Endotoxin-Directed Innate Immunity in Tracheal Aspirates of Mechanically Ventilated Human Neonates

Abstract

Background Mechanical ventilation of preterm infants is associated with pulmonary inflammation. Intubated infants often develop bacterial tracheal colonization but little is known about endotoxin in tracheal aspirates (TAs) or the mobilization of innate immunity towards endotoxin, a potent stimulus that contributes to inflammatory disease. We characterized mobilization of endotoxin-directed innate immunity in TAs from an observational cohort of mechanically ventilated neonates. Methods TA supernatants (n=42; GA=23-40 weeks, postnatal age=1-71 days) were assayed for endotoxin (Limulus amoebocyte lysate assay) and endotoxin-modulating proteins: bactericidal/permeability-increasing protein (BPI), LPS-binding protein (LBP) and soluble CD14 (sCD14). TA cellular BPI was measured by ELISA, western blot, flow cytometry, and bactericidal assay. TA mRNAs encoding endotoxin-modulating proteins were measured by quantitative RT-PCR. Results Endotoxin in TA supernatants was proportional to both postnatal age and polymorphonuclear leukocytes (PMN). Neonatal TAs were rich in PMN containing BPI and expressed mRNAs encoding Toll-like receptor 4 (TLR4), CD14, and MD-2. Extracellular BPI was consistently detectable and correlated with TA PMN and GA. Both extracellular- and cellular-BPI increased during the first postnatal week. TA extracellular BPI, LBP, and sCD14 were positively correlated. Conclusions TAs from intubated neonates demonstrate endotoxin accumulation and mobilization of endotoxin-directed innate immunity, potentially contributing to pulmonary inflammation.

Role of Innate Host Defenses in Susceptibility to Early-Onset Neonatal Sepsis

“…oxygen or aspiration). Intubation is also associated with the progressive accumulation of colonizing bacteria and bacterial endotoxin in respiratory fluids with concomitant mobilization to the airway of endotoxin-modulating host defense proteins49. Neonates with surfactant deficiency lack host defense proteins with valuable immune function such as surfactant proteins A and D that are absent in commercially available exogenous surfactants due to destruction during preparation50.…”

“…Reduced basal β-defensin concentrations in tracheal aspirates from preterm neonates may contribute to the risk of early pulmonary infection51. Mobilization of APPs is evident by increases in airway fluid concentrations of BPI, LL-37, and β-defensins that occur with mechanical ventilation, pneumonia, or systemic infection49, 206, 224. Reduced lactoferrin and lysozyme concentrations have been described in tracheal aspirates of neonates with BPD and may contribute to chronic cellular inflammation due to poor clearance of bacteria225.…”

“…Of note, intubated preterm newborns demonstrate accumulation of endotoxin in airway fluids corresponding to duration of intubation, raising the possibility that endotoxin contributes to pulmonary inflammation and its consequences49. In this context, improvement of pulmonary mucosal immune function through the administration of anti-infective proteins such as surfactant proteins A and D, β-defensins, or endotoxin antagonists may help to reduce local and systemic inflammation associated with pneumonia.…”

Deficient expression of bactericidal/permeability-increasing protein in immunocompromised hosts: translational potential of replacement therapy

“…A population that might particularly benefit from such intervention may be very low birthweight premature infants who are at high risk for Gram-negative sepsis [59] and/or other conditions associated with endotoxemia, such as necrotizing enterocolitis [32]. Of note, prolonged intubation of critically ill preterm newborns is associated with endotoxin accumulation and endotoxin-directed innate immune responses in respiratory fluid [60], raising the possibility that respiratory administration of an endotoxin antagonist such as rBPI 21 , that can be bioactive in vivo by the intranasal route [61], may provide benefit by reducing lung inflammation and bronchopulmonary dysplasia [62].…”

The bactericidal/permeability-increasing protein (BPI) in the innate defence of the lower airways

“…aeruginosa [19]. In tracheal aspirates of mechanically ventilated newborns, who are prone to bacterial colonization and lung inflammation, extracellular BPI positively correlates with the number of infiltrating PMNs, arguing for PMNs as the most important source of BPI in the airways [20]. Besides the long known phagocytosis and degranulation processes, neutrophils were found in the last few years to employ a new third antimicrobial strategy.…”