| Abstract |
Interstitial lung diseases (ILDs) in general, and idiopathic pulmonary fibrosis(IPF), in particular, are complex disorders with various disease behaviour profiles and different responses to treatment. In IPF epithelial dysfunction is considered major driver of the disease whilst in other ILDs inflammation pathways are considered as the original triggers. Evidence suggests that to some extend there are common pathobiologic pathways that drive tissue fibrosis and progression despite the specific underlying cause of fibrosis. In all tissues, macrophages and monocytes are known regulators of the balance between tissue repair and fibrosis. In the lungs, resident alveolar macrophages (AMs) have anti-inflammatory and injury-resolution properties. However, after a fibrotic injury, bone-marrow derived monocytes are recruited in the lungs and differentiate into alveolar macrophages. Studies have linked this monocyte-derived AMs to initiation and promotion of fibrosis.
The inflammasome is one central cellular mechanism for chemokine release in macrophages. Inflammasomes are cytosolic multiprotein complexes that act as innate immune system sensors. Upon triggering, inflammasomes cleave IL-1β, a potent pro-inflammatory cytokine associated with acute lung injury and fibrosis. NLRP3 is activated by a variety of stimuli including ATP, nigericin and ROS. AIM2 inflammasome is activated by double stranded DNA (dsDNA), while NLRC4 by flagellin. Several inflammasome triggers have been identified as potential pathogenetic factors in lung fibrosis.
Mitochondrial dysfunction and subsequent mitochondrial oxidation is considered hallmark of IPF pathogenesis. Additionally, microbiota changes have been described in IPF and several bacterial genres have been associated with progression, exacerbation as well as specific immune cell cytokine release.
In this study we hypothesised that the inflammasome is overactivated in lung fibrosis-AMs as a result of mitochondrial oxidation and microbiota perturbation.
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We prospectively recruited patients with lung fibrosis (IPF or other ILDs) and healthy controls. Subjects underwent bronchoscopy and bronchoalveolar lavage fluid (BALF) was obtained. From the BALF we isolated AMs which were subsequently cultured and stimulated to activate NLRP3, AIM2 and NLRC4. In parallel, we measured mitochondrial ROS by flowcytometry in unstimulated AMs and microbial burden in the BALF.
There was no notable difference in the baseline activation of inflammasomes in lung fibrosis. However, upon stimulation, NLRP3 could be overactivated in IPF and other ILDs compared to controls. NLRP3 overactivation was also associated with FVC decline in ILDs. AIM2 activation was also more inducible in IPF compared to controls, with similar trends observed in Non-IPF-ILDs. NLRC4 activation was similar across groups.
mtROS was significantly associated with heightened NLRP3 and AIM2 activation. NLRP3 activation coincided with a burst of mtROS and this could be abrogated by mitochondrial antioxidant. Similarly antioxidant therapy inhibited inflammasome activation. The microbial burden was measured in ILDs and it was linked to baseline IL-1β release from AMs and AIM2 and IL-18 relative expression independently of mtROS.
In conclusion, the above findings suggest a link between the overactivation of NLRP3 and AIM2 inflammasomes, driven by mitochondrial oxidation, in the pathogenesis of lung fibrosis while changes in the microbiota may prime the inflammasome in the lungs.
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The interplay between Inflammasome activation, mitochondrial oxidation, and bacterial burden in Interstitial Lung Diseases
