respectively. Virtually no NKT cells were generated in the H-2(k/k) 2C Tgm (neutral selecting background). CD8(+) NKT cells in the H-2(b/b) 2C Tgm expressed CD8alphabeta heterodimers, whereas those in the H-2(b/d) 2C Tgm expressed CD8alphaalpha homodimers. These findings suggest that development of a subpopulation of NKT cells is influenced by the H-2 molecules. Upon stimulation with anti-CD3 mAb, tg-TCR(+) NKT cells generated in the H-2(b/b) and H-2(b/d) backgrounds produced IFN-gamma, but not IL-4.

The CD8alphabeta co-receptor on double-positive thymocytes binds with differing affinities to the products of distinct class I MHC loci

The CD8 co-receptor is essential for TCR-dependent immune recognition and T cell development involving peptides bound to MHC class I (MHCI) molecules. The dominant interaction of CD8 alpha alpha and alpha beta co-receptors is with the alpha3 domain of an MHCI molecule. Whether this interaction is different for the products of various MHCI loci is currently unknown. Here we examine the interaction between H-2K(b) and H-2D(b), the two MHCI molecules in the C57BL / 6 mouse, and CD8 using H-2K(b) and H-2D(b) tetramers. The MHCI molecules bind to the CD8alpha beta co-receptor on double-positive thymocytes with different avidities (H-2K(b) > D(b)). The differences are linked to their respective alpha3 domains. Hence, an H-2D(b)K(b) tetramer comprising D(b)alpha1--alpha2 and K(b)alpha3 domains shows more binding than H-2D(b). We also quantitated the monomeric affinities of CD8alpha alpha and CD8alpha beta for H-2K(b) and H-2D(b). The H-2K(b) interaction with CD8alpha alpha and CD8alpha beta is stronger than that of H-2D(b). Given that T cell repertoire selection of DP thymocytes is a function of both TCR-pMHCI and CD8alpha beta-pMHCI avidities, these differences may explain the dominant role of H-2K(b) as compared to H-2D(b) in CD8 T cell development of C57BL / 6 mice. The influence of allelic and non-allelic alpha3 polymorphisms on thymic selection processes are discussed.

Notch1 confers a resistance to glucocorticoid-induced apoptosis on developing thymocytes by down-regulating SRG3 expression

We previously have reported that SRG3 is required for glucocorticoid (GC)-induced apoptosis in the S49.1 thymoma cell line. Activation of Notch1 was shown to induce GC resistance in thymocytes. However, the specific downstream target of Notch1 that confers GC resistance on thymocytes is currently unknown. We found that the expression level of SRG3 was critical in determining GC sensitivity in developing thymocytes. The expression of SRG3 also was down-regulated by the activated form of Notch1 (NotchIC). The promoter activity of the SRG3 gene also was down-regulated by NotchIC. Expression of transgenic SRG3 resulted in the restoration of GC sensitivity in thymocytes expressing transgenic Notch1. These results suggest that SRG3 is the downstream target of Notch1 in regulating GC sensitivity of thymocytes.

Strength of signaling by CD4 and CD8 coreceptor tails determines the number but not the lineage direction of positively selected thymocytes

The present study has assessed the impact of the intracellular domains of CD4 and CD8 on positive selection and lineage direction of MHC class I-restricted thymocytes. Contrary to current presumption, we found that the CD4 tail promotes the generation of both CD4+ and CD8+ T cells without preference for the CD4+ T cell lineage. We also found that the identity of the coreceptor tail and hence the strength of coreceptor signaling determine the number of thymocytes undergoing positive selection but not their ultimate CD4/CD8 phenotype. These findings demonstrate that the strength of coreceptor signaling has a significant quantitative but not qualitative impact on positive selection and provide a simple explanation for the greater numbers of CD4+ than CD8+ T cells selected in the normal thymus.

CTLA-4 blockade enhances the CTL responses to the p53 self-tumor antigen

p53 is an attractive target for cancer immunotherapy because it is overexpressed in a high proportion of many different types of tumors. However, it is also expressed in normal tissues and acts as a toleragen in vivo. Previously, detailed examination of the repertoire specific for the murine p53(261-269) epitope in conventional and p53-deficient mice demonstrated that because of expression of p53, the CD8(+) T cells that respond to this epitope express low-affinity TCRs. It has been reported that tolerance to tumor Ags can be broken by in vivo administration of anti-CTLA-4 mAb. With the goal of overriding tolerance and achieving optimal activation of p53-specific CTL, the current study has assessed the effect of anti-CTLA-4 mAb on the p53-specific repertoire. It was found that blockade of CTLA-4 engagement at the time of antigenic stimulation induced a vigorous amplification of the CTL responses to p53 as well as proportionate expansion of the memory T cell pool. This effect was dependent on the presence of CD4(+) T cell help and correlated with an enhancement of helper function. However, anti-CTLA-4 treatment did not enhance the avidity of the resultant p53-specific CTL populations and, therefore, could not reverse this important consequence of tolerance.

Molecular interactions of coreceptor CD8 and MHC class I: the molecular basis for functional coordination with the T-cell receptor

In recent years, substantial progress has been made towards understanding the molecular basis for CD8 binding to class I MHC and the coreceptor's role in cytotoxic T-cell activation. Here, we review the structural, mechanistic and functional studies that point to a model of coordination of T-cell receptor and CD8 signaling that might provide the key to cytotoxic T-cell activation.

Expression of functional CD8alpha Beta heterodimer on rat gamma delta T cells does not correlate with the CDR3 length of the TCR delta chain predicted for MHC class I-restricted antigen recognition

In accordance with their lack of MHC restriction, most mouse and human gamma delta T cells express neither the CD4 nor CD8(alpha beta) coreceptor. In striking contrast, up to 80% of splenic rat gamma delta T cells express the CD8alpha beta isoform of CD8, which for the alpha beta T cell subset serves as a marker for MHC class I-restricted cells. We compared CD8 on alpha beta and gamma delta T cells with regard to co-stimulatory function and correlation of CD8 expression with TCRDV usage and CDR3delta length. In both subsets, CD8 acted as a co-stimulatory molecule in vitro and was found to bind the kinase lck efficiently. No differences between the CDR3delta length spectra of CD8+ and CD8- gamma delta T cells or between unselected thymic and peripheral gamma delta T cells were found. As seen in man and mice, CDR3delta were rather long, a structural feature which can be expected to interfere with an alpha beta TCR-like mode of MHC class I binding. In summary, CD8 expressed by rat gamma delta T cells is a molecule with the potential to act as a coreceptor, but its expression gives no indication for antigen recognition analogous to that of MHC class I-restricted alpha beta T cells.