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

Analysis of transmembrane signalling and T cell defects associated with idiopathic thrombocytopenic purpura (ITP)

Adult chronic idiopathic thrombocytopenic purpura (ITP) is an autoimmune disease characterized by production of autoreactive antibodies to platelet antigens. It is now becoming clear that autoantibody production, in general, is regulated by T helper (Th) cells. Several recent studies have examined potential defects in T cell function in this disease and have demonstrated that patients with ITP possess abnormal lymphocyte activation and Th1/Th2-mediated cytokine production. Although the underlying cause(s) of aberrant T cell function in this disease are not known, studies from other models of autoimmune disease indicate that defects in T cell transmembrane signalling can be causally linked to abnormal T cell activation and cytokine production. This review will present some of the major T cell signalling pathways and discuss how altered T cell signalling may be linked to autoimmunity with an emphasis on ITP. Recent preliminary findings of a potential defect in the signal transduction apparatus in lymphocytes from three patients with ITP will also be presented.

Signal transduction during NK cell activation: balancing opposing forces

Significant progress has been made in our understanding of the basic signaling mechanisms regulating NK cell activation. Advances have been fueled in part by the molecular characterization of specific activating receptors (e.g., the Fc gamma RIII multi-subunit complex) and inhibitory receptors (e.g., novel MHC-recognizing inhibitory receptors). However, certain aspects of these analyses are complicated by the heterogeneous nature of the receptor-ligand interactions utilized during the development of a cytotoxic response. Future advances will depend in part on the further molecular characterization of the involved receptors and second messengers and on the development of experimental models for genetically manipulating the signaling elements. It will remain important to understand both activating and inhibitory signaling pathways as the emerging theme is that the balance of these two opposing forces determines the functional outcome of an NK cells interaction with its target.

T cell receptor-initiated calcium release is uncoupled from capacitative calcium entry in Itk-deficient T cells

Itk, a Tec family tyrosine kinase, plays an important but as yet undefined role in T cell receptor (TCR) signaling. Here we show that T cells from Itk-deficient mice have a TCR-proximal signaling defect, resulting in defective interleukin 2 secretion. Upon TCR stimulation, Itk-/- T cells release normal amounts of calcium from intracellular stores, but fail to open plasma membrane calcium channels. Since thapsigargin-induced store depletion triggers normal calcium entry in Itk-/- T cells, an impaired biochemical link between store depletion and channel opening is unlikely to be responsible for this defect. Biochemical studies indicate that TCR-induced inositol 1,4,5 tris-phosphate (IP3) generation and phospholipase C gamma1 tyrosine phosphorylation are substantially reduced in Itk-/- T cells. In contrast, TCR-zeta and ZAP-70 are phosphorylated normally, suggesting that Itk functions downstream of, or in parallel to, ZAP-70 to facilitate TCR-induced IP3 production. These findings support a model in which quantitative differences in cytosolic IP3 trigger distinct responses, and in which only high concentrations of IP3 trigger the influx of extracellular calcium.

Genetic evidence of a role for Lck in T-cell receptor function independent or downstream of ZAP-70/Syk protein tyrosine kinases

T-cell antigen receptor (TCR) engagement results in sequential activation of the Src protein tyrosine kinases (PTKs) Lck and Fyn and the Syk PTKs, ZAP-70 and Syk. While the Src PTKs mediate the phosphorylation of TCR-associated signaling subunits and the phosphorylation and activation of the Syk PTKs, the lack of a constitutively active Syk PTK has prohibited the analysis of Lck function downstream of these initiating signaling events. We describe here the generation of an activated Syk family PTK by substituting the kinase domain of Syk for the homologous region in ZAP-70 (designated as KS for kinase swap). Expression of the KS chimera resulted in its autophosphorylation, the phosphorylation of cellular proteins, the upregulation of T-cell activation markers, and the induction of interleukin-2 gene synthesis in a TCR-independent fashion. The KS chimera and downstream ZAP-70 or Syk substrates, such as SLP-76, were still phosphorylated when expressed in Lck-deficient JCaM1.6 T cells. However, expression of the KS chimera in JCaM1.6 cells failed to rescue downstream signaling events, demonstrating a functional role for Lck beyond the activation of the ZAP-70 and Syk PTKs. These results indicate that downstream TCR signaling pathways may be differentially regulated by ZAP-70 and Lck PTKs and provide a mechanism by which effector functions may be selectively activated in response to TCR stimulation.

Analysis of antigen receptor signalling using mouse gene targeting

Gene targeting in mice has enabled the study of antigen receptor signalling in primary lymphocytes. Furthermore, it has provided the tools to directly assess the function of individual signalling proteins by mutation of the genes that code for them. Some of the results that gene targeting has produced have confirmed previous views of the function of particular proteins. Others have given surprising results and overturned accepted viewpoints.

Genetic evidence for differential coupling of Syk family kinases to the T-cell receptor: reconstitution studies in a ZAP-70-deficient Jurkat T-cell line

T-cell antigen receptor (TCR) engagement activates multiple protein tyrosine kinases (PTKs), including the Src family member, Lck, and the Syk-related PTK, ZAP-70. Studies in ZAP-70-deficient humans have demonstrated that ZAP-70 plays crucial roles in T-cell activation and development. However, progress toward a detailed understanding of the regulation and function of ZAP-70 during TCR signaling has been hampered by the lack of a suitable T-cell model for biochemical and genetic analyses. In this report, we describe the isolation and phenotypic characterization of a Syk- and ZAP-70-negative somatic mutant derived from the Jurkat T-cell line. The P116 cell line displays severe defects in TCR-induced signaling functions, including protein tyrosine phosphorylation, intracellular Ca2+ mobilization, and interleukin-2 promoter-driven transcription. These signaling defects were fully reversed by reintroduction of catalytically active versions of either Syk or ZAP-70 into the P116 cells. However, in contrast to ZAP-70 expression, Syk expression triggered a significant degree of cellular activation in the absence of TCR ligation. Transfection experiments with ZAP-70-Syk chimeric proteins indicated that both the amino-terminal regulatory regions and the carboxy-terminal catalytic domains of Syk and ZAP-70 contribute to the distinctive functional properties of these PTKs. These studies underscore the crucial role of ZAP-70 in TCR signaling and offer a powerful genetic model for further analyses of ZAP-70 regulation and function in T cells.

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.

Cytoskeletal polarization of T cells is regulated by an immunoreceptor tyrosine-based activation motif-dependent mechanism

Binding of a T cell to an appropriate antigen-presenting cell (APC) induces the rapid reorientation of the T cell cytoskeleton and secretory apparatus towards the cell-cell contact site in a T cell antigen receptor (TCR) and peptide/major histocompatibility complex-dependent process. Such T cell polarization directs the delivery of cytokines and cytotoxic mediators towards the APC and contributes to the highly selective and specific action of effector T cells. To study the signaling pathways that regulate cytoskeletal rearrangements in T lymphocytes, we set up a conjugate formation assay using Jurkat T cells as effectors and cell-sized latex beads coated with various antibodies as artificial APCs. Here, we report that beads coated with antibodies specific for the TCR-CD3 complex were sufficient to induce T cell polarization towards the bead attachment site, as judged by reorientation of the microtubule-organizing center (MTOC) and localized actin polymerization. Thus, these cytoskeletal changes did not depend on activation of additional coreceptors. Moreover, single subunits of the TCR complex, namely TCR-zeta and CD3epsilon, were equally effective in inducing cytoskeletal polarization. However, mutagenesis of the immunoreceptor tyrosine-based activation motifs (ITAMs), present three times in TCR-zeta and once in CD3epsilon, revealed that the induction of cytoskeletal rearrangements required the presence of at least one intact ITAM. In agreement with this result, lack of functional Lck, the protein tyrosine kinase responsible for ITAM phosphorylation, abolished both MTOC reorientation and polarized actin polymerization. Both inhibitor and transient overexpression studies demonstrated that MTOC reorientation could occur in the absence of Ras activation. Our results suggest that APC-induced T cell polarization is a TCR-mediated event that is coupled to the TCR by the same signaling motif as TCR-induced gene activation, but diverges in its distal signaling requirements.

The gene for severe combined immunodeficiency disease in Athabascan-speaking Native Americans is located on chromosome 10p

Severe combined immunodeficiency disease (SCID) consists of a group of heterogeneous genetic disorders. The most severe phenotype, T-B- SCID, is inherited as an autosomal recessive trait and is characterized by a profound deficiency of both T cell and B cell immunity. There is a uniquely high frequency of T-B- SCID among Athabascan-speaking Native Americans (A-SCID). To localize the A-SCID gene, we conducted a genomewide search, using linkage analysis of approximately 300 microsatellite markers in 14 affected Athabascan-speaking Native American families. We obtained conclusive evidence for linkage of the A-SCID locus to markers on chromosome 10p. The maximum pairwise LOD scores 4.53 and 4.60 were obtained from two adjacent markers, D10S191 and D10S1653, respectively, at a recombination fraction of straight theta=.00. Recombination events placed the gene in an interval of approximately 6.5 cM flanked by D10S1664 and D10S674. Multipoint analysis positioned the gene for the A-SCID phenotype between D10S191 and D10S1653, with a peak LOD score of 5.10 at D10S191. Strong linkage disequilibrium was found in five linked markers spanning approximately 6.5 cM in the candidate region, suggesting a founder effect with an ancestral mutation that occurred sometime before 1300 A.D.

Regulation of TCR Signal Transduction in Murine Thymocytes by Multiple TCR ζ-Chain Signaling Motifs

The αβ TCR is a multimeric protein complex comprising ligand-binding and signal-transducing subunits. The signal transduction processes are mediated by the immunoreceptor tyrosine-based activation motifs (ITAMs), and up to 10 ITAMs are present within a single TCR complex. This multiplicity may allow for signal amplification and/or the formation of qualitatively distinct intracellular signals. Notably, the TCR-ζ subunit contains three ITAMs, and exists as a disulfide-linked homodimer in the TCR complex. In normal murine thymocytes and peripheral T cells, a proportion of TCR-ζ molecules is constitutively tyrosine phosphorylated and associated with the ZAP-70 protein tyrosine kinase. We examined the contribution of the different TCR-ζ ITAMs in regulating the constitutive phosphorylation of the TCR-ζ subunit in thymocytes by analyzing TCR-ζ-deficient mice that had been reconstituted with either full-length or single ITAM-containing TCR-ζ subunits. We report in this work that in the absence of a full-length TCR-ζ subunit, there is no apparent constitutive phosphorylation of the remaining TCR/CD3 ITAMs. Following TCR ligation, all of the CD3 ITAMs become inducibly phosphorylated and associate with the ZAP-70 protein tyrosine kinase. Regardless of the number of TCR-ζ ITAMs present in the TCR complex, we report that a number of molecules involved in downstream signaling events, such as ZAP-70, SLP-76, and pp36, are all inducibly tyrosine phosphorylated following TCR ligation. These results support the notion that the different TCR ITAMs function in a quantitative rather than qualitative manner.

Functional role for Syk tyrosine kinase in natural killer cell-mediated natural cytotoxicity

Natural killer (NK) cells are named based on their natural cytotoxic activity against a variety of target cells. However, the mechanisms by which sensitive targets activate killing have been difficult to study due to the lack of a prototypic NK cell triggering receptor. Pharmacologic evidence has implicated protein tyrosine kinases (PTKs) in natural killing; however, Lck-deficient, Fyn-deficient, and ZAP-70-deficient mice do not exhibit defects in natural killing despite demonstrable defects in T cell function. This discrepancy implies the involvement of other tyrosine kinases. Here, using combined biochemical, pharmacologic, and genetic approaches, we demonstrate a central role for the PTK Syk in natural cytotoxicity. Biochemical analyses indicate that Syk is tyrosine phosphorylated after stimulation with a panel of NK-sensitive target cells. Pharmacologic exposure to piceatannol, a known Syk family kinase inhibitor, inhibits natural cytotoxicity. In addition, gene transfer of dominant-negative forms of Syk to NK cells inhibits natural cytotoxicity. Furthermore, sensitive targets that are rendered NK-resistant by major histocompatibility complex (MHC) class I transfection no longer activate Syk. These data suggest that Syk activation is an early and requisite signaling event in the development of natural cytotoxicity directed against a variety of cellular targets.

A dual participation of ZAP-70 and scr protein tyrosine kinases is required for TCR-induced tyrosine phosphorylation of Sam68 in Jurkat T cells

Sam68 has been initially described as a substrate of src kinases during mitosis in fibroblasts. Recent evidence suggests that in T lymphocytes Sam68 may act as an adaptor protein and participate in the early biochemical cascade triggered after CD3 stimulation. A direct interaction between Sam68 and the two src kinases involved in T cell activation, p59(fyn) and p56(lck), as well as a partnership of Sam68 with various key downstream signaling molecules, like phospholipase Cgamma-1 and Grb2, has been shown. In this study we analyze the contribution of p56(lck), as well as the role of ZAP-70, the second class of protein tyrosine kinase involved in T cell activation, in Sam68 tyrosine phosphorylation in the human Jurkat T cell line. Using the src inhibitor PP1 [4-amino-5-(4-methylphenyl)7-(t-butyl) pyrazolo [3,4-d] pyrymidine] and cell variants with defective expression of p56(lck) or expressing a dominant negative form of ZAP-70, we demonstrate that, while both p56(lck) and ZAP-70 are dispensable for the low constitutive phosphorylation of Sam68 observed in Jurkat cells, a cooperation between the two kinases is required to increase its rapid phosphorylation observed in vivo after CD3 stimulation. We also show that recombinant forms of both p56(lck) and ZAP-70 phosphorylate Sam68 in vitro. However, using CD2 stimulated cells, we observe that p56(lck) activation by itself does not induce Sam68 tyrosine phosphorylation. We conclude that p59(fyn) and p56(lck) differently participate in regulating the phosphorylation state of Sam68 in T cells and that ZAP-70 may contribute to Sam68 tyrosine phosphorylation and to the specific recruitment of this molecule after CD3 stimulation.

Regulation of ZAP-70 intracellular localization: visualization with the green fluorescent protein

To investigate the cellular dynamics of ZAP-70, we have studied the distribution and regulation of its intracellular location using a ZAP-70 green fluorescent protein chimera. Initial experiments in epithelial cells indicated that ZAP-70 is diffusely located throughout the quiescent cell, and accumulates at the plasma membrane upon cellular activation, a phenotype enhanced by the coexpression of Lck and the initiation of ZAP-70 kinase activity. Subsequent studies in T cells confirmed this phenotype. Intriguingly, a large amount of ZAP-70, both chimeric and endogenous, resides in the nucleus of quiescent and activated cells. Nuclear ZAP-70 becomes tyrosine phosphorylated upon stimulation via the T cell receptor, indicating that it may have an important biologic function.

Analysis of immunoreceptor tyrosine-based activation motif (ITAM) binding to ZAP-70 by surface plasmon resonance

The signaling function of the T cell antigen receptor (TCR) is mediated via CD3 polypeptides, the cytoplasmic sequences of which bear conserved immunoreceptor tyrosine-based activation motifs (ITAM). ITAM are defined by two YxxL/I sequences separated by a six-eight amino acid long spacer. Upon antigen recognition, ITAM become phosphorylated on both tyrosine residues, creating a high affinity binding site for the tandem SH2 domains found in the protein tyrosine kinase ZAP-70. Using surface plasmon resonance, we further dissected the sequences required for the binding of ZAP-70 to each TCR-associated ITAM. First, we generated protein tyrosine phosphatase-resistant ITAM peptide analogs, in which difluorophosphonomethyl phenylalanyl (F2p) replaced both phosphotyrosines, and showed that those protein tyrosine phosphatase-resistant analogs bind ZAP-70 with high affinity, establishing a rational strategy for the design of novel pharmacological tools capable of interfering with TCR signaling function. Second, we substituted the five amino acids separating the two YxxL/I sequences of the CD3 zeta 1 ITAM with a non-peptidic linker made up of gamma-amino butyric acid units and demonstrated that the length of this intervening sequence rather than its chemical composition is essential for high affinity binding of phosphorylated ITAM to the ZAP-70 SH2 domains.

An enhancer that directs lineage-specific expression of CD8 in positively selected thymocytes and mature T cells

Positive selection of CD4+CD8+ T cells to the CD4+CD8- helper and CD4- CD8+ cytotoxic lineages is a multistep process that involves complex regulation of coreceptor gene expression. By analyzing expression of a reporter gene in transgenic mice, we have identified a DNA segment, located between the murine CD8beta and CD8alpha genes, that has enhancer activity restricted to CD8 lineage cells. Remarkably, this enhancer functions in thymocytes undergoing positive selection to the CD4-CD8+ phenotype but not in immature double-positive thymocytes. The enhancer also functions in gut intraepithelial lymphocytes that express CD8alpha but not CD8beta, suggesting that it is specific for CD8alpha expression. The tight correlation between activation of this enhancer and the final step in positive selection has important implications for understanding the mechanism of lineage commitment in thymocytes.

Evidence that calcineurin is rate-limiting for primary human lymphocyte activation

Cyclosporine (CsA) is both a clinical immunosuppressive drug and a probe to dissect intracellular signaling pathways. In vitro, CsA inhibits lymphocyte gene activation by inhibiting the phosphatase activity of calcineurin (CN). In clinical use, CsA treatment inhibits 50-75% of CN activity in circulating leukocytes. We modeled this degree of CN inhibition in primary human leukocytes in vitro in order to study the effect of partial CN inhibition on the downstream signaling events that lead to gene activation. In CsA-treated leukocytes stimulated by calcium ionophore, the degree of reduction in CN activity was accompanied by a similar degree of inhibition of each event tested: dephosphorylation of nuclear factor of activated T cell proteins, nuclear DNA binding, activation of a transfected reporter gene construct, IFN-gamma and IL-2 mRNA accumulation, and IFN-gamma production. Furthermore, the degree of CN inhibition was reflected by a similar degree of reduction in lymphocyte proliferation and IFN-gamma production in the allogeneic mixed lymphocyte cultures. These data support the conclusion that CN activity is rate-limiting for the activation of primary human T lymphocytes. Thus, the reduction of CN activity observed in CsA-treated patients is accompanied by a similar degree of reduction in lymphocyte gene activation, and accounts for the immunosuppression observed.

Proximity and orientation underlie signaling by the non-receptor tyrosine kinase ZAP70

Signaling by the antigen receptor of T lymphocytes initiates different developmental transitions, each of which require the tyrosine kinase ZAP70. Previous studies with agonist and antagonist peptides have indicated that ZAP70 might respond differently to different structures of the TCR-CD3 complex induced by bound peptides. The roles of membrane proximity and orientation in activation of ZAP70 signaling were explored using synthetic ligands and their binding proteins designed to produce different architectures of membrane-bound complexes composed of ZAP70 fusion proteins. Transient membrane recruitment of physiological levels of ZAP70 with the membrane-permeable synthetic ligand FK1012A leads to rapid phosphorylation of ZAP70 and activation of the ras/MAPK and Ca2+/calcineurin signaling pathways. ZAP70 SH2 domains are not required for signaling when the kinase is artifically recruited to the membrane, indicating that the SH2 domains function solely in recruitment and not in kinase activation. Using additional synthetic ligands and their binding proteins that recruit ZAP70 equally well but orient it at the cell membrane in different ways, we define a requirement for a specific presentation of ZAP70 to its downstream targets. These results provide a mechanism by which ZAP70, bound to the phosphorylated receptor, could discriminate between conformational changes induced by the binding of different MHC-peptide complexes to the antigen receptor and introduce an approach to exploring the role of spatial orientation of signaling complexes in living cells.

The molecular basis of intractable diarrhoea of infancy

The intractable diarrhoeas of infancy present very major problems of clinical management. However, the conceptual importance of these conditions lies in the information that they may provide about normal small-intestinal function in humans: among such infants will be found the human equivalents of the 'knock-out' mice, in which targeted gene disruption allows sometimes unexpected insight into the regulation of intestinal function. The challenge posed by the intractable diarrhoeal syndromes, of working backwards from an apparently common phenotype to probably multiple genotypes, is, however, immense. Very few of these conditions have been described at the genetic level, although the molecular basis of pathogenesis has been better explored in recent years. The two major groups of intractable diarrhoea are due to (1) primary epithelial abnormalities (which usually present within the first few days of life) and (2) immunologically mediated (which generally present after the first few weeks). The high prevalence of autoimmune enteropathy among infantile autoimmune disease, in contrast to adult autoimmunity, is intriguing and may reflect constitutive abnormality of extrathymic lymphocyte maturation. The use of potent immunosuppressive drugs and increasing expertise with parenteral nutrition are improving the outlook of these previously fatal conditions. Viewed globally, however, the pressing problem is to treat effectively the millions of infants who die from severe persistent diarrhoea and wasting, which would certainly not be considered intractable in wealthy countries.

Restoration of thymocyte development and function in zap-70-/- mice by the Syk protein tyrosine kinase

The Syk family of protein tyrosine kinases, consisting of ZAP-70 and Syk, associate with the pre- and alphabeta T cell antigen receptors (TCRs) and undergo tyrosine phosphorylation and activation following receptor engagement. Thymocyte development in zap-70-/- mice is blocked at the CD4+CD8+ TCR(lo) stage. The presence of Syk in the thymus has raised the possibility that Syk may be able to mediate TCR function. To determine if Syk can play a role in thymocyte development, we generated zap-70-/- mice expressing a human syk cDNA. Syk expression restored both thymocyte development and function. In addition, Syk function required the CD45 transmembrane protein tyrosine phosphatase. Hence, ZAP-70 and Syk can play overlapping functions and exhibit similar regulatory mechanisms in mediating alphabeta T cell development.

Protein tyrosine kinases in thymocyte development

Much has been learned over the past few years about how protein tyrosine kinases mediate pre-TCR and mature alphabetaTCR function. The highlights include understanding the roles and the distinct effects of the Src and Syk families of protein tyrosine kinases in thymocyte development and function.

Regulation of T-cell antigen receptor signalling by Syk tyrosine protein kinase

T-cell antigen receptor (TCR) signalling has been shown to involve two classes of tyrosine protein kinases: the Src-related kinases p56(lck) and p59(fyr), and the Zap-70/Syk family kinases. Lck and FynT are postulated to initiate TCR-triggered signal transduction by phosphorylating the CD3 and zeta subunits of the TCR complex. This modification permits the recruitment of Zap-70 and Syk, which are presumed to amplify the TCR-triggered signal, by phosphorylating additional intracellular proteins. While Zap-70 is expressed in all T cells, Syk is present in thymocytes and mature T-cell populations such as intraepithelial gammadelta T cells and naive alphabeta T cells. To better understand the role of Syk in these cells, its impact on the physiology of an antigen-specific T-cell line was tested. Our results showed that compared to Zap-70 alone, Syk was a strong positive regulator of antigen receptor-induced signals in BI-141 cells. Surprisingly, they indicated that, like Src family kinases, Syk augmented TCR-triggered tyrosine phosphorylation of CD3/zeta. Syk, but not Zap-70 alone, could also stimulate tyrosine phosphorylation of a zeta-bearing chimera in transiently transfected Cos-1 cells. Finally, evidence was provided that Syk has the capacity to directly phosphorylate a zeta-derived peptide in vitro. These findings suggested that Syk may have a unique role in T cells, as a consequence of its ability to efficiently phosphorylate multiple components of the TCR signalling cascade. Furthermore, they raised the possibility that Syk can regulate the initiation of TCR signalling, by promoting phosphorylation of the immunoreceptor tyrosine-based activation motifs of the TCR complex.

Phenotypic features of selective T cell deficiency characterized by absence of CD8+ T lymphocytes and undetectable mRNA for ZAP-70 kinase

Selective T cell deficiency is a rare immune deficiency characterized by the absence of CD8+ T lymphocytes and depressed/absent T cell function. This syndrome has been associated with mutations in the gene for ZAP-70, a tyrosine kinase that has profound effects on signaling via the T cell receptor. In this paper we describe a patient with selective T cell deficiency and certain phenotypic features that are unique among the small number of patients described. The patient had virtually absent T cell function, hypogammaglobulinemia, and no response to vaccination. The T lymphocytes failed to respond to mitogenic stimuli, even in the presence of exogenous interleukin 2. Similar to other patients with this disorder, the T cells were capable of proliferating when stimulated by pharmacologic agents such as phorbol ester and ionomycin. While peripheral blood T cells had limited capability to increase cytosolic Ca2+ levels in response to mitogenic stimulation, thymocytes responded to a large panel of antibodies and mitogens. This report broadens the spectrum of clinical presentations associated with selective T cell deficiency and, for the first time, compares the responses of both peripheral T cells and thymocytes. The data support the concept that the defect in signal transduction resulting from the absence of ZAP-70 is primarily manifested following export of T lymphocytes from the thymus and that selection of CDS-positive T cells is dependent on the presence of ZAP-70.

Differential T cell receptor-mediated signaling in naive and memory CD4 T cells

Naive and memory CD4 T cells differ in cell surface phenotype, function, activation requirements, and modes of regulation. To investigate the molecular bases for the dichotomies between naive and memory CD4 T cells and to understand how the T cell receptor (TCR) directs diverse functional outcomes, we investigated proximal signaling events triggered through the TCR/CD3 complex in naive and memory CD4 T cell subsets isolated on the basis of CD45 isoform expression. Naive CD4 T cells signal through TCR/CD3 similar to unseparated CD4 T cells, producing multiple tyrosine-phosphorylated protein species overall and phosphorylating the T cell-specific ZAP-70 tyrosine kinase which is recruited to the CD3zeta subunit of the TCR. Memory CD4 T cells, however, exhibit a unique pattern of signaling through TCR/CD3. Following stimulation through TCR/CD3, memory CD4 T cells produce fewer species of tyrosine-phosphorylated substrates and fail to phosphorylate ZAP-70, yet unphosphorylated ZAP-70 can associate with the TCR/CD3 complex. Moreover, a 26/28-kDa phosphorylated doublet is associated with CD3zeta in resting and activated memory but not in naive CD4 T cells. Despite these differences in the phosphorylation of ZAP-70 and CD3-associated proteins, the ZAP-70-related kinase, p72syk, exhibits similar phosphorylation in naive and memory T cell subsets, suggesting that this kinase could function in place of ZAP-70 in memory CD4 T cells. These results indicate that proximal signals are differentially coupled to the TCR in naive versus memory CD4 T cells, potentially leading to distinct downstream signaling events and ultimately to the diverse functions elicited by these two CD4 T cell subsets.

The interferon regulatory transcription factor IRF-1 controls positive and negative selection of CD8+ thymocytes

Little is known about the molecular mechanisms and transcriptional regulation that govern T cell selection processes and the differentiation of CD4+ and CD8+ T cells. Mice lacking the interferon regulatory transcription factor-1 (IRF-1) have reduced numbers of mature CD8+ cells within the thymus and peripheral lymphatic organs. Here we show that positive and negative T cell selection of two MHC class I-restricted TCR alphabeta transgenes, H-Y and P14, are impaired in IRF-1-/- mice. The absence of IRF-1 resulted in decreased expression of LMP2, TAP1, and MHC class I on thymic stromal cells. Despite decreased MHC class I expression on IRF-1-/- thymic stromal cells, the defect in CD8+ T cells development did not reside in the thymic environment, and IRF-1-/- stromal cells can fully support development of CD8+ thymocytes in in vivo bone marrow chimeras and in vitro reaggregation cultures. Moreover, IRF-1-/- thymocytes displayed impaired TCR-mediated signal transduction, and the induction of negative selection in TCR Tg thymocytes from IRF-1-/- mice required a 1000-fold increase in selecting peptide. We also provide evidence that IRF-1 is mainly expressed in mature, but not immature, thymocytes and that expression of IRF-1 in immature thymocytes is induced after peptide-specific TCR activation. These results indicate that IRF-1 regulates gene expression in developing thymocytes required for lineage commitment and selection of CD8+ thymocytes.

Uncoupling of T-cell antigen receptor and downstream protein tyrosine kinases in common variable immunodeficiency

Patients with common variable immunodeficiency (CVID) are heterogeneous in the clinical manifestations of the disease and the underlying mechanisms leading to the immunodeficiency. Although the overt defect is an impairment in B-cell function, there is increasing evidence of primary T-cell dysfunctions in a proportion of patients with CVID. We have analyzed T-cells from six CVID patients for activation of both early and late events in response to TCR triggering. The data showed that T-cells from three of six CVID patients were defective in the capacity to initiate the TCR/CD3 signaling pathway by activating intracellular tyrosine kinases, associated with impaired proliferative responses to TCR/CD3 triggering. Since both surface expression of the TCR/CD3 complex and intracellular expression of key tyrosine kinases such as p56lek and ZAP-70 were normal in these patients, our data suggest a defect in the earliest step of TCR signal transduction.

Perturbation of the T lymphocyte lineage in transgenic mice expressing a constitutive repressor of nuclear factor (NF)-kappaB

Members of the nuclear factor (NF)-kappaB/Rel family transcription factors are induced during thymic selection and in mature T lymphocytes after ligation of the T cell antigen receptor (TCR). Despite these findings, disruption of individual NF-kappaB/Rel genes has revealed no intrinsic defect in the development of mature T cells, perhaps reflecting functional redundancy. To circumvent this possibility, the T cell lineage was targeted to express a trans-dominant form of IkappaBalpha that constitutively represses the activity of multiple NF-kappaB/Rel proteins. Transgenic cells expressing this inhibitor exhibit a significant proliferative defect, which is not reversed by the addition of exogenous interleukin-2. Moreover, mitogenic stimulation of splenocytes leads to increased apoptosis of transgenic T cells as compared with controls. In addition to deregulated T cell growth and survival, transgene expression impairs the development of normal T cell populations as evidenced by diminished numbers of TCRhi CD8 single-positive thymocytes. This defect was significantly amplified in the periphery and was accompanied by a decrease in CD4(+) T cells. Taken together, these in vivo findings indicate that the NF-kappaB/Rel signaling pathway contains compensatory components that are essential for the establishment of normal T cell subsets.

A spontaneously arising mutation in the DLAARN motif of murine ZAP-70 abrogates kinase activity and arrests thymocyte development

Development of immature CD4+ CD8+ thymocytes into functionally mature CD4+ and CD8+ T cells is driven by selection events that require signals transduced through the T cell antigen receptor (TCR). Transduction of TCR signals in the thymus involves tyrosine phosphorylation of the protein tyrosine kinase ZAP-70 by p56(lck) and results in induction of ZAP-70 enzymatic activity. We have identified a novel, spontaneously arising point mutation within a highly conserved motif (DLAARN) in the kinase domain of murine ZAP-70 that uncouples tyrosine phosphorylation of ZAP-70 from induction of ZAP-70 kinase activity. Mice homozygous for this mutation are devoid of mature T cells because thymocyte development is arrested at the CD4+ CD8+ stage of differentiation. The developmental arrest is due to the inability of CD4+ CD8+ thymocytes to propagate TCR signals in the absence of ZAP-70 kinase activity despite tyrosine phosphorylation of TCR-associated ZAP-70 molecules.

T cell antigen receptor signal transduction

The T cell antigen receptor (TCR) initiates signal transduction by activating multiple cytoplasmic protein tyrosine kinases (PTKs). Considerable progress in the field of TCR signal transduction has been made in three areas recently: first, in understanding the structure and function of the PTK ZAP-70; second, in the elucidation of the function of the substrates and pathways downstream of the PTKs; and third, in the identification of molecules that negatively regulate TCR signalling.

Production and characterization of a novel monoclonal antibody against phosphorylated T cell receptor zeta chain

One of the earliest events following T cell receptor (TCR) triggering is the activation of the protein kinase Lck and induction of tyrosine phosphorylation of zeta, the major signal transduction subunit of the T cell receptor complex. Here we report the generation and characterization of a monoclonal antibody specific for human phosphozeta. The antibody was produced by immunizing mice with a truncated recombinant form of human zeta together with the Lck enzyme. The C415.9A antibody recognizes recombinant as well as cellular phosphozeta but is unreactive with unphosphorylated zeta or other tyrosine phosphorylated proteins. Using this antibody, we have demonstrated aberrant TCR-zeta tyrosine phosphorylation in Jurkat T cell transduction mutants. Therefore, this antibody can be used to elucidate T cell signal transduction mechanisms by analyzing and monitoring tyrosine phosphorylation of zeta in vitro and in vivo directly. Furthermore, this antibody could find application in the analysis of abnormal T cell signaling in autoimmune disease, cancer, and immunodeficiency disorders.

Human severe combined immunodeficiency: genetic, phenotypic, and functional diversity in one hundred eight infants

OBJECTIVE

To determine the relative frequencies of the different genetic forms of severe combined immunodeficiency (SCID) and whether there are distinctive characteristics of the particular genotypes.

STUDY DESIGN

The demographic, genetic, and immunologic features of 108 infants with SCID who were treated consecutively at Duke University Medical Center were analyzed.

RESULTS

Eighty-nine subjects were boys and 19 were girls; there were 84 white infants, 16 black infants, and 8 Hispanic infants. Forty-nine had X-linked SCID with mutations of common cytokine receptor gamma chain (gamma c), 16 had adenosine deaminase (ADA) deficiency, 8 had Janus kinase 3 (Jak3) deficiency, 21 had unknown autosomal recessive mutations, 1 had reticular dysgenesis, 1 had cartilage hair hypoplasia, and 12 (all boys) had SCID of undetermined type. Deficiency of ADA caused the most profound lymphopenia; gamma c or Jak3 deficiency resulted in the most B cells and fewest natural killer (NK) cells; NK cells and function were highest in autosomal recessive and unknown types of SCID.

CONCLUSIONS

Different SCID genotypes are associated with distinctive lymphocyte characteristics. The presence of NK function in ADA-deficient, autosomal recessive, and unknown type SCIDs, and low NK function in a majority of gamma c and Jak3 SCIDs indicates that some molecular lesions affect T, B, and NK cells (gamma c and Jak3), others primarily T cells (ADA deficiency), and others just T and B cells.

ITIMs and ITAMs. The Yin and Yang of antigen and Fc receptor-linked signaling machinery

The initial stages of an immune response are regulated at the level of the cell-surface antigen and Fc receptors. The extracellular portions of these receptors provide immune specificity and determine the nature of the responding effector cells, whereas the intracellular portion transduces signals into the cell and determines the intensity and duration of the immune response. Recent studies led to the identification of two types of modules within the cytoplasmic region of receptor subunits that are critical for the activation and termination of signal transduction pathways. Phosphorylation of the conserved tyrosine residues within the two modules, the immunoreceptor tyrosine-based activation motif (ITAM) and the immunoreceptor tyrosine-based inhibition motif (ITIM), is followed by the recruitment of different sets of SH2-containing molecules to the receptor site. These proteins regulate the receptor-linked signal transduction pathways in a positive or a negative fashion, which is a reminiscent of the ancestral Yin-Yang principle.

Development and function of T cells in T cell antigen receptor/CD3 zeta knockout mice reconstituted with Fc epsilon RI gamma

Engagement of alpha-beta T cell receptors (TCRs) induces many events in the T cells bearing them. The proteins that transduce these signals to the inside of cells are the TCR-associated CD3 polypeptides and zeta-zeta or zeta-eta dimers. Previous experiments using knockout (KO) mice that lacked zeta (zeta KO) showed that zeta is required for good surface expression of TCRs on almost all T cells and for normal T cell development. Surprisingly, however, in zeta KO mice, a subset of T cells in the gut of both zeta KO and normal mice bore nearly normal levels of TCR on its surface. This was because zeta was replaced by the Fc epsilon RI gamma (FcR gamma). These cells were relatively nonreactive to stimuli via their TCRs. In addition, a previous report showed that zeta replacement by the FcR gamma chain also might occur on T cells in mice bearing tumors long term. Again, these T cells were nonreactive. To understand the consequences of zeta substitution by FcR gamma for T cell development and function in vivo, we produced zeta KO mice expressing FcR gamma in all of their T cells (FcR gamma TG zeta KO mice). In these mice, TCR expression on immature thymocytes was only slightly reduced compared with controls, and thymocyte selection occurred normally and gave rise to functional, mature T cells. Therefore, the nonreactivity of the FcR gamma + lymphocytes in the gut or in tumor-bearing mice must be caused by some other phenomenon. Unexpectedly, the TCR levels of mature T cells in FcR gamma TG zeta KO mice were lower than those of controls. This was particularly true for the CD4+ T cells. We conclude that FcR gamma can replace the functions of zeta in T cell development in vivo but that TCR/CD3 complexes associated with FcR gamma rather than zeta are less well expressed on cells. Also, these results revealed a difference in the regulation of expression of the TCR/CD3 complex on CD4+ and CD8+ T cells.

Peripheral expression of Jak3 is required to maintain T lymphocyte function

The Jak family tyrosine kinase, Jak3, is involved in signaling through cytokine receptors that utilize the common gamma chain (gammac), such as those for IL-2, IL-4, IL-7, IL-9, and IL-15. Recent studies of Jak3-deficient mice and humans have demonstrated that Jak3 plays a critical role in B and T lymphocyte maturation and function. The T lymphocyte defects in Jak3-deficient mice include a small thymus, a decrease in peripheral CD8+ cells, an increase in the surface expression of activation markers, and a severe reduction in proliferative and cytokine secretion responses to mitogenic stimuli. To determine whether the peripheral T lymphocyte defects result from aberrant maturation in the thymus or from the absence of Jak3 protein in peripheral T cells, we generated reconstituted mice that express normal levels of Jak3 protein in the thymus but lose Jak3 expression in peripheral T cells. Jak3 expression in the thymus restores normal T cell development, including CD8+, gammadelta, and natural killer cells. However, the loss of Jak3 protein in peripheral T cells leads to the Jak3-/- phenotype, demonstrating that Jak3 is constitutively required to maintain T cell function.

Naturally occurring primary deficiencies of the immune system

Naturally occurring genetic disorders of the immune system provide many models for the study of its development and function. In a way, their analysis complements the information provided by the generation of genetic defects in mice created using homologous recombination techniques. In this review, the recent findings made in three areas are focused upon deficiencies in T cell differentiation and in T lymphocyte activation, and on the control process of peripheral immune response.

Leukocyte protein tyrosine kinases: potential targets for drug discovery

Intracellular signal transduction following the extracellular ligation of a wide variety of different types of surface molecules on leukocytes involves the activation of protein tyrosine kinases. The dependence of successful intracellular signaling on the functions of the nontransmembrane class of protein tyrosine kinases coupled with the cell type-specific expression patterns for several of these enzymes makes them appealing targets for therapeutic intervention. Development of drugs that can interfere with the catalytic functions of the nontransmembrane protein tyrosine kinases or that can disrupt critical interactions with regulatory molecules and/or substrates should find clinical applications in the treatment of allergic diseases, autoimmunity, transplantation rejection, and cancer.

Identification and characterization of a substrate specific for the T cell protein tyrosine kinase ZAP-70

ZAP-70 is a protein tyrosine kinase (PTK) that plays a critical role in T cell activation. To study the role of ZAP-70 catalytic activity in this process, a substrate capable of distinguishing between the activities of ZAP-70 and other PTKs would be useful, especially since it has recently been shown that ZAP-70 interacts with another T cell PTK, Lck. We have thus identified a site of phosphorylation on the cytoplasmic fragment of the erythrocyte band 3 protein that is recognized by ZAP-70, but not Lck. A synthetic peptide based on this site has been demonstrated to be a good in vitro substrate for ZAP-70 and a poor substrate for the T cell PTKs Lck and Itk. This peptide molecule should thus prove useful to many investigators working in the field of T cell activation.

The kinase, SH3, and SH2 domains of Lck play critical roles in T-cell activation after ZAP-70 membrane localization

Antigenic stimulation of the T-cell antigen receptor initiates signal transduction through the immunoreceptor tyrosine-based activation motifs (ITAMs). When its two tyrosines are phosphorylated, ITAM forms a binding site for ZAP-70, one of the cytoplasmic protein tyrosine kinases essential for T-cell activation. The signaling process that follows ZAP-70 binding to ITAM has been analyzed by the construction of fusion proteins that localize ZAP-70 to the plasma membrane. We found that membrane-localized forms of ZAP-70 induce late signaling events such as activation of nuclear factor of activated T cells without any stimulation. This activity was observed only when Lck was expressed and functional. In addition, each mutation that affects the function of Lck in the kinase, Src homology 2 (SH2), and SH3 domains greatly impaired the signaling ability of the chimeric protein. Therefore, Lck functions in multiple manners in T-cell activation for the steps following ZAP-70 binding to ITAM.

Enhancement of lymphocyte responsiveness by a gain-of-function mutation of ZAP-70

The protein tyrosine kinase ZAP-70 plays an essential role in T-cell activation and development. After T-cell receptor stimulation, ZAP-70 is associated with the receptor and is phosphorylated on many tyrosine residues, including tyrosine 292 (Y-292), in the region between the C-terminal SH2 domain and the kinase domain (interdomain B). Here we show that a mutation of Y-292 (292F) or deletion of interdomain B enhanced the ability of ZAP-70 to reconstitute B-cell receptor stimulation-dependent NF-AT induction in a B-cell line deficient in Syk. In contrast, in a T-cell line, expression of 292F led to basal NF-AT induction independent of T-cell receptor stimulation. These results demonstrate that the role of Y-292 is to negatively regulate the function of ZAP-70 in lymphocytes. This appears to be a dominant function of interdomain B because deletion of most of interdomain B also resulted in a mutant of ZAP-70 with enhanced ability to reconstitute Syk-deficient DT-40 B cells. Since our biochemical studies did not reveal an effect of the 292F mutation on either the kinase activity of ZAP-70 or on the ability of ZAP-70 to bind to the receptor, we propose a model in which Y-292 interacts with an inhibitory protein to negatively regulate ZAP-70 function.

alpha beta T cell development is abolished in mice lacking both Lck and Fyn protein tyrosine kinases

Two families of protein tyrosine kinases (PTKs), the Src and Syk/ZAP-70 families, are required for T cell development. Lck is the major Src family member required for thymopoiesis, since there is a severe deficit of CD4+CD8+ thymocytes and mature T cells in its absence. However, some peripheral T cells are evident in these mice, suggesting that additional PTKs may contribute to T cell development. Here we show that the combined disruption of Lck and Fyn (lck(-/-)fyn(-/-)) completely arrests alpha beta T cell development at the CD4-CD8- stage. The development of V gamma 3+ dendritic epidermal T cells is also severely impaired, but natural killer cell development and cytolytic activity is unaffected in lck(-/-)fyn(-/-) mice. These findings reveal the potential for redundant functions mediated by Src family PTKs while emphasizing crucial roles for Lck and Fyn in T cell development.

Reconstitution of T cell receptor signaling in ZAP-70-deficient cells by retroviral transduction of the ZAP-70 gene

A variant of severe combined immunodeficiency syndrome (SCID) with a selective inability to produce CD8 single positive T cells and a signal transduction defect in peripheral CD4+ cells has recently been shown to be the result of mutations in the ZAP-70 gene. T cell receptor (TCR) signaling requires the association of the ZAP-70 protein tyrosine kinase with the TCR complex. Human T cell leukemia virus type I-transformed CD4+ T cell lines were established from ZAP-70-deficient patients and normal controls. ZAP-70 was expressed and appropriately phosphorylated in normal T cell lines after TCR engagement, but was not detected in T cell lines from ZAP-70-deficient patients. To determine whether signaling could be reconstituted, wild-type ZAP-70 was introduced into deficient cells with a ZAP-70 retroviral vector. High titer producer clones expressing ZAP-70 were generated in the Gibbon ape leukemia virus packaging line PG13. After transduction, ZAP-70 was detected at levels equivalent to those observed in normal cells, and was appropriately phosphorylated on tyrosine after receptor engagement. The kinase activity of ZAP-70 in the reconstituted cells was also appropriately upregulated by receptor aggregation. Moreover, normal and transduced cells, but not ZAP-70-deficient cells, were able to mobilize calcium after receptor ligation, indicating that proximal TCR signaling was reconstituted. These results indicate that this form of SCID may be corrected by gene therapy.

Mechanism of activation for Zap-70 catalytic activity

There is a growing body of evidence, including data from human genetic and T-cell receptor function studies, which implicate a zeta-associated protein of M(r) 70,000 (Zap-70) as a critical protein tyrosine kinase in T-cell activation and development. During T-cell activation, Zap-70 becomes associated via its src homology type 2 (SH2) domains with tyrosine-phosphorylated immune-receptor tyrosine activating motif (ITAM) sequences in the cytoplasmic zeta chain of the T-cell receptor. An intriguing conundrum is how Zap-70 is catalytically activated for downstream phosphorylation events. To address this question, we have used purified Zap-70, tyrosine phosphorylated glutathione S-transferase (GST)-Zeta, and GST-Zeta-1 cytoplasmic domains, and various forms of ITAM-containing peptides to see what effect binding of zeta had upon Zap-70 tyrosine kinase activity. The catalytic activity of Zap-70 with respect to autophosphorylation increased approximately 5-fold in the presence of 125 nM phosphorylated GST-Zeta or GST-Zeta-1 cytoplasmic domain. A 20-fold activity increase was observed for phosphorylation of an exogenous substrate. Both activity increases showed a GST-Zeta concentration dependence. The increase in activity was not produced with nonphosphorylated GST-Zeta, phosphorylated zeta, or phosphorylated ITAM-containing peptides. The increase in Zap-70 activity was SH2 mediated and was inhibited by phenylphosphate, Zap-70 SH2, and an antibody specific for Zap-70 SH2 domains. Since GST-Zeta and GST-Zeta-1 exist as dimers, the data suggest Zap-70 is activated upon binding a dimeric form of phosphorylated zeta and not by peptide fragments containing a single phosphorylated ITAM. Taken together, these data indicate that the catalytic activity of Zap-70 is most likely activated by a trans-phosphorylation mechanism.

RAG mutations in human B cell-negative SCID

Patients with human severe combined immunodeficiency (SCID) can be divided into those with B lymphocytes (B+ SCID) and those without (B- SCID). Although several genetic causes are known for B+ SCID, the etiology of B- SCID has not been defined. Six of 14 B- SCID patients tested were found to carry a mutation of the recombinase activating gene 1 (RAG-1), RAG-2, or both. This mutation resulted in a functional inability to form antigen receptors through genetic recombination and links a defect in one of the site-specific recombination systems to a human disease.

HIV infection--induced posttranslational modification of T cell signaling molecules associated with disease progression

In attempt to elucidate the mechanism of the HIV infection induced T cell unresponsiveness, we studied signal-transducing molecules proximal to the T cell receptor (TCR) in T lymphocytes of HIV-infected individuals. Total amounts of protein tyrosine kinases (PTKs) Lck, Fyn, and ZAP-70 and the zeta chain of the TCR were found significantly decreased in T cells of symptomatic/AIDS patients as well as in T cells of individuals in acute and early asymptomatic stages of HIV infection. Unexpectedly, the detection of Lck, Fyn, and ZAP-70 was reversed after the treatment of cell lysates with dithiothreitol. This suggests that PTKs Lck, Fyn, and ZAP-70 were modified by a mechanism altering the status of sulfhydryl groups. Moreover, this mechanism seems to affect selectively T cells of HIV infected patients since B cell PTKs Syk and Lyn were detected structurally and functionally intact. Interestingly, similar alterations of signaling molecules were not detected in T cells of HIV-infected long-term asymptomatic individuals. Modification of T cell PTKs may thus underlie the HIV-induced impairment of lymphocyte function and may potentially predict disease progression.

Differential intrinsic enzymatic activity of Syk and Zap-70 protein-tyrosine kinases

Syk and Zap-70 are related protein-tyrosine kinases implicated in antigen and Fc receptor signaling. While Zap-70 is restricted to T-cells and natural killer cells, Syk accumulates in B-cells, mast cells, platelets, and immature T-cells. In addition, we found that an isoform of Syk (SykB), which carries a 23-amino acid deletion in the "linker" region, is prominently expressed in bone marrow. To better understand the relative impact of Syk, SykB, and Zap-70 on signal transduction, we compared their intrinsic enzymatic properties in transiently transfected COS-1 cells and in hemopoietic cells. Using modified versions of these enzymes bearing a common Myc epitope at the amino terminus, we determined that the ability of Syk and SykB to undergo autophosphorylation and to phosphorylate erythrocyte band 3 in immune complex kinase reactions was at least 100-fold greater than that of Zap-70. Similarly, Syk and SykB, but not Zap-70, caused prominent tyrosine phosphorylation of p120(c-)cbl in COS-1 cells. A similar pattern of activity was also noted for endogenous Syk and Zap-70 from hemopoietic cells. To understand the structural basis for these characteristics, we also created and analyzed a series of chimeras between Syk and Zap-70. These studies indicated that the catalytic domain of Syk and Zap-70, but not their SH2 domains, linker region or carboxyl-terminal tail, was responsible for their respective activity. Taken together, these data demonstrated that the intrinsic enzymatic activity of Syk and SykB is superior to that of Zap-70 and that such a distinction relates to structural variations in the catalytic domain.

Severe combined immunodeficiency due to defective binding of the nuclear factor of activated T cells in T lymphocytes of two male siblings

Peripheral blood lymphocytes (PBL) and alloreactive T cell lines of two male infants born to consanguinous parents and presenting with severe combined immunodeficiency (SCID) showed a pronounced deficiency in T cell activation. Although phenotypically normal, the proliferative response of the childrens' T cells was strongly reduced but could be improved by the addition of interleukin-2 (IL-2). Furthermore both childrens' T cells were unable to produce the cytokines IL-2, interferon-gamma (IFN-gamma), IL-4 and tumor necrosis factor-alpha (TNF-alpha). This multiple cytokine production deficiency could not be restored by IL-2 or co-stimulatory signals provided by antigen-presenting cells (APC). Moreover, mRNA for IL-2 and IFN-gamma could not be detected. In contrast, expression of the activation-dependent cell surface markers CD25 and CD69 was within normal limits. To determine whether the functional defect of the patients' T cells was due to the absence or abnormal binding of transcription factors involved in cytokine gene expression, electrophoretic mobility shift assays were used to examine the DNA binding of AP-1, Oct, CREB, SP1, NF-kappa B and the nuclear factor of activated T cells (NF-AT) to their respective response elements in the promoter of the IL-2 gene. Whereas AP-1, NF-kappa B, Oct, CREB and SP1 displayed normal binding activities in nuclear extracts, the binding of NF-AT to its IL-2 promoter response element was barely detectable both before and after T cell stimulation. Our results strongly suggest that this NF-AT/DNA binding defect is responsible for the multiple cytokine deficiency and the SCID phenotype observed in the two infant brothers.