Helga Schneider

The CD28-Related Molecule ICOS Is Required for Effective T Cell-Dependent Immune Responses

While CD28 is critical for expansion of naive T cells, recent evidence suggests that the activation of effector T cells is largely independent of CD28/B7. We suggest that ICOS, the third member of the CD28/CTLA-4 family, plays an important role in production of IL-2, IL-4, IL-5, and IFNgamma from recently activated T cells and contributes to T cell-dependent B help in vivo. Inhibition of ICOS attenuates lung mucosal inflammation induced by Th2 but not Th1 effector populations. Our data indicate a critical function for the third member of the CD28 family in T cell-dependent immune responses.

CD28 and CTLA-4 coreceptor expression and signal transduction

Summary:  T‐cell activation is mediated by antigen‐specific signals from the TCRζ/CD3 and CD4–CD8–p56lck complexes in combination with additional co‐signals provided by coreceptors such as CD28, inducible costimulator (ICOS), cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4), programmed death (PD‐1), and others. CD28 and ICOS provide positive signals that promote and sustain T‐cell responses, while CTLA‐4 and PD‐1 limit responses. The balance between stimulatory and inhibitory co‐signals determines the ultimate nature of T‐cell responses where response to foreign pathogen is achieved without excess inflammation and autoimmunity. In this review, we outline the current knowledge of the CD28 and CTLA‐4 signaling mechanisms [involving phosphatidylinositol 3 kinase (PI3K), growth factor receptor‐bound protein 2 (Grb2), Filamin A, protein kinase C θ (PKCθ), and phosphatases] that control T‐cell immunity. We also present recent findings on T‐cell receptor‐interacting molecule (TRIM) regulation of CTLA‐4 surface expression, and a signaling pathway involving CTLA‐4 activation of PI3K and protein kinase B (PKB)/AKT by which cell survival is ensured under conditions of anergy induction.

Reversal of the TCR stop signal by CTLA-4.

The coreceptor cytotoxic T lymphocyte–associated antigen 4 (CTLA-4) is pivotal in regulating the threshold of signals during T cell activation, although the underlying mechanism is still not fully understood. Using in vitro migration assays and in vivo two-photon laser scanning microscopy, we showed that CTLA-4 increases T cell motility and overrides the T cell receptor (TCR)–induced stop signal required for stable conjugate formation between T cells and antigen-presenting cells. This event led to reduced contact periods between T cells and antigen-presenting cells that in turn decreased cytokine production and proliferation. These results suggest a fundamentally different model of reverse stop signaling, by which CTLA-4 modulates the threshold for T cell activation and protects against autoimmunity.

Stratospheric Mean Ages and Transport Rates from Observations of Carbon Dioxide and Nitrous Oxide

Measurements of stratospheric carbon dioxide (CO2) and nitrous oxide (N2O) concentrations were analyzed to investigate stratospheric transport rates. Temporal variations in tropospheric CO2 were observed to propagate into the stratosphere, showing that tropospheric air enters the lower tropical stratosphere continuously, ascends, and is transported rapidly (in less than 1 month) to both hemispheres. The mean age A of stratospheric air determined from CO2 data is approximately 5 years in the mid-stratosphere. The mean age is mathematically equivalent to a conserved tracer analogous to exhaust from stratospheric aircraft. Comparison of values for A from models and observations indicates that current model simulations likely underestimate pollutant concentrations from proposed stratospheric aircraft by 25 to 100 percent.

Selective CD28pYMNM mutations implicate phosphatidylinositol 3-kinase in CD86-CD28-mediated costimulation

CD28 costimulatory signals are required for lymphokine production and T cell proliferation. CD28 signaling recruits the intracellular proteins PI 3-kinase, ITK, and GRB-2/SOS. PI 3-kinase and GRB-2/SOS bind the CD28 cytoplasmic pYMNM motif via SH2 domains. We generated CD28 pYMNM mutants and found that Y191 mutation (Y191CD28F) disrupted both PI 3-kinase and GRB-2 binding, while M194 mutation (M194CD28C) disrupted only PI 3-kinase binding. Both mutants still bound ITK. We have assessed the ability of these selective mutants to support IL-2 production upon TCR zeta/CD3 ligation in the presence of CHO-CD86 (B7-2) cells. Both Y191CD28F and M194CD28C mutants failed to generate IL-2. These data directly implicate PI 3-kinase in CD28-mediated costimulation leading to IL-2 secretion. Wortmannin, an inhibitor of PI 3-kinase, induced cell apoptosis and as such was unsuitable for use in this study.

SKAP-55 regulates integrin adhesion and formation of T cell-APC conjugates

Src kinase–associated phosphoprotein of 55 kDa (SKAP-55; encoded by SCAP1) is a T cell adaptor protein of unknown function that contains a pleckstrin homology and an SH3 domain. Here we show that SKAP-55 regulates integrin-mediated adhesion and conjugate formation between T cells and antigen-presenting cells (APCs). SKAP-55 enhances adhesion to fibronectin and intercellular adhesion molecule-1 (ICAM-1), colocalizes with actin at the T cell–APC synapse and promotes the clustering of lymphocyte-associated antigen-1 (LFA-1). Enhanced conjugation is comparable to that induced by adhesion and degranulation–promoting adaptor protein (ADAP), a binding partner of SKAP-55, and is abrogated by deletion of the SKAP-55 SH3 domain. Conjugate formation is accompanied by the translocation of SKAP-55 to membrane rafts, an event that is regulated by both LFA-1 and T cell receptor ligation. Our findings identify a mechanism by which SKAP-55 modulates T cell responses to antigen.

CTLA-4 disrupts ZAP70 microcluster formation with reduced T cell/APC dwell times and calcium mobilization

CTLA‐4 is a co‐receptor that modulates the threshold of T cell activation and autoimmunity. We previously showed that CTLA‐4 reverses the TCR‐mediated stop signal needed for T cell/APC interactions [Schneider et al., Science 2006, 313: 1972]. In this study, using a different T cell system, we show that CTLA‐4 expression changed the behavior of T8.1 T cells by reducing the contact time between T cell and APC, preventing re‐inforced contacts, and reducing the contact area at the immunological synapse. This led to a major reduction in Ca2+ influx/mobilization and interleukin‐2 production. Further, anti‐CD3/CTLA‐4 increased T cell motility on antibody‐coated glass slides, concurrent with an abrogation of ZAP70 microcluster formation. Our findings further support a role for CTLA‐4 in limiting the interaction between T cell and APC that is needed for optimal activation.

CTLA-4 trafficking and surface expression

The T-cell co-receptor cytotoxic T-cell antigen 4 (CTLA-4) has a strong inhibitory role as shown by the lymphoproliferative phenotype of CTLA-4-deficient mice. Despite its potent effects on T-cell function, CTLA-4 is primarily an intracellular antigen whose surface expression is tightly regulated by restricted trafficking to the cell surface and rapid internalisation. Recently, several signalling molecules such as Trim, PLD, ARF-1 and TIRC7 have been described to be involved in the transport of CTLA-4 to the cell surface. Minor changes in surface expression levels have major effects on the outcome of T-cell activation. Optimal regulation of CTLA-4 surface expression is crucial for the balance of stimulatory and inhibitory signals to maximize protective immune responses while maintaining immunological tolerance and preventing autoimmunity.

Resting lymphocyte kinase (Rlk/Txk) targets lymphoid adaptor SLP-76 in the cooperative activation of interleukin-2 transcription in T-cells.

Rlk/Txk is a T-cell-specific member of the Btk/Tec family of tyrosine kinases, whereas SLP-76 is a lymphoid adaptor that is essential for pre-TcR and mature TcR signaling. Although Rlk deficient T-cells show partial defects in T-cell proliferation, Rlk can complement ITK−/− cells with multiple defects in TcR initiated early events and interleukin (IL)-2 production. A key question is the nature of the target of Rlk responsible for bridging the TcR with the activation of IL-2 transcription. In this study, we identify a pathway in which Rlk phosphorylates SLP-76 leading to the phosphorylation of PLCγ1, activation of ERKs, and the synergistic up-regulation of TcR-driven IL-2 NFAT/AP-1 transcription. Rlk phosphorylated the N-terminal region of SLP-76, a region that has been previously shown to serve as a target for ZAP-70. Loss of N-terminal YESP/YEPP sites of SLP-76 or the Rlk kinase activity attenuated cooperativity between Rlk and SLP-76. These observations support a model where the TcR can utilize Rlk (as well as ZAP-70) in the phosphorylation of key sites in SLP-76 leading to the up-regulation of Th1 preferred cytokine IL-2.

Lymphocyte signaling: Cbl sets the threshold for autoimmunity

Cbl, a negative regulator of immune signaling, has recently been shown to act as a ubiquitin-protein ligase. Further, two new papers describing Cbl-b-deficient mice suggest that Cbl-b sets the threshold of signaling in T and B cells and prevents the development of autoimmunity.