The Peptidoglycan-Degrading Property of Lysozyme Is Not Required for Bactericidal Activity In Vivo

Abstract

Lysozyme is an abundant, cationic antimicrobial protein that plays an important role in pulmonary host defense. Increased concentration of lysozyme in the airspaces of transgenic mice enhanced bacterial killing whereas lysozyme deficiency resulted in increased bacterial burden and morbidity. Lysozyme degrades peptidoglycan in the bacterial cell wall leading to rapid killing of Gram-positive organisms; however, this mechanism cannot account for the protective effect of lysozyme against Gram-negative bacteria. The current study was therefore designed to test the hypothesis that the catalytic activity (muramidase activity) of lysozyme is not required for bacterial killing in vivo. Substitution of serine for aspartic acid at position 53 (D53S) in mouse lysozyme M completely ablated muramidase activity. Muramidase-deficient recombinant lysozyme (LysMD53S) killed both Gram-positive and Gram-negative bacteria in vitro. Targeted expression of LysMD53S in the respiratory epithelium of wild-type (LysM+/+/LysMD53S) or lysozyme Mnull mice (LysM−/−/LysMD53S) resulted in significantly elevated lysozyme protein in the airspaces without any increase in muramidase activity. Intratracheal challenge of transgenic mice with Gram-positive or Gram-negative bacteria resulted in a significant increase in bacterial burden in LysM−/− mice that was completely reversed by targeted expression of LysMD53S. These results indicate that the muramidase activity of lysozyme is not required for bacterial killing in vitro or in vivo.

pH modulates the activity and synergism of the airway surface liquid antimicrobials β-defensin-3 and LL-37

“…Synergistic effects may occur when one peptide in a conformation parallel to the bacterial membrane facilitates insertion of other peptides (63). We speculate that such an effect might account for synergistic activity between hBD-3 and LL-37 as well as synergism for the combination of LL-37 and lysozyme; in addition to its enzymatic activity, lysozyme has nonenzymatic antibacterial activity dependent on its cationic and hydrophobic domains that disrupt bacterial plasma membranes (64)(65)(66). A possibility that we cannot exclude is that variations in pH might render S. aureus and P. aeruginosa more or less susceptible to antimicrobial factors.…”

New Insights into Upper Airway Innate Immunity

“…36,37 It catalyzes the hydrolysis of the ␤-1,4-glycosidic linkages in the peptidoglycan layer of bacterial cell walls. 38 For this reason, lysozyme has a higher affinity for gram-positive bacteria due to their thick peptidoglycan; however, effects against gram-negative bacteria 39,40 and fungi 41 have also been observed. Several studies found that the amount of lysozyme produced by epithelial cells is well correlated with the elimination of pathogens.…”

Bioengineered lysozyme in combination therapies forPseudomonas aeruginosalung infections

“…13 In addition to its ability to enzymatically degrade peptidoglycan, lysozyme exhibits catalysis-independent modes of action related to membrane disruption and dysregulation of bacterial autolysins. [14][15][16] While its broad-spectrum activity and endogenous role in human immunity make hLYS an interesting drug candidate, the wild type enzyme has limitations in therapeutic applications. For example, there is extensive evidence that hLYS is subject to electrostatic mediated inhibition by anionic biopolymers, which are known to accumulate in the infected and inflamed lung.…”