Evaluation of an F Protein-Based Recombinant Protein for Immunization Against Respiratory Syncytial Virus

Respiratory syncytial virus (RSV) is one of the most important etiologies of acute respiratory infections that cause bronchiolitis in children under 5 years of age. Treatments are expensive, no vaccine is available, and this is an important cause of hospitalization. Costimulatory molecules have been reported to be good inducers of antiviral type 1 immune response. This study aimed to generate a recombinant vaccine with the RSV F protein fused to 4-1BBL to evaluate the activation of an antiviral response <i>in vitro</i> and the production of neutralizing antibodies <i>in vivo</i>. The codon-optimized F gene was subcloned into an expression vector as follows: streptavidin core, gene F, and costimulatorytumor necrosis factor receptor superfamily member 9 -TNFRS9- ligand (4-1BBL). After the induction of expression in <i>Escherichia coli</i> C43, the recombinant protein (SA-F3x-4-1BBL, denominated SF4) was purified and verified by western blotting. Cultured RAW264.7 macrophages were stimulated with SF4 protein, then tumor necrosis factor (TNF) receptor-associated factor 2 (TRAF2), p38, and nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB) were analyzed by western blot, and mRNA cytokines were analyzed by RT-qPCR. Finally, male C57BL/6 mice were inoculated with SF4, and the generation of anti-RSV neutralizing antibodies and serum cytokines was examined. SF4 had a size of 84.4 kDa with a 5.6% yield. SA-F-4-1BBL upregulated TRAF2, TNF-α, and interferon (IFN)-γ expression levels and activated p38 mitogen-activated protein kinase and NF-κΒ pathways in RAW264.7 cells. Importantly, antibodies capable of neutralizing RSV infection and producing type 1 cytokines were detected in the sera of immunized animals. These results suggest that the fusion protein SF4 activates the 4-1BBL signaling pathway, resulting in an effective antiviral response mediated by neutralizing antibodies and antiviral cytokines.