Antigen-induced translocation of PKC-theta to membrane rafts is required for T cell activation

Protein kinase C-theta (PKC-theta) is essential for mature T cell activation; however, the mechanism by which it is recruited to the TCR signaling machinery is unknown. Here we show that T cell stimulation by antibodies or peptide-major histocompatibility complex (MHC) induces translocation of PKC-theta to membrane lipid rafts, which localize to the immunological synapse. Raft translocation was mediated by the PKC-theta regulatory domain and required Lck but not ZAP-70. In addition, PKC-theta was associated with Lck in the rafts. An isolated PKC-straight theta catalytic fragment did not partition into rafts or activate the transcription factor NF-kappa B, although addition of a Lck-derived raft-localization sequence restored these functions. Thus, physiological T cell activation translocates PKC-theta to rafts, which localize to the T cell synapse; this PKC-theta translocation is important for its function.

NF- B activation induced by T cell receptor/CD28 costimulation is mediated by protein kinase C-

Protein kinase C-theta (PKCtheta) is a Ca(2+)-independent member of the PKC family that is selectively expressed in skeletal muscle and T lymphocytes and plays an important role in T cell activation. However, the molecular basis for the important functions of PKCtheta in T cells and the manner in which it becomes coupled to the T cell receptor-signaling machinery are unknown. We addressed the functional relationship between PKCtheta and CD28 costimulation, which plays an essential role in T cell receptor-mediated IL-2 production. Here, we provide evidence that PKCtheta is functionally coupled to CD28 costimulation by virtue of its selective ability to activate the CD28RE/activator protein-1 (AP-1) element in the IL-2 gene promoter. First, CD28 costimulation enhanced the membrane translocation and catalytic activation of PKCtheta. Second, among several PKC isoforms, PKCtheta was the only one capable of activating NF-kappaB or CD28RE/AP-1 reporters in T cells (but not in 293T cells). Third, wild-type PKCtheta synergized with CD28/CD3 signals to activate CD28RE/AP-1. In addition, PKCtheta selectively synergized with Tat to activate a CD28RE/AP-1 reporter. Fourth, CD3/CD28-induced CD28RE/AP-1 activation and NF-kappaB nuclear translocation were blocked by a selective PKCtheta inhibitor. Last, PKCtheta-mediated activation of the same reporter was inhibited by the proteasome inhibitor MG132 (which blocks IkappaB degradation) and was found to involve IkappaB-kinase beta. These findings identify a unique PKCtheta-mediated pathway for the costimulatory action of CD28, which involves activation of the IkappaB-kinase beta/IkappaB/NF-kappaB-signaling cascade.

A novel functional interaction between Vav and PKCtheta is required for TCR-induced T cell activation

Vav and PKCtheta play an early and important role in the TCR/CD28-induced stimulation of MAP kinases and activation of the IL-2 gene. Vav is also essential for actin cytoskeleton reorganization and TCR capping. Here, we report that PKCtheta function was selectively required in a Vav signaling pathway that mediates the TCR/CD28-induced activation of JNK and the IL-2 gene and the upregulation of CD69 expression. Vav also promoted PKCtheta translocation from the cytosol to the membrane and cytoskeleton and induced its enzymatic activation in a CD3/CD28-initiated pathway that was dependent on Rac and on actin cytoskeleton reorganization. These findings reveal that the Vav/Rac pathway promotes the recruitment of PKCtheta to the T cell synapse and its activation, essential processes for T cell activation and IL-2 production.