Toll-like receptors (TLR) recognize pathogen-associated molecular patterns (PAMPs) and endogenous danger-associated molecular patterns (DAMPs) via their leucine-rich repeat (LRR) domains and initiate innate immune system signaling in response to detected threats. TLRs are critical for host defense and dysfunctional TLR signaling is strongly associated with the pathogenesis of inflammatory and autoimmune diseases. TLRs contain a cytoplasmic TIR (Toll/interleukin-1 receptor/resistance protein) domain, upon PAMP/DAMP binding by the non-cytoplasmic TLR LRR domain. The cytoplasmic TIR domain recruits downstream adaptors and effector enzymes such as MAL (MyD88 adaptor-like) and MyD88 (myeloid differentiation primary response gene 88) to initiate immune system signaling.
Among ten identified TLRs, TLR7, TLR8, and TLR9 are localized to endosomal membranes and sense nucleic acids. Once activated, TLR7/8/9 recruit MyD88 to the endosome membrane, initiating the downstream signaling. Abnormal activation of TLR7/8/9 signaling leads to autoimmune diseases, including psoriasis and systemic lupus erythematosus. However, the underlying mechanism causing these diseases remains elusive. Given the similarity of TLR7/8/9, this project aims to 1) express and purify the protein TLR7/8/9 TIR; 2) identify the structural basis of human TLR7/8/9 TIR, 3) analyze the intermolecular interactions between human TLR7/8/9 TIR and their adaptor proteins MAL or MyD88. The project hypotheses are 1) TLR7/8/9TIR dimerize and interact with adaptor proteins through TIR-TIR interactions, 2) TLR7/8/9TIR directly interact with MAL or MyD88 to initiate immune system signaling, 3) TIR domains of TLR7/8/9 can induce the formation of MAL or MyD88 assemblies. Our research will shed light on drugs targeting on autoimmune diseases.