Intrathymic T cell receptor (TcR) targeting in mice lacking CD4 or major histocompatibility complex (MHC) class II: rescue of CD4 T cell lineage without co-engagement of TcR/CD4 by MHC class II

In Vivo Treatment of Class II MHC-Deficient Mice with Anti-TCR Antibody Restores the Generation of Circulating CD4 T Cells and Optimal Architecture of Thymic Medulla

TCR ligation by the self-peptide-associated MHC molecules is essential for T cell development in the thymus, so that class II MHC-deficient mice do not generate CD4(+)CD8(-) T cells. The present results show that the administration of anti-TCR mAb into class II MHC-deficient mice restores the generation of CD4(+)CD8(-) T cells in vivo. The CD4 T cells were recovered in the thymus, peripheral blood, and the spleen, indicating that the anti-TCR treatment is sufficient for peripheral supply of newly generated CD4 T cells. Unlike peripheral CD4 T cells that disappeared within 5 wk after the treatment, CD4(+)CD8(-) thymocytes remained undiminished even after 5 wk, suggesting that CD4 T cells in the thymus are maintained separately from circulating CD4 T cells and even without class II MHC molecules. It was also found that the mass of medullary region in the thymus, which was reduced in class II MHC-deficient mice, was restored by the anti-TCR administration, suggesting that the medulla for CD4(+)CD8(-) thymocytes is formed independently of the medulla for CD4(-)CD8(+) thymocytes. These results indicate that in vivo anti-TCR treatment in class II MHC-deficient mice restores the generation of circulating CD4 T cells and optimal formation of the medulla in the thymus, suggesting that anti-TCR Ab may be useful for clinical treatment of class II MHC deficiencies.

Thymic selection revisited: how essential is it?

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

Effect of murine thymic epithelial cell line (MTEC1) on the functional expression of CD4(+)CD8(-) thymocyte subgroups

To determine the effect of thymic stromal cells on the functional maturation of CD4 single-positive (SP) thymocytes, the functional status of isolated CD4 SP thymocyte subgroups was investigated by means of cell proliferation and cytokine production in response to concanavalin A (Con A) prior and after co-culturing with a murine thymic epithelial cell line (MTEC1). Mouse medullary CD4 SP thymocytes were phenotypically divided into seven discrete subgroups predicted to reflect the maturation pathway from newly emerging CD4 SP thymocytes to terminally differentiated cells. For functional analysis, six major subgroups (6C10(+)CD69(+), 6C10(-)CD69(+), 6C10(-)CD69(-)3G11(+)Qa-2(-), 6C10(-)CD69(-)3G11(+)Qa-2(+), 6C10(-)CD69(-)3G11(-)Qa-2(-) and 6C10(-)CD69(-)3G11(-)Qa-2(+)) cells were isolated and their functional status in response to Con A stimulation assessed. A functional hierarchy is revealed among these subgroups, consistent with their phenotypic maturation status, which may imply that these cells undergo a functional maturation process within thymic medulla. The function of cytokine production by CD4 SP thymocytes is acquired in a stepwise manner from a low to high level and characterized by T(h)0-type cytokines in the main stream of differentiation pathway. However, a minor subgroup that appeared at the late stage as 3G11(-)6C10(-) cells was biased to produce T(h)2-type cytokines. Nevertheless, the functional capacity of the final two Qa-2(+) subgroups of CD4 SP thymocytes was still significantly lower than that of spleen CD4(+) T cells. After co-cultivation with MTEC1 cells, four subgroups of TCRalphabeta(+)CD4(+)CD8(-) thymocytes exhibited significantly higher levels of proliferation capability and modulation in cytokine production capability. However, co-culturing with MTEC1 cells did not change the pattern of T(h)0- or T(h)2-like cytokine production by respectively medullary CD4 SP thymocyte subgroups nor could MTEC1 induce CD4 SP thymocytes to secrete T(h)1-type cytokines. The results suggest that MTEC1 can regulate the functional status of these thymocyte subgroups.

Activation of the Extracellular Signal-Related Kinase/Mitogen-Activated Protein Kinase Pathway Discriminates CD4 Versus CD8 Lineage Commitment in the Thymus

We have investigated the role of the mitogen-activated protein kinase (MAPK) pathway in the differentiation of CD4+ and CD8+ T cells by looking specifically at the effects of inhibitors of MAPK-activating enzyme, MAPK/extracellular signal-related kinase (ERK) kinase (MEK), during the positive selection step from double-positive to single-positive (SP) thymocytes. Using a variety of transgenic/knockout mouse strain combinations that fail to differentiate individual lineages of SP thymocytes together with genetically engineered F(ab′)2 reagents that induce maturation preferentially to either the CD4 or CD8 subpopulations, we show that induction of CD4 differentiation cells is highly sensitive to levels of MEK inhibition that have no effect on CD8 maturation. In addition, the presence of MEK inhibitor is able to modify signals that normally induce CD4 differentiation to instead promote CD8 differentiation. Finally, we show that continuous culture in the presence of inhibitor interferes with TCR up-regulation in SP thymocytes, suggesting that MAPK signaling may be involved in final maturation steps for both lineages. These data indicate that there is discrimination in the biochemical pathways that are necessary to specify CD4 and CD8 lineage commitment and can reconcile previously conflicting reports on the influence of MAPK activation in commitment and maturation of thymocytes.

Positive selection of CD4+ T cells by TCR-specific antibodies requires low valency TCR cross-linking: implications for repertoire selection in the thymus

The developmental fate of immature CD4+ 8+ thymocytes is determined by intrathymic signals transduced by surface TCR complexes. In particular, TCR signals are required for immature CD4+ 8+ thymocytes to further differentiate into CD4+ 8- or CD4- 8+ T cells, a process referred to as positive selection. It is generally thought that positive selection results from low affinity TCR interactions with self antigens which engage the relatively few surface TCR complexes that are on immature CD4+ 8+ thymocytes. However, we now demonstrate with TCR-specific antibodies that positive selection of CD4+ T cells requires low valency cross-linking of surface TCR complexes on immature thymocytes. That is, positive selection signals are only generated within a narrow range of TCR cross-linking: cross-linking either too few or too many surface TCR complexes fails to signal positive selection. We interpret these results as indicating that positive selection of CD4+ T cells is not signaled by low affinity TCR interactions per se, but rather can be signaled by any combination of TCR affinity and ligand density that induces low valency TCR cross-linking on immature thymocytes.

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.

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.

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.

Opposite CD4/CD8 Lineage Decisions of CD4+8+ Mouse and Rat Thymocytes to Equivalent Triggering Signals: Correlation with Thymic Expression of a Truncated CD8α Chain in Mice But Not Rats

Unselected CD4+8+ rat thymocytes, generated in vitro from their direct precursors, are readily converted to functional TCRhigh T cells by stimulation with immobilized TCR-specific mAb plus IL-2. Lineage decision invariably occurs toward CD4−8+, regardless of the timing of TCR stimulation after entry into the CD4+8+ compartment or the concentration of TCR-specific mAb used for stimulation. CD4-specific mAb synergizes with suboptimal TCR-specific mAb in inducing T cell maturation, but lineage decision remains exclusively CD4−8+. These results contrast with those obtained in mice, in which Abs to the TCR complex were shown to promote CD4+8− T cell maturation from CD4+8+ thymocytes. Surprisingly, when rat and mouse CD4+8+ thymocytes were stimulated with PMA/ionomycin under identical conditions, the opposite lineage commitment was observed, i.e., mouse thymocytes responded with the generation of CD4+8− and rat thymocytes with the generation of CD4−8+ cells. It thus seems that CD4+8+ thymocytes of the two species respond with opposite lineage decisions to strong activating signals such as given by TCR-specific mAb or PMA/ionomycin. A possible key to this difference lies in the availability of p56lck for coreceptor-supported signaling. We show that in contrast to mouse CD4+8+ thymocytes, which express both a complete and a truncated CD8α-chain (CD8α′) unable to bind p56lck, rat thymocytes only express full-length CD8α molecules. Mice, but not rats, therefore may use CD8α′ as a “dominant negative” coreceptor chain to attenuate the CD8 signal, thereby facilitating MHC class II recognition through the higher amount of p56lck delivered, and rats may use a different mechanism for MHC class distinction during positive selection.

Two separable T cell receptor signals reconstitute positive selection of CD4 lineage T cells in vivo

Positive selection is an obligatory step during intrathymic T cell differentiation. It is associated with rescue of short-lived, self major histocompatibility complex (MHC)-restricted thymocytes from programmed cell death, CD4/CD8 T cell lineage commitment, and induction of lineage-specific differentiation programs. T cell receptor (TCR) signaling during positive selection can be closely mimicked by targeting TCR on immature thymocytes to cortical epithelial cells in situ via hybrid antibodies. We show that selection of CD4 T cell lineage cells in mice deficient for MHC class I and MHC class II expression can be reconstituted in vivo by two separable T cell receptor signaling steps, whereas a single TCR signal leads only to induction of short-lived CD4+CD8lo intermediates. These intermediates remain susceptible to a second TCR signal for 12-48 h providing an estimate for the duration of positive selection in situ. While both TCR signals induce differentiation steps, only the second one confers long-term survival on immature thymocytes. In further support of the two-step model of positive selection we provide evidence that CD4 T cell lineage cells rescued by a single hybrid antibody pulse in MHC class II-deficient mice are pre-selected by MHC class I.

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.

Intrathymic maturation of CD4+ T-lymphocytes in an MHC class II deficient transplant model

Major histocompatibility complex (MHC) class II knockout (class II-) mice fail to generate CD4+ CD8- T-lymphocytes. We were interested in determining whether these class II- mice could be reconstituted with CD4+ CD8- T-lymphocytes following marrow transplantation from normal (class II+) donors. Transplantation of class II+ marrow into lethally irradiated class II- recipients failed to generate peripheral CD4+ CD8- T-lymphocytes. Unexpectedly, however, transplantation of class II marrow into class II+ recipients also resulted in a deficiency of CD4+ CD8- cells. Analysis of intrathymic T cells showed normal distribution of CD4 and CD8 single and double positive or negative thymocytes in normal recipients, while class II- recipients always lacked CD4+ CD8- T cells intrathymically. These results suggest, therefore, that T-cell maturation in mice requires the presence of MHC class II antigens not only in the thymus but also on immature, marrow-derived pre-thymocytes.