Respiratory syncytial virus (RSV) is a leading cause of severe lower respiratory tract infections in infants and toddlers, contributing to significant morbidity and mortality worldwide. Severe cases can lead to long-term complications such as airway hypersensitivity and an increased risk of asthma. In addition to infants, RSV poses a considerable threat to the elderly and immunocompromised individuals. Despite its global impact, RSV infection does not confer long-lasting immunity, highlighting the urgent need for an effective vaccine. However, vaccine development has been historically hindered by the phenomenon of vaccine-enhanced respiratory disease (ERD), first observed in a 1966 clinical trial with a formalin-inactivated (FI-) RSV vaccine. Until a few years ago, no RSV vaccine was available, with prevention relying solely on monoclonal antibody therapies such as Palivizumab and recently also Nirsevimab. Only since 2023 the Food and Drug Administration (FDA) has approved two intramuscular RSV vaccines, Arexvy and ABRYSVO. Both vaccines utilize a stabilized pre-fusion F protein to elicit strong neutralizing antibody responses. ABRYSVOTM is also licensed for maternal immunization, and has shown to provide protection to neonates for up to six months postpartum. Although these vaccines represent significant progress, current strategies predominantly focus on systemic immunization. The lack of mucosal vaccine approaches remains a major limitation, as the establishment of local immunity could be crucial for rapid viral clearance and reduced transmission. The development of effective mucosal RSV vaccines remains improtant in global health research and demands detailed safety analyses of new vaccine candidates. The aim of this work was to establish an effective prime / boost vaccination protocol using recombinant adenoviral vectors (rAd) and plasmid DNA and additionally to evaluate rAd19a as a potential new vaccine vector. Furthermore, it was of interest to understand the mechanisms underlying successful protection or the development of ERD during RSV infection after immunization. It became evident that an adenoviral boost following an intramuscular DNA prime was a promising vaccination protocol in which rAd19a-F demonstrated its suitability compared to rAd5-F. Although the immunogenicity of both vectors differed, they were able to protect mice from severe RSV infection. Mice that received a systemic prime followed by an intranasal boost with rAd19a-F or rAd5-F exhibited similarly minimal weight loss. In contrast, animals boosted intramuscularly with rAd19a-F showed pronounced weight loss within the first days of infection—a phenomenon not observed in non-vaccinated, infected animals. Notably, intramuscular rAd19a-F boosted mice clearly showed signs of ERD, which were even more severe in the group with a single DNA immunization (rAd19a-Mock boost). In the detailed kinetic analysis, mucosally treated animals achieved faster viral control, as evidenced by lower viral loads and reduced lung tissue damage early after challenge. Ex vivo characterization of lung-infiltrating T-cells and innate immune cells, alongside the analysis of mucosal and systemic antibody responses, revealed that the presence of lung-resident memory CD8+ T-cells and IgA responses was critical for superior protection. Conversely, the predominance of TNF-producing circulating CD4+ and CD8+ T-cells correlated with ERD development. As the rAd19a-Mock group displayed the most pronounced ERD outcome, this cohort was selected to investigate the involvement of the specific contribution of T-cell subsets to this effect. A direct link between the worsened disease outcome and a single T-cell population could not be clearly established. The depletion of either CD8+ or CD4+ T-cells resulted in impaired viral control in the lungs. However, CD4+ T-cells likely contributed to a pro-inflammatory and cytotoxic environment while still being crucial for adequate CD8+ T-cell and B-cell responses. Simultaneously, RSV-specific CD8+ T-cells may mitigate side effects that may be driven by CD4+ T-cells. Furthermore, the role of the adaptive immune response in the absence of B-cells was examined. B-cell-deficient mice exhibited a significantly higher number of F-specific CD8+ T-cells compared to equally immunized wild-type animals, while the functionality of T-cells remained comparable. The absence of a B-cell response was compensated by a robust T-cell response, which provided protection against severe disease progression. However, following RSV challenge, the total protein concentration in the BALF as indicator for epithelial barrier disruption was elevated across all immunized animals compared to the naive group. This observation may be attributed to the heightened presence of cytotoxic T-cells in the lung at the onset of infection. In conclusion, a systemic prime followed by a mucosal boost proved to be the most effective approach in preventing RSV infection while minimizing ERD. Lung-resident memory CD8+ T-cells and IgA were crucial for protection, while TNF-producing T-cells were associated with the development of ERD. Additionally, a balanced immune response was essential for optimal protection. These findings offer valuable insights for the optimization of future RSV vaccine research.
Jana Fuchs (2026) studied this question.