CELLULAR AND MOLECULAR CONTRIBUTIONS OF TLR9 TO AUTOREACTIVE B CELLS IN A MURINE MODEL OF SYSTEMIC LUPUS ERYTHEMATOSUS

One of the hallmarks of the autoimmune disease systemic lupus erythematosus (SLE) is the production of pathogenic anti-nuclear antibodies (ANA). These autoantibodies are the result of a break in tolerance that allows for the activation and differentiation of autoreactive B cells into antibody-secreting plasma cells. ANA can form immune complexes with self nucleic acids, which can bind to and deposit in small blood vessels and instigate immunopathology. The CD45E613R knock-in model of SLE causes different phenotypic consequences on several murine genetic backgrounds, indicating that this signaling mutation is sensitive to genetic modifiers. Despite similar dysregulation of phosphatase activity, CD45E613R mice on a C57BL/6 (B6) background are resistant to SLE phenotypes, while CD45E613R.BALB/c are sensitive to ANA but not end organ damage. In an unbiased screen for genetic modifiers of ANA, we identify TLR9 as a putative modifier for the production of a specific ANA subtype, anti-dsDNA, in the context of CD45E613R.Here, we examine whether TLR9 modifies autoantibody production in the CD45E613R model between the resistant CD45E613R.B6 and sensitive CD45E613R.BALB/c model. Since CD45E613R alters Src family kinase and immunoreceptor signaling in several immune lineages, we generated a series of mixed bone marrow chimeras in the sensitive BALB/c genetic background to test which lineages require CD45E613R. TLR9 is also broadly expressed, so we examined which cell lineages require TLR9 and whether CD45E613R and TLR9 are necessary in cis or trans for anti-dsDNA IgG production in the BALB/c background. Upon finding that both CD45E613R and TLR9 are necessary in B cells for autoreactivity, we examined B cells from both genetic backgrounds. TLR9 stimulation induces elevated signaling in B cells from B6 mice compared to BALB/c, and these signals are not mediated by CD45E613R or co-stimulation via the BCR. We find that TLR9 negatively regulates ANA production in resistant CD45E613R.B6 mice by altering central B cell tolerance. In contrast, TLR9 positively regulates ANA production in BALB/c mice, likely because the strength of signal downstream of TLR9 is not sufficient for tolerance. These results indicate that TLR9 is a genetic modifier of autoreactivity in the context of dysregulated CD45 signaling