Mucosal Vaccination with UV-Inactivated Chlamydia suis in Pre-Exposed Outbred Pigs Decreases Pathogen Load and Induces CD4 T-Cell Maturation into IFN-γ⁺ Effector Memory Cells

Chlamydia trachomatis (Ct) infections are the most frequent bacterial sexually transmitted disease, and they can lead to ectopic pregnancy and infertility. Despite these detrimental long-term sequelae, a vaccine is not available. Success in preclinical animal studies is essential for vaccines to move to human clinical trials. Pigs are the natural host to Chlamydia suis (Cs)-a chlamydia species closely related to Ct, and are susceptible to Ct, making them a valuable animal model for Ct vaccine development. Before making it onto market, Ct vaccine candidates must show efficacy in a high-risk human population. The high prevalence of human Ct infection combined with the fact that natural infection does not result in sterilizing immunity, results in people at risk likely having been pre-exposed, and thus having some level of underlying non-protective immunity. Like human Ct, Cs is highly prevalent in outbred pigs. Therefore, the goal of this study was to model a trial in pre-exposed humans, and to determine the immunogenicity and efficacy of intranasal Cs vaccination in pre-exposed outbred pigs. The vaccine candidates consisted of UV-inactivated Cs particles in the presence or absence of an adjuvant (TriAdj). In this study, both groups of vaccinated pigs had a lower Cs burden compared to the non-vaccinated group; especially the TriAdj group induced the differentiation of CD4⁺ cells into tissue-trafficking CCR7^- IFN-γ-producing effector memory T cells. These results indicate that Cs vaccination of pre-exposed pigs effectively boosts a non-protective immune response induced by natural infection; moreover, they suggest that a similar approach could be applied to human vaccine trials.