Background/Purpose . T-cell selection in the thymus is primarily determined by the avidity of T cell receptor (TCR) for self-ligand-MHC. Since this process is dependent on the somatically generated receptors against the internal antigenic environment, all T-cells are inherently self-reactive to some degree. Hence, a signaling threshold must be established whereby overtly high self-avidity, potentially pathogenic, T-cells are removed, while allowing other lower self-avidity T-cells to survive. Using a avidity-based TCR transgenic model system, we found that besides the TCR, there are other signal regulating molecules that play a role in establishing signal threshold during a process known as TCR tuning, which modulates the intensity of TCR signaling (Pinkhasov et al, manuscript in preparation). Here, we determined the expression of potential TCR signal fine-tuning (TFT) molecules in a model of multi-system autoimmune disease, and investigated their role in T-cell signaling defects and mechanisms of their alteration.

Methods . The availability of murine strains that develop autoimmune disease resembling human SLE enables one to study the preclinical events in the pathogenesis. Here, we used MRL/MpJ-Fas^+/+ (MRL+/+) mice that develop SLE at 8–10-months of age, congenic MRL/MpJ-Fas^lpr/lpr (MRL/lpr) mice that develop accelerated SLE due to a mutation of the fas gene, and MHC-matched C3H control mice. To analyze TFTs in antigen-specific T cells, studies were repeated in MRL/lpr, MRL+/+ and healthy B10.BR mice carrying the AND TCR transgene. 

Results . We found the altered expression patterns of a battery of TFTs on thymocytes from MRL/lpr and MRL+/+ mice compared to C3H controls; specifically, negative regulator TFTs were reduced, while a positive regulator was increased. Thymocytes from young MRL+/+ and MRL/lpr mice showed increased activation and phosphokinase signal upon ex vivo stimulation, which were restored to near normal levels in the presence of a CD5 agonistic antibody. Unexpectedly, we observed a drastic reduction of TFTs in activated peripheral T-cells after disease onset.  The reduced levels of negative TFTs correlated with increased responsiveness to TCR stimulation and to weak antigenic ligands. The reduced expression of TFTs in periphery was not due to their decreased transcripts, but rather, to activation-induced post-translational modification due to increased ubiquitination leading to targeted protein degradation. This was associated with an altered expression of E3 ubiquitin ligases cbl-b, traf6, grail and itch in MRL mice. 

Conclusion . These results suggest that T-cells are able to tune the expression levels of TFTs post-development, likely by targeted protein degradation using the ubiquitin cycling pathway. We propose that the ability of T-cells to alter their internal signal threshold by altering TFT expression is a novel mechanism for T-cells to escape peripheral tolerance and perpetuate autoimmune disease. Restoration of TCR signals to normal upon increased signaling through a TFT raises hope for a new avenue of treating systemic autoimmune diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/t-cell-signaling-defects-can-be-corrected-by-manipulating-tcr-signal-fine-tuning-molecules-that-are-altered-due-to-increased-ubiquitination-in-systemic-autoimmune-disease/