Subglottic host defence in prolonged intubation

Abstract

Ventilator-associated pneumonia (VAP) is pneumonia developing 48-hours or more after tracheal tube insertion and mechanical ventilation. VAP is associated with significant morbidity, mortality and costs. The excessive use of antimicrobial agents in VAP treatment and prevention is contributing to antibiotic resistance. Introduction of the tracheal tube is thought to be a key contributing factor in VAP development, providing a conduit for invasion of the airways with virulent microorganisms. The subglottis (the region located immediately below the vocal cords, and directly above the tracheal tube cuff) is key to VAP development. The subglottis in intubated ICU patients has not previously been characterised, and therefore therapeutic targets for preventing VAP are limited. I have demonstrated the presence of a diverse population of oropharyngeal commensal bacteria in the subglottic mucus of newly intubated, non critically ill control patients. In a cohort of ICU patients, intubated for four or more days, there was a less diverse population of more virulent bacterial and fungal organisms. I have also established that the subglottic mucus of long-term ventilated ICU patients, compared to newly intubated controls, is more viscous, has increased neutrophil counts, and increased concentrations of mucins, cytokines and neutrophil proteases. I was able to demonstrate in vitro that neutrophils isolated from the blood of healthy volunteers, when incubated with purified ICU-derived mucin, showed impaired chemotaxis, phagocytosis and bacterial killing, which was reversible upon treatment with a mucolytic agent. These effects were concentration dependent and only found at mucin levels found in ICU patients, not controls. Finally I describe the establishment of a successful method to culture primary human subglottic epithelial cells, at air-liquid interface (ALI). This provides the first in vitro model of this region of the airway. This model suggests that high concentrations of purified ICU-derived mucin applied to ALI cultures, may induce bacterial growth and invasion of the epithelial membrane.