Cathelicidin regulates homeostasis of innate immune responses

The human antimicrobial peptide cathelicidin acts as effector molecule of the innate immune system with direct antimicrobial and immunomodulatory functions. The aim of this study was to test whether the cathelicidin LL-37 modulates the response of neutrophils to microbial stimulation. Furthermore we wanted to investigate whether the presence of cathelicidin reduces pulmonary emphysema and enhances of pulmonary epithelial repair after acute lung injury induced by naphthalene. Human neutrophils were stimulated with LPS, Staphylococcus aureus and Pseudomonas aeruginosa following incubation with LL-37. Cytokine release was measured by ELISA. Reactive Oxygen Species (ROS) production of neutrophils was determined by luminometric and a flow cytometric methods. Peritoneal mouse neutrophils isolated from CRAMP deficient and wildtype animals were treated with LPS and TNF-a was measured in the supernatant by ELISA. Antimicrobial activity of neutrophils was detected by incubating neutrophils isolated from CRAMP knockout and wildtype mice with bacteria. Pulmonary emphysema was induced in mice by intratracheal instillation of elastase and induction of emphysema was evaluated depending on morphological parameter like mean linear intercept (Lm). To test whether cathelicidin enhances lung tissue repair, a selective injury was induced to mouse nonciliated bronchiolar epithelial cells (clara) with naphthalene. The repair of clara cells were determined by immunohistochemical staining for CC10 protein. Incubation with LL-37 significantly decreased the release of proinflammatory cytokines from human neutrophils stimulated with TLR ligands or whole bacteria. LL-37 induced the production of ROS and the increased engulfment of bacteria into neutrophils. Neutrophils from CRAMP deficient mice released significantly more TNF-a after LPS stimulation and showed decreased antimicrobial activity as compared to cells from wildtype animals. Absence of cathelicidin in CRAMP deficient mice decreases significantly the repair of airway epithelium and increases the induction of pulmonary emphysema-induced by application of elastase. In conclusion, LL-37 modulates the response of various innate immune mechanisms involved in tissue homeostasis and inflammation. Cathelicidin controls the release of inflammatory mediators while increasing neutrophils antimicrobial activity.