mRNA-LNP vaccine encoding CDV hemagglutinin confers effective protection in raccoon dogs

Canine distemper virus (CDV) is a highly contagious morbillivirus that causes severe disease in carnivores, with raccoon dogs (Nyctereutes procyonoides) serving as major reservoirs for transmission. To address the limitations of current vaccines-such as virulence reversion, multiple doses, and poor efficacy in raccoon dogs-we developed a novel mRNA-lipid nanoparticle (LNP) vaccine encoding a secreted, tetrameric CDV hemagglutinin (H) protein fused to a signal peptide and Tetrabrachion domain to enhance stability and immunogenicity. The vaccine showed high purity, uniform particle size (~108 nm), and efficient in vivo expression of glycosylated H protein. In BALB/c mice, it induced strong H-specific IgG and increased splenic CD4⁺ T cells, suggesting a Th-skewed immune response. In raccoon dogs, high neutralizing antibody titers were maintained post-immunization, and all vaccinated animals survived lethal CDV challenge, whereas controls showed 100% mortality. Compared with a live-attenuated CDV vaccine, the mRNA-LNP vaccine markedly reduced clinical signs, preserved intestinal integrity, and nearly eliminated viral RNA in mucosal and systemic tissues. This study provides the first evidence that an mRNA-LNP vaccine encoding CDV hemagglutinin can confer complete protection against CDV challenge in raccoon dogs, highlighting its potential for veterinary and wildlife disease prevention.