The impact of microbial exposure and outdoor housing on vaccine responses in laboratory mice
Preclinical vaccination research is typically conducted using laboratory mice that are bred and housed under specific pathogen-free (SPF) conditions. However, an increasing number of studies describe that the microbiome and immune system of these SPF-housed laboratory mice is underdeveloped compared to those of mice that have been exposed to environmental conditions. Furthermore, the immune system of mice exposed to environmental microbial stimulation more closely resembles that of humans. As a result, microbe-exposed mouse models may better predict clinical study outcomes. In this study, we determined whether increased microbial exposure has an impact on vaccination responses of laboratory mice.  To investigate the impact of microbial exposure, groups of 24 female C57BL/6J mice were either housed under normal SPF conditions, with or without priming with a mix of inactivated murine pathogens or kept outdoor in cages open to environmental air exposure. Within each condition, eight mice received either the pneumococcal polysaccharide conjugate vaccine PCV13, the protein antigen-based vaccine Boostrix, or salt solution as a negative control. Vaccines were administered three times at two-week intervals. Serum and faeces samples were collected regularly. Eleven days after the last vaccination, mice were euthanized after which blood, bronchoalveolar lavage and splenocytes were collected. Antibody levels were measured in serum and mucosal samples in a bead-based multiplex immunoassay, the immune cell composition in the spleen was determined by spectral flow cytometry and the gut bacteriome was characterized through 16S rRNA gene sequencing.