Effects of different doses of Poly I:C on immune responses and protective efficacy induced by intranasal immunization with HRSV pre-F protein in mice

Background Human respiratory syncytial virus (HRSV) primarily infects the respiratory mucosa, making local mucosal immunity an important target for vaccine development. Poly I:C is a promising mucosal adjuvant, but the effects of different doses on immune responses and protection induced by intranasal HRSV prefusion F (Pre-F) protein vaccination remain unclear. Methods Female BALB/c mice were assigned to five groups ( n = 6 per group): Pre-F + 10 μg CpG, Pre-F + 10 μg Poly I:C, Pre-F + 25 μg Poly I:C, Pre-F + 50 μg Poly I:C, and PBS. Mice were immunized intranasally on days 0 and 21. Serum Pre-F-specific antibodies and HRSV-neutralizing antibodies were measured after the second immunization. Mice were challenged intranasally with HRSV Long strain 14 days after the second immunization. Body weight, splenic cytokine-secreting lymphocytes, lung Pre-F-specific IgA, HRSV RNA loads, and lung histopathology were evaluated. Results All immunized groups developed Pre-F-specific antibody responses. Neutralizing antibody titers in the 10, 25, and 50 μg Poly I:C groups were 262, 340, and 1,173, respectively, while lung Pre-F-specific IgA titers were 1,185, 1,405, and 1,838. All Poly I:C groups induced IFN- γ -secreting lymphocyte responses, with the strongest response observed in the 50 μg group. Following HRSV challenge, immunized mice showed less body weight loss, lower lung HRSV RNA loads, and reduced histopathological injury than PBS controls. Among the Poly I:C groups, the 50 μg group had the lowest lung HRSV RNA load (30.6 copies/mg), approximately 1/25 of that in the PBS group (766.5 copies/mg). Conclusion Poly I:C enhanced systemic, mucosal, and cellular immune responses induced by intranasal HRSV Pre-F immunization and improved protection against HRSV challenge. Within the tested dose range, 50 μg Poly I:C showed the most favorable overall profile among the Poly I:C groups and warrants further dose-optimization and safety evaluation.