Toll-like receptor (TLR)3 recognizes dsRNA and activates the signaling cascade leading to production of IFN-beta via an adaptor protein, TICAM-1 (also called TRIF). The interface between ligand recognition and signal transduction by TLR3 remains largely unknown. The crystalized ectodomain of human TLR3 revealed a horseshoe-shaped solenoid assembled from 23 leucine-rich repeats (LRRs). Here, we constructed LRR deletion mutants and tested the participation of each LRR in the IFN-inducing ability of TLR3. Only 3 of the 23 LRRs (LRR4, LRR11 and LRR17) were dispensable for the TLR3 function. Among the 20 dysfunctional mutants, LRR20- and LRR22-deleted mutants acted as dominant-negative inhibitors of wild-type TLR3. The LRR20-deleted mutant lost the poly(I:C)-binding ability, while LRR22-deleted mutant possessed it. Strikingly, the LRR21-deleted mutant functioned as a constitutively active form. These three mutants formed homodimers regardless of their different functional features and reacted with TLR3.7, a function-blocking anti-human TLR3 mAb whose epitope resided in LRR10-LRR16, suggesting that the intact conformation around the central solenoid was retained in the C-terminal mutants. These results suggest that the extracellular domains are a crucial trigger of cytoplasmic IFN signaling in TLR3. The altered molecular topology resulting from the deletion of LRR20, LRR21 or LRR22 critically affects the functional assembly of cytoplasmic TLR3, resulting in dysregulation of receptor-receptor association and signal transmission from the outside ectodomain to the inside TIR domain.