Adjuvant Mucosal Strategies Confer Safe and Effective Immunity Against Mycoplasma pneumoniae and Overcome Vaccine-Associated Enhanced Lung Pathology

Background/Objectives: The global spread of Mycoplasma pneumoniae (MP) poses a significant threat to public health; however, no licensed vaccine for human use is currently available. The development of a safe and effective vaccine is a critical priority. This study systematically evaluated the protective efficacy and safety of an inactivated MP vaccine using different adjuvants and immunization routes. Methods: Mice were immunized with inactivated vaccines via either intramuscular (IM) injection with aluminum hydroxide (alum) or a combination of CpG+QS21 (CQ) or via intranasal (IN) administration of Flagellin from Salmonella Typhimurium (FLA-ST), a potent Toll-like receptor 5 (TLR5) agonist, as a mucosal adjuvant. Vaccine-induced immunogenicity, protective efficacy against MP challenge, and associated lung pathology were assessed. Results: Both IM-vaccinated groups (alum and CQ) exhibited robust systemic immune responses. However, upon subsequent MP challenge, these groups exhibited significant inflammatory pathology in the lung tissues. Notably, the CQ-adjuvanted group displayed severe pulmonary inflammatory infiltration. In stark contrast, compared with the IM-vaccinated group, the IN-immunized group with the FLA-ST mucosal adjuvant achieved significant clearance of MP from the lungs and showed markedly milder histopathological lung damage. Conclusions: Our findings suggest that IM immunization with CQ-adjuvanted inactivated vaccines may represent a suboptimal strategy for MP, given the risk of exacerbating lung immunopathology. Conversely, a mucosal immunization approach using the FLA-ST adjuvant demonstrates considerable promise, offering an effective balance between bacterial clearance and an improved safety profile, highlighting its potential for future MP vaccine development.