Thymic selection of cytotoxic T cells independent of CD8 alpha-Lck association

Uncovering a novel role of PLCβ4 in selectively mediating TCR signaling in CD8+ but not CD4+ T cells

Because of their common signaling molecules, the main T cell receptor (TCR) signaling cascades in CD4⁺ and CD8⁺ T cells are considered qualitatively identical. Herein, we show that TCR signaling in CD8⁺ T cells is qualitatively different from that in CD4⁺ T cells, since CD8α ignites another cardinal signaling cascade involving phospholipase C β4 (PLCβ4). TCR-mediated responses were severely impaired in PLCβ4-deficient CD8⁺ T cells, whereas those in CD4⁺ T cells were intact. PLCβ4-deficient CD8⁺ T cells showed perturbed activation of peripheral TCR signaling pathways downstream of IP₃ generation. Binding of PLCβ4 to the cytoplasmic tail of CD8α was important for CD8⁺ T cell activation. Furthermore, GNAQ interacted with PLCβ4, mediated double phosphorylation on threonine 886 and serine 890 positions of PLCβ4, and activated CD8⁺ T cells in a PLCβ4-dependent fashion. PLCβ4-deficient mice exhibited defective antiparasitic host defense and antitumor immune responses. Altogether, PLCβ4 differentiates TCR signaling in CD4⁺ and CD8⁺ T cells and selectively promotes CD8⁺ T cell–dependent adaptive immunity.

Structural basis of the CD8 alpha beta/MHC class I interaction: focused recognition orients CD8 beta to a T cell proximal position

In the immune system, B cells, dendritic cells, NK cells, and T lymphocytes all respond to signals received via ligand binding to receptors and coreceptors. Although the specificity of T cell recognition is determined by the interaction of T cell receptors with MHC/peptide complexes, the development of T cells in the thymus and their sensitivity to Ag are also dependent on coreceptor molecules CD8 (for MHC class I (MHCI)) and CD4 (for MHCII). The CD8alphabeta heterodimer is a potent coreceptor for T cell activation, but efforts to understand its function fully have been hampered by ignorance of the structural details of its interactions with MHCI. In this study we describe the structure of CD8alphabeta in complex with the murine MHCI molecule H-2D(d) at 2.6 A resolution. The focus of the CD8alphabeta interaction is the acidic loop (residues 222-228) of the alpha3 domain of H-2D(d). The beta subunit occupies a T cell membrane proximal position, defining the relative positions of the CD8alpha and CD8beta subunits. Unlike the CD8alphaalpha homodimer, CD8alphabeta does not contact the MHCI alpha(2)- or beta(2)-microglobulin domains. Movements of the CD8alpha CDR2 and CD8beta CDR1 and CDR2 loops as well as the flexibility of the H-2D(d) CD loop facilitate the monovalent interaction. The structure resolves inconclusive data on the topology of the CD8alphabeta/MHCI interaction, indicates that CD8beta is crucial in orienting the CD8alphabeta heterodimer, provides a framework for understanding the mechanistic role of CD8alphabeta in lymphoid cell signaling, and offers a tangible context for design of structurally altered coreceptors for tumor and viral immunotherapy.

Genomic organization and expression of CD8alpha and CD8beta genes in fugu Takifugu rubripes

We have cloned cDNAs encoding the alpha and beta chains of CD8 from the tiger pufferfish (fugu), Takifugu rubripes. The cDNA sequences encode a putative leader peptide, extracellular immunoglobulin variable region-like domain, stalk region, transmembrane region, and cytoplasmic tail. A protein tyrosine kinase p56lck binding motif was not found in the putative fugu CD8alpha cytoplasmic tail. O-linked glycosylation sites were found in the stalk of both CD8 chains, suggesting possible stalk formation. Phylogenetic analysis showed that fugu CD8alpha and CD8beta chains cluster with other vertebrate CD8alpha and CD8beta chains, respectively. The fugu CD8 genes comprise six exons separated by five introns. The genes are tandemly aligned 3.6 kb apart and are in the same transcription orientation. Quantitative RT-PCR analysis demonstrated that fugu CD8 is expressed predominantly in lymphoid tissues. In situ hybridization showed that fugu CD8 genes are expressed in thymocytes and lymphocytes within lymphoid organs. Molecular characterization of CD8 in fish provides the basis for development of specific antibodies to identify T-cell subsets, as well as potentially important insights into the evolution of CD8 and the adaptive immunity.

A CD8/Lck transgene is able to drive thymocyte differentiation

Efficient development of thymocytes requires participation of a CD8 or CD4 coreceptor in the TCR:MHC interaction. Both CD8 and CD4 coreceptor cytoplasmic domains associate with Lck. In this study, we attempted to delineate the role of CD8alpha-associated Lck in driving CD8 single positive (SP) thymocyte development. We used a chimeric molecule encoding the extracellular and transmembrane domains of CD8alpha fused to full-length Lck. In mice deficient for CD8alpha and transgenic for 2C, a MHC class I-restricted TCR, robust reconstitution of CD8 SP thymocytes occurred both centrally and peripherally. The reconstituted CD8 SP population was phenotypically and functionally comparable to 2C wild-type counterparts expressing endogenous CD8alpha. A CD8alpha/Lck kinase-dead chimera also resulted in reconstitution of CD8 SP thymocytes. Our results suggest that CD8alpha-associated Lck is sufficient to drive CD8 SP thymocyte development. Furthermore, this CD8 SP development may not necessarily depend on Lck kinase activity.

An antagonist peptide mediates positive selection and CD4 lineage commitment of MHC class II-restricted T cells in the absence of CD4

The CD4 coreceptor works together with the T cell receptor (TCR) to deliver signals to the developing thymocyte, yet its specific contribution to positive selection and CD4 lineage commitment remains unclear. To resolve this, we used N3.L2 TCR transgenic, RAG-, and CD4-deficient mice, which are severely impaired in positive selection, and asked whether altered peptide ligands can replace CD4 function in vivo. Remarkably, in the presence of antagonist ligands that normally deleted CD4⁺ T cells in wild-type mice, we induced positive selection of functional CD4 lineage T cells in mice deficient in CD4. We show that the kinetic threshold for positive and negative selection was lowered in the absence of CD4, with no evident skewing toward the CD8 lineage with weaker ligands. These results suggest that CD4 is dispensable as long as the affinity threshold for positive selection is sustained, and strongly argue that CD4 does not deliver a unique instructional signal for lineage commitment.

The CD8 T cell coreceptor exhibits disproportionate biological activity at extremely low binding affinities

T lymphocytes recognize peptides presented in the context of major histocompatibility complex (MHC) molecules on the surface of antigen presenting cells. Recognition specificity is determined by the alphabeta T cell receptor (TCR). The T lymphocyte surface glycoproteins CD8 and CD4 enhance T cell antigen recognition by binding to MHC class I and class II molecules, respectively. Biophysical measurements have determined that equilibrium binding of the TCR with natural agonist peptide-MHC (pMHC) complexes occurs with KD values of 1-50 microm. The pMHCI/CD8 and pMHCII/CD4 interactions are significantly weaker than this (KD >100 microm), and the relative roles of TCR/pMHC and pMHC/coreceptor affinity in T cell activation remain controversial. Here, we engineer mutations in the MHCI heavy chain and beta2-microglobulin that further reduce or abolish the pMHCI/CD8 interaction to probe the significance of pMHC/coreceptor affinity in T cell activation. We demonstrate that the pMHCI/CD8 coreceptor interaction retains the vast majority of its biological activity at affinities that are reduced by over 15-fold (KD > 2 mm). In contrast to previous reports, we observe that the weak interaction between HLA A68 and CD8, which falls within this spectrum of reduced affinities, retains substantial functional activity. These findings are discussed in the context of current concepts of coreceptor dependence and the mechanism by which TCR coreceptors facilitate T cell activation.

The influence of the src-family kinases, Lck and Fyn, on T cell differentiation, survival and activation

The src-family kinases p56lck (Lck) and p59fyn (Fyn) are expressed in T cells and are among the first signaling molecules to be activated downstream of the T cell receptor (TCR). Evidence is emerging that although closely related, these signaling molecules have discrete functions during development, maintenance and activation of peripheral T cells. For example, during thymopoiesis Lck is uniquely able to provide all the signals required for pre-TCRbeta selection, although Fyn can substitute for a subset of these. Positive selection of CD4 single-positive (SP) cells is also critically dependent on the expression of Lck but not Fyn, while differentiation of CD8 SP cells proceeds relatively efficiently in the absence of Lck. In naïve peripheral T cells either Lck or Fyn can transmit TCR-mediated survival signals, and yet only Lck is able to trigger TCR-mediated expansion signals under conditions of lymphopenia. Stimulation of naïve T cells by antigenic stimuli is also severely compromised in the absence of Lck, but more subtly impaired by the absence of Fyn. We discuss recent experiments addressing how these two src-kinase family members interface with downstream signaling pathways to regulate these diverse aspects of T cell behavior.

The human CD8 coreceptor effects cytotoxic T cell activation and antigen sensitivity primarily by mediating complete phosphorylation of the T cell receptor zeta chain

Recognition of antigen by cytotoxic T lymphocytes (CTL) is determined by interaction of both the T cell receptor and its CD8 coreceptor with peptide-major histocompatibility complex (pMHC) class I molecules. We examine the relative roles of these receptors in the activation of human CTL using mutations in MHC class I designed to diminish or abrogate the CD8/pMHC interaction. We use surface plasmon resonance to determine that point mutation of the alpha3 loop of HLA A2 abrogates the CD8/pMHC interaction without affecting the affinity of the T cell receptor/pMHC interaction. Antigen-presenting cells expressing HLA A2 which does not bind to CD8 fail to activate CTL at any peptide concentration. Comparison of CTL activation by targets expressing HLA A2 with normal, abrogated, or diminished CD8/pMHC interaction show that the CD8/pMHC interaction enhances sensitivity to antigen. We determine that the biochemical basis for coreceptor dependence is the activation of the 23-kDa phosphoform of the CD3zeta chain. In addition, we produce mutant MHC class I multimers that specifically stain but do not activate CTL. These reagents may prove useful in circumventing undesirable activation-related perturbation of intracellular processes when pMHC multimers are used to phenotype antigen-specific CD8+ lymphocytes.

Positive and negative selection of thymocytes depends on Lck interaction with the CD4 and CD8 coreceptors

Considerable evidence supports a role for the Src family protein tyrosine kinase Lck in regulating multiple aspects of thymocyte development. In this report, we establish that early events in T lymphopoiesis are restored to Lck-deficient mice by provision of a transgene encoding a version of Lck that cannot interact with the coreceptors CD4 and CD8. In addition, we demonstrate that later events in thymocyte development, specifically, the processes of positive and negative selection, are compromised in mice where the only Lck available cannot associate with either CD4 or CD8. We conclude that not only is Lck activity required for positive and negative selection, but that that activity must be coupled to the CD4 and CD8 coreceptors.

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.

Defective p56Lck activity in T cells from an adult patient with idiopathic CD4+ lymphocytopenia

Idiopathic CD4(+) lymphocytopenia (ICL) is defined by a stable loss of CD4(+) T cells in the absence of any known cause of immune deficiency. This syndrome is still of undetermined origin. It affects adult patients, some of them displaying opportunistic infections similar to HIV-infected subjects. The hypothesis that the cellular immune defect may be due to biochemical failures of the CD3-TCR pathway is investigated here in a patient associating a severe selective CD4(+) lymphocytopenia with an increased CD8(+) T cell count discovered in the course of a cryptococcal meningitidis. A 40% reduction of T cell proliferation to CD3-TCR stimulation is observed only in the CD4(+) subpopulation. The early CD3-induced protein tyrosine phosphorylations are conserved in both CD4(+) and CD8(+) subsets, and the levels of the T cell protein tyrosine kinases p56(Lck), p59(Fyn) and ZAP-70 are normal. However, we find a 50% reduction of p56(Lck) kinase activity in the patient's T cells compared to a healthy control donor. p59(Fyn) activity does not appear to be altered. Nevertheless, we do not find any genetic abnormality of p56(Lck). These results thus suggest that a defect of an unknown protein regulating p56(Lck) activity takes place in this patient's T cells. Taken together, these findings reveal p56(Lck) alteration in ICL and confirm the critical role of this kinase in the maintenance of the peripheral CD4(+) T cell subpopulation.

Description of an ectothermic TCR coreceptor, CD8 alpha, in rainbow trout

We have cloned the first CD8 alpha gene from an ectothermic source using a degenerate primer for Ig superfamily V domains. Similar to homologues in higher vertebrates, the rainbow trout CD8 alpha gene encodes a 204-aa mature protein composed of two extracellular domains including an Ig superfamily V domain and hinge region. Differing from mammalian CD8 alpha V domains, lower vertebrate (trout and chicken) sequences do not contain the extra cysteine residue (C strand) involved in the abnormal intrachain disulfide bridging within the CD8 alpha V domain of mice and rats. The trout membrane proximal hinge region contains the two essential cysteine residues involved in CD8 dimerization (alpha alpha or alpha beta) and threonine, serine, and proline residues which may be involved in multiple O-linked glycosylation events. Although the transmembrane region is well conserved in all CD8 alpha sequences analyzed to date, the putative trout cytoplasmic region differs and, in fact, lacks the consensus p56lck motif common to other CD8 alpha sequences. We then determined that the trout CD8 alpha genomic structure is similar to that of humans (six exons) but differs from that of mice (five exons). Additionally, Northern blotting and RT-PCR demonstrate that trout CD8 alpha is expressed at high levels within the thymus and at weaker levels in the spleen, kidney, intestine, and peripheral blood leukocytes. Finally, we show that trout CD8 alpha can be expressed on the surface of cells via transfection. Together, our results demonstrate that the basic structure and expression of CD8 alpha has been maintained for more than 400 million years of evolution.

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.

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.

Antagonism of cytotoxic T-lymphocyte activation by soluble CD8

The CD8 co-receptor is important in the differentiation and selection of class I MHC-restricted T cells during thymic development, and in the activation of mature T lymphocytes in response to antigen. Here we show that soluble CD8alphaalpha receptor, despite an extremely low affinity for MHC, inhibits activation of cytotoxic lymphocytes by obstructing CD3 zeta-chain phosphorylation. We propose a model for this effect that involves interference of productive receptor multimerization at the T-cell surface. These results provide new insights into the mechanism of T-cell activation and evidence that CD8 function is exquisitely sensitive to disruption, an effect that might be exploited by molecular therapeutics.

The Cytoplasmic Domain of CD8β Regulates Lck Kinase Activation and CD8 T Cell Development

Previous studies have shown that CD8β plays a role in both enhancing CD8α-associated Lck kinase activity and promoting the development of CD8-lineage T cells. To examine the role of this enhancement in the maturation of CD8-lineage cells, we assessed CD8α-associated Lck kinase activity in both T cell hybridomas and thymocytes of mice expressing CD8β mutations known to impair CD8 T cell development. Lack of CD8β expression or expression of a cytoplasmic domain-deleted CD8β resulted in a severalfold reduction in CD8α-associated Lck kinase activity compared with that observed with cells expressing wild-type CD8β chain. This analysis indicated a critical role for the cytoplasmic domain of CD8β in the regulation of CD8α-associated Lck activity. Decreased CD8α-associated Lck activity observed with the various CD8β mutations also correlated with diminished in vivo cellular tyrosine phosphorylation. In addition, analysis of CD8β mutant mice (CD8β−/− or cytoplasmic domain-deleted CD8β transgenic) indicated that the degree of reduction in CD8α-associated Lck activity associated with each mutation correlated with the severity of developmental impairment. These results support the importance of CD8β-mediated enhancement of CD8α-associated Lck kinase activity in the differentiation of CD8 single-positive thymocytes.

Signaling checkpoints during the development of T lymphocytes

Two major lineage decisions face immature T cells as they develop in the thymus. At an early stage in their development, they must first commit to either the gamma delta or alpha beta lineages. If they opt for the alpha beta lineage, then at a later stage they must also choose between a CD4+ or CD8+ fate before they can pass through the thymic medulla and exit to the periphery. Thymocyte survival at key developmental checkpoints is determined by signaling from cytokine receptors and the T-cell receptor. Recent advances have been made in contemporary understanding of the signals that regulate thymocyte survival, proliferation and lineage decisions.

CD4 regulates the efficiency of an endogenous superantigen-induced clonal deletion of TCRV beta 11+ cells in the periphery

Peripheral T-cell antigen receptor V beta (TCRV beta) repertoire is influenced by clonal deletion both in the thymus and periphery. Developing thymocytes expressing certain TCRV beta are deleted by endogenous superantigens presented on major histocompatibility complex (MHC) molecules in the thymus. Likewise, mature T cells bearing particular TCRV beta chains can be clonally deleted by superantigens in the periphery. The efficiency with which T cells expressing particular V beta subunits are deleted differs depending upon which coreceptor is expressed. Indeed, while deletion of V beta 11+ splenic T cells in CBA/J (Mls-1, a I-E, + MTV 9+) mice is quite efficient for CD4+ spleen T cells, it is much less efficient for CD8+ splenic T cells. If the difference in the efficiency of deletion is due solely to the coreceptor expressed, then a transgene encoding CD4 should increase the efficiency with which CD8+ cells are deleted. To address this question, we have produced CD4 transgenic (TG) mice that express physiologic levels of CD4 on all thymocytes and peripheral CD8 T cells. CD4 molecules expressed on CD8+ splenic T cells were associated with P56lck tyrosine kinase, and were functional as evidenced by their ability to facilitate class II alloreactivity. Furthermore, we found that ectopic expression of TG CD4 molecules on CD8+ cells was able to affect the efficiency of deletion in response to superantigen stimulation. In particular, deletion of TCRV beta 11+ T cells was much less efficient for CD8+ than for CD4+ T-cell subpopulations in (CBA/J x B6) F1 mice. However, expression of the CD4 transgene on CD8+ splenic T cells from these mice increased the efficiency of deletion in the CD8+ V beta 11 T cells. Interestingly, this effect was not observed in a mature CD8+ thymocyte subpopulation. The results in this report demonstrate that CD4 molecules are involved in peripheral deletion of TCRV beta 11+ T cells in (CBA/J x B6) F1 mice, and that the TCRV beta repertoire can be altered by ectopic expression of CD4 on all T-lineage cells.

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.

Peptide-induced positive selection of TCR transgenic thymocytes in a coreceptor-independent manner

T cell receptor (TCR) transgenic thymocytes specific for the LCMV gp peptide are normally positively selected to the CD8 lineage. Transgenic thymocyte development was substantially reduced in the absence of these CD8 coreceptors. However, efficient positive selection was restored when TCR transgenic CD8-/- fetal thymic lobes were cultured with a peptide variant of the wild-type ligand. These mature thymocytes were functional, as shown by their ability to respond against strong peptide agonists. Additional experiments demonstrated that transgenic positive selection was peptide-specific. These results prove that CD8 does not possess essential signaling properties that are necessary for T cell development. In addition, the unilateral commitment of transgenic thymocytes to mature CD4-TCR(hi) T cells expressing intracellular perforin suggests that there must be some instructive component to CD4 down-regulation and lineage commitment during thymocyte selection.

Surface molecules that drive T cell development in vitro in the absence of thymic epithelium and in the absence of lineage-specific signals

Differentiation of immature double positive (DP) CD4+ CD8+ thymocytes into single positive (SP) CD4+ and CD8+ T cells is referred to as positive selection and requires physical contact with thymic cortical epithelium. We now have identified "coinducer" molecules on DP thymocytes that, together with TCR, signal DP thymocytes to differentiate into SP T cells in vitro in the absence of thymic epithelium. A remarkable number of different molecules on DP thymocytes possessed "coinducing" activity, including CD2, CD5, CD24, CD28, CD49d, CD81, and TSA-1. Interestingly, in vitro differentiation occurred in the absence of lineage-specific signals, yet resulted in the selective generation of CD4+CD8- T cells. Thus, the present study has identified surface molecules that can signal DP thymocytes to differentiate into SP T cells in the absence of thymic epithelium and has characterized a default pathway for CD4+ T cell differentiation.

Co-receptor-independent signal transduction in a mismatched CD8+ major histocompatibility complex class II-specific allogeneic cytotoxic T lymphocyte

The contribution of co-receptors in signal transduction upon T cell receptor (TCR)-mediated recognition of major histocompatibility complex (MHC) class II antigen by mature T lymphocytes expressing TCR derived from the apparently co-receptor-independent, I-Ak-specific allogeneic CD8+ CTL clone QM11 has been examined. Mature double-negative, CD8+ and CD4+ bulk T cell lines and clones expressing TCR(QM11) were developed from TCR(QM11) transgenic mice. All these T cells, irrespective of co-receptor expression, showed specific lytic activity on cells expressing I-Ak. Furthermore, co-receptorless mutants were obtained from a CD4+ and CD8+ clone. The responses of these co-receptorless mutants upon specific recognition of the alloantigen, as judged by cytolytic activity, granule exocytosis, lymphokine production, proliferation, and tyrosine phosphorylation of the zeta chain, were comparable to those of the original clones. Thus, the results proved the co-receptor independence of the recognition of I-Ak by TCR(QM11) and further indicated there is no indispensable unique signal transduced by co-receptors. However, when the amount of the available antigen was limited by anti-I-Ak antibody, the CD4+ T cell clone showed a remarkable resistance to the inhibition whereas the mismatched CD8+ clone was readily inhibitable. The anti-I-Ak-resistant component of the CD4+ clone showed dependency on the CD4 molecule. Taken collectively, the results indicate that the role played by a co-receptor molecule in mature T cells is purely quantitative amplification of the signal through the formation of a TCR/MHC/co-receptor ternary complex, and also indicate that the role of co-receptor molecules as TCR-independent adhesion molecules is at best minimal.

Feedback regulation of T cell development: manifestations in aging

Recent findings have indicated that mature T cells may regulate thymocytopoiesis in an age-related differential manner. The studies were based on T lymphocyte development in mouse fetal thymus stroma colonized with immature thymocytes and CD4+ T cells from young or old donors. In the present study, we used mathematical modeling and computer simulations in order to identify the thymocyte subsets that are targets for this type of regulation, and the processes affected by it. Our results suggest that thymocyte development is subject to regulation through 2 feedback loops: mature CD4+ cells exert a negative feedback on the double-negative to double-positive transition and on double-positive subset growth, and a positive feedback on the double-positive to CD4 single-positive transition. These effects may operate, in young mice, through a reduction in the rate of death of CD4+8- thymocytes, and a faster maturation of double-positive cells. In old mice, our simulations suggest that there may additionally be a reduction in double-positive proliferation rate. In some, but not all, of the simulations of old donor- derived thymocytes, we also had to assume a reduction in double-negative to double-positive differentiation, an increase in double-positive death rates, an increase of CD4+8- cell division rate, and a decrease of differentiation to the CD8 lineage.

Thymocytes can become mature T cells without passing through the CD4+ CD8+, double-positive stage

T cells bearing the class II-restricted, DO-T cell receptor (TCR) are CD4+ if their thymocyte precursors are positively selected on the class II protein, IAd, but they are almost all CD4- after positive selection on a class II for which they have higher avidity, IAb. DO-TCR+ T cells mature in H-2b mice lacking CD4. CD4- DO-TCR+ T cells appear in H-2b mice at the same rate as their CD4+ counterparts appear in H-2d animals, suggesting that the CD4- cells are not the product of some minor pathway of thymocyte development and selection. In H-2b CD4 knock out mice expressing human CD2 under the control of the mouse CD4 promoter, mature DO-TCR+ cells did not express human CD2. These results suggest that the CD4-CD8-, DO-TCR+ mature T cells have developed without ever passing through the equivalent of a CD4+,CD8+ stage. The early expression of alpha/beta receptors (TCRs) on thymocytes in TCR transgenic mice may allow maturation of this type. Passage through the equivalent of the CD4+ CD8+, double-positive stage is not essential for differentiation of thymocytes into mature T cells.

The p56lck SH2 domain mediates recruitment of CD8/p56lck to the activated T cell receptor/CD3/zeta complex

The CD4 or CD8 co-receptors and the T cell receptor (TCR) are though to interact with the same antigen-presenting major histocompatibility complex molecule in a stable ternary complex. Therefore, the TCR and its co-receptor need to come into close proximity on the surface of the T cell. We have previously shown that the interaction of the p56lck SH2 domain with zeta-associated, tyrosine phosphorylated ZAP-70 and Syk kinases leads to an enhanced association of CD4 with TCR/CD3/zeta complex after CD3 stimulation of Jurkat cells. In this report, we analyzed whether a similar mechanism can mediate recruitment of the CD8 alpha alpha and CD8 alpha beta isoforms to the TCR. We demonstrate in vivo in association of CD8 alpha alpha/p56lck with the tyrosine kinase ZAP-70 after CD3 stimulation of Jurkat cells. A phosphopeptide competing in vitro for the binding of tyrosine phosphorylated proteins to the SH2 domain of p56lck specifically impedes the association of ZAP-70 with CD8 alpha alpha/p56lck without affecting the zeta/ZAP-70 interaction. The same peptide is able to compete for the activation-dependent association of the CD8 alpha alpha or CD8 alpha beta isoform with the TCR/CD3/zeta complex. Moreover, co-precipitation of the TCR with both CD8 isoforms was observed after CD3 stimulation. These findings strongly suggest that the p56lck SH2 domain mediates recruitment of CD8/p56lck to the activated TCR/CD3/zeta complex.

Requirement for p56lck tyrosine kinase activation in T cell receptor-mediated thymic selection

The nonreceptor protein tyrosine kinase p56lck (Lck) serves as a fundamental regulator of thymocyte development by delivering signals from the pre-T cell receptor (pre-TCR) that permit subsequent maturation. However, considerable evidence supports the view that Lck also participates in signal transduction from the mature TCR. We have tested this conjecture by expressing a dominant-negative form of Lck under the control of a promoter element (the distal lck promoter) that directs high expression in CD4+CD8+ thymocytes, mature thymocytes, and peripheral T cells, thereby avoiding, complications that result from the well-documented ability of dominant-negative Lck to block very early events in thymocyte maturation. Here we report that expression of the catalytically inactive Lck protein at twice normal concentrations inhibits thymocyte positive selection by as much as 80%, while leaving other aspects of cell maturation intact. This effect was studied in more detail in mice simultaneously bearing the male-specific H-Y alpha/beta TCR transgene and ovalbumin-specific DO10 alpha/beta TCR transgene, where even equimolar expression of the dominant-negative Lck protein substantially vitiated the positive selection process. Although deletion of H-Y alpha/beta thymocytes proceeded normally in male mice despite the presence of catalytically inactive Lck, modest inhibition of superantigen-mediated deletion was in some cases observed. These data further implicate Lck in the propagation of all TCR-derived signals, and indicate that even very modest deficiencies in the representation of functional Lck molecules could in humans, profoundly alter the character of the peripheral TCR repertoire.

Cellular interactions in thymocyte development

Interactions between stromal cells and thymocytes play a crucial role in T cell development. The thymic stroma is complex and consists of epithelial cells derived from the pharyngeal region during development, together with macrophages and dendritic cells of bone marrow origin. In addition, fibroblasts and matrix molecules permeate the whole framework. It is now apparent that these individual stromal components play specialized roles at different stages of T cell differentiation. Thus, at the early CD4-8- stage of development, T cell precursors require fibroblast as well as epithelial cell interactions. Later, at the CD4+8+ stage, as well as providing low avidity TCR/MHC-peptide interactions, thymic epithelial cells have been shown to possess unique properties essential for positive selection. Dendritic cells, on the other hand, are probably efficient mediators of negative selection, but they may not be solely responsible for this activity. Alongside the functional roles of stromal cells, considerable progress is being made in unraveling the nature of the signaling pathways involved in T cell development. Identification of the pre-T cell receptor (pre-TCR) and associated signaling molecules marks an important advance in understanding the mechanisms that control gene rearrangement and allelic exclusion. In addition, a better understanding of the signaling pathways that lead to positive selection on the one hand and negative selection on the other is beginning to emerge. Many issues remain unresolved, and some are discussed in this review. What, for example, is the nature of the chemotactic factor(s) that attract stem cells to the thymus? What is the molecular basis of the essential interactions between early thymocytes and fibroblasts, and early thymocytes and epithelial cells? What is special about cortical epithelial cells in supporting positive selection? These and other issues are ripe for analysis and can now be approached using a combination of modern molecular and cellular techniques.

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.

Insights into T cell development and signal transduction provided by TCR-zeta chain deficient mice

The T cell antigen receptor (TCR) transduces signals that mediate different responses depending on the stage of development of the T cell and the nature of the ligand it engages. The presence of multiple signal transducing subunits (CD3-gamma-delta,-epsilon and zeta chain) suggests the potential to control these responses by altering the subunit composition of the TCR. zeta chain represents an especially important signalling molecule as it contains multiple signalling motifs within its cytoplasmic tail. The generation and analysis of zeta deficient (zeta-/-) and zeta-transgenic mice has provided insight into the role of zeta as well as the CD3 subunits in TCR surface expression, T cell activation and thymocyte development. Herein, we discuss the results from such experiments which suggest distinct roles for zeta chain and the CD3 components at different stages of T cell development.

A mutation in zap-70 protein tyrosine kinase results in a selective immunodeficiency

We have previously described a new type of selective T-cell deficiency characterized by persistent infections reminiscent of severe combined immunodeficiency. We show here that selective T-cell deficiency patients carry a mutation of zap-70 protein tyrosine kinase, resulting in a loss of the activity of this kinase. The thymus of zap-70(-1-1) patients shows the presence of CD4CD8 double-positive cells in the cortex, however, only CD4, and not CD8, single-positive cells are present in the medulla. Peripheral CD4+ T cells from the zap-70(-1-1) patients exhibit markedly reduced tyrosine phosphorylation, fail to produce interleukin-2, and do not proliferate in response to T-cell receptor stimulation by mitogens or antigens. Thus zap-70 kinase appears to be indispensable for the development of CD8 single-positive T cells as well as for the signal transduction and function of single-positive CD4 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.

CD8 beta chain influences CD8 alpha chain-associated Lck kinase activity

The CD8 molecule plays an important role in the differentiation of CD8+ T cells in the thymus and in their normal function in the periphery. CD8 exists on the cell surface in two forms, the alpha alpha homodimer and the alpha beta heterodimer. Recent studies indicate an important role for the CD8 beta chain in thymic development of CD8+ T cells and suggest that signaling via CD8 alpha beta may be distinct from CD8 alpha alpha. To better understand these differences, we introduced the CD8 beta gene into a T cell hybridoma which only expressed the CD8 alpha alpha homodimer. In the parent hybridoma, cross-linking of the CD8 alpha chain led to minimal enhancement of CD8-associated Lck tyrosine kinase activity. In the CD8 beta+ transfectants, several observations suggested that CD8 beta modifies CD8 alpha-associated Lck tyrosine kinase activity: (a) in in vitro kinase assays, antibody-mediated crosslinking of CD8 alone, or CD8 cross-linking with the TCR, resulted in 10-fold greater activation of Lck kinase activity, compared to cells expressing CD8 alpha alpha alone; (b) in vivo, markedly enhanced tyrosine phosphorylation of several intracellular proteins was observed upon CD8 cross-linking with the TCR in CD8 alpha beta-expressing cells, compared to cells expressing CD8 alpha alpha alone; and (c) Lck association with CD8 alpha was stabilized by the coexpression of CD8 beta. Thus, the differential Lck kinase activation and tyrosine phosphorylation seen with CD8 alpha alpha vs. CD8 alpha beta may reflect the unique signaling capabilities of the CD8 beta molecule. These differences in signaling may, in part, account for the diminished ability to generate CD8 single positive thymocytes in mice bearing a homozygous disruption of the CD8 beta gene.

Early human T cell development: analysis of the human thymus at the time of initial entry of hematopoietic stem cells into the fetal thymic microenvironment

To determine events that transpire during the earliest stages of human T cell development, we have studied fetal tissues before (7 wk), during (8.2 wk), and after (9.5 wk to birth) colonization of the fetal thymic rudiment with hematopoietic stem cells. Calculation of the approximate volumes of the 7- and 8.2-wk thymuses revealed a 35-fold increase in thymic volumes during this time, with 7-wk thymus height of 160 microM and volume of 0.008 mm3, and 8.2-wk thymus height of 1044 microM and volume of 0.296 mm3. Human thymocytes in the 8.2-wk thymus were CD4+ CD8 alpha+ and cytoplasmic CD3 epsilon+ cCD3 delta+ CD8 beta- and CD3 zetta-. Only 5% of 8-wk thymocytes were T cell receptor (TCR)-beta+, < 0.1% were TCR-gamma+, and none reacted with monoclonal antibodies against TCR-delta. During the first 16 wk of gestation, we observed developmentally regulated expression of CD2 and CD8 beta (appearing at 9.5 wk), CD1a,b, and c molecules (CD1b, then CD1c, then CD1a), TCR molecules (TCR-beta, then TCR-delta), CD45RA and CD45RO isoforms, CD28 (10 wk), CD3 zeta (12-13 wk), and CD6 (12,75 wk). Whereas CD2 was not expressed at the time of initiation of thymic lymphopoiesis, a second CD58 ligand, CD48, was expressed at 8.2 wk, suggesting a role for CD48 early in thymic development. Taken together, these data define sequential phenotypic and morphologic changes that occur in human thymus coincident with thymus colonization by hematopoietic stem cells and provide insight into the molecules that are involved in the earliest stages of human T cell development.

Apoptosis and interleukin 7 gene expression in chronic B-lymphocytic leukemia cells

mRNA for interleukin 7 (IL-7) was readily detected in leukemic cells immediately upon their removal from patients with chronic B-lymphocytic leukemia (B-CLL). IL-7 mRNA expression and IL-7 gene transcription were down regulated, however, when B-CLL cells were placed in culture at 37 degrees C for 4 hr. Down regulation of the IL-7 gene was prevented in cells maintained at 4 degrees C. Continued culture of B-CLL cells at 37 degrees C resulted in programmed cell death, or apoptosis, as evidenced by DNA fragmentation. The coincident kinetics of IL-7 gene down regulation and apoptosis suggested that IL-7 gene expression may be required for maintenance of CLL viability in vivo. Signals for IL-7 gene regulation and apoptosis induction were thus examined. Activation of normal B cells through their immunoglobulin receptors did not result in upregulation of IL-7 gene expression. Reagents required for CLL cell purification and culture also did not contribute to IL-7 gene regulation and apoptosis induction. IL-7 gene expression was retained and apoptosis was prevented, however, in CLL cells cultured on a monolayer of EA.hy926 human umbilical cord endothelial hybrid cells. Signals specifically presented by EA.hy926 cells supported both CLL cell viability and IL-7 gene expression, whereas culture of CLL cells on A549/8 carcinoma cells, the fusion partner used to generate the EA.hy926 cells, did not. Cell-cell contact was required, as culture supernatants did not prevent apoptosis. Specifically, IL-7 mRNA expression was retained and apoptosis was prevented only by contact with the endothelial cell hybrids. Preliminary data indicated that integrins expressed on CLL cells affected modulation of apoptosis and IL-7 gene regulation, suggesting that integrins may play significant roles in regulating viability of CLL cells.

Bcl-2 expression during T-cell development: early loss and late return occur at specific stages of commitment to differentiation and survival

During T-cell development CD3-CD4-CD8- (double-negative) thymocytes proliferative and produce an enormous number of CD3loCD4+CD8+ (double-positive) thymocytes which are destined to die intrathymically unless rescued by positive selection. Those which survive become mature CD3hiCD4/8+ (single-positive) cells and are the precursor of peripheral blood lymphocytes. The product of the bcl-2 protooncogene has been implicated in preventing programmed cell death and is required for prolonged lymphocyte survival following maturation. Previously we and others have reported that Bcl-2 protein expression is biphasic, being high in proliferating double-negative stem cells, low in all double-positive thymocytes except for 1-5% of these cells, and restored in mature, single-positive thymocytes. However, it remained unclear which signaling and selection events regulate Bcl-2 during T-cell maturation. Now we have utilized four-color flow cytometry in normal and genetically altered mice for a detailed analysis of Bcl-2 expression as it relates to T-cell receptor (TCR) expression and positive selection. These studies show that (i) expression of a transgenic TCR in double-negative thymocytes does not lead to premature loss of Bcl-2; thus, Bcl-2 downregulation is not solely due to TCR expression; (ii) Bcl-2 expression is lost at the early transitional CD3-/loCD4-CD8+ stage, prior to expression of CD4; (iii) the Bcl-2+ double-positive thymocytes are those which have undergone positive selection; and (iv) upregulation of Bcl-2 during positive selection requires participation of the CD4 or CD8 co-receptor. These results demonstrate that Bcl-2 and TCR expression are regulated independently during T-cell development, and suggest a role for the CD4 or CD8 co-receptor in Bcl-2 induction during positive selection.

Protein tyrosine kinase p56-Lck regulates lymphocyte function-associated 1 adhesion molecule expression, granule exocytosis, and cytolytic effector function in a cloned T cell

Elevated levels of p56-Lck kinase activity were achieved in an interleukin 2 (IL-2)-dependent cloned cytolytic T cell CTLL-2 through gene transfer approaches. CTLL-2-Lck cells remained dependent on IL-2 for growth and survival in culture but exhibited markedly elevated, IL-2-independent cytolytic activity against a variety of tumor targets. This immune cell effector function was similar to the non-major histocompatibility complex-restricted cytolytic activity previously described for lymphokine activated killer (LAK) cells, and involved a cytolytic mechanism that was independent of protein synthesis in either the T cells or the tumor targets. Characterization of CTLL-2-Lck cells revealed markedly elevated levels of both the alpha (CD11a) and beta (CD18) chains of the cell adhesion molecule lymphocyte function-associated 1 (LFA-1) and increased binding of these T cells to a recombinant protein representing the extracellular domain of the LFA-ligand, intercellular adhesion molecule 1 (ICAM-1). Antibodies to CD11a partially abrogated cytolytic killing of tumor target cells by CTLL-2-Lck cells, suggesting that the upregulation in LFA protein levels potentially accounts at least in part for the phenotype of these T cells. Gene transfer-mediated elevations in p56-Lck kinase activity in an IL-3-dependent myeloid cell clone 32D.3 also resulted in increased LFA-1 expression, demonstrating that the findings are not unique to CTLL-2 cells. In addition to upregulation of LFA-1 expression, CTLL-Lck cells also exhibited more efficient exocytosis of cytotoxic granules upon activation with Ca(2+)-ionophore and phorbol ester, relative to control transfected and untransfected CTLL-2 cells. The findings functionally link the Lck kinase to a T cell effector pathway involved in cell-mediated cytotoxicity.

Analysis of coreceptor versus accessory molecule function of CD8 as a correlate of exogenous peptide concentration

Substitution in the alpha 3 domain of class I molecules can ablate the recognition of target cells by CD8 dependent cytotoxic T lymphocytes. This effect has been attributed to a destruction of the CD8 alpha binding site on the class I molecule, a hypothesis which is consistent with results obtained in conjugate binding assays. To assess the relative contribution to CTL activation of CD8 functioning as either a coreceptor or an accessory molecule, we have compared the ability of H-2Kb ovalbumin reactive CTL to lyse M12.C3 or T2 cells transfected with an H-2Kb gene encoding a wild type or mutant (CD8 nonbinding) alpha 3 domain. To establish that the substitution in the alpha 3 domain does not alter the ability of the H-2Kb molecule to bind the antigenic peptide, we have compared the binding of the ovalbumin derived H-2Kb restricted peptide (SIINFEKL) to T2 cells expressing either the CD8 binding or the CD8 nonbinding form of H-2Kb. This peptide conjugated with FITC bound equally well to T2 cells expressing either form of H-2Kb. Upon binding of this peptide, both forms of the H-2Kb molecule underwent the same conformational change as revealed by increases in the expression of particular serological epitopes. Furthermore, inhibition of the binding of the SIINFEKL peptide to both the wild type and mutant H-2Kb was observed following pretreatment of the cells with similar amounts of other H-2Kb restricted peptides derived from Sendai and Vesicular Stomatitis viruses. When the transfected M12 cells were tested for their ability to serve as targets for an anti-H-2Kb ovalbumin CTL clone, cells expressing the mutant H-2Kb molecule required the addition of 100-fold more exogenous peptide than did cells expressing the wild type molecule in order to obtain significant lysis. These data strengthen the previous hypothesis that CD8 functions much more efficiently as a coreceptor than as an accessory molecule for T cell effector function.

Human severe combined immunodeficiency due to a defect in ZAP-70, a T cell tyrosine kinase

A homozygous mutation in the kinase domain of ZAP-70, a T cell receptor-associated protein tyrosine kinase, produced a distinctive form of human severe combined immunodeficiency. Manifestations of this disorder included profound immunodeficiency, absence of peripheral CD8+ T cells, and abundant peripheral CD4+ T cells that were refractory to T cell receptor-mediated activation. These findings demonstrate that ZAP-70 is essential for human T cell function and suggest that CD4+ and CD8+ T cells depend on different intracellular signaling pathways to support their development or survival.

Adhesion receptors in lymphocyte activation

The past several years have seen significant progress in understanding the role of T lymphocyte coreceptors in adhesion and activation. New insights have been gained in several areas: the avidity regulation of beta 1 and beta 2 integrins and their role in signal transduction; the regulation of CD8 avidity; the role of Lck in CD4 coreceptor activity; and the novel role for CD2 adhesion in the T cell antigen response.

Resting and activated T cells display different requirements for CD8 molecules

Clonotype-positive (1B2+) T cells from 2C T cell receptor (TCR) transgenic mice were used to define the role of CD8 molecules in the induction phase vs. the effector phase of the primary response to class I alloantigens. Three main findings are reported. First, in the presence of exogenous lymphokines, resting CD8+ 2C cells gave strong proliferative responses to two alloantigens, Ld and Kbm11. In the absence of added lymphokines, however, CD8+ 2C cells responded only to Ld and not to Kbm11; Ld stimulated both interleukin 2 (IL-2) and IL-2 receptor (R) synthesis, whereas Kbm11 elicited only IL-2R synthesis. The primary response of CD8+ 2C cells was thus helper-independent (HI) to Ld but helper-dependent (HD) to Kbm11, presumably reflecting that Ld is a stronger antigen than Kbm11. Second, in contrast to CD8+ cells, CD8- 2C cells mounted only an HD and not an HI response to the strong Ld antigen; conversely, selecting for a minor subset of CD8hi cells enabled 2C cells to mount an HI response to the weak Kbm11 antigen. These findings, together with experiments with heterozygous vs. homozygous stimulator cells, suggest that HI and HD responses reflect differences in the overall avidity of T antigen presenting cell (APC) interaction: high-avidity interaction leads to strong intracellular signaling and an HI response, whereas low-avidity interaction causes weak signaling and an HD response; high-avidity T/APC interaction is heavily dependent on CD8 expression. Third, CD8 expression was found to be less important for CTL activity than for primary proliferative responses. Thus, in contrast to HI proliferative responses, CTL responses of 2C cells to Ld were CD8 independent. However, 2C lysis of Ld targets became strongly CD8 dependent in the presence of limiting doses of anti-TCR (1B2) antibody. Collectively, the data suggest that, both for T cell induction and the expression of effector function, CD8 molecules play a decisive role in augmenting TCR-mediated signaling, either by promoting TCR contact with antigen or delivering kinases (p56lck) to the TCR/CD3 complex, or both.

T-cell development and function in gene-knockout mice

The use of mice with defined genetic defects engineered by homologous recombination in embryonic stem cells has greatly enhanced our understanding of immune functions at the single-gene level. The complex molecular interactions involved in T-cell development and antigen recognition have been especially targeted for detailed analysis via knockout technology.

Thymocyte lineage commitment: is it instructed or stochastic?

Thymocytes co-expressing the CD4 and CD8 co-receptors differentiate into mature T cells that express either CD4 or CD8 and have helper or cytotoxic functions, respectively. Recent studies indicate that commitment to the CD4+ or CD8+ lineages occurs stochastically, but retention of the appropriate co-receptor is required to complete development.

Signaling mechanisms in thymocyte selection

The processes known as positive and negative selection that determine the fate of T and B cells depend on finely tuned interactions between the T-cell receptor complex, CD4 or CD8 co-receptors, and a peptide-MHC complex. New work indicates that the avidity of this interaction is critical in the determination of its outcome. The effects of these interactions on developing thymocytes are also a function of the unique activation properties with which thymocytes are programmed just before they undergo selection.

A subset of CD4+ thymocytes selected by MHC class I molecules

To complete their maturation, most immature thymocytes depend on the simultaneous engagement of their antigen receptor [alpha beta T cell receptor (TCR)] and their CD4 or CD8 coreceptors with major histocompatibility complex class II or I ligands, respectively. However, a normal subset of mature alpha beta TCR+ thymocytes did not follow these rules. These thymocytes expressed NK1.1 and a restricted set of alpha beta TCRs that are intrinsically class I-reactive because their positive selection was class I-dependent but CD8-independent. These cells were CD4+ and CD4-8- but never CD8+, because the presence of CD8 caused negative selection. Thus, neither CD4 nor CD8 contributes signals that direct their maturation into the CD4+ and CD4-8- lineages.