A Point Mutation in the Amino Terminus of TLR7 Abolishes Signaling without Affecting Ligand Binding

TLR7is the mammalian receptor for ssRNA and some nucleotide-like small molecules. We have generated a mouse by N- nitrose-N′-ethyl urea mutagenesis in which threonine 68 of TLR7was substituted with isoleucine. Cells bearing this mutant TLR7lost the sensitivity to the small-molecule TLR7agonist resiquimod, hence the name TLR7rsq1 . In this work, we report the characterization of this mutant protein. Similar to the wild-type counterpart, TLR7rsq1 localizes to the endoplasmic reticulum and is expressed at normal levels in both primary cellsand reconstituted 293T cells. In addition to small-molecule TLR7agonists, TLR7rsq1 fails to be activated by ssRNA. Whole-transcriptome analysis demonstrates that TLR7is the exclusive and indispensable receptor for both classes of ligands, consistent with the fact that both ligands induce highly similar transcriptional signatures in TLR7wt/wt splenocytes. Thus, TLR7rsq1 is a bona fide phenocopyof the TLR7null mouse. Because TLR7rsq1 binds to ssRNA, our studies imply that the N-terminal portion of TLR7triggers a yet to be identified event on TLR7. TLR7rsq1 mice might represent a valuable tool to help elucidate novel aspects of TLR7biology.