Cathelicidin is a "fire alarm", generating protective NLRP3-dependent airway epithelial cell inflammatory responses during infection with Pseudomonas aeruginosa.

Brian J Mchugh,Emily L Gwyer Findlay,Donald J Davidson,Hsin-Ni Li,David Brough,Julia R Dorin,Holly Stevens,Rebekah Kells,Adriano G Rossi,Paula E Beaumont,Robert D Gray,Rongling Wang,Lisa Young

doi:10.1371/journal.ppat.1007694

Abstract

Pulmonary infections are a major global cause of morbidity, exacerbated by an increasing threat from antibiotic-resistant pathogens. In this context, therapeutic interventions aimed at protectively modulating host responses, to enhance defence against infection, take on ever greater significance. Pseudomonas aeruginosa is an important multidrug-resistant, opportunistic respiratory pathogen, the clearance of which can be enhanced in vivo by the innate immune modulatory properties of antimicrobial host defence peptides from the cathelicidin family, including human LL-37. Initially described primarily as bactericidal agents, cathelicidins are now recognised as multifunctional antimicrobial immunomodulators, modifying host responses to pathogens, but the key mechanisms involved in these protective functions are not yet defined. We demonstrate that P. aeruginosa infection of airway epithelial cells promotes extensive infected cell internalisation of LL-37, in a manner that is dependent upon epithelial cell interaction with live bacteria, but does not require bacterial Type 3 Secretion System (T3SS). Internalised LL-37 acts as a second signal to induce inflammasome activation in airway epithelial cells, which, in contrast to myeloid cells, are relatively unresponsive to P. aeruginosa. We demonstrate that this is mechanistically dependent upon cathepsin B release, and NLRP3-dependent activation of caspase 1. These result in LL-37-mediated release of IL-1β and IL-18 in a manner that is synergistic with P. aeruginosa infection, and can induce caspase 1-dependent death of infected epithelial cells, and promote neutrophil chemotaxis. We propose that cathelicidin can therefore act as a second signal, required by P. aeruginosa infected epithelial cells to promote an inflammasome-mediated altruistic cell death of infection-compromised epithelial cells and act as a “fire alarm” to enhance rapid escalation of protective inflammatory responses to an uncontrolled infection. Understanding this novel modulatory role for cathelicidins, has the potential to inform development of novel therapeutic strategies to antibiotic-resistant pathogens, harnessing innate immunity as a complementation or alternative to current interventions.

Highlights

Pulmonary diseases caused by bacterial or viral infections are a common cause of morbidity and account for 1 in 5 deaths in the UK [1]
We show that when Pseudomonas aeruginosa infects these cells, LL-37 can provide a second signal, acting like a fire alarm, instructing the compromised cells to signal the danger, to recruit host defence cells to the site, and to activate their own altruistic death
We demonstrate the mechanism of this process, from bacterial and host cell perspectives, occurring by activation of an intracellular sensing platform with a novel synergy between the infection and the impact of LL-37, with implications for innovative new approaches to treat multi-drug resistant infections

Summary

Introduction

Pulmonary diseases caused by bacterial or viral infections are a common cause of morbidity and account for 1 in 5 deaths in the UK [1]. There is an increasing global threat from antibiotic-resistant bacterial infections and newly emerging viral infections In this context, therapeutic interventions aimed at protectively modulating host responses, to enhance defence against infection, take on greater significance as alternative or complementary future approaches. An important component of first line defences against such infections is the innate immune response of airway epithelial cells, which constitute the principal barrier first encountered by respiratory pathogens. These innate epithelial cell responses include the secretion of antimicrobial host defence peptides (HDP) [2, 3], and the release of chemokines and cytokines to help orchestrate the response of other immune effector cells. HDP are produced by innate immune effector cells in response to infection, damage and inflammation [4] Understanding their effect on epithelial cell responses to infection is important in elucidating the potential of HDP as targets for development of future antimicrobial interventions

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