The role of IL-1R8 in TLRs and IL-1Rs signaling

Abstract

Innate immune signaling relies on the precise assembly of receptor-adaptor complexes to initiate inflammatory responses while preventing excessive activation. Interleukin-1 receptor 8 (IL-1R8, SIGIRR) is a key negative regulator of Toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) pathways. Although its intracellular TIR domain was reported to be sufficient to inhibit LPS-induced TLR4 signaling, the structural basis of this inhibition has remained unclear. This thesis aimed to elucidate the structural and functional mechanisms by which IL-1R8TIR regulates TLR signaling. Using negative-stain electron microscopy and biochemical assays, IL-1R8TIR was shown to co-assemble with MALTIR into filaments morphologically distinct from MALTIR-only assemblies. The cryo-EM structure of these filaments was determined at 4.3 Å, revealing that IL-1R8TIR subunits engage MALTIR laterally rather than through the canonical head-to-tail interfaces seen in signaling-competent TIR assemblies. Additionally, IL-1R8 subunits interact with each other via previously uncharacterized interfaces. These findings suggest that IL-1R8TIR may modulate MALTIR filament formation in either a signaling-incompetent (inhibitory) or, under as-yet-unidentified conditions, a signaling-competent manner. Complementary assays further confirmed that IL-1R8TIR disrupts TRAMTIR filament formation, extending its inhibitory role to the TRIF-dependent branch of TLR4 signaling. Collectively, this thesis provides the first structural evidence of how IL-1R8TIR engages MALTIR to form higher-order assemblies, highlighting its potential dual role in regulating TLR-mediated innate immune responses.