Development and characterisation of a murine model for oral replication competent adenovirus vaccines

Oral vaccination offers many immunological and practical advantages. Nontheless immunize by per os administration necessitates to overcome oral tolerance mechanisms. Viral vectors, such as adenoviruses (AdVs) that potentially combine an efficient delivery system to a powerfull adjuvant effect could be an interesting approach in that regard. Since the 1970s, replication-competent human AdVs 4 and 7 have been used as oral vaccines to protect US soldiers against the severe respiratory diseases caused by these viruses. Though this succesfull application make replication-competent AdVs attractive candidates as oral vaccine vectors, this potential has been poorly studied until now. Such study is limited by the restrictive host specificity of AdVs that hamper human AdV replication in laboratory animals. Here, we used the mouse AdV type 1 (MAV-1) in BALB/c mice to establish a model of oral replication-competent AdV based vaccines.
Firstly, we evaluated the protection confered by a MAV-1 oral infection against a respiratory challenge with the same virus. We showed that MAV-1 oral administration recapitulates in BALB/c mice the homologous protection observed in human with AdVs 4 and 7 vaccines. Beside, our results indicated that live oral MAV-1 vaccine better protected from a respiratory challenge than inactivated intramuscular vaccine. This protection was associated with the presence of MAV-1 specific antibodies and with a better recruitment of effector CD8 T cells. However, we also found that such oral replication-competent vaccine generated a subclinical, but systemic infection that spread all over the body, raising important safety questions.
After showing that MAV-1 is a consistent model to study replication-competent oral AdV based homologous vaccines in mouse, we adapted this model to investigate oral replicative AdV vectors for heterologous vaccination. We constructed MAV-1 recombinants expressing either the entire hemagglutinine (HA) of influenza or the stalk domain of this protein which, as a more conserved region, could be a “universal” influenza antigen. We then vaccinated mice orally with these mutants to evaluate the protection confered against a subsequent intranasal influenza challenge. While the truncated HA vector did not generate a significant humoral or cellular immune response to influenza, a single oral immunization with the full-length HA vaccine generated influenza specific and neutralizing antibodies, and completely protected mice against clinical signs and viral replication. This data confirmed that replication competent AdVs could constitute a very efficient vaccine platform for oral immunization. Nontheless, further investigations are still needed to establish their safety and rationnaly exploit their potential.
Finally, to better figure out the mechanisms underlying AdV oral infection, we generated a luciferase expressing MAV-1 allowing to study AdVs in vivo through bioluminescence imaging. Using this tool, we compared several route of MAV-1 infection, in BALB/c and SJL mice respectively described as relatively resistant and highly sensitive. This difference was confirmed by intraperitoneal administration which gave rise to an intense signal in the brain and nasal horns of SJL mice, while these organes remained non luminescent in BALB/c mice. On the contrary, intranasal and intramuscular administrations respectively generated local lung and muscle signals, regardless of the mouse strain. Surprisingly, in both murine strains, no signal was detected following oral administration. These data illustrate the importance of the route of entry in the fate of AdV infection and suggests that the mouse strain is determining for pathogenesis specifically in case of intraperitoneal administration. Moreover, though our work leaves outstanding issues about the oral MAV-1 infection processes, it also provides a very usefull model to further decipher this phenomenon.
We established here an innovative host specific animal model for oral replication competent AdV vaccines. While confirming the potential of replication competent AdVs as oral vaccine platform, our study highlights a significant risk of systemic infection linked to these vaccines. A better understanding of AdV infection and immunization mechanisms is therefore mandatory to attempt to utilize this potential in safe conditions. Our model should constitute a precious tool for the future development of such vaccine platforms.