A Chlamydia trachomatis CPAF-STING agonist conjugate vaccine administered intramuscularly and intradermally is immunogenic in the pig model

Introduction Chlamydia trachomatis ( Ct ) is the most common bacterial cause of sexually transmitted infection worldwide. Given the limited effectiveness of current Ct screening and treatment programs, alongside the lack of lasting immunity from natural infection, developing an effective vaccine is crucial. Pigs serve as a valuable model for chlamydia research due to their physiological similarity to humans and natural susceptibility to Chlamydia suis ( Cs ), a close relative of Ct . Thus, we utilized the pig model to evaluate the immunogenicity of three Chlamydial Protease-like Activity Factor (CPAF)-based vaccine candidates. Because stimulator of interferon genes (STING) pathway agonists have shown promise as adjuvants for subunit vaccines but are limited by rapid diffusion from the injection site and poor cellular uptake, we evaluated two improved STING agonist formulations: direct conjugation of the agonist to CPAF (CPAF-STG1151) and adsorption onto phytoglycogen nanoparticles (CPAF/NanoST), and compared their performance to a conventional oil-in-water microemulsion adjuvant (CPAF/IMS1313). Methods Pigs received two intramuscular (IM) or two intradermal (ID) doses and adaptive immune responses were assessed weekly by three-color FluoroSpot assays, multi-parameter flow cytometry and anti-CPAF ELISAs. Results Among the vaccine candidates tested, the novel CPAF-STG1151 conjugate elicited the strongest systemic T cell response. It induced a robust cell-mediated immune response characterized by IFNγ production with or without TNFα coproduction and this response was equally mediated by all T cell subsets (CD4, CD8 and γδ T cells). The route of administration (IM vs. ID) had no significant effect on either the magnitude or the cytokine profile of the cell-mediated immune response. The highest anti-CPAF IgG serum levels were also observed following vaccination with the CPAF-STG1151 conjugate vaccine candidate. Discussion Thus, conjugation of the STING pathway agonist STG1151 to the Chlamydia protein CPAF resulted in a highly immunogenic subunit vaccine. Future studies will determine mucosal administration routes and assess the vaccine efficacy of this promising Ct vaccine candidate.