S-TIR-ring up TLR7 and TLR9: signaling domain substitutions clarify the TLR paradox.

In systemic lupus erythematosus (SLE), autoimmunity often develops toward self nucleic acids. The nucleic acid receptors TLR7 and TLR9, which sense RNA and DNA, respectively, are critical for the generation of pathogenic autoimmune antibodies. Despite similarities in their downstream signaling cascades, these receptors play opposing roles in most mouse lupus models: TLR7 promotes disease, while TLR9 provides protection - an observation often referred to as "the TLR paradox." To understand the basis of this dichotomy, Leibler et al. created genetically edited lupus-prone mice in which TLR7 receptors express the TLR intracellular signaling domain (TIR) that corresponds to TLR9, or vice versa. Their results revealed that the TIR domains contribute to the receptors' opposing roles in SLE, shedding light into the TLR paradox in autoimmunity.