Investigation of the Role of the NF-kB Pathway in Mediating the Effects of Hypercapnic Acidosis in Prolonged Systemic Sepsis and Ventilation Induced Acute Lung Injury
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Background: Lung protective ventilation is integral to the management of ARDS. The resulting respiratory acidosis [HCA], termed 'permissive hypercapnia', is protective in diverse non-septic acute lung injury [ALI] models, but worsens bacterial pneumonia induced ALI. NF-kB is a key transcription factor regulating lung injury, inflammation, and repair. Our aim was to investigate the role of NF-kB pathway mediating the effects of HCA in in vivo septic and non-septic ALI models. We hypothesised that the effects of HCA are mediated by NF-kB inhibition, and that direct inhibition of NF-kB would exert similar protective effects. Methods: Following model development pilot studies, four sets of experiments were performed. (1) Caecal ligation and puncture [CLP] was used to develop an in vivo prolonged sepsis induced ALI model. Sprague-Dawley rats were exposed to normocapnia or HCA for 96 hours after CLP. (2) Two in vivo ventilation induced lung injury [VILI] models with distinct injury severities were developed. Sprague-Dawley rats were exposed to normocapnia or HCA while were ventilated with either moderate or severe VILI protocols for 4 hours. (3) An IkB-alpha-super-repressor containing adeno-associated viral vector system [rAAV6-IkBalpha-SR] was constructed. Sprague-Dawley rats received rAAV6-IkBalpha-SR, viral vector without transgene or surfactant via intratracheal instillation. After 96 hours animals underwent VILI. In all studies, animal survival, physiologic and structural indices of lung injury severity, cytokine concentrations, and indices of activation of the NF-kB pathway [including transgene expression where relevant] were assessed. Results: (1) Sustained HCA attenuated systemic sepsis induced ALI and reduced NF-kB activity. Importantly, HCA did not increase bacterial load in CLP sepsis. (2) HCA protected against VILI by inhibiting the NF-kB pathway. (3) Intrapulmonary delivery of AAV6-IkBalpha-SR conferred direct protection against VILI. Conclusion: HCA protected against sepsis and ventilation induced ALI and this appears to have been mediated, at least in part, by NF-kB pathway inhibition.