Unbiased modifier screen reveals that signal strength determines the regulatory role murine TLR9 plays in autoantibody production1

The autoimmune disease systemic lupus erythematosus (SLE) has a complex environmental and multi-factorial genetic basis. Genome wide association studies have recently identified numerous disease-associated polymorphisms, but it remains unclear in which cells and during which step of pathogenesis specific polymorphisms interact to cause disease. Using a mouse model in which the same activating mutation (CD45E613R) causes distinct genetic background-dependent disease phenotypes, we perform a screen for genetic modifiers of autoreactivity between anti-nuclear antibody (ANA)-resistant CD45E613R.B6 and ANA-permissive CD45E613R.BALB/c mice. Within a novel autoreactivity-associated locus on chromosome 9, we identify a putative modifier, TLR9. Validating a role for TLR9 in modifying autoreactivity in the context of the CD45E613R mutation, manipulation of TLR9 gene dosage eliminates ANA in CD45E613R.BALB/c, but confoundingly permits ANA in CD45E613R.B6. We demonstrate that sensitivity to ANA is modulated by strength of TLR9 signal, since stronger TLR9 B6 signals, but not weaker TLR9 BALB/c signals, negatively regulate CD45E613R B cell development during competitive reconstitution at the central tolerance checkpoint. Our results identify a novel autoreactivity-associated locus and validate Tlr9 as a candidate gene within the locus. We further demonstrate a novel role for TLR9 signal strength in central tolerance, providing insight into the interplay of disease-associated polymorphisms at a discrete step of SLE pathogenesis.