MyD88 Deficiency Markedly Worsens Tissue Inflammation and Bacterial Clearance in Mice Infected with Treponema pallidum, the Agent of Syphilis Adam C. Silver 1 , Dana W. Dunne 1 , Caroline J. Zeiss 2 , Linda K. Bockenstedt 3 , Justin D. Radolf 4,5,6,7,8 , Juan C. Salazar 5,7,8 , Erol Fikrig 1,9 * 1 Section of Infectious Diseases, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut, United States of America, 2 Section of Comparative Medicine, Yale School of Medicine, New Haven, Connecticut, United States of America, 3 Section of Rheumatology, Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut, United States of America, 4 Department of Medicine, University of Connecticut Health Center, Farmington, Connecticut, United States of America, 5 Department of Pediatrics, University of Connecticut Health Center, Farmington, Connecticut, United States of America, 6 Department of Genetics and Developmental Biology, University of Connecticut Health Center, Farmington, Connecticut, United States of America, 7 Department of Immunology, University of Connecticut Health Center, Farmington, Connecticut, United States of America, 8 Division of Pediatric Infectious Diseases, Connecticut Children’s Medical Center, Hartford, Connecticut, United States of America, 9 Howard Hughes Medical Institute, Chevy Chase, Maryland, United States of America Abstract Research on syphilis, a sexually transmitted infection caused by the non-cultivatable spirochete Treponema pallidum, has been hampered by the lack of an inbred animal model. We hypothesized that Toll-like receptor (TLR)-dependent responses are essential for clearance of T. pallidum and, consequently, compared infection in wild-type (WT) mice and animals lacking MyD88, the adaptor molecule required for signaling by most TLRs. MyD88-deficient mice had significantly higher pathogen burdens and more extensive inflammation than control animals. Whereas tissue infiltrates in WT mice consisted of mixed mononuclear and plasma cells, infiltrates in MyD88-deficient animals were predominantly neutrophilic. Although both WT and MyD88-deficient mice produced antibodies that promoted uptake of treponemes by WT macrophages, MyD88- deficient macrophages were deficient in opsonophagocytosis of treponemes. Our results demonstrate that TLR-mediated responses are major contributors to the resistance of mice to syphilitic disease and that MyD88 signaling and FcR-mediated opsonophagocytosis are linked to the macrophage-mediated clearance of treponemes. Citation: Silver AC, Dunne DW, Zeiss CJ, Bockenstedt LK, Radolf JD, et al. (2013) MyD88 Deficiency Markedly Worsens Tissue Inflammation and Bacterial Clearance in Mice Infected with Treponema pallidum, the Agent of Syphilis. PLoS ONE 8(8): e71388. doi:10.1371/journal.pone.0071388 Editor: Adam J. Ratner, Columbia University, United States of America Received May 14, 2013; Accepted June 28, 2013; Published August 5, 2013 Copyright: ß 2013 Silver et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: EF is an investigator of the Howard Hughes Medical Institute. This work was also supported in part by National Institutes of Health (NIH) grant AI49200 (EF), and NIH/National Institute of Allergy and Infectious Diseases AI26756 (JDR) and AI090166AI (JCS). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: erol.fikrig@yale.edu Introduction Syphilis is a multistage, sexually transmitted illness caused by the obligate human pathogen Treponema pallidum subsp. pallidum and characterized by protean clinical manifestations [1,2,3]. Following inoculation, usually in the genital region, spirochetes replicate locally, inducing an inflammatory response that results in the distinctive, painless chancre of primary syphilis. Within weeks, the chancre heals, indicating the local clearance of T. pallidum, by which time spirochetes have disseminated to various tissues and organs [1,2,3]. Secondary syphilis, resulting from the hematoge- nous dissemination of organisms, typically occurs six to eight weeks after infection. This stage of the disease most commonly involves the skin, mucous membranes and lymph nodes but can affect virtually any organ including the central nervous system [3,4,5]. T. pallidum is thought to be cleared by macrophages via antibody- mediated opsonophagocytosis [5,6,7,8,9]. Infection is contained but often not eliminated – spirochetes have the capacity to persist for years at sites of dissemination without causing symptoms [2,3,6]. For unclear reasons, approximately one-third of patients with latent infection develop one of the recrudescent forms of disease, collectively known as tertiary syphilis [2,3,6]. Despite its global importance as a human pathogen, little is known regarding the pathogenesis of syphilis and the strategies T. pallidum employs to evade the cellular and humoral responses it elicits within its obligate human host [2,6,7]. While several animal models for syphilis have been described over the years, the rabbit model has been the most widely used [8,10,11,12]. In addition to being extremely susceptible to treponemal infection [13], T. pallidum-infected rabbits develop histopathologic changes, serologic responses, and gross lesions partially resembling those of humans [6,8,10,11,12]. Nevertheless, the use of outbred animals, which cannot easily be genetically manipulated, poses serious limitations, which are compounded further by the lack of reagents for studying inflammatory processes in rabbits. In the mid-twentieth century, investigators reported that mice could be infected with T. pallidum and that spirochetes persist within inoculated mice; however, unlike rabbits, symptomatic infection was not observed [10,14,15,16]. In 1980, Klein and colleagues [17] reported that certain mouse strains develop cutaneous lesions after T. pallidum PLOS ONE | www.plosone.org 1 August 2013 | Volume 8 | Issue 8 | e71388