Essential role for Toll-like receptor 7 (TLR7)-unique cysteines in an intramolecular disulfide bond, proteolytic cleavage and RNA sensing

Toll-like receptor 7 (TLR7) an innate immune sensor for microbial RNA, erroneously responds to self-derived RNA. To avoid autoimmune responses, TLR7 is suggested to be silenced until the N-terminal half of the TLR7 ectodomain (TLR7N) is cleaved off. Resultant truncated TLR7 (TLR7C) is thought to signal microbial RNA. We here show that TLR7N remains associated with TLR7C through a disulfide bond. By N-terminal amino acid sequencing, TLR7C was found to start at 461E or 462A. The newly established monoclonal anti-TLR7N showed that endogenous TLR7 in bone marrow-derived dendritic cells was almost all cleaved and cleaved TLR7N remained in endolysosomes. TLR7N in endolysosomes was linked with TLR7C by a disulfide bond. In contrast, TLR9 did not have a disulfide bond between TLR9N and TLR9C fragments. Among the cysteines unique to the ectodomain of TLR7 but not TLR9 (Cys98, Cys445, Cys475 and Cys722), Cys98 in TLR7N and Cys475 in TLR7C were required for an intramolecular disulfide bond. These cysteines were also needed for proteolytic cleavage of and RNA sensing by TLR7, but not for TLR7 trafficking from endoplasmic reticulum to endosomes. No response was seen in TLR7 mutants lacking the proteolytic cleavage site or TLR7C alone. These results demonstrate requirement for proteolytic cleavage and TLR7N in TLR7 responses and indicate RNA sensing by TLR7N + TLR7C.