Direct T cell activation by chimeric single chain Fv-Syk promotes Syk-Cbl association and Cbl phosphorylation

Redirection of T-cell effector functions for cancer therapy: bispecific antibodies and chimeric antigen receptors

T cells are the most potent cells of the immune system; however, they fail in the immunosurveillance of tumors. In previous decades, scientists began studying methods to take advantage of T-cell potency in cancer therapy by redirecting them against tumors independently from the T-cell receptor-defined specificity. Among different approaches, the most promising are the use of bispecific antibodies and T-cell engineering to create chimeric antigen receptors. Bispecific antibodies, by simultaneously recognizing target antigen and an activating receptor on the surface of an immune effector cell, offer an opportunity to redirect immune effector cells to kill cancer cells. The other approach is the generation of chimeric antigen receptors by fusing extracellular antibodies to intracellular signaling domains. Chimeric antigen receptor-engineered T cells are able to specifically kill tumor cells in a MHC-independent way. The efficacy of these reagents in different formats has been clinically validated and will be presented here.

Cbl enforces Vav1 dependence and a restricted pathway of T cell development

Extensive studies of pre-TCR- and TCR-dependent signaling have led to characterization of a pathway deemed essential for efficient T cell development, and comprised of a cascade of sequential events involving phosphorylation of Lck and ZAP-70, followed by phosphorylation of LAT and SLP-76, and subsequent additional downstream events. Of interest, however, reports from our lab as well as others have indicated that the requirements for ZAP-70, LAT, and SLP-76 are partially reversed by inactivation of c-Cbl (Cbl), an E3 ubiquitin ligase that targets multiple molecules for ubiquitination and degradation. Analysis of signaling events in these Cbl knockout models, including the recently reported analysis of SLP-76 transgenes defective in interaction with Vav1, suggested that activation of Vav1 might be a critical event in alternative pathways of T cell development. To extend the analysis of signaling requirements for thymic development, we have therefore assessed the effect of Cbl inactivation on the T cell developmental defects that occur in Vav1-deficient mice. The defects in Vav1-deficient thymic development, including a marked defect in DN3-DN4 transition, were completely reversed by Cbl inactivation, accompanied by enhanced phosphorylation of PLC-γ1 and ERKs in response to pre-TCR/TCR cross-linking of Vav1^-/-Cbl^-/- DP thymocytes. Taken together, these results suggest a substantially modified paradigm for pre-TCR/TCR signaling and T cell development. The observed consensus pathways of T cell development, including requirements for ZAP-70, LAT, SLP-76, and Vav1, appear to reflect the restriction by Cbl of an otherwise much broader set of molecular pathways capable of mediating T cell development.

Re-targeting T-cells against cancer by gene-transfer of tumor-reactive receptors

BACKGROUND

Adoptive transfer of T-lymphocytes is a promising treatment for a variety of malignancies, but is often not feasible due to difficulties in generating T-cells reactive with the targeted antigen from patients. To facilitate rapid generation of cells for therapy, T-cells can be programmed with genes encoding for an antigen-specific T-cell receptor (TCR) or chimeric receptors.

OBJECTIVE

To discuss the molecular design and selected pitfalls of TCR gene modified T-cells and T-cells expressing chimeric receptors, so called T-bodies.

METHODS

A selected review of the recent literature.

CONCLUSION

Clinical trials report so far only limited efficacy of adoptively transferred genetically modified T-cells. However, the recent progress in engineering tumor-reactive T cells is providing a promising basis to further explore this treatment modality.

Genetic modification of T lymphocytes for adoptive immunotherapy

Adoptive transfer of T lymphocytes is a promising therapy for malignancies-particularly of the hemopoietic system-and for otherwise intractable viral diseases. Efforts to broaden the approach have been limited by the physiology of the T cells themselves and by a range of immune evasion mechanisms developed by tumor cells. In this review we show how genetic modification of T cells is being used preclinically and in patients to overcome these limitations, by incorporation of novel receptors, resistance mechanisms, and control genes. We also discuss how the increasing safety and effectiveness of gene transfer technologies will lead to an increase in the use of gene-modified T cells for the treatment of a wider range of disorders.

Tolerance and cancer: mechanisms of tumor evasion and strategies for breaking tolerance

The development of malignant disease might be seen as a failure of immune surveillance. However, not all tumors are naturally immunogenic, and even among those that are immunogenic, the uncontrolled rapid growth of a tumor may sometimes out-run a robust immune response. Nevertheless, recent evidence suggests that mechanisms of tolerance that normally exist to prevent autoimmune disease may also preclude the development of an adequate antitumor response and that tumors themselves have the ability to thwart the development of effective immune responses against their antigens. A major challenge has been to develop approaches to breaking this tolerance in tumor-bearing hosts, and recent advances in our understanding of antigen presentation and tolerance have led to some promising strategies. An alternative approach is to use T cells from nontumor-bearing, allogeneic hosts in the form of lymphocyte infusions, with or without hematopoietic cell transplantation. Immunotherapy may occur in this setting via the response of nontolerant, tumor antigen-specific T cells from nontumor-bearing hosts or via the powerful destructive effect of an alloresponse directed against antigens shared by malignant cells in the recipient. Approaches to exploiting this beneficial effect without the deleterious consequence of graft-versus-host disease in allogeneic hematopoietic cell recipients are discussed.

Inhibition of Src family kinases blocks epidermal growth factor (EGF)-induced activation of Akt, phosphorylation of c-Cbl, and ubiquitination of the EGF receptor

Stimulation of T47D cells with epidermal growth factor (EGF) results in the activation of the intrinsic tyrosine kinases of the receptor and the phosphorylation of multiple cellular proteins including the receptor, scaffold molecules such as c-Cbl, adapter molecules such as Shc, and the serine/threonine protein kinase Akt. We demonstrate that EGF stimulation of T47D cells results in the activation of the Src protein-tyrosine kinase and that the Src kinase inhibitor PP1 blocks the EGF-induced phosphorylation of c-Cbl but not the activation/phosphorylation of the EGF receptor itself. PP1 also blocks EGF-induced ubiquitination of the EGF receptor, which is presumably mediated by phosphorylated c-Cbl. Src is associated with c-Cbl, and we have previously demonstrated that the Src-like kinase Fyn can phosphorylate c-Cbl at a preferred binding site for the p85 subunit of phosphatidylinositol 3'-kinase. PP1 treatment blocks EGF-induced activation of the anti-apoptotic protein kinase Akt suggesting that Src may regulate activation of Akt, perhaps by a Src --> c-Cbl --> phosphatidylinositol 3'-kinase --> Akt pathway.

Beyond the RING: CBL proteins as multivalent adapters

Following discovery of c-Cbl, a cellular form of the transforming retroviral protein v-Cbl, multiple Cbl-related proteins have been identified in vertebrate and invertebrate organisms. c-Cbl and its homologues are capable of interacting with numerous proteins involved in cell signaling, including various molecular adapters and protein tyrosine kinases. It appears that Cbl proteins play several functional roles, acting both as multivalent adapters and inhibitors of various protein tyrosine kinases. The latter function is linked, to a substantial extent, to the E3 ubiquitin-ligase activity of Cbl proteins. Experimental evidence for these functions, interrelations between them, and their biological significance are addressed in this review, with the main accent placed on the adapter functions of Cbl proteins.

Chimeric antigen receptors for the retargeting of cytotoxic effector cells

T lymphocytes recognize specific antigens through interaction of the T cell receptor (TCR) with short peptides presented by major histocompatibility complex (MHC) class I or II molecules. For initial activation and clonal expansion, naïve T cells are dependent on professional antigen-presenting cells (APCs) that provide additional co-stimulatory signals. TCR activation in the absence of co-stimulation can result in unresponsiveness and clonal anergy. To bypass immunization, different approaches for the derivation of cytotoxic effector cells with grafted recognition specificity have been developed. Chimeric antigen receptors have been constructed that consist of binding domains derived from natural ligands or antibodies specific for cell-surface antigens, genetically fused to effector molecules such as the TCR alpha and beta chains, or components of the TCR-associated CD3 complex. Upon antigen binding, such chimeric receptors link to endogenous signaling pathways in the effector cell and generate activating signals similar to those initiated by the TCR complex. Since the first reports on chimeric antigen receptors, this concept has steadily been refined and the molecular design of chimeric receptors has been optimized. Aided by advances in recombinant antibody technology, chimeric antigen receptors targeted to a wide variety of antigens on the surface of cancer cells and of cells infected by human immunodeficiency virus (HIV) have been generated. In initial clinical studies, infusion of such cells into patients proved to be safe and transient therapeutic effects have been observed.

The Cbl protooncoprotein: a negative regulator of immune receptor signal transduction

The Cbl protooncoprotein has recently emerged as a component of tyrosine kinase-mediated signal transduction in a variety of cell types. Here, we discuss evidence that supports a role for Cbl as a novel negative regulator of immune receptor signaling, and present models for its mode of function.

Impact of chimeric immune receptor extracellular protein domains on T cell function

Chimeric immune receptors (CIR) encompass tumor- or virus-specific ligands or antibodies fused to the signaling domains of either the T cell receptor or Fc receptor. T cells expressing these receptors recapitulate the cytopathic effects mediated by the T cell receptor and allow the targeting of tumor or virus infected cells in an MHC-independent manner. With this technology, large numbers of T cells with redirected target specificity can be generated. To define the structural features of recombinant CIRs required for optimal function, a panel of five closely related CIRs with identical target specificity were generated. These receptors recognized HIVenv through the single chain Fv (scFv) of an anti-gp 120 antibody. These scFv-zeta receptors were constructed to include alternative extracellular spacer and transmembrane protein domains derived from members of the immunoglobulin supergene family. The effect of these alternative extracellular protein domains on receptor stability, antigen affinity and T cell activity was assessed. We demonstrate that modifying the extracellular protein domains of the anti-HIVenv CIRs significantly impacted receptor stability and substrate binding affinity and that these effects, and not simply the level of cell surface expression, correlated most strongly with changes in CIR-mediated killing. These studies will aid in the rationale design of recombinant CIRs for the immunotherapy of viral infections, cancer and other diseases.

Fyn associates with Cbl and phosphorylates tyrosine 731 in Cbl, a binding site for phosphatidylinositol 3-kinase

We have investigated the interaction between Cbl and the Src-related tyrosine kinase Fyn. Fyn was observed to be constitutively associated with Cbl in lysates of several different cell types including the interleukin-3-dependent murine myeloid cell line 32Dcl3, and the prolactin-dependent rat thymoma cell line Nb2. Binding studies indicated that Cbl could bind to glutathione S-transferase (GST) fusion proteins encoding the unique, Src homology domain 3 (SH3), and SH2 domains of Fyn, Hck, or Lyn. Fusion proteins encoding either the SH3 or SH2 domains of Fyn bound to Cbl as effectively as the fusion protein encoding the unique, SH3, and SH2 domains of Fyn. The Fyn SH2 domain bound to both tyrosine-phosphorylated and nonphosphorylated Cbl, implying that this interaction might be phosphotyrosine-independent. Binding of the Fyn SH2 domain to Cbl was not disrupted by the addition of phosphotyrosine, phosphoserine, or phosphothreonine. A GST fusion protein encoding the proline-rich region of Cbl bound to Fyn present in a total cell lysate. Far Western blot analysis also indicated that the SH3 domain of Fyn bound preferentially to the proline-rich region of Cbl. The addition of [gamma-32P]ATP to either anti-Cbl immunoprecipitates or anti-Fyn immunoprecipitates resulted in the phosphorylation of both Cbl and Fyn as demonstrated by immunoprecipitation of the phosphorylated proteins with specific antisera. Fyn directly phosphorylated a GST fusion protein containing the C-terminal region of Cbl (GST-CBL-LZIP). In contrast, immunoprecipitated JAK2 was not able to phosphorylate this same region of Cbl. The GST-CBL-LZIP fusion protein contains a binding site for the SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase, which mapped to Tyr731, which is present in the sequence YEAM. Mutation of Tyr731 in GST-CBL-LZIP eliminated binding of the p85 subunit of phosphatidylinositol 3-kinase and substantially reduced the phosphorylation of this fusion protein by Fyn, despite the presence of four other tyrosine residues in this fusion protein. These data are consistent with the hypothesis that Cbl represents a substrate for Src-like kinases that are activated in response to the engagement of cell surface receptors, and that Src-like kinases are responsible for the phosphorylation of a tyrosine residue in Cbl that may regulate activation of phosphatidylinositol 3-kinase.

Cbl-mediated negative regulation of the Syk tyrosine kinase. A critical role for Cbl phosphotyrosine-binding domain binding to Syk phosphotyrosine 323

The proto-oncogene product Cbl has emerged as a potential negative regulator of the Syk tyrosine kinase; however, the nature of physical interactions between Cbl and Syk that are critical for this negative regulation remains unclear. Here we show that the phosphotyrosine-binding (PTB) domain within the N-terminal transforming region of Cbl (Cbl-N) binds to phosphorylated Tyr323 in the linker region between the Src homology 2 and kinase domains of Syk, confirming recent results by another laboratory using the yeast two-hybrid approach (Deckert, M., Elly, C., Altman, A., and Liu, Y. C. (1998) J. Biol. Chem. 273, 8867-8874). A PTB domain-inactivating point mutation (G306E), corresponding to a loss-of-function mutation in the Caenorhabditis elegans Cbl homologue SLI-1, severely compromised Cbl-N/Syk binding in vitro and Cbl/Syk association in transfected COS-7 cells. Using heterologous expression in COS-7 cells, we investigated the role of Cbl PTB domain binding to Syk Tyr323 in the negative regulation of Syk. Co-expression of Cbl with Syk in COS-7 cells led to a dose-dependent decrease in the autophosphorylated pool of Syk and in phosphorylation of an in vivo substrate, CD8-zeta. Unexpectedly, these effects were largely due to the loss of Syk protein. Both the decrease in Syk and CD8-zeta phosphorylation and reduction in Syk protein levels were blocked by either G306E mutation in Cbl or by Y323F mutation in Syk. These results demonstrate a critical role for the Cbl PTB domain in the recruitment of Cbl to Syk and in Cbl-mediated negative regulation of Syk.

Characterization of T cell-expressed chimeric receptors with antibody-type specificity for the CD4 binding site of HIV-1 gp120

Chimeric T cell receptors (cTCR) with an antibody specificity have been proposed in several models as a combination of antibody and cellular immunotherapy without MHC restriction. Such a tool could be of a limited use in HIV infection because of the great variability of the virus. The human single-chain antibody (ScFv-b12) derives from the b12 antibody directed to the CD4 binding site of gp120, a potent neutralizer of different HIV-1 strains, including a large panel of primary isolates. A single-chain fragment variable (ScFv) bearing the VH Pro-->Glu mutation that improves b12 affinity 54-fold, called ScFv-b12E, was also constructed. The ScFv were linked to the signal-transducing y chain of the Fc(gamma)RIII, with or without spacer region, and expressed in the murine MD45 T cell line. The different cTCR formats behave similarly in terms of ScFv surface expression, but differ according to their activation threshold. T cell transfectants can be stimulated with immobilized gp120 derived from all HIV strains tested. BHK cells infected with Semliki forest virus (SFV) carrying an HIV-1 envelope gene (SFV-env) derived from either HIV-1 laboratory strains (LAI, MN12, HXB2) or field isolates (BX08, CHAR or 133) were used as targets for the transfectants. All gp120-expressing cells induced cTCR-specific activation. The latter result is contrasting with the lack of specific recognition of SFV-CHAR- or 133-infected cells by the native b12 antibody, as measured by cytofluorometric analysis. Finally, HeLa cells (which constitutively express the coreceptor CXCR4) are able to bind HIV-1 gp160 when transfected with the chimeric receptor ScFv-b12-gamma, but, importantly, do not become infected by the virus. Our results therefore suggest that cTCR with b12 specificity can confer to T cells broad anti-HIV reactivity without making them susceptible to HIV infection.

ZAP-70 tyrosine kinase is required for LFA-1-dependent T cell migration

The ZAP-70 tyrosine kinase is essential for T cell activation by the T cell receptor. We show that ZAP-70 is also required for migration of T cells that is dependent on the integrin LFA-1. Invasion of TAM2D2 T cell hybridoma cells into fibroblast monolayers, which is LFA-1-dependent, was blocked by overexpression of dominant-negative ZAP-70 and by piceatannol but not by herbimycin A. The Syk inhibitor piceatannol blocks the Syk homologue ZAP-70, which is expressed by TAM2D2 cells, with the same dose dependence as the inhibition of invasion. Dominant-negative ZAP-70 completely inhibited the extensive metastasis formation of TAM2D2 cells to multiple organs upon i.v. injection into mice. Migration of TAM2D2 cells through filters coated with the LFA-1 ligand ICAM-1, induced by 1 ng/ml of the chemokine SDF-1, was blocked by anti-LFA-1 mAb and also abrogated by dominant-negative ZAP-70 and piceatannol. In contrast, migration induced by 100 ng/ml SDF-1 was independent of both LFA-1 and ZAP-70. LFA-1 cross-linking induced tyrosine phosphorylation, which was blocked by dominant-negative ZAP-70 and piceatannol. We conclude that LFA-1 engagement triggers ZAP-70 activity that is essential for LFA-1-dependent migration.

Regulation of beta 1-integrin-mediated cell adhesion by the Cbl adaptor protein

BACKGROUND

Leukocyte activation results in a rapid increase in adhesion to the extracellular matrix due to the activation of beta 1 integrin receptors. A role for phosphatidylinositol (PI) 3-kinase in integrin activation has been proposed, as activation of integrins by many receptors can be blocked by PI 3-kinase inhibitors. One receptor that regulates integrins is the CD28 surface antigen; here, we investigated the mechanisms responsible for CD28-mediated integrin activation.

RESULTS

CD28-mediated integrin activation was blocked by mutation of the binding site for the p85 catalytic subunit of PI 3-kinase in the CD28 cytoplasmic domain, and by expression of a dominant-negative form of the p85 subunit. Substitution of the Src homology 2 (SH2)-binding motif in the CD28 cytoplasmic domain for the corresponding motif in the CD28-related CTLA-4 surface antigen also blocked integrin activation but did not affect the recruitment and activation of PI 3-kinase. Mutations of the CD28 cytoplasmic domain that blocked integrin activation also impaired the tyrosine phosphorylation of the Cbl adaptor protein and the activation of the PI 3-kinase that was associated with Cbl. This Cbl-associated PI 3-kinase was distinct from the PI 3-kinase that coprecipitated with the CD28 cytoplasmic domain. CD28-mediated activation of beta 1 integrins was inhibited by expression of a mutant Cbl protein that shows reduced association with PI 3-kinase.

CONCLUSIONS

Cbl is required for PI-3-kinase-dependent regulation of integrin receptors by CD28. Furthermore, CD28 is coupled to two distinct pools of PI 3-kinase, one directly associated with the CD28 cytoplasmic tail and the other associated with Cbl.

Cbl: complex formation and functional implications

Chbl, a 120-kDa proto-oncogene product, whose gene was first identified as part of a transforming gene of a murine retrovirus and whose expression is predominant in haematopoietic cells, consists of an amino-terminal transforming region, a zinc Ring finger, multiple proline-rich stretches, and several potential phosphotyrosine-containing motifs. Cbl is rapidly tyrosine-phosphorylated in response to stimulation of a variety of cell-surface receptors and becomes associated with a number of intracellular signalling molecules such as protein tyrosine kinases, phosphatidylinositol 3-kinase, Crk, and 14-3-3 proteins through different protein-interacting modules, leading to the formation of multimolecular signalling complexes. Cbl and its transforming mutants have been shown to display both negative and positive regulatory activities in protein tyrosine kinase- and Ras-mediated signalling pathways. Nevertheless, the exact biological function of this adaptor protein remains largely unknown. The present review summarizes recent progress in our understanding of the structure, regulation and biological function of Chl and defines open questions for future research.

Tyrosine kinase chimeras for antigen-selective T-body therapy

Protein tyrosine kinases (PTKs) transmit activation signals in almost every cell type, including immune effector cells. The aberrant or constitutive activation of PTKs can often cause neoplastic transformation. The use of chimeric receptors based on PTKs may enable us to elucidate the signaling pathways of normal immune cells and other cell types, and the abnormal events that can lead to malignant transformation. In this review, we focus on antigen specific chimeric PTKs in which antibody-derived scFv are joined to the Syk family of PTKs. These chimeric receptors yielded reagents that can selectively redirect immune effector cells and specifically activate them to produce cytokines or lyse their target. The advantages of using such PTK-based chimeras to redirect lymphocytes to tumor targets and their potential as an immunotherapeutic approach to malignant disease is discussed.

Fyn, Yes, and Syk phosphorylation sites in c-Cbl map to the same tyrosine residues that become phosphorylated in activated T cells

Protooncogenic protein c-Cbl undergoes tyrosine phosphorylation in response to stimulation through the receptors for antigens, immunoglobulins, cytokines, and growth factors as well as through the integrins. Tyrosine phosphorylation of c-Cbl may play a functional role in signal transduction, since c-Cbl interacts with many crucial signaling molecules including protein-tyrosine kinases, adaptor proteins, and phosphatidylinositol 3'-kinase. Therefore, it is essential for our understanding of the functions of c-Cbl in signal transduction to identify its tyrosine phosphorylation sites, to determine the protein-tyrosine kinases that phosphorylate these sites, and to elucidate the role of these sites in the interactions of c-Cbl with other signaling proteins. In this report, we demonstrate that tyrosines 700, 731, and 774 are the major tyrosine phosphorylation sites of c-Cbl in T cells in response to pervanadate treatment, as well as in response to TcR/CD3 ligation. Coexpression experiments in COS cells demonstrate that among T cell-expressed Src- and Syk-related protein-tyrosine kinases, Fyn, Yes, and Syk appear to play a major role in phosphorylation of c-Cbl, whereas Lck and Zap phosphorylate c-Cbl ineffectively. Fyn, Yes, and Syk phosphorylate the same sites of c-Cbl that become phosphorylated in stimulated T cells. Among these kinases, Fyn and Yes demonstrate strong binding to c-Cbl, which involves both phosphotyrosine-dependent and phosphotyrosine-independent mechanisms.

Interleukin-2 stimulation induces tyrosine phosphorylation of p120-Cbl and CrkL and formation of multimolecular signaling complexes in T lymphocytes and natural killer cells

Interleukin (IL)-2, a major growth and differentiation factor for T lymphocytes, was found to induce tyrosine phosphorylation of the proto-oncogene products p120-Cbl and CrkL in IL-2-dependent cell lines. We established that, in unstimulated lymphocytes, the Src homology 2 (SH2) and SH3 domain-containing protein Grb2 and the p85 subunit of phosphatidylinositol 3-kinase, associate constitutively with Cbl via their SH3 domains. Furthermore, IL-2 stimulation increased the level of interaction of phosphorylated Cbl with the p85 SH2 domains, and we provide evidence that the preformed Cbl-Grb2 complex recruits the phosphorylated p52 Shc adaptor protein. In addition, we demonstrate that the SH2-SH3-SH3 adaptor protein CrkL is tyrosine-phosphorylated in an IL-2-dependent manner and, via its SH2 domain, associates with a large proportion of phosphorylated Cbl. We also show that p85 is preassociated with the CrkL SH3 domain. Furthermore, the association of CrkL and p85 is increased after IL-2 treatment by a mechanism involving intermediary tyrosine-phosphorylated proteins that remain to be identified. Our results show that CrkL associates independently with Cbl or p85 and suggest that it also participates in larger complexes containing Cbl and p85. Although the precise roles of Cbl and CrkL remain to be elucidated, their tyrosine phosphorylation, in addition to the multiple protein interactions described here, strongly suggest that Cbl and CrkL may play pivotal roles in the early steps of IL-2 signal transduction.

Harnessing Syk Family Tyrosine Kinases as Signaling Domains for Chimeric Single Chain of the Variable Domain Receptors: Optimal Design for T Cell Activation

T cells of tumor bearers often show defective TCR-mediated signaling events and, therefore, exhibit impaired immune responses. As such, patients with heavy tumor burden are often not amenable to adoptive T cell therapy. To overcome this limitation, we have developed a chimeric receptor that joins an extracellular single chain Fv (scFv) of a specific Ab for Ag recognition to an intracellular protein tyrosine kinase (PTK) for signal propagation. Stimulation through the scFv-PTK receptor should bypass defective TCR-proximal events and directly access the T cell’s effector mechanisms. In this study we describe the optimization of a scFv-PTK configuration, leading to complete T cell activation. The cytosolic PTK Syk is superior to its family member, Zap-70, for intracellular signaling. As a transmembrane (TM) domain, CD4 performs better than CD8 when plastic-immobilized Ag serves as a stimulator. However, when APC are used to trigger chimeric receptors, the need for a flexible spacer between the scFv and TM domains becomes apparent. The CD8α-derived hinge successfully performs this task in chimeric scFv-Syk receptors regardless of its cysteine content. A cytotoxic T cell hybridoma expressing chimeric receptor genes composed of scFv-CD8hinge-CD8TM-Syk or scFv-CD8hinge-CD4TM-Syk is efficiently stimulated to produce IL-2 upon interaction with APC and specifically lyses appropriate target cells in a non-MHC-restricted manner.

The Cbl phosphotyrosine-binding domain selects a D(N/D)XpY motif and binds to the Tyr292 negative regulatory phosphorylation site of ZAP-70

The Cbl protooncogene product has emerged as a novel negative regulator of receptor and non-receptor tyrosine kinases through currently undefined mechanisms. Therefore, determining how Cbl physically interacts with tyrosine kinases is of substantial interest. We recently identified a phosphotyrosine binding (PTB) domain residing within the N-terminal transforming region of Cbl (Cbl-N), which mediated direct binding to ZAP-70 tyrosine kinase. Here, we have screened a degenerate phosphopeptide library and show that the Cbl-PTB domain selects a D(N/D)XpY motif, reminiscent of but distinct from the NPXpY motif recognized by the PTB domains of Shc and IRS-1/2. A phosphopeptide predicted by this motif and corresponding to the in vivo negative regulatory phosphorylation site of ZAP-70 (Tyr(P)292) specifically inhibited binding of ZAP-70 to Cbl-N. A ZAP-70/Y292F mutant failed to bind to Cbl-N, whereas a D290A mutant resulted in a 64% decrease in binding, confirming the importance of the Tyr(P) and Y-2 residues in Cbl-PTB domain recognition. Finally the ZAP-70/Y292F mutant also failed to associate with Cbl-N or full-length Cbl in vivo. These results identify a potential Cbl-PTB domain-dependent role for Cbl in the negative regulation of ZAP-70 and predict potential Cbl-PTB domain binding sites on other protein tyrosine kinases known to interact with Cbl.