Evaluation of MPLA and chimeric TLR agonist adjuvants in RSV virus-like particle vaccines delivered by the intramuscular route.

Respiratory syncytial virus (RSV) remains a major cause of severe respiratory disease in infants and older adults, and despite recent advances in prefusion F-based vaccines and monoclonal antibody prophylaxis, additional vaccine strategies remain of interest. Virus-like particle (VLP) platforms offer a promising approach by mimicking the native virion structure while maintaining an excellent safety profile. In this study, we evaluated the immunogenicity and protective efficacy of RSV-VLPs containing prefusogenic F, G and matrix (M) proteins, administered intramuscularly with or without adjuvants targeting pattern-recognition receptors (MPLA, CL413 and CL429). Immunization of mice with adjuvanted RSV-VLPs induced robust systemic IgG responses, enhanced binding to RSV-F and G proteins and significantly higher neutralizing antibody titers compared to unadjuvanted VLPs and alum-adjuvanted FI-RSV controls. Analysis of IgG subclasses indicated a balanced or Th1-skewed antibody response. Splenocyte cytokine profiling revealed elevated IFN-γ, TNF, IL-2 and IL-6 secretion in groups receiving MPLA- or CL429-adjuvanted formulations, while IL-4 remained low, confirming Th1 dominance. Notably, CL413 promoted detectable IL-17 A production, indicative of Th17 activation. Upon viral challenge, all adjuvanted VLP formulations conferred effective protection against RSV-induced pulmonary inflammation, with markedly reduced lung pathology scores. Collectively, these results demonstrate that chimeric TLR-based adjuvants performed equally or better than MPLA in certain cases and enhance the quality but not magnitude of immune responses to RSV-VLPs delivered intramuscularly. These findings underscore the translational potential of adjuvanted VLP vaccines for safe and effective parenteral RSV immunization.