The oral vaccine intrOv is attenuated in infecting the lower airway and inducing chronic pathology

The Chlamydia muridarum mutant intrOv is an intracellular oral vaccine vector, as oral inoculation with it induces protection in extra-gut mucosal tissues. Although intrOv is attenuated in both genital and gastrointestinal mucosal tissues, it remains unclear whether intrOv induces pathology in the airway, as wild-type C. muridarum causes severe lung pathology and body weight loss. The current study revealed that following intranasal inoculation, intrOv failed to induce significant weight loss and shed fewer live organisms than wild-type. Acute infection with intrOv was mainly restricted to the upper airway, with no live intrOv recovered from lung tissue, whereas the similarly inoculated wild-type disseminated throughout the airway. Despite the absence of live intrOv in the lung, intranasally inoculated intrOv still induced acute lung inflammatory infiltration, consistent with the detection of intrOv genomes in the lung. However, the intrOv-induced acute lung inflammation was fully resolved by day 28, whereas the wild-type-induced lung inflammation persisted with progression to fibrosis. Thus, intrOv is highly attenuated in both its infectivity and pathogenicity in the lower airway. Despite significant attenuation, the airway intrOv still induced chlamydial antigen-specific serum antibodies with a titer comparable to that induced by wild-type C. muridarum. Thus, we have demonstrated the airway safety of intrOv as an oral vaccine and provided evidence supporting its development as an airway vaccine to induce systemic immunity.