SARS-CoV-2 RBD fusion ameliorates enhanced disease induced by RSV G CCD mRNA vaccine in BALB/c mice

Abstract Respiratory syncytial virus (RSV) is a primary pathogen responsible for lower respiratory tract infections in both infants and the elderly. However, vaccine development has been consistently hampered by the risk of vaccine-enhanced disease (VED), while the role of the RSV attachment (G) glycoprotein has long been overlooked. Using an mRNA vaccine platform, this study systematically evaluated the roles of two critical RSV G protein domains—the central conserved domain (CCD) and the mucin-like domains—in mediating immune protection and VED. Results indicate that the mucin-like domains confer no protection and induce marked VED. Surprisingly, although CCD immunization elicits potent humoral immunity and significantly reduces viral load, it also induces Th2-biased responses and exacerbates pulmonary inflammation following RSV challenge. However, fusion of CCD with the receptor-binding domain (RBD) of SARS-CoV-2 in an mRNA vaccine (CCD + RBD) successfully redirected the Th2-biased immune responses toward a balanced Th1/Th2 profile, while significantly attenuating CCD-induced immunopathology. CD8⁺ T-cell depletion and adoptive-transfer experiments further supported a contribution of RBD-responsive CD8⁺ T cells to the reduction in pathology. These findings provide proof of concept that heterologous antigen fusion can reshape CCD-associated immunopathology in a BALB/c RSV challenge model.