New Insights into Clinical and Mechanistic Heterogeneity of the Acute Respiratory Distress Syndrome: Summary of the Aspen Lung Conference 2021

Lung endothelium, tau, and amyloids in health and disease

Lung endothelia in the arteries, capillaries, and veins are heterogeneous in structure and function. Lung capillaries in particular represent a unique vascular niche, with a thin yet highly restrictive alveolar-capillary barrier that optimizes gas exchange. Capillary endothelium surveys the blood while simultaneously interpreting cues initiated within the alveolus and communicated via immediately adjacent type I and type II epithelial cells, fibroblasts, and pericytes. This cell-cell communication is necessary to coordinate the immune response to lower respiratory tract infection. Recent discoveries identify an important role for the microtubule-associated protein tau that is expressed in lung capillary endothelia in the host-pathogen interaction. This endothelial tau stabilizes microtubules necessary for barrier integrity, yet infection drives production of cytotoxic tau variants that are released into the airways and circulation, where they contribute to end-organ dysfunction. Similarly, beta-amyloid is produced during infection. Beta-amyloid has antimicrobial activity, but during infection it can acquire cytotoxic activity that is deleterious to the host. The production and function of these cytotoxic tau and amyloid variants are the subject of this review. Lung-derived cytotoxic tau and amyloid variants are a recently discovered mechanism of end-organ dysfunction, including neurocognitive dysfunction, during and in the aftermath of infection.

Pulmonary Surfactant in Adult ARDS: Current Perspectives and Future Directions

Acute respiratory distress syndrome (ARDS) is a major cause of hypoxemic respiratory failure in adults, leading to the requirement for mechanical ventilation and poorer outcomes. Dysregulated surfactant metabolism and function are characteristic of ARDS. A combination of alveolar epithelial damage leading to altered surfactant synthesis, secretion, and breakdown with increased functional inhibition from overt alveolar inflammation contributes to the clinical features of poor alveolar compliance and alveolar collapse. Quantitative and qualitative alterations in the bronchoalveolar lavage and tracheal aspirate surfactant composition contribute to ARDS pathogenesis. Compared to neonatal respiratory distress syndrome (nRDS), replacement studies of exogenous surfactants in adult ARDS suggest no survival benefit. However, these studies are limited by disease heterogeneity, variations in surfactant preparations, doses, and delivery methods. More importantly, the lack of mechanistic understanding of the exact reasons for dysregulated surfactant remains a significant issue. Moreover, studies suggest an extremely short half-life of replaced surfactant, implying increased catabolism. Refining surfactant preparations and delivery methods with additional co-interventions to counteract surfactant inhibition and degradation has the potential to enhance the biophysical characteristics of surfactant in vivo.

Clinical Sepsis Phenotypes in Critically Ill Patients

Sepsis, defined as the life-threatening dysregulated host response to an infection leading to organ dysfunction, is considered as one of the leading causes of mortality worldwide, especially in intensive care units (ICU). Moreover, sepsis remains an enigmatic clinical syndrome, with complex pathophysiology incompletely understood and a great heterogeneity both in terms of clinical expression, patient response to currently available therapeutic interventions and outcomes. This heterogeneity proves to be a major obstacle in our quest to deliver improved treatment in septic critical care patients; thus, identification of clinical phenotypes is absolutely necessary. Although this might be seen as an extremely difficult task, nowadays, artificial intelligence and machine learning techniques can be recruited to quantify similarities between individuals within sepsis population and differentiate them into distinct phenotypes regarding not only temperature, hemodynamics or type of organ dysfunction, but also fluid status/responsiveness, trajectories in ICU and outcome. Hopefully, we will eventually manage to determine both the subgroup of septic patients that will benefit from a therapeutic intervention and the correct timing of applying the intervention during the disease process.

Gamma secretase activating protein promotes end-organ dysfunction after bacterial pneumonia

Pneumonia elicits the production of cytotoxic beta amyloid (Aβ) that contributes to end-organ dysfunction, yet the mechanism(s) linking infection to activation of the amyloidogenic pathway that produces cytotoxic Aβ is unknown. Here, we tested the hypothesis that gamma-secretase activating protein (GSAP), which contributes to the amyloidogenic pathway in the brain, promotes end-organ dysfunction following bacterial pneumonia. First-in-kind Gsap knockout rats were generated. Wild-type and knockout rats possessed similar body weights, organ weights, circulating blood cell counts, arterial blood gases, and cardiac indices at baseline. Intratracheal Pseudomonas aeruginosa infection caused acute lung injury and a hyperdynamic circulatory state. Whereas infection led to arterial hypoxemia in wild-type rats, the alveolar-capillary barrier integrity was preserved in Gsap knockout rats. Infection potentiated myocardial infarction following ischemia-reperfusion injury, and this potentiation was abolished in knockout rats. In the hippocampus, GSAP contributed to both pre- and postsynaptic neurotransmission, increasing the presynaptic action potential recruitment, decreasing neurotransmitter release probability, decreasing the postsynaptic response, and preventing postsynaptic hyperexcitability, resulting in greater early long-term potentiation but reduced late long-term potentiation. Infection abolished early and late long-term potentiation in wild-type rats, whereas the late long-term potentiation was partially preserved in Gsap knockout rats. Furthermore, hippocampi from knockout rats, and both the wild-type and knockout rats following infection, exhibited a GSAP-dependent increase in neurotransmitter release probability and postsynaptic hyperexcitability. These results elucidate an unappreciated role for GSAP in innate immunity and highlight the contribution of GSAP to end-organ dysfunction during infection.NEW & NOTEWORTHY Pneumonia is a common cause of end-organ dysfunction, both during and in the aftermath of infection. In particular, pneumonia is a common cause of lung injury, increased risk of myocardial infarction, and neurocognitive dysfunction, although the mechanisms responsible for such increased risk are unknown. Here, we reveal that gamma-secretase activating protein, which contributes to the amyloidogenic pathway, is important for end-organ dysfunction following infection.

Ficolin A knockout alleviates sepsis-induced severe lung injury in mice by restoring gut Akkermansia to inhibit S100A4/STAT3 pathway

Acute lung injury (ALI) is a life-threatening disease with high morbidity and mortality. Our previous results demonstrated that Ficolin A (FcnA) protected against lipopolysaccharide (LPS)-induced mild ALI via activating complement, however the mechanism of severe lung damage caused by sepsis remains unclear. This study aimed to investigate whether FcnA modulated gut microbiota to affect the progression of sepsis-induced severe ALI. Fcna^-/- and Fcnb^-/- C57BL/6 mice were applied to establish the ALI model by injection of LPS intraperitoneally. Mice were treated with antibiotics, fecal microbiota transplantation (FMT), and intratracheal administration of recombinant protein S100A4. Changes in body weight of mice were recorded, and lung injury were assessed. Then lung tissue wet/dry weight was calculated. We found knockout of FcnA, but not FcnB, alleviated sepsis-induced severe ALI evidenced by increased body weight change, decreased wet/dry weight of lung tissue, reduced inflammatory infiltration, decreased lung damage score, decreased Muc-2, TNF-α, IL-1β, IL-6, and Cr levels, and increased sIgA levels. Furthermore, knockout of FcnA restored gut microbiota homeostasis in mice. Correlation analysis showed that Akkermansia was significantly negatively associated with TNF-α, IL-1β, and IL-6 levels in serum and bronchoalveolar lavage fluid (BALF). Moreover, knockout of FcnA regulated gut microbiota to protect ALI through S100A4. Finally, we found knockout of FcnA alleviated ALI by inhibiting S100A4 via gut Akkermansia in mice, which may provide further insights and new targets into treating sepsis-induced severe lung injury.

The role of the alveolar epithelial glycocalyx in acute respiratory distress syndrome

The alveolar epithelial glycocalyx is a dense anionic layer of glycosaminoglycans (GAGs) and proteoglycans that lines the apical surface of the alveolar epithelium. In contrast to the pulmonary endothelial glycocalyx, which has well-established roles in vascular homeostasis and septic organ dysfunction, the alveolar epithelial glycocalyx is less understood. Recent preclinical studies demonstrated that the epithelial glycocalyx is degraded in multiple murine models of acute respiratory distress syndrome (ARDS), particularly those that result from inhaled insults (so-called "direct" lung injury), leading to shedding of GAGs into the alveolar airspaces. Epithelial glycocalyx degradation also occurs in humans with respiratory failure, as quantified by analysis of airspace fluid obtained from ventilator heat moisture exchange (HME) filters. In patients with ARDS, GAG shedding correlates with the severity of hypoxemia and is predictive of the duration of respiratory failure. These effects may be mediated by surfactant dysfunction, as targeted degradation of the epithelial glycocalyx in mice was sufficient to cause increased alveolar surface tension, diffuse microatelectasis, and impaired lung compliance. In this review, we describe the structure of the alveolar epithelial glycocalyx and the mechanisms underlying its degradation during ARDS. We additionally review the current state of knowledge regarding the attributable effect of epithelial glycocalyx degradation in lung injury pathogenesis. Finally, we address glycocalyx degradation as a potential mediator of ARDS heterogeneity, and the subsequent value of point-of-care quantification of GAG shedding to potentially identify patients who are most likely to respond to pharmacological agents aimed at attenuating glycocalyx degradation.

Advancing Precision Medicine for the Diagnosis and Treatment of Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a common and life-threatening cause of respiratory failure. Despite decades of research, there are no effective pharmacologic therapies to treat this disease process and mortality remains high. The shortcomings of prior translational research efforts have been increasingly attributed to the heterogeneity of this complex syndrome, which has led to an increased focus on elucidating the mechanisms underlying the interpersonal heterogeneity of ARDS. This shift in focus aims to move the field towards personalized medicine by defining subgroups of ARDS patients with distinct biology, termed endotypes, to quickly identify patients that are most likely to benefit from mechanism targeted treatments. In this review, we first provide a historical perspective and review the key clinical trials that have advanced ARDS treatment. We then review the key challenges that exist with regards to the identification of treatable traits and the implementation of personalized medicine approaches in ARDS. Lastly, we discuss potential strategies and recommendations for future research that we believe will aid in both understanding the molecular pathogenesis of ARDS and the development of personalized treatment approaches.

Linkage of NAMPT promoter variants to eNAMPT secretion, plasma eNAMPT levels, and ARDS severity

BACKGROUND AND OBJECTIVES

eNAMPT (extracellular nicotinamide phosphoribosyltransferase), a novel DAMP and TLR4 ligand, is a druggable ARDS therapeutic target with NAMPT promoter SNPs associated with ARDS severity. This study assesses the previously unknown influence of NAMPT promoter SNPs on NAMPT transcription, eNAMPT secretion, and ARDS severity.

METHODS AND DESIGN

Human lung endothelial cells (ECs) transfected with NAMPT promoter luciferase reporters harboring SNPs G-1535A, A-1001 C, and C-948A, were exposed to LPS or LPS/18% cyclic stretch (CS) and NAMPT promoter activity, NAMPT protein expression, and secretion assessed. NAMPT genotypes and eNAMPT plasma measurements (Days 0/7) were assessed in two ARDS cohorts (DISCOVERY n = 428; ALVEOLI n = 103).

RESULTS

Comparisons of minor allelic frequency (MAF) in both ARDS cohorts with the 1000 Human Genome Project revealed the G-1535A and C-948A SNPs to be significantly associated with ARDS in Blacks compared with controls and trended toward significance in non-Hispanic Whites. LPS-challenged and LPS/18% CS-challenged EC harboring the -1535G wild-type allele exhibited significantly increased NAMPT promoter activity (compared with -1535A) with the -1535G/-948A diplotype exhibiting significantly increased NAMPT promoter activity, NAMPT protein expression, and eNAMPT secretion compared with the -1535A/-948 C diplotype. Highly significant increases in Day 0 eNAMPT plasma values were observed in both DISCOVERY and ALVEOLI ARDS cohorts (compared with healthy controls). Among subjects surviving to Day 7, Day 7 eNAMPT values were significantly increased in Day 28 non-survivors versus survivors. The protective -1535A SNP allele drove -1535A/-1001A and -1535A/-948 C diplotypes that confer significantly reduced ARDS risk (compared with -1535G, -1535G/-1001 C, -1535G/-948A), particularly in Black ARDS subjects. NAMPT SNP comparisons within the two ARDS cohorts did not identify significant association with either APACHE III scores or plasma eNAMPT levels.

CONCLUSION

NAMPT SNPs influence promoter activity, eNAMPT protein expression/secretion, plasma eNAMPT levels, and ARDS severity. NAMPT genotypes are a potential tool for stratification in eNAMPT-focused ARDS clinical trials.

Acute respiratory distress syndrome: causes, pathophysiology, and phenotypes

Acute respiratory distress syndrome (ARDS) is a common clinical syndrome of acute respiratory failure as a result of diffuse lung inflammation and oedema. ARDS can be precipitated by a variety of causes. The pathophysiology of ARDS is complex and involves the activation and dysregulation of multiple overlapping and interacting pathways of injury, inflammation, and coagulation, both in the lung and systemically. Mechanical ventilation can contribute to a cycle of lung injury and inflammation. Resolution of inflammation is a coordinated process that requires downregulation of proinflammatory pathways and upregulation of anti-inflammatory pathways. The heterogeneity of the clinical syndrome, along with its biology, physiology, and radiology, has increasingly been recognised and incorporated into identification of phenotypes. A precision-medicine approach that improves the identification of more homogeneous ARDS phenotypes should lead to an improved understanding of its pathophysiological mechanisms and how they differ from patient to patient.