Matrikines are bioactive fragments of the extracellular matrix (ECM) that are fundamental in regulating a diverse array of physiological processes. The tripeptide Proline-Glycine-Proline (PGP) is a ...collagen-derived matrikine that has classically been described as a neutrophil chemoattractant. In this article, we describe our current understanding of the pathways that generate, degrade and modify PGP to dictate its bioavailability and stability. Additionally, we discuss our expanding appreciation of the capacity of PGP to regulate diverse cell types and biological processes, independent of its activity on neutrophils, including a putative role in wound repair. We argue that PGP functions as a primitive and conserved damage-associated molecular pattern, which is generated during infection or injury and subsequently acts to shape ensuing inflammatory and repair processes. As a fragment of the ECM that accumulates at the epicentre of the action, PGP is perfectly positioned to focus neutrophils to the exact site required and direct a localised repair response. However, it is essential that PGP is efficiently degraded, as if this matrikine is allowed to persist then pathology can ensue. Accordingly, we discuss how this pathway is subverted in chronic lung diseases giving rise to persistent inflammation and pathological tissue remodelling.
Bioactive matrix fragments (matrikines) have been identified in a myriad of disorders, but their impact on the evolution of airway inflammation has not been demonstrated. We recently described a ...pathway where the matrikine and neutrophil chemoattractant proline-glycine-proline (PGP) could be degraded by the enzyme leukotriene A4 hydrolase (LTA4H). LTA4H classically functions in the generation of pro-inflammatory leukotriene B4, thus LTA4H exhibits opposing pro- and anti-inflammatory activities. The physiological significance of this secondary anti-inflammatory activity remains unknown. Here we show, using readily resolving pulmonary inflammation models, that loss of this secondary activity leads to more pronounced and sustained inflammation and illness owing to PGP accumulation. PGP elicits an exacerbated neutrophilic inflammation and protease imbalance that further degrades the extracellular matrix, generating fragments that perpetuate inflammation. This highlights a critical role for the secondary anti-inflammatory activity of LTA4H and thus has consequences for the generation of global LTA4H inhibitors currently being developed.
Alveolar macrophages are sentinels of the airways that must exhibit immune restraint to innocuous antigens but elicit a robust inflammatory response to pathogenic threats. How distinction between ...these dichotomous functions is controlled is poorly defined.Neutrophils are the first responders to infection, and we hypothesised that they may free alveolar macrophages from their hyporesponsive state, promoting their activation. Activation of the inflammasome and interleukin (IL)-1β release is a key early inflammatory event that must be tightly regulated. Thus, the role of neutrophils in defining inflammasome activation in the alveolar macrophage was assessed.
Mice were infected with the X31 strain of influenza virus and the role of neutrophils in alveolar macrophage activation established through administration of a neutrophil-depleting (1A8) antibody.
Influenza elicited a robust IL-1β release that correlated (r=0.6849; p<0.001) with neutrophil infiltrate and was ablated by neutrophil depletion. Alveolar macrophages were shown to be the prominent source of IL-1β during influenza infection, and virus triggered the expression of Nod-like receptor protein 3 (NLRP3) inflammasome and pro-IL-1β in these cells. However, subsequent activation of the inflammasome complex and release of mature IL-1β from alveolar macrophages were critically dependent on the provision of a secondary signal, in the form of antimicrobial peptide mCRAMP, from infiltrating neutrophils.
Neutrophils are critical for the activation of the NLRP3 inflammasome in alveolar macrophages during respiratory viral infection. Accordingly, we rationalise that neutrophils are recruited to the lung to confront a viable pathogenic threat and subsequently commit alveolar macrophages to a pro-inflammatory phenotype to combat infection.
Acetylcholine (ACh) from neuronal and non-neuronal sources plays an important role in the regulation of immune responses and is associated with the development of several disease pathologies. We have ...previously demonstrated that group 2 innate lymphoid cell (ILC2)-derived ACh is required for optimal type 2 responses to parasitic infection and therefore sought to determine whether this also plays a role in allergic inflammation.
mice (in which ILC2s cannot synthesize ACh) were exposed to an allergenic extract of the fungus
, and immune responses in the airways and lung tissues were analyzed. Airway neutrophilia and expression of the neutrophil chemoattractants CXCL1 and CXCL2 were enhanced 24 h after exposure, suggesting that ILC2-derived ACh plays a role in limiting excessive pulmonary neutrophilic inflammation. The effect of non-selective depletion of ACh was examined by intranasal administration of a stable parasite-secreted acetylcholinesterase. Depletion of airway ACh in this manner resulted in a more profound enhancement of neutrophilia and chemokine expression, suggesting multiple cellular sources for the release of ACh. In contrast, depletion of ACh inhibited
-induced activation of ILC2s, suppressing the expression of IL-5, IL-13, and subsequent eosinophilia. Depletion of ACh reduced macrophages with an alternatively activated M2 phenotype and an increase in M1 macrophage marker expression. These data suggest that ACh regulates allergic airway inflammation in several ways, enhancing ILC2-driven eosinophilia but suppressing neutrophilia through reduced chemokine expression.
Abstract
The pro-inflammatory mediator leukotriene B
4
(LTB
4
) is implicated in the pathologies of an array of diseases and thus represents an attractive therapeutic target. The enzyme leukotriene A
...4
hydrolase (LTA
4
H) catalyses the distal step in LTB
4
synthesis and hence inhibitors of this enzyme have been actively pursued. Despite potent LTA
4
H inhibitors entering clinical trials all have failed to show efficacy. We recently identified a secondary anti-inflammatory role for LTA
4
H in degrading the neutrophil chemoattractant Pro-Gly-Pro (PGP) and rationalized that the failure of conventional LTA
4
H inhibitors may be that they inadvertently prevented PGP degradation. We demonstrate that these inhibitors do indeed fail to discriminate between the dual activities of LTA
4
H, and enable PGP accumulation in mice. Accordingly, we have developed novel compounds that potently inhibit LTB
4
generation whilst leaving PGP degradation unperturbed. These novel compounds could represent a safer and superior class of LTA
4
H inhibitors for translation into the clinic.
Proline–glycine–proline (PGP) is a bioactive fragment of collagen generated by the action of matrix metalloproteinase-9 (MMP-9) and prolylendopeptidase (PE), and capable of eliciting neutrophil ...chemotaxis and epithelial remodelling. PGP is normally then degraded by leukotriene A4 hydrolase (LTA4H) to limit inflammation and remodelling. This study hypothesized that early and persistent airway neutrophilia in Cystic Fibrosis (CF) may relate to abnormalities in the PGP pathway and sought to understand underlying mechanisms.
Broncho-alveolar lavage (BAL) fluid was obtained from 38 CF (9 newborns and 29 older children) and 24 non-CF children. BAL cell differentials and levels of PGP, MMP-9, PE and LTA4H were assessed.
Whilst PGP was present in all but one of the older CF children tested, it was absent in non-CF controls and the vast majority of CF newborns. BAL levels of MMP-9 and PE were elevated in older children with CF relative to CF newborns and non-CF controls, correlating with airway neutrophilia and supportive of PGP generation. Furthermore, despite extracellular LTA4H commonly being greatly elevated concomitantly with inflammation to promote PGP degradation, this was not the case in CF children, potentially owing to degradation by neutrophil elastase.
A striking imbalance between PGP-generating and -degrading enzymes enables PGP accumulation in CF children from early life and potentially supports airway neutrophilia.
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•PGP is a collagen-derived pro-neutrophilic mediator.•PGP accumulates in the airways of children with cystic fibrosis.•Imbalance of PGP generation (MMP/PE) vs degradation (LTA4H) enables PGP build-up.•LTA4H is destroyed by neutrophil elastase to prevent PGP degradation.•Infections are key to spiking PGP generation in setting it cannot be degraded.
Neutrophil mobilization, recruitment, and clearance must be tightly regulated as overexuberant neutrophilic inflammation is implicated in the pathology of chronic diseases, including asthma. Efforts ...to target neutrophils therapeutically have failed to consider their pleiotropic functions and the implications of disrupting fundamental regulatory pathways that govern their turnover during homeostasis and inflammation. Using the house dust mite (HDM) model of allergic airway disease, we demonstrate that neutrophil depletion unexpectedly resulted in exacerbated T helper 2 (T
2) inflammation, epithelial remodeling, and airway resistance. Mechanistically, this was attributable to a marked increase in systemic granulocyte colony-stimulating factor (G-CSF) concentrations, which are ordinarily negatively regulated in the periphery by transmigrated lung neutrophils. Intriguingly, we found that increased G-CSF augmented allergic sensitization in HDM-exposed animals by directly acting on airway type 2 innate lymphoid cells (ILC2s) to elicit cytokine production. Moreover, increased systemic G-CSF promoted expansion of bone marrow monocyte progenitor populations, which resulted in enhanced antigen presentation by an augmented peripheral monocyte-derived dendritic cell pool. By modeling the effects of neutrophil depletion, our studies have uncovered previously unappreciated roles for G-CSF in modulating ILC2 function and antigen presentation. More broadly, they highlight an unexpected regulatory role for neutrophils in limiting T
2 allergic airway inflammation.
It is anticipated that bioactive fragments of the extracellular matrix (matrikines) can influence the development and progression of chronic diseases. The enzyme leukotriene A
hydrolase (LTA
H) ...mediates opposing proinflammatory and anti-inflammatory activities, through the generation of leukotriene B
(LTB
) and degradation of proneutrophilic matrikine Pro-Gly-Pro (PGP), respectively. We show that abrogation of LTB
signaling ameliorated inflammation and airway hyperresponsiveness (AHR) in a murine asthma model, yet global loss of LTA
H exacerbated AHR, despite the absence of LTB
This exacerbated AHR was attributable to a neutrophil-independent capacity of PGP to promote pathological airway epithelial remodeling. Thus, we demonstrate a disconnect between airway inflammation and AHR and the ability of a matrikine to promote an epithelial remodeling phenotype that negatively affects lung function. Subsequently, we show that substantial quantities of PGP are detectable in the sputum of moderate-severe asthmatics in two distinct cohorts of patients. These studies have implications for our understanding of remodeling phenotypes in asthma and may rationalize the failure of LTA
H inhibitors in the clinic.
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