Influenza A, rhinovirus and the human respiratory syncytial virus (RSV) are leading causes of severe lower respiratory tract infections in young children, immunocompromised patients and the elderly. Up to 50% of the children under the age of 16 with these viral infections represent with bacterial co-infections. There are hints from the literature that these bacterial co-infections may aggravate the clinical symptoms substantially, while the reason for this observation especially in premature babies is still unknown. In search for a clinical relevant animal model of RSV-infection we compared BALB/c with DBA/2 N mice during infection, the later suffering from a complement factor 5 (C5) deficiency. It turned out that DBA/2 N mice, especially when co-infected with mycoplasma spp, showed significantly more clinical symptoms, higher viral replication in lung homogenate, strong influx of inflammatory cells and increased airway hyperreactivity (AHR). This data was confirmed by co-infection of RSV and mycoplasma spp. in a C5a-receptor knock-out (C5aR1-KO) also showing enhanced clinical signs of infection like weight loss, prominent invasion of inflammatory cells into the lung and AHR. The connection between infection based AHR and induction of cysteinyl leukotrienes (CysLT) by viral pathogens is known; interestingly, we observed that co-infection in C5aR1-KO-mice led to higher levels of CysLT, which in turn resulted in a more distinctive AHR. Cell influx into BALF and AHR could be reduced by treatment with Zileuton inhibiting the synthesis of CysLT by the 5-Lipoxygenase. Furthermore, the application of recombinant C5a dampened the inflammatory effect after co-infection. In summary our data show a possible link between signaling through the C5aR and the regulation of the synthesis of CysLT. Since CysLT was already recognized as important targets in inflammatory airway disease the modulation of this signaling axis may be another meaningful concept to treat symptoms of severe airway infections.

The Respiratory Syncytial Virus (RSV) and Influenza A Virus (IAV) are both two major causative agents of severe respiratory tract infections in humans leading to hospitalization and thousands of deaths each year. In this study, we evaluated the immunogenicity and efficacy of a combinatory DNA vaccine in comparison to the single component vaccines against both diseases in a mouse model. Intramuscular electroporation with plasmids expressing the hemagglutinin (HA) of IAV and the F protein of RSV induced strong humoral immune responses regardless if they were delivered in combination or alone. In consequence, high neutralizing antibody titers were detected, which conferred protection against a lethal challenge with IAV. Furthermore, the viral load in the lungs after a RSV infection could be dramatically reduced in vaccinated mice. Concurrently, substantial amounts of antigen-specific, polyfunctional CD8+ T-cells were measured after vaccination. Interestingly, the cellular response to the hemagglutinin was significantly reduced in the presence of the RSV-F encoding plasmid, but not vice versa. Although these results indicate a suppressive effect of the RSV-F protein, the protective efficacy of the combinatory vaccine was comparable to the efficacy of both single-component vaccines. In conclusion, the novel combinatory vaccine against RSV and IAV may have great potential to reduce the rate of severe respiratory tract infections in humans without increasing the number of necessary vaccinations.