The Rho family guanine nucleotide exchange factor Vav-2 regulates the development of cell-mediated cytotoxicity

Locked and Loaded: Mechanisms Regulating Natural Killer Cell Lytic Granule Biogenesis and Release

NK cell-mediated cytotoxicity is a critical element of our immune system required for protection from microbial infections and cancer. NK cells bind to and eliminate infected or cancerous cells via direct secretion of cytotoxic molecules toward the bound target cells. In this review, we summarize the current understanding of the molecular regulations of NK cell cytotoxicity, focusing on lytic granule development and degranulation processes. NK cells synthesize apoptosis-inducing proteins and package them into specialized organelles known as lytic granules (LGs). Upon activation of NK cells, LGs converge with the microtubule organizing center through dynein-dependent movement along microtubules, ultimately polarizing to the cytotoxic synapse where they subsequently fuse with the NK plasma membrane. From LGs biogenesis to degranulation, NK cells utilize several strategies to protect themselves from their own cytotoxic molecules. Additionally, molecular pathways that enable NK cells to perform serial killing are beginning to be elucidated. These advances in the understanding of the molecular pathways behind NK cell cytotoxicity will be important to not only improve current NK cell-based anti-cancer therapies but also to support the discovery of additional therapeutic opportunities.

The Rac activator DOCK2 regulates natural killer cell-mediated cytotoxicity in mice through the lytic synapse formation

Natural killer (NK) cells play an important role in protective immunity against viral infection and tumor progression, but they also contribute to rejection of bone marrow grafts via contact-dependent cytotoxicity. Ligation of activating NK receptors with their ligands expressed on target cells induces receptor clustering and actin reorganization at the interface and triggers polarized movement of lytic granules to the contact site. Although activation of the small GTPase Rac has been implicated in NK cell-mediated cytotoxicity, its precise role and the upstream regulator remain elusive. Here, we show that DOCK2, an atypical guanine nucleotide exchange factor for Rac, plays a key role in NK cell-mediated cytotoxicity. We found that although DOCK2 deficiency in NK cells did not affect conjugate formation with target cells, DOCK2-deficienct NK cells failed to effectively kill leukemia cells in vitro and major histocompatibility complex class I-deficient bone marrow cells in vivo, regardless of the sorts of activating receptors. In DOCK2-deficient NK cells, NKG2D-mediated Rac activation was almost completely lost, resulting in a severe defect in the lytic synapse formation. Similar results were obtained when the Rac guanine nucleotide exchange factor activity of DOCK2 was selectively abrogated. These results indicate that DOCK2-Rac axis controls NK cell-mediated cytotoxicity through the lytic synapse formation.

WASH knockout T cells demonstrate defective receptor trafficking, proliferation, and effector function

WASH is an Arp2/3 activator of the Wiskott-Aldrich syndrome protein superfamily that functions during endosomal trafficking processes in collaboration with the retromer and sorting nexins, but its in vivo function has not been examined. To elucidate the physiological role of WASH in T cells, we generated a WASH conditional knockout (WASHout) mouse model. Using CD4(Cre) deletion, we found that thymocyte development and naive T cell activation are unaltered in the absence of WASH. Surprisingly, despite normal T cell receptor (TCR) signaling and interleukin-2 production, WASHout T cells demonstrate significantly reduced proliferative potential and fail to effectively induce experimental autoimmune encephalomyelitis. Interestingly, after activation, WASHout T cells fail to maintain surface levels of TCR, CD28, and LFA-1. Moreover, the levels of the glucose transporter, GLUT1, are also reduced compared to wild-type T cells. We further demonstrate that the loss of surface expression of these receptors in WASHout cells results from aberrant accumulation within the collapsed endosomal compartment, ultimately leading to degradation within the lysosome. Subsequently, activated WASHout T cells experience reduced glucose uptake and metabolic output. Thus, we found that WASH is a newly recognized regulator of TCR, CD28, LFA-1, and GLUT1 endosome-to-membrane recycling. Aberrant trafficking of these key T cell proteins may potentially lead to attenuated proliferation and effector function.

The N-terminal 20-amino acid region of guanine nucleotide exchange factor Vav1 plays a distinguished role in T cell receptor-mediated calcium signaling

Vav1 is a guanine nucleotide exchange factor (GEF) specifically expressed in hematopoietic cells. It consists of multiple structural domains and plays important roles in T cell activation. The other highly conserved isoforms of Vav family, Vav2 and Vav3, are ubiquitously expressed in human tissues including lymphocytes. All three Vav proteins activate Rho family small GTPases, which are involved in a variety of biological processes during T cell activation. Intensive studies have demonstrated that Vav1 is indispensable for T cell receptor (TCR)-mediated signal transduction, whereas Vav2 and Vav3 function as GEFs that overlap with Vav1 on TCR-induced cytoskeleton reorganization. T cells lacking Vav1 exhibited severe defect in TCR-mediated calcium elevation, indicating that the co-existing Vav2 and Vav3 did not compensate Vav1 in calcium signaling. What is the functional particularity of Vav1 in lymphocytes? In this study, we identified the N-terminal 20 amino acids of Vav1 in the calponin homology (CH) domain to be essential for its interaction with calmodulin (CaM) that leads to TCR-induced calcium mobilization. Substitution of the 1-20 amino acids of Vav1 with those of Vav2 or Vav3 abolished the association with CaM, and the N-terminal mutations of Vav1 failed to potentiate normal TCR-induced calcium mobilization, that in turn, suspended nuclear factor of activated T cells (NFAT) activation and IL-2 production. This study highlights the importance of the N-terminal 20 aa of Vav1 for CaM binding, and provides new insights into the distinguished and irreplaceable role of Vav1 in T cell activation and signal transduction.

PIPKI gamma 90 negatively regulates LFA-1-mediated adhesion and activation in antigen-induced CD4+ T cells

T cell activation requires the formation and maintenance of stable interactions between T cells and APCs. The formation of stable T cell-APC contacts depends on the activation of the integrin LFA-1 (CD11aCD18). Several positive regulators of LFA-1 activation downstream of proximal TCR signaling have been identified, including talin; however, negative regulators of LFA-1 activity remain largely unexplored. Extended isoform of phosphatidylinositol phosphate kinase type I γ (PIPKIγ90) is a member of the type I phosphatidylinositol phosphate kinase family that has been shown previously to modulate talin activation of integrins through production of phosphatidylinositol 4,5-bisphosphate and direct binding to talin. In this study, we show that PIPKIγ90 negatively regulates LFA-1-mediated adhesion and activation of T cells. Using CD4(+) T cells from PIPKIγ90-deficient mice, we show that CD4(+) T cells exhibit increased LFA-1-dependent adhesion to ICAM-1 and increased rates of T cell-APC conjugate formation with enhanced LFA-1 polarization at the synapse. In addition to increased adhesiveness, PIPKIγ90-deficient T cells exhibit increased proliferation both in vitro and in vivo and increased production of IFN-γ and IL-2. Together, these results demonstrate that PIPKIγ90 is a negative regulator of Ag-induced T cell adhesion and activation.

Tyrosine-phosphorylated caveolin-1 blocks bacterial uptake by inducing Vav2-RhoA-mediated cytoskeletal rearrangements

During the early stages of infection, Neisseria gonorrhoeae triggers a phosphotyrosine-dependent Cav1-Vav2-RhoA signaling cascade that promotes the pathogen's extracellular state.

Certain bacterial adhesins appear to promote a pathogen's extracellular lifestyle rather than its entry into host cells. However, little is known about the stimuli elicited upon such pathogen host-cell interactions. Here, we report that type IV pili (Tfp)-producing Neisseria gonorrhoeae (P⁺GC) induces an immediate recruitment of caveolin-1 (Cav1) in the host cell, which subsequently prevents bacterial internalization by triggering cytoskeletal rearrangements via downstream phosphotyrosine signaling. A broad and unbiased analysis of potential interaction partners for tyrosine-phosphorylated Cav1 revealed a direct interaction with the Rho-family guanine nucleotide exchange factor Vav2. Both Vav2 and its substrate, the small GTPase RhoA, were found to play a direct role in the Cav1-mediated prevention of bacterial uptake. Our findings, which have been extended to enteropathogenic Escherichia coli, highlight how Tfp-producing bacteria avoid host cell uptake. Further, our data establish a mechanistic link between Cav1 phosphorylation and pathogen-induced cytoskeleton reorganization and advance our understanding of caveolin function.

Like many bacterial pathogens, successful attachment of Neisseria gonorrhoeae—the causative agent of the sexually transmitted disease gonorrhoea—to its host cells depends on specialized structures on the bacterial surface called type IV pili (Tfp). Pathogen attachment induces changes within host cells that may facilitate and promote infection. In this study, we identify some of the earliest cellular signals elicited by N. gonorrhoeae during infection, which, in this case, prevent the organism from entering the cell precociously. After attachment to host cells the bacteria form microcolonies on the cell surface. Underneath these microcolonies, so-called cortical plaques form within the host cell—these contain the cytoskeleton protein actin and a range of signaling proteins. We show that N. gonorrhoeae recruits a host cell protein called caveolin-1 to the cell membrane where the bacteria are attached; here, caveloin-1 effectively impedes uptake of the bacteria by activating a signaling cascade that involves its phosphorylation on a tyrosine residue and subsequent interactions with proteins that regulate the cytoskeleton. Thus, these proteins play a pivotal role in maintaining N. gonorrhoeae in the extracellular milieu. By extrapolating our findings to another Tfp-producing bacterium, the enteropathogenic Escherichia coli, we argue that the establishment and maintenance of this extracellular state benefits certain pathogens by giving them time to express proteins required for subsequent steps of infection.

Identification of the Eph receptor pathway as a novel target for eicosapentaenoic acid (EPA) modification of gene expression in human colon adenocarcinoma cells (HT-29)

Background

The health benefits of polyunsaturated fatty acids (PUFAs), particularly those of the n-3 series are well documented. The mechanisms by which these effects are mediated are not fully clarified.

Methods

We used microarrays to assess the effects on gene expression in HT29 colon adenocarcinoma cells of exposure to the n-3 fatty acid eicosapentaenoic acid (EPA). HT29 cells were cultured with EPA (150 μM) for up to 24 hr prior to harvesting and isolation of RNA. Microarray results were analyzed within the statistical package 'R', and GeneGo MetaCore was used to identify key pathways of altered gene expression.

Results

EphB4, Vav2 and EphA1 gene expression were identified as significantly altered by EPA treatment. Statistically significant changes in gene expression after HT29 exposure to EPA were confirmed in a second experiment by real-time RT-PCR (TaqMan), This experiment also compared the effects of exposure to EPA to arachadonic acid (AA, n-6). Corresponding changes in protein expression were also assessed by Western blotting.

Conclusions

Eph receptor mediated signaling is an entirely novel signaling pathway through which EPA may promote a wide range of health benefits, in particular in relation to reduction of colorectal cancer progression.

Dynamin 2 regulates granule exocytosis during NK cell-mediated cytotoxicity

NK cells are innate immune cells that can eliminate their targets through granule release. In this study, we describe a specialized role for the large GTPase Dynamin 2 (Dyn2) in the regulation of these secretory events leading to cell-mediated cytotoxicity. By modulating the expression of Dyn2 using small interfering RNA or by inhibiting its activity using a pharmacological agent, we determined that Dyn2 does not regulate conjugate formation, proximal signaling, or granule polarization. In contrast, during cell-mediated killing, Dyn2 localizes with lytic granules and polarizes to the NK cell-target interface where it regulates the final fusion of lytic granules with the plasma membrane. These findings identify a novel role for Dyn2 in the exocytic events required for effective NK cell-mediated cytotoxicity.

The Gammaherpesvirus m2 protein manipulates the Fyn/Vav pathway through a multidocking mechanism of assembly

To establish latent infections in B-cells, gammaherpesviruses express proteins in the infected B-cells of the host that spuriously activate signalling pathways located downstream of the B-cell receptor. One such protein is M2, a murine gammaherpesvirus 68-encoded molecule that activates the Vav1/Rac1 pathway via the formation of trimolecular complexes with Scr family members. Previous reports have shown that the formation of this heteromolecular complex involves interactions between a proline rich region of M2 and the Vav1 and Fyn SH3 domains. Here, we show that the optimal association of these proteins requires a second structural motif encompassing two tyrosine residues (Tyr120 and 129). These residues are inducibly phosphorylated by Fyn in non-hematopoietic cells and constitutively phosphorylated in B-cells. We also demonstrate that the phosphorylation of Tyr120 creates specific docking sites for the SH2 domains of both Vav1 and Fyn, a condition sine qua non for the optimal association of these two signalling proteins in vivo. Interestingly, signaling experiments indicate that the expression of M2 in B-cells promotes the tyrosine phosphorylation of Vav1 and additional signaling proteins, a biological process that requires the integrity of both the M2 phosphotyrosine and proline rich region motifs. By infecting mice with viruses mutated in the m2 locus, we show that the integrity of each of these two M2 docking motifs is essential for the early steps of murine gammaherpesvirus-68 latency. Taken together, these results indicate that the M2 phosphotyrosine motif and the previously described M2 proline rich region work in a concerted manner to manipulate the signaling machinery of the host B-cell.

RasGRF2, a guanosine nucleotide exchange factor for Ras GTPases, participates in T-cell signaling responses

The Ras pathway is critical for the development and function of T lymphocytes. The stimulation of this GTPase in T cells occurs primarily through the Vav1- and phospholipase C-gamma1-dependent activation of RasGRP1, a diacylglycerol-responsive Ras GDP/GTP exchange factor. Here, we show that a second exchange factor, RasGRF2, also participates in T-cell signaling. RasGRF2 is expressed in T cells, translocates to immune synapses, activates Ras, and stimulates the transcriptional factor NF-AT (nuclear factor of activated T cells) through Ras- and phospholipase C-gamma1-dependent routes. T-cell receptor-, Vav1-, and Ca2+-elicited pathways synergize with RasGRF2 for NF-AT stimulation. The analysis of RasGRF2-deficient mice indicates that this protein is required for the induction of bona fide NF-AT targets such as the cytokines tumor necrosis factor alpha and interleukin 2, while it plays minor roles in Ras activation itself. The comparison of lymphocytes from Vav1-/-, Rasgrf2-/-, and Vav1-/-; Rasgrf2-/- mice demonstrates that the RasGRF2 pathway cooperates with the Vav1/RasGRP1 route in the blasting transformation and proliferation of mature T cells. These results identify RasGRF2 as an additional component of the signaling machinery involved in T-cell receptor- and NF-AT-mediated immune responses.

TCR/CD28-stimulated actin dynamics are required for NFAT1-mediated transcription of c-rel leading to CD28 response element activation

TCR/CD28 engagement triggers the initiation of a variety of signal transduction pathways that lead to changes in gene transcription. Although reorganization of the actin cytoskeleton is required for T cell activation, the molecular pathways controlled by the actin cytoskeleton are ill defined. To this end, we analyzed TCR/CD28-stimulated signaling pathways in cytochalasin D-treated T cells to determine the cytoskeletal requirements for T cell activation. Cytochalasin D treatment impaired T cell activation by causing a reduction in TCR/CD28-mediated calcium flux, and blocked activation of two regulatory elements within the IL-2 promoter, NFAT/AP-1 and CD28RE/AP. Treatment had no effect on signaling leading to the activation of either AP-1 or NF-kappaB. Significantly, we found that NFAT1 is required for optimal c-rel up-regulation in response to TCR/CD28 stimulation. In fact, NFAT1 could be detected bound at the c-rel promoter in response to TCR/CD28 stimulation, and targeting of NFAT1 using RNA interference in human CD4(+) T cells abrogated c-rel transcription. Overall, these findings establish that disrupting actin cytoskeletal dynamics impairs TCR/CD28-mediated calcium flux required for NFAT1-mediated c-rel transcription and, thus, activation of the CD28RE/AP.

Differential regulation of human NK cell-mediated cytotoxicity by the tyrosine kinase Itk

NK cells are effector lymphocytes that can recognize and eliminate virally infected and transformed cells. NK cells express distinct activating receptors, including an ITAM-containing FcR complex that recognizes Ab-coated targets, and the DNAX-activating protein of 10 kDa-containing NKG2D receptor complex that recognizes stress-induced ligands. The regulatory role of specific tyrosine kinases in these pathways is incompletely understood. In this study, we show that, in activated human NK cells, the tyrosine kinase IL-2-inducible T cell kinase (Itk), differentially regulates distinct NK-activating receptors. Enhanced expression of Itk leads to increases in calcium mobilization, granule release, and cytotoxicity upon stimulation of the ITAM-containing FcR, suggesting that Itk positively regulates FcR-initiated cytotoxicity. In contrast, enhanced Itk expression decreases cytotoxicity and granule release downstream of the DNAX-activating protein of 10 kDa-containing NKG2D receptor, suggesting that Itk is involved in a pathway of negative regulation of NKG2D-initiated granule-mediated killing. Using a kinase mutant, we show that the catalytic activity of Itk is required for both the positive and negative regulation of these pathways. Complementary experiments where Itk expression was suppressed also showed differential regulation of the two pathways. These findings suggest that Itk plays a complex role in regulating the functions initiated by distinct NK cell-activating receptors. Moreover, understanding how these pathways may be differentially regulated has relevance in the setting of autoimmune diseases and antitumor immune responses where NK cells play key regulatory roles.

Formins regulate the actin-related protein 2/3 complex-independent polarization of the centrosome to the immunological synapse

T cell receptor (TCR)-mediated cytoskeletal reorganization is considered to be actin-related protein (Arp) 2/3 complex dependent. We therefore examined the requirement for Arp2/3- and formin-dependent F-actin nucleation during T cell activation. We demonstrated that without Arp2/3-mediated actin nucleation, stimulated T cells could not form an F-actin-rich lamellipod, but instead produced polarized filopodia-like structures. Moreover, the microtubule-organizing center (MTOC, or centrosome), which rapidly reorients to the immunological synapse through an unknown mechanism, polarized in the absence of Arp2/3. Conversely, the actin-nucleating formins, Diaphanous-1 (DIA1) and Formin-like-1 (FMNL1), did not affect TCR-stimulated F-actin-rich structures, but instead displayed unique patterns of centrosome colocalization and controlled TCR-mediated centrosome polarization. Depletion of FMNL1 or DIA1 in cytotoxic lymphocytes abrogated cell-mediated killing. Altogether, our results have identified Arp2/3 complex-independent cytoskeletal reorganization events in T lymphocytes and indicate that formins are essential cytoskeletal regulators of centrosome polarity in T cells.

Strongly reduced alloreactivity and long-term survival times of cardiac allografts in Vav1- and Vav1/Vav2-knockout mice

Vav proteins mediate T- and B-cell activation by functioning as GTP/GDP exchange factors for small GTPases. We have studied the role of Vav1 and Vav2 in allogeneic T-cell activation, antibody responses and allograft rejection. Alloantigen-induced proliferation of T cells from Vav1- and Vav1/Vav2-knockout (ko) mice was decreased by >90% in a mixed lymphocyte reaction. In whole-blood cultures, Vav deficiency led to markedly impaired T- and B-cell activation. Expansion of Vav1- or Vav1/Vav2-ko T cells (C57BL/6) was reduced after transfer into severe combined immune deficiency/beige recipient mice (BALB/c). After priming with 2,4-dinitrophenyl (DNP)-keyhole limpet hemocyanin, T-cell-dependent anti-DNP IgM and IgG antibody levels were normal in Vav1-ko mice but undetectable in Vav1/Vav2-ko mice. The median survival time of BALB/c cardiac allografts transplanted into C57BL/6 Vav1-ko mice (n = 13) or Vav1/Vav2-ko mice (n = 5) was >100 and >77 days, compared with 8-9 days in the corresponding wild-type mice. Vav1/Vav2-ko mice with <100 days graft survival developed bacterial skin infections and were prematurely killed with beating cardiac allograft. Long-term surviving transplants of single and double ko mice showed mild cellular interstitial rejection and mild to severe vascular remodeling. In conclusion, our studies show for the first time that the absence of Vav1 and Vav1/Vav2 in ko mice strongly reduces alloreactivity and results in long-term allograft survival, whereas antibody responses were only affected in Vav1/Vav2 ko mice.

High-efficient lentiviral vector-mediated gene transfer into primary human NK cells

OBJECTIVE

The long-term transfection of genes into primary natural killer (NK) cells without disrupting normal cellular functions has been proven to be difficult with currently available gene-transfer methods. In this study, we establish a lentiviral vector-based technique for improved gene transfer into human NK cells in vitro and we report on high-efficient transduction of freshly isolated as well as cultured primary NK cells.

METHODS

Freshly isolated or primary cultured human NK cells, as well as the human NK cell line YTS, were transduced with replication-incompetent human immunodeficiency virus (HIV)-based lentiviral vector bearing a GFP reporter gene or a gene of interest under the control of the elongation factor 1alpha (EF1alpha) promoter. Transduction efficiencies were monitored by flow cytometry.

RESULTS

A long-term transgene expression was detected in up to 98% of YTS NK cells, whereas in freshly isolated or primary cultured NK cells exposed to interleukin (IL)-2 plus IL-12 upon infection, efficiency was in the range of 50% to 90%. Moreover, in freshly isolated quiescent NK cells a transfection efficiency of 18% to 20% was achieved without stimulation. Notably, no major phenotypic and functional modifications were observed in transduced cells with respect to control cells: the expression levels of activating receptors, CD69-antigen induction as well as cytotoxic function were unaffected.

CONCLUSION

Results of our study demonstrate that NK cells can be efficiently transduced by lentiviral vectors.

HS1 functions as an essential actin-regulatory adaptor protein at the immune synapse

HS1, the leukocyte-specific homolog of cortactin, regulates F-actin in vitro and is phosphorylated in response to TCR ligation, but its role in lymphocyte activation has not been addressed. We demonstrate that HS1-deficient T cells fail to accumulate F-actin at the immune synapse (IS) and, upon TCR ligation, form actin-rich structures that are disordered and unstable. Early TCR activation events are intact in these cells, but Ca2+ influx and IL-2 gene transcription are defective. Importantly, HS1 tyrosine phosphorylation is required for its targeting to the IS and for its function in regulating actin dynamics and IL-2 promoter activity. Phosphorylation also links HS1 to multiple signaling proteins, including Lck, PLCgamma1, and Vav1, and is essential for the stable recruitment of Vav1 to the IS. Taken together, our studies show that HS1 is indispensable for signaling events leading to actin assembly and IL-2 production during T cell activation.

NKG2D-mediated signaling requires a DAP10-bound Grb2-Vav1 intermediate and phosphatidylinositol-3-kinase in human natural killer cells

NKG2D is an important immunosurveillance receptor that responds to stress-induced ligand expression on tumors and virus-infected cells. Human natural killer cells express NKG2D and require the transmembrane adaptor DAP10 to initiate their full cytotoxic activation. However, DAP10 has no immunoreceptor tyrosine-based activation motif and thus the mechanism of recruiting 'downstream' effector proteins is unclear. We show here that binding of the p85 subunit of phosphatidylinositol-3- kinase to DAP10 could not by itself trigger cell-mediated cytotoxicity and that binding of an intermediate consisting of the DAP10 binding partner Grb2 and the effector molecule Vav1 (Grb2-Vav1) to DAP10 was sufficient to initiate tyrosine-phosphorylation events. For full calcium release and cytotoxicity to occur, both Grb2-Vav1 and p85 had to bind to DAP10. These findings identify a previously unknown mechanism by which NKG2D-DAP10 mediates cytotoxicity and provides a framework for evaluating activation by other receptor complexes that lack immunoreceptor tyrosine-based activation motifs.

Cholecystokinin stimulates the recruitment of the Src–RhoA–phosphoinositide 3-kinase pathway by Vav-2 downstream of Gα13 in pancreatic acini

In isolated rat pancreatic acini, Src, RhoA, PI3-K, Vav-2, G(alpha12), and G(alpha13) were detected by immunoblotting. CCK enhanced the levels of these proteins, and the levels of Src and RhoA were reduced by the Src inhibitor herbimycin A and the Rho inhibitor pravastatin. The PI3-K inhibitor wortmannin reduced the level of PI3-K. These inhibitors also decreased amylase secretion in CCK-treated pancreatic acini without altering basal secretion. Immunoprecipitation studies indicated that CCK caused Src to associate with Vav-2, RhoA, and PI3-K and RhoA and Src to associate with Vav-2. Ras, RasGAP, and SOS did not coimmunoprecipitate with Vav-2, and RasGAP and SOS did not coimmunoprecipitate with RhoA. CCK also enhanced Vav-2 and RhoA to coimmunoprecipitate with G(alpha13). We conclude that CCK stimulates the recruitment of the Src-RhoA-PI3-K signaling pathway by Vav-2 downstream of G(alpha13) in pancreatic acini.

Identification of Differentially Expressed Proteins in Imatinib Mesylate-resistant Chronic Myelogenous Cells

Resistance to imatinib mesylate (also known as Gleevec, Glivec, and STI571) often becomes a barrier to the treatment of chronic myelogenous leukemia (CML). In order to identify markers of the action of imatinib mesylate, we used a mass spectrometry approach to compare protein expression profiles in human leukemia cells (K562) and in imatinib mesylate-resistant human leukemia cells (K562-R) in the presence and absence of imatinib mesylate. We identified 118 differentially regulated proteins in these two leukemia cell-lines, with and without a 1 microM imatinib mesylate challenge. Nine proteins of unknown function were discovered. This is the first comprehensive report regarding differential protein expression in imatinib mesylate-treated CML cells.

Vav-family proteins in T-cell signalling

The Vav family proteins (Vav1, Vav2, Vav3) are cytoplasmic guanine nucleotide exchange factors (GEFs) for Rho-family GTPases. T-cell antigen receptor (TCR) signalling results in the tyrosine phosphorylation of Vav proteins and hence their activation. Results from mice deficient in one or more Vav proteins has shown that they play critical roles in T-cell development and activation. Vav1 is required for TCR-induced calcium flux, activation of the ERK MAP kinase pathway, activation of the NF-kappaB transcription factor, inside-out activation of the integrin LFA-1, TCR clustering, and polarisation of the T cell. Although many of these processes may require the GEF activity of Vav1, it is possible that Vav1 also has adaptor-like functions. Recent evidence suggests that Vav1 might also function in the nucleus, where it undergoes arginine methylation. An emerging theme is that Vav proteins may have important functions downstream of receptors other than the TCR, such as integrins and chemokine receptors.

Arf6: a new player in FcγRIIIA lymphocyte-mediated cytotoxicity

The activation of phosphoinositide metabolism represents a critical step in the signaling pathways leading to the activation of cytolytic machinery, but its regulation is partially understood. We report here that the stimulation of the low-affinity receptor for immunoglobulin G (IgG) (FcγRIIIA, CD16) on primary human natural killer (NK) cells induces a phosphatidylinositol 3-kinase (PI3K)–dependent activation of the small G protein Arf6. We first demonstrate a functional role for Arf6-dependent signals in the activation of the antibody-dependent cellular cytotoxicity (ADCC) attributable to the control of secretion of lytic granule content. We also show that Arf6 couples CD16 to the lipid-modifying enzymes phosphatidylinositol4phosphate 5-kinase type I alpha (PI5KIα) and phospholipase D (PLD) that are involved in the control of granule secretion; Arf6, but not Rho family small G proteins RhoA and Rac1, is required for receptor-induced PI5KIα membrane targeting as well as for PI5KIα and PLD activation. Our findings suggest that Arf6 plays a crucial role in the generation of a phosphatidylinositol4,5-bisphosphate (PIP2) plasma membrane pool required for cytolytic granule-mediated target cell killing.

Defective Vav expression and impaired F-actin reorganization in a subset of patients with common variable immunodeficiency characterized by T-cell defects

Common variable immunodeficiency (CVID) is a primary immune disorder characterized by impaired antibody production, which is in many instances secondary to defective T-cell function (T-CVID). We have previously identified a subset of patients with T-CVID characterized by defective T-cell receptor (TCR)-dependent protein tyrosine phosphorylation. In these patients, ZAP-70 fails to be recruited to the TCR as the result of impaired CD3zeta phosphorylation, which is, however, not dependent on defective Lck expression or activity. Here we show that neither Fyn nor CD45 is affected in these patients. On the other hand, T-CVID T cells show dramatic defects in the Vav/Rac pathway controlling F-actin dynamics. A significant deficiency in Vav protein was indeed observed; in 3 of 4 patients with T-CVID, it was associated with reduced VAV1 mRNA levels. The impairment in Vav expression correlated with defective F-actin reorganization in response to TCR/CD28 co-engagement. Furthermore, TCR/CD28-dependent up-regulation of lipid rafts at the cell surface, which requires F-actin dynamics, was impaired in these patients. The actin cytoskeleton defect could be reversed by reconstitution of Vav1 expression in the patients' T cells. Results demonstrate an essential role of Vav in human T cells and strongly suggest Vav insufficiency in T-CVID.

Homocysteine activates NADH/NADPH oxidase through ceramide-stimulated Rac GTPase activity in rat mesangial cells

BACKGROUND

We recently demonstrated that homocysteine (Hcys) increases superoxide (O2-) production via NADH/NADPH oxidase in renal mesangial cells. This O2- production contributes to increased expression of tissue inhibitor of metalloproteinase (TIMP-1) and consequent deposition of collagen in response to Hcys. However, the mechanism by which Hcys activates NADH/NADPH oxidase remains unknown. Given that ceramide is an intracellular activator of this oxidase in several cell types, the present study tests the hypothesis that Hcys activates NADH/NADPH oxidase through a ceramide-mediated signaling pathway in rat mesangial (MG) cells, resulting in O2- production.

METHODS

Rat MG cells were incubated with L-homocysteine (L-Hcys) to determine the mechanism by which Hcys activates NADH/NADPH oxidase. Thin layer chromatography (TLC), Western blot analysis, Rac GTPase activity pull down assay, and NADH/NADPH oxidase activity measurements were performed.

RESULTS

TLC analysis demonstrated that L-Hcys increased de novo production of ceramide in MG cells. L-Hcys and increased ceramide did not alter the amount of NADH/NADPH oxidase subunit p47phox and p67phox in both membrane and cytosolic fractions from MG cells. However, L-Hcys or ceramide markedly increased the level of GTP-bound Rac, which was accompanied by enhanced activity of NADH/NADPH oxidase. These Hcys or ceramide-induced actions were substantially blocked by a Rac GTPase inhibitor, GDPbetaS, and a de novo ceramide synthesis inhibitor, fumonisin B1 (FB1).

CONCLUSION

These results indicate that Hcys activates NADH/NADPH oxidase by stimulating de novo ceramide synthesis, and subsequently enhancing Rac GTPase activity in rat MG cells. This ceramide-Rac GTPase signaling pathway may mediate Hcys-induced oxidative stress in these glomerular cells.

Dynamin 2 regulates T cell activation by controlling actin polymerization at the immunological synapse

Actin reorganization at the immunological synapse is required for the amplification and generation of a functional immune response. Using small interfering RNA, we show here that dynamin 2 (Dyn2), a large GTPase involved in receptor-mediated internalization, did not alter antibody-mediated T cell receptor internalization but considerably affected T cell receptor-stimulated T cell activation by regulating multiple biochemical signaling pathways and the accumulation of F-actin at the immunological synapse. Moreover, Dyn2 interacted directly with the Rho family guanine nucleotide exchange factor Vav1, and this interaction was required for T cell activation. These data identify a functionally important interaction between Dyn2 and Vav1 that regulates actin reorganization and multiple signaling pathways in T lymphocytes.

Anatomy of a murder—signal transduction pathways leading to activation of natural killer cells

Natural killer (NK) cells control the early phases of viral infections, modulate antigen-specific immune responses, and participate in the rejection of tumours and bone marrow grafts. A fine balance between inhibitory and activating receptors tightly regulates NK cell activation. Biochemical studies in human cell lines and primary cells have revealed some of the activating NK cell signalling pathways, however animal models are instrumental to understand the physiological implications of these findings for immune responses in vivo. Gene targeting in mice and biochemical studies in cells are helping to dissect out the various signal transduction pathways that control NK cell activation. A clearer view of these pathways may eventually help designing more effective immune therapies based on the use of NK cells.

Ectopic expression of VAV1 reveals an unexpected role in pancreatic cancer tumorigenesis

Herein, we show that the hematopoietic-specific GEF VAV1 is ectopically expressed in primary pancreatic adenocarcinomas due to demethylation of the gene promoter. Interestingly, VAV1-positive tumors had a worse survival rate compared to VAV1-negative tumors. Surprisingly, even in the presence of oncogenic KRAS, VAV1 RNAi abrogates neoplastic cellular proliferation in vitro and in vivo, thus identifying Vav1 as a growth-stimulatory protein in this disease. Vav1 acts synergistically with the EGF receptor to stimulate pancreatic tumor cell proliferation. Mechanistically, the effects of Vav1 require its GEF activity and the activation of Rac1, PAK1, and NF-kappaB and involve cyclin D1 upregulation. Thus, the discovery of prooncogenic pathways regulated by Vav1 makes it an attractive target for therapeutic intervention.

Vav1 and vav3 have critical but redundant roles in mediating platelet activation by collagen

Vav family proteins are guanine nucleotide exchange factors for the Rho/Rac family of small GTP-binding proteins. In addition, they have domains that mediate protein-protein interactions, including one Src homology 2 (SH2) and two Src homology 3 (SH3) domains. Vav1, Vav2, and Vav3 play a crucial role in the regulation of phospholipase C gamma (PLC gamma) isoforms by immuno-tyrosine-based activation motif (ITAM)-coupled receptors, including the T- and B-cell antigen receptors. We have reported in platelets, however, that Vav1 and Vav2 are not required for activation of PLC gamma 2 in response to stimulation of the ITAM-coupled collagen receptor glycoprotein VI (GPVI). Here we report that Vav3 is tyrosinephosphorylated upon activation of GPVI but that Vav3-deficient platelets also exhibit a normal response upon activation of the ITAM receptor. In sharp contrast, platelets deficient in both Vav1 and Vav3 show a marked inhibition of aggregation and spreading upon activation of GPVI, which is associated with a reduction in tyrosine phosphorylation of PLC gamma 2. The phenotype of Vav1/2/3 triple-deficient platelets is similar to that of Vav1/3 double-deficient cells. These results demonstrate that Vav3 and Vav1 play crucial but redundant roles in the activation of PLC gamma 2 by GPVI. This is the first time that absolute redundancy between two protein isoforms has been observed with respect to the regulation of PLC gamma 2 in platelets.

Differential requirements for Vav proteins in DAP10- and ITAM-mediated NK cell cytotoxicity

Natural killer (NK) cells express multiple activating receptors that initiate signaling cascades through DAP10- or immunoreceptor tyrosine-based activation motif–containing adapters, including DAP12 and FcRγ. Among downstream signaling mediators, the guanine nucleotide exchange factor Vav1 carries out a key role in activation. However, whether Vav1 regulates only some or all NK cell–activating pathways is matter of debate. It is also possible that two other Vav family molecules, Vav2 and Vav3, are involved in NK cell activation. Here, we examine the relative contribution of each of these exchange factors to NK cell–mediated cytotoxicity using mice lacking one, two, or all three Vav proteins. We found that Vav1 deficiency is sufficient to disrupt DAP10-mediated cytotoxicity, whereas lack of Vav2 and Vav3 profoundly impairs FcRγ- and DAP12-mediated cytotoxicity. Our results provide evidence that these three Vav proteins function specifically in distinct pathways that trigger NK cell cytotoxicity.

Differential regulation of TCR-mediated gene transcription by Vav family members

Although all three Vav family members are expressed in T lymphocytes, the role that Vav3 plays in T cell activation is poorly defined. Here we show that, like Vav1, Vav3 undergoes rapid tyrosine phosphorylation after T cell receptor (TCR) cross-linkage and interacts with the adaptor molecules SLP76 and 3BP2 in a SH2-dependent manner. However, depletion of Vav1 but not Vav3 protein by RNA interference affects TCR-mediated IL-2 promoter activity. In contrast, Vav3 function is specifically required for coupling TCR stimulation to serum response element–mediated gene transcription. These data indicate that, although both Vav proteins are biochemically coupled to the TCR, they regulate distinct molecular pathways leading to defined gene transcriptional events.

The Rac-activating toxin cytotoxic necrotizing factor 1 oversees NK cell-mediated activity by regulating the actin/microtubule interplay

The cell cytoskeleton is widely acknowledged as a master for NK cell function. Specifically, actin filaments guide the NK cell binding to target cells, engendering the formation of the so-called immunological synapse, while microtubules direct the killer behavior. All these cytoskeleton-dependent activities are competently governed by the Rho GTPases, a family of regulatory molecules encompassing the three different subfamilies, Rho, Rac, and Cdc42. By using a Rac GTPase-activating bacterial protein toxin from Escherichia coli named cytotoxic necrotizing factor 1 (CNF1), we obtained results supporting the activation of Rac GTPase as a booster for effector cell-binding efficiency, recruitment ability, and, consequently, cytotoxicity. In particular, the augmented killer capacity of CNF1-treated NK cells was associated with the increased expression of certain cell adhesion or activation-associated molecules and the reshaping of the actin and microtubule networks. Importantly, CNF1 counteracted the activity exerted by toxins disrupting the cytoskeletal architecture. Hence, the activation of Rho GTPases, particularly Rac, induced by CNF1, appears to orchestrate a dynamic cross talk between microtubules and actin filaments, leading to a fruitful NK cell activity and polarization state. Our findings suggest that protein toxins might be viewed as modulators of NK cell cytotoxic activity and could possibly be regarded as useful pharmacological tools for certain Rho-linked immune diseases in the near future.

NKG2D-DAP10 triggers human NK cell-mediated killing via a Syk-independent regulatory pathway

The immune recognition receptor complex NKG2D-DAP10 on natural killer cells is stimulated by specific ligands carried on virus-infected and malignant cells. Because DAP10 does not have an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic tail, its ability to trigger killing has been debated. Here we show that a crucial Tyr-Ile-Asn-Met amino acid motif in the cytoplasmic tail of DAP10 couples receptor stimulation to the downstream activation of phosphatidylinositol 3-kinase, Vav1, Rho family GTPases and phospholipase C. Unlike that of ITAM-containing receptors, the activation of NKG2D-DAP10 proceeds independently of Syk family protein tyrosine kinases. Yet the signals initiated by NKG2D-DAP10 are fully capable of inducing killing. Our findings identify a previously unknown mechanism by which receptor complexes that lack ITAM motifs can trigger lymphocyte activation.

Regulation of lymphocyte-mediated killing by GTP-binding proteins

Exocytosis of granules containing apoptosis-inducing proteins is one mechanism of target cell killing by cytotoxic T lymphocytes (CTLs) and natural killer (NK) cells. Granules containing perforin and granzymes are redistributed to the area of cell contact initiated by specific interactions between surface ligands on a target cell and receptors on an effector lymphocyte. The formation of a stable conjugate between a cytotoxic lymphocyte and its potential target cell, followed by the directed delivery of granule components to the target cell are prerequisites of lymphocyte-mediated killing. Critical to understanding the development of cytotoxic function by CTLs and NK cells is the delineation of the second messenger pathways that specifically control the reorganization of the actin cytoskeleton during cell-mediated cytotoxicity. The low molecular weight guanosine 5'-triphosphate-binding proteins of the Rho family play a central role in these regulatory events controlling cytotoxic lymphocyte activation.

SH2-containing inositol phosphatase (SHIP-1) transiently translocates to raft domains and modulates CD16-mediated cytotoxicity in human NK cells

Membrane recruitment of the SH2-containing 5' inositol phosphatase 1 (SHIP-1) is responsible for the inhibitory signals that modulate phosphatidylinositol 3-kinase (PI3K)-dependent signaling pathways. Here we have investigated the molecular mechanisms underlying SHIP-1 activation and its role in CD16-mediated cytotoxicity. We initially demonstrated that a substantial fraction of SHIP-1-mediated 5' inositol phosphatase activity associates with CD16 zeta chain after receptor cross-linking. Moreover, CD16 stimulation on human primary natural killer (NK) cells induces the rapid and transient translocation of SHIP-1 in the lipid-enriched plasma membrane microdomains, termed rafts, where it associates with tyrosine-phosphorylated zeta chain and shc adaptor protein. As evaluated by confocal microscopy, CD16 engagement by reverse antibody-dependent cellular cytotoxicity (ADCC) rapidly induces SHIP-1 redistribution toward the area of NK cell contact with target cells and its codistribution with aggregated rafts where CD16 receptor also colocalizes. The functional role of SHIP-1 in the modulation of CD16-induced cytotoxicity was explored in NK cells infected with recombinant vaccinia viruses encoding wild-type or catalytic domain-deleted mutant SHIP-1. We found a significant SHIP-1-mediated decrease of CD16-induced cytotoxicity that is strictly dependent on its catalytic activity. These data demonstrate that CD16 engagement on NK cells induces membrane targeting and activation of SHIP-1, which acts as negative regulator of ADCC function.

Vav1, but not Vav2, contributes to platelet aggregation by CRP and thrombin, but neither is required for regulation of phospholipase C

We have investigated the role of the Rho and Rac family small guanine triphosphate (GTP) exchange factors (RhoGEFs), Vav1 and Vav2, in the activation of platelets by the immunoreceptor tyrosine-based activation motif (ITAM)-coupled collagen receptor GPVI and by the G protein-coupled receptor agonist thrombin. The glycoprotein VI (GPVI)-specific agonist collagen-related peptide (CRP) and thrombin stimulated tyrosine phosphorylation of Vav1 but not Vav2 in human platelets. Surprisingly, however, CRP did not activate the low-molecular-weight G protein Rac and stimulated only a small increase in activity of p21-associated kinase 2 (PAK2), despite the fact that both proteins are regulated downstream of Vav1 in other cells. Further, activation of Rac and PAK2 by thrombin was maintained in platelets from mice deficient in Vav1. Activation of phospholipase C (PLC) by GPVI and thrombin was unaltered in Vav1-, Vav2-, and Vav1/Vav2-deficient platelets. A weak inhibition of late-stage aggregation to CRP and thrombin was observed in platelets deficient in Vav1 but not Vav2, whereas spreading on fibrinogen was not changed. The present results demonstrate that neither Vav1 nor Vav2 lie upstream of PLC or Rac in platelets, highlighting an important difference in their role in signaling by ITAM-coupled receptors in other cell types. The present study has provided evidence for a possible role of Vav1 but not Vav2 in the later stages of platelet aggregation.

Pleiotropic defects in TCR signaling in a Vav-1-null Jurkat T-cell line

The Rac/Rho-specific guanine nucleotide exchange factor, Vav-1, is a key component of the T-cell antigen receptor (TCR)-linked signaling machinery. Here we have used somatic cell gene-targeting technology to generate a Vav-1-deficient Jurkat T-cell line. The J.Vav1 cell line exhibits dramatic defects in TCR-dependent interleukin (IL)-2 promoter activation, accompanied by significant reductions in the activities of the NFAT(IL-2), NFkappaB, AP-1 and REAP transcription factors that bind to the IL-2 promoter region. In contrast, loss of Vav-1 had variable effects on early TCR-stimulated signaling events. J.Vav1 cells display a selective defect in sustained Ca(2+) signaling during TCR stimulation, and complementation of this abnormality by exogenously introduced Vav-1 is dependent on the Vav-1 calponin homology domain. While JNK activation was severely impaired, the stimulation of Ras, ERK and protein kinase C-theta activities, as well as the mobilization of lipid rafts, appeared normal in the J.Vav1 cells. Finally, evidence is presented to suggest that the alternative Vav family members, Vav-2 and Vav-3, are activated during TCR ligation, and partially compensate for the loss of Vav-1 in Jurkat T cells.

Natural killer cell activation in mice and men: different triggers for similar weapons?

The signaling pathways that regulate B and T lymphocytes are remarkably conserved between humans and mice. However, recent evidence suggests that the pathways regulating natural killer (NK) cell activation may actually differ between these two species. We discuss the controversies in the field and propose that this divergence could be deceptive: despite some clear differences between human and mouse NK cell receptors, the many ways of activating NK cells and their functions may well be conserved.

VAV proteins as signal integrators for multi-subunit immune-recognition receptors

In recent years, substantial progress has been made towards the identification of intracellular signalling molecules that couple multi-subunit immune-recognition receptors (MIRRs) to their various effector functions. Among these, the VAV proteins have been observed to have a crucial role in regulating some of the earliest events in receptor signalling. VAV proteins function, in part, as guanine-nucleotide exchange factors (GEFs) for the RHO/RAC family of GTPases. This review focuses on the role of VAV proteins in the regulation of lymphocyte development and function, and emphasizes the regulatory roles that these proteins have through both GEF-dependent and -independent mechanisms.

Rac activation induces NADPH oxidase activity in transgenic COSphox cells, and the level of superoxide production is exchange factor-dependent

Transient expression of constitutively active Rac1 derivatives, (G12V) or (Q61L), was sufficient to induce phagocyte NADPH oxidase activity in a COS-7 cell model in which human cDNAs for essential oxidase components, gp91(phox), p22(phox), p47(phox), and p67(phox), were expressed as stable transgenes. Expression of constitutively active Rac1 in "COS(phox)" cells induced translocation of p47(phox) and p67(phox) to the membrane. Furthermore, translocation of p47(phox) was induced in the absence of p67(phox) expression, even though Rac does not directly bind p47(phox). Rac effector domain point substitutions (A27K, G30S, D38A, Y40C), which can selectively eliminate interaction with different effector proteins, impaired Rac1V12-induced superoxide production. Activation of endogenous Rac1 by expression of constitutively active Rac-guanine nucleotide exchange factor (GEF) derivatives was sufficient to induce high level NADPH oxidase activity in COS(phox) cells. The constitutively active form of the hematopoietic-specific GEF, Vav1, was the most effective at activating superoxide production, despite detection of higher levels of Rac1-GTP upon expression of constitutively active Vav2 or Tiam1 derivatives. These data suggest that Rac can play a dual role in NADPH oxidase activation, both by directly participating in the oxidase complex and by activating signaling events leading to oxidase assembly, and that Vav1 may be the physiologically relevant GEF responsible for activating this Rac-regulated complex.

A role for a RhoA/ROCK/LIM-kinase pathway in the regulation of cytotoxic lymphocytes

Polarization of lipid rafts and granules to the site of target contact is required for the development of cell-mediated killing by cytotoxic lymphocytes. We have previously shown that these events require the activation of proximal protein tyrosine kinases. However, the downstream intracellular signaling molecules involved in the development of cell-mediated cytotoxicity remain poorly defined. We report here that a RhoA/ROCK/LIM-kinase axis couples the receptor-initiated protein tyrosine kinase activation to the reorganization of the actin cytoskeleton required for the polarization of lipid rafts and the subsequent generation of cell-mediated cytotoxicity. Pharmacologic and genetic interruption of any element of this RhoA/ROCK/LIM-kinase pathway inhibits both the accumulation of F-actin and lipid raft polarization to the site of target contact and the subsequent delivery of the lethal hit. These data define a specialized role for a RhoA-->ROCK-->LIM-kinase pathway in cytotoxic lymphocyte activation.

Vav-1 regulates NK T cell development and NK cell cytotoxicity

The hematopoietic-specific Rho-family GTP exchange factor Vav-1 is a regulator of lymphocyte antigen receptor signaling and mediates normal maturation and activation of B and T cells. Recent findings suggest that Vav-1 also forms part of signaling pathways required for natural and antibody dependent cellular cytotoxicity (ADCC) of human NK cells. In this study, we show that Vav-1 is also expressed in murine NK cells. Vav-1(-/-) mice had normal numbers of splenic NK cells, and these displayed a similar expression profile of NK cell receptors as wild-type mice. Unexpectedly, IL-2-activated Vav-1(-/-) NK cells retained normal ADCC. Fc-receptor mediated activation of ERK, JNK, and p38 was also normal. In contrast, Vav-1(-/-) NK cells exhibited reduced natural cytotoxicity against EL4, C4.4.25, RMA and RMA/S. Together, the results demonstrate that Vav-1 is dispensable for mainstream NK cell development, but is required for NK natural cytotoxicity. Unlike the findings for NK cells, NK T cells were dramatically diminished in Vav-1(-/-) mice and splenocytes from Vav-1 mutant mice failed to produce IL-4 in response to in vivo CD3 stimulation. These data highlight the important role of Vav-1 in NK T cell development and NK cell function.

Signal transduction through Vav-2 participates in humoral immune responses and B cell maturation

B and T lymphocytes develop normally in mice lacking the guanine nucleotide exchange factor Vav-2. However, the immune responses to type II thymus-independent antigen as well as the primary response to thymus-dependent (TD) antigen are defective. Vav-2-deficient mice are also defective in their ability to switch immunoglobulin class, form germinal centers and generate secondary immune responses to TD antigens. Mice lacking both Vav-1 and Vav-2 contain reduced numbers of B lymphocytes and display a maturational block in the development of mature B cells. B cells from Vav-1(-/-)Vav-2(-/-) mice respond poorly to antigen receptor triggering, both in terms of proliferation and calcium release. These studies show the importance of Vav-2 in humoral immune responses and B cell maturation.

Functional dichotomy in natural killer cell signaling: Vav1-dependent and -independent mechanisms

The product of the protooncogene Vav1 participates in multiple signaling pathways and is a critical regulator of antigen-receptor signaling in B and T lymphocytes, but its role during in vivo natural killer (NK) cell differentiation is not known. Here we have studied NK cell development in Vav1-/- mice and found that, in contrast to T and NK-T cells, the absolute numbers of phenotypically mature NK cells were not reduced. Vav1-/- mice produced normal amounts of interferon (IFN)-gamma in response to Listeria monocytogenes and controlled early infection but showed reduced tumor clearance in vivo. In vitro stimulation of surface receptors in Vav1-/- NK cells resulted in normal IFN-gamma production but reduced tumor cell lysis. Vav1 was found to control activation of extracellular signal-regulated kinases and exocytosis of cytotoxic granules. In contrast, conjugate formation appeared to be only mildly affected, and calcium mobilization was normal in Vav1-/- NK cells. These results highlight fundamental differences between proximal signaling events in T and NK cells and suggest a functional dichotomy for Vav1 in NK cells: a role in cytotoxicity but not for IFN-gamma production.

Vav2 activates c-fos serum response element and CD69 expression but negatively regulates nuclear factor of activated T cells and interleukin-2 gene activation in T lymphocyte

Vav1 and Vav2 are members of the Dbl family of guanine nucleotide exchange factors for the Rho family of small GTPases. Although the role of Vav1 during lymphocyte development and activation is well characterized, the function of Vav2 is still unclear. In this study, we compared the signaling pathways regulated by Vav1 and Vav2 following engagement of the T cell receptor (TCR). We show that Vav2 is tyrosine-phosphorylated upon TCR stimulation and by co-expressed Src and Syk family kinases. Using glutathione S-transferase fusion proteins, we observed that the Src homology 2 domain of Vav2 binds tyrosine-phosphorylated proteins from TCR-stimulated Jurkat T cell lysates, including c-Cbl and SLP-76. Like Vav1, Vav2 cooperated with TCR stimulation to increase extracellular signal-regulated kinase activation and to promote c-fos serum response element transcriptional activity. Moreover, both proteins displayed a similar action in increasing the expression of the early activation marker CD69 in Jurkat T cells. However, in contrast to Vav1, Vav2 dramatically suppressed TCR signals leading to nuclear factor of activated T cells (NF-AT)-dependent transcription and induction of the interleukin-2 promoter. Vav2 appears to act upstream of the phosphatase calcineurin because a constitutively active form of calcineurin rescued the effect of Vav2 by restoring TCR-induced NF-AT activation. Interestingly, the Dbl homology and Src homology 2 domains of Vav2 were necessary for its inhibitory effect on NF-AT activation and for induction of serum response element transcriptional activity. Taken together, our results indicate that Vav1 and Vav2 exert overlapping but nonidentical functions in T cells. The negative regulatory pathway elicited by Vav2 might play an important role in regulating lymphocyte activation processes.

Regulation of NK cell-mediated cytotoxicity by the adaptor protein 3BP2

Stimulation of lymphocytes through multichain immune recognition receptors activates multiple signaling pathways. Adaptor proteins play an important role in integrating these pathways by their ability to simultaneously bind multiple signaling components. Recently, the 3BP2 adaptor protein has been shown to positively regulate the transcriptional activity of T cells. However, the mechanisms by which signaling components are involved in this regulation remain unclear, as does a potential role for 3BP2 in the regulation of other cellular functions. Here we describe a positive regulatory role for 3BP2 in NK cell-mediated cytotoxicity. We also identify p95(vav) and phospholipase C-gamma isoforms as binding partners of 3BP2. Our results show that tyrosine-183 of 3BP2 is specifically involved in this interaction and that this residue critically influences 3BP2-dependent function. Therefore, 3BP2 regulates NK cell-mediated cytotoxicity by mobilizing key downstream signaling effectors.

Vav-2 controls NFAT-dependent transcription in B- but not T-lymphocytes

We show here that Vav-2 is tyrosine phosphorylated following antigen receptor engagement in both B- and T-cells, but potentiates nuclear factor of activated T cells (NFAT)-dependent transcription only in B cells. Vav-2 function requires the N-terminus, as well as functional Dbl homology and SH2 domains. More over, the enhancement of NFAT-dependent transcription by Vav-2 can be inhibited by a number of dominant-negative GTPases. The ability of Vav-2 to potentiate NFAT-dependent transcription correlates with its ability to promote a sustained calcium flux. Thus, Vav-2 augments the calcium signal in B cells but not T cells, and a truncated form of Vav-2 can neither activate NFAT nor augment calcium signaling. The CD19 co-receptor physically interacts with Vav-2 and synergistically enhances Vav-2 phosphorylation induced by the B-cell receptor (BCR). In addition, we found that Vav-2 augments CD19-stimulated NFAT- dependent transcription, as well as transcription from the CD5 enhancer. These data suggest a role for Vav-2 in transducing BCR signals to the transcription factor NFAT and implicate Vav-2 in the integration of BCR and CD19 signaling.