Thrombin inhibitors reduce intrapulmonary accumulation of fibrinogen and procoagulant activity of bronchoalveolar lavage fluid during acute lung injury induced by pulmonary overdistention in newborn piglets

Targeting coagulation activation in severe COVID-19 pneumonia: lessons from bacterial pneumonia and sepsis

Novel coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome-coronavirus-2 (SARS-CoV-2), has rapidly spread throughout the world, resulting in a pandemic with high mortality. There are no effective treatments for the management of severe COVID-19 and current therapeutic trials are focused on antiviral therapy and attenuation of hyper-inflammation with anti-cytokine therapy. Severe COVID-19 pneumonia shares some pathological similarities with severe bacterial pneumonia and sepsis. In particular, it disrupts the haemostatic balance, which results in a procoagulant state locally in the lungs and systemically. This culminates in the formation of microthrombi, disseminated intravascular coagulation and multi-organ failure. The deleterious effects of exaggerated inflammatory responses and activation of coagulation have been investigated in bacterial pneumonia and sepsis and there is recognition that although these pathways are important for the host immune response to pathogens, they can lead to bystander tissue injury and are negatively associated with survival. In the past two decades, evidence from preclinical studies has led to the emergence of potential anticoagulant therapeutic strategies for the treatment of patients with pneumonia, sepsis and acute respiratory distress syndrome, and some of these anticoagulant approaches have been trialled in humans. Here, we review the evidence from preclinical studies and clinical trials of anticoagulant treatment strategies in bacterial pneumonia and sepsis, and discuss the importance of these findings in the context of COVID-19.

Low anticoagulant heparin blocks thrombin-induced endothelial permeability in a PAR-dependent manner

Acute lung injury and acute respiratory distress syndrome are accompanied by thrombin activation and fibrin deposition that enhance lung inflammation, activate endothelial cells and disrupt lung paracellular permeability. Heparin possesses anti-inflammatory properties but its clinical use is limited by hemorrhage and heparin induced thrombocytopenia. We studied the effects of heparin and low anticoagulant 2-O, 3-O desulfated heparin (ODSH) on thrombin-induced increases in paracellular permeability of cultured human pulmonary endothelial cells (ECs). Pretreatment with heparin or ODSH blocked thrombin-induced decrease in the EC transendothelial electrical resistance (TER), attenuated thrombin-stimulated paracellular gap formation and actin cytoskeletal rearrangement. Our data demonstrated that heparin and ODSH had inhibitory effects on thrombin-induced RhoA activation and intracellular calcium elevation. Thrombin-stimulated phosphorylation of the cytoskeletal regulatory proteins, myosin light chain and ezrin/radixin/moesin was also reduced. In these effects, low anticoagulant ODSH was more potent than heparin. Heparin or ODSH alone produced decreases in the EC TER that were abolished by siRNA-mediated depletion of the thrombin receptor, PAR-1. We also demonstrated that, in contrast to heparin, ODSH did not possess thrombin-binding activity. Results suggest that heparin and low anticoagulant ODSH can interfere with thrombin-activated signaling.

Bioinformatic identification of novel early stress response genes in rodent models of lung injury

Acute lung injury is a complex illness with a high mortality rate (>30%) and often requires the use of mechanical ventilatory support for respiratory failure. Mechanical ventilation can lead to clinical deterioration due to augmented lung injury in certain patients, suggesting the potential existence of genetic susceptibility to mechanical stretch ( 6 , 48 ), the nature of which remains unclear. To identify genes affected by ventilator-induced lung injury (VILI), we utilized a bioinformatic-intense candidate gene approach and examined gene expression profiles from rodent VILI models (mouse and rat) using the oligonucleotide microarray platform. To increase statistical power of gene expression analysis, 2,769 mouse/rat orthologous genes identified on RG_U34A and MG_U74Av2 arrays were simultaneously analyzed by significance analysis of microarrays (SAM). This combined ortholog/SAM approach identified 41 up- and 7 downregulated VILI-related candidate genes, results validated by comparable expression levels obtained by either real-time or relative RT-PCR for 15 randomly selected genes. K-mean clustering of 48 VILI-related genes clustered several well-known VILI-associated genes (IL-6, plasminogen activator inhibitor type 1, CCL-2, cyclooxygenase-2) with a number of stress-related genes (Myc, Cyr61, Socs3). The only unannotated member of this cluster ( n = 14) was RIKEN_1300002F13 EST, an ortholog of the stress-related Gene33/Mig-6 gene. The further evaluation of this candidate strongly suggested its involvement in development of VILI. We speculate that the ortholog-SAM approach is a useful, time- and resource-efficient tool for identification of candidate genes in a variety of complex disease models such as VILI.

Science review: searching for gene candidates in acute lung injury

Acute lung injury (ALI) is a complex and devastating illness, often occurring within the setting of sepsis, and carries an annual mortality rate of 30-50%. Although the genetic basis of ALI has not been fully established, an increasing body of evidence suggests that genetic predisposition contributes to disease susceptibility and severity. Significant difficulty exists, however, in defining the exact nature of these genetic factors, including large phenotypic variance, incomplete penetrance, complex gene-environment interactions, and strong potential for locus heterogeneity. We utilized the candidate gene approach and an ortholog gene database to provide relevant gene ontologies and insights into the genetic basis of ALI. We employed a Medline search of selected basic and clinical studies in the English literature and studies sponsored by the HopGene National Institutes of Health sponsored Program in Genomic Applications. Extensive gene expression profiling studies in animal models of ALI (rat, murine, canine), as well as in humans, were performed to identify potential candidate genes http://www.hopkins-genomics.org/. We identified a number of candidate genes for ALI, with blood coagulation and inflammation gene ontologies being the most highly represented. The candidate gene approach coupled with extensive gene profiling and novel bioinformatics approaches is a valuable way to identify genes that are involved in ALI.

Orthologous gene-expression profiling in multi-species models: search for candidate genes

Microarray-driven gene-expression profiles are generally produced and analyzed for a single specific experimental model. We have assessed an analytical approach that simultaneously evaluates multi-species experimental models within a particular biological condition using orthologous genes as linkers for the various Affymetrix microarray platforms on multi-species models of ventilator-associated lung injury. The results suggest that this approach may be a useful tool in the evaluation of biological processes of interest and selection of process-related candidate genes.

Direct thrombin inhibition reduces lung collagen, accumulation, and connective tissue growth factor mRNA levels in bleomycin-induced pulmonary fibrosis

Dramatic activation of the coagulation cascade has been extensively documented for pulmonary fibrosis associated with acute and chronic lung injury. In addition to its role in hemostasis, thrombin exerts profibrotic effects via activation of the major thrombin receptor, protease-activated receptor-1. In this study, we examined the effect of the direct thrombin inhibitor, UK-156406 on fibroblast responses in vitro and on bleomycin-induced pulmonary fibrosis in rats. UK-156406 significantly inhibited thrombin-induced fibroblast proliferation, procollagen production, and connective tissue growth factor (CTGF) mRNA levels when used at equimolar concentration to the protease. Thrombin levels in bronchoalveolar lavage fluid and expression of thrombin and protease-activated receptor-1 in lung tissue were increased after intratracheal instillation of bleomycin. The characteristic doubling in lung collagen in bleomycin-treated animals (38.4 +/- 2.0 mg versus 17.1 +/- 1.4 mg, P < 0.01) was preceded by significant elevations in alpha1(I) procollagen and CTGF mRNA levels (3.0 +/- 0.4-fold and 6.3 +/- 0.4-fold respectively, (P < 0.01), and total inflammatory cell number. UK-156406, administered at an anticoagulant dose, attenuated lung collagen accumulation in response to bleomycin by 35 +/- 12% (P < 0.05), inhibited alpha1(I) procollagen and CTGF mRNA levels by 50% and 35%, respectively (P < 0.05), but had no effect on inflammatory cell recruitment. This is the first report showing that direct thrombin inhibition abrogates lung collagen accumulation in bleomycin-induced pulmonary fibrosis.

Do clinical markers of barotrauma and oxygen toxicity explain interhospital variation in rates of chronic lung disease? The Neonatology Committee for the Developmental Network

OBJECTIVE

To explore the hypothesis that variation in respiratory management among newborn intensive care units (NICUs) explains differences in chronic lung disease (CLD) rates.

DESIGN

Case-cohort study.

SETTING

NICUs at 1 medical center in New York (Babies' and Children's Hospital [Babies']) and 2 in Boston (Beth Israel Hospital and Brigham and Women's Hospital [Boston]).

STUDY POPULATION

Four hundred fifty-two infants born at 500 to 1500 g birth weight between January 1991 and December 1993, who were enrolled in an epidemiologic study of neonatal intracranial white matter disorders.

CASE DEFINITION

Supplemental oxygen required at 36 weeks' postmenstrual age.

RESULTS

The prevalence rates of CLD differed substantially between the centers: 4% at Babies' and 22% at the 2 Boston hospitals, despite similar mortality rates. Initial respiratory management at Boston was more likely than at Babies' to include mechanical ventilation (75% vs 29%) and surfactant treatment (45% vs 10%). Case and control infants at Babies' were more likely than were those at Boston to have higher partial pressure of carbon dioxide and lower pH values on arterial blood gases. However, measures of oxygenation and ventilator settings among case and control infants were similar at the 2 medical centers in time-oriented logistic regression analyses. In multivariate logistic regression analyses, the initiation of mechanical ventilation was associated with increased risk of CLD: after adjusting for other potential confounding factors, the odds ratios for mechanical ventilation were 13.4 on day of birth, 9.6 on days 1 to 3, and 6.3 on days 4 to 7. Among ventilated infants, CLD risk was elevated for maximum peak inspiratory pressure >25 and maximum fraction of inspired oxygen = 1.0 on the day of birth, lowest peak inspiratory pressure >20 and maximum partial pressure of carbon dioxide >50 on days 1 to 3, and lowest white blood count <8 K on days 4 to 7. Even after adjusting for white blood count <8 K and the 4 respiratory care variables, infants in Boston continued to be at increased risk of CLD, compared with premature infants at Babies' Hospital.

CONCLUSION

In multivariate analyses, a number of specific measures of respiratory care practice during the first postnatal week were associated with the risk of a very low birth weight infant developing CLD. However, after adjusting for baseline risk, most of the increased risk of CLD among very low birth weight infants hospitalized at 2 Boston NICUs, compared with those at Babies' Hospital, was explained simply by the initiation of mechanical ventilation.

Thrombin upregulates interleukin-8 in lung fibroblasts via cleavage of proteolytically activated receptor-I and protein kinase C-gamma activation

Acute and chronic interstitial lung diseases are accompanied by evidence of inflammation and vascular injury. Thrombin activity in bronchoalveolar lavage fluid from such conditions is often increased, as well as interleukin (IL)-8. We observed that conditioned medium from lung fibroblasts exposed to thrombin has chemotactic activity for polymorphonuclear cells, and that this activity can be abolished by antibody to IL-8. We report that thrombin stimulates expression of IL-8 in human lung fibroblasts on both the messenger RNA and protein levels in a time- and dose-dependent manner. Stimulation of IL-8 expression by thrombin is inhibited by specific thrombin inhibitors. Synthetic thrombin receptor agonist peptide-14 mimics thrombin's stimulation of IL-8 expression in a dose-dependent manner consistent with the idea that upregulation of IL-8 by thrombin in human lung fibroblasts requires cleavage of proteolytically activated receptor-I. We demonstrate further that thrombin-induced IL-8 synthesis is regulated by protein kinase (PK) C. PKC-gamma may be involved in the upregulation of lung fibroblast IL-8 by thrombin because stimulation of lung fibroblasts with thrombin caused significant upregulation of PKC-gamma and because PKC-gamma antisense oligonucleotides inhibited the accumulation of PKC-gamma protein and IL-8 protein. Our data suggest that the PKC-gamma isoform increase observed after thrombin stimulation is required for thrombin-induced IL-8 formation by human lung fibroblasts.

Heparin improves gas exchange during experimental acute lung injury in newborn piglets

Although intrapulmonary fibrin deposition is a pathognomonic feature of acute lung injury, it remains uncertain whether thrombin inhibitors affect clinically important outcomes. We hypothesized that both heparin and antithrombin (AT) concentrate improve gas exchange during experimental respiratory distress syndrome. We also tested whether combination therapy is more beneficial than monotherapy. Forty-eight newborn piglets were randomized within 12 litters to one of four groups in a factorial design: (1) AT; (2) heparin; (3) AT plus heparin; (4) untreated control animals. After lung lavage and 4 h of barovolutrauma, mechanical ventilation was continued for 24 h during which ventilator pressures and inspired oxygen were adjusted to maintain normal blood gases. The arterial/ alveolar oxygen tension ratio (a/A ratio) and the ventilator efficiency index (VEI) at 18 and 24 h were compared by repeated measures analysis of variance (ANOVA). In contrast to our hypothesis, only heparin improved gas exchange, and we found little evidence of an interaction with AT. The a/A ratio was 0.48 +/- 0.27 (mean +/- SD) in the presence of heparin versus 0.33 +/- 0.26 in its absence; p = 0.01. Corresponding VEI was 0.30 +/- 0.12 versus 0.25 +/- 0.14; p = 0.04. Hyaline membrane formation was also decreased in heparin-treated animals (p = 0.02).

Influence of mechanical stretch on thrombin regulation by fetal mixed lung cells

Respiratory distress syndrome (RDS) is characterized by intrapulmonary fibrin deposition, which can adversely affect surfactant function, and stimulate fibroblast proliferation, which may contribute to the development of bronchopulmonary dysplasia (BPD). We speculated that the premature lung may have impaired regulation of thrombin, thus making preterm infants susceptible to fibrin formation within the lung. Therefore, we studied the effect of stretch, which simulates fetal breathing movements (FBMs), on the generation and inhibition of a key hemostatic enzyme-thrombin-by rat fetal mixed lung cells (FMLCs). Our results showed that stretch induced glycosaminoglycan production with increased antithrombin activity due to an increase in the concentration of active chondroitin sulfate. Stretch downregulated secretion of tissue factor procoagulant activity, which may lead to decreased thrombin generation on the surface of FMLCs. Overall, stretch enhanced the local control of thrombin by FMLCs. These results suggest that premature infants, who will have experienced less FBM, may have impaired thrombin regulation. Impaired thrombin regulation likely contributes to increased fibrin deposition and, potentially, the development of BPD.

A placebo-controlled randomized trial of antithrombin therapy in neonatal respiratory distress syndrome

Neonatal respiratory distress syndrome (RDS) is associated with decreased plasma activity of antithrombin (AT) and increased formation of thrombin. We tested whether AT reduces thrombin formation, improves gas exchange, and decreases the duration of mechanical ventilation and supplemental oxygen. One hundred twenty-two infants were randomized to pasteurized AT concentrate or to placebo. Two ml/kg (equivalent to 100 IU AT/kg) were followed by 1 ml/kg (50 IU/kg) every 6 h for 48 h. Outcome measures included plasma AT activity, thrombin-AT (TAT) complex, prothrombin fragment (F1+2), the ratio of arterial to alveolar oxygen pressure [(a/A)PO2], and the ventilator efficiency index (VEI). In the AT group (n = 61), mean (SD) birth weight was 1,198 (301) g, mean (SD) gestational age (GA) was 28.3 (2.0) wk, 54% were male. In the placebo group (n = 61), mean (SD) birth weight was 1,201 (315) g, mean (SD) GA was 28.8 (2. 3) wk, 51% were male. In treated infants, AT activity was raised to means of 1.69 and 2.25 U/ml at 24 and 48 h, respectively. Corresponding means in control infants were 0.37 and 0.44 U/ml (p < 0.0001). F1+2, but not TAT, was significantly reduced by AT (p = 0. 004). VEI and (a/A)PO2 were similar in both groups throughout the first week of life. Median days receiving mechanical ventilation were 7.1 (AT) versus 4.8 (placebo), p = 0.0014. Median days receiving supplemental oxygen were 7.9 (AT) versus 5.5 (placebo), p < 0.0001. There were seven (11.5%) deaths in the AT group and three (4.9%) deaths in the placebo group. We conclude that treatment with AT cannot be recommended in premature infants with RDS.

[18F]fluorodeoxyglucose uptake in neonatal acute lung injury measured by positron emission tomography

The objective of this study was to evaluate positron emission tomography (PET) of [18F]fluorodexoyglucose (18FDG) uptake as a measure of neonatal acute lung injury. Inasmuch as intrapulmonary sequestration of neutrophils is a hallmark of acute lung injury, quantification of neutrophil activity using 18FDG may offer a novel, in vivo technique to examine the progression and resolution of this disease. Ten newborn piglets were studied: six received bronchoalveolar lavage followed by 4 h of high pressure ventilation of create acute lung injury. Four healthy piglets served as controls. 18FDG (0.8 mCi/kg; 29.6 MBq) was given i.v. and PET (ECAT 953/31, Siemens) was performed for 90 min. During PET, all animals were sedated, paralyzed, and ventilated to maintain normal blood gases. The time course of radioactivity in lung regions and in plasma was used to calculate the rate constant for the metabolic trapping of 18FDG in tissue according to the method of C. S. Patlak. Median 18FDG influx constants were significantly higher in injured piglets (0.0187 min-1) than in control piglets (0.0052 min-1) (p < 0.01). Moreover, consistent with the 18FDG uptake data, injured piglets had moderate to severe injury on lung histology whereas control piglets had only slight and focal histologic changes. We conclude that PET of 18FDG uptake is an accurate, readily repeatable in vivo measure of neonatal acute lung injury.