Respiratory Syncytial Virus Recombinant F Protein (Residues 255–278) Induces a Helper T Cell Type 1 Immune Response in Mice

We have developed and evaluated an immunodominant respiratory syncytial virus (RSV) F antigen in a mouse model. The antigenic region corresponding to amino acids 255–278 of the RSV F protein was cloned into a vector containing the ctxA₂B gene of cholera toxin (CT). The recombinant protein was expressed in Escherichia coli and analyzed on sodium dodecyl sulfate–polyacrylamide gels. The purified protein was evaluated by immunoblot and ganglioside GM₁ enzyme-linked immunosorbent assay to confirm the expression of the RSV F protein and to correct association of the recombinant protein to form a holotoxin-like chimera, respectively. We hypothesized that genetic fusion of modified CT-based adjuvant with RSV F immunodominant epitopes (rRF-255) would induce protective humoral and cellular immune responses in mice. Intranasal immunization of mice with rRF-255 overall induced higher concentrations of anti-RSV F-specific antibodies in both serum and saliva as compared with mice immunized intranasally with RSV or phosphate-buffered saline (PBS). Antibody isotype analysis (IgA, IgG1, IgG2a, and IgG2b) was also performed. The predominant IgG2a antibody isotype response in combination with cytokine analysis of helper T cell type 1 (interferon-γ, interleukin [IL]-2, IL-12 p70, and tumor necrosis factor-α) and helper T cell type 2 (IL-4 and IL-10) responses revealed that rRF-255 antigen induces a prominent helper T cell type 1 immune response in mice. The rRF-255 antigen also induced serum neutralizing antibodies in immunized mice. Analysis of RSV load in lungs showed that rRF-255 immunization provided significant protection compared with PBS control animals.