Respiratory infections cause the release of extracellular vesicles: implications in exacerbation of asthma/COPD

Abstract

Background: Infection-related exacerbations of respiratory diseases are a major health concern; thus understanding the mechanisms driving them is of paramount importance. Despite distinct inflammatory profiles and pathological differences, asthma and COPD share a common clinical facet: raised airway ATP levels. Furthermore, evidence is growing to suggest that infective agents can cause the release of extracellular vesicle (EVs) in vitro and in bodily fluids. ATP can evoke the P2X7/caspase 1 dependent release of IL-1β/IL-18 from EVs; these cytokines are associated with neutrophilia and are increased during exacerbations. Thus we hypothesized that respiratory infections causes the release of EVs in the airway and that the raised ATP levels, present in respiratory disease, triggers the release of IL-1β/IL-18, neutrophilia and subsequent disease exacerbations.

Methods: To begin to test this hypothesis we utilised human cell-based assays, ex vivo murine BALF, in vivo pre-clinical models and human samples to test this hypothesis.

Results: Data showed that in a murine model of COPD, known to have increased airway ATP levels, infective challenge causes exacerbated inflammation. Using cell-based systems, murine models and samples collected from challenged healthy subjects, we showed that infection can trigger the release of EVs. When exposed to ATP the EVs release IL-1β/IL-18 via a P2X7/caspase-dependent mechanism. Furthermore ATP challenge can cause a P2X7 dependent increase in LPS-driven neutrophilia.

Conclusions: This preliminary data suggests a possible mechanism for how infections could exacerbate respiratory diseases and may highlight a possible signalling pathway for drug discovery efforts in this area.

Conflict of interest statement

Competing Interests: Co-author Dr Stevenson was employed by Novartis during the course of the study. Dr. Stevenson is currently affiliated to Respivert. This does not alter the authors' adherence to PLOS ONE policies on sharing data and materials.

Figures

Figure 1. Demonstration that LPS-induced release of EVs can enhance IL-1β and neutrophil levels and change disease phenotype in model known to have increased levels of ATP.

Mice (n = 8 per treatment group) were exposed to either room air (control) or CS (3R4F cigarettes) using a negative pressure system. Mice were subjected to 2 periods of CS exposure (500 ml/minute) per day (4 hours apart) for 3 consecutive days. On the morning of the third challenge day, the mice were exposed to aerosolised vehicle of endotoxin free saline or LPS (1 mg/ml) in Perspex chambers for 30 minutes. Animals were culled and BALF and lung tissue samples were collected 24 hours after LPS treatment. IL-1β levels were measured in the BALF and neutrophil numbers were determined in the BALF and lung tissue. In separate BALF samples collected from parallel smoke or LPS driven challenges ATP levels were measured (Panel A). Data shown as mean +/− S.E.M. (A: ATP #  = P = 0.0023, Mann-Whitney; B: IL-1β #  = P = 0.0009, Mann-Whitney; C: BALF neutrophil number, #  = P = 0.0431, Students T test; D: lung tissue neutrophil number; #  = P = 0.0006, Mann-Whitney).

Figure 2. Demonstration of the concept that a bacterial mimetic insult can cause EV release.

THP-1 cells were treated with RPMI (vehicle) or LPS (0.1 µM) and incubated overnight and samples were collected and centrifuged to remove the cells. The supernatants were collected and split into two equal fractions: non-ultracentrifuged (EV-rich – left side) and ultracentrifuged (EV-deficient – right side). The samples were pre-treated with vehicle (DMSO, 0.1%, V/V) or P2X7 antagonist (AZ 11645373; 10−7 M). Samples were incubated for one hour and then treated with vehicle (PBS) or exogenous ATPγS (10−3 M). The samples were then incubated for a further 4 hours prior to ELISA assessment for cytokines (A: IL-1β, B: IL-18, C: TNFα, D: MMP-9). The data is shown as mean +/− S.E.M.

Figure 3. Determining if a bacterial mimetic (LPS) can cause the release of EVs in the lung – Electron Microscopy.

Mice were challenged with the aerosolised vehicle of endotoxin-free saline or LPS (1 mg/ml) in Perspex chambers for 30 minutes. Animals were sacrificed and BALF obtained 6 hours after challenge. The samples were then centrifuged (900 g) to remove the white blood cells and debris. The presence of EVs was imaged using EM (top panel – vehicle, middle panels – vehicle challenge, bottom panels – LPS challenge).

Figure 4. Determining if a bacterial mimetic (LPS) can cause the release of EVs in the lung – Nanosight imaging.

Mice were challenged with the aerosolised vehicle of endotoxin-free saline or LPS (1 mg/ml) in Perspex chambers for 30 minutes. Animals were sacrificed and BALF obtained 6 hours after challenge. The samples were then centrifuged (900 g) to remove the white blood cells and debris. The presence of EVs was imaged using Nanosight technology.

Figure 5. Determining if a bacterial mimetic (LPS) can cause the release of EVs in the lung – Cytokine release.

Mice were challenged with the aerosolised vehicle of endotoxin-free saline or LPS (1 mg/ml) in Perspex chambers for 30 minutes. Animals were sacrificed and BALF obtained 6 hours after challenge. The samples were centrifuged (900 g) to remove the white blood cells and debris, and then treated with vehicle (PBS) or ATPγS (10−3 M), and incubated for a further 4 hours and subsequent cytokine release was analysed by ELISA. Data shown as mean +/− S.E.M. (A: IL-1β, B: IL-18, C: IL-1α).

Figure 6. Determining if a bacterial mimetic (LPS) can cause the release of EVs in the lung – Signalling.

Mice (n = 6 per group) were challenged with the aerosolised vehicle of endotoxin-free saline or LPS (1 mg/ml) in Perspex chambers for 30 minutes. Animals were sacrificed and BALF obtained 6 hours after challenge. The samples were then centrifuged (900 g) to remove the white blood cells and then pre-treated with inhibitors (P2X7 antagonist A 438079 (10−6 M) or caspase-1 inhibitor VX 765 (10−7 M)) and incubated for 1 hour. Samples were then treated with vehicle (PBS) or ATPγS (10−3 M), and incubated for a further 4 hours and subsequent cytokine release was analysed by ELISA. Data shown as mean +/− S.E.M. (A: IL-1β, B: TNFα). *  = P = 0.0138 (One way ANOVA followed by a Bonferroni's Multiple Comparison test).

Figure 7. Human translation data: exogenous ATP increases IL-1β/IL-18 level in samples collected from LPS challenged healthy subjects.

Healthy subjects were challenged with inhaled LPS and BALF was collected 6(10−3 M) and incubated for 4 hours; cytokine release was analysed by ELISA. Panel A shows the paired IL-1β data. Panel B, C and D represents the levels of IL-1β, IL-18 and TNFα, respectively. Data shown as mean +/− S.E.M. Statistical analysis using a paired T-test.

Figure 8. Determining if live bacteria or a viral mimetic can cause the release of EVs in the lung.

Live bacterial model: Mice (n = 6 per group) were intranasally challenged with Haemophilus influenzae (1×107 colony forming units serotype b) in sterile phosphate buffered saline (PBS). Mice were sacrificed and BALF samples were collected at 6, 24 and 72 hours. The samples were then centrifuged (900 g) to remove the white blood cells and then treated with vehicle (PBS) or ATPγS (10−3 M), incubated for 4 hours and analysed by ELISA. Data shown as mean +/− S.E.M. (A: IL-1β, B: TNFα). #  = P = 0.0206 (Mann-Whitney). Viral mimetic model: Mice (n = 6 per group) were challenged with vehicle (saline, approximately 25 µl per nostril) or the viral mimetic Poly I:C (0.6 mg/ml) under inhaled isoflurane. Mice were sacrificed and BALF samples were collected at 2, 6 and 24 hours. The samples were then centrifuged (900 g) to remove the white blood cells and then treated with vehicle (PBS) or ATPγS (10−3 M), incubated for 4 hours and analysed by ELISA. Data shown as mean +/− S.E.M. (C: IL-1β, D: TNFα).

Figure 9. Determining whether the ATP/P2X7 axis is central to the exacerbation response in vivo.

Mice (n = 8 per group) were challenged with the aerosolised vehicle of endotoxin-free saline or LPS (1 mg/ml) in Perspex chambers for 30 minutes. Four hours later the mice were intranasally dosed with saline (2 ml/kg) or ATPγs (0.001 mg/kg) whilst under light anaesthesia (4% isoflurane in oxygen). The mice received oral vehicle or P2X7 inhibitor, A438079, 30 minutes prior to the ATP challenge, 4 hours after the challenge and 1 hour prior to cull. Twenty four hours after the LPS exposure the mice were culled and lavaged. IL-1β (A) and neutrophil (B) numbers were measured in the BALF. Data shown as mean +/− S.E.M. An unpaired T-test was used for the statistical analysis. *  = P = 0.0378 (Panel A); *  = P = 0.0162 (Panel B).