Increased Neutrophil-Subset Associated With Severity/Mortality In ARDS And COVID19-ARDS Expresses The Dual Endothelin-1/VEGFsignal-Peptide Receptor (DEspR): An Actionable Therapeutic Target

Neutrophil-mediated secondary tissue injury underlies acute respiratory distress syndrome (ARDS) and progression to multi-organ-failure (MOF) and death, processes linked to severe COVID19. This ‘innocent bystander’ tissue injury arises in dysregulated hyperinflammatory states from neutrophil functions and neutrophil extracellular traps (NETs) intended to kill pathogens, but injure cells instead, causing MOF. Insufficiency of prior therapeutic approaches suggest need to identify dysregulated neutrophil-subset(s) and induce subset-specific apoptosis critical for neutrophil function-shutdown and clearance. We hypothesized that neutrophils expressing the pro-survival dual endothelin-1/signal peptide receptor, DEspR, are apoptosis-resistant just like DEspR+ cancer cells, hence comprise a consequential pathogenic neutrophil-subset in ARDS and COVID19-ARDS. Here, we report correlation of circulating DEspR+CD11b+ activated neutrophils (DESpR+actNs) and NETosing-neutrophils with severity in ARDS and in COVID19-ARDS, increased DEspR+ neutrophils and monocytes in post-mortem ARDS-patient lung sections, and neutrophil DEspR/ET1 receptor/ligand autocrine loops in severe COVID19. Unlike DEspR[−] neutrophils, ARDS patient DEspR+actNs exhibit apoptosis-resistance, which decreased upon ex vivo treatment with humanized anti-DEspR-IgG4S228P antibody, hu6g8. Ex vivo live-cell imaging of non-human primate DEspR+actNs showed hu6g8 target-engagement, internalization, and induction of apoptosis. Altogether, data differentiate DEspR+actNs as a targetable neutrophil-subset associated with ARDS and COVID19-ARDS severity, and suggest DEspR-inhibition as a potential therapeutic paradigm.


Introduction
Acute respiratory distress syndrome (ARDS) and progression to multi-organ failure (MOF) comprise a pathological spectrum of secondary 'bystander' tissue injury arising when one's in ammatory response to an inciting 'primary injury' -be it infectious or non-infectious -becomes dysregulated and excessive. 1 Stopping this feed-forward destructive in ammation in ARDS and MOF remains an important unmet need, as there is no FDA-approved pharmacotherapy able to reduce the high mortality in ARDS from MOF. 2 The lethality of destructive in ammation is highlighted by the COVID19 pandemic as progression to ARDS and multi-organ failure are accelerated in COVID19, and comprise the major cause of death in severe COVID- 19. 3 Notably, destructive in ammation often exhibits a feed-forward progression to multi-organ failure and death even if the inciting primary injury is resolving, if not nite, such as decreasing bacterial or viral load, one-time sterile trauma. This supports the scienti c rationale that effective targeted inhibitors of destructive in ammation can be agnostic of the different primary injury etiologies in ARDS -be it bacterial infection at any site, SARS CoV2, or sterile injury.
As an approach to address the pressing need for novel therapies for ARDS and COVID19-ARDS, we reasoned that comparative analysis and identi cation of pathogenic commonalities in ARDS and COVID19-ARDS could help identify novel therapies. A priori, this pinpoints cytokine storm inhibitors common to both ARDS and COVID10-ARDS. However, the redundancies among cytokine-mediated pathways, and the partial-only reduction in mortality by an IL-6 receptor inhibitor in severe COVID19, 4 suggest that targeted inhibition of the cellular effectors of the cytokine storm will be required. Among cytokine effector cells, neutrophils have long been implicated in ARDS and progression to multi-organ failure. 1,5 Activated neutrophils play key roles in multi-organ dysfunction and progression to failure 6 through neutrophil-mediated microvascular endothelial injury, capillary permeability, 7 and neutrophilextracellular trap (NET)-associated endothelial and lung epithelial injury. 8,9 The central role of neutrophils is supported by the association of increased neutrophil-lymphocyte ratios (NLR) with worse ARDS prognosis 10 as well as with more severe COVID19 and poor prognosis 11 .
More recently, comparative single cell RNA-sequencing (scRNA-seq) analysis of mild and critically ill COVID19-patients, and non-infected healthy controls, demonstrated molecular evidence supporting the central role of neutrophils, 12 concordant with other reports [13][14][15] . However, inhibiting neutrophils effectively to mitigate neutrophil-driven secondary tissue injury safely and effectively has been elusive despite preclinical e cacy in animal models of acute lung injury. 5 The cumulative low translatability, due most likely to species differences in neutrophil biology and to multifactorial complexities in ARDS pathogenesis not present in preclinical models of acute lung injury, provides scienti c basis for ex vivo analysis of ARDS patient whole blood samples. The ex vivo study of neutrophils within the pathobiological context of other immune cells in the circulation during progression of ARDS towards multi-organ failure or resolution is imperative in the validation of putative therapeutic targets.
Based on insights from the study of DEspR + cancer stem-like cells (CSCs) exhibiting aberrant apoptosisresistance associated with myeloid cell leukemia (Mcl1) levels, a key apoptosis-evasion protein in cancer, 16 we reasoned that DEspR + neutrophils would also have survival advantages as Mcl1 levels correlate with neutrophil survival. 17 Since neutrophil apoptosis is required for efferocytosis, function shutdown and active resolution of in ammation 18 , longer neutrophil survival increases risk for dysregulation and subsequent excessive or hyper-in ammation. Additionally, given that endothelin-1 (ET-1) levels, one of two DEspR ligands, 19 are elevated in ARDS, 20 and since ET1 is known to enhance neutrophil activation and functionality, 21 ET1-mediated DEspR activation could play key pathogenic role(s) in neutrophil-mediated secondary tissue injury in ARDS.
We therefore tested the unifying hypothesis that DEspR + neutrophils comprise an activated neutrophil subset with pathogenic survival advantage over other DESpR [-] activated neutrophils, and whose cumulative increase drives neutrophil-mediated secondary tissue injury in ARDS and COVID19-ARDS leading to multi-organ failure. Here, we studied 1) whether DEspR + neutrophils comprise a neutrophilsubset associated with ARDS clinical severity, mortality, and reported biomarkers of ARDS-severity, as well as, with higher levels of circulating neutrophil extracellular trap formation (NETosis), 2) whether identi cation of the DEspR + neutrophil subset is reproducible in different research labs and concordant with scRNA-seq ndings in severe COVID19, and 3) whether DEspR + neutrophils can be safely inhibited to restore neutrophil apoptosis as a targetable therapeutic neutrophil subset.

Results
DEspR + CD11b + human neutrophil subset increased by TLR4 activation To determine DEspR-expression in human neutrophils as a potential therapeutic target, we analyzed protein levels by immuno uorescence, western blot and ow cytometry analyses. First, we performed immuno uorescence staining of normal human volunteer (NHV) neutrophils that have survived ex vivo for 24-hours. We used a humanized anti-DEspR antibody, cross reactive to human, monkey and rodent DEspR, with a hinge-stabilized [S228P]IgG4 backbone, hu6g8, developed by us and validated for detection of cancer cells expressing DEspR by immuno uorescence, western blot analysis and ow cytometry, and for in vitro and in vivo e cacy in inducing apoptosis in DEspR + tumor cells. 16 Direct immuno uorescence of 24-hrs old surviving neutrophils detected DEspR expression in multiple compartments: neutrophil nuclei, cell membrane and cytoplasm ( Fig. 1-A), consistent with membrane-cytoplasmic-nuclear shuttling observed in cancer cells. 16 Immuno uorescence staining also detected DEspR + neutrophils with extruded DNA and still intact cell membranes suggesting DEspR+ "vital NETosis" 22,23 , as well as DEspR[-] neutrophils (Fig. 1A). Notably, majority of 24-hour old neutrophils and NETosing neutrophils were DEspR + in these ex vivo experimental conditions (Fig. 1B). Next, parallel western blot analysis of whole cell protein isolates detected the expected size DEspR protein in the NHV neutrophils, thus con rming DEspR + expression. Western blot analysis also detected a larger DEspR protein in human kidney (Fig. 1C, Fig. S1A) due to PNGase-sensitive glycosylation, as shown previously in cancer cells. 24 To further assess DEspR + expression on neutrophils in preparation for ARDS patient studies, we performed ow cytometry (FCM) analysis with double-immunostaining for cell-surface co-expression of DEspR and CD11b. We selected CD11b as a marker of activated neutrophils as CD11b mediates neutrophil-complement system crosstalk, and since CD11b + neutrophils are increased in ARDS patient peripheral blood and in broncho-alveolar uid. 25 Flow cytometry analysis of normal human volunteer (NHV) EDTA-anticoagulated whole blood samples detected minimal, if any, DEspR + expression on the cell surface of intact CD11b + activated neutrophils (actNs), monocytes and lymphocytes in baseline conditions, and no expression on red blood cells (RBCs) (Fig. 1D). DEspR expression was increased by standard RBC-lysis step done before antibody binding ( Supplementary Fig. S1-B), likely in response to damage associated molecular patterns (DAMPs) released during RBC hemolysis. 26 Notably, in response to cell stress expected in any ex vivo condition, DEspR expression also increased with time from blood sampling > 1-hour be it at 4 °C or at 37 °C (Supplementary Fig. S1-C).
Relevant to ex vivo analysis, these observations indicate that EDTA-anticoagulated blood exhibit less susceptibility to ex vivo experimental changes with increases in time and temperature (Fig. 1E, Fig. S1-C). For quantitative ex vivo ARDS patient sample analysis, we therefore used only EDTA anti-coagulated whole blood processed within 1-hour from sampling from hereon, in order to minimize confounders that increase DEspR-expression ex vivo. This will avoid overestimating actual circulating levels in patient samples and false positives.

DEspR + neutrophils and monocytes/macrophages in ARDS patient lung sections
To demonstrate pathological basis to study DEspR + immune cells in ARDS patient whole blood samples, immunohistochemistry analyses of post-mortem serial lung sections from patients with ARDS (n = 8) in regions of diffuse alveolar damage ( Fig. 2A-2I) as well as, in areas of acute alveolar injury ( Fig. 2J-2K) were performed. Using an anti-DEspR mouse-recombinant mAb of hu6g8, hu6g8-m, immunohistochemistry with DAB chromogen (IHC-DAB) was optimized and detected DEspR + neutrophils in intrabronchiolar exudate, along with some DEspR(-) neutrophils with characteristic polylobulated nuclei  (Fig. 2J), immunohistochemistry detected DEspR + neutrophils and monocytes in the intravascular, parenchymal and intra-alveolar spaces (Fig. 2K). These observations validate measuring peripheral DEspR + CD11b + neutrophil-and monocyte-subset levels by ow cytometry as a window into projected pulmonary levels in ARDS or COVID19-ARDS.

DEspR core-expression network increased in COVID19 neutrophils
To determine pathogenic basis to study DEspR + neutrophils in COVID19-ARDS, we analyzed single cell RNA-sequencing (scRNA-seq) database of healthy control (n = 5), mild (n = 8) and severe (n = 11) COVID19 patient samples. Since DEspR's ADAR1 RNA-edited transcript 16 is not discernable in scRNA-sequencing limited to 300 nucleotides from the poly-A sequence of each transcript to ascertain speci city and equivalent representation, we focused on DEspR's essential expression-network comprised of DEspR's modulators, ligands, and bioeffect marker represented in the scRNA-seq database of immune cells and epithelial cells in nasopharyngeal and broncho-lavage uid samples from COVID19 subjects. 12 Comparative scRNA-seq analysis showed that positive modulators of DEspR-transcription (TLR4 and Hif1-α), DEspR RNA-editing for translation (ADAR-1 RNA-editase), DEspR cell-surface mobilization (TLR4), TLR4-endogenous activators (alarmins S100A8/S100A9) and bioeffect prosurvival marker (Mcl1) are predominantly expressed in neutrophils, along with DEspR's two ligands, endothelin-1 (ET1 or EDN1), and the signal peptide in VEGFA-propeptide (spVEGF) (Fig. 3A). Interestingly, neutrophil expression of the endothelin converting enzyme (ECE1), required for release of ET1 from its pro-peptide ( Notably, expression levels of all 9 genes representing DEspR's expression and functional network are signi cantly increased in COVID19 compared to healthy controls (Fig. 3B), and in neutrophils compared to monocytes-macrophages in nasopharyngeal and broncho-lavage COVID19 samples (Fig. 3C). The basis to study neutrophils is further supported by scRNA-seq documentation of increased expression of receptors to cytokines increased in ARDS 32 and/or in COVID19-ARDS 33 , such as: IL-6, IL-8, IL-1β, IL-18, and TNF-α 34 ( Supplementary Fig. S2G). These observations support the role of neutrophils as effectors of the "cytokine storm" leading to destructive in ammation manifesting as ARDS and/or multi-organ failure, be it COVID19 3 or bacterial pneumonia. 32 Increased DEspR + CD11b + neutrophil-subset in ARDS and COVID19-ARDS To study the potential clinical relevance of the DEspR + neutrophil-subset, we performed a prospective pilot observational study of consented patients diagnosed with ARDS based on the Berlin De nition, regardless of acute disease etiologies (sepsis, pneumonia, cardiac arrest, surgery) and comorbidities (Supplementary Table S1 for demographics). First, we ascertained DEspR-speci c immunotyping of whole blood samples from ARDS patients by validating our gating strategy for ow cytometry (Supplementary Similarly, we also observed differential DEspR + expression in COVID19-ARDS patients in the ICU requiring mechanical ventilatory support (Supplementary Table S1 for demographics). For safety reasons, we studied disinfected (4% paraformaldehyde or PFA) whole blood samples from COVID19-ARDS patients, and performed FCM analysis within 1 hour from blood draw. FCM analysis representing extremes of the clinical severity spectrum also revealed increased total #DEspR + N-counts and monocytes (M)-counts in a patient with severe COVID19-ARDS requiring 61days intensive care unit (ICU)-care (Fig. 4D), compared with a patient with milder COVID19-ARDS discharged after 6 days in the ICU (Fig. 4E).
Observing marked differential levels at the polar ends of the clinical spectrum of severity, we next strati ed mortality outcomes in ARDS (Fig. 4F) and COVID19-ARDS ( Fig. 4G) patients by respective levels of DEspR + CD11b + neutrophil-counts (thousand K/µL whole blood). Per level of DEspR + N-counts, we graphed the length of stay in the ICU until discharge or death, and observed a parallel trend of increased DEspR + N-counts with mortality in the graphs. Aside from corroborating the identi cation of the DEspR + CD11b + neutrophil subset, these observations provide scienti c basis for quantitative analysis of emerging differential modulation between survivors and non-survivors in ARDS and COVID19-ARDS.
Similarly, in COVID19-ARDS pilot group, Spearman rank correlation matrix analysis also showed signi cant, strong, negative correlation of DEspR + CD11b + neutrophil-counts with ICU-free days at day 28 from ARDS diagnosis, and with ARDS severity S/F ratio (Fig. 5F, Table 2). Interestingly, the sum of %DEspR+[monocytes and neutrophils] correlated with ICU-free days at day 28 with higher Spearman rho correlation coe cient, signi cance and power than either alone (Table 2). This observation suggests DEspR + CD11b+[neutrophil-monocyte] intravascular-interactions likely involved in systemic tissue injury in ARDS progression to multi-organ failure, as observed in acute glomerular injury. 35 Notably, the neutrophil lymphocyte ratio (NLR) showed signi cant albeit less robust correlation with ICUfree days at day 28 in COVID19-ARDs. Comparative analysis of COVID19-ARDS survivors and nonsurvivors showed signi cant differences in means with large effect size for both NLR (Fig. 5G) and DEspR + CD11b + neutrophil-counts (Fig. 5H). A retrospective analysis of COVID19-ARDS patients requiring ventilatory support at BMC corroborate signi cant differences in NLR (Fig. S4A-S4D), concordant with reports that increased NLR is an independent preidcotr of mortality in ARDS and COVID19. 36 In contrast, C-reactive protein did not show correlation ( Supplementary Fig. S4E to S4H).
In COVID19-ARDS patients, the number (#) of DEspR + CD11b + NETosing neutrophils correlated strongly with three clinical measures: 1] outcome at day-28 (ICU-free days at day-28), 2] degree of hypoxemia (SpO2/FiO2 or S/F ratio), and 3] severity of multi-organ failure (SOFA score at end of ICU-stay) (Fig. 5F, Table 2). Signi cant differences in means between survivors and non-survivors was also detected (Fig.   5I). This contrasts D-dimer levels obtained during ICU which did not exhibit signi cant difference in means be it peak levels or average levels while patients were in the ICU ( Supplementary Fig. S4-I to S4-L). Notably, scRNA-seq pro le for PADI4 linked to suicidal NETosis is minimally expressed in neutrophils with only 1.4% of neutrophils expressing PADI4 > 2X fold (Fig. S2E). Having detected DEspR + CD11b + cytoplasts with NETosing neutrophils on immunostained whole blood cytology slides (Fig. 6A, middle panel), we analyzed levels of cytoplasts in the circulation by ow cytometry, since cytoplasts are released during suicidal NETosis. 39 Flow cytometry analysis detected elevated DEspR + CD11b + cytoplast levels in ARDS subjects (Fig. 5A, Table 2) and in COVID19-ARDS (Fig.   6H), however, no signi cant correlations were observed (Fig. 5A, Table 2). Elevated DEspR + cytoplasts and DEspR + CD11b + neutrophils were detected in an independent pilot study of patients with sepsis, and sepsis-ARDS in contrast to none in healthy donors ( Supplementary Fig. S5A-S5D) using a different methodology wherein whole blood samples were enriched for white blood cells via inertial micro uidic separation from RBCs 40 ( Supplementary Fig. S5D-S5F).

Ex vivo DEspR-inhibition induces apoptosis in ARDS-patient and NHP neutrophils
To determine targetability and bioeffects of DEspR-inhibition, we analyzed bioeffects of ex vivo treatment of ARDS patient whole blood with humanized anti-DEspR IgG4 S228P antibody, hu6g8, for 17-20 hours overnight with rotation to prevent aggregation. Controls comprised of patient-speci c mock-treated control and baseline control (Fig. 7A). Comparative FCM-analysis showed that compared to baseline levels and To further test that DEspR-inhibition induces apoptosis in DEspR + CD11b + neutrophils, we performed live cell imaging of non-human primate (NHP) neutrophils exposed to uorescently labeled hu6g8-AF568 or uorescently labeled human IgG4-AF568 isotype control for 20 minutes at 4°C to avoid non-speci c endocytosis. We selected NHPs as model system as NHP-to-human neutrophils are more similar than human-to-mouse neutrophils. 41 With this set-up, we rst show suitability of Rhesus macaque NHPs as model for study by presence of circulating DEspR + CD11b + neutrophils detected via ow cytometry using identical conditions to ex vivo analysis of ARDS patient samples ( Supplementary Fig. S6A-H).
Next, we tested for target engagement, internalization, and induction of characteristic apoptosis cellbudding bioeffects by confocal live cell imaging of NHP neutrophils. We rst exposed NHP neutrophils to either AF568-labeled anti-DEspR hu6g8 antibody (treatment) or IgG4-isotype (mock-treatment) control for 20 minutes at 4 °C to avoid non-speci c endocytosis. After removing excess unbound antibody, 24-hour live cell imaging was initiated with video-recordings. At t-45 minutes, live-cell images detected target engagement and internalization of hu6g8-AF568 antibody ( Supplementary Fig. S6-I) but not in the isotype control. Speci cities were con rmed throughout with representative t-12 hr timepoint images (Fig. 7E, Fig.   7F). Live cell imaging showed more apoptotic cell budding changes in NHP-neutrophils with internalized hu6g8. In the isotype control, apoptotic cell budding was detected concordant with neutrophil constitutive apoptosis. SytoxGreen impermeable dye uptake marked loss of cell viability. Both cell death indicators increased with time.
At the 12-hour midpoint, quantitation of apoptotic cell changes and SytoxGreen-positive non-viablity were done. Representative 12-hr live cell images show hu6g8-target engagement, internalization and apoptotic cell budding in DEspR + neutrophils (Fig. 7E) compared to minimal uptake of isotype-AF568 control by NHP neutrophils (Fig. 7F). Quantitative analysis of 18 high power elds (HPFs) with 20-50 cells/HPF representing three independent experimental elds of view showed that hu6g8 induced apoptosis in DEspR + neutrophils signi cantly greater than levels seen in isotype-treated control NHP cells (Fig. 7G).
Importantly, hu6g8 induced apoptosis greater than constitutive apoptosis occurring in DEspR[-] cells unaffected by hu6g8 treatment (Fig. 7H). Interestingly, loss-of-viability staining by Sytox Green occurred in neutrophils not undergoing apoptotic cell budding, and was also slightly greater in hu6g8-treated neutrophils compared with isotype mock-Tx controls (Fig. 7H), indicating that DEspR-inhibition may facilitate other programmed cell-death in neutrophils via decreased CIAP2 as observed in anti-DEspR mAbtreated pancreatic cancer stem cells. 42

Discussion
Data showing the correlation of DEspR + CD11b + activated neutrophil levels with severity and mortality in both ARDS and COVID19-ARDS delineate a pathogenically relevant neutrophil-subset, DEspR+ "rogue" neutrophil-subset, that exhibits longer survival than DEspR[-] neutrophils and a predisposition to NETosis in circulation. Notably, these neutrophil phenotypes -delayed apoptosis and pro-NETosis -are concordant with previous observations of pathogenic neutrophil phenotypes in ARDS patients. 1,43 The subset-speci c expression pattern of DEspR and its co-expression with CD11b but not in all CD11b + activated neutrophils, are concordant with constraints arising from the need for concurrent expression of DEspR's multiple modulators: Hif1α and TLR4 for DEspR transcription, ADAR1 for RNA-editing of the DEspR transcript for translation, and activated TLR4 for mobilization to the cell surface. Among the modulators, expression of ADAR1 could be the 'gate-keeper' of DEspR + neutrophil subsets as ADAR1 is detected in 29.2% of neutrophils, in contrast to higher levels of neutrophil expression for Hif1a (54%) in critically ill COVID19 broncho-lavage uid and nasopharyngeal sample neutrophils. This observation is supported by increased ADAR1 localized to neutrophils in ARDS patient lung Sect. 44 Presence in pulmonary vascular lumen and in lung areas with diffuse alveolar damage (DAD) and acute alveolar injury, pathological hallmarks in ARDS, validate informativeness of ow cytometry analysis of DEspR + CD11b + neutrophil-subset levels in whole blood samples. Notably, the cell-surface mobilization of DEspR upon TLR4-activation ties the DEspR + neutrophil subset with neutrophil TLR4-activation upon docking of SARS-CoV2 spike protein with neutrophil TLR4 at higher a nity in silico than with the spike protein receptor, ACE2 45 . This provides a pathogenic mechanism for direct, hence early activation of TLR4 + neutrophils without need for neutrophilic infection as observed. 12 Additionally, direct TLR4-activation and CD11b + induction in neutrophils by serum S100A8/A9 alarmins, the prototype DAMPS 46 found to be elevated in ARDS 47 and COVID19-ARDS 48 , further ties the DEspR + CD11b + neutrophil-subset to ARDS and COVID19-ARDS. Functionally, as alarmins and TLR4-activation provide a self-sustaining neutrophil activation loop, activated TLR4-induced DEspR upregulation provides a mechanism for delayed apoptosis. This combination of functionalities provides a putative mechanism for feed-forward progression of neutrophil-mediated secondary tissue injury as seen in severe ARDS and COVID19-ARDS, and would be concordant with the observed association of increased DEspR + CD11b + neutrophils with severity and mortality in ARDS and COVID19-ARDS.
The detection by direct visualization of neutrophils with characteristic extruded DNA and retained cell membrane 49  Intuitively, the observed DNA-strand and interconnections among NETosing neutrophils with their extruded-but still attached DNA fragments could be projected to contribute intravascular biophysical impedance to vascular ow and concomitant low-ow ischemia, thus predisposing to multiorgan dysfunction, as well as microvascular occlusion with or without micro-thromboses. These observations provide insight into why low-ow or micro-ischemic events in different organs persist despite pharmacological thromboprophylaxis or anti-thrombotic treatment. 52 Additionally, the microvascular ow impedance from DNA-strand and/or from vital-NETosing neutrophil interconnections provide pathogenic concordance with reported severe hypoxemia despite high lung compliance deduced to be due to ventilation/circulation ow mismatch. 53 More importantly, the detection of DEspR + expression on vital NETosing neutrophils provides an actionable therapeutic target to pre-empt NETosing neutrophils in the circulation.
Data showing that DEspR-inhibition leads to apoptosis in ARDS patient samples and NHP samples supports DEspR as an actionable therapeutic target to induce apoptosis in the dysregulated, apoptosisresistant neutrophil-subset 54 implicated in progressive secondary tissue injury leading to ARDS and/or multi-organ failure 55 . Coupled with strong correlation with multiple clinical severity measures, targetedinhibition of DEspR with endpoint induction of neutrophils apoptosis comprises a much-needed therapeutic paradigm with potential advantages. First, data showing that anti-DEspR hu6g8 induced neutrophil apoptosis and prevented terminal complex of complement sC5b9 increase, suggests functionshutdown of neutrophil-complement system reciprocal-interactions, 56 and possibly also NETs-induced complement activation. 57 Second, since neutrophil apoptosis is required for neutrophil function shutdown, clearance and neutrophil-initiation of resolution, 58 restoration of neutrophil constitutive apoptosis upon DEspR-inhibition could then be expected to promote active resolution of dysregulated hyperin ammation.
While more studies are needed to elucidate mechanisms, the emerging mode-of-action of DEspR inhibition presents a valid pathway to meet therapeutic goals required to stop neutrophil-mediated tissue injury 1 by inducing neutrophil apoptosis for function shutdown. Additionally, with 99.1% of neutrophils in a representative COVID19 scRNA-pro le not expressing CD47 "don't eat me signal," induction of neutrophil apoptosis by anti-DEspR antibody can be expected to proceed to efferocytosis. Similarly, with 98.6% of neutrophils not expressing PADI4, anti-DEspR therapy can play a pivotal role in decreasing NETosismediated pathogenesis in ARDS and COVID19-ARDS. Third, non-inhibition of DESpR[-] CD11b + activated neutrophil subsets to ght infections and initiate active resolution mechanisms elucidates an inherent safety pro le.
Lastly, consideration for potential side effects, especially in the context of acute kidney injury as part of multi-organ failure in ARDS, highlights known DEspR + expression in human medullary tubular epithelial cells. In the presence of immunoglobinuria, anti-DEspR antibody passing through the glomerulus could present potential on-target tubular epithelial effects, but unlikely as antibody functionality will be altered in the increasingly acidic and hyperosmotic milieu in the kidney medullary lumen. Altogether, data identify the DEspR + CD11b + neutrophil subset as an actionable therapeutic target whose targeted inhibition has the potential to slow progression of multi-organ failure in ARDS and COVID19-ARDS, with minimal, if any, projected side effects. These data and insights provide foundational basis for further study. For disinfected COVID19 blood samples (2%PFA-xed), samples were washed 3 times with 8 volumes of HBSS + 2% FBS to remove residual xative prior to processing for ow cytometry as described above.
Each test sample run in duplicates.
At BWH, EDTA-anticoagulated whole blood samples were processed 2-3 hours from sampling and white blood cells were separated from RBCs via Inertial Micro uidic Separation validated previously for neutrophil characterization. 40  At Fraunhofer ITEM, heparinized whole blood was stored on ice until processing and used within 1-hour after collection. Whole blood (100 µl) samples were washed with 1 ml of ice cold assay buffer, and cells were incubated in 100 µl of assay buffer containing bacterial endotoxin lipopolysaccharide LPS (100 ng/ml; Escherichia coli serotype 0111:B4) or assay buffer as control for 1 h at 37°C. The reaction was then stopped, cells washed, then resuspended and cells were stained with hu6g8-PE (10 µg/ml) and CD11b-FITC for 30 min on ice under constant stirring in the dark. Cells were washed to remove unbound antibodies, xed for 10 min at 4°C, followed by RBC lysis. The cell pellet was resuspended in 250 µl ow cytometry buffer and was analyzed within 2 hours using a Beckman Coulter Navios 3L 10C ow cytometer and data analyzed using Beckman Coulter Kaluza 2.1 Software.
At BUSM, 100µl EDTA-anticoagulated whole blood samples (n = 6) were exposed to 75-100 µg/ml LPS at 37 °C x 1-hour, then subjected to FCM analysis as described above. To compare the levels of mitochondrial to nuclear DNA in human plasma samples we used the NovaQUANT™ Human Mitochondrial to Nuclear DNA Ratio Kit (SIGMA-Aldrich cat# 72620-1KIT) as per manufacturer's instructions. The kit measures the mtDNA copy number to that of nuclear DNA by Real-Time PCR of speci c mitochondrial and nuclear genes. Plasma DNA was isolated from 200 uL of plasma using the Quick-cfDNA Serum & Plasma Kit (Zymo Research, cat# D4076) as per manufacturer's instruction.
Immuno uorescence staining of NETosing neutrophils Cytology slides were prepared by capillary action from EDTA anticoagulated whole blood (10 µL) samples on a Superfrost Plus Microscope slide (Fisher Scienti c, cat# 12-550-15) within 1-hour from blood sampling. Cytology smears were air dried for 10 minutes then xed with 100 % Methanol (chilled to -20 C) for 10 min. Fixed slides were stored dry in -20°C freezer for future immunostaining.
Immuno uorescence (IF)-staining to detect NETosing neutrophils was done as described previously. 59 Fixed Cell Imaging of Blood Smears (NETosing quanti cation) Immuno uorescence imaging was performed as contract research service at Nikon Imaging Laboratory Ex-vivo anti-DEspR treatment of ARDS patient blood samples One ml of freshly obtained blood samples were incubated overnight at 37 o C with or without anti-DEspR mAb (hu6g8 at 100 µg/ml). After incubation half of the samples were subjected to FACS analysis as described above and the other half was processed for plasma isolation. Plasma MPO and C5b-9 levels were determined with corresponding ELISA kits as described above.
Quantitation of apoptotic cell changes and viability after anti-DEspR treatment of non-human primate (NHP) DEspR + CD11b + neutrophils by live cell imaging See Supplementary Methods for details. Brie y, whole blood from Rhesus macaque NHP provided by Biomere (Biomere Biomedical Research Models, Inc., Worcester MA) was analyzed by ow cytometry to determine the number of DEspR + CD11b + activated neutrophils. White blood cells (WBCs) were then obtained, washed and resuspended in Hank's Balanced Salt Solution (HBSS) + 2% Fetal Bovine Serum (FBS). WBCs were counted, divided into aliquots and incubated with 10 µg/ml Alexa Fluor 568-conjugated hu6g8 antibody or Alexa Fluor 568-conjugated IgG4 isotype antibody for 20 minutes at 4°C. Cells were washed to remove unbound antibody, then concentrated at to approximately 10 8 cells/mL, then loaded into imaging device. Live cell imaging was performed using a micro uidic chip with three parallel conjoined micro uidic channels, and a confocal microscope (Ti2-E microscope equipped with Nikon A1R HD25 point scanner and 60X Plan Apo λ Oil objective) housed within a temperature and CO 2 -controlled incubator. Images were then acquired every minute for the rst 9 hours, and then every 5 minutes for 15 hours thereafter, for a total of 24 hours