PKCtheta/Cbl-b pathway in immunological tolerance

The immune system ensures that an organism can successfully deal with the permanent attacks of various pathogens like bacteria, viruses or parasites. In the course of immunosurveillance, it is even capable of eliminating developing tumors. On the other hand, inappropriate immune reactions can lead to autoimmune diseases like multiple sclerosis or lupus erythematodes. Therefore, it is crucial that there is an intrinsic fine tuning to activate the immune system only when it is needed but to silence it when it is potentially harmful. Over the last years, the diverse family of E3 ubiquitin ligases has emerged more and more as key players in the negative regulation of immune responses, thereby limiting the immune system`s potential to attack selftissues. Especially the RING type E3 ligase Cbl-b was shown to be crucial for inducing immune tolerance and restricting autoimmunity. However, compared to the obvious importance of Cbl-b in maintaining immune homeostasis, surprisingly little is known about the regulation of Cbl-b itself. Moreover, the target proteins that are ubiquitinated and thereby negatively regulated by Cbl-b are largely unidentified. Therefore, we plan to investigate the T cell receptor/Cbl-b pathway in detail concerning the upstream requirements for Cbl-b activation vs. inactivation as well as downstream proteins that are direct ubiquitination targets of Cbl-b. Our research will lay a special focus on the PKCtheta/Cbl-b connection, which was recently identified by our group as a central module for setting T cell activation thresholds. Our project will address the following main topics: Posttranslational regulation of Cbl-b as a pivotal mechanism in maintaining the balance between a productive immune response and the induction of immunologic tolerance upon various strengths of antigen signaling. Role of the PKCtheta/Cbl-b/c-Cbl interaction in T cells depending on variation of signal strength with special regard to Akt/PKB signaling. Evaluation of Cbl-b functions in T cells and search for critical downstream target proteins that are being ubiquitinated by Cbl-b upon T cell activation. To elucidate the PKCtheta/Cbl-barget(s) pathway would represent a major advance in basic T cell research but would also have implications for our efforts to alleviate immune response aberrations.

TGF? is mainly expressed in the immune system and one of the key regulators of immune homeostasis and peripheral T cell tolerance. It has been shown that TGF? inhibits T cell activity, and so suppresses autoimmunity. On the other hand, it could be shown that cancer cells secrete TGF? into the tumor tissue, which essentially contributes to tumor cells escaping the immune system. In addition to the immunosuppressive properties of TGF?, this cytokine is essential for the differentiation of T cells in different effector T cell subsets.Due to this central immunoregulatory role of TGF?, it is important to understand the signals triggered by this cytokine in T cells. Disturbed signal transduction from the TGF?-receptor into the cell can cause autoimmune diseases, but also prevent effective T cell responses against cancer cells.The research group led by Dr. Thomas Gruber of the Section of Cell Genetics (Department of Medical Genetics, Molecular and Clinical Pharmacology), headed by Univ.-Prof. Gottfried Baier could now shed some further light upon the TGF? signaling pathway. It has been shown previously that Cbl-b, a protein from the class of ubiquitin ligases, plays an essential role in mediating the intracellular effects of TGF?. T cells, where the gene for Cbl-b has been removed, are resistant to the effects of TGF?. As a result, mice lacking the Cbl-b gene very efficiently repel tumors, but they are also more prone to autoimmune diseases. Until recently, however, it was unclear how Cbl-b regulates the TGF? signaling pathway.The research group led by Dr. Gruber has now shown that Cbl-b is responsible for the degradation of a specific inhibitor of TGF? signaling. This protein called Smad7 specifically inhibits the effects of TGF?, with the result that the T cells respond only weakly to this cytokine. As a consequence, the cells are hyperactive, i.e. they can attack healthy tissue, but also tumor cells. The group found that Smad7 is tagged ("ubiquitinated") by Cbl-b and thereby prepared for degradation. By this process, Cbl-b ensures that the T cells are sensitized to TGF? and the organism is protected against the development of autoimmune diseases. The other side of the coin is that efficient tumor rejection is prevented. The group has already demonstrated in previous publications that Cbl-b inactivation actually leads to increased tumor rejection. A pharmacological modulation of this pathway has the effect that the T cells are more aggressive against tumor cells. With this direct and functionally crucial Cbl-b/Smad7 interaction, the research group has now identified the underlying molecular regulatory mechanism.