The immunological synapse and the actin cytoskeleton: molecular hardware for T cell signaling

Dangerous liaisons at the virological synapse

Cell-to-cell viral transmission facilitates the propagation of HIV-1 and human T cell leukemia virus type 1. Mechanisms of cell-to-cell transmission by retroviruses were not well understood until the recent description of virological synapses (VSs). VSs function as specialized sites of immune cell-to-cell contact that direct virus infection. Deciphering the molecular mechanisms of VS formation provides a fascinating insight into how pathogens subvert immune cell communication programs and achieve viral spread.

Stop and Go Traffic to Tune T Cell Responses

Adaptive immune responses are initiated by interactions of T cells with antigen-presenting cells, but the basic nature of these interactions during an immune response in vivo has been a matter of speculation. While some in vitro systems provide evidence for stable interactions, referred to as immunological synapses, compelling evidence supports T cell activation through serial transient interactions. Deep tissue intravital and organ culture microscopy studies suggest that both modes of interaction are employed, but new issues have emerged. This review will discuss in vitro results that framed the hypotheses that are currently being tested in vivo. I present a model in which TCR stop signals compete with chemokine-mediated go signals to adjust the duration of immunological synapse formation and tune the immune response between tolerance and full activation.

Cutting edge: selective requirement for the Wiskott-Aldrich syndrome protein in cytokine, but not chemokine, secretion by CD4+ T cells

The mechanism of cytokine secretion is not well understood, but cytokines appear to be synthesized and released in a polarized fashion toward an Ag-specific target cell. In this study, we demonstrate that the Wiskott-Aldrich syndrome protein (WASp) is an essential component of the cytokine secretory pathway in CD4(+) T cells. Murine WASp-deficient CD4(+) T cells fail to polarize cytokines toward a target and show an unexpected and striking block in cytokine secretion. In contrast, chemokine secretion and trafficking of plasma membrane proteins, transported via the constitutive secretory pathway, are unaffected by the lack of WASp. These results suggest that CD4(+) T cell cytokines require a specialized, WASp-dependent pathway for cellular traffic and/or vesicle release that is distinct from that required for chemokine release. We propose that the use of different secretory pathways for cytokines and chemokines enables CD4(+) T cell activity to be further fine-tuned to serve specialized effector functions.

Requirement of Rac1 and Rac2 expression by mature dendritic cells for T cell priming

Upon maturation, dendritic cells (DCs) acquire the unique ability to activate naïve T cells. We used time-lapse video microscopy and two-photon imaging of intact lymph nodes to show that after establishing initial contact between their dendrites and naïve T lymphocytes, mature DCs migrate toward the contacted lymphocytes. Subsequently, the DCs tightly entrap the T cells within a complex net of membrane extensions. The Rho family guanosine triphosphatases Rac1 and Rac2 but not Rho itself control the formation of dendrites in mature DCs, their polarized short-range migration toward T cells, and T cell priming.

Macrophages from lupus-prone MRL mice are characterized by abnormalities in Rho activity, cytoskeletal organization, and adhesiveness to extracellular matrix proteins

Macrophages (mphi) from prediseased mice of the major murine models of lupus have an identical defect in cytokine expression that is triggered by serum and/or apoptotic cells. It is striking that cytokine expression in the absence of serum and apoptotic cells is equivalent to that of nonautoimmune mice. Here, we show that mphi from prediseased lupus-prone MRL/MpJ (MRL/+) or MRL/MpJ-Tnfrsf6(lpr) (MRL/lpr) mice also have reversible abnormalities in morphology, cytoskeletal organization, and adhesive properties. In the presence of serum, MRL mphi adhered in increased numbers to a variety of extracellular matrix proteins compared with mphi from two nonautoimmune strains. However, in the absence of serum, adhesion by MRL mphi was similar to that of nonautoimmune mphi. Increased adhesion by MRL mphi was also observed in the presence of apoptotic, but not necrotic, cells. The morphology and actin-staining pattern of adherent MRL mphi were consistent with reduced activity of Rho, a cytoskeletal regulator. Indeed, MRL mphi cultured in the presence of serum had markedly decreased levels of active Rho compared with nonautoimmune mphi. It is remarkable that when cultured in the absence of serum, MRL mphi displayed normal Rho activity and cytoskeletal morphology. Addition of a Rho inhibitor to normal mphi reproduced the morphologic and cytoskeletal abnormalities observed in MRL mphi. Taken together, our findings support the hypothesis that mphi from MRL and other systemic lupus erythematosus-prone mice have an apoptotic, cell-dependent, autoimmune phenotype that affects a broad range of mphi functions, including cytokine gene expression and Rho-dependent cytoskeletal regulation.

Relevance of CD6-Mediated Interactions in T Cell Activation and Proliferation

CD6 is a cell surface receptor expressed on immature thymocytes and mature T and B1a lymphocytes. The ultimate function of CD6 has not been deciphered yet, but much evidence supports a role for CD6 in T cell activation and differentiation. In this study, we show that a fraction of CD6 molecules physically associates with the TCR/CD3 complex by coimmunoprecipitation, cocapping, and fluorescence resonance energy transfer experiments. Image analysis of Ag-specific T-APC conjugates demonstrated that CD6 and its ligand, activated leukocyte cell adhesion molecule (CD166), colocalize with TCR/CD3 at the center of the immunological synapse, the so-called central supramolecular activation cluster. The addition of a soluble rCD6 form significantly reduced the number of mature Ag-specific T-APC conjugates, indicating that CD6 mediates early cell-cell interactions needed for immunological synapse maturation to proceed. This was in agreement with the dose-dependent inhibition of CD3-mediated T cell proliferation induced by soluble rCD6. Taken together, our data illustrate the important role played by the intra- and intercellular molecular interactions mediated by CD6 during T cell activation and proliferation processes.

IFN-α Subtypes Differentially Affect Human T Cell Motility

The type I IFN family includes 14 closely related antiviral cytokines that are produced in response to viral infections. They bind to a common receptor, and have qualitatively similar biological activities. The physiological relevance of this redundancy is still unclear. In this study, we analyzed and compared the effects of two potent antiviral type I IFNs, IFN-alpha 2 and IFN-alpha 8, on the motility of various populations of human T lymphocytes in vitro. In this study, we show that IFN-alpha 2 induces chemokinesis of both CD4(+) and CD8(+) T cells at various stages of differentiation, and induces functional changes that result in enhanced T cell motility, including up-regulation of the integrins LFA-1 and VLA-4, and subsequently, increased ICAM-1- and fibronectin-dependent migration. In contrast, IFN-alpha 8 did not affect T cell motility, despite having similar antiviral properties and similar effects on the induction of the antiviral protein MxA. However, transcription of other IFN-stimulated genes showed that transcription of these genes is selectively activated by IFN-alpha 2, but not IFN-alpha 8, in T cells. Finally, while the antiviral activity of the two subtypes is inhibited by Abs against the two subunits of the IFN-alpha receptor, the chemokinetic effect of IFN-alpha 2 is selectively blocked by Abs against the A1 receptor subunit. These observations are consistent with the possibility that subtype-specific intracellular signaling pathways are activated by type I IFNs in T lymphocytes.

Inhibition of Activator Protein 1 by Barbiturates Is Mediated by Differential Effects on Mitogen-Activated Protein Kinases and the Small G Proteins Ras and Rac-1

Barbiturates are known to suppress protective immunity, and their therapeutic use is associated with nosocomial infections. Although barbiturates inhibit T cell proliferation, differentiation, and cytokine synthesis, only thiobarbiturates markedly reduce the activation of immune regulatory transcription factors such as nuclear factor-kappaB and nuclear factor of activated T cells. In this study, we investigated barbiturate-mediated effects on the regulation of the transcription factor activator protein 1 (AP-1) in primary T lymphocytes. We show that both thiobarbiturates and their oxy-analogs inhibit AP-1-dependent gene expression and AP-1 complex formation at clinically relevant doses. Furthermore, mitogen-activated protein (MAP) kinase activity, which transcriptionally and posttranslationally regulates AP-1 complex formation, is suppressed by most barbiturates. CD3/CD28- or phorbol 12-myristate 13-acetate (PMA)/ionomycin-induced p38 and extracellular signal-regulated kinase 1/2 phosphorylation or c-jun NH2-terminal kinase (JNK) 1/2 kinase activity was significantly diminished by pentobarbital, thiamylal, secobarbital, or methohexital treatment. These barbiturates also inhibited the initiators of the MAP kinase cascade, the small G proteins ras and rac-1, and prevented binding to their partners raf-1 and PAK, respectively. Thiopental, unlike the other barbiturates, only reduced ras and JNK activity upon direct CD3/CD28 receptor engagement. Contrarily, upon PMA/ionomycin stimulation, thiopental blocked AP-1-dependent gene expression independently of the small G protein ras and MAP kinases, thus suggesting an additional, unknown mechanism of AP-1 regulation. In conclusion, our results contribute to the explanation of a clinically manifested immune suppression in barbiturate-treated patients and support the idea of a MAP kinase-independent regulation of AP-1 by PKC and calcium in human T cells.

Tec Kinase Migrates to the T Cell-APC Interface Independently of Its Pleckstrin Homology Domain

Tec is the prototypical member of the Tec tyrosine kinases family, which plays an important role in T cell signaling. We show in this study that Tec translocates to the immunological synapse when a T cell contacts a dendritic cell. Surprisingly, the presence of the pleckstrin homology (PH) domain of Tec is not required for this accumulation, and despite a strong activation of 3'-phosphorylated phosphoinositide lipids synthesis during the synapse formation, the Tec PH domain is not redistributed to the T cell plasma membrane. In contrast, we demonstrate that an active Src homology 3 domain is absolutely required, underlining the essential role played by this part of the molecule in the recruitment and/or stabilization of Tec at the immunological synapse. Our results nevertheless suggest that the PH domain controls the kinase activity of the molecule in vivo. We finally demonstrate that the two domains are necessary to trigger transcriptional events following Ag presentation. These data support a model in which the plasma membrane recruitment of the PH-containing protein Tec is not dependent on the production of 3'-phosphorylated phosphoinositide lipids by the PI3K, but rather on an intact Src homology 3 domain.

Alterations of the expression of T-cell-related costimulatory CD28 and downregulatory CD152 (CTLA-4) molecules in patients with B-cell chronic lymphocytic leukaemia

In the present study, we have examined the kinetics and magnitude of expression of the CD28 and CD152 molecules on unstimulated and anti-CD3+rIL-2-stimulated peripheral blood CD4+ and CD8+ T cells in patients with chronic lymphocytic leukaemia (B-CLL) and controls. The mean percentages of both CD3+/CD4+/CD28+ and CD3+/CD8+/CD28+ cells were significantly lower in B-CLL than in controls before culture, decreased rapidly, reaching their lowest levels between 24 and 48 h, and returned to basal levels after 72 h of culture. In controls, the lowest proportions of CD3+/CD4+/CD28+ and CD3+/CD8+/CD28+ cells were found after 24 h and returned to prestimulation levels after 48 h of stimulation. We observed significantly higher proportions of unstimulated CD3+/CD4+/CD152+ and CD3+/CD8+/CD152+ cells in B-CLL patients than in controls. The highest percentages of CD3+/CD4+/CD152+ and CD3+/CD8+/CD152+ cells were observed in controls after 72 h, and in B-CLL patients after 24 h, and remained statistically higher after 48, 72 and 96 h of stimulation. CD152 molecule expression returned to prestimulation levels after 96 h of culture in controls, and after 120 h in B-CLL patients. The abnormal kinetics and levels of CD28 and CD152 expression on T cells in B-CLL may lead to a state of hyporesponsiveness or anergy and could be one of the mechanisms of immune deficiency in this disease.

Formation of a central supramolecular activation cluster is not required for activation of naive CD8+ T cells

Although both naive and effector T lymphocytes interact with antigen-expressing cells, the functional outcome of these interactions is distinct. Naive CD8(+) T cells are activated to proliferate and differentiate into effector cytolytic T lymphocytes (CTL), whereas CTL interact with specific targets, such as tumor cells, to induce apoptotic death. We recently observed that several molecules linked to actin cytoskeleton dynamics were up-regulated in effector vs. naive CD8(+) T cells, leading us to investigate whether T cell differentiation is accompanied by changes in actin-dependent processes. We observed that both naive and effector CD8(+) T cells underwent T cell receptor capping and formed stable conjugates with antigen-specific antigen-presenting cells. However, the characteristics of the immunological synapse were distinct. Whereas accumulation of signaling molecules at the T cell/antigen-presenting cell contact site was detectable in both naive and effector CD8(+) T cells, only effector cells developed a central supramolecular activation cluster as defined by punctate focusing of PKC theta, phospho-PKC theta, and phospho-ZAP70. Extended kinetics, CD28 costimulation, and high-affinity antigenic peptide did not promote PKC theta focusing in naive cells. Nonetheless, naive CD8(+) T cells polarized the microtubule organizing center, produced IL-2, proliferated, and differentiated into effector cells. Our results suggest that the formation of a central supramolecular activation cluster is not required for activation of naive CD8(+) T cells and support the notion that one role of an organized immune synapse is directed delivery of effector function.

HDAC6 deacetylase activity links the tubulin cytoskeleton with immune synapse organization

We investigated the role of acetylated microtubules in the antigen-specific interaction of T helper and antigen-presenting cells (APCs). In T cells, acetylated microtubules concentrated at contact site with APCs, surrounding clusters of CD3 and LFA-1. TcR engagement induced a transient deacetylation of microtubules at early times and an enhanced acetylation at late times. Confocal videomicroscopy studies revealed that the HDAC6 tubulin deacetylase was translocated and concentrated at the contact site of T cells with APCs. Overexpression of HDAC6 but not a dead deacetylase mutant in T cells disorganized CD3 and LFA-1 at the immune synapse. This effect was reverted by treatment with the deacetylase inhibitor trichostatin A. The antigen-specific translocation of the microtubule organizing center (MTOC) and IL-2 production were also severely impaired by overexpression of HDAC6. Our results underscore the key role for HDAC6 in the organization of the immune synapse and the antigen-specific reorientation of the MTOC.

Human immunodeficiency virus type 1 Nef activates p21-activated kinase via recruitment into lipid rafts

The Nef protein of human immunodeficiency virus type 1 is an important factor in AIDS pathogenesis. In addition to downregulating CD4 and major histocompatibility complex class I molecules from the cell surface, as well as increasing virion infectivity, Nef triggers activation of the T-cell receptor (TCR) cascade to facilitate virus spread. Signaling pathways that are induced by Nef have been identified; however, it is unclear how and in which subcellular compartment Nef triggers signaling. Nef recruits a multiprotein complex to activate the cellular Pak kinase that mediates downstream effector functions. Since a subpopulation of Nef is present in detergent-insoluble microdomains (lipid rafts) from where physiological TCR signaling is initiated, we tested whether lipid rafts are instrumental for Nef-mediated Pak activation. In flotation analysis, Nef-associated Pak activity exclusively fractionated with lipid rafts. Activation of Pak in the presence of Nef coincided with lipid raft recruitment of the kinase, which was otherwise excluded from detergent-insoluble microdomains. Experimental solubilization of lipid rafts interfered with the association of Pak activity with Nef. To analyze the importance of the raft localization for Nef function more rigorously, we generated a palmitoylated Nef (PalmNef). PalmNef was highly enriched in lipid rafts and associated with significantly higher levels of Pak activity than Nef. Notably, activation of Pak by its physiological activators, Cdc42 and Rac, also occurred in lipid rafts and required raft integrity. Together, these data suggest that Nef induces signal transduction via the recruitment of a signaling machinery including Pak into lipid rafts, thereby mimicking a physiological cellular mechanism to initiate the TCR cascade.

F-actin-dependent Translocation of the Rap1 GDP/GTP Exchange Factor RasGRP2

RasGRPs constitute a new group of diacylglycerol-dependent GDP/GTP exchange factors that activate Ras subfamily GTPases. Despite a common structure, Ras-GRPs diverge in their GTPase specificity, subcellular distribution, and downstream biological effects. The more divergent family member is RasGRP2, a Rap1-specific exchange factor with low affinity toward diacylglycerol. The regulation of RasGRP2 during signal transduction has remained elusive up to now. In this report, we show that the subcellular localization of Ras-GRP2 is highly dependent on actin dynamics. Thus, the induction of F-actin by cytoskeletal regulators such as Vav, Vav2, Dbl, and Rac1 leads to the shift of RasGRP2 from the cytosol to membrane ruffles and its co-localization with F-actin. Treatment of cells with cytoskeletal disrupting drugs abolishes this effect, leading to an abnormal localization of RasGRP2 in cytoplasmic clusters of actin. The use of Rac1 effector mutants indicates that the RasGRP2 translocation is linked exclusively to actin polymerization and is independent of other pathways such as p21-activated kinase JNK, or superoxide production. Biochemical experiments demonstrate that the translocation of RasGRP2 to membrane ruffles is mediated by the direct association of this protein with F-actin, a property contained within its 150 first amino acids. Finally, we show that the RasGRP2/F-actin interaction promotes the regionalized activation of Rap1 in juxtamembrane areas of the cell. These results reveal a novel function of the actin cytoskeleton in mediating the spatial activation of Ras subfamily GTPases through the selective recruitment of GDP/GTP exchange factors.

HIV type 1 Nef increases the association of T cell receptor (TCR)-signaling molecules with T cell rafts and promotes activation-induced raft fusion

HIV-1 Nef (Nef) is a myristoylated protein that contributes to HIV disease pathogenesis. Nef has a modulatory effect on T cell receptor (TCR) signaling, resulting in up-regulation of interleukin-2 (IL-2) production in stimulated T cells. Recent studies have shown that efficient TCR signaling requires enhanced association of TCR-signaling molecules with plasma membrane microdomains (lipid rafts) and fusion of rafts into larger structures. We utilized Jurkat T cell lines expressing wild-type Nef (Nef(wt)) and a myristoylation-deficient form of Nef (Nef(G)2(A)), from an inducible promoter, to determine the effects of Nef on the association of TCR-signaling molecules with rafts in nonstimulated T cells. In addition, the effect of Nef on raft size, before and after TCR activation by CD3 cross-linking, was also examined. Following induction, Nef(wt) was associated with both rafts and nonrafts, while Nef(G)2(A) was almost exclusively cytosolic. Induction of Nef(wt), but not Nef(G)2(A), coincided with an increased association of the src family tyrosine kinase, Lck, and TCRzeta with rafts, but not with nonrafts. Further, rafts were found to be significantly larger in CD3-activated T cells in the presence of Nef(wt) when compared to nonexpressing cells. We propose that myristoylated, raft-localized Nef primes resting T cells for activation by increasing the levels of signaling molecules within rafts, and that TCR activation is enhanced by the capacity of Nef to promote raft fusion.

Membranes as messengers in T cell adhesion signaling

Talin and RapL are components of molecular pathways that regulate the avidity of the integrin lymphocyte function-associated antigen 1 (LFA-1) for its ligand, intercellular adhesion molecule 1. In this review, we discuss recent advances in our understanding of LFA-1 affinity regulation and signaling and discuss a scenario for how Talin and Rap1 might act in synergy to achieve regulation of LFA-1 that is tailored to the specific functional requirements of different situations. Speedy delivery of signals may be crucial, and membrane trafficking from endosomes and the Golgi apparatus seem to be essential in delivering the messages from spatially segregated surface receptors.

Control of TCR-Mediated Activation of β1 Integrins by the ZAP-70 Tyrosine Kinase Interdomain B Region and the Linker for Activation of T Cells Adapter Protein

One of the earliest functional responses of T lymphocytes to extracellular signals that activate the Ag-specific CD3/TCR complex is a rapid, but reversible, increase in the functional activity of integrin adhesion receptors. Previous studies have implicated the tyrosine kinase zeta-associated protein of 70 kDa (ZAP-70) and the lipid kinase phosphatidylinositol 3-kinase, in the activation of beta(1) integrins by the CD3/TCR complex. In this report, we use human ZAP-70-deficient Jurkat T cells to demonstrate that the kinase activity of ZAP-70 is required for CD3/TCR-mediated increases in beta(1) integrin-mediated adhesion and activation of phosphatidylinositol 3-kinase. A tyrosine to phenylalanine substitution at position 315 in the interdomain B of ZAP-70 inhibits these responses, whereas a similar substitution at position 292 enhances these downstream signals. These mutations in the ZAP-70 interdomain B region also specifically affect CD3/TCR-mediated tyrosine phosphorylation of residues 171 and 191 in the cytoplasmic domain of the linker for activation of T cells (LAT) adapter protein. CD3/TCR signaling to beta(1) integrins is defective in LAT-deficient Jurkat T cells, and can be restored with expression of wild-type LAT. Mutant LAT constructs with tyrosine to phenylalanine substitutions at position 171 and/or position 191 do not restore CD3/TCR-mediated activation of beta(1) integrins in LAT-deficient T cells. Thus, these studies demonstrate that the interdomain B region of ZAP-70 regulates beta(1) integrin activation by the CD3/TCR via control of tyrosine phosphorylation of tyrosine residues 171 and 191 in the LAT cytoplasmic domain.

Treatment with anti-LFA-1 delays the CD8+ cytotoxic-T-lymphocyte response and viral clearance in mice with primary respiratory syncytial virus infection

Cytotoxic T lymphocytes (CTLs) play an important role in the immune response against respiratory syncytial virus (RSV) infection. The cell surface molecule lymphocyte function-associated antigen 1 (LFA-1) is an important contributor to CTL activation, CTL-mediated direct cell lysis, and lymphocyte migration. In an attempt to determine the role of LFA-1 during RSV infection, we treated BALB/c mice with monoclonal antibodies to LFA-1 at days -1, +1, and +4 relative to primary RSV infection. Anti-LFA-1 treatment during primary RSV infection led to reduced illness and delayed clearance of virus-infected cells. CTLs from RSV-infected mice that were treated with anti-LFA-1 exhibited diminished cytolytic activity and reduced gamma interferon production. In addition, studies with BrdU (5-bromo-2'-deoxyuridine)- and CFSE [5-(and 6)-carboxyfluorescein diacetate succinimidyl ester]-labeled lymphocytes showed that anti-LFA-1 treatment led to delayed proliferation during RSV infection. These results indicate that LFA-1 plays a critical role in the initiation of the immune response to RSV infection by facilitating CTL activation. These results may prove useful in the development of new therapies to combat RSV infection or other inflammatory diseases.

Impact of polyunsaturated fatty acids on cytoskeletal linkage of L-selectin

Polyunsaturated fatty acid [omega-3 polyunsaturated fatty acids (omega-3PUFAs)] incorporation into cell membranes has been shown to have potent anti-inflammatory activity, though the mechanisms involved are only partially characterized. Here, we show that PUFA enrichment of T cell membranes decreased the overall expression of L-selectin as well as a highly conserved epitope on L-selectin that may serve as a marker for optimal protein function. Additionally, PUFA enrichment inhibited L-selectin cytoskeletal association, which is thought to be important for optimal functional activity. In support of this, PUFA enrichment of gammadelta T cell membranes reduced L-selectin-dependent rolling interactions under conditions mimicking physiological flow. Taken together, these data suggest that the anti-inflammatory activity of omega-3 polyunsaturated fatty acids may be due, in part, to a novel effect on L-selectin, namely PUFA reduction or prevention of cytoskeletal association of L-selectin.

Leishmania-induced inhibition of macrophage antigen presentation analyzed at the single-cell level

A number of studies have previously examined the capacity of intracellular Leishmania parasites to modulate the capacity of macrophages to process and present Ags to MHC class II-restricted CD4(+) T cells. However, the bulk culture approaches used for assessing T cell activation make interpretation of some of these studies difficult. To gain a more precise understanding of the interaction between Leishmania-infected macrophages and effector T cells, we have analyzed various parameters of T cell activation in individual macrophage-T cell conjugates. Leishmania-infected macrophages efficiently stimulate Ag-independent as well as Ag-dependent, TCR-mediated capping of cortical F-actin in DO.11 T cells. However, infected macrophages are less efficient at promoting the sustained TCR signaling necessary for reorientation of the T cell microtubule organizing center and for IFN-gamma production. A reduced ability to activate these T cell responses was not due to altered levels of surface-expressed MHC class II-peptide complexes. This study represents the first direct single-cell analysis of the impact of intracellular infection on the interaction of macrophages with T cells and serves to emphasize the subtle influence Leishmania has on APC function.

Regulation of ZAP-70 activation and TCR signaling by two related proteins, Sts-1 and Sts-2

T cells play a central role in the recognition and elimination of foreign pathogens. Signals through the T cell receptor (TCR) control the extent and duration of the T cell response. To ensure that T cells are not inappropriately activated, signaling pathways downstream of the TCR are subject to multiple levels of positive and negative regulation. Herein, we describe two related proteins, Sts-1 and Sts-2, that negatively regulate TCR signaling. T cells from mice lacking Sts-1 and Sts-2 are hyperresponsive to TCR stimulation. The phenotype is accompanied by increased Zap-70 phosphorylation and activation, including its ubiquitinylated forms. Additionally, hyperactivation of signaling proteins downstream of the TCR, a marked increase in cytokine production by Sts1/2(-/-) T cells, and increased susceptibility to autoimmunity in a mouse model of multiple sclerosis is observed. Therefore, Sts-1 and Sts-2 are critical regulators of the signaling pathways that regulate T cell activation.

Gap junctions and connexin-mediated communication in the immune system

Gap junctions and connexins are present in the immune system. In haematopoiesis, connexin 43, the most widely distributed gap junction protein, appears to be a key player in the development of progenitor cells and their communication with stromal cells. Connexin 43 is expressed by macrophages, neutrophils and mast cells. Lymphocytes also express connexin 43, and inhibition of gap junction channels in these cells by using highly specific connexin mimetic reagents has profound effects on immunoglobulin secretion and synthesis of cytokines. Lymphocytes and leukocytes also communicate directly in vitro with endothelial cells via gap junctions. Connexins are implicated in inflammatory reactions in a range of tissues. Their involvement in atherosclerotic plaque formation in the vascular system is also a current growth point in research, and could lead to the development of therapeutic interventions.

Identification of a novel isoform of ZAP-70, truncated ZAP kinase

We identified a novel cDNA encoding truncated ZAP-70, which lacked the SH2 domain and a part of interdomain B, and named it truncated ZAP kinase (TZK). TZK was expressed in the thymus, spleen, and lymph nodes with ZAP-70. TZK was expressed in CD44+CD25- thymocytes up to mature T cells, but ZAP-70 was not expressed in CD44+CD25- or CD44+CD25+ thymocytes. ZAP-70 or TZK was transfected into P116 cells derived from a Jurkat T-cell line deficient in ZAP-70. The P116 cells with ZAP-70 induced the T-cell receptor-mediated signal transduction, but the cells expressing TZK did not. While ZAP-70 was accumulated at the immune synapse, TZK was not. Meanwhile, impaired phosphorylation of SLP-76, one of the substrates of ZAP-70, in P116 cells upon pervanadate stimulation was rescued in the cells expressing TZK. These findings show that TZK is a novel isoform of ZAP-70, which is expressed in pre-T-cell receptor-minus thymocytes and functions as a kinase not associated with T-cell receptor.

ERM proteins regulate cytoskeleton relaxation promoting T cell–APC conjugation

During activation, T cells associate with antigen-presenting cells, a dynamic process that involves the formation of a broad area of intimate membrane contact known as the immunological synapse. The molecular intermediates that link initial antigen recognition to the cytoskeletal changes involved in this phenomenon have not yet been defined. Here we demonstrate that ezrin-radixin-moesin proteins are rapidly inactivated after antigen recognition through a Vav1-Rac1 pathway. The resulting disanchoring of the cortical actin cytoskeleton from the plasma membrane decreased cellular rigidity, leading to more efficient T cell-antigen-presenting cell conjugate formation. These findings identify an antigen-dependent molecular pathway that favors immunological synapse formation and the subsequent development of an effective immune response.

Complex and dynamic redistribution of NF-kappaB signaling intermediates in response to T cell receptor stimulation

The central zone of the supramolecular activation cluster (c-SMAC) is a zone of T cell receptor (TCR) enrichment that forms at a T cell/antigen-presenting cell (APC) junction in response to antigen stimulation. We demonstrate that there is a surprisingly complex relocalization process that brings PKC and Bcl10, two intermediates in TCR activation of NF-kappaB, to the cytoplasmic face of the c-SMAC. TCR activation causes enrichment of PKC at the c-SMAC, followed by Bcl10 relocalization to punctate cytoplasmic structures, often at sites distant from the c-SMAC. These Bcl10 structures then undergo further relocalization, becoming enriched at the c-SMAC. TCR activation of NF-kappaB therefore involves the dynamic relocalization of multiple signaling intermediates, with distinct phases proximal to and distant from the c-SMAC.

NF-kappaB activation pathways induced by T cell costimulation

Analysis of knockout mice and of T cells deficient for individual signaling proteins allowed the identification of novel members of the costimulation-induced NF-kappaB activation pathway while biochemical approaches started to unveil their functional mechanisms. These results show that NF-kappaB activation depends on an early wave of tyrosine phosphorylation that allows the inducible formation of multiprotein complexes containing several proteins required for NF-kappaB activation: adaptor proteins including Src homology 2 domain-containing leukocyte phosphoprotein 76 (SLP-76) and proteins with enzymatic activity, such as phospholipase C (PLC) gamma and the exchange factor Vav1. While Vav1 contributes to Rac-dependent reorganization of the actin cytoskeleton, activated PLCgamma1 generates the protein kinase C (PKC) activator diacylglycerol. In T cells, the novel PKC isoform PKCtheta is indispensable for NF-kappaB activation and its enzymatic activity depends on recruitment to the immunological synapse. Downstream from PKCtheta, the caspase recruitment domain (CARD) proteins CARD11/CARMA1 and Bcl10 relay T cell receptor-derived signals to the IkappaB kinase (IKK) complex. Many members of the NF-kappaB activation cascade, including the IKKs, are either constitutively or inducibly translocated to the lipid raft fraction, showing a highly organized spatial distribution of these NF-kappaB activating proteins.

The soluble form of CD83 dramatically changes the cytoskeleton of dendritic cells

CD83 is the best-known surface marker for mature dendritic cells (DC) and recently we could show that a soluble form of CD83 inhibits DC maturation. In addition, this soluble form inhibits DC-mediated T cell proliferation in vitro and in vivo. Furthermore, several viruses induce CD83 degradation or shedding in infected DC. A soluble form of CD83 was also found in plasma and serum of healthy individuals and interestingly at highly elevated levels in a number of haematological malignancies. Thus, CD83 also has functional implications for the immune response. However, the molecular mechanism is not well defined. Here we describe for the first time that soluble CD83 completely changed the cytoskeleton (analysed using phalloidin-, tubulin- and fascin-specific antibodies) when administered at a concentration of 10 microg/ml to mature DC. The cells rounded off and had only short, truncated, or no veils at all. Furthermore, soluble CD83-treated cells were completely inhibited in their ability to form clusters with T cells, an absolute prerequisite in order to stimulate T cells.

Dendritic Cell Maturation Controls Adhesion, Synapse Formation, and the Duration of the Interactions with Naive T Lymphocytes

The initiation of adaptive immune responses requires the direct interaction of dendritic cells (DCs) with naive T lymphocytes. It is well established that the maturation state of DCs has a critical impact on the outcome of the response. We show here that mature DCs form stable conjugates with naive T cells and induce the formation of organized immune synapses. Immature DCs, in contrast, form few stable conjugates with no organized immune synapses. A dynamic analysis revealed that mature DCs can form long-lasting interactions with naive T cells, even in the absence of Ag. Immature DCs, in contrast, established only short intermittent contacts, suggesting that the premature termination of the interaction prevents the formation of organized immune synapses and full T cell activation.

Surface T-cell antigen receptor expression and availability for long-term antigenic signaling

It is important to understand how T-cell antigen receptor (TCR) engagement and signaling are regulated throughout an immune response. This review examines the dynamics of surface TCR expression and signaling capacity during thymic and effector T-cell development. Although the TCR can undergo vast changes in surface expression, T cells remain capable of sustaining TCR engagement for long periods of time. This may be achieved by a combination of mechanisms that involve (a) controlling the quantity of surface TCR available for ligand interaction and (b) controlling the quality of surface TCR expression during T-cell activation. TCR signaling itself appears to be one of the main quantitative modulators of surface TCR expression, and it can cause both downregulation and upregulation at different times of T-cell activation. Recent studies indicate that the degree of upregulation is tunable by the strength of antigenic stimulation. There is evidence that qualitatively distinct forms of the TCR exist, and their potential role in sustained antigenic signaling is also discussed. A goal of future studies will be to better characterize these modulations in surface TCR expression and to clarify their impact on the regulation of immune responses.

Uncoupling between Immunological Synapse Formation and Functional Outcome in Human γδ T Lymphocytes

Human T lymphocytes expressing the Vgamma9Vdelta2 TCR recognize non-peptidic Ags, referred to as phosphoantigens, produced by microbial pathogens and by human tumor cells. Here we show that gammadelta T cells establish a mature immunological synapse (IS) with the myelomonocytic THP-1 tumoral cell line. This synapse is characterized by an enrichment for phosphotyrosine, CD2, and gammadelta TCR together with the exclusion of CD45. The CD94 and NKG2D receptors are also recruited to the signaling area, while the C-lectin-like activation marker CD69 segregates out of the synapse. gammadelta T cell conjugation to THP-1 increases upon stimulation by soluble phosphoantigen, is paralleled by the metabolic activation of gammadelta T cells and leads to cytokine production. Molecular segregation of the above molecules also occurs at the gammadelta T cell/THP-1 interface in the absence of exogenously added phosphoantigen, although it does not result in intracellular signaling and cytokine production under these conditions. Hence the molecular interactions at the gammadelta T cell-THP-1 target cell interface are sufficient to induce the formation of an IS, but cytokine production requires the full engagement of gammadelta TCR by a strong agonist. Thus in gammadelta T cells, formation of the IS is uncoupled from its functional outcome.

Mouse EphrinB3 Augments T-cell Signaling and Responses to T-cell Receptor Ligation

Ephrins (EFN) are cell-surface ligands of Ephs, the largest family of cell-surface receptor tyrosine kinases. The function of EFNs in the immune system has not been well studied, although some EFNs and Ephs are expressed at high levels on certain leukocytes. We report here that EFNB3 and its receptors (collectively called EFNB3Rs, as EFNB3 binds to multiple EphBs) were expressed in peripheral T cells and monocytes/macrophages, with T cells being the dominant EFNB3+ and EFNB3R+ cell type. Solid-phase EFNB3-Fc in the presence of suboptimal anti-CD3 crosslinking enhanced T-cell responses in terms of proliferation, activation marker expression, interferon-gamma but not interleukin-2 production, and cytotoxic T-cell activity. EFNB3R costimulation in the presence of phorbol 12-myristate 13- acetate was insensitive to cyclosporin A, similar to CD28 costimulation, suggesting they might share a part of the signaling pathway. After crosslinking, T-cell receptor and EFNB3R congregated into aggregated rafts, and this provided a morphological basis for signaling pathways of T-cell receptor and EFNB3R to interact. Solid-phase EFNB3-Fc augmented p38 and p44/42 MAPK activation further downstream of the signaling pathway. These data suggest that EFNB3 is important in T-cell/T-cell and T-cell/antigen-presenting cell collaboration to enhance T-cell activation and function.

T cell receptor engagement leads to the recruitment of IBP, a novel guanine nucleotide exchange factor, to the immunological synapse

Reorganization of the actin cytoskeleton is crucial to the formation and function of the immunological synapse. Rho GTPases are critical mediators of cytoskeletal reorganization, and their activity at the synapse is likely to be stringently regulated. Guanine nucleotide exchange factors (GEFs) belonging to the Dbl family of proteins represent one major class of proteins that regulate the activity of Rho GTPases. Here we demonstrate that IBP, a homologue of SWAP-70, is a novel GEF for Rac1 and Cdc42 in T lymphocytes, which is recruited to the immunological synapse upon engagement of the antigen receptor. Mutational analysis supports a model whereby IBP is inactive in unstimulated cells. Upon engagement of the T cell receptor, its GEF activity is enhanced by tyrosine phosphorylation, as well as by binding newly generated phosphatidylinositol 3,4,5-trisphosphate. Although it is known that T cell receptor engagement leads to the recruitment of Vav to the immunological synapse, these findings indicate that other GEFs, such as IBP, also relocalize to this intercellular region. The recruitment and activation of distinct classes of GEFs may allow for precise control of Rho GTPase function at the crucial interface between T cells and antigen presenting cells.

Lymphocyte function-associated antigen-1-mediated T cell adhesion is impaired by low molecular weight phosphotyrosine phosphatase-dependent inhibition of FAK activity

Protein tyrosine phosphorylation is one of the earliest signaling events detected in response to lymphocyte function-associated antigen-1 (LFA-1) engagement during lymphocyte adhesion. In particular, the focal adhesion kinase p125FAK, involved in the modulation and rearrangement of the actin cytoskeleton, seems to be a crucial mediator of LFA-1 signaling. Herein, we investigate the role of a FAK tyrosine phosphatase, namely low molecular weight phosphotyrosine phosphatase (LMW-PTP), in the modulation of LFA-1-mediated T cell adhesion. Overexpression of LMW-PTP in Jurkat cells revealed an impairment of LFA-1-dependent cell-cell adhesion upon T cell receptor (TCR) stimulation. Moreover, in these conditions LMW-PTP causes FAK dephosphorylation, thus preventing the activation of FAK downstream pathways. Our results also demonstrated that, upon antigen stimulation, LMW-PTP-dependent FAK inhibition is associated to a strong reduction of LFA-1 and TCR co-clustering toward a single region of T cell surface, thus causing an impairment of receptor activity by preventing changes in their avidity state. Because co-localization of both LFA-1 and TCR is an essential event during encounters of T cells with antigen-presenting cells and immunological synapse (IS) formation, we suggest an intriguing role of LMW-PTP in IS establishment and stabilization through the negative control of FAK activity and, in turn, of cell surface receptor redistribution.

Antigen-driven T cell anergy and defective memory T cell response via deregulated Rap1 activation in SPA-1-deficient mice

SPA-1 is a principal Rap1 GTPase-activating protein in the hematopoietic progenitors and peripheral T cells, and SPA-1-deficient mice develop a spectrum of myeloproliferative stem cell disorders of late onset. In the present study, we show that SPA-1-deficient mice develop age-dependent T cell unresponsiveness preceding the myeloid disorders, whereas the T cell numbers remained unchanged. Progression of the T cell dysfunction was attributed to the age-dependent increase in CD44high T cell population that was unresponsive to T cell receptor stimulation. Younger SPA-1-deficient mice exhibited selectively impaired recall T cell responses against a T-dependent antigen with normal primary antibody response. These results suggested that the unresponsiveness of CD44high T cells was antigen-driven in vivo. T cells from younger SPA-1-/- mice showed much greater and more persisted Rap1 activation by anti-CD3 stimulation than control T cells. Furthermore, freshly isolated T cells from SPA-1-/- mice exhibited progressive accumulation of Rap1GTP as mice aged. T cells from aged SPA-1-/- mice with high amounts of Rap1GTP showed normal or even enhanced Ras activation with little extracellular signal-regulated kinase activation in response to anti-CD3 stimulation, indicating that excess Rap1GTP induced the uncoupling of Ras-mediated extracellular signal-regulated kinase activation. These results suggested that antigenic activation of naïve T cells in SPA-1-/- mice was followed by anergic rather than memory state due to the defective down-regulation of Rap1 activation, resulting in the age-dependent progression of overall T cell immunodeficiency.

SLP-76 coordinates Nck-dependent Wiskott-Aldrich syndrome protein recruitment with Vav-1/Cdc42-dependent Wiskott-Aldrich syndrome protein activation at the T cell-APC contact site

We have shown previously that Wiskott-Aldrich syndrome protein (WASP) activation at the site of T cell-APC interaction is a two-step process, with recruitment dependent on the proline-rich domain and activation dependent on binding of Cdc42-GTP to the GTPase binding domain. Here, we show that WASP recruitment occurs through binding to the C-terminal Src homology 3 domain of Nck. In contrast, WASP activation requires Vav-1. In Vav-1-deficient T cells, WASP recruitment proceeds normally, but localized activation of Cdc42 and WASP is disrupted. The recruitment and activation of WASP are coordinated by tyrosine-phosphorylated Src homology 2 domain-containing leukocyte protein of 76 kDa, which functions as a scaffold, bringing Nck and WASP into proximity with Vav-1 and Cdc42-GTP. Taken together, these findings reconstruct the signaling pathway leading from TCR ligation to localized WASP activation.

The beta1 and beta3 integrins promote T cell receptor-mediated cytotoxic T lymphocyte activation

Recognition by CD8+ cytotoxic T lymphocytes (CTLs) of antigenic peptides bound to major histocompatibility class (MHC) I molecules on target cells leads to sustained calcium mobilization and CTL degranulation resulting in perforin-dependent killing. We report that beta1 and beta3 integrin-mediated adhesion to extracellular matrix proteins on target cells and/or surfaces dramatically promotes CTL degranulation. CTLs, when adhered to fibronectin but not CTL in suspension, efficiently degranulate upon exposure to soluble MHC.peptide complexes, even monomeric ones. This adhesion induces recruitment and activation of the focal adhesion kinase Pyk2, the cytoskeleton linker paxillin, and the Src kinases Lck and Fyn in the contact site. The T cell receptor, by association with Pyk2, becomes part of this adhesion-induced activation cluster, which greatly increases its signaling.

Restricted clonal expression of IL-2 by naive T cells reflects differential dynamic interactions with dendritic cells

Limited frequencies of T cells express IL-2 in primary antigenic responses, despite activation marker expression and proliferation by most clonal members. To define the basis for restricted IL-2 expression, a videomicroscopic system and IL-2 reporter transgenic model were used to characterize dendritic cell (DC)-T cell interactions. T cells destined to produce IL-2 required prolonged interactions with DCs, whereas most T cells established only transient interactions with DCs and were activated, but did not express IL-2. Extended conjugation of T cells with DCs was not always sufficient to initiate IL-2 expression. Thus, there is intrinsic variability in clonal T cell populations that restricts IL-2 commitment, and prolonged engagement with mature DCs is necessary, but not sufficient, for IL-2 gene transcription.

Ephrin B2 induces T cell costimulation

Eph kinases form the largest family of receptor tyrosine kinases, and their ligands are ephrins (EFNs), which are cell surface proteins. Some Eph kinases and EFNs are expressed on T cells, B cells, and dendritic cells, but their functions in the immune system are largely unknown. In this study, we investigated the effect of EFNB2 on murine T cells. EFNB2 mRNA was expressed in the cortex of the thymus and white pulp of the spleen. At the protein level, it was expressed on T cells and monocytes/macrophages, but not on B cells. EFNB2Rs were expressed mainly on T cells. Solid-phase EFNB2 along with suboptimal anti-CD3 strongly stimulated T cell proliferation, with concomitant augmentation of IFN-gamma but not IL-2 or IL-4 secretion. The activity of cytotoxic T cells was also significantly enhanced in the presence of solid-phase EFNB2. These results indicate that EFNB2R cross-linking results in costimulation of T cells. EFNB2Rs were normally scattered on the T cell surface; after TCR cross-linking, they rapidly congregated to capped TCR complexes and then to patched rafts. This provides a morphological base for EFNB2Rs to participate in T cell costimulation. We also demonstrated that EFNB2R signaling led to augmented p38 and p44/42 mitogen-activated protein kinase activation. Our study shows that EFNB2 plays important roles in immune regulation.

Thromboxane A2 modulates interaction of dendritic cells and T cells and regulates acquired immunity

Physical interaction of T cells and dendritic cells (DCs) is essential for T cell proliferation and differentiation, but it has been unclear how this interaction is regulated physiologically. Here we show that DCs produce thromboxane A2 (TXA2), whereas naive T cells express the thromboxane receptor (TP). In vitro, a TP agonist enhances random cell movement (chemokinesis) of naive but not memory T cells, impairs DC-T cell adhesion, and inhibits DC-dependent proliferation of T cells. In vivo, immune responses to foreign antigens are enhanced in TP-deficient mice, which also develop marked lymphadenopathy with age. Similar immune responses were seen in wild-type mice treated with a TP antagonist during the sensitization period. Thus, TXA2-TP signaling modulates acquired immunity by negatively regulating DC-T cell interactions.

DOCK2 is essential for antigen-induced translocation of TCR and lipid rafts, but not PKC-theta and LFA-1, in T cells

DOCK2 is a mammalian homolog of Caenorhabditis elegans CED-5 and Drosophila melanogaster Myoblast City which are known to regulate actin cytoskeleton. DOCK2 is critical for lymphocyte migration, yet the role of DOCK2 in TCR signaling remains unclear. We show here that DOCK2 is essential for TCR-mediated Rac activation and immunological synapse formation. In DOCK2-deficient T cells, antigen-induced translocation of TCR and lipid rafts, but not PKC-theta and LFA-1, to the APC interface was severely impaired, resulting in a significant reduction of antigen-specific T cell proliferation. In addition, we found that the efficacy of both positive and negative selection was reduced in DOCK2-deficient mice. These results suggest that DOCK2 regulates T cell responsiveness through remodeling of actin cytoskeleton via Rac activation.

CD44v7 interferes with activation-induced cell death by up-regulation of anti-apoptotic gene expression

Blockade of CD44v7 was described to cure trinitrobenzene sulfonic acid-induced colitis, a disease not developed by mice with targeted deletion of the CD44v7 exon. There was evidence for a reduction in activation-induced cell death on lamina propria lymphocytes of control as compared with CD44v7-deficient mice. To elucidate the mechanism underlying the relative apoptosis resistance of CD44v7-competent as compared with CD44v7-deficient lymphocytes, T cell activation and induction of apoptosis were analyzed on mesenteric lymph node cells and Peyer's patch lymphocytes of CD44v7-deficient and CD44v4-v7-transgenic mice, which overexpress rat CD44v4-v7 on T lymphocytes. CD44v7 deficiency was characterized by an increase in the percentage of apoptotic cells after stimulation, increased numbers of CD95L- and CD152-positive cells, low levels of the anti-apoptotic proteins Bcl-2 and Bcl-Xl, and decreased phosphorylation of the pro-apoptotic protein BAD. Also, lymphocytes from CD44v4-v7-transgenic mice displayed reduced levels of CD95L, low numbers of apoptotic cells, and constitutively elevated levels of Bcl-Xl. When stimulating lymphocytes by CD3 cross-linking, CD44v7 was not recruited toward the immunological synapse and preferentially associated with the cytoskeletal-linker protein ezrin. Thus, as opposed to the CD44 standard isoform, CD44v7 does not function as an accessory molecule; instead, it supports survival of activated T cells by interfering with activation-induced cell death.

Spontaneous clustering and tyrosine phosphorylation of NK cell inhibitory receptor induced by ligand binding

Inhibition of NK cell cytotoxicity by killer cell Ig-like receptors (KIR) depends on phosphorylation of cytoplasmic tyrosines in KIR, which recruit tyrosine phosphatase Src homology protein tyrosine phosphatase 1. It is not clear how KIR, whose function lies downstream of a tyrosine kinase, succeeds in blocking proximal NK cell activation signals upon binding HLA class I on target cells. Here we show that mixing NK cells with insect cells expressing HLA-C was sufficient to induce clustering of KIR, and phosphorylation of KIR and SHP-1. Transient phosphorylation of KIR was detected in the presence of pervanadate, an inhibitor of protein tyrosine phosphatases, at suboptimal concentration. Phosphorylation of KIR was specifically induced by ligand binding because it was detected only when HLA-C was loaded with a peptide that permits KIR binding. KIR phosphorylation was not dependent on ICAM-1-mediated adhesion and was not blocked by inhibition of actin polymerization, but required Zn(2+). Fluorescence resonance energy transfer between HLA-C molecules revealed close molecular interactions induced by KIR binding. These results demonstrate tight clustering of KIR and rapid KIR phosphorylation induced simply by binding to HLA-C. The unique property of KIR to become phosphorylated in the absence of adhesion and of actin cytoskeleton rearrangement explains how KIR can efficiently block early activation signals during NK-target cell contacts.

Analysis of the mobility of signaling molecules in lymphocytes using fluorescence photobleaching techniques

The earliest biochemical events at the plasma membrane that lead to gene activation appear to depend not only on the local concentration of signaling molecules, but also on the mobility of these molecules at the site of signaling. To elucidate the process of signal transduction after receptor engagement in the immune system, it is important to analyze the mobility of signaling molecules in living lymphocytes. Current knowledge of the changes in intracellular localization and dynamic movements of signaling molecules during lymphocyte activation is limited. Here, we describe a method for known as fluorescence recovery after photobleaching, used to measure the diffusion mobility of a signaling molecule in a T cell line after T cell receptor stimulation. This method is a useful tool in studies of spatiotemporal regulation in immunoreceptor signaling.

What it takes to become an effector T cell: the process, the cells involved, and the mechanisms

When activated, CD4(+) T cells differentiate into two major sub-populations differing in their profiles of secreted cytokines. Type One, or TH1, cells secrete IL-2, IFNgamma, and TNFbeta and mediate a cellular immune response. Type Two, or TH2, cells secrete IL-4, IL-5, IL-6, IL-10, and IL-13 and potentiate a humoral response. The nature of any specific immune response depends on the interaction of antigen-presenting cells and T cells. The role of antigen-presenting cells is to respond to the nature of the immune challenge and signal differentiation of CD4(+) T cells. A number of factors are involved in the effector phenotype of T cells-nature and affinity of antigen, co-receptors signals, and cytokine environment. T-cell differentiation is a complex process comprising four defined developmental stages: activation of particular cytokine genes, commitment of the cells, silencing of the opposing cytokine genes, and stabilization of the phenotype. In each of these stages, the cells respond to the products of many signaling cascades from many membrane-bound receptors. The stages in development are mediated by different molecular mechanisms, involving control of gene expression and chromatin remodeling. This review centers on the factors, cellular interactions, and molecular mechanisms involved in the maturation of naïve CD4(+) T lymphocytes into fully effector cells.

Cytoskeletal remodeling in lymphocyte activation

The formerly distinct fields of lymphocyte signal transduction and cytoskeletal remodeling have recently become linked, as proteins involved in transducing signals downstream of lymphocyte antigen receptors have also been implicated in actin cytoskeleton remodeling, microtubule dynamics and regulation of cell polarity. These discoveries have fuelled interest in understanding both the role of the actin cytoskeleton as an integral component of lymphocyte activation and the interplay between lymphoid cell-cell contact sites (immunological synapse), retractile pole structures (uropod, distal pole complex), and Rho-family GTPases (Rac, Rho, Cdc42), their upstream activators (Dbl-family guanine nucleotide exchange factors) and their downstream effectors (WASp, Arp2/3, ADAP). To understand how these complex regulatory networks are wired, a new breed of computational biologists uses mathematical language to reproduce and simulate signaling circuits 'in silico'.

The MAGUK family protein CARD11 is essential for lymphocyte activation

Members of the MAGUK family proteins cluster receptors and intracellular signaling molecules at the neuronal synapse. We report that genetic inactivation of the MAGUK family protein CARD11/Carma1/Bimp3 results in a complete block in T and B cell immunity. CARD11 is essential for antigen receptor- and PKC-mediated proliferation and cytokine production in T and B cells due to a selective defect in JNK and NFkappaB activation. Moreover, B cell proliferation and JNK activation were impaired upon stimulation of TLR4 with lipopolysaccharide, indicating that CARD11 is involved in both the innate and adaptive immune systems. Our results show that the same family of molecules are critical regulators of neuronal synapses and immune receptor signaling.