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Department of Emergency and Disaster Medicine, Gifu University Graduate School of Medicine, Gifu, JapanLaboratory of Molecular Biology, Department of Biofunctional Analysis, Gifu Pharmaceutical University, Gifu, Japan
Department of Parasitology and Infectious Diseases, Gifu University Graduate School of Medicine, Gifu, JapanDomain of Integrated Life Systems, Center for Highly Advanced Integration of Nano and Life Sciences (G-CHAIN), Gifu University, Gifu, Japan
Neutrophil elastase (NE) is necessary for effective sterilization of phagocytosed bacterial and fungal pathogens; however, NE increases alveolocapillary permeability and induces proinflammatory cytokine production in sepsis-induced acute respiratory distress syndrome. Under septic conditions, the pulmonary endothelial glycocalyx covering on the healthy endothelium surface is injured, but the contribution of NE to this injury remains unknown. Our aim was to examine whether NE-induced pulmonary endothelial injury is associated with endotoxemia. Lipopolysaccharide (LPS; 20 mg/kg) was injected intraperitoneally into 9- to 12-week–old granulocyte colony-stimulating factor knockout (G-CSFKO) mice, which harbor few neutrophils, and littermate control mice; in a second assay, mice were injected with the NE-inhibitor sivelestat (0.2 mg/kg) at 3, 6, 9, and 12 hours after LPS administration. Subsequently, vascular endothelial injury was evaluated through ultrastructural analysis. At 48 hours after LPS injection, survival rate was more than threefold higher among G-CSFKO than control mice, and degradation of both thrombomodulin and syndecan-1 was markedly attenuated in G-CSFKO compared with control mice. Ultrastructural analysis revealed attenuated vascular endothelial injury and clear preservation of the endothelial glycocalyx in G-CSFKO mice. Moreover, after LPS exposure, survival rate was approximately ninefold higher among sivelestat-injected mice than control mice, and sivelestat treatment potently preserved vascular endothelial structures and the endothelial glycocalyx. In conclusion, NE is associated with pulmonary endothelial injury under LPS-induced endotoxemic conditions.
Sepsis is defined as serious organ dysfunction caused by an uncontrollable host response to infection.
Among the multiple organ dysfunctions that occur, the development of acute respiratory distress syndrome (ARDS) is associated with short- and long-term morbidity, mortality, prolonged hospitalization, and high costs.
Furthermore, we recently reported that glycocalyx disruption participated in microvascular endothelial injury that is distinctive of sepsis-induced ARDS; we found that the endothelial glycocalyx in the lung was obviously disrupted by lipopolysaccharide (LPS).
The endothelial glycocalyx plays an important role in regulating microvascular homeostasis by maintaining endothelial permeability and microvascular tone, preserving an oncotic gradient across the endothelial barrier, and modulating adhesion/migration of leukocytes.
However, to our knowledge, no study has investigated whether neutrophils and NE injure the pulmonary endothelial glycocalyx under septic conditions. Therefore, we hypothesized that neutrophils and NE injure the pulmonary endothelial glycocalyx during sepsis, and to test this we evaluated the condition of the pulmonary endothelial glycocalyx after LPS administration in granulocyte colony-stimulating factor knockout (G-CSFKO) mice and in wild-type (WT) mice treated with the NE inhibitor sivelestat.
In Vivo Animal Studies
This study conformed to the Guide for the Care and Use of Laboratory Animals
and was approved by the Institutional Animal Research Committee of Gifu University (Gifu, Japan). Male G-CSFKO mice (B6; 129P2-Csf3tm1Ard/J) were purchased from The Jackson Laboratory (Bar Harbor, ME) and bred and genotyped as described
; 9- to 12-week–old G-CSFKO and littermate control mice were used in this study. After 16 hours of starvation, mice were intraperitoneally administered LPS (20 mg/kg; MilliporeSigma, Burlington, MA), and in certain assays the NE-inhibitor sivelestat (gifted from Ono Pharmaceutical Co., Ltd., Osaka, Japan) was intraperitoneally administrated at 3, 6, 9, and 12 hours after LPS injection. Survival rate was determined at 12, 24, 36, and 48 hours after LPS administration, and surviving mice were sacrificed, and lung specimens were collected.
Serum Preparation and Enzyme-Linked Immunosorbent Assay
Blood samples were collected from the maxillary artery, allowed to clot at room temperature for 2 hours, and centrifuged at 2000 × g for 4°C for 20 minutes. The supernatant was collected as the serum for measuring NE levels by using enzyme-linked immunosorbent assay quantitation kits for mouse NE (E-EL-M0444; Elabscience, Houston, TX).
Flow Cytometric Analysis of Peripheral Neutrophils
Before and at 6 hours after LPS administration, blood samples were collected in the same manner. Samples were prepared as previously described.
Cells were stained with the following antibodies (BioLegend, San Diego, CA) for 5 minutes at room temperature: anti-mouse CD45–peridinin chlorophyll protein complex (clone: 30-F11), anti-mouse CD11b-phycoerythrin (clone: M1/70), anti-mouse Ly6G-fluorescein isothiocyanate (clone: 1A8), and anti-mouse F4/80-Alexa Fluor 647 (clone: BM8). Neutrophils were defined as the CD45+CD11b+Ly6G+F4/80− white blood cell population. Flow cytometry was performed on a BD FACSCalibur instrument (Becton and Dickinson Company, Franklin Lakes, NJ), and data were analyzed with FlowJo software version 10.5.3 (TreeStar LLC, Ashland, OR).
After deparaffinization, sections were cut (4 μm thick), lung tissues were counterstained with hematoxylin and eosin and were scored by a certificated pathologist as follows for neutrophilic infiltration: 1, absent to rare solitary neutrophils; 2, detectable extravasated neutrophils observed as small loose cellular accumulates in one to a few airways and/or alveoli; 3, detectable extravasated neutrophils observed as loose to compact cellular accumulates in multiple to coalescing airway and/or alveoli with some effacement of lung architecture; and 4, detectable extravasated neutrophils observed as compact cellular accumulates effacing most adjacent pulmonary structure. Pulmonary edema was scored as 1, absent; 2, detectable seroproteinaceous fluid in one to a few alveoli; and 3, seroproteinaceous fluid filling alveoli in a multifocal to coalescing pattern in lung.
Lung sections incubated with primary antibodies against the neutrophil surface marker Gr-1 (ab8592; Abcam, Cambridge, UK) and the endothelial-injury surface marker thrombomodulin (ab6980; Abcam). Sections were immunostained with the Vectastain Elite ABC system (Vector Laboratories, Burlingame, CA) as described.
A trained pathologist counted the incidence of thrombomodulin-positive cells in the total endothelial cells. The cell count was performed on five randomly chosen high-power field (HPF) in each section. The number of endothelial cells evaluated was 253 ± 21 cells per lung.
Western Blot Analysis
Tissue lysate preparation and electrophoresing were performed as previously described.
The membranes were probed with antibodies against thrombomodulin (Abcam), syndecan-1 (ab34164; Abcam), or α-tubulin (sc-5546; Santa Cruz Biotechnology, Dallas, TX), and then immunoreactive bands were visualized with enhanced chemiluminescence (GE Healthcare UK Ltd., Little Chalfont, Buckinghamshire, UK), and signal intensities were quantified (as arbitrary units) with the use of ImageJ software version 1.51j8 (NIH, Bethesda, MD; http://imagej.nih.gov/ij). Western blot analysis was performed on four samples from each group.
Electron microscopy analysis of the endothelial glycocalyx was performed as previously described.
Briefly, mice were anesthetized and then perfused with a solution composed of 2% glutaraldehyde, 2% sucrose, 0.1 mol/L sodium cacodylate buffer (pH 7.3), and 2% lanthanum nitrate, at a steady flow-rate of 1 mL/min, through a cannula placed in the left ventricle. After the mice were sacrificed, lung samples were fixed in a solution without glutaraldehyde and then washed in alkaline (0.03 mol/L NaOH) 2% sucrose solution. The freeze-fracture method was used to prepare samples for scanning electron microscopy (S-4800; Hitachi High-Technologies Global, Tokyo, Japan). To prepare samples for transmission electron microscopy (TEM), specimens were embedded in epoxy resin and then ultrathin (90-nm) sections were generated, stained with uranyl acetate and lead citrate, and subjected to TEM analysis (HT-7700, Hitachi High-Technologies Global, Tokyo, Japan). To prepare samples for conventional electron microscopy, 2.5% glutaraldehyde in 0.1 mol/L phosphate buffer (pH 7.4) without lanthanum nitrate was used as the fixative.
Data are presented as means ± SEM. Two-tailed t-test was used for comparing two groups, and survival data were analyzed with the log-rank test; P < 0.05 was considered significant. All calculations were performed with Prism software version 7.02 (GraphPad, La Jolla, CA).
Neutrophils and NE in G-CSFKO Mice under LPS Administration
Analysis of the enzyme-linked immunosorbent assay revealed serum G-CSF deficiency, and flow cytometric analysis revealed neutropenia in G-CSFKO mice (Figure 1, A and B ), confirming the original report.
To produce LPS-induced experimental endotoxemia model mice, 20 mg/kg LPS was injected intraperitoneally to 9- to 12-week–old G-CSFKO and littermate control male mice. At 48 hours after LPS administration, the survival rate of G-CSFKO mice (77%, 24 of 31) was markedly higher (P < 0.05) than that of control WT mice (23%, 6 of 26) (Figure 1C).
To determine pulmonary injury 48 hours after LPS injection, a scoring system was used (Figure 1, D–G).
After LPS administration, levels of neutrophil infiltration and pulmonary edema increased compared with before LPS injection. Conversely, G-CSFKO mice had a significant decrease in neutrophil infiltration and pulmonary edema.
In WT mice, serum NE concentration was increased at 6 hours after LPS administration and then the level decreased gradually; by comparison, serum NE was considerably lower before and at 6 and 12 hours after LPS injection in G-CSFKO mice (Figure 2A). Furthermore, peripheral neutrophil numbers were markedly lower in G-CSFKO mice than WT mice, both before and at 6 hours after LPS administration, the time at which serum NE concentration peaked (Figure 2B), and at 6 hours after LPS injection substantially fewer Gr-1-positive cells were present in the pulmonary tissues of G-CSFKO mice (24 ± 4 cells/HPF) than WT mice (104 ± 4 cells/HPF) (Figure 2, C and D).
Pulmonary Endothelial Injury and Endothelial Glycocalyx Injury Are Attenuated in G-CSFKO Mice
To investigate pulmonary endothelial injury, the localization of thrombomodulin, an endothelial-injury marker, was examined. Immunohistochemical analysis revealed that thrombomodulin was expressed on pulmonary capillaries in both WT and G-CSFKO mice under sham treatment (Figure 3, A and B), but little thrombomodulin was expressed in WT mice after LPS administration. LPS treatment markedly reduced thrombomodulin expression in WT mice but not in G-CSFKO mice (Figure 3, A–D). In accordance with these results, the expression of syndecan-1, an endothelial glycocalyx injury marker, was considerably lower in LPS-treated WT mice than in untreated WT mice (Figure 3, E and F). Conversely, syndecan-1 expression in LPS-treated G-CSFKO mice was not different from that in untreated WT mice (Figure 3, E and F). These results suggest that pulmonary endothelial injury and endothelial glycocalyx injury in LPS-treated G-CSFKO mice were diminished relative to those in LPS-treated WT mice.
Pulmonary Endothelial Injury Are Attenuated in G-CSFKO Mice under Endotoxemic Conditions
The ultrastructure of the endothelium and endothelial glycocalyx was analyzed with electron microscopy. Conventional scanning electron microscopy results showed that pulmonary capillaries were of the continuous type, characterized by the presence of an uninterrupted endothelium and a continuous basal lamina, in untreated WT and G-CSFKO mice (Figure 4A ). Conventional TEM revealed that after LPS injection the endothelial wall became edematous, but the extent was lesser in G-CSFKO mice than in WT mice (Figure 4B).
Scanning electron microscopy and TEM analyses performed with lanthanum revealed an endothelial glycocalyx that featured a moss-like structure on the surface of the vascular endothelium in both WT and G-CSFKO mice under normal conditions; however, after LPS injection, the endothelial glycocalyx was peeled away in WT mice, and this was again attenuated in G-CSFKO mice (Figure 4, C–P).
Sivelestat Increases Survival Rate under Endotoxemia
Next, to determine the contribution of NE in septic mice, 0.2 mg/kg sivelestat was injected intraperitoneally into mice at 3, 6, 9, and 12 hours after LPS administration. Sivelestat forms the acyl complex with an active region of the NE and inhibits NE activity.
At 48 hours after LPS treatment, the survival rate was significantly higher (P < 0.05) among sivelestat-injected mice (82%, 22 of 27) than control mice (9%, 3 of 32) (Figure 5A).
In sivelestat-treated mice, the levels of neutrophil infiltration and pulmonary edema were attenuated compared with untreated mice 48 hours after LPS injection (Figure 5, B–D).
At 24 hours after LPS administration, serum NE concentration was markedly lower (P < 0.05) in sivelestat-treated mice than in control (saline-treated) mice (Figure 5E). Moreover, the Gr-1–positive cell numbers within pulmonary tissues were obviously increased (P < 0.01) in LPS-injected mice (108 ± 4 cells/HPF) compared with that in untreated mice (12 ± 1 cells/HPF), but there were no significant differences between the numbers of peripheral neutrophils in untreated and sivelestat-treated mice (Figure 5F) or the numbers of extravasated Gr-1–positive cells in the lung sections from these mice at 6 hours after LPS administration (Figure 5, G and H).
Sivelestat Attenuates Pulmonary Endothelial Injury under Endotoxemia
Finally, to examine the protective effect of sivelestat treatment against pulmonary endothelial injury and endothelial glycocalyx injury under endotoxemia, thrombomodulin and syndecan-1 expression was analyzed. In sivelestat-treated mice, thrombomodulin was detected on pulmonary capillaries after LPS administration in the same manner as in untreated control mice (Figure 5, I and J); however, quantitative Western blot analyses showed that thrombomodulin and syndecan-1 expression in lung tissue after LPS administration was enhanced in sivelestat-injected mice compared with that in saline-injected mice (Figure 5, K–N). Notably, ultrastructural analysis revealed that after LPS administration, the endothelial surface was less edematous and the endothelial glycocalyx was preserved to a greater extent in sivelestat-treated WT mice than in saline-treated WT mice (Figure 6). These results suggest that pulmonary endothelial injury and endothelial glycocalyx injury were ameliorated in sivelestat-treated mice compared with that in untreated mice after LPS injection.
This study revealed that G-CSFKO mice and sivelestat-treated WT mice showed attenuated pulmonary endothelial glycocalyx injury after LPS administration compared with LPS-treated WT mice. Our findings indicate that the absence of G-CSF and the inhibition of NE critically contribute to glycocalyx protection.
The inner surface of vascular endothelial cells is coated with a glycocalyx of membrane-bound macromolecules, including glycoproteins, hyaluronan, sulfated proteoglycans, and plasma proteins that adhere to this surface matrix.
and in a previous report we presented three-dimensional images that revealed exacerbated pulmonary vascular permeability and degradation of the endothelial glycocalyx on the inner surface of endothelial cells under septic conditions.
We presented ultrastructural imaging of the endothelial glycocalyx in the present study. The endothelial glycocalyx forms a continuous structure that allows for vascular homeostasis. Other than confirming the location of the existence of glycocalyx with the use of a lectin staining technique, the resolution of an optical microscope is not enough to accurately detect the structure. We recently reported that the structures of the endothelial glycocalyx differ greatly among the brain, heart, and lung.
In this report, the pulmonary capillary glycocalyx is constructed of a thin layer; therefore, it is particularly difficult to detect its structure without ultrastructural imaging. Likewise, in a quantitative assessment of protein expression by Western blot analysis, it is impossible to confirm that the endothelial glycocalyx actually exists on the inner surface of the vascular endothelium. Therefore, we believe that the glycocalyx ultrastructural imaging is a helpful tool in the present study.
Sepsis induces alterations of neutrophil deformability and neutrophil entrapment in lung capillaries; neutrophil entrapment is followed by permeability alterations and edema formation.
In G-CSFKO mice, fewer neutrophils are present than in WT mice, even under endotoxemic conditions. Given that endothelial glycocalyx injury after LPS injection was markedly attenuated in G-CSFKO mice compared with that in WT mice, neutrophils might exert harmful effects on the endothelial glycocalyx under septic conditions. However, two explanations can account for the increased survival rate and attenuated endothelial glycocalyx injury in G-CSFKO mice after LPS administration: neutropenia produced a protective effect against endotoxemia in mice; and the amount of neutrophil-secreted mediators was insufficient for causing endothelial injury.
NE was previously reported to play a pivotal role during the pathogenesis of acute lung injury/ARDS,
These reports support our present results. We demonstrated that the pulmonary endothelial glycocalyx injury of sivelestat-treated WT mice was attenuated compared with that of saline-treated WT mice under endotoxemia. Because sivelestat is a competitive antagonist of NE, we suspect that NE concentrations did not differ substantially between sivelestat-treated and untreated mice in this study. However, a previous clinical study indicated that intravenous sivelestat injection exerted no effect on all-cause mortality or ventilator-free days in patients with heterogeneous acute lung injury.
One reason for this loss of effect of NE inhibition on neutrophil function could be related to tissue penetration. If sivelestat can be delivered selectively to injury sites by using new drug-delivery systems in the future, sivelestat administration might emerge as an effective clinical treatment against sepsis.
Sepsis is an extremely complicated disease compared with simple endotoxemia in an experimental model. Because our focus here was to investigate the direct relationship between endothelial glycocalyx injury and neutrophils, we used an endotoxemia model that does not reflect certain typical septic conditions such as bacterial infection, and this represents one limitation of our study. A previous study showed that KO mice that lack neutrophil elastase have impaired host defenses to Gram-negative bacteria,
although the present study showed that neutrophil elastase was deleterious to the host. It is thought that the difference may be due to the presence or absence of bacteria. Neutrophils are one of the first responders of inflammatory cells and work against acute inflammation as a result of bacterial infection. Neutrophil elastase injures the endothelium through the endothelial glycocalyx while also requiring innate immunity to successfully battle a bacterial infection. Therefore, innate immunity may not be sufficient in the bacteremia model because G-CSFKO mice have depleted neutrophils. In other words, the neutrophil has two roles. In the present study, it was not confirmed whether neutrophil elastase altered endothelial glycocalyx directly. Additional studies that use a bacteremia model to further explore the role of endothelial glycocalyx in endothelial injury during sepsis are needed.
Another limitation of this study is that we did not examine inflammatory cytokines such as IL-6; the serum concentration of IL-6 in particular was reported to contribute to the survival rate in sepsis.
Comparative study of glucocorticoids, cyclosporine A, and JTE-607 [(-)-Ethyl-N[3,5-dichloro-2-hydroxy-4-[2-(4-methylpiperazin-1-yl)ethoxy]benzoyl]-L-phenylalaninate dihydrochloride] in a mouse septic shock model.
In addition, the aforementioned report suggested that the endothelial injury was caused by several factors, including superoxides. The present study has not analyzed the influences of superoxides in endotoxemia mice.
NE plays a pivotal role in pulmonary endothelial glycocalyx degradation; thus, inhibition of NE might offer a potential means to protect the endothelial glycocalyx.
K.S. and H.O. wrote the manuscript; G.T., C.T., and N.M. performed transmission electron microscopy (TEM) imaging; K.S., H.O., and K.O. performed scanning electron microscopy (SEM) imaging; C.T. prepared samples for TEM imaging; K.S., H.O., C.T., A.K., H.Y., and R.Z. prepared samples for SEM imaging; K.S., K.M., and H.T. performed immunohistochemical analysis; K.S. and T.F. performed the enzyme-linked immunosorbent assay study; K.S., H.O., A.K., H.Y., R.Z., S.M., A.U., S.S., T.K., T.W., T.Y., H.U., and S.Y. conducted the animal studies; K.S., A.K., S.S., and Y.M. performed the fluorescence-activated cell sorting analysis; S.O. supervised the animal studies; H.O. and G.T. revised and edited the manuscript; all authors read and approved the final manuscript.
van der Poll T.
The Third International Consensus Definitions for Sepsis and Septic Shock (Sepsis-3).
Comparative study of glucocorticoids, cyclosporine A, and JTE-607 [(-)-Ethyl-N[3,5-dichloro-2-hydroxy-4-[2-(4-methylpiperazin-1-yl)ethoxy]benzoyl]-L-phenylalaninate dihydrochloride] in a mouse septic shock model.
Supported in part by the Ministry of Education, Science and Culture of Japan grants-in-aid for scientific research 18K08914 (K.S.), 17K11569 (S.O.), 16H05497 (H.O.), 16K20381 (K.S.), and 15K10973 (T.Y.). Sivelestat, a neutrophil elastase competitive antagonist, was gifted from Ono Pharmaceutical Co., Ltd., Osaka, Japan.