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WASp verprolin homology, cofilin homology, and acidic region domain-mediated actin polymerization is required for T cell development.

All members of the Wiskott-Aldrich syndrome protein (WASp) family contain a carboxyl-terminal verprolin homology, cofilin homology, and acidic region (VCA) domain that binds and activates the Arp2/3 complex, thereby linking these proteins to the induction of actin polymerization. Although the VCA domain imbues WASp and other WASp family members with the capacity to modulate cytoskeletal organization, little is known about the impact of this domain activity on lymphoid cell function. Here we demonstrate that T cell-restricted expression of VCA domain-deleted WASp (WASpdeltaVCA) in WAS(-/-) mice engenders a severe early block in T lymphopoiesis associated with impaired T cell antigen receptor alphabeta expression and a consequent failure to generate single-positive CD4(+) and CD8(+) T cells. These latter defects, which are not observed in WAS(-/-) mice, are associated with impaired induction of cellular actin polymerization and a failure in the terminal differentiation of double-negative thymocytes. These findings indicate that WASp family proteins play an essential role in modulating the signaling events required for early thymocyte development and reveal their capacity to subserve this role to depend on VCA domain-mediated actin polymerization.

Actin Depolymerizing Factors↗

Synergistic activation of the germline epsilon promoter mediated by Stat6 and C/EBP beta.

Transcription of the Ig H chain germline transcripts is a prerequisite for class switching. Expression of the epsilon germline transcript is induced by IL-4 and requires the integrity of a composite IL-4 response element. The element is bound by the IL-4-inducible transcription factor Stat6 and one or more members of the CAAT/enhancer-binding protein (C/EBP) family, a constitutively expressed class of transcription factors. Here, we show that Stat6 and C/EBP beta cooperate to synergistically activate transcription from the epsilon element. The effect was most pronounced in lymphoid cells, and the activation domains of both proteins were required to achieve this synergy. Although other members of the C/EBP family are able to bind the element, very little cooperativity was seen with C/EBP alpha and none with C/EBP gamma. In fact, C/EBP gamma was able to inhibit IL-4-induced reporter activity. Stat6 and C/EBP beta bind the IL-4 response element simultaneously. The fast dissociation rate apparent when Stat6 binds this DNA element alone is slowed when C/EBP beta binds at the neighboring site. These data suggest a mechanism whereby C/EBP beta stabilizes Stat6 binding at this element, thereby increasing the likelihood that both of their activation domains will interact, possibly with other factors, to activate transcription in an IL-4-dependent manner.

CCAAT-Enhancer-Binding Proteins↗

Detection of Epstein-Barr virus BGLF4 protein kinase in virus replication compartments and virus particles.

BGLF4 is the only serine/threonine protein kinase identified in Epstein-Barr virus (EBV); it is known to phosphorylate viral DNA polymerase processivity factor, EA-D (BMRF1), EBNA-LP, EBNA-2, cellular EF-1delta and nucleoside analogue ganciclovir. However, the expression and biological functions of BGLF4 have not yet been clearly demonstrated in EBV-infected cells. To reveal authentic functions of BGLF4 protein within viral-replicating cells, a panel of specific monoclonal antibodies was generated and characterized. The major immunogenic regions of BGLF4 were mapped to aa 27-70 and 327-429. Using these antibodies, the expression kinetics and localization of BGLF4 were analysed in reactivated EBV-positive lymphoid and epithelial cells. BGLF4 was expressed as a phosphoprotein at the early lytic stage and was detected predominantly in the nucleus of EBV-positive cells, but small amounts of BGLF4 were observed in cytosolic and heavy membrane fractions at the late phase of virus replication. Additionally, it was demonstrated that BGLF4 co-localizes with viral DNA polymerase processivity factor, EA-D (BMRF1), in the virus replication compartment and that it is a virion component. Finally, possible functional domains at the N terminus of BGLF4 were analysed and it was found that aa 1-26 of BGLF4 are dispensable for EA-D phosphorylation, whereas deletion of aa 27-70 reduced kinase activity.

Animals↗

Inhibition of generation of cytotoxic T lymphocyte activity by a CCL19/macrophage inflammatory protein (MIP)-3beta antagonist.

Chemokines constitute a group of over 40 secreted peptides that are important for the control of leukocyte migration both during homeostasis and inflammation. Recent studies have implicated the ligands CCL19 and CCL21 and their receptor, CCR7, in the specific migration of naïve lymphocytes and mature dendritic cells to secondary lymphoid organs during immune homeostasis. However, the role that these molecules play during immune priming is not well understood. In this study, using CCL19((8-83)), a novel N-terminal truncation mutant, we have investigated the role of CCL19 in a primary allogeneic immune response, a response of particular relevance to transplant rejection. This antagonist specifically inhibited wild type CCL19-induced chemotaxis and intracellular calcium mobilization without affecting that of CCL21. The treatment of mice with CCL19((8-83)) did not globally inhibit the recruitment of cells into lymph nodes; however, it inhibited the generation of cytotoxic T lymphocytes toward allogeneic dendritic cells. This is the first evidence that CCL19 plays a role in immune priming.

Amino Acid Sequence↗

Domain interactions in protein tyrosine kinase Csk.

Csk (C-terminal Src kinase) is a protein tyrosine kinase that phosphorylates Src family member C-terminal tails, resulting in downregulation of Src family members. It is composed of three principal domains: an SH3 (Src homology 3) domain, an SH2 (Src homology 2) domain, and a catalytic domain. The impact of the noncatalytic domains on kinase catalysis was investigated. The Csk catalytic domain was expressed in Escherichia coli as a recombinant glutathione S-transferase-fusion protein and demonstrated to have 100-fold reduced catalytic efficiency. Production of the catalytic domain by proteolysis of full-length Csk afforded a similar rate reduction. This suggested that the reduction in catalytic efficiency of the recombinant catalytic domain was intrinsic to the sequence and not an artifact related to faulty expression. This rate reduction was similar for peptide and protein substrates and was due almost entirely to a reduced k(cat) rather than to effects on substrate K(m)s. Viscosity experiments on the catalytic fragment kinase reaction demonstrated that the chemical (phosphoryl transfer) step had a reduced rate. While the Csk SH2 domain had no intermolecular effect on the kinase activity of the Csk catalytic domain, the SH3 domain and SH3-SH2 fragment led to a partial rescue (4-5-fold) of the lost kinase activity. This rescue was not achieved with two other SH3 domains (lymphoid cell kinase, Abelson kinase). The extrapolated K(d) of interaction for the Csk catalytic domain with the Csk SH3 domain was 2.2 microM and that of the Csk catalytic domain with the Csk SH3-SH2 fragment was 8.8 microM. Taken together, these findings suggest that there is likely an intramolecular interaction between the catalytic and SH3 domains in full-length Csk that is important for efficient catalysis. By employing a Csk SH3 specific type II polyproline helix peptide and carrying out site-directed mutagenesis, it was established that the SH3 surface that interacts with the catalytic domain was distinct from the surface that binds type II polyproline helix peptides. This finding suggests a novel mode of protein-protein interaction for an SH3 domain. The implications for Csk substrate selectivity, regulation, and function are discussed.

Amino Acid Sequence↗

Marek's disease virus latent protein MEQ: delineation of an epitope in the BR1 domain involved in nuclear localization.

Marek's disease virus latent protein MEQ (MDV Eco Q) is abundantly expressed and consistently detected in MDV-induced tumors and cell lines. Deletion mutants were constructed to study the domain structure of MEQ. Four deletion mutants were obtained in the basic regions of MEQ, namely basic region 1 (DeltaBR1), basic region 2 (DeltaBR2), basic regions 1 and 2 (DeltaBR1 and 2), and the C-terminal (bZIP) domain. The BR1 and BR2 are nuclear localization signals and either is sufficient to cause transport of MEQ into the nucleus. In addition, the BR2 is also responsible for MEQ's nucleolar localization. A monoclonal antibody (Mab 23B46) was produced using recombinant fowlpox virus (rFPV) expressing MEQ (rFPV/MEQ) as a source of protein. The isotype of Mab 23B46 is IgG1 and immunoprecipitated a band in rFPV/MEQ infected cells with molecular weight of 60 kDa specific to MEQ protein. We detected abundant expression of MEQ in (rFPV/MEQ), recombinant baculovirus (rBac) (rBac/MEQ), and lymphoid tumors induced by MDV. In order to delineate the epitope of MEQ reactive with Mab 23B46, we used four deletion mutants from the basic and bZIP domains. We found the deletions in the N-terminal region including BR1 (DeltaBR1), and (DeltaBR1 and 2) completely abolished the specific binding with Mab 23B46 as shown by Western blot analysis and immunofluoresence test. Deletion of BR2 (DeltaBR2) and the C-terminal (bZIP) domain had no effect on antibody binding. These data provide direct evidence that monoclonal antibody reactive epitope is localized in the BR1 domain of the molecule. Since both BR1 and BR2 domains contain sequences important for nuclear entry, we now have reagent to further study and elucidate the mechanism of MEQ's involvement in nuclear and nucleolar localization.

Amino Acid Sequence↗

Mutations in the v-Rel transactivation domain indicate altered phosphorylation and identify a subset of NF-kappaB-regulated cell death inhibitors important for v-Rel transforming activity.

Consistent with the constitutive activation of Rel/NF-kappaB in human hematopoietic tumors, the v-Rel oncoprotein induces aggressive leukemia/lymphomas in animal models. v-Rel is thus a valuable tool to characterize the role of Rel/NF-kappaB in cancer and the mechanisms involved. Prior studies by our group identified a serine-rich domain in v-Rel that was required for biological activity. Here, we investigated the molecular basis for the transformation defect of specific serine mutants. We show that the transforming efficiency of these mutants in primary lymphoid cells is correlated with their ability to mediate kappaB site-dependent transactivation and with specific changes in phosphorylation profiles. Interestingly, coexpression of the death antagonists Bcl-xL and Bcl-2 significantly increased their oncogenicity, whereas other NF-kappaB-regulated death inhibitors showed little or no effect. The fact that a subset of apoptosis inhibitors could rescue v-Rel transactivation mutants suggests that their reduced transcriptional activity may critically affect expression of defined death antagonists essential for oncogenesis. Consistent with this hypothesis, we observed selection for high endogenous expression of Bcl-2-related death antagonists in cells transformed by weakly transforming v-Rel mutants. These results emphasize the need for Rel/NF-kappaB to efficiently activate expression of a subset of antiapoptotic genes from the Bcl-2 family to manifest its oncogenic phenotype.

Animals↗

Mutations in apoptosis genes: a pathogenetic factor for human disease.

Cell death by apoptosis is exerted by the coordinated action of many different gene products. Mutations in some of them, acting at different levels in the apoptosis process, have been identified as cause or contributing factor for human diseases. Defects in the transmembrane tumor necrosis factor receptor 1 (TNF-R1) lead to the development of familial periodic fever syndromes. Mutations in the homologous receptor Fas (also named CD95; Apo-1) are observed in malignant lymphomas, solid tumors and the autoimmune lymphoproliferative syndrome type I (ALPS I). A mutation in the ligand for Fas (Fas ligand; CD95 ligand, Apo-1 ligand), which induces apoptosis upon binding to Fas, was described in a patient with systemic lupus erythematodes and lymphadenopathy. Perforin, an other cytotoxic protein employed by T- and NK-cells for target cell killing, is mutated in chromosome 10 linked cases of familial hemophagocytic lymphohistiocytosis. Caspase 10, a representative of the caspase family of proteases, which plays a central role in the execution of apoptosis, is defect in autoimmune lymphoproliferative syndrome type II (ALPS II). The intracellular pro-apoptotic molecule bcl-10 is frequently mutated in mucosa-associated lymphoid tissue (MALT) lymphomas and various non-hematologic malignancies. The p53, an executioner of DNA damage triggered apoptosis, and Bax, a pro-apoptotic molecule with the ability to perturb mitochondrial membrane integrity, are frequently mutated in malignant neoplasms. Anti-apoptotic proteins like bcl-2, cellular-inhibitor of apoptosis protein 2 (c-IAP2) and neuronal apoptosis inhibitory protein 1 (NAIP1) are often altered in follicular lymphomas, MALT lymphomas and spinal muscular atrophy (SMA), respectively. This article reviews the current knowledge on mutations of apoptosis genes involved in the pathogenesis of human diseases and summarises the gradual transformation of discoveries in apoptosis research into benefits for the clinical management of diseases.

Adaptor Proteins, Signal Transducing↗

2D7 diabody bound to the alpha2 domain of HLA class I efficiently induces caspase-independent cell death against malignant and activated lymphoid cells.

A mouse monoclonal antibody (2D7 mAb), which specifically bound to the alpha2 domain of HLA class I, rapidly induces cell aggregation accompanied by weak cytotoxicity against ARH-77 cells, suggesting that 2D7 mAb had a potential for agonist antibody. In order to enhance this cytotoxicity, 2D7 mAb was engineered to be a small bivalent antibody fragment, 2D7 diabody. The resultant 2D7 diabody showed a strong cytotoxicity against ARH-77 cells. As a notable characteristic feature, the lethal effect of 2D7 diabody was quite rapid, mediated by a caspase-independent death pathway. Furthermore, 2D7 diabody also showed cytotoxicity against several leukemia and lymphoma cell lines, and mitogen-activated peripheral blood mononuclear cells (PBMC), but not for normal resting PBMC and adherent cell lines such as HUVEC. These results suggest that 2D7 diabody could be expected as a novel therapeutic antibody for hematological malignancies as well as inflammatory diseases.

Animals↗

A recombinant CD7-specific single-chain immunotoxin is a potent inducer of apoptosis in acute leukemic T cells.

A recombinant immunotoxin was constructed from the hybridoma antibody TH-69 directed against human CD7, a surface antigen of leukemic T cells. The antibody was subcloned as a single chain Fv (scFv) fragment and genetically linked to a truncated Pseudomonas exotoxin A fragment containing the catalytic domains II and III but lacking the receptor binding domain I. Domain I was replaced by the scFv, thus conferring restricted specificity for CD7-positive cells. The bacterially expressed and purified toxin retained binding specificity for CD7-positive cells. It promoted apoptosis in two CD7-positive cell lines derived from T-lineage acute lymphoblastic leukemias, CEM and Jurkat, but not in the CD7-negative B-lymphoid lines REH, Nalm-6, and SEM. Maximum killing in excess of 95% was reached after 96 h in CEM and Jurkat cells with a single dose of 100 ng/ml. Cells treated with a similarly constructed scFv-exotoxin A immunotoxin against melanoma-associated chondroitin sulfate proteoglycan, an antigen absent from leukemic T cells, remained unaffected. Lysis of target cells occurred via apoptosis as evidenced by staining with Annexin V and specific cleavage of poly(ADP-ribose) polymerase. Approximately 20% of leukemic cells from a patient with CD7-positive acute T-cell leukemia kept in long-term primary culture for 30 cell generations were killed within 96 h after treatment with the toxin. These findings justify further evaluation of the agent in view of potential therapeutic applications.

ADP Ribose Transferases↗

Requirement for Abl kinases in T cell receptor signaling.

BACKGROUND: The c-Abl and Arg proteins comprise a unique family of nonreceptor tyrosine kinases that have been implicated in the regulation of cell proliferation and survival, cytoskeletal reorganization, cell migration, and the response to oxidative stress and DNA damage. Targeted deletion or mutation of c-Abl in mice results in a variety of immune system phenotypes, including splenic and thymic atrophy, lymphopenia, and an increased susceptibility to infection. However, despite the generation of these mice over a decade ago, little is known regarding the mechanisms responsible for these phenotypes or the immune-related consequences of ablation of both the c-Abl and Arg kinases, which are coexpressed in lymphoid tissues. RESULTS: Here, we report that T cell receptor (TCR) stimulation results in activation of the endogenous Abl kinases. We demonstrate that Zap70 and the transmembrane adaptor linker for activation of T cells (LAT) are targets of the Abl kinases, and that loss of Abl kinase activity reduces TCR-induced Zap70 phosphorylation at tyrosine 319. This correlates with diminished LAT tyrosine phosphorylation, as well as reduced tyrosine phosphorylation and recruitment of phospholipase Cgamma1 to LAT. Significantly, we show that Abl kinase activity is required for maximal signaling leading to transcription of the IL-2 promoter, as well as TCR-induced IL-2 production and proliferation of primary T cells. CONCLUSIONS: We conclude that the Abl kinases have a role in the regulation of TCR-mediated signal transduction leading to IL-2 production and cell proliferation.

Adaptor Proteins, Signal Transducing↗

Cell-free expression and functional reconstitution of CALM in clathrin assembly.

Clathrin-mediated vesicle formation is an essential step in the intracellular trafficking of the protein and lipid. Binding of clathrin assembly protein to clathrin triskelia induces their assembly into clathrin-coated vesicles (CCVs). In order to better understand a possible role of post-translational modification of CALM (clathrin assembly protein lymphoid myeloid), the homologue of AP180, in the assembly of CCVs, CALM was expressed in the cell-free reticulocyte translation system that is capable of carrying out post-translational modification. The apparent molecular weight of the expressed recombinant CALM was estimated as 105 kD. Alkaline phosphatase treatment of CALM resulted in a mobility shift on SDS-PAGE. We found that CALM was associated with the proteins harboring SH3 domain, promote assembly of clathrin triskelia into clathrin cage and bound to the preformed clathrin cage. CALM was also proteolyzed by caspase 3 and calpain but not by caspase 8. These results indicated that the post-translationally modified CALM, expressed in the eukaryotic cell-free reticulocyte translation system was able to mediate the assembly of clathrin and the coated-vesicle formation.

Adaptor Proteins, Vesicular Transport↗

Association of distinct tetraspanins with MHC class II molecules at different subcellular locations in human immature dendritic cells.

Dendritic cells have the capacity to trigger T cell responses in lymphoid organs against antigens captured in the periphery. T cell stimulation depends on the ability of MHC class II molecules to present peptides at the cell surface that are acquired in MHC class II compartments. The high capacity of dendritic cells to stimulate T lymphocytes is related to their ability to regulate the distribution of MHC class II molecules intracellularly. To analyze the molecular components involved in the generation of MHC class II-peptide complexes in human immature dendritic cells, mAb were raised against purified MHC class II compartments. One of the antigens turned out to be CD63, a member of the tetraspanin superfamily. CD63 localized exclusively intracellularly where it associated with peptide-loaded class II molecules. In contrast, the tetraspanins CD9, CD53 and CD81 associated with class II molecules at the plasma membrane. Selective association of distinct tetraspanins may be involved in the regulation of MHC class II distribution in human dendritic cells.

Animals↗

HIP-55 is important for T-cell proliferation, cytokine production, and immune responses.

Engagement of the T-cell receptor (TCR) triggers a series of signaling events that lead to the activation of T cells. HIP-55 (SH3P7 or mAbp1), an actin-binding adaptor protein, interacts with and is tyrosine phosphorylated by ZAP-70, which is a crucial proximal protein tyrosine kinase for TCR signaling. HIP-55 is important for JNK and HPK1 activation induced by TCR signaling. In this study, we report the generation and characterization of HIP-55 knockout mice. We found that HIP-55 knockout mice were viable and fertile but showed decreased body weight and increased occurrence of death within the first 4 weeks after birth. The lymphoid organs in HIP-55 knockout mice showed cellularity and T-cell development comparable to that of the wild-type mice. HIP-55 knockout T cells displayed defective T-cell proliferation, decreased cytokine production, and decreased up-regulation of the activation markers induced by TCR stimulation. TCR internalization was slightly increased in HIP-55 knockout T cells. These phenotypes were accompanied by reduced immune responses, including antigen-specific antibody production and T-cell proliferation in HIP-55 knockout mice. The TCR-induced signaling events, including LAT/phospholipase Cgamma1 phosphorylation and HPK1/JNK activation, were partially defective in HIP-55 knockout T cells. These results demonstrate the importance of HIP-55 as an adaptor protein in the TCR signaling and immune system.

Actins↗

Three-dimensional clustering of human RAG2 gene mutations in severe combined immune deficiency.

The V(D)J recombination, which leads to the somatic rearrangement of variable, diversity, and joining segments, is the mechanism accountable for the diversity of T cell receptor- and Ig-encoding genes. The products of the RAG1 and RAG2 genes are the lymphoid-specific factors responsible for the initiation of the V(D)J recombination through the generation of a DNA double strand break. RAG1 or RAG2 gene inactivation in the mouse leads to abortion of the V(D)J rearrangement process, early block in both T and B cell maturation, and, ultimately, to severe combined immune deficiency (SCID). A human SCID condition is also characterized by an absence of mature T and B lymphocytes and is associated with mutations in either RAG1- or RAG2-encoding genes. Based on the predicted beta-propeller three-dimensional structure model for RAG2, we found that six out of the seven mutations described to date in T-B-SCID patients are clustered on one side of the propeller, in regions exposed to solvent. This finding reinforces the biological significance of this predicted model and suggests that RAG1 interacts with RAG2 on one of the side of the scaffold formed by the beta-propeller.

Amino Acid Sequence↗

Jamip1 (marlin-1) defines a family of proteins interacting with janus kinases and microtubules.

Jamip1 (Jak and microtubule interacting protein), an alias of Marlin-1, was identified for its ability to bind to the FERM (band 4.1 ezrin/radixin/moesin) homology domain of Tyk2, a member of the Janus kinase (Jak) family of non-receptor tyrosine kinases that are central elements of cytokine signaling cascades. Jamip1 belongs to a family of three genes conserved in vertebrates and is predominantly expressed in neural tissues and lymphoid organs. Jamip proteins lack known domains and are extremely rich in predicted coiled coils that mediate dimerization. In our initial characterization of Jamip1 (73 kDa), we found that it comprises an N-terminal region that targets the protein to microtubule polymers and, when overexpressed in fibroblasts, profoundly perturbs the microtubule network, inducing the formation of tight and stable bundles. Jamip1 was shown to associate with two Jak family members, Tyk2 and Jak1, in Jurkat T cells via its C-terminal region. The restricted expression of Jamip1 and its ability to associate to and modify microtubule polymers suggest a specialized function of these proteins in dynamic processes, e.g. cell polarization, segregation of signaling complexes, and vesicle traffic, some of which may involve Jak tyrosine kinases.

Adaptor Proteins, Signal Transducing↗

Expression of vascular cell adhesion molecule-1 by follicular dendritic cells.

Follicular dendritic cells are the major supporting cell of the germinal center microenvironment. The major function of follicular dendritic cells is to present antigen to B cells in secondary lymphoid tissues. Through cell-cell interactions, FDCs are hypothesized to be central to the regulation of normal B cell growth and differentiation. The major receptor-ligand pair which mediates B cell-FDC adhesion is the beta 1 integrin VLA-4, present on B cells and VCAM-1 expressed on FDCs. Follicular non-Hodgkin's lymphomas similarly employ this mechanism to bind to neoplastic germinal centers. The VCAM-1 molecule can exist as a 6 or 7 immunoglobulin domain form. The major form of VCAM-1 on activated endothelium is the 7 domain form. In this report we have determined by polymerase chain reaction of purified FDCs that they express predominantly mRNA for 7 domain VCAM-1. It is likely that the two forms of VCAM-1 are associated with distinct functions, therefore the expression of 7 domain VCAM-1 may be important in normal and neoplastic B cell-FDC interactions.

Base Sequence↗

Deletion of the RAG2 C terminus leads to impaired lymphoid development in mice.

The recombination-activating gene (RAG)1 and RAG2 proteins comprise the lymphocyte-specific components of the V(D)J recombinase and are required for the assembly of antigen-receptor variable-region genes. A mutant truncated RAG2 protein ("core" RAG2) lacking the C-terminal 144 amino acids, together with core RAG1, is able to mediate the basic biochemical steps required for V(D)J recombination in vitro and in transfected cell lines. Here we examine the effect of replacing the endogenous RAG2 locus in mice with core RAG2. These mice generate substantial numbers of B and T cells, demonstrating that the core RAG2 protein retains significant in vivo function. However, core RAG2 mice display a reduction in the total number of B and T cells, reflecting impaired lymphocyte development at the progenitor stage associated with reduced chromosomal V(D)J recombination. We discuss potential roles of the RAG2 C terminus in mediating rearrangement of endogenous antigen-receptor loci.

Animals↗