Assessing pathogen persistence and the host response to Treponema pallidum in a newly developed mouse model of congenital syphilis

Syphilis is a sexually transmitted, multistage systemic infectious disease with ~6-12M new infections reported globally each year. Treponema pallidum subspecies pallidum (Tp) mother-to-child transmission results in adverse pregnancy outcomes including miscarriage, stillbirth, and congenital syphilis (CS) in newborns. If infected infants do not receive adequate treatment, infection can result in developmental defects, skeletal abnormalities and even development of blindness, deafness etc. CDC has described a substantial increase in CS cases over the years with 3,755 reported in 2022, which showed >11-fold increase just within one decade. The major goal of this project is to establish a mouse model of CS, determine changes in host immune responses, and lay foundation for diagnostics and vaccine development in the future. C4-deficient (C4-D) guinea pigs were previously used to study CS by Dr. Wicher; however, the model has not been adopted by any other laboratory in the last three decades. Under our ‘International Collaboration on an Infectious Diseases Research (ICIDR) project, U01-AI115497 with Dr. Giacani at University of Washington and the late Dr. Arturo Centurion-Lara at Cayetano Heredia University in Peru, we described the function of two outer membrane associated lipoproteins and identified Tp0954 as the first placental cell binding adhesin. We also tested vaccine candidates against syphilis in the rabbit infection model in that project. Rabbits have been used to propagate Tp, and determine vaccines efficacy against syphilis; however, not for assessing CS. Furthermore, rabbits cannot be examined by the existing In Vivo Imaging System (IVIS) to determine colonization at different stages of infection by live imaging. Finally, not many immunological reagents are available to study innate and adaptive immune responses against Tp in the rabbit model. Our collaborator, Dr. Giacani has generated a GFP-expressing infectious SS14 strain of Tp that he generously provided to us to generate preliminary data for this proposal. Using this Tp strain and Balb/c mice, we could detect Tp colonization of different organs in both parents and pups by live imaging using IVIS-200. The results were confirmed by recovery of live, motile spirochetes from organs into medium. We hypothesize that mouse model could be used to study sexual and vertical transmission of Tp, examine by live imaging at different stages and assess inflammatory responses, and induction of protective humoral immunity that could prevent vertical transmission to the later generations of pups. The following aims will test our hypothesis. (1). Determine whether infection of Balb/c mice with Tp triggers immune responses to facilitate vertical and potentially horizontal transmission of this spirochete. (2). Determine whether specific humoral immunity generated during Tp infection prevents/reduces mother-to-pups transmission in the later generations. The major significance of this study is to establish a mouse model for CS, determine host immune responses, conduct longitudinal studies to identify vaccine candidates against syphilis/CS to be tested in the future.