MicroRNAs have been shown to be important regulators of inflammatory and immune responses and are implicated in several immune disorders including systemic lupus erythematosus and rheumatoid arthritis, but their role in Lyme borreliosis remains unknown. We performed a microarray screen for expression of miRNAs in joint tissue from three mouse strains infected with Borrelia burgdorferi. This screen identified upregulation of miR-146a, a key negative regulator of NF-κB signaling, in all three strains, suggesting it plays an important role in the in vivo response to B. burgdorferi. Infection of B6 miR-146a −/− mice with B. burgdorferi revealed a critical nonredundant role of miR-146a in modulating Lyme arthritis without compromising host immune response or heart inflammation. The impact of miR-146a was specifically localized to the joint, and did not impact lesion development or inflammation in the heart. Furthermore, B6 miR-146a −/− mice had elevated levels of NF-κB-regulated products in joint tissue and serum late in infection. Flow cytometry analysis of various lineages isolated from infected joint tissue of mice showed that myeloid cell infiltration was significantly greater in B6 miR-146a −/− mice, compared to B6, during B. burgdorferi infection. Using bone marrow-derived macrophages, we found that TRAF6, a known target of miR-146a involved in NF-κB activation, was dysregulated in resting and B. burgdorferi-stimulated B6 miR-146a −/− macrophages, and corresponded to elevated IL-1β, IL-6 and CXCL1 production. This dysregulated protein production was also observed in macrophages treated with IL-10 prior to B. burgdorferi stimulation. Peritoneal macrophages from B6 miR-146a −/− mice also showed enhanced phagocytosis of B. burgdorferi. Together, these data show that miR-146a-mediated regulation of TRAF6 and NF-κB, and downstream targets such as IL-1β, IL-6 and CXCL1, are critical for modulation of Lyme arthritis during chronic infection with B. burgdorferi.
Lyme Disease is caused by infection with the bacteria Borrelia burgdorferi, is transmitted through infected deer ticks ( Ixodes scapularis), and often leads to arthritis that can persist, even after antibiotic treatment. Here, we have identified a microRNA that is critical in modulating Lyme arthritis, but not carditis. This microRNA, miR-146a, is a negative regulator of NF-κB signaling, known to be important in host defense against pathogens, and long suspected to play a role in Lyme arthritis development. Mice lacking miR-146a develop more severe arthritis and show signs of hyperactive NF-κB activation during the persistent phase of infection. Heart manifestations of disease were not altered. Furthermore, this severe arthritis is independent of host defense, since these mice are better able to clear invading bacteria in joints, and bacterial numbers are similar in heart and ear tissue. We identified TRAF6 as an important target of miR-146a-mediated NF-κB regulation of pro-inflammatory cytokines IL-6 and IL-1β, as well as chemokines CXCL1 and CXCL2. Our data demonstrate the importance of maintaining appropriate regulation of amplitude and resolution of NF-κB activation during Borrelia burgdorferi infection, and provide a novel model for elucidating the role of NF-κB in Lyme arthritis development, independent of effect on host defense.