An ORFV F1L mRNA Vaccine Candidate: Preparation, Immunogenicity, and Comparison with a Commercial Live Vaccine

Orf virus (ORFV) is a major pathogen in goats and sheep, and control currently depends mainly on commercial live vaccines. Although mRNA vaccines have revolutionized human medicine, their use in veterinary settings is largely unexplored. In this study, an mRNA vaccine candidate encoding the ORFV F1L protein (F1L-mRNA-LNP) was developed via in vitro transcription and encapsulated in lipid nanoparticles. BALB/c mice were divided into five groups (n = 14 each): three receiving different doses of F1L-mRNA-LNP (5, 10, or 15 μg), one receiving a commercial live vaccine (CV), and a PBS control group. Mice were immunized intramuscularly and boosted after 14 days; immune responses were assessed 14 days later following ARRIVE 2.0 guidelines. Both the F1L-mRNA-LNP and CV vaccines induced specific antibodies versus PBS (p < 0.01). The 10 μg mRNA group showed Th1 cytokine and CD8⁺ T cell responses comparable to CV (p > 0.05), whereas IL-4 (Th2) was significantly higher in the CV group (p < 0.05). Neutralizing antibody titers did not differ between groups, indicating that the mRNA vaccine induces comparable Th1 cellular immunity but weaker Th2 humoral immunity. Upon ORFV challenge, the 10 μg F1L-mRNA-LNP vaccine protected BALB/c mice, as evidenced by stable body weight, no clinical symptoms, and reduced viral load, with efficacy comparable to CV (p > 0.05). This study provides strong evidence supporting the optimization of ORFV mRNA vaccines and highlights the translational potential of the F1L-mRNA-LNP candidate vaccine for veterinary applications.