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Borrelia burgdorferi Arthritis–Associated Locus Bbaa1 Regulates Lyme Arthritis and K/B×N Serum Transfer Arthritis through Intrinsic Control of Type I IFN Production

Localized upregulation of type I IFN was previously implicated in development of Borrelia burgdorferi–induced arthritis in C3H mice, and was remarkable due to its absence in the mildly arthritic C57BL/6 (B6) mice. Independently, forward genetics analysis identified a quantitative trait locus on Chr4, termed B. burgdorferi–associated locus 1 (Bbaa1), that regulates Lyme arthritis severity and includes the 15 type I IFN genes. Involvement of Bbaa1 in arthritis development was confirmed in B6 mice congenic for the C3H allele of Bbaa1 (B6.C3-Bbaa1), which developed more severe Lyme arthritis and K/B×N model of rheumatoid arthritis (RA) than did parental B6 mice. Administration of a type I IFN receptor blocking mAb reduced the severity of both Lyme arthritis and RA in B6.C3-Bbaa1 mice, formally linking genetic elements within Bbaa1 to pathological production of type I IFN. Bone marrow–derived macrophages from Bbaa1 congenic mice implicated this locus as a regulator of type I IFN induction and downstream target gene expression. Bbaa1-mediated regulation of IFN-inducible genes was upstream of IFN receptor–dependent amplification; however, the overall magnitude of the response was dependent on autocrine/paracrine responses to IFN-β. In addition, the Bbaa1 locus modulated the functional phenotype ascribed to bone marrow–derived macrophages: the B6 allele promoted expression of M2 markers, whereas the C3H allele promoted induction of M1 responses. This report identifies a genetic locus physically and functionally linked to type I IFN that contributes to the pathogenesis of both Lyme and RA.