The Ubiquitin-like modifier FAT10 in tolerance induction

Abstract

HLA-F adjacent transcript 10 (FAT10) is a ubiquitin-like modifier which is predominantly expressed in lymphoid tissue like thymus, lymph node and spleen. The expression of FAT10 is inducible in various cell types with pro-inflammatory cytokines tumor necrosis factor-alpha and interferon-gamma. FAT10 is able to target proteins for proteasomal degradation, but unlike ubiquitin, it is degraded along with its substrates. Only a few biological functions of FAT10 have been described, for instance, regulation of apoptosis, Toll-like receptor signaling, and cell cycle control. Under proteasomal inhibition, FAT10 can also target proteins to the aggresome and promote protein degradation via the autophagic-lysosomal pathway. In the first part of this thesis, high expression of FAT10 in the thymus could be assigned to medullary thymic epithelial cells. These cells play an essential role in negative selection and central tolerance induction as they present self-antigens to developing thymocytes and promote elimination of self-reactive T cells. FAT10 expression was primarily detected in terminally differentiated medullary thymic epithelial cells that express the autoimmune regulator Aire and display high antigen-presenting capacity. Analysis of T cell receptor transgenic OT-2 and Smarta mice deficient for FAT10 revealed that thymic selection of transgenic OT-2 cells, but not of Smarta cells was altered in the absence of FAT10. Therefore, FAT10 seems to have an epitope dependent effect on thymic section. Furthermore, FAT10 prevented OT-2 T cells from excessive negative selection which was investigated in RIP-OVAhi mice. Additionally, FAT10-deficient TCR-transgenic Smarta mice showed defective negative selection leading to severe lethality in a carrier model of lymphocytic choriomeningitis virus. In the second part, four new interaction partners of FAT10 were identified: 1. Bat3, 2. AATF, 3. Tollip, and 4. Moesin. FAT10 interacts non-covalently with Bat3, a protein that plays an important role in the removal of aggregated proteins. Endogenous Bat3 and FAT10 also interact under proteasomal inhibition. Additionally, FAT10 also interacts non-covalently with the apoptosis-antagonizing transcription factor AATF pointing out to further involvement of FAT10 in the regulation of apoptosis. Furthermore, it was shown that FAT10 interacts non-covalently with Tollip, an important repressor of Toll-like receptor signaling. Moesin, a protein involved in the organization of membranes and cell polarity, interacts non-covalently with FAT10. Additionally, it may represent a new target of covalent FAT10-modification as it seemed to be modified by one or two FAT10 moieties. Taken together, FAT10 was identified as a novel player in thymic selection as it influences the selection of immature T cells in the thymus and thereby co-determines the T cell repertoire. The identification of four new FAT10 interaction partners provides new insights into the physiological role of FAT10.