A novel adjuvant system combining STING agonist-antigen conjugation with liposomal QS-21 exhibits potent synergistic Chlamydia vaccine immunogenicity

Chlamydia trachomatis (CT) is the most prevalent bacterial sexually transmitted infection worldwide. Over 70% of infections in women and approximately 50% in are asymptomatic, leading to underdiagnosis and sustained transmission. In women, untreated infection can result in severe reproductive complications, including pelvic inflammatory disease, ectopic pregnancy, and infertility. An effective vaccine is therefore urgently needed. A safe and effective vaccine against Chlamydia trachomatis remains an urgent global health need. Protective immunity requires robust Th1-biased CD4 T cell responses, yet subunit vaccines often fail to elicit sufficient cellular immunity. Here, we describe a next-generation adjuvant strategy based on site-specific conjugation of the chlamydial antigen CPAF to a synthetic cyclic dinucleotide STING agonist (CPAF–STG1151), with and without combination adjuvant formulation including QS-21 liposomes. Intramuscular immunization with CPAF–STG1151 induced potent, CD4-dominant Th1 responses across multiple mouse strains and both sexes while requiring a 100-fold lower agonist dose compared to admixed STING agonist formulation. Addition of QS-21 further amplified innate cytokine production, antibody titers, and polyfunctional Th1 responses in mice and synergistically enhanced interferon and inflammasome-associated cytokine responses in human macrophages in vitro. Notably, CPAF-STG1151 alone elicited robust cellular and humoral immunity, highlighting its potential as a simplified, scalable vaccine platform. Together, these findings demonstrate that antigen–STING agonist conjugation, with optional combined saponin adjuvant formulation, provides a versatile and translatable strategy for inducing Th1 immunity against Chlamydia and other intracellular pathogens.