Defining the Physiological and Molecular Role of Pellino3 in TLR3 Signalling

Abstract

Innate immunity utilises a series of pathogen associated molecular pattern receptors such as the Toll-like receptors (TLRs) to detect and respond to invading microorganisms. The different TLRs play a pivotal role in the first line of defence by recognising the type of pathogenic threat encountered and by responding in kind. TLR pathogen interaction triggers the production of a number of pro-inflammatory cytokines and anti-viral mediators to initiate the process of microbial elimination. However, if left unregulated, such immunostimulatory and cytotoxic factors have the potential to cause grievous harm to the host and can facilitate and propagate autoimmune disease. Therefore, the TLR signalling response is under the control of stringent regulatory mechanisms to prevent self-harm. In this thesis, the generation and characterisation of a genetically deficient Pellino3 mouse is described. Pellino3 is a member of the highly evolutionary conserved Pellino family of E3 ubiquitin ligases. Using this model, the physiological role of Pellino3 in TLR signalling is delineated. Peli3-deficient mice display no obvious abnormalities in cytokine production in response to pathogenic bacterial and fungal ligands. Pellino3 functions as a specific regulator of anti-viral type 1 interferon production in response to double stranded viral RNA recognition by TLR3. Pellino3-deficient mice are more resistant to the pathogenic and lethal effects of encephalomycarditis virus (EMCV) infection. Pellino3 functions in a novel auto-regulatory mechanism to specifically prevent excessive TLR3 induced expression of type 1 IFNs but leaves pro-inflammatory cytokine production intact. TLR3 signalling induces Pellino3 expression which in turn interacts with TRAF6 and facilitates its polyubiquitination. This modification of TRAF6 suppresses its ability to bind and ubiquitinate a key anti-viral transcription factor IRF7, thereby down regulating the type 1 IFN response. The findings outlined in this thesis define for the first time, a physiological role for Pellino3