Differential effects of zeta and eta transgenes on early alpha/beta T cell development

NuRD complex recruitment to Thpok mediates CD4+ T cell lineage differentiation

Although BTB-zinc finger (BTB-ZF) transcription factors control the differentiation of multiple hematopoietic and immune lineages, how they function is poorly understood. The BTB-ZF factor Thpok controls intrathymic CD4⁺ T cell development and the expression of most CD4⁺ and CD8⁺ lineage genes. Here, we identify the nucleosome remodeling and deacetylase (NuRD) complex as a critical Thpok cofactor. Using mass spectrometry and coimmunoprecipitation in primary T cells, we show that Thpok binds NuRD components independently of DNA association. We locate three amino acid residues within the Thpok BTB domain that are required for both NuRD binding and Thpok functions. Conversely, a chimeric protein merging the NuRD component Mta2 to a BTB-less version of Thpok supports CD4⁺ T cell development, indicating that NuRD recruitment recapitulates the functions of the Thpok BTB domain. We found that NuRD mediates Thpok repression of CD8⁺ lineage genes, including the transcription factor Runx3, but is dispensable for Cd4 expression. We show that these functions cannot be performed by the BTB domain of the Thpok-related factor Bcl6, which fails to bind NuRD. Thus, cofactor binding critically contributes to the functional specificity of BTB-ZF factors, which control the differentiation of most hematopoietic subsets.

CD5-mediated inhibition of TCR signaling proceeds normally in the absence of SHP-1

The CD5 transmembrane glycoprotein functions as a co-receptor in the signaling pathway linking T-cell antigen receptor (TCR) engagement to activation and differentiation. Although CD5 effects on TCR signaling have been shown to be primarily inhibitory, the underlying mechanisms remain unclear. In view of recent data revealing the ability of CD5 to associate with the SHP-1 tyrosine phosphatase, a protein that also downregulates TCR signaling, we examined the role of SHP-1 in modulating CD5 function using thymocytes from SHP-1-deficient viable motheaten (me^v) mice. The results revealed the association of SHP-1 with CD5 to be markedly increased following TCR stimulation and indicated that this interaction was enhanced by and was dependent on CD5 tyrosine phosphorylation. However, there was no difference of the tyrosine phosphorylation status of CD5 between resting and TCR-stimulated cells in SHP-1-deficient compared to wild-type thymocytes. Lack of SHP-1 activity did not affect the levels of CD5 surface expression, CD5 co-immunoprecipitable tyrosine phosphatase activity and intracellular calcium increase following co-crosslinking of the TCR and CD5. Similarly, an analysis of T-cell thymocyte populations in me^v mice expressing an H-Y transgene as well as a construct mediating T-cell restricted CD5 overexpression, revealed that the reduction in the positive selection conferred by CD5 overexpression was unaffected by SHP-1 deficiency. CD5 is not a SHP-1 substrate and SHP-1 is not required for and possibly not involved in the CD5-mediated modulation of TCR signaling.

Lmo2 induces hematopoietic stem cell-like features in T-cell progenitor cells prior to leukemia

LIM domain only 2 (Lmo2) is frequently deregulated in sporadic and gene therapy-induced acute T-cell lymphoblastic leukemia (T-ALL) where its overexpression is an important initiating mutational event. In transgenic and retroviral mouse models, Lmo2 expression can be enforced in multiple hematopoietic lineages but leukemia only arises from T cells. These data suggest that Lmo2 confers clonal growth advantage in T-cell progenitors. We analyzed proliferation, differentiation, and cell death in CD2-Lmo2 transgenic thymic progenitor cells to understand the cellular effects of enforced Lmo2 expression. Most impressively, Lmo2 transgenic T-cell progenitor cells were blocked in differentiation, quiescent, and immortalized in vitro on OP9-DL1 stromal cells. These cellular effects were concordant with a transcriptional signature in Lmo2 transgenic T-cell progenitor cells that is also present in hematopoietic stem cells (HSCs) and early T-cell precursor ALL. These results are significant in light of the crucial role of Lmo2 in the maintenance of the HSC. The cellular effects and transcriptional effects have implications for LMO2-dependent leukemogenesis and the treatment of LMO2-induced T-ALL.

LIM domain only-2 (LMO2) induces T-cell leukemia by two distinct pathways

The LMO2 oncogene is deregulated in the majority of human T-cell leukemia cases and in most gene therapy-induced T-cell leukemias. We made transgenic mice with enforced expression of Lmo2 in T-cells by the CD2 promoter/enhancer. These transgenic mice developed highly penetrant T-ALL by two distinct patterns of gene expression: one in which there was concordant activation of Lyl1, Hhex, and Mycn or alternatively, with Notch1 target gene activation. Most strikingly, this gene expression clustering was conserved in human Early T-cell Precursor ALL (ETP-ALL), where LMO2, HHEX, LYL1, and MYCN were most highly expressed. We discovered that HHEX is a direct transcriptional target of LMO2 consistent with its concordant gene expression. Furthermore, conditional inactivation of Hhex in CD2-Lmo2 transgenic mice markedly attenuated T-ALL development, demonstrating that Hhex is a crucial mediator of Lmo2's oncogenic function. The CD2-Lmo2 transgenic mice offer mechanistic insight into concordant oncogene expression and provide a model for the highly treatment-resistant ETP-ALL subtype.

The ability of Sos1 to oligomerize the adaptor protein LAT is separable from its guanine nucleotide exchange activity in vivo

The activation of the small guanosine triphosphatase Ras by the guanine nucleotide exchange factor (GEF) Sos1 (Son of Sevenless 1) is a central feature of many receptor-stimulated signaling pathways. In developing T cells (thymocytes), Sos1-dependent activation of extracellular signal-regulated kinase (ERK) is required to stimulate cellular proliferation and differentiation. We showed that in addition to its GEF activity, Sos1 acted as a scaffold to nucleate oligomerization of the T cell adaptor protein LAT (linker for activation of T cells) in vivo. The scaffold function of Sos1 depended on its ability to bind to the adaptor protein Grb2. Furthermore, the GEF activity of Sos1 and the Sos1-dependent oligomerization of LAT were separable functions in vivo. Whereas the GEF activity of Sos1 was required for optimal ERK phosphorylation in response to T cell receptor (TCR) stimulation, the Sos1-dependent oligomerization of LAT was required for maximal TCR-dependent phosphorylation and activation of phospholipase C-γ1 and Ca(2+) signaling. Finally, both of these Sos1 functions were required for early thymocyte proliferation. Whereas transgenic restoration of either the GEF activity or the LAT oligomerization functions of Sos1 alone failed to rescue thymocyte development in Sos1-deficient mice, simultaneous reconstitution of these two signals in the same cell restored normal T cell development. This ability of Sos1 to act both as a RasGEF and as a scaffold to nucleate Grb2-dependent adaptor oligomerization may also occur in other Grb2-dependent pathways, such as those activated by growth factor receptors.

Interchangeability of Themis1 and Themis2 in thymocyte development reveals two related proteins with conserved molecular function

Themis1, a recently identified T cell protein, has a critical function in the generation of mature CD4(+)CD8(-) and CD4(-)CD8(+) (CD4 and CD8 single-positive [SP]) thymocytes and T cells. Although Themis1 has been shown to bind to the adaptor proteins LAT and Grb2, previous studies have yielded conflicting results regarding whether thymocytes from Themis1(-/-) mice exhibit TCR-mediated signaling defects. In this study, we demonstrate that, in the absence of Themis1, TCR-mediated signaling is selectively impaired in CD4 SP and CD8 SP thymocytes but is not affected in CD4(+)CD8(+) double-positive thymocytes despite high expression of Themis1 in double-positive thymocytes. Like Themis1, Themis2, a related member of the Themis family, which is expressed in B cells and macrophages, contains two conserved cysteine-based domains, a proline-rich region, and a nuclear localization signal. To determine whether Themis1 and Themis2 can perform similar functions in vivo, we analyzed T cell development and TCR-mediated signaling in Themis1(-/-) mice reconstituted with either Themis1 or Themis2 transgenes. Notably, Themis1 and Themis2 exhibited the same potential to restore T cell development and TCR-mediated signaling in Themis1(-/-) mice. Both proteins were tyrosine phosphorylated and were recruited within Grb2 signaling complexes to LAT following TCR engagement. These results suggest that conserved molecular features of the Themis1 and Themis2 proteins are important for their biological activity and predict that Themis1 and Themis2 may perform similar functions in T and B cells, respectively.

Invariant NKT cell development requires a full complement of functional CD3 zeta immunoreceptor tyrosine-based activation motifs

Invariant NKT (iNKT) cells regulate early immune responses to infections, in part because of their rapid release of IFN-gamma and IL-4. iNKT cells are proposed to reduce the severity of Lyme disease following Borrelia burgdorferi infection. Unlike conventional T cells, iNKT cells express an invariant alphabeta TCR that recognizes lipids bound to the MHC class I-like molecule, CD1d. Furthermore, these cells are positively selected following TCR interactions with glycolipid/CD1d complexes expressed on CD4+CD8+ thymocytes. Whereas conventional T cell development can proceed with as few as 4/10 CD3 immunoreceptor tyrosine-based activation motifs (ITAMs), little is known about the ITAM requirements for iNKT cell selection and expansion. We analyzed iNKT cell development in CD3 zeta transgenic lines with various tyrosine-to-phenylalanine substitutions (YF) that eliminated the functions of the first (YF1,2), third (YF5,6), or all three (YF1-6) CD3 zeta ITAMs. iNKT cell numbers were significantly reduced in the thymus, spleen, and liver of all YF mice compared with wild type mice. The reduced numbers of iNKT cells resulted from significant reductions in the expression of the early growth response 2 and promyelocytic leukemia zinc finger transcription factors. In the mice with few to no iNKT cells, there was no difference in the severity of Lyme arthritis compared with wild type controls, following infections with the spirochete B. burgdorferi. These findings indicate that a full complement of functional CD3 zeta ITAMs is required for effective iNKT cell development.

Cutting Edge: Localization of Linker for Activation of T Cells to Lipid Rafts Is Not Essential in T Cell Activation and Development

It has been proposed that upon T cell activation, linker for activation of T cells (LAT), a transmembrane adaptor protein localized to lipid rafts, orchestrates formation of multiprotein complexes and activates signaling cascades in lipid rafts. However, whether lipid rafts really exist or function remains controversial. To address the importance of lipid rafts in LAT function, we generated a fusion protein to target LAT to nonraft fractions using the transmembrane domain from a nonraft protein, linker for activation of X cells (LAX). Surprisingly, this fusion protein functioned well in TCR signaling. It restored MAPK activation, calcium flux, and NFAT activation in LAT-deficient cells. To further study the function of this fusion protein in vivo, we generated transgenic mice that express this protein. Analysis of these mice indicated that it was fully capable of replacing LAT in thymocyte development and T cell function. Our results demonstrate that LAT localization to lipid rafts is not essential during normal T cell activation and development.

Evidence that the mouse 3' kappa light chain enhancer confers position-independent transgene expression in T- and B-lineage cells

One of the major obstacles for successful application of murine leukemia virus (MLV) vectors to genetic therapy of lymphocyte disorders is low levels of transgene expression or the eventual loss of expression. To overcome this problem, an improved retroviral vector was constructed utilizing the myeloproliferative sarcoma virus (MPSV) long terminal repeat (LTR), which provided a significantly higher level of transgene expression in human lymphoid cells than did MLV vectors. Nevertheless, transgene expression remained low in a large percentage of transduced cells. To address whether lymphocyte enhancer elements might improve transgene expression mediated by retroviral vectors in lymphocytes, we cloned the mouse immunoglobulin 3' kappa light chain enhancer gene (mE3') into the MPSV vector. We found that the mE3' conferred a higher, more uniform and sustained level of expression in transduced T- and B-cell lines, and in primary T cells, than did the control vector lacking this element. Integration sites were diverse and a single copy of the proviral genome was present in all examined transduced cells. The mE3' failed to enhance transgene expression in most nonlymphoid cells, indicating it is relatively lineage-specific. Taken together, these results provide strong evidence that the mE3' functions as a locus control region (LCR) in conferring enhanced integration-site-independent expression of a retroviral transgene.

Signaling through MHC in transgenic mice generates a population of memory phenotype cytolytic cells that lack TCR

We constructed a chimeric molecule, composed of the T-cell receptor (TCR)-zeta chain fused to the extracellular domains of a prototypical allogeneic major histocompatibility complex (MHC) class I molecule, Dd, to assess whether such a construct could affect Dd allospecific responses in vitro and in vivo. To generate cytotoxic T lymphocytes (CTLs) expressing the construct, Dd-zeta was targeted to lymphocyte populations in transgenic mice by placing its expression under control of the CD2 promoter. In response to ligation of Dd, lymphocytes from transgenic mice expressing high levels of Dd-zeta are activated to proliferate and kill cells binding to Dd, despite the near total loss of CD8+ T cells in these mice. Thus, the Dd-zeta cytolytic cell was found not to be a conventional CD8+ CTL, but rather an unusual T lineage cell (CD3-CD5+Thy1.1+) that lacked alphabeta or gammadelta TCRs, as well as CD4 and CD8 coreceptors, but expressed surface markers strikingly similar to memory CTLs, including CD44, Ly-6C, and CD122. These cells originate in the thymus and potently veto responses to Dd in vitro. Lacking TCRs, these veto cells are unlikely to mediate graft-versus-host disease (GVHD) and thus may be useful as a cellular therapy for therapeutic deletion of alloreactive T cells in the settings of graft rejection and GVHD.

Regulation of constitutive TCR internalization by the zeta-chain

The ability of a T cell to be activated is critically regulated by the number of TCRs expressed on the plasma membrane. Cell surface TCR expression is influenced by dynamic processes such as synthesis and transport of newly assembled receptors, endocytosis of surface TCR, and recycling to the plasma membrane of internalized receptors. In this study, the internalization of fluorescently labeled anti-TCR Abs was used to analyze constitutive endocytosis of TCRs on T cells, and to investigate the role of the zeta-chain in this process. We found that cell surface TCRs lacking zeta were endocytosed more rapidly than completely assembled receptors, and that reexpression of full-length zeta led to a dose-dependent decrease in the rate of TCR internalization. Rapid TCR internalization was also observed with CD4(+)CD8(+) thymocytes from zeta-deficient mice, whereas TCR internalization on thymocytes from CD3-delta deficient animals was slow, similar to that of wild-type thymocytes. This identifies a specific role for zeta in the regulation of constitutive receptor internalization. Furthermore, chimeric zeta molecules containing non-native intracellular amino acid sequences also led to high levels of TCR expression and reduced TCR cycling. These effects were dependent solely on the length of the intracellular tail, ruling out a role for intracellular zeta-specific interactions with other molecules as a mechanism for regulating TCR internalization. Rather, these findings strongly support a model in which the zeta-chain stabilizes TCR residency on the cell surface, and functions to maintain cell surface receptor expression by sterically blocking internalization sequences in other TCR components.

Imprint control element-mediated secondary methylation imprints at the Igf2/H19 locus

Understanding the molecular basis of monoallelic expression as observed at imprinted loci is helpful in understanding the mechanisms underlying epigenetic regulation. Genomic imprinting begins during gametogenesis with the establishment of epigenetic marks on the chromosomes such that paternal and maternal chromosomes are rendered distinct. During embryonic development, the primary imprint can lead to generation of secondary epigenetic modifications (secondary imprints) of the chromosomes. Eventually, either the primary imprints or the secondary imprints interfere with transcription, leading to parent-of-origin-dependent silencing of one of the two alleles. Here we investigated several aspects pertaining to the generation and functional necessity of secondary methylation imprints at the Igf2/H19 locus. At the H19 locus, these secondary imprints are, in fact, the signals mediating paternal chromosome-specific silencing of that gene. We first demonstrated that the H19 secondary methylation imprints are entirely stable through multiple cell divisions, even in the absence of the primary imprint. Second, we generated mouse mutations to determine which DNA sequences are important in mediating establishment and maintenance of the silent state of the paternal H19 allele. Finally, we analyzed the dependence of the methylation of Igf2DMR1 region on the primary methylation imprint about 90 kilobases away.

A potential role for CD69 in thymocyte emigration

The early activation marker, CD69, is transiently expressed on activated mature T cells and on thymocytes that are undergoing positive or negative selection in the thymus. CD69 is a member of the NK gene complex family of C-type lectin-like signaling receptors; however, its function is unknown. In this report, we describe the characterization of mice that constitutively express high levels of surface CD69 on immature and mature T cells throughout development. Constitutive surface expression of CD69 did not affect T cell maturation, signaling through the TCR or thymocyte selection. However, phenotypically and functionally mature thymocytes accumulated in the medulla of CD69 transgenic mice and failed to be exported from the thymus. The retention of mature thymocytes correlated with transgene dose and CD69 surface levels. These results identify a potential role for CD69 in controlling thymocyte export, and suggest that the transient expression of CD69 on thymocytes and T cells may function to regulate thymocyte and T cell trafficking.

Early TCRalpha expression generates TCRalphagamma complexes that signal the DN-to-DP transition and impair development

Clonotypic T cell receptor (TCR) genes undergo ordered rearrangement and expression in the thymus with the result that TCRalpha and TCRgamma proteins are not expressed in the same cell at the same time. Such "TCRalpha/gamma exclusion" is a feature of normal thymocyte differentiation, but it is abrogated in TCR-transgenic mice, which prematurely express transgenic TCRalpha proteins in early double-negative (DN) thymocytes. We report here that early expression of TCRalpha proteins results in the formation of TCRalphagamma complexes that efficiently signal the differentiation of DN into double-positive thymocytes independently of pre-TCR and TCRbeta expression. Thus, abrogation of TCRalpha/gamma exclusion by early TCRalpha expression results in the formation of isotypically mixed TCRalphagamma complexes whose in vivo signals circumvent TCRbeta selection and redirect thymocyte development along an aberrant developmental pathway.

Fine tuning of TCR signaling by CD5

Current data indicate that CD5 functions as an inhibitor of TCR signal transduction. Consistent with this role, thymocyte selection in TCR transgenic/CD5(-/-) mice is altered in a manner suggestive of enhanced TCR signaling. However, the impact of CD5 deletion on thymocyte selection varies depending on the transgenic TCR analyzed, ranging from a slight to a marked shift from positive toward negative selection. An explanation for the variable effect of CD5 on selection is suggested by the observation that CD5 surface expression is regulated by TCR signal intensity during development and CD5 surface levels on mature thymocytes and T cells parallel the avidity of the positively selecting TCR/MHC/ligand interaction. In this study, we generated mice that overexpress CD5 during thymocyte development (CD5-tg), and then examined the effect of CD5 overexpression or CD5 deletion (CD5(-/-)) on selection of thymocytes that express the same TCR transgenes. The results demonstrate that the effect on thymocyte selection of altering CD5 expression depends on the avidity of the selecting interaction and, consequently, the level of basal (endogenous) CD5 surface expression. Substitution of endogenous CD5 with a transgene encoding a truncated form of the protein failed to rescue the CD5(-/-) phenotype, demonstrating that the cytoplasmic domain of CD5 is required for its inhibitory function. Together, these results indicate that inducible regulation of CD5 surface expression during thymocyte selection functions to fine tune the TCR signaling response.

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

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

Critical requirement for the membrane-proximal cytosolic tyrosine residue for CD28-mediated costimulation in vivo

The YMNM motif that exists in the CD28 cytoplasmic domain is known as a binding site for phosphatidylinositol 3-kinase and Grb-2 and is considered to be important for CD28-mediated costimulation. To address the role of the YMNM motif in CD28 cosignaling in primary T cells, we generated transgenic mice on a CD28 null background that express a CD28 mutant lacking binding ability to phosphatidylinositol 3-kinase and Grb-2. After anti-CD3 and anti-CD28 Ab stimulation in vitro, the initial proliferative response and IL-2 secretion in CD28 Y189F transgenic T cells were severely compromised, while later responses were intact. In contrast to anti-CD3 and anti-CD28 Ab stimulation, PMA and anti-CD28 Ab stimulation failed to induce IL-2 production from CD28 Y189F transgenic T cells at any time point. Using the graft-vs-host reaction system, we assessed the role of the YMNM motif for CD28-mediated costimulation in vivo and found that CD28 Y189F transgenic spleen cells failed to engraft and could not induce acute graft-vs-host reaction. Together, these results suggest that the membrane-proximal tyrosine of CD28 is required for costimulation in vivo. Furthermore, these results indicate that the results from in vitro assays of CD28-mediated costimulation may not always correlate with T cell activation in vivo.

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

The contribution of the CD8beta subunit to CD8 coreceptor function is poorly understood. We now demonstrate that the CD8beta extracellular domain increases the avidity of CD8 binding to MHC I, and that the intracellular domain of CD8beta enhances association with two intracellular molecules required for TCR signal transduction, Lck and LAT. By assessing CD8+ T cell differentiation in CD8beta-deficient mice reconstituted with various transgenic CD8beta chimeric molecules, we also demonstrate that the intracellular and extracellular domains of CD8beta can contribute independently to CD8+ T cell development, but that both CD8beta domains together are most efficient. Thus, this study identifies the molecular functions of the CD8beta intracellular and extracellular domains and documents their contributions to CD8+ T cell development.

A role for the Tec family tyrosine kinase Txk in T cell activation and thymocyte selection

Recent data indicate that several members of the Tec family of protein tyrosine kinases function in antigen receptor signal transduction. Txk, a Tec family protein tyrosine kinase, is expressed in both immature and mature T cells and in mast cells. By overexpressing Txk in T cells throughout development, we found that Txk specifically augments the phospholipase C (PLC)-gamma1-mediated calcium signal transduction pathway upon T cell antigen receptor (TCR) engagement. Although Txk is structurally different from inducible T cell kinase (Itk), another Tec family member expressed in T cells, expression of the Txk transgene could partially rescue defects in positive selection and signaling in itk(-)(/)(-) mice. Conversely, in the itk(+/+) (wild-type) background, overexpression of Txk inhibited positive selection of TCR transgenic thymocytes, presumably due to induction of cell death. These results identify a role for Txk in TCR signal transduction, T cell development, and selection and suggest that the Tec family kinases Itk and Txk perform analogous functions.

Antigen Presentation Determines the Fate of the T Memory Response In Vivo After Sublethal Gamma-Irradiation

The survival of memory T cells is critical to vaccination strategies for infectious diseases and cancer, whereas their elimination may be crucial for treatment of autoimmune states. We examined the consequences of gamma-irradiation, which induces apoptosis of memory T cells in vitro, on the memory response to MHC class I alloantigen in vivo. Sublethal gamma-irradiation of primed mice eliminated accelerated rejection of skin allografts but failed to induce tolerance. Accelerated rejection was restored in irradiated mice by infusion of bone marrow cells expressing the priming alloantigen on immunostimulatory APCs (dendritic cells), whereas the memory response was not restored by infusion of bone marrow cells expressing the priming alloantigen on nonstimulatory APCs (B cells). Strikingly, irradiated mice infused with nonstimulatory bone marrow APCs exhibited long-term survival or tolerance to skin grafts expressing the priming MHC class I alloantigen. The mechanism of tolerance in this setting is explored.

Involvement of the SHP-1 Tyrosine Phosphatase in Regulation of T Cell Selection

The selection events shaping T cell development in the thymus represent the outcome of TCR-driven intracellular signaling cascades evoked by Ag receptor interaction with cognate ligand. In view of data indicating TCR-evoked thymocyte proliferation to be negatively modulated by the SHP-1 tyrosine phosphatase, a potential role for SHP-1 in regulating selection processes was investigated by analysis of T cell development in H-Y TCR transgenic mice rendered SHP-1 deficient by introduction of the viable motheaten mutation or a dominant negative SHP-1-encoding transgene. Characterization of thymocyte and peripheral T cell populations in H-Y TCR-viable motheaten mice revealed TCR-evoked proliferation as well as the positive and negative selection of H-Y-specific thymocytes to be enhanced in these mice, thus implicating SHP-1 in the negative regulation of each of these processes. T cell selection processes were also augmented in H-Y TCR mice carrying a transgene driving lymphoid-restricted expression of a catalytically inert, dominant-negative form of SHP-1. SHP-1-negative effects on thymocyte TCR signaling were not influenced by co-cross-linking of the CD28 costimulatory and/or CTLA-4 inhibitory receptors and appear, accordingly, to be realized independently of these comodulators. These observations indicate that SHP-1 raises the signaling threshold required for both positive and negative selection and reveal the inhibitory effects of SHP-1 on TCR signaling to be cell autonomous. The demonstrated capacity for SHP-1 to inhibit TCR-evoked proliferation and selection indicate SHP-1 modulatory effects on the magnitude of TCR-generated signal to be a key factor in determining the cellular consequences of TCR-ligand interaction.

The Extracellular Domain of the ζ-Chain Is Essential for TCR Function

The ζ-chain homodimer is a key component in the TCR complex and exerts its function through its cytoplasmic immunoreceptor-tyrosine activation motif (1). The ζ-chain extracellular (EC) domain is highly conserved; however, its functional and structural contributions to the TCR signaling have not been elucidated. We show that the EC domain of the ζ homodimer is essential for TCR surface expression. To gain a more detailed structural and functional information about the ζ-chain EC domain, we applied a cysteine scanning mutagenesis to conserved amino acids of the short domain. The results showed that the interchain disulfide bridge can be displaced by seven or eight amino acids along the EC domain. The TCR signaling efficacy was dramatically reduced during peptide/MHC engagement in the ζ mutants containing the displaced disulfide bond. These signaling defective ζ mutants produced an unconventional early tyrosine phosphorylation pattern. While the tyrosine phosphorylated forms of ζ (p21 and p23) could be observed during Ag stimulation, downstream signaling events such as the generation of phospho-p36, higher m.w. forms of phospho-ζ, and phospho-ζ/ZAP-70 complexes were impaired. Together these results suggest an important function of the phylogenetically conserved ζ-EC domain.

Specific requirement for CD3epsilon in T cell development

T cell antigen receptor (TCR) and pre-TCR complexes are composed of clonotypic heterodimers in association with dimers of signal transducing invariant subunits (CD3gamma, -delta, -epsilon, and zeta). The role of individual invariant subunits in T cell development has been investigated by generating gene-specific mutations in mice. Mutation of CD3gamma, -delta, or zeta results in an incomplete block in development, characterized by reduced numbers of mature T cells that express low levels of TCR. In contrast, mature T cells are absent from CD3epsilon-/- mice, and thymocyte development is arrested at the early CD4(-)CD8(-) stage. Although these results suggest that CD3epsilon is essential for pre-TCR and TCR expression/function, their interpretation is complicated by the fact that expression of the CD3gamma and CD3delta genes also is reduced in CD3epsilon-/- mice. Thus, it is unclear whether the phenotype of CD3epsilon-/- mice reflects the collective effects of CD3gamma, -delta, and -epsilon deficiency. By removing the selectable marker (PGK-NEO) from the targeted CD3epsilon gene via Cre/loxP-mediated recombination, we generated mice that lack CD3epsilon yet retain normal expression of the closely linked CD3gamma and CD3delta genes. These (CD3epsilonDelta/Delta) mice exhibited an early arrest in T cell development, similar to that of CD3epsilon-/- mice. Moreover, the developmental defect could be rescued by expression of a CD3epsilon transgene. These results identify an essential role for CD3epsilon in T cell development not shared by the CD3gamma, CD3delta, or zeta-family proteins and provide further evidence that PGK-NEO can influence the expression of genes in its proximity.

Cutting Edge: Induction of Antigen-Specific Hyporesponsiveness by Transplantation of Hemopoietic Cells Containing an MHC Class I Transgene Regulated by a Lymphocyte-Specific Promoter

We explored a novel approach to tolerance induction by the transplantation of bone marrow (BM) cells (BMCs) that themselves do not express a foreign histocompatibility Ag, but which give rise to mature lymphocytes that do so. Lines of transgenic (FVB) mice were generated that contained an MHC class I Dd cDNA regulated by a CD2 promoter. Because the CD2 promoter is lymphocyte-specific and activated relatively late in lymphocyte ontogeny, Dd is expressed on most mature lymphocytes in the periphery but only on developing B cells in the BM of transgenic mice. Transgenic BMCs are tolerogenic and reproducibly engraft in nontransgenic mice using a conditioning regimen that is nonpermissive for the engraftment of conventional (MHC promoter) Dd-transgenic BMCs. Engrafted BMCs generate transgene-expressing lymphocytes and confer a state of Ag-specific hyporesponsiveness on the host that is primarily attributable to a peripheral mechanism. The strategies by which tolerance can be optimized in this system are discussed.

Elf-1 Regulates Basal Expression from the T Cell Antigen Receptor ζ-Chain Gene Promoter

In mature T cells, limited synthesis of the TCR-ζ subunit is primarily responsible for regulating surface expression of TCRs. Transcription of ζ is directed by a complex promoter that includes two potential binding sites for the Ets family of transcription factors at −52 (zEBS1) and −135 (zEBS2). Mutation of these two sites results in a marked reduction of transcription from this promoter. Using electrophoretic mobility shift analysis, Elf-1 was demonstrated to be the Ets family member that binds to these sites. One site, zEBS1, matches the optimal Elf-1 consensus sequence in eight of nine bases, making it the best match of any known mammalian Elf-1 binding site. A role for Elf-1 in TCR-ζ trans-activation was confirmed by ectopic expression of Elf-1 in COS-7 cells. This resulted in an increase in TCR-ζ promoter activity that mapped to zEBS1 and zEBS2. Additional support for the involvement of Elf-1 in TCR-ζ trans-activation derives from the finding that a GAL4-Elf-1 fusion protein trans-activated TCR-ζ promoter constructs that had been modified to contain GAL4 DNA binding sites. These results demonstrate that Elf-1 plays an essential role in the trans-activation of a constitutively expressed T cell-specific gene, and that trans-activation occurs in the context of the native promoter in both lymphoid and nonlymphoid cells. Taken together with the existing literature, these data also suggest that the requirement for inducible factors in Elf-1-mediated trans-activation may decrease as the affinity and number of Elf-1 sites increase.

Variable expression of CD3-zeta and associated protein tyrosine kinases in lymphocytes from patients with myeloid malignancies

In myeloid malignancies, T-cell and NK function has been shown to deteriorate with transformation from pre-leukaemia to advanced disease. Immune dysfunction in solid tumours has been attributed to abnormal signal transduction, possibly through altered expression of intracellular components of the TCR/CD3 complex (e.g. CD3-zeta), receptors on NK cells and their associated protein tyrosine kinases (PTKs; p56lck, p59fyn and ZAP-70). Using a flow cytometric method to detect dual-expression of surface proteins and intracellular components of the TCR/CD3 complex, we have studied 46 patients with myeloid malignancies. CD3-zeta expression was abnormal in 64% of patients, and was more prominent in those with advanced disease. Three patients with reduced CD3-zeta were analysed both pre- and post-treatment, and recovery of CD3-zeta expression was associated with successful remission induction (expression of PTKs was variable and reduced levels were seen all disease stages). The results of this study suggest that loss of signalling proteins is not a result of direct contact of leukaemic cells with lymphocytes per se or the extent of the leukaemia burden, but to a specific property of some myeloid malignancies, which is more frequently acquired with greater malignant transformation.

Changes in expression of signal transduction proteins in T lymphocytes of patients with leprosy

Advanced stages of mycobacterial diseases such as leprosy and tuberculosis are characterized by a loss of T-cell function. The basis of this T-cell dysfunction is not well understood. The present report demonstrates major alterations in the expression of signal transduction molecules in T cells of leprosy patients. These alterations were most frequently observed in lepromatous leprosy (LL) patients. Of 29 LL patients, 69% had decreased T-cell receptor zeta-chain expression, 48% had decreased p56(lck) tyrosine kinase, and 63% had a loss of nuclear transcription factor NF-kappaB p65. An electrophoretic mobility shift assay with the gamma interferon core promoter region revealed a loss of the Th1 DNA-binding pattern in LL patients. In contrast, tuberculoid leprosy patients had only minor signal transduction alterations. These novel findings might improve our understanding of the T-cell dysfunction observed in leprosy and other infectious diseases and consequently might lead to better immunologic evaluation of patients.

Delayed maturation of CD4- CD8- Fc gamma RII/III+ T and natural killer cell precursors in Fc epsilon RI gamma transgenic mice

Fc epsilon RI gamma (gamma) is a member of a group of related proteins (the zeta-family dimers) that function as signal-transducing components of both Fc receptors and the T cell antigen receptor (TCR). Analysis of gamma expression during fetal thymus ontogeny revealed that it is expressed in early thymocytes, before the initiation of clonotypic TCR-alpha and TCR-beta gene rearrangement but is down-regulated in most adult thymocytes. To explore a possible role for gamma in thymocyte development, we generated transgenic mice in which this protein was overexpressed at all stages of ontogeny. Overexpression of gamma inhibited the maturation of T cells as well as natural killer (NK) cells. The developmental effects were transgene dose related and correlated with markedly delayed maturation of fetal CD4-CD8- FcRII/III+ thymocytes, cells thought to include the progenitors of both T and NK cells. These results suggest that the zeta and gamma chains serve distinctive functions in thymocyte development and indicate that Fc receptor(s) may play an important role in regulating the differentiation of early progenitor cells within the thymus.

Altered peptide ligand-induced partial T cell activation: molecular mechanisms and role in T cell biology

The elucidation of the phenomena of T cell antagonism and partial activation by altered peptide ligands has necessitated a revision in the traditional concepts of TCR recognition of antigen and subsequent signal transduction. Whereas previous models supported a single ligand specificity for any particular T cell, many studies using analogs of immunogenic peptides have now demonstrated a flexibility in this recognition. Moreover, interaction with such altered peptide ligands can result in dramatically different phenotypes of the T cells, ranging from inducing selective stimulatory functions to completely turning off their functional capacity. Investigations of the biochemical basis leading to these phenotypes have shown that altered peptide ligands can induce a qualitatively different pattern of signal transduction events than does any concentration of the native ligand. Such observations imply that several signaling modules are directly linked to the TCR/CD3 complex and that they can be dissociated from each other as a direct result of the nature of the ligand bound. Interestingly, many in vivo models of T cell activation are compatible with a selective signaling model, and several studies have shown that peptide analogs can play a role in various T cell biologic phenomena. These data strongly suggest that naturally occurring altered peptide ligands for any TCR exist in the repertoire of self-peptides or, in nature, derived from pathogens, and recent reports provide compelling evidence that this is indeed the case. The concept of altered peptide ligands, their effects on T cell signaling, the hypothesized mechanisms by which they exert their effects, and their possible roles in shaping the T cell immune response are the scope of this review.

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.

Developmental arrest of NK1.1+ T cell antigen receptor (TCR)-alpha/beta+ T cells and expansion of NK1.1+ TCR-gamma/delta+ T cell development in CD3 zeta-deficient mice

The relationship between the structure of the T cell antigen receptor (TCR)-CD3 complex and development of NK1.1+ T cells was investigated. The TCR complex of freshly isolated NK1.1+ TCR-alpha/beta+ thymocytes contained CD3 zeta homodimers and CD zeta-FcR gamma heterodimers, whereas that of the majority of NK1.1- T cells did not contain FcR gamma. The function of CD3 zeta and FcR gamma in the development of NK1.1+ T cells was determined by analyzing CD3 zeta- and FcR gamma-deficient mice. The NK1.1+ T cells from wild-type and CD3 zeta-deficient mice had equal levels of CD3 expression. However, the development of NK1.1+ TCR-alpha/beta+ T cells was almost completely disrupted in thymus and spleen in CD3 zeta-deficient mice, whereas no alteration was observed in FcR gamma-deficient mice. In contrast, the number of novel NK1.1+ TCR-gamma/delta+ thymocytes expressing a surface phenotype similar to NK1.1+ TCR-alpha/beta+ thymocytes increased approximately six times in CD3 zeta-deficient mice. These findings establish the distinct roles of the CD3 zeta chain in the development of the following different thymic T cell compartments: NK1.1- TCR+, NK1.1+ TCR-alpha/beta+, and NK1.1+ TCR-gamma/delta+ thymocytes, which cannot be replaced by CD3 eta or FcR gamma chains.

The trouble with transgenic mice

Observations on many antigen-receptor transgenic models with anti-self specificities have been interpreted as proof for clonal deletion or for mechanisms involved in clonal deletion. At the same time, there is increasing evidence that many lymphocytes that recognize self exist, are activated and produce end products, even in individuals without clinical autoimmunity. Except perhaps for the amount of anti-self activity, there is little agreement on what distinguishes immune products normally recognizing self from those associated with clinical autoimmunity. To resolve this paradox, the tendency in immunology is to accept conclusions from transgenic models as normal, while judging those from the unmanipulated state as suspect. However, transgenics have a major weakness. Transgenes encoding antigen-receptors are derived from highly selected mature lymphocytes and are expressed in developing lymphocytes that normally do not display the antigen-receptors of mature lymphocytes. Such precocious expression of antigen-receptors could have profound abnormal effects on lymphocyte development. Other transgenic models suggest that processes in lymphocyte differentiation not involving antigen-receptor binding specificity exert powerful influences on lymphocyte development; therefore, mechanisms other than classical positive and negative selection are important.

Defective lymphoid development in mice lacking expression of the common cytokine receptor gamma chain

The common gamma chain (gamma c) of the IL-2, IL-4, IL-7, IL-9, and IL-15 receptors is defective in humans with XSCID. Mice lacking gamma c expression had hypoplastic thymuses; the thymocytes responded to gamma c-independent mitogens, but not gamma c-dependent stimuli. Splenic T cells were diminished at 3 weeks of age, but CD4+ T cells markedly increased by 4 weeks. B cells were greatly diminished in contrast with the situation in XSCID. NK cells, gamma delta intestinal intraepithelial lymphocytes, dendritic epidermal T cells, peripheral lymph nodes, and gut-associated lymphoid tissue were absent. These findings underscore the importance of gamma c in lymphoid development. Moreover, differences in humans and mice lacking gamma c expression indicate species-specific differences in the roles of gamma c-dependent cytokines or in the existence of redundant pathways. These mice provide an important model for studying the pathophysiology provide an important model for studying the pathophysiology of and gene therapy for human XSCID.

T cell development in CD3-zeta mutant mice

Increasing evidence points to multiple pathways of T lymphocyte development. The well characterized thymus-dependent pathway gives rise to T cells bearing TCR alpha beta heterodimers and either CD4 or CD8 alpha beta co-receptors. T cells of this lineage populate peripheral lymphoid compartments including lymph nodes, spleen, skin, and Peyer's patches. By comparison, factors which govern extrathymic T cell development are poorly understood. A variety of experiments have shown that intestinal intraepithelial lymphocytes (IELs) develop outside of the thymic environment, e.g., in the gut of nude, SCID, and beta 2m-/- mutant mice, and after transplanting bone marrow or fetal liver cells into irradiated thymectomized adult mice. This review focuses on the role of the CD3-zeta subunit in the development of both thymically and extrathymically derived T cells as determined by gene-targeting experiments in mice. Data from these and other T cell-related mutations continue to define crucial stages in thymocyte differentiation. Most interestingly, CD3-zeta mutant mice contain a unique population of intestinal IELs that develops independently of thymic selective processes and expresses a novel TCR/CD3 complex.

The B cell antigen receptor complex: mechanisms and implications of tyrosine kinase activation

The B cell receptor is a multimeric receptor complex whose constituent chains appear to mediate distinct and possibly interrelated functions. In this review we have focused on how one chain, immunoglobulin (Ig)-alpha, may function to activate tyrosine kinases and the consequences of that activation. The cytoplasmic domain of Ig-alpha contains a consensus sequence, the antigen recognition homology 1 (ARH 1) motif, which is found in Ig-beta and other antigen recognition receptor associated chains. We argue that this conserved structure reflects an underlying conserved mechanism of secondary effector activation. Our data also indicates that the specificity of each motif (i.e., the elements which restrict secondary effector binding to particular motifs) is encoded within divergent sequences found in each ARH 1 motif. In the particular case of kinase activation by Ig-alpha, the subsequent phosphorylation of multiple tyrosines on Ig-alpha, Ig-beta, CD19, CD22 and possibly other functionally related chains form recruitment sites for a myriad of secondary signal transducers. In this model, proximal tyrosine kinases and phosphatases do not function so much to mediate the linear transfer of information as to establish and modulate an interrelated network of signal transducers capable of driving complicated cellular responses.

Role of TCR zeta chain in T cell development and selection

Signals mediated by the T cell receptor (TCR) are required for thymocyte maturation and selection. To examine the role of TCR zeta chain signals in development, TCR expression was restored in zeta-deficient mice with transgenic zeta chains that partially or completely lacked sequences required for signal transduction. The zeta chain played a role in thymic development by promoting TCR surface expression, but zeta-mediated signals were not essential because TCRs that contained signaling-deficient zeta chains promoted T cell maturation and transduced signals associated with thymic selection.

Transcriptional regulation of the T cell antigen receptor zeta subunit: identification of a tissue-restricted promoter

The cell surface expression of T cell antigen receptors (TCR) is regulated in part by the limiting synthesis of the zeta subunit. Utilizing fragments from the 5' region of the human zeta gene, two discrete regions that promote transcription were characterized. Both of these elements are located within 125 bases of the most 3' site of transcription initiation. The more proximal (3') promoter exhibits activity in lymphoid as well as nonlymphoid cells. In contrast, the more distal (5') promoter element functions in a tissue-restricted fashion. The tissue-specific promoter is localized to a 29-base fragment. The sequence of this region is remarkable for a stretch of 11 consecutive purines that are required for activity. This element constitutes the only known tissue-specific promoter for an invariant TCR subunit. Consistent with the unique role served by the zeta subunit in assembly of the TCR, this study demonstrates that the expression of the zeta gene is regulated in a fashion distinct from other TCR components.