Tyrosine kinases and tyrosine-based activation motifs. Current research on activation via the T cell antigen receptor

CD3ζ ITAMs enable ligand discrimination and antagonism by inhibiting TCR signaling in response to low-affinity peptides

The T cell antigen receptor (TCR) contains ten immunoreceptor tyrosine-based activation motif (ITAM) signaling sequences distributed within six CD3 subunits; however, the reason for such structural complexity and multiplicity is unclear. Here we evaluated the effect of inactivating the three CD3ζ chain ITAMs on TCR signaling and T cell effector responses using a conditional 'switch' mouse model. Unexpectedly, we found that T cells expressing TCRs containing inactivated (non-signaling) CD3ζ ITAMs (6F-CD3ζ) exhibited reduced ability to discriminate between low- and high-affinity ligands, resulting in enhanced signaling and cytokine responses to low-affinity ligands because of a previously undetected inhibitory function of CD3ζ ITAMs. Also, 6F-CD3ζ TCRs were refractory to antagonism, as predicted by a new in silico adaptive kinetic proofreading model that revises the role of ITAM multiplicity in TCR signaling. Finally, T cells expressing 6F-CD3ζ displayed enhanced cytolytic activity against solid tumors expressing low-affinity ligands, identifying a new counterintuitive approach to TCR-mediated cancer immunotherapy.

Structural principles of B cell antigen receptor assembly

The B cell antigen receptor (BCR) is composed of a membrane-bound class M, D, G, E or A immunoglobulin for antigen recognition1-3 and a disulfide-linked Igα (also known as CD79A) and Igβ (also known as CD79B) heterodimer (Igα/β) that functions as the signalling entity through intracellular immunoreceptor tyrosine-based activation motifs (ITAMs)4,5. The organizing principle of the BCR remains unknown. Here we report cryo-electron microscopy structures of mouse full-length IgM BCR and its Fab-deleted form. At the ectodomain (ECD), the Igα/β heterodimer mainly uses Igα to associate with Cµ3 and Cµ4 domains of one heavy chain (µHC) while leaving the other heavy chain (µHC') unbound. The transmembrane domain (TMD) helices of µHC and µHC' interact with those of the Igα/β heterodimer to form a tight four-helix bundle. The asymmetry at the TMD prevents the recruitment of two Igα/β heterodimers. Notably, the connecting peptide between the ECD and TMD of µHC intervenes in between those of Igα and Igβ to guide TMD assembly through charge complementarity. Weaker but distinct density for the Igβ ITAM nestles next to the TMD, suggesting potential autoinhibition of ITAM phosphorylation. Interfacial analyses suggest that all BCR classes utilize a general organizational architecture. Our studies provide a structural platform for understanding B cell signalling and designing rational therapies against BCR-mediated diseases.

Modulating binding affinity, specificity and configurations by multivalent interactions

Biological functions of proteins rely on their specific interactions with binding partners. Many proteins contain multiple domains, which can bind to their targets that often have more than one binding site, resulting in multivalent interactions. While it has been shown that multivalent interactions play an crucial role in modulating binding affinity and specificity, other potential effects of multivalent interactions are less explored. Here, we developed a broadly applicable transfer matrix formalism and used it to investigate the binding of two-domain ligands to targets with multiple binding sites. We show that 1) ligands with two specific binding domains can drastically boost both the binding affinity and specificity and down-shift the working concentration range, compared to single-domain ligands, 2) the presence of a positive domain-domain cooperativity or containing a non-specific binding domain can down-shift the working concentration range of ligands by increasing the binding affinity without compromising the binding specificity, 3) the configuration of the bound ligands has a strong concentration dependence, providing important insights into the physical origin of phase-separation processes taking place in living cells. In line with previous studies, our results suggest that multivalent interactions are utilized by cells for highly efficient regulation of target binding involved in a diverse range of cellular processes such as signal transduction, gene transcription, antibody-antigen recognition.

Tyrosine-phosphorylation of the scaffold protein ADAP and its role in T cell signaling

INTRODUCTION

The Adhesion and Degranulation promoting Adaptor Protein (ADAP) is phosphorylated upon T cell activation and acts as a scaffold for the formation of a signaling complex that integrates molecular interactions between T cell or chemokine receptors, the actin cytoskeleton, and integrin-mediated cellular adhesion and migration.

AREAS COVERED

This article reviews current knowledge of the functions of the adapter protein ADAP in T cell signaling with a focus on the role of individual phosphotyrosine (pY) motifs for SH2 domain mediated interactions. The data presented was obtained from literature searches (PubMed) as well as the authors own research on the topic. Expert commentary: ADAP can be regarded as a paradigmatic example of how tyrosine phosphorylation sites serve as dynamic interaction hubs. Molecular crowding at unstructured and redundant sites (pY595, pY651) is contrasted by more specific interactions enabled by the three-dimensional environment of a particular phosphotyrosine motif (pY571).

The mouse B cell-specific mb-1 gene encodes an immunoreceptor tyrosine-based activation motif (ITAM) protein that may be evolutionarily conserved in diverse species by purifying selection

The B-lymphocyte accessory molecule Ig-alpha (Ig-α) is encoded by the mouse B cell-specific gene (mb-1), and along with the Ig-beta (Ig-β) molecule and a membrane bound immunoglobulin (mIg) makes up the B-cell receptor (BCR). Ig-α and Ig-β form a heterodimer structure that upon antigen binding and receptor clustering primarily initiates and controls BCR intracellular signaling via a phosphorylation cascade, ultimately triggering an effector response. The signaling capacity of Ig-α is contained within its immunoreceptor tyrosine-based activation motif (ITAM), which is also a key component for intracellular signaling initiation in other immune cell-specific receptors. Although numerous studies have been devoted to the mb-1 gene product, Ig-α, and its signaling mechanism, an evolutionary analysis of the mb-1 gene has been lacking until now. In this study, mb-1 coding sequences from 19 species were compared using Bayesian inference. Analysis revealed a gene phylogeny consistent with an expected species divergence pattern, clustering species from the primate order separate from lower mammals and other species. In addition, an overall comparison of non-synonymous and synonymous nucleotide mutational changes suggests that the mb-1 gene has undergone purifying selection throughout its evolution.

Pre-existing clusters of the adaptor Lat do not participate in early T cell signaling events

Engaged T cell antigen receptors (TCRs) initiate signaling through the adaptor protein Lat. In quiescent T cells, Lat is segregated into clusters on the cell surface, which raises the question of how TCR triggering initiates signaling. Using super-resolution fluorescence microscopy, we found that pre-existing Lat domains were neither phosphorylated nor laterally transported to TCR activation sites, which suggested that these clusters do not participate in TCR signaling. Instead, TCR activation resulted in the recruitment and phosphorylation of Lat from subsynaptic vesicles. Studies of Lat mutants confirmed that recruitment preceded and was essential for phosphorylation and that both processes were independent of surface clustering of Lat. Our data suggest that TCR ligation preconditions the membrane for vesicle recruitment and bulk activation of the Lat signaling network.

Characterization of CD8+ leukocytes in fugu (Takifugu rubripes) with antiserum against fugu CD8alpha

We have investigated the characteristics of CD8+ leukocytes by using an anti-CD8alpha antiserum raised in mouse by DNA-immunization. The magnetically sorted CD8alpha+ peripheral blood leukocyte (PBL) population comprised lymphocytes/thrombocytes and monocytes, whereas CD8alpha- PBLs consisted of lymphocytes/thrombocytes, monocytes, and neutrophils. Expression analysis demonstrated that both groups of cells expressed the CD3epsilon and TCRalpha genes. The CD8alpha and CD8beta genes were detected only in CD8alpha+ cells, whereas expression of CD4 and immunoglobulin light chain (IgL) was observed only in CD8alpha- cells. These results suggest that fugu CD8alpha+ leukocytes contain CD8+ T cells, but not CD4+ T cells or B cells. Furthermore, mitogenesis of the CD8+ lymphocyte/thrombocyte population was induced by phytohemaglutinin stimulation, suggesting that fish CD8+ lymphocytes/thrombocytes (probably CD8+ T cells) have characteristics similar to mammalian CD8+ T cells. Neutrophils and monocytes/macrophages infiltrating a subcutaneous inflammatory site expressed only CD8alpha, but not CD8beta, CD4, TCRalpha, or IgL. This result suggests that similar to mammalian dendritic cells, fugu monocytes/macrophages express CD8alpha.

T-B+NK+ severe combined immunodeficiency caused by complete deficiency of the CD3zeta subunit of the T-cell antigen receptor complex

CD3zeta is a subunit of the T-cell antigen receptor (TCR) complex required for its assembly and surface expression that also plays an important role in TCR-mediated signal transduction. We report here a patient with T(-)B(+)NK(+) severe combined immunodeficiency (SCID) who was homozygous for a single C insertion following nucleotide 411 in exon 7 of the CD3zeta gene. The few T cells present contained no detectable CD3zeta protein, expressed low levels of cell surface CD3epsilon, and were nonfunctional. CD4(+)CD8(-)CD3epsilon(low), CD4(-)CD8(+)CD3epsilon(low), and CD4(-)CD8(-)CD3epsilon(low) cells were detected in the periphery, and the patient also exhibited an unusual population of CD56(-)CD16(+) NK cells with diminished cytolytic activity. Additional studies demonstrated that retrovirally transduced patient mutant CD3zeta cDNA failed to rescue assembly of nascent complete TCR complexes or surface TCR expression in CD3zeta-deficient MA5.8 murine T-cell hybridoma cells. Nascent transduced mutant CD3zeta protein was also not detected in metabolically labeled MA5.8 cells, suggesting that it was unstable and rapidly degraded. Taken together, these findings provide the first demonstration that complete CD3zeta deficiency in humans can cause SCID by preventing normal TCR assembly and surface expression.

PAG-associated FynT regulates calcium signaling and promotes anergy in T lymphocytes

Phosphoprotein associated with glycolipid-enriched membranes (PAG), also named Csk-binding protein (Cbp), is a transmembrane adaptor associated with lipid rafts. It is phosphorylated on multiple tyrosines located in the cytoplasmic domain. One tyrosine, tyrosine 314 (Y314) in the mouse, interacts with Csk, a protein tyrosine kinase that negatively regulates Src kinases. This interaction enables PAG to inhibit T-cell antigen receptor (TCR)-mediated T-cell activation. PAG also associates with the Src-related kinase FynT. Genetic studies indicated that FynT was required for PAG tyrosine phosphorylation and binding of PAG to Csk in T cells. Herein, we investigated the function and regulation of PAG-associated FynT. Our data showed that PAG was constitutively associated with FynT in unstimulated T cells and that this association was rapidly lost in response to TCR stimulation. Dissociation of the PAG-FynT complex preceded PAG dephosphorylation and PAG-Csk dissociation after TCR engagement. Interestingly, in anergic T cells, the association of PAG with FynT, but not Csk, was increased. Analyses of PAG mutants provided evidence that PAG interacted with FynT by way of tyrosines other than Y314. Enforced expression of a PAG variant interacting with FynT, but not Csk, caused a selective enhancement of TCR-triggered calcium fluxes in normal T cells. Furthermore, it promoted T-cell anergy. Both effects were absent in mice lacking FynT, implying that the effects were mediated by PAG-associated FynT. Hence, besides enabling PAG tyrosine phosphorylation and the PAG-Csk interaction, PAG-associated FynT can stimulate calcium signals and favor T-cell anergy. These data improve our comprehension of the function of PAG in T cells. They also further implicate FynT in T-cell anergy.

Migration inhibitory factor up-regulates vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 via Src, PI3 kinase, and NFkappaB

Cell adhesion molecules are critical in monocyte (MN) recruitment in immune-mediated and hematologic diseases. We investigated the novel role of recombinant human migration inhibitory factor (rhMIF) in up-regulating vascular cell adhesion molecule-1 (VCAM-1) and intercellular adhesion molecule-1 (ICAM-1) and their signaling pathways in human MNs. rhMIF-induced expression of VCAM-1 and ICAM-1 was significantly higher compared with nonstimulated MNs. rhMIF induced MN VCAM-1 and ICAM-1 expression in a concentration-dependent manner (P < .05). Antisense oligodeoxynucleotides (ODNs) and inhibitors of Src, PI3K, p38, and NFkappaB significantly reduced rhMIF-induced MN VCAM-1 and ICAM-1 expression (P < .05). However, Erk1/2 and Jak2 were not involved. Silencing RNA directed against MIF, and inhibitors of Src, PI3K, NFkappaB, anti-VCAM-1, and anti-ICAM-1 significantly inhibited rhMIF-induced adhesion of HL-60 cells to human dermal microvascular endothelial cells (HMVECs) or an endothelial cell line, HMEC-1, in cell adhesion assays, suggesting the functional significance of MIF-induced adhesion molecules (P < .05). rhMIF also activated MN phospho-Src, -Akt, and -NFkappaB in a time-dependent manner. rhMIF induced VCAM-1 and ICAM-1 up-regulation in 12 hours via Src, PI3K, and NFkappaB as shown by Western blotting and immunofluorescence. MIF and MIF-dependent signaling pathways may be a potential target for treating diseases characterized by up-regulation of cell adhesion molecules.

Structural basis for the function and regulation of the receptor protein tyrosine phosphatase CD45

CD45 is the prototypic member of transmembrane receptor-like protein tyrosine phosphatases (RPTPs) and has essential roles in immune functions. The cytoplasmic region of CD45, like many other RPTPs, contains two homologous protein tyrosine phosphatase domains, active domain 1 (D1) and catalytically impaired domain 2 (D2). Here, we report crystal structure of the cytoplasmic D1D2 segment of human CD45 in native and phosphotyrosyl peptide-bound forms. The tertiary structures of D1 and D2 are very similar, but doubly phosphorylated CD3ζ immunoreceptor tyrosine-based activation motif peptide binds only the D1 active site. The D2 “active site” deviates from the other active sites significantly to the extent that excludes any possibility of catalytic activity. The relative orientation of D1 and D2 is very similar to that observed in leukocyte common antigen–related protein with both active sites in an open conformation and is restrained through an extensive network of hydrophobic interactions, hydrogen bonds, and salt bridges. This crystal structure is incompatible with the wedge model previously suggested for CD45 regulation.

The magnitude of TCR engagement is a critical predictor of T cell anergy or activation

Fast dissociation rate of peptide-MHC complexes from TCR has commonly been accepted to cause T cell anergy. In this study, we present evidence that peptides that form transient complexes with HLA-DR1 induce anergy in T cell clones in vitro and specific memory T cells in vivo. We demonstrate that similar to the low densities of long-lived agonist peptide-MHC, short-lived peptide-MHC ligands induce anergy by engagement of approximately 1000 TCR and activation of a similar pattern of intracellular signaling events. These data strongly suggest that short-lived peptides induce anergy by presentation of low densities of peptide-MHC complexes. Moreover, they suggest that the traditional antagonist peptides might also trigger anergy by a similar molecular mechanism. The use of short-lived peptides to induce T cells anergy is a potential strategy for the prevention or treatment of autoimmune diseases.

Signal transduction of phagocytosis

The interaction of particles with certain cell surface receptors initiates intracellular signalling pathways that ultimately lead to submembranous actin filament assembly, pseudopod extension, and the ingestion of the particles. Here, Steven Greenberg reviews recent evidence implicating various signalling events in phagocytosis--in particular, activation of tyrosine kinases and phosphatidylinositol 3-kinase--and speculates how they might regulate the actin cytoskeleton.

B-cell antigen-receptor signalling in lymphocyte development

Signalling through the B-cell antigen receptor (BCR) is required throughout B-cell development and peripheral maturation. Targeted disruption of BCR components or downstream effectors indicates that specific signalling mechanisms are preferentially required for central B-cell development, peripheral maturation and repertoire selection. Additionally, the avidity and the context in which antigen is encountered determine both cell fate and differentiation in the periphery. Although the signalling and receptor components required at each stage have been largely elucidated, the molecular mechanisms through which specific signalling are evoked at each stage are still obscure. In particular, it is not known how the pre-BCR initiates the signals required for normal development or how immature B cells regulate the signalling pathways that determine cell fate. In this review, we will summarize the recent studies that have defined the molecules required for B-cell development and maturation as well as the theories on how signals may be regulated at each stage.

Activation of Syk tyrosine kinase is required for c-Cbl-mediated ubiquitination of Fcepsilon RI and Syk in RBL cells

Engagement of the high affinity receptor for IgE (FcepsilonRI) on mast cells and basophils results in FcepsilonRI beta and gamma subunits ubiquitination by an as yet undefined mechanism. Here we show that, upon FcepsilonRI engagement on RBL-2H3 cells Syk undergoes ubiquitination and Syk kinase activity is required for its own ubiquitination and that of FcepsilonRI beta and gamma chains. This requirement was demonstrated by overexpression of Syk wild-type or its kinase-dead mutant in RBL cells or using an Syk-deficient RBL-derived cell line transfected with wild-type or a kinase inactive form of Syk. We also identify c-Cbl as the E3 ligase responsible for both Syk and receptor ubiquitination. Furthermore, we demonstrate that Syk controls tyrosine phosphorylation of Syk-associated Cbl induced after receptor engagement. These data suggest a mutual regulation between Syk and Cbl activities. Finally, we show that a selective inhibitor of proteasome degradation induces persistence of tyrosine-phosphorylated receptor complexes, of activated Syk, and of FcepsilonRI-triggered degranulation. Our results provide a molecular mechanism for down-regulation of engaged receptor complexes by targeting ubiquitinated FcepsilonRI and activated Syk to the proteasome for degradation.

Differentiation of the human monocyte cell line, U937, with dibutyryl cyclicAMP induces the expression of the inhibitory Fc receptor, FcgammaRIIb

FC receptor for IgG receptor (Fcgamma) mediated activation of macrophages is essential for the clearance of immune complexes and control of inflammation. However, activated macrophages play an integral role in tissue destruction associated with autoimmune and inflammatory disease processes. Understanding the mechanisms which balance activating and inhibitory signals generated by immune complexes are therefore of critical importance to human disease. Here, we demonstrate that differentiation of the human monocytic U937 cell line to a macrophage phenotype with dibutyryl cyclicAMP induces both mRNA and protein expression of the inhibitory IgG receptor, FcgammaRIIb1. We further demonstrate that, following receptor aggregation, FcgammaRII transiently recruits the 5'-inositol phosphatase, SHIP. These data define a role for FcgammaRIIb in the modulation of immune complex mediated macrophage activation in a human model system.

Defective expression and tyrosine phosphorylation of the T cell receptor zeta chain in peripheral blood T cells from systemic lupus erythematosus patients

We have reported that tyrosine phosphorylation and expression of the T cell receptor zeta chain (TCR zeta) was decreased in two systemic lupus erythematosus (SLE) patients with an abnormal TCR zeta lacking exon-7. To examine further the TCR zeta defect and any possible relationship with specific clinical features, we studied the expression of TCR zeta in peripheral blood T cells from 44 patients with SLE, 53 with other rheumatic diseases (30 rheumatoid arthritis (RA), 11 systemic sclerosis (SSc) and 12 primary Sjögren's syndrome(SjS)) and 39 healthy individuals. Flow cytometric analysis demonstrated a significant decrease in the expression of TCR zeta in SLE (P < 0.001), but not in the other rheumatic diseases. Immunoprecipitation experiments confirmed that the expression of TCR zeta in SLE T cells was decreased dramatically (normal: 111.4 +/- 22.6%, SLE: 51.6 +/- 37.4%, P < 0.0001). The decrease in TCR zeta did not correlate with disease activity, or with the dose of prednisolone (PSL). There were, however, three SLE patients in whom the level of TCR zeta expression normalized after treatment, suggesting that mechanisms responsible for the TCR zeta defect appear to be heterogeneous. These results confirm the defective expression and altered tyrosine phosphorylation of TCR zeta in a large proportion of SLE patients, suggesting that it may play an important role in T cell dysfunction in SLE.

The lymphoid protein tyrosine phosphatase Lyp interacts with the adaptor molecule Grb2 and functions as a negative regulator of T-cell activation

OBJECTIVE

Following activation of T cells, phosphorylation of tyrosine residues occurs through a complex signaling process involving protein tyrosine kinases, phosphatases, and a variety of adapter molecules including Grb2. We have attempted to identify new signaling molecules that are important for the activation response.

METHODS

Using a protein interaction screening protocol based on phage display, T-cell signaling components that associate with the adapter molecule, Grb2, the lymphoid-specific tyrosine phosphatase Lyp was identified. Using transcriptional reporter assays, the role of Lyp in T-cell activation was studied by overexpression of wild-type or catalytically inactive mutants of Lyp.

RESULTS

A GST fusion containing the C-terminal SH3 domain of Grb2 bound to the nucleotide exchange factor Sos or Grb2-associated binder 2 (Gab2). In contrast, the N-terminal SH3-containing fusion bound to the protein tyrosine phosphatase Lyp. Grb2 was co-immunoprecipitated with Lyp in 293T cells overexpressing both proteins. Using Northern blot analysis, Lyp was found to be expressed predominantly in hematopoietic tissue, including spleen, lymph node, thymus, peripheral blood leukocytes, bone marrow, and fetal liver. Two human T-cell lines, Jurkat and HuT78, expressed both Lyp mRNA and protein. Overexpression of wild-type Lyp or a catalytically inactive, substrate-trapping mutant (D195A) in Jurkat cells inhibited transcriptional activity initiated by anti-CD3 and anti-CD28 antibodies. In contrast, two other catalytically inactive mutants (R233M or C227S) had no effect.

CONCLUSION

These data demonstrate a novel interaction between the phosphatase Lyp and the adaptor Grb2 and are consistent with a negative regulatory role for Lyp in T-cell signaling.

Piceatannol is an effective inhibitor of IgE-mediated secretion from human basophils but is neither selective for this receptor nor acts on syk kinase at concentrations where mediator release inhibition occurs

BACKGROUND

Syk kinase is probably an early necessary tyrosine kinase involved in IgE-mediated secretion from human basophils. Causal testing of the role of syk kinase in the secretion requires a selective pharmacological agent. Piceatannol has previously been used to demonstrate the causal role of syk in secretion but its selectively has recently come into question.

OBJECTIVE

To determine whether piceatannol inhibits IgE-mediated signalling events in a manner consistent with its putative inhibitory effects on syk kinase and at concentrations relevant to its inhibition of mediator release.

METHODS

Human basophils were examined for the effects of piceatannol on mediator release or various signalling steps.

RESULTS

We show that while piceatannol has an IC50 for inhibition of IgE-mediated histamine release of 3-5 microm, these same concentrations inhibit secretion of phorbol 12-myristate 13-acetate (PMA)-induced histamine release (as previously shown) and leukotriene C (LTC)4 release induced by fMLP. Concentrations of piceatannol up to 100 microm also did not inhibit IgE-mediated phosphorylation of shc, a immediate downstream target of syk kinase. Similar concentrations also did not inhibit IgE-mediated cytosolic calcium elevations, another downstream signal thought to be dependent on syk kinase. In contrast, piceatannol did modify the cytosolic calcium response that follows stimulation with formyl methionyl-leucyl-phenylalanine (fMLP).

CONCLUSION

Taken together with published studies using other cell types, we conclude that piceatannol does not inhibit secretion from human basophils by inhibiting the activity of syk kinase.

Alterations in transcription factor binding at the IL-2 promoter region in anergized human CD4+ T lymphocytes

BACKGROUND

The mechanisms responsible for the induction of clonal anergy are not well understood. We have utilized an in vitro model of human T cell anergy to explore the perturbations in cell signaling at the level of interleukin (IL)-2 gene transcription and to define the contribution of other cytokines to this effect.

METHODS

An in vitro model of clonal anergy was established by using CD4+ T lymphocytes from healthy human donors. Cells were anergized by prestimulation with an anti-CD3 monoclonal antibody (mAb) followed by restimulation 72 hr later with anti-CD3 mAb with or without anti-CD28.

RESULTS

CD4+ T cells, anergized with anti-CD3 monoclonal antibody (OKT3) prestimulation, displayed a marked reduction in proliferation (P=0.0036) and IL-2 production (P<0.0001). Co-incubation with IL-10 reduced cellular proliferation in OKT3/CD28 pretreated cells by 19% (P=NS) and reduced IL-2 production by 40% (P=0.0024). Anergized T cells demonstrated a reduced binding activity of the AP-1 complex to the IL-2 promoter. Supershift experiments and Western blots confirmed that the binding of c-Fos, JunB, and JunD, but not of FosB, was reduced in anergized cells. At the sis-inducible element (SIE)-binding region of the c-Fos promoter, Stat3 binding was reduced.

CONCLUSIONS

T cell anergy, induced by prestimulation with OKT3, is characterized by reduced proliferation and a profound decrease in IL-2 production. Anergy can be prevented by co-incubation with anti-CD28 and partially re-established by IL-10. Anergy is accompanied by a reduction in AP-1 binding to the IL-2 promoter, with selective reduction in binding of c-Fos, JunB, and JunD. Defective binding for Stat3 at the c-Fos promoter suggests an involvement of the Jak-Stat pathway.

Signal transduction events in human peripheral blood mononuclear cells stimulated by Schistosoma mansoni antigens

Activation of protein tyrosine kinases (PTKs) is a common step of T cell stimulation. However, the relationship between PTKs and activation of peripheral blood mononuclear cells (PBMC) from intestinal chronic schistosomiasis patients has not been explored yet. In this study, we investigated the participation of Lck and ZAP-70 protein tyrosine kinases (PTKs), as well as PLC-gamma1 and Shc proteins in PBMC activation by Schistosoma mansoni antigens. PBMC were stimulated with SEA (soluble egg antigen) or SWAP (soluble worm preparation), lysed, precipitated with specific antibodies and the level of tyrosine phosphorylation evaluated. Our results show that Lck and Shc were phosphorylated upon stimulation of the cells with SWAP, as well as with SEA. However, the phosphorylation level was more pronounced in SWAP than in SEA-stimulated cells. Phosphorylation of ZAP-70 was observed only in SWAP stimulated cells. Additionally, PLC-gamma1 phosphorylation was not observed in PBMC stimulated with SEA. Together, these results indicate that SEA and SWAP induce PBMC proliferation through distinct intracellular signaling pathways. Moreover, the weaker response of PBMC to SEA compared to SWAP stimulation suggests down-regulation of cells from intestinal chronic schistosomiasis patients to SEA, which may occur during immunomodulation to S. mansoni response.

Soluble E-selectin induces monocyte chemotaxis through Src family tyrosine kinases

Cellular adhesion molecules such as E-selectin function to recruit leukocytes into the inflammatory lesions of diseases such as rheumatoid arthritis (RA) and atherosclerosis. Monocytes are the key components of the cellular infiltrates present in these disorders. We hypothesized that soluble E-selectin (sE-selectin) might mediate the chemotaxis of monocytes. In this report, we show that sE-selectin induced normal human peripheral blood monocyte migration in the nanomolar range in a concentration-dependent manner. Neutralization studies using RA human joint synovial fluids and anti-E-selectin antibody showed a mean 31% reduction in RA synovial fluid-mediated monocyte chemotaxis (p < 0.05), indicating that sE-selectin is a major monocyte recruiter in RA. Next, we investigated the role of tyrosine phosphorylation pathways in sE-selectin-induced monocyte chemotaxis. Human peripheral blood monocytes stimulated with sE-selectin showed a time-dependent increase in the tyrosine phosphorylation of a broad range of cellular proteins, predominantly in the molecular size range of Src family kinases (50-60 kDa) and mitogen-activated protein kinases (MAPKs). Western blot analysis of Src family kinases showed a time-dependent increase in Src, Hck, and Lyn phosphorylation. The pretreatment of monocytes with the Src inhibitor AG1879: 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolol[3,4-d]pyrimidine (PP2) prior to stimulation with sE-selectin markedly inhibited Hck and Lyn phosphorylation, whereas the phosphorylation of Src was partially inhibited. In addition, the sE-selectin stimulation of monocytes resulted in the increased phosphorylation of extracellular signal-related kinase (ERK1/2) and p38 MAPK. The pretreatment of monocytes with PP2 showed 89 and 83% inhibition of ERK1/2 and p38 MAPK phosphorylation, respectively. sE-selectin also showed a time-dependent activation of Ras kinase. Furthermore, the pretreatment of monocytes with PP2 completely inhibited sE-selectin-mediated monocyte chemotaxis. Taken together, our data demonstrate a novel function for sE-selectin as a monocyte chemotactic agent and suggest that sE-selectin might be mediating its biological functions through the Src-MAPK pathway.

The clustered Fcgamma receptor II is recruited to Lyn-containing membrane domains and undergoes phosphorylation in a cholesterol-dependent manner

Phosphorylation of clustered Fcgamma receptor II (FcgammaRII) by Src family tyrosine kinases is the earliest event in the receptor signaling cascade. However, the molecular mechanisms for the interaction between FcgammaRII and these kinases are not elucidated. To asses this problem we isolated high molecular weight complexes of cross-linked FcgammaRII from non-ionic detergent lysates of U937 monocytic cells. CD55, a glycosylphosphatidylinositol-anchored protein, a ganglioside GM1 and Lyn, a Src family tyrosine kinase, were also located in these complexes. Gradient centrifugation demonstrated that the complexes containing cross-linked FcgammaRII displayed a low buoyant density. The FcgammaRII present in the complexes underwent tyrosine phosphorylation. Cross-linked FcgammaRII and Lyn occupied common 100-200 nm detergent-resistant membrane fragments, as demonstrated by immunoprecipitation and microscopy studies. Pretreatment of the cells with beta-cyclodextrin, a cholesterol acceptor, depleted membrane cholesterol and released CD55, GM1 and Lyn from the detergent-resistant complexes. In parallel, the association of Lyn with cross-linked FcgammaRII was disrupted and phosphorylation of the receptor inhibited. Reincorporation of cholesterol evoked the relocation of Lyn into the detergent-resistant membrane fraction and restored both Lyn association with cross-linked FcgammaRII and tyrosine phosphorylation of the receptor. Our data demonstrate that cholesterol-enriched membrane rafts can facilitate tyrosine phosphorylation of clustered FcgammaRII by Lyn kinase.

The neutrophil: function and regulation in innate and humoral immunity

The neutrophil is a critical effector cell in humoral and innate immunity and plays vital roles in phagocytosis and bacterial killing. Discussed here are the neutrophil components necessary for these processes and the diseases in which these components are either lacking or dysfunctional, illustrating that normal neutrophil function is vital for health.

Targeting Src homology 2 domain-containing tyrosine phosphatase (SHP-1) into lipid rafts inhibits CD3-induced T cell activation

To study the mechanism by which protein tyrosine phosphatases (PTPs) regulate CD3-induced tyrosine phosphorylation, we investigated the distribution of PTPs in subdomains of plasma membrane. We report here that the bulk PTP activity associated with T cell membrane is present outside the lipid rafts, as determined by sucrose density gradient sedimentation. In Jurkat T cells, approximately 5--10% of Src homology 2 domain-containing tyrosine phosphatase (SHP-1) is constitutively associated with plasma membrane, and nearly 50% of SHP-2 is translocated to plasma membrane after vanadate treatment. Similar to transmembrane PTP, CD45, the membrane-associated populations of SHP-1 and SHP-2 are essentially excluded from lipid rafts, where other signaling molecules such as Lck, linker for activation of T cells, and CD3 zeta are enriched. We further demonstrated that CD3-induced tyrosine phosphorylation of these substrates is largely restricted to lipid rafts, unless PTPs are inhibited. It suggests that a restricted partition of PTPs among membrane subdomains may regulate protein tyrosine phosphorylation in T cell membrane. To test this hypothesis, we targeted SHP-1 into lipid rafts by using the N-terminal region of Lck (residues 1--14). The results indicate that the expression of Lck/SHP-1 chimera inside lipid rafts profoundly inhibits CD3-induced tyrosine phosphorylation of CD3 zeta/epsilon, IL-2 generation, and nuclear mobilization of NF-AT. Collectively, these results suggest that the exclusion of PTPs from lipid rafts may be a mechanism that potentiates TCR/CD3 activation.

The tyrosine kinases p53/56lyn and p72syk are differentially expressed at the protein level but not at the messenger RNA level in nonreleasing human basophils

Within the general population, individuals can be found whose basophils do not secrete after stimulation through the immunoglobulin (Ig) E receptor. In this study we compared two groups of donors, those whose basophils responded with 65+/-16% histamine release to an optimal concentration of anti-IgE antibody and those whose basophil response was not statistically different from nonstimulated release (1+/-1%). We show that these so-called nonreleasing basophils have at least 10-fold lower expression of the tyrosine kinases, lyn and syk, but normal expression of the tyrosine kinase Btk when compared with the panel of releasing basophils. Indeed, maximum histamine release correlated with expression of both syk (Spearman rank correlation coefficient [Rs] = 0.98) and lyn (Rs = 0.93). In contrast, equivalent levels of messenger RNA (mRNA) for lyn and syk kinase were found for both groups. By sequencing a critical region in the syk mRNA, our results also demonstrate that the frame shift mutation in syk leading to a premature stop codon which has been observed in other cell types is not present in nonreleasing human basophils. Our results suggest that there may be translational or post-translational regulatory mechanisms specific to the expression of two important FcepsilonRI-associated signaling elements in basophils.

Differential involvement of Src family kinases in Fc gamma receptor-mediated phagocytosis

The tyrosine phosphorylation cascade originated from Fc gamma receptors (Fc gamma Rs) is essential for macrophage functions including phagocytosis. Although the initial step is ascribed to Src family tyrosine kinases, the role of individual kinases in phagocytosis signaling is still to be determined. In reconstitution experiments, we first showed that expression in the RAW 264.7 cell line of C-terminal Src kinase (Csk) inhibited and that of a membrane-anchored, gain-of-function Csk abolished the Fc gamma R-mediated signaling that leads to phagocytosis in a kinase-dependent manner. We next tested reconstruction of the signaling in the membrane-anchored, gain-of-function Csk-expressing cells by introducing Src family kinases the C-terminal negative regulatory sequence of which was replaced with a c-myc epitope. Those constructs derived from Lyn and Hck (a-Lyn and a-Hck) that associated with detergent-resistant membranes successfully reconstructed Fc gamma R-mediated Syk activation, filamentous actin rearrangement, and phagocytosis. In contrast, c-Src-derived construct (a-Src), that was excluded from detergent-resistant membranes, could not restore the series of phagocytosis signaling. Tyrosine phosphorylation of Vav and c-Cbl was restored in common by a-Lyn, a-Hck, and a-Src, but Fc gamma RIIB tyrosine phosphorylation, which is implicated in negative signaling, was reconstituted solely by a-Lyn and a-Hck. These findings suggest that Src family kinases are differentially involved in Fc gamma R-signaling and that selective kinases including Lyn and Hck are able to fully transduce phagocytotic signaling.

Syk kinase, tyrosine-phosphorylated proteins and actin filaments accumulate at forming phagosomes during Fcgamma receptor-mediated phagocytosis

Phagocytosis mediated by Fcgamma receptors (FcgammaRs) is thought to be regulated by a cascade of tyrosine phosphorylation events that finally leads to the rearrangement of submembranous actin-based cytoskeleton and internalization of particles. Suggestions concerning the functional relationship between protein tyrosine kinases, their substrates, and actin filament reorganization prompted us to determine cellular distribution of these elements during uptake of IgG-coated particles in murine thio-macrophages. We found that the onset of uptake of the particles was accompanied by tyrosine phosphorylation of several proteins, among which 90, 50, 40, 30, and 25 kDa polypeptides were distinguished. In most of the proteins the tyrosine hyperphosphorylation persisted up to 3 min of the uptake; however, kinetics of the phosphorylation of individual proteins varied. Immunofluorescence data showed that the phosphotyrosine-bearing proteins were localized in regions of the particle uptake, being concentrated at phagocytic cups and nascent phagosomes. The local enrichment in tyrosine phosphorylated proteins was correlated with accumulation of actin filaments at these early stages of phagosome formation. During phagosome maturation, both tyrosine phosphorylated proteins and microfilaments disappeared from the periphagosomal regions. Syk, one of the tyrosine kinases, was translocated to the regions where FcgammaR-mediated phagocytosis had started. On the contrary, no enrichment in phosphatidylinositol 3-kinase was detected in these places.

Stimulation of 92-kd gelatinase (matrix metalloproteinase 9) production by interleukin-17 in human monocyte/macrophages: a possible role in rheumatoid arthritis

OBJECTIVE

To examine the cellular mechanisms by which the proinflammatory cytokine interleukin-17 (IL-17) induces the synthesis of 92-kd gelatinase (matrix metalloproteinase 9 [MMP-9]) by human monocyte/ macrophages in primary culture.

METHODS

IL-17-stimulated human monocytes isolated from the peripheral blood of healthy donors were cultured in the presence of antiinflammatory cytokines, neutralizing antibodies against IL-1beta, tumor necrosis factor alpha (TNFalpha), or IL-1 receptor antagonist, and with protein kinase inhibitors of diverse specificity. MMP measurements were performed using specific enzyme-linked immunosorbent assays, while the expression of specific messenger RNA was determined by Northern blotting. Detection of phosphorylated proteins and specific transcriptional factors was performed by Western blotting and by gel retardation experiments, respectively.

RESULTS

Biologically active IL-17 was detected in the synovial fluid of patients with rheumatoid arthritis. IL-17-induced MMP-9 production in human monocyte/ macrophages was dependent on endogenous prostaglandin E2 synthesis and related to autocrine stimulation by TNFalpha, but was IL-1beta independent. This activation involves both p42/44 and p38 kinases and nuclear factor kappaB. IL-17-inducible activator protein 1 and signal transducer and activator of transcription 1/3 may transactivate the MMP-9 promoter.

CONCLUSION

IL-17 may contribute to an unbalanced production of proinflammatory cytokines and MMP-9 in diseased articular joint tissues by interacting with the macrophages in the rheumatoid synovium.

Engineering mouse T lymphocytes specific to type II collagen by transduction with a chimeric receptor consisting of a single chain Fv and TCR zeta

The chimeric cell surface receptor scC2Fv/CD8/zeta was constructed to engineer primary mouse T lymphocytes with antibody-type specificity to type II collagen (CII). Such cells could be used as gene carriers in the anti-inflammatory gene therapy of an autoimmune arthritis. This receptor includes the single chain Fv domain (scFv) of the anti-CII monoclonal antibody (mAb) C2, hinge region of CD8alpha and the transmembrane and cytoplasmic domains of TCRzeta. The scC2Fv/CD8/zeta gene was transduced into T cell hybridomas and primary mouse lymphocytes using retrovirus-mediated gene transfer. The chimeric receptor scC2Fv/CD8/zeta forms covalently bound homodimers, as demonstrated in T cell hybridomas and packaging fibroblasts. It does not associate with endogenous signalling subunits of the TCR complex. When scC2Fv/CD8/zeta-expressing clones of T cell hybridomas MD.45 and HCQ6 were stimulated with CII they produced IL-2. The level of their IL-2 response correlated with the expression level of the chimeric receptor on the cell surface. Splenocytes isolated from DBA/1 mice were stimulated with Con A in vitro to facilitate retrovirus-mediated transfer of the scC2Fv/CD8/zeta gene. As a result of transduction, approximately 4% of the Con A-activated splenocytes expressed the chimeric receptor scC2Fv/CD8/zeta on the cell surface. These cells proliferated in response to stimulation with CII.

Temperature-sensitive ZAP70 mutants degrading through a proteasome-independent pathway. Restoration of a kinase domain mutant by Cdc37

CD8 deficiency is an autosomal recessive form of severe combined immunodeficiency diseases characterized by the absence of CD8(+) T lymphocytes and impaired T cell functions. We identified two novel mis-sense mutations in the zap70 genes of a CD8-deficiency patient. One mutation (P80Q) affects a residue in an SH2 domain and another (M572L) in the kinase subdomain XI. Both mutations cause a degradation of ZAP70 protein in a temperature-sensitive manner through an ATP-dependent and proteasome-independent pathway. We further demonstrated that Cdc37, a protein kinase-specific chaperone, bound to M572L but not P80Q mutant and restored the expression of the M572L mutant when overexpressed. The restoration of M572L mutant by Cdc37 required the function of HSP90. These results indicate that Cdc37 in conjunction with HSP90 functions as a molecular chaperone for a temperature-sensitive kinase domain mutant of ZAP70.

Expression of zeta molecules is decreased in NK cells from HIV-infected patients

Cytolysis by natural killer (NK) cells is impaired in HIV infection. We investigated whether the expression of zeta (zeta) molecules, essential elements of signalling initiated upon ligation of, e.g., CD16, is reduced and if so, whether this reduction could be involved in defective cytolysis. FACS analysis revealed significantly lower levels of zeta in NK cells from AIDS patients compared to cells from patients without AIDS and healthy controls. CD16-dependent cytolysis by NK cells correlated with expression of zeta molecules and CD16, the latter possibly related to zeta expression. No correlation was observed between CD16-independent cytolysis and zeta expression. Reduced expression of zeta molecules by NK cells from HIV-infected patients thus correlates with disease progression and may, in part, explain the defective cytolysis by these cells.

Age-related impairments in TCR/CD3 activation of ZAP-70 are associated with reduced tyrosine phosphorylations of zeta-chains and p59fyn/p56lck in human T cells

The expression and catalytic activity of the protein tyrosine kinase (PTK) ZAP-70 are needed for normal intracellular signaling through the T-cell receptor (TCR)/CD3 complex. However, the possible effect of aging on the catalytic activity of ZAP-70 in human peripheral blood T cells stimulated via the TCR/CD3 complex is unknown. The current studies show that T cells from a substantial proportion of elderly humans (12) exhibit significant reductions in the catalytic activity, but not expression of ZAP-70 when stimulated by ligation of the TCR/CD3 with cross-linked anti-CD3epsilon monoclonal antibody OKT3. In addition, the reduced catalytic activity of ZAP-70 in T cells from elderly subjects was not restored to the normal levels in response to ligation of CD4 receptors, suggesting defects in PTKs linked to both CD3 and CD4 receptors. Other experiments demonstrated that the age-related impairments of ZAP-70 activation in anti-CD3-stimulated T cells were accompanied by decreased tyrosine phosphorylations of zeta-chains and autophosphorylations of the PTKs p561ck/p59fyn. Moreover, the age-related defects in these early TCR/CD3-mediated phosphorylation events were readily detectable in both CD45RO+ memory and CD45RA+ naive T cells. Thus, these results suggest that defects in early TCR/CD3-mediated phosphorylation events among CD45RO+ memory and CD45RA+ naive T cells from certain elderly humans may con tribute to impaired induction of ZAP-70 catalytic activity.

Inefficient Phospholipase C Activation and Reduced Lck Expression Characterize the Signaling Defect of Umbilical Cord T Lymphocytes

Adult and neonatal immunocompetent cells exhibit important functional distinctions, including differences in cytokine production and susceptibility to tolerance induction. We have investigated the molecular features that characterize the immune response of cord blood-derived T lymphocytes compared with that of adult T lymphocytes. Our findings demonstrate that phospholipase C (PLC) isozymes, which play a pivotal role in the control of protein kinase C activation and Ca2+ mobilization, are differently expressed in cord and adult T lymphocytes. PLCβ1 and δ1 are expressed at higher levels in cord T cells, while PLCβ2 and γ1 expression is higher in adult T lymphocytes. PLCδ2 and γ2 appear to be equally expressed in both cell types. In addition, a functional defect in PLC activation via CD3 ligation or pervanadate treatment, stimuli that activate tyrosine kinases, was observed in cord blood T cells, whereas treatment with aluminum tetrafluoride (AlF4−), a G protein activator, demonstrated a similar degree of PLC activation in cord and adult T cells. The impaired PLC activation of cord blood-derived T cells was associated with a a very low expression of the Src kinase, Lck, along with a reduced level of ZAP70. No mitogenic response to CD3 ligation was observed in cord T cells. However, no signaling defect was apparent downstream of PLC activation, as demonstrated by the mitogenic response of cord T cells to the pharmacologic activation of protein kinase C and Ca2+ by treatment with PMA and ionomycin. Thus, neonatal cord blood-derived T cells show a signaling immaturity associated with inadequate PLCγ activation and decreased Lck expression.

Role of protein kinase C in signal attenuation following T cell receptor engagement

T lymphocyte activation through stimulation of the T cell receptor complex and co-stimulatory receptors is associated with acute tyrosine phosphorylation of intracellular proteins, which in turn mediate downstream signaling events that regulate interleukin-2 expression and cell proliferation. The extent of protein tyrosine phosphorylation is rapidly attenuated after only 1-2 min of stimulation as a means of tightly controlling the initial signaling response. Here we show that this attenuation of tyrosine phosphorylation of Shc, CrkL, and the proto-oncogene Cbl is mimicked by treatment of mouse T lymphocytes or cultured Jurkat cells with phorbol 12-myristate 13-acetate. This effect is blocked by the specific protein kinase C inhibitor GF109203X, but not by PD98059, an inhibitor of MEK1/2 kinase. Activation of protein kinase C by phorbol ester also causes rapid (t(1)/(2) = 2 min) dissociation of both CrkL and p85/phosphoinositide 3-kinase from Cbl concomitant with Cbl tyrosine dephosphorylation. More important, GF109203X treatment of Jurkat cells prior to T cell receptor stimulation by anti-CD3/CD4 antibodies results in an enhanced (2-fold) peak of Cbl phosphorylation compared with that observed in control cells. Furthermore, the rate of attenuation of both Cbl tyrosine phosphorylation and its association with CrkL following stimulation with anti-CD3/CD4 antibodies is much slower in Jurkat cells treated with GF109203X. Taken together, these data provide strong evidence that one or more isoforms of phorbol ester-responsive protein kinase C play a key role in a feedback mechanism that attenuates tyrosine phosphorylation of proteins and reverses formation of signaling complexes in response to T cell receptor activation.

Immunocytochemical localization of phospholipase C isozymes in cord blood and adult T-lymphocytes

The response of T-cells to peptide antigen plus major histocompatibility complex (MHC) consists of a series of cellular events collectively called T-cell activation. An essential component of this pathway is phospholipase C (PLC)gamma1, whose hydrolytic activity increases rapidly after binding of ligands to the T-cell receptor (TCR) and consequent activation of tyrosine kinases. Recent studies also suggest a GTP binding protein-dependent activation of PLCbeta during the early steps of T-cell activation. On the basis of these findings, we first checked the expression of PLC isoforms by Western blotting and by confocal and electron microscopy techniques, and then we looked for the phosphoinositide breakdown induced by CD3 engagement in cord and adult T-lymphocytes. Our results indicated that PLCbeta1 was almost exclusively expressed in cord T-cells, whereas PLCbeta2 was more strongly represented in the adult. The amount of PLCgamma1 was found to be larger in the adult than in cord cells. No significant differences were found in PLCgamma2 and delta2 expression. PLCdelta1 was scarcely detectable. On CD3 stimulation, adult lymphocytes gave rise, as expected, to a dramatic increase in phosphoinositide breakdown, whereas in cord cells this response was scarcely detected. These results indicate that a shift in PLC expression occurs in the postnatal period and that this change is associated with induction of the capability to respond to CD3 engagement with phosphoinositide hydrolysis.

Signaling pathways in phagocytosis

Phagocytosis is an uptake of large particles governed by the actin-based cytoskeleton. Binding of particles to specific cell surface receptors is the first step of phagocytosis. In higher Eucaryota, the receptors able to mediate phagocytosis are expressed almost exclusively in macrophages, neutrophils, and monocytes, conferring immunodefence properties to these cells. Receptor clustering is thought to occur upon particle binding, that in turn generates a phagocytic signal. Several pathways of phagocytic signal transduction have been identified, including the activation of tyrosine kinases and (or) serine/threonine kinase C in pivotal roles. Kinase activation leads to phosphorylation of the receptors and other proteins, recruited at the sites of phagocytosis. Monomeric GTPases of the Rho and ARF families are likely to be engaged downstream of activated receptors. The GTPases, in cooperation with phosphatidylinositol 4-phosphate 5-kinase and phosphatidylinositol 3-kinase lipid modifying enzymes, can modulate locally the assembly of the submembranous actin filament system leading to particle internalization.

Functional Expression of the High Affinity Receptor for IgE (FcɛRI) in Human Platelets and Its’ Intracellular Expression in Human Megakaryocytes

The high affinity IgE receptor (FcɛRI) expressed on the cell surface of mast cells and basophils is the key molecule in triggering the IgE-mediated allergic reaction. Recently, it was elucidated that the FcɛRI is expressed on a variety of other cells like Langerhans cells, monocytes, and eosinophils, and the functional importance of the FcɛRI expression in Langerhans cells was also shown. Some studies suggest that human platelets may play important roles in allergic inflammation through the cell-surface expression of the FcɛRII and FcγRII. Here, we report that human platelets and megakaryocytes constitutively express the messenger RNA and protein for the FcɛRI. Although the FcɛRI is expressed on the cell surface of human platelets, it is only detected in the cytoplasm of human megakaryocytes. We also confirmed that human platelets express the genes for the , β, and γ chains of the FcɛRI without any defined mutations. Furthermore, stimulation of human platelets via the FcɛRI induced the release of serotonin and RANTES (Regulated on Activation, Normal T Expressed, and presumably Secreted). Taken together, these results suggest a novel and important role for human platelets in perpetuating allergic inflammation through the expression of and activation via the FcɛRI.

Functional Expression of the High Affinity Receptor for IgE (FcɛRI) in Human Platelets and Its’ Intracellular Expression in Human Megakaryocytes

The high affinity IgE receptor (FcɛRI) expressed on the cell surface of mast cells and basophils is the key molecule in triggering the IgE-mediated allergic reaction. Recently, it was elucidated that the FcɛRI is expressed on a variety of other cells like Langerhans cells, monocytes, and eosinophils, and the functional importance of the FcɛRI expression in Langerhans cells was also shown. Some studies suggest that human platelets may play important roles in allergic inflammation through the cell-surface expression of the FcɛRII and FcγRII. Here, we report that human platelets and megakaryocytes constitutively express the messenger RNA and protein for the FcɛRI. Although the FcɛRI is expressed on the cell surface of human platelets, it is only detected in the cytoplasm of human megakaryocytes. We also confirmed that human platelets express the genes for the , β, and γ chains of the FcɛRI without any defined mutations. Furthermore, stimulation of human platelets via the FcɛRI induced the release of serotonin and RANTES (Regulated on Activation, Normal T Expressed, and presumably Secreted). Taken together, these results suggest a novel and important role for human platelets in perpetuating allergic inflammation through the expression of and activation via the FcɛRI.

Age-related impairment of p56lck and ZAP-70 activities in human T lymphocytes activated through the TcR/CD3 complex

Cellular immune responses decrease with aging. Lymphocytes of aged individuals do not perform as well as cells from young subjects in a number of in vitro assays including cell proliferation, cytokine production, and protection against apoptosis. Here, we have tested the hypothesis that a decrease in T cell responses in tymphocytes from elderly subjects could parallel a decrease in the activity of protein tyrosine kinases (PTK) associated with signal transduction in T lymphocytes. We report that anti-CD3-triggered T lymphocyte proliferation was significantly decreased in T lymphocytes from elderly subjects, but the decrease was not due to an alteration of the percentage or mean fluorescence intensities of CD3, CD4, and CD45. Of significance, the activities of p56lck and ZAP-70 in vitro were significantly decreased in T lymphocytes from elderly subjects compared to young individuals. However, the level of expression of the two kinases did not change with aging. The activity of p59fyn did not show changes with aging, suggesting that p59fyn did not compensate for the decreased activity of p56lck. We also found that the extent of tyrosine phosphorylation of the adaptor protein p95vav was similar in activated T lymphocytes from elderly and young subjects. Our results suggest that the altered cellular immune responses observed in T lymphocytes with aging may be the result, at least in part, of an alteration in early events associated with signal transduction through the TcR/CD3 complex that translates into decreased activities of p56lck and ZAP-70. Impairment in the activities of these twokey components of T cell signaling may contribute to reduced immune functions associated with aging.

STAT5 interaction with the T cell receptor complex and stimulation of T cell proliferation

The role of STAT (signal transducer and activator of transcription) proteins in T cell receptor (TCR) signaling was analyzed. STAT5 became immediately and transiently phosphorylated on tyrosine 694 in response to TCR stimulation. Expression of the protein tyrosine kinase Lck, a key signaling protein in the TCR complex, activated DNA binding of transfected STAT5A and STAT5B to specific STAT inducible elements. The role of Lck in STAT5 activation was confirmed in a Lck-deficient T cell line in which the activation of STAT5 by TCR stimulation was abolished. Expression of Lck induced specific interaction of STAT5 with the subunits of the TCR, indicating that STAT5 may be directly involved in TCR signaling. Stimulation of T cell clones and primary T cell lines also induced the association of STAT5 with the TCR complex. Inhibition of STAT5 function by expression of a dominant negative mutant STAT5 reduced antigen-stimulated proliferation of T cells. Thus, TCR stimulation appears to directly activate STAT5, which may participate in the regulation of gene transcription and T cell proliferation during immunological responses.

Loss of Original Antigenic Specificity in T Cell Hybridomas Transduced with a Chimeric Receptor Containing Single-Chain Fv of an Anti-Collagen Antibody and FcεRI-Signaling γ Subunit

T cell hybridomas HCQ6 and MD.45 acquired Ab-type specificity to collagen type II, when engrafted with a chimeric cell surface receptor, scC2Fv/γ, which includes the single-chain Fv domain (scFv) of the anti-collagen type II mAb C2 and the signaling γ subunit of the FcεRI. When transduced into MD.45 cells, scC2Fv/γ or its mutated form lacking immunoreceptor tyrosine-based activation motif (ITAM), scC2Fv/γIC−, formed mainly homodimers. A small proportion of these molecules formed heterodimers with endogenous CD3ζ in these hybridoma cells. By contrast, in HCQ6 cells, the majority of scC2Fv/γ and scC2Fv/γIC− molecules formed heterodimers with CD3ζ, and only a small proportion of them was expressed as homodimers. Stimulation with plastic-immobilized collagen induced IL-2 production in scC2Fv/γ-transduced MD.45 cells, but not in MD.45 cells transduced with the ITAM-less chimera scC2Fv/γIC−. HCQ6 cells transduced with scC2Fv/γ responded to plastic-bound collagen. Due to the high content of CD3ζ-associated chimeras, HCQ6 cells transduced with the ITAM-less scC2Fv/γIC− chimera were also responsive to plastic-bound collagen. When cells were stimulated with collagen in solution, MD.45 cells transduced with scC2Fv/γ produced IL-2, whereas transduced HCQ6 cells were unresponsive, hence suggesting that the ability of cells transduced with scC2Fv chimeras to respond to soluble collagen correlated with predominant expression of divalent scC2Fv/γ homodimers, but not monovalent scC2Fv/γ-CD3ζ or scC2Fv/γIC−-CD3ζ heterodimers. Of interest, expression of CD3 subunits in hybridomas transduced with scC2Fv chimeras was reduced, resulting in decreased response to cognate Ags.

Zeta chain of the T-cell receptor interacts with nef of simian immunodeficiency virus and human immunodeficiency virus type 2

A truncated version of the nef gene of simian immunodeficiency virus SIVmac239 capable of encoding amino acids 98 to 263 was used as bait to screen a cDNA library from activated lymphocytes in a yeast two-hybrid system. The zeta chain of the T-cell receptor (TCRzeta) was found to interact specifically not only with truncated SIV nef in yeast cells but also with full-length glutathione S-transferase (GST)-SIVnef fusion protein in vitro. Coimmunoprecipitation of TCRzeta with full-length SIV nef was demonstrated in transfected Jurkat cells and in Cos 18 cells which express the cytoplasmic domain of TCRzeta fused to the external domain of CD8 via the CD8 transmembrane domain. Using a series of nef deletion mutants, we have mapped the binding site within the central core domain of nef (amino acids 98 to 235). Binding of TCRzeta was specific for nef isolated from SIVmac239, SIVsmH4, and human immunodeficiency virus (HIV)-2ST and was not detected with nef from five different HIV-1 isolates. An active tyrosine kinase was coprecipitated with nef-TCRzeta complexes from Jurkat cells but not from J.CAM1.6 cells which lack a functional Lck tyrosine kinase. These results demonstrate a specific association of SIV and HIV-2 nef, but not HIV-1 nef, with TCRzeta.

ZAP-70 association with T cell receptor zeta (TCRzeta): fluorescence imaging of dynamic changes upon cellular stimulation

The nonreceptor protein tyrosine kinase ZAP-70 is a critical enzyme required for successful T lymphocyte activation. After antigenic stimulation, ZAP-70 rapidly associates with T cell receptor (TCR) subunits. The kinetics of its translocation to the cell surface, the properties of its specific interaction with the TCRzeta chain expressed as a chimeric protein (TTzeta and Tzetazeta), and its mobility in different intracellular compartments were studied in individual live HeLa cells, using ZAP-70 and Tzetazeta fused to green fluorescent protein (ZAP-70 GFP and Tzetazeta-GFP, respectively). Time-lapse imaging using confocal microscopy indicated that the activation-induced redistribution of ZAP-70 to the plasma membrane, after a delayed onset, is of long duration. The presence of the TCRzeta chain is critical for the redistribution, which is enhanced when an active form of the protein tyrosine kinase Lck is coexpressed. Binding specificity to TTzeta was indicated using mutant ZAP-70 GFPs and a truncated zeta chimera. Photobleaching techniques revealed that ZAP-70 GFP has decreased mobility at the plasma membrane, in contrast to its rapid mobility in the cytosol and nucleus. Tzetazeta- GFP is relatively immobile, while peripherally located ZAP-70 in stimulated cells is less mobile than cytosolic ZAP-70 in unstimulated cells, a phenotype confirmed by determining the respective diffusion constants. Examination of the specific molecular association of signaling proteins using these approaches has provided new insights into the TCRzeta-ZAP-70 interaction and will be a powerful tool for continuing studies of lymphocyte activation.

Phosphorylation and coupling of zeta-chains to activated T-cell receptor (TCR)/CD3 complexes from peripheral blood T-cells of elderly humans

Aging is often accompanied by altered T-cell signaling and functions. Signals mediated through the T-cell receptor (TCR)/CD3 complex are associated with tyrosine phosphorylations of zeta-chains by the regulated activities of protein tyrosine kinases p56(lck) and p59(fyn) as well as protein tyrosine phosphatases. In the present investigation, the coupling and phosphorylation of zeta-chains to TCR/CD3 immunocomplexes were examined in peripheral blood T-cells from 13 elderly and young humans stimulated by ligation of the TCR/CD3 with cross-linked anti-CD3epsilon monoclonal antibody OKT3. Western blots analyzing the non-covalent coupling of zeta-chains to TCR/CD3 immunocomplexes from Brij-96 detergent lysates of anti-CD3 ligated T-cells showed that the levels of zeta-chains within TCR/CD3 immunocomplexes from T-cells of elderly and young subjects did not significantly differ. By contrast, the levels of phosphorylated zeta-chains generated during in vitro phosphorylations of TCR/CD3 immunocomplexes from elderly subjects were significantly reduced and averaged 44% of those observed for anti-CD3epsilon ligated T-cells from young subjects. Analyses of the levels of zeta-chain coupling and phosphorylations in T-cells from each of the 13 elderly individuals also showed that the reductions in zeta-chain phosphorylations were heterogeneous and unrelated to modest reductions in coupling. Furthermore, the age-related decreases in zeta-chain phosphorylations were not due to diminished frequencies of CD3epsilon+ cells or densities of CD3epsilon surface receptors and could be observed without reductions in epsilon-chain phosphorylations. These results suggest that aberrancies of zeta-chain phosphorylations can occur in T-cells of elderly humans independent from any uncoupling of zeta-chains to activated TCR/CD3 complexes.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.