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

Inactivation of misselected CD8 T cells by CD8 gene methylation and cell death

Misselected CD8 cells that express T cell receptors (TCRs) that do not recognize class I major histocompatibility complex (MHC) protein can emerge from thymic selection. A postthymic quality control mechanism that purges these cells from the repertoire is defined here. The failure of mature CD8 cells to simultaneously engage their TCR and CD8 coreceptor triggers an activation process that begins with inhibition of CD8 gene expression through remethylation and concludes with up-regulation of surface Fas and Fas ligand and cellular apoptosis. Thus, inhibition of a death signal through continued TCR-CD8 coengagement of MHC molecules is a key checkpoint for the continued survival of correctly selected T cells. Molecular defects that prevent delivery of the death signal to mistakenly selected T cells underlie the expansion of double-negative T cells, which is the cellular signature of a subset of systemic autoimmune diseases.

Reciprocal Expression in CD4 or CD8 Subsets of Different Members of the Vα11 Gene Family Correlates with Sequence Polymorphism

Previous staining studies with TCR Vα11-specific mAbs showed that Vα11.1/11.2 (AV11S1 and S2) expression was selectively favored in the CD4+ peripheral T cell population. As this phenomenon was essentially independent of the MHC haplotype, it was suggested that AV11S1 and S2 TCRs exert a preference for recognition of class II MHC molecules. The Vα segment of the TCR α-chain is suggested to have a primary role in shaping the T cell repertoire due to selection for class I or II molecules acting through the complementarity determining regions (CDR) 1α and CDR2α residues. We have analyzed the repertoire of Vα11 family members expressed in C57BL/6 mice and have identified a new member of this family; AV11S8. We show that, whereas AV11S1 and S2 are more frequent in CD4+ cells, AV11S3 and S8 are more frequent in CD8+ cells. The sequences in the CDR1α and CDR2α correlate with differential expression in CD4+ or CD8+ cells, a phenomenon that is also observed in BALB/c mice. With no apparent restriction in TCR Jα usage or CDR3α length in C57BL/6, these findings support the idea of Vα-dependent T cell repertoire selection through preferential recognition of MHC class I or class II molecules.

Alterations in CD4-Binding Regions of the MHC Class II Molecule I-Ek Do Not Impede CD4+ T Cell Development

The T cell coreceptors CD4 and CD8 enhance T cell responses to TCR signals by participating in complexes containing TCR, coreceptor, and MHC molecules. These ternary complexes are also hypothesized to play a seminal role during T cell development, although the precise timing, frequency, and consequences of TCR-coreceptor-MHC interactions during positive selection and lineage commitment remain unclear. To address these issues, we designed transgenic mice expressing mutant I-Ek molecules with reduced CD4-binding capability. These transgenic lines were crossed to three different lines of I-Ek-specific TCR transgenic mice, and the efficiency of production of CD4+ lineage cells in the doubly transgenic progeny was assessed. Surprisingly, replacing wild-type I-Ek molecules with these mutant molecules did not affect the production of CD4+CD8− thymocytes or CD4+ peripheral T cells expressing any of the three TCRs examined. These data, when considered together with other experiments addressing the role of coreceptor during development, suggest that not all MHC class II-specific thymocytes require optimal and simultaneous TCR-CD4-MHC interactions to mature. Alternatively, it is possible that these particular alterations of I-Ek do not disrupt the CD4-MHC interaction adequately, potentially indicating functional differences between I-A and I-E MHC class II molecules.

Effects of complementarity determining region mutations on the affinity of an alpha/beta T cell receptor: measuring the energy associated with CD4/CD8 repertoire skewing

It has been proposed that the generally low affinities of T cell receptors (TCRs) for their peptide-major histocompatibility complex (pMHC) ligands (Kd approximately 10(-4) to 10(-7) M) are the result of biological selection rather than an intrinsic affinity limitation imposed by the TCR framework. Using a soluble version of the 2C TCR, we have used complementarity determining region (CDR)-directed mutagenesis to investigate whether the affinity of this receptor for its allogeneic pMHC ligand can be improved upon. We report that several mutants at positions lying within CDR3alpha and CDR2beta showed increased affinities for pMHC compared with the wild-type receptor. Additionally, we have investigated whether Valpha mutations that have been implicated in the phenomenon of CD8(+) repertoire skewing achieve this skewing by means of generalized increases in affinity for MHC-I molecules. Two mutants (S27F and S51P), which each promote skewing toward a CD8(+) phenotype, exhibited significantly reduced affinity for pMHC-I, consistent with a quantitative-instructional model of CD4/CD8 lineage commitment. This model predicts that CD8 is downregulated on thymocytes that have TCR-ligand interactions above a minimal energy threshold. Together, the results (a) demonstrate that engineering higher affinity TCRs is feasible, and (b) provide TCR-pMHC energy values associated with CD4/CD8 repertoire skewing.

Visualization of CD4/CD8 T cell commitment

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

CD5 expression is developmentally regulated by T cell receptor (TCR) signals and TCR avidity

Recent data indicate that the cell surface glycoprotein CD5 functions as a negative regulator of T cell receptor (TCR)-mediated signaling. In this study, we examined the regulation of CD5 surface expression during normal thymocyte ontogeny and in mice with developmental and/or signal transduction defects. The results demonstrate that low level expression of CD5 on CD4(-)CD8(-) (double negative, DN) thymocytes is independent of TCR gene rearrangement; however, induction of CD5 surface expression on DN thymocytes requires engagement of the pre-TCR and is dependent upon the activity of p56(lck). At the CD4(+)CD8(+) (double positive, DP) stage, intermediate CD5 levels are maintained by low affinity TCR-major histocompatibility complex (MHC) interactions, and CD5 surface expression is proportional to both the surface level and signaling capacity of the TCR. High-level expression of CD5 on DP and CD4(+) or CD8(+) (single positive, SP) thymocytes is induced by engagement of the alpha/beta-TCR by (positively or negatively) selecting ligands. Significantly, CD5 surface expression on mature SP thymocytes and T cells was found to directly parallel the avidity or signaling intensity of the positively selecting TCR-MHC-ligand interaction. Taken together, these observations suggest that the developmental regulation of CD5 in response to TCR signaling and TCR avidity represents a mechanism for fine tuning of the TCR signaling response.

Coreceptor-Independent T Cell Activation in Mice Expressing MHC Class II Molecules Mutated in the CD4 Binding Domain

We have previously reported that efficient selection of the mature CD4+ T cell repertoire requires a functional interaction between the CD4 coreceptor on the developing thymocyte and the MHC class II molecule on the thymic epithelium. Mice expressing a class II protein carrying the EA137/VA142 double mutation in the CD4 binding domain develop fewer than one-third the number of CD4+ T cells found in wild-type mice. In this report we describe the functional characteristics of this population of CD4+ T cells. CD4+ T cells that develop under these conditions are predicted to be a CD4-independent subset of T cells, bearing TCRs of sufficient affinity for the class II ligand to undergo selection despite the absence of accessory class II-CD4 interactions. We show that CD4+ T cells from the class II mutant mice are indeed CD4 independent in their peripheral activation requirements. Surprisingly, we find that CD4+ T cells from the class II mutant mice, having been selected in the absence of a productive class II-CD4 interaction, fail to functionally engage CD4 even when subsequently provided with a wild-type class II ligand. Nevertheless, CD4+ T cells from EA137/VA142 class II mutant mice can respond to T-dependent Ags and support Ig isotype switching.

Correlating notch signaling with thymocyte maturation

The Notch receptor and its ligands are involved in many developmental processes. They are highly expressed in the thymus and have been implicated in the CD4 versus CD8 lineage decision. We identified the constitutively active intracellular fragment of murine Notch-1 as capable of rendering thymomas resistant to glucocorticoid-induced apoptosis. This effect was confirmed in other T cell lines and in CD4+ CD8+ DP thymocytes. Activation of the Notch signaling pathway also upregulated a number of other markers that, like steroid resistance, correlate with DP maturation into both the CD4 and CD8 lineages. These results suggest that Notch signaling is critically involved in the maturation of DP thymocytes into both CD4+ and CD8+ SP thymocytes.

Normal Thymic Selection of TCR Transgenic CD4 T Cells, but Impaired Survival in the Periphery Despite the Presence of Selecting MHC Molecules

In this paper, we investigate selection in the thymus and survival in the periphery of CD4 T cells, which carry a major histocompatibility class II-restricted transgenic TCR (A18 TCRtg) specific for a natural self Ag, the fifth component of complement (C5). A18 TCRtg thymocytes develop normal numbers of CD4 single-positive (SP) thymocytes, but do not show pronounced overselection as do some other TCR transgenic strains. CD4 SP cells are mature as judged by termination of CD8 synthesis, resistance to cortisone, and functional competence. The kinetics of positive selection, determined by BrdU labeling, are very fast. CD4 SP thymocytes are demonstrable within 2 days of labeling, and within 8 days after labeling a large proportion (20%) of lymph node T cells are recent thymic emigrants. The high number of recent thymic emigrants suggests rapid turnover of CD4 T cells in the periphery, which was confirmed by thymectomy and determination of CD4 T cell life spans. A18 TCRtg T cells have a t1/2 of ∼6 wk, despite the presence of selecting MHC molecules. This explains the failure to accumulate high numbers of peripheral T cells and suggests that the MHC-bound ligand(s) responsible for initiating survival signals is limiting for the selection and maintenance of A18 transgenic CD4 T cells.

CD4/CD8 lineage commitment: matching fate with competence

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

Molecular requirements for lineage commitment in the thymus--antibody-mediated receptor engagements reveal a central role for lck in lineage decisions

Recent experiments in our laboratory have focused on the receptor engagements required for the differentiation of fully mature, single positive thymocytes from their double positive precursors. We have used a novel approach which involves the ligation of surface receptors on immature thymocytes with genetically engineered F(ab1)2 reagents, which, unlike conventional antibodies, do not aggregate the CD3 complex to such an extent as to induce extensive deletion of these cells. The experimental data presented in this review indicate that differentiation of the two mature CD4 and CD8 lineages occurs in response to distinct intracellular signals induced by particular receptor engagements. The data suggest that the tyrosine kinase p56lck (lck) plays a crucial role in determining lineage choice, in that maturation of thymocytes into the CD4 lineage occurs upon recruitment of active lck to the T-cell receptor (TCR)/CD3 complex, whereas CD8 maturation can be induced by CD3 ligation in the absence of co-receptor-mediated lck recruitment. A central role for lck activity in determining the threshold for differentiation of the CD4 lineage is revealed in experiments with thymi deficient for a regulator of lck activity, CD45. A model of thymocyte differentiation is presented in which we propose that the relative balance of signals delivered by TCR engagement and lck activation determines lineage choice.

T-cell fate

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

Antagonist peptide selects thymocytes expressing a class II major histocompatibility complex-restricted T cell receptor into the CD8 lineage

CD4/CD8 lineage decision is an important event during T cell maturation in the thymus. CD8 T cell differentiation usually requires corecognition of major histocompatibility complex (MHC) class I by the T cell receptor (TCR) and CD8, whereas CD4 T cells differentiate as a consequence of MHC class II recognition by the TCR and CD4. The involvement of specific peptides in the selection of T cells expressing a particular TCR could be demonstrated so far for the CD8 lineage only. We used mice transgenic for an MHC class II-restricted TCR to investigate the role of antagonistic peptides in CD4 T cell differentiation. Interestingly, antagonists blocked the development of CD4(+) cells that normally differentiate in thymus organ culture from those mice, and they induced the generation of CD8(+) cells in thymus organ culture from mice impaired in CD4(+) cell development (invariant chain-deficient mice). These results are in line with recent observations that antagonistic signals direct differentiation into the CD8 lineage, regardless of MHC specificity.

CD4 Regulation of TCR Signaling and T Cell Differentiation Following Stimulation with Peptides of Different Affinities for the TCR

To define the role of CD4 in modulating T cell signaling pathways and regulating Th1 and Th2 differentiation, we have examined the activation and differentiation characteristics of naive T cells from CD4 mutant mice. Using peptides with differing affinities for the moth cytochrome c-specific TCR, we test the hypothesis that differences in coreceptor recruitment and signaling explain the qualitatively distinct signaling pathways seen in CD4 T cells following high affinity agonist and low affinity altered peptide ligand (APL) ligation. We find that the absence of CD4 signaling during stimulation with a strong agonist peptide does not qualitatively change the pattern of early TCR-mediated biochemical signaling events into a pattern resembling the response of CD4+ T cells to APLs. In contrast, the response to APL stimulation, by T cells bearing the same TCR, does require a component of CD4 signaling. The proliferative response and calcium signals normally seen following APL stimulation are markedly diminished in the absence of CD4. In addition, we find that naive T cell differentiation into Th2 effector cells is impaired in the absence of CD4. These data suggest that the altered pattern of biochemical signals generated by APLs require CD4 coreceptor function and that some of these signals may be required to initiate Th2 differentiation.

Pig MHC Mediates Positive Selection of Mouse CD4+ T Cells with a Mouse MHC-Restricted TCR in Pig Thymus Grafts

Remarkably normal immune function and specific T cell tolerance to discordant xenogeneic donors can be achieved by grafting fetal pig thymus and liver (FP THY/LIV) tissue to T cell and NK cell-depleted, thymectomized (ATX) mice. To determine whether or not host class II MHC molecules participate in the positive selection of mouse CD4+ T cells in FP THY/LIV grafts, we compared their development in ATX “AND” TCR-transgenic mice with positive selecting or nonselecting host MHC genotypes. Mouse TCR-transgenic CD4 single positive T cells repopulated the periphery significantly and to a similar extent in both T/NK cell-depleted, ATX AND mice with positive-selecting or nonselecting MHC backgrounds after grafting with FP THY/LIV. Therefore, MHC molecules from a widely disparate xenogeneic species can positively select T cells bearing a host class II MHC-restricted TCR without a contribution from the host MHC. These results, in combination with previous studies performed in this model, suggest that the T cell repertoire that is generated by the combination of positive selection on xenogeneic MHC and negative selection on both recipient and xenogeneic porcine MHC is tolerant of both donor and recipient and has sufficient cross-reactivity with host MHC/foreign peptide complexes to confer a high level of immunocompetence. The results have implications for the potential clinical applicability of xenogeneic thymic transplantation and also suggest a predominant role for the TCR recognition of species-conserved MHC residues in positive selection.

HD mice: a novel mouse mutant with a specific defect in the generation of CD4(+) T cells

We have identified a spontaneous mutation in mice, which we term HD for "helper T cell deficient." This mouse is distinguished by the virtual absence of peripheral T cells of the CD4(+)8(-) major histocompatibility complex (MHC) class II-restricted T helper subset due to a specific block in thymic development. The developmental defect is selective for CD4(+)8(-) cells; the maturation of CD4(-)8(+) and gamma delta T cells is normal. The autosomal recessive mutation underlying the HD phenotype is unrelated to MHC class II, since it segregates independently of the MHC class II locus. Moreover, the HD phenotype is not caused by a defect of the CD4 gene. Bone marrow transfer experiments demonstrate that the defect is intrinsic to cells of the hematopoietic lineage, i.e., most likely to developing thymocytes themselves. The frequency of CD4(+)8(low) intermediate cells is markedly increased in HD mice, suggesting that class II-restricted thymocytes are arrested at this stage. This is the first genetic defect of its kind to be described in the mouse and may prove highly informative in understanding the molecular pathways underlying lineage commitment.

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.

Cellular Mechanisms Involved in Protection Against Influenza Virus Infection in Transgenic Mice Expressing a TCR Receptor Specific for Class II Hemagglutinin Peptide in CD4+ and CD8+ T Cells

Mice transgenic for a TCR that recognizes peptide110–120 of hemagglutinin of PR8 influenza virus in the context of MHC class II I-Ed molecules express the transgenes in both CD4+ and CD8+ T cells. We have found that these TCR-hemagglutinin (TCR-HA) transgenic mice display a significantly increased resistance to the primary infection with PR8 virus compared with the wild-type mice. The TCR-HA transgenic mice mounted significant MHC type II and enhanced MHC type I-restricted cytotoxicity as well as increased cytokine responses in both spleen and lungs after infection with PR8 virus. In contrast, the primary humoral response against PR8 virus was not significantly different from that of the wild-type mice. In vivo depletion and adoptive cell transfer experiments demonstrated that both CD4+ and CD8+ TCR-HA+ T cell subsets were required for the complete clearance of pulmonary virus following infection with a dose that is 100% lethal in wild-type mice. Whereas CD4+ TCR-HA+ T cells were necessary for effective activation and local recruitment of CD8+ T cells, CD8+ TCR-HA+ T cells showed a Th1-biased pattern and MHC type II-restricted cytotoxicity. However, in the absence of in vivo expression of MHC type I molecules on the infected cells, the protection conferred by the TCR-HA+ T cells was impaired, indicating that the enhanced MHC class I-restricted cytotoxicity due to TCR-HA+ CD4+ Th cells was a critical element for clearance of the pulmonary virus by the transgenic mice.

CD3 ligation on immature thymocytes generates antagonist-like signals appropriate for CD8 lineage commitment, independently of T cell receptor specificity

The signals that direct differentiation of T cells to the CD4 or CD8 lineages in the thymus remain poorly understood. Although it has been relatively easy to direct differentiation of CD4 single positive (CD4+) cells using combinations of antibodies and pharmacological agents that mimic receptor engagements, equivalent stimuli do not induce efficient maturation of CD8+ cells. Here we report that, irrespective of the MHC-restriction specificity of the TCR, differentiation of mature CD8+ thymocytes can be induced by ligation of CD3 polypeptides on immature thymocytes with a F(ab')2 reagent (CD3fos-F(ab')2). The tyrosine phosphorylation patterns stimulated by CD3fos-F(ab')2 have been shown to resemble those delivered to mature T cells by antagonist peptides, which are known to direct positive selection of CD8+ cells, and we can show that this reagent exhibits potent antagonistic-like activity for primary T cell responses. Our results suggest a distinction in the signals that specify lineage commitment in the thymus. We present a model of thymocyte differentiation that proposes that the relative balance of signals delivered by TCR engagement and by p56lck activation is responsible for directing commitment to the CD8 or CD4 lineages.

Disruption of the CD4-major histocompatibility complex class II interaction blocks the development of CD4(+) T cells in vivo

The experiments presented in this report were designed to specifically examine the role of CD4-major histocompatibility complex (MHC) class II interactions during T cell development in vivo. We have generated transgenic mice expressing class II molecules that cannot interact with CD4 but that are otherwise competent to present peptides to the T cell receptor. MHC class II expression was reconstituted in Abeta gene knock-out mice by injection of a transgenic construct encoding either the wild-type I-Abetab protein or a construct encoding a mutation designed to specifically disrupt binding to the CD4 molecule. We demonstrate that the mutation, EA137 and VA142 in the beta2 domain of I-Ab, is sufficient to disrupt CD4-MHC class II interactions in vivo. Furthermore, we show that this interaction is critical for the efficient selection of a complete repertoire of mature CD4(+) T helper cells as evidenced by drastically reduced numbers of conventional CD4(+) T cells in animals expressing the EA137/VA142 mutant I-Ab and by the failure to positively select the transgenic AND T cell receptor on the mutated I-Ab. These results underscore the importance of the CD4-class II interaction in the development of mature peripheral CD4(+) T cells.

T-cell selection

Major histocompatibility complex (MHC) molecules in the thymus select from the repertoire of germline receptors those that will be most useful in mounting responses to antigen in the periphery. Recent data has shed light on the involvement of self peptides presented by the MHC in this process, and has indicated that a requirement for the constant recognition of 'self' may be essential for T-cell longevity in the periphery.

The Level of CD4 Surface Protein Influences T Cell Selection in the Thymus

During T cell development thymocytes are subjected to positive and negative selection criteria to ensure that the mature T cell repertoire is MHC restricted, yet self tolerant at the same time. The CD4 and CD8 coreceptors are thought to play a crucial role in this developmental process. To elucidate the role of CD4 in T cell selection, we have produced a mouse strain that expresses CD4 at a reduced level. We used homologous recombination in embryonic stem cells to insert neo into the 3′ untranslated region of CD4. The resulting mice have a reduction in the percentage of CD4+ cells in the thymus and a concomitant increase in CD8+ cells. In addition, breeding two individual class II-restricted TCR transgenic mice onto the CD4low (low level of CD4) mutant background affects the selection of each TCR differentially. In one case (AND TCR transgenic), significantly fewer CD4+ cells with the transgenic TCR develop on the CD4low mutant background, whereas in the other (5C.C7 TCR transgenic), selection to the CD4 lineage is only slightly reduced. These data support the differential avidity model of positive and negative selection. With little or no avidity, the cell succumbs to programmed cell death, low to moderate avidity leads to positive selection, and an avidity above a certain threshold, presumably above one that would lead to autoreactivity in the periphery, results in clonal deletion. These data also support the idea that a minimum avidity threshold for selection exists and that CD4 plays a crucial role in determining this avidity.

Pertussis toxin can replace T cell receptor signals that induce positive selection of CD8 T cells

CD4+ helper T lymphocytes and CD8+ killer T lymphocytes are both generated in the thymus from common precursor cells expressing CD4 and CD8. The development of immature CD4 CD8+ thymocytes into mature 'single-positive' T cells requires T cell antigen-receptor (TCR)-mediated positive selection signals. Although it is known that the recognition specificity of TCR expressed by CD4+ CD8+ thymocytes determines their fate to become either CD4+ or CD8+ T cells, the molecular signals that direct precursor thymocytes to become CD4+ and CD8+ T cells are unclear. By using ZAP-70 mutant thymus organ cultures in which T cell development is arrested at the CD4+ CD8+ thymocyte stage, the present study shows that distinct biochemical treatments can selectively restore the generation of mature CD4+ and CD8+ T cells, bypassing TCR-induced positive selection signals. The combination of phorbol ester and ionomycin selectively restores the generation of CD4+ CD8- TCR(high) cells, consistent with previous results. On the other hand, we find that the generation of CD4- CD8+ TCR(high) cells is selectively induced by pertussis toxin. Interestingly, the signals generated by pertussis toxin, which increase Notch expression, can dominate the signals by phorbol ester and ionomycin, steering thymocyte development to CD8 lineage. These results indicate that distinct biochemical signals replace TCR signals that selectively induce positive selection of CD4+ and CD8+ T cells, and that biochemical treatment can manipulate the development and choice of CD4+ and CD8+ T cells.

The influence of the MAPK pathway on T cell lineage commitment

During development, progenitor thymocytes differentiate into either CD4 or CD8 T cells, and this fate decision depends on the specificity of the T cell antigen receptor (TCR) for MHC class II or class I molecules. Based on the mechanisms of fate specification known for simple metazoan organisms, we sought to determine whether the extracellular signal-related kinases (ERKs) play a role in T cell differentiation and lineage commitment. Using a dominant gain-of-function mutant of the erk2 gene, we show that differentiation into the CD4 lineage is favored. We also show that, conversely, the addition of a pharmacological inhibitor of the ERK pathway favors differentiation into the CD8 lineage. We present a quantitative selection model that incorporates these results as well as those of recent reports on the role of Notch in T cell lineage specification.

Commitment of immature CD4+8+ thymocytes to the CD4 lineage requires CD3 signaling but does not require expression of clonotypic T cell receptor (TCR) chains

As a consequence of positive selection in the thymus, immature CD4(+)8(+) double-positive, [DP] thymocytes selectively terminate synthesis of one coreceptor molecule and, as a result, differentiate into either CD4(+) or CD8(+) T cells. The decision by individual DP thymocytes to terminate synthesis of one or the other coreceptor molecule is referred to as lineage commitment. Previously, we reported that the intrathymic signals that induced commitment to the CD4 versus CD8 T cell lineages were markedly asymmetric. Notably, CD8 commitment appeared to require lineage-specific signals, whereas CD4 commitment appeared to occur in the absence of lineage-specific signals by default. Consequently, it was unclear whether CD4 commitment, as revealed by selective termination of CD8 coreceptor synthesis, occurred in all DP thymocytes, or whether CD4 commitment occurred only in T cell receptor (TCR)-CD3-signaled DP thymocytes. Here, we report that selective termination of CD8 coreceptor synthesis does not occur in DP thymocytes spontaneously. Rather, CD4 commitment in DP thymocytes requires signals transduced by either CD3 or zeta chains, which can signal CD4 commitment even in the absence of clonotypic TCR chains.

Direct, MHC-dependent presentation of the drug sulfamethoxazole to human alphabeta T cell clones

T cells can recognize small molecular compounds like drugs. It is thought that covalent binding to MHC bound peptides is required for such a hapten stimulation. Sulfamethoxazole, like most drugs, is not chemically reactive per se, but is thought to gain the ability to covalently bind to proteins after intracellular drug metabolism. The purpose of this study was to investigate how sulfamethoxazole is presented in an immunogenic form to sulfamethoxazole-specific T cell clones. The stimulation of four CD4(+) and two CD8(+) sulfamethoxazole-specific T cell clones by different antigen-presenting cells (APC) was measured both by proliferation and cytolytic assays. The MHC restriction was evaluated, first, by inhibition using anti-class I and anti-class II mAb, and second, by the degree of sulfamethoxazole-induced stimulation by partially matched APC. Fixation of APC was performed with glutaraldehyde 0.05%. The clones were specific for sulfamethoxazole without cross-reaction to other sulfonamides. The continuous presence of sulfamethoxazole was required during the assay period since pulsing of the APC was not sufficient to induce proliferation or cytotoxicity. Stimulation of clones required the addition of MHC compatible APC. The APC could be fixed without impairing their ability to present sulfamethoxazole. Sulfamethoxazole can be presented in an unstable, but MHC-restricted fashion, which is independent of processing. These features are best explained by a direct, noncovalent binding of sulfamethoxazole to the MHC-peptide complex.

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.

CD8 lineage commitment in the absence of CD8

The absence of cytotoxic T lymphocyte activity and the failure of MHC class I-restricted T cell receptor (TCR) transgenic thymocytes to mature in CD8alpha-deficient mice suggest that CD8 may be essential for CD8 lineage commitment. We report that variants of the antigenic peptide that delete TCR transgenic thymocytes from CD8 wild-type but not CD8alpha-deficient mice can restore positive selection of CD8 lineage cells in the absence of CD8. The positively selected cells down-regulate CD4, up-regulate TCR, respond to the antigenic peptide, and express CD8beta mRNA. Interestingly, there was no enhanced selection of CD4+ T cells, implying that the TCR-MHC interaction, even in the absence of CD8, provided instructive signaling for commitment to the CD8 lineage. Our results are discussed in terms of recent models of T cell lineage commitment.

CD8 T cells from major histocompatibility complex class II-deficient mice respond vigorously to class II molecules in a primary mixed lymphocyte reaction

Mature CD4+ and CD8+ T cells are restricted by major histocompatibility complex (MHC) class II and class I molecules, respectively. In a primary mixed lymphocyte reaction (MLR), CD8+ T cells from C57BL/6 (B6) mice can respond to allo-class I molecules, but not allo-class II molecules. However, a significant fraction of CD8+ T cells from C57BL/6 class II-deficient (B6Aalpha-) mice violate this rule by responding vigorously in a MLR to class II molecules. The frequency of responding cells is approximately 50% of that of B6 CD8+ T cells responding to B6bm1 allo-class I molecules. This response requires neither appropriate co-receptor, i.e. CD4, nor exogenous lymphokines, indicating that interactions between the T cell receptors (TCR) and class II molecules are remarkably efficient. Since these CD8+ T cells are positively selected by class I molecules in the thymus of class II-deficient mice, these CD8+ T cells should interact with both classes of MHC molecules. The absence of thymic negative selection by class II molecules may result in the production of these CD8+ T cells. The data imply that a substantial fraction of CD4+CD8+ double-positive thymocytes in wild-type mice interacts with both classes of MHC molecules prior to thymic selection.

Asynchronous coreceptor downregulation after positive thymic selection: prolonged maintenance of the double positive state in CD8 lineage differentiation due to sustained biosynthesis of the CD4 coreceptor

In several experimental systems analyzing the generation of single positive (SP) thymocytes from double positive (DP) thymocytes, CD4 SP cells have been shown to appear before CD8 SP cells. This apparent temporal asymmetry in the maturation of CD4 SP and CD8 SP thymocytes could either be due to divergent molecular differentiation programs of the two T cell lineages, or merely to slower degradation kinetics of the CD4 protein. To study this question in unmanipulated in vivo differentiation, we developed a four-color flow cytometry protocol which identifies a recently activated TCRintCD69pos thymocyte population containing DP cells and early CD4 SP cells but no CD8 SP cells. We show that these TCRintCD69pos thymocytes represent a transitory stage in the mainstream alphabeta T cell lineage. The precursors of the CD8 SP cells are contained in this population as incompletely selected DP cells. Moreover, we show that expression of both coreceptors in the TCRintCD69pos population depends on transcriptional and translational activity, thus excluding differences in turnover rates of the CD4 and CD8 proteins as the cause of the asynchrony in differentiation of the CD4 and CD8 lineages.

Thymic stromal organization is regulated by the specificity of T cell receptor/major histocompatibility complex interactions

The thymic architecture is normally compartmentalized into a central medulla surrounded by a peripheral cortical region. We investigated how compartmentalization of the thymic stroma is regulated using T cell receptor (TCR)-transgenic mouse models. Our studies show that the signals generated by TCR/peptide/major histocompatibility complex interactions regulate thymic stromal cell compartmentalization. In TCR-transgenic mice, normal stromal cell compartmentalization occurs when the transgenic TCR is expressed on a background that does not result in skewing toward either positive or negative selection. In models representing strong positive selection, the thymic stromal elements do not fully organize into a central medulla. Instead, small medullary foci are dispersed throughout the thymus with some regions residing directly under the capsule. The highest degree of disorganization in medullary epithelial regions is observed in TCR-transgenic mice that exhibit negative selection. Although the medullary foci lack central organization, the expression in these regions of CD80, CD86 and CD40, as well as the clustering of dendritic cells, is similar to that observed in medullae of wild-type mice. Thus, the organization of the medulla appears to occur in two stages: (1) small medullary epithelial regions that are dispersed in fetal thymi expand and associate with antigen-presenting cells, and (2) the expanded medullary foci organize into a central medullary compartment. Our data suggest a model in which this second stage of stromal cell organization is increasingly inhibited as the normal balance of TCR-mediated signals is skewed by higher-avidity interactions between thymocytes and antigen-presenting cells.

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

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