Natural IgM attenuate formation of neutrophil extracellular traps induced by oxidation-specific epitopes in vitro and in vivo

Background

Neutrophil extracellular traps (NETs) emerged as important drivers of thrombus formation in acute myocardial infarction (AMI). Therefore, mechanistic understanding of inducers and modulators of NETosis at the site of occlusion is pivotal for new diagnostic and therapeutic approaches. Many triggers of NET formation have been described, among them oxidations-specific epitopes (OSE), products of lipid peroxidation. Extracellular vesicles (EV) carrying OSE are elevated in AMI. OSE-EV can be recognized by natural IgM antibodies exerting protective functions in cardiovascular disease by reducing the pro-inflammatory response to OSE.

Purpose

We hypothesized that EV-induced NET formation could be dampened by OSE-IgM in vitro and in vivo.

Methods

Patients were recruited after diagnosis of ST-segment elevation myocardial infarction (STEMI, n=51). Blood was aspirated from the culprit site during percutaneous coronary intervention and at several follow-up time points. Myocardial function was assessed by cardiac magnetic resonance (CMR) at 72 hours and 6 months. EV were isolated by ultracentrifugation and characterized by flow cytometry staining for CD45, CD41a, CD144, and OSE. Isolated EV were used for neutrophil stimulation in vitro and in vivo employing a murine injection model. NETs were visualized by immunofluorescence, staining for DNA, histones, citrullinated histone 3, and myeloperoxidase. NET markers and natural IgM recognizing OSE (OSE-IgM) in murine and patient plasma were measured by ELISA.

Results

EVs of endothelial, leukocyte, and platelet origin revealed a prominent absolute and relative increase of the OSE-carrying population as recognized by the malondialdehyde-specific antibody LR04. Plasma derived from the site of occlusion contained more and proportionally higher levels of OSE-EV derived from CD45+ cells than the intra-patient peripheral control. No difference was observed for EV of platelet origin, and the endothelial-derived subset of OSE-EV was decreased at the culprit site. OSE-EV and NET markers were positively associated in the circulation. Decreasing OSE-IgM levels during hospital stay indicated consumption of protective antibodies. EV isolated from AMI patient plasma were revealed to induce NET formation in neutrophils in vitro and after injection into mice in vivo, as measured by cell-free DNA and fluorescence microscopy of citrullinated histones in neutrophils. The LR04 IgM antibody, but not a control IgM, reduced the NETogenic effect of EV in both models. Consistently, higher circulating levels of EV and lower OSE-IgM were associated with a reduced ejection fraction in AMI patients at 72 hours and six months.

Conclusion

EV from AMI patients induced NETosis in vitro and in vivo. IgM recognizing malondialdehyde-epitopes diminished this effect indicating that the balance between OSE-EV and OSE-IgM during AMI may represent a potential prognostic and therapeutic target with impact on heart function.

Funding Acknowledgement

Type of funding sources: Private grant(s) and/or Sponsorship. Main funding source(s): Vienna Science and Technology Fund (WWTF)

Imbalance between plasma double-stranded DNA and deoxyribonuclease activity predicts mortality after out-of-hospital cardiac arrest

AIM

Despite an increased rate of return of spontaneous circulation (ROSC) in out-of-hospital cardiac arrest (OHCA) patients, almost half of patients do not survive up to hospital discharge. Understanding pathophysiological mechanisms of post-cardiac arrest syndrome is essential for developing novel therapeutic strategies. During systemic inflammatory responses and concomitant cell death, double-stranded (ds) DNA is released into circulation, exerting pro-inflammatory effects. Deoxyribonuclease (DNase) degrades dsDNA. The role of DNase activity in OHCA survivors and impact on clinical outcome has not been analyzed yet.

METHODS

In a prospective, single-center study, dsDNA and DNase activity were determined at hospital admission (acute phase) and 24 h (subacute phase) after ROSC. The ratio between dsDNA levels and DNase activity was calculated to determine the extent of dsDNA release in relation to the patients' capacity of degradation. Thirty-day mortality was defined as study end point.

RESULTS

We enrolled 64 OHCA survivors, of whom 26.6% (n = 17) died within 30 days. A peak of circulating dsDNA was observed at admission which decreased within 24 h. DNase activity did not differ between acute and subacute phase, while dsDNA load per DNase activity significantly decreased. The ratio between dsDNA levels and DNase activity in the subacute phase was the strongest predictor of 30-day mortality with an adjusted HR per 1 SD of 3.59 (95% CI, 1.80-7.18, p < 0.001).

CONCLUSION

Disproportionally increased dsDNA levels uncompensated by DNase activity are a strong predictor of mortality in OHCA survivors. This pilot study points to a potentially protective effect of DNase activity in patients undergoing cardiac arrest.

P6010Deficiency in milk fat globule-epidermal growth factor 8 delays thrombus resolution

Background

Chronic thromboembolic pulmonary hypertension (CTEPH) is characterized by the obstruction of pulmonary vessels by organized thrombotic and fibrotic lesions. Efferocytosis refers to the engulfment of apoptotic cells (ACs) by phagocytes, a process that is facilitated by bridging proteins. Milk fat globule-epidermal growth factor 8 (MFG-E8) connects phosphatidylserine on ACs with integrin alpha-v beta-III on phagocytes. MFG-E8-deficient mice develop auto-immune disease closely resembling systemic lupus erythematosus. In humans, decreased MFG-E8 levels were observed in patients with coronary heart disease and chronic obstructive pulmonary disease. Whether defective efferocytosis is involved in failure to resolve thrombi in CTEPH remains unknown.

Purpose

We aimed to assess whether deficiency in MFG-E8 is responsible for of chronic non-resolving thrombosis in CTEPH.

Methods

We employed a murine model of chronic thrombosis by inferior vena cava ligation, in MFG-E8 knockout (KO) or wild-type (WT) mice to assess thrombus formation and resolution. Thrombus size at days 3, 7, 14 and 28 after ligation was assessed using either histologic trichrome stainings (n=4–13 per group and time point) or in vivo high-frequency ultrasound (n=10 per group and time point). We furthermore recruited CTEPH patients (n=60, 53% female, mean age 56±11 years) and sex- and age-matched healthy controls for measurement of MFG-E8 plasma levels using ELISA. In CTEPH patients, hemodynamic measurements were performed. Human lung specimens harvested during surgery for CTEPH or from healthy controls, and isolated monocytes from whole blood of CTEPH patients or controls were analyzed using RT-qPCR.

Results

We observed substantially increased thrombus volume in MFG-E8 KO mice compared to WT, which persisted until day 14 after ligation. In human CTEPH patients, MFG-E8 in plasma was increased compared to healthy controls. Similarly, CTEPH monocytes displayed higher concentrations of MFG-E8 mRNA. Conversely, MFG-E8 expression of CTEPH pulmonary artery specimens was downregulated. No correlations between MFG-E8 levels and hemodynamic parameters were observed.

Conclusion

MFG-E8 plays an important role in thrombus resolution. In CTEPH, dysregulation of efferocytosis via impaired MFG-E8 expression in the pulmonary arteries, might drive persistence of thrombus in pulmonary arteries. The absence of a correlation between MFG-E8 and hemodynamic measures argues against pressure as a confounder of the observation.

P6396LDL cholesterol promotes neutrophil extracellular trap formation

Background

Pro-protein convertase subtilisin/kexin 9 (PCSK9) is a regulator of low density lipoprotein (LDL) receptor (LDLR) expression and has gained attention in the treatment of hyperlipidemia. Serum levels of LDL are correlated with numbers of activated neutrophils in the circulation of hyperlipidemic patients. Activated neutrophils can form neutrophil extracellular traps (NETs) by expelling their chromatin, and NETs have been recognized as important risk factors for acute myocardial infarction (AMI) and stroke.

Purpose

We analyzed the influence of serum LDL levels on neutrophil activation, their propensity to form NETs, and the correlation of NET surrogate markers with systemic inflammatory responses in AMI.

Methods

We recruited 249 consecutive patients with AMI (mean age 58 years, 20% females) and assessed laboratory parameters, inflammatory markers, and serum lipid profiles 72 h after primary percutaneous coronary intervention. Double-stranded DNA (dsDNA) and citrullinated histone H3 were determined from plasma as markers of in vivo NET formation. In a subset of patients (n=25), ex vivo NET formation in response to PCSK9 and ionomycin was analyzed, neutrophils were stained for CD11b, LDLR, and lectin-like oxidized LDLR (LOX-1) using flow cytometry at baseline and after activation with phorbol myristate acetate. PCSK9 levels were measured by ELISA.

Results

Patients with serum LDL levels above the median [median (IQR) LDL 111 mg/dl (87–141), mean ± SD LDL 115±41 mg/dl] had significantly higher concentrations of circulating dsDNA.

Levels of dsDNA [median 121 ng/ml, IQR 101–156] were linked with levels of the specific NET marker citH3 [median (IQR) 252 ng/ml (655–1600), rs=0.157]. Plasma concentrations of dsDNA and citH3 correlated significantly with CRP (dsDNA rs=0.328; citH3 rs=0.209), proBNP (dsDNA rs=0.286; citH3 rs=0.192), troponin T (dsDNA rs=0.282; citH3 rs=0.197), and IL-6 (dsDNA rs=0.242; citH3 rs=0.216).

In the subset of patients in whom neutrophils were characterized, pre-treatment of neutrophils with PCSK9 could significantly decrease ionomycin induced NET release in a dose-dependent manner. Levels of serum LDL were associated with spontaneous NET formation (r=0.504) ex vivo and activated CD11b on neutrophils (r=0.495).

LOX-1 expression correlated significantly with LDLR expression (rs=0.670) and PCSK9 levels (r=0.520). Patient neutrophil stimulation in whole blood led to a significant decrease of the LDLR positive neutrophil population. This effect was greater when PCSK9 levels were higher.

Conclusion

Our data indicate a role for LDL in boosting neutrophil activation depending on PCSK9.