The immune system uses several mechanisms of central and peripheral tolerance in order to prevent the activation of T lymphocytes toward self-antigens. Although the importance of immune self-tolerance has been established for a long time, some essential cellular and molecular mechanisms of T-cell tolerance have only been recently revealed. Once thought to be a recycling system, protein ubiquitylation by E3 ligases has now emerged as a regulated and crucial modulator of immune responses, and more importantly as a key signaling pathway involved in T-cell tolerance. In this review, we highlight our current understanding of the transcriptional and molecular signaling mechanisms involved in ubiquitylation-mediated T-cell tolerance.Key words: Autoimmunity . Cbl-b . E3 ligase . T-cell tolerance . Ubiquitylation
IntroductionUpon activation, the immune system orchestrates robust and potentially destructive responses intended to eliminate the immunogenic antigen. Thus, immune system activation is under stringent control to prevent a detrimental immune response against self-antigens. The processes that ensure unresponsiveness toward self-antigens are known as immunological tolerance and, in their absence or failure, autoimmunity occurs [1]. Central tolerance mechanisms operate in the thymus to eliminate potentially self-reactive thymocytes and generate (T reg ) suppressor cells [2]. In addition, several peripheral tolerance mechanisms act to prevent self-reactivity once mature T cells have reached the periphery. These peripheral mechanisms of tolerance include the following: (i) immunological ignorance to selfantigens expressed at low levels or in immunoprivileged sites; (ii) active immunosuppression by Foxp31 T reg cells; (iii) activation-induced cell death of autoreactive T cells and (iv) induction of a state of unresponsiveness known as T-cell anergy [3].During the past few years, extensive research has revealed new insights into the cellular and molecular mechanisms necessary to induce and maintain T-cell tolerance. In particular, ubiquitylation of proteins by E3 ligases has emerged as a novel and indispensable signaling pathway that regulates T-cell tolerance toward self-antigens [4][5][6].
The ubiquitylation processUbiquitylation is an important mechanism of post-translational modification of proteins by which the 76-aa polypeptide ubiquitin, Ub, is covalently attached to a substrate protein. This process occurs in a stepwise manner, requiring the participation of three enzymes: a ubiquitin-activating enzyme E1, responsible for the first binding and activation of the Ub; a second ubiquitinconjugating enzyme E2, which receives the activated Ub from the E1; and a third ubiquitin-ligase E3 that catalyzes the final covalent transfer of the Ub from the E2 enzyme to the protein substrate [7]. Once thought to only mediate proteosomal degradation, the ubiquitylation of a protein can have multiple effects, including altered subcellular localization, regulation of protein-protein interactions, and functional modulation. The fa...