PLUNC: a multifunctional surfactant of the airways

Abstract

PLUNC (palate, lung and nasal epithelium clone) protein is an abundant secretory product of epithelia throughout the mammalian conducting airways. Despite its homology with the innate immune defence molecules BPI (bactericidal/permeability-increasing protein) and LBP (lipopolysaccharide-binding protein), it has been difficult to define the functions of PLUNC. Based on its marked hydrophobicity and expression pattern, we hypothesized that PLUNC is an airway surfactant. We found that purified recombinant human PLUNC exhibited potent surfactant activity by several different measures, and experiments with airway epithelial cell lines and primary cultures indicate that native PLUNC makes a significant contribution to the overall surface tension in airway epithelial secretions. Interestingly, we also found that physiologically relevant concentrations of PLUNC-inhibited Pseudomonas aeruginosa biofilm formation in vitro without acting directly as a bactericide. This finding suggests that PLUNC protein may inhibit biofilm formation by airway pathogens, perhaps through its dispersant properties. Our data, along with reports from other groups on activity against some airway pathogens, expand on an emerging picture of PLUNC as a multifunctional protein, which plays a novel role in airway defences at the air/liquid interface.

Figure 1. PLUNC potently reduces surface tension at the air/liquid interface

Dynamic surface tension was measured for various solutions using the pulsating bubble surfactometer. Grey bars represent the mean minimum surface tension achieved after 5 min of pulsation. Results are means ± S.E.M. Asterisks indicate samples that exhibited statistically significant reductions in surface tension relative to the buffer control, as determined by Student’s t test (*P < 0.01; **P < 0.0001).

Figure 2. PLUNC inhibits Ps. aeruginosa biofilm formation at the air/liquid interface in vitro

Upper panel: static broth cultures of Ps. aeruginosa PA14 were grown for 48 h, producing biofilms (pellicles) at the air/liquid interface. In contrast with the untreated and buffer-treated cultures, bacteria grown in the presence of PLUNC (100 μg/ml) exhibited significantly decreased pellicle formation in this assay. Lower panel: following the removal of medium, PLUNC-treated cultures exhibited reduced biomass adhering to the sides of culture tubes after 72 h of growth.