Intranasal immunization with a helper-dependent adenoviral vector expressing the codon-optimized fusion glycoprotein of human respiratory syncytial virus elicits protective immunity in BALB/c mice

Abstract

Background: Human respiratory syncytial virus (RSV) is a serious pediatric pathogen of the lower respiratory tract. Currently, there is no clinically approved vaccine against RSV infection. Recent studies have shown that helper-dependent adenoviral (HDAd) vectors may represent effective and safe vaccine vectors. However, viral challenge has not been investigated following mucosal vaccination with HDAd vector vaccines.

Methods: To explore the role played by HDAd as an intranasally administered RSV vaccine vector, we constructed a HDAd vector encoding the codon optimized fusion glycoprotein (Fsyn) of RSV, designated HDAd-Fsyn, and delivered intranasally HDAd-Fsyn to mice.

Results: RSV-specific humoral and cellular immune responses were generated in BALB/c mice, and serum IgG with neutralizing activity was significantly elevated after a homologous boost with intranasal (i.n.) application of HDAd-Fsyn. Humoral immune responses could be measured even 14 weeks after a single immunization. Immunization with i.n. HDAd-Fsyn led to effective protection against RSV infection on challenge.

Conclusion: The results indicate that HDAd-Fsyn can induce powerful systemic immunity against subsequent i.n. RSV challenge in a mouse model and is a promising candidate vaccine against RSV infection.

Figure 1

Characterization of antibody responses following intranasal administration of HDAd-Fsyn in BALB/c mice. Six BALB/c mice were immunized intranasally with HDAd-Fsyn in weeks 0 and 4. ***P<0.0001. Values represent mean ± SEM. (A) Serum anti-RSV F IgG responses induced by HDAd-Fsyn. RSV F-specific antibody titers were measured by ELISA in weeks 2 and 6 after primary immunization. The results represent log₁₀ endpoint values for six individual mice. (B) IgG subtype in sera from HDAd-Fsyn-immunized mice. Sera were obtained in week 6 after primary immunization and ELISA was performed as described in Materials and Methods. Results represent absorbance (450 nm) for samples from six individual mice. (C) Anti-RSV F IgA levels induced by HDAd-Fsyn in lung homogenates. RSV F-specific IgA antibody titers were measured by ELISA in week 6 after primary immunization. Results represent log₁₀ endpoint values for six individual mice. (D) Virus-neutralizing activity in sera from animals vaccinated with HDAd-Fsyn. Virus-neutralizing antibody titers were analyzed by immunoenzyme assay in week 6 after primary immunization. Results are expressed as neutralizing titers corresponding to the serum dilution giving 50% inhibition of plaque formation.

Figure 2

CD8⁺ T-cell responses after immunization with HDAd-Fsyn. Six BALB/c mice were immunized intranasally with HDAd-Fsyn in weeks 0 and 4. RSV F-specific CD8⁺ T-cell responses were assessed using ELISPOT in week 6 after the primary immunization. Results are expressed as the average number of IFN-γ-producing CD8⁺ T cells per million input splenocytes. ***P<0.0001. Values represent mean ± SEM.

Figure 3

HDAd-Fsyn induced protective immunity against intranasal RSV challenge. Six BALB/c mice were immunized intranasally with HDAd-Fsyn in weeks 0 and 4 and then challenged 3 weeks after the final immunization with intranasal administration of 100 μl of subgroup A RSV Long strain (10⁶ pfu/ml). *P<0.05; ***P<0.0001. Values represent mean ± SEM.

Figure 4

Duration of the immune responses stimulated by single intranasal administration of HDAd-Fsyn in BALB/c mice. Six BALB/c mice were immunized intranasally with one dose of HDAd-Fsyn in week 0. *P<0.05; ***P<0.0001. Values represent mean ± SEM. (A) Serum anti-RSV F IgG induced by HDAd-Fsyn. RSV F-specific antibody titers were measured by ELISA at 2-week intervals up to 14 weeks after immunization. The results represent log₁₀ endpoint values for six individual mice. (B) Virus-neutralizing activity in sera from mice immunized with HDAd-Fsyn. Virus-neutralizing titers in sera were measured by immunoenzyme assay at 2-week intervals up to 14 weeks after immunization. Results are expressed as neutralizing titers corresponding to the serum dilution giving 50% inhibition of plaque formation.