Red blood cells function as reservoirs of tumor DNA.

Novel screening techniques for early detection of lung cancer are urgently needed. Profiling circulating tumor cell-free DNA (ctDNA) has emerged as a promising tool for biopsy-free tumor genotyping. However, both the scarcity and short half-life of ctDNA substantially limit the sensitivity and clinical utility of ctDNA detection methodologies. Our discovery that red blood cells (RBCs) sequester mitochondrial DNA opens a new avenue for detecting circulating nucleic acids, as RBCs represent an unrecognized reservoir of circulating nucleic acid. Here, we show that RBCs acquire tumor DNA following coculture with lung cancer cell lines harboring Kirsten rat sarcoma viral oncogene homolog (KRAS) and epidermal growth factor receptor (EGFR) mutations. RBC-bound tumor DNA is detectable in patients with early-stage non-small cell lung cancer (NSCLC) but not in healthy controls by qPCR. Our results collectively uncover a previously unrecognized yet easily accessible reservoir of tumor DNA, offering a promising foundation for future RBC-based tumor diagnostics.NEW & NOTEWORTHY We present a novel method for lung cancer detection by revealing RBCs as a reservoir for tumor DNA, overcoming the limitations of current circulating tumor ctDNA methodologies. By demonstrating that RBCs can capture tumor DNA, including critical mutations found in lung cancer, we provide a promising, biopsy-free avenue for early cancer diagnostics. This discovery opens up exciting possibilities for developing RBC-based diagnostic tools, significantly enhancing the sensitivity and clinical utility of noninvasive cancer detection.

The ultimate tradeoff: how red cell adaptations to malaria alter the host response during critical illness.

The human immune system evolved in response to pathogens. Among these pathogens, malaria has proven to be one of the deadliest and has exerted the most potent selective pressures on its target cell, the red blood cell. Red blood cells have recently gained recognition for their immunomodulatory properties, yet how red cell adaptations contribute to the host response during critical illness remains understudied. This review will discuss how adaptations that may have been advantageous for host survival might influence immune responses in modern critical illness. We will highlight the current evidence for divergent host resilience arising from the adaptations to malaria and summarize how understanding evolutionary red cell adaptations to malaria may provide insight into the heterogeneity of the host response to critical illness, perhaps driving future precision medicine approaches to syndromes affecting the critically ill such as sepsis and acute respiratory distress syndrome (ARDS).

Immunomodulatory roles of red blood cells.

PURPOSE OF THE REVIEW: To discuss recent advances supporting the role of red blood cells (RBCs) in the host immune response. RECENT FINDINGS: Over the last century, research has demonstrated that red blood cells exhibit functions beyond oxygen transport, including immune function. Recent work indicates that the nucleic acid sensing receptor, toll-like receptor 9 (TLR9), is expressed on the RBC surface and implicated in innate immune activation and red cell clearance during inflammatory states. In addition to this DNA-sensing role of RBCs, there is growing evidence that RBCs may influence immune function by inducing vascular dysfunction. RBC proteomics and metabolomics have provided additional insight into RBC immune function, with several studies indicating changes to RBC membrane structure and metabolism in response to severe acute respiratory syndrome coronavirus 2 infection. These structural RBC changes may even provide insight into the pathophysiology of the 'long-coronavirus disease 2019' phenomenon. Finally, evidence suggests that RBCs may influence host immune responses via complement regulation. Taken together, these recent findings indicate RBCs possess immune function. Further studies will be required to elucidate better how RBC immune function contributes to the heterogeneous host response during inflammatory states. SUMMARY: The appreciation for nongas exchanging, red blood cell immune functions is rapidly growing. A better understanding of these RBC functions may provide insight into the heterogeneity observed in the host immune response to infection and inflammation.

Trefoil Factor Family: A Troika for Lung Repair and Regeneration.

Tissue damage in the upper and lower airways caused by mechanical abrasion, noxious chemicals, or pathogenic organisms must be followed by rapid restorative processes; otherwise, persistent immunopathology and disease may ensue. This review will discuss evidence for the important role served by trefoil factor (TFF) family members in healthy and diseased airways of humans and rodents. Collectively, these peptides serve to both maintain and restore homeostasis through their regulation of the mucous layer and their control of cell motility, cell differentiation, and immune function in the upper and lower airways. We will also discuss important differences in which trefoil member tracks with homeostasis and disease between humans and mice, which poses a challenge for research in this area. Moreover, we discuss new evidence supporting newly identified receptor binding partners in the leucine-rich repeat and immunoglobulin-like domain-containing NoGo (LINGO) family in mediating the biological effects of TFF proteins in mouse models of epithelial repair and infection. Recent advances in our knowledge regarding TFF peptides suggest that they may be reasonable therapeutic targets in the treatment of upper and lower airway diseases of diverse etiologies. Further work understanding their role in airway homeostasis, repair, and inflammation will benefit from these newly uncovered receptor-ligand interactions.

Update on the Features and Measurements of Experimental Acute Lung Injury in Animals: An Official American Thoracic Society Workshop Report.

Advancements in methods, technology, and our understanding of the pathobiology of lung injury have created the need to update the definition of experimental acute lung injury (ALI). We queried 50 participants with expertise in ALI and acute respiratory distress syndrome using a Delphi method composed of a series of electronic surveys and a virtual workshop. We propose that ALI presents as a "multidimensional entity" characterized by four "domains" that reflect the key pathophysiologic features and underlying biology of human acute respiratory distress syndrome. These domains are 1) histological evidence of tissue injury, 2) alteration of the alveolar-capillary barrier, 3) presence of an inflammatory response, and 4) physiologic dysfunction. For each domain, we present "relevant measurements," defined as those proposed by at least 30% of respondents. We propose that experimental ALI encompasses a continuum of models ranging from those focusing on gaining specific mechanistic insights to those primarily concerned with preclinical testing of novel therapeutics or interventions. We suggest that mechanistic studies may justifiably focus on a single domain of lung injury, but models must document alterations of at least three of the four domains to qualify as "experimental ALI." Finally, we propose that a time criterion defining "acute" in ALI remains relevant, but the actual time may vary based on the specific model and the aspect of injury being modeled. The continuum concept of ALI increases the flexibility and applicability of the definition to multiple models while increasing the likelihood of translating preclinical findings to critically ill patients.

Erythrocytes identify complement activation in patients with COVID-19.

COVID-19, the disease caused by the SARS-CoV-2 virus, can progress to multisystem organ failure and viral sepsis characterized by respiratory failure, arrhythmias, thromboembolic complications, and shock with high mortality. Autopsy and preclinical evidence implicate aberrant complement activation in endothelial injury and organ failure. Erythrocytes express complement receptors and are capable of binding immune complexes; therefore, we investigated complement activation in patients with COVID-19 using erythrocytes as a tool to diagnose complement activation. We discovered enhanced C3b and C4d deposition on erythrocytes in COVID-19 sepsis patients and non-COVID sepsis patients compared with healthy controls, supporting the role of complement in sepsis-associated organ injury. Our data suggest that erythrocytes may contribute to a precision medicine approach to sepsis and have diagnostic value in monitoring complement dysregulation in COVID-19-sepsis and non-COVID sepsis and identifying patients who may benefit from complement targeted therapies.

Collateral damage: necroptosis in the development of lung injury.

Cell death is increasingly recognized as a driving factor in the development of acute lung injury. Necroptosis, an immunogenic regulated cell death program important in innate immunity, has been implicated in the development of lung injury in a diverse range of conditions. Characterized by lytic cell death and consequent extracellular release of endogenous inflammatory mediators, necroptosis can be both beneficial and deleterious to the host, depending on the context. Here, we review recent investigations linking necroptosis and the development of experimental lung injury. We assess the consequences of necroptosis during bacterial pneumonia, viral infection, sepsis, and sterile injury, highlighting increasing evidence from in vitro studies, animal models, and clinical studies that implicates necroptosis in the pathogenesis of ARDS. Lastly, we highlight current challenges in translating laboratory findings to the bedside.

RAGE interacts with the necroptotic protein RIPK3 and mediates transfusion-induced danger signal release.

RBC transfusion is associated with increased morbidity and mortality in critically ill patients. Endothelial cell necroptosis and subsequent damage-associated molecular pattern (DAMP) release has been identified as a mechanism of injury following RBC transfusion. Mounting evidence implicates the pro-inflammatory pattern recognition receptor, Receptor for Advanced Glycation End Products (RAGE), in initiating cell death programmes such as necroptosis. Here, we demonstrate the role of RAGE in endothelial necroptosis, as deletion of RAGE attenuates necroptotic cell death in response to TNFα, LPS or CpG-DNA. We show direct interaction of RAGE with the critical mediator of necroptosis, Receptor Interacting Protein Kinase 3 (RIPK3), during necroptosis. Furthermore, we observe decreased plasma High Mobility Group Box 1 (HMGB1) and RIPK3 levels in RAGE deficient mice compared to WT mice post-transfusion, substantiating the role for RAGE in transfusion-induced DAMP release in vivo. Collectively, these findings underscore RAGE as an essential mediator of regulated necrosis and post-transfusion DAMP release. Further studies to understand the role of RAGE and the necroptotic pathway in transfusion-induced organ injury may offer key targets to mitigate transfusion-related risks, including the risk of ARDS, in susceptible hosts.

The Evolving Erythrocyte: Red Blood Cells as Modulators of Innate Immunity.

The field of red cell biology is undergoing a quiet revolution. Long assumed to be inert oxygen carriers, RBCs are emerging as important modulators of the innate immune response. Erythrocytes bind and scavenge chemokines, nucleic acids, and pathogens in circulation. Depending on the conditions of the microenvironment, erythrocytes may either promote immune activation or maintain immune quiescence. We examine erythrocyte immune function through a comparative and evolutionary lens, as this framework may offer perspective into newly recognized roles of human RBCs. Next, we review the known immune roles of human RBCs and discuss their activity in the context of sepsis where erythrocyte function may prove important to disease pathogenesis. Given the limited success of immunomodulatory therapies in treating inflammatory diseases, we propose that the immunologic function of RBCs provides an understudied and potentially rich area of research that may yield novel insights into mechanisms of immune regulation.

Plasma receptor interacting protein kinase-3 levels are associated with acute respiratory distress syndrome in sepsis and trauma: a cohort study.

BACKGROUND: Necroptosis, a form of programmed cell death mediated by receptor interacting serine/threonine-protein kinase-3 (RIPK3), is implicated in murine models of acute respiratory distress syndrome (ARDS). We hypothesized that plasma RIPK3 concentrations in sepsis and trauma would be associated with ARDS development and that plasma RIPK3 would reflect changes in lung tissue RIPK3 in a murine model of systemic inflammation. METHODS: We utilized prospective cohort studies of critically ill sepsis (n = 120) and trauma (n = 180) patients and measured plasma RIPK3 at presentation and 48 h. Patients were followed for 6 days for ARDS by the Berlin definition. We used multivariable logistic regression to determine the association of plasma RIPK3 with ARDS in each cohort, adjusting for confounders. In mice, we determined whether plasma and lung tissue RIPK3 levels rise concomitantly 4 h after injection with lipopolysaccharide and ZVAD-FMK, an apoptosis inhibitor. RESULTS: The change in plasma RIPK3 from presentation to 48 h (ΔRIPK3) was associated with ARDS in sepsis (OR 1.30, 95% CI 1.03-1.63, per ½ standard deviation) and trauma (OR 1.79, 95% CI 1.33-2.40). This association was not evident for presentation RIPK3 levels. Secondary analyses showed similar findings for the association of ΔRIPK3 with acute kidney injury and 30-day mortality. Mice injected with lipopolysaccharide and ZVAD-FMK had significantly higher plasma (p < 0.001) and lung (p = 0.005) RIPK3 than control mice. CONCLUSIONS: The change in plasma RIPK3 from presentation to 48 h in both sepsis and trauma patients is independently associated with ARDS, and plasma RIPK3 may reflect RIPK3 activity in lung tissue.

Red Blood Cells Homeostatically Bind Mitochondrial DNA through TLR9 to Maintain Quiescence and to Prevent Lung Injury.

RATIONALE: Potentially hazardous CpG-containing cell-free mitochondrial DNA (cf-mtDNA) is routinely released into the circulation and is associated with morbidity and mortality in critically ill patients. How the body avoids inappropriate innate immune activation by cf-mtDNA remains unknown. Because red blood cells (RBCs) modulate innate immune responses by scavenging chemokines, we hypothesized that RBCs may attenuate CpG-induced lung inflammation through direct scavenging of CpG-containing DNA. OBJECTIVES: To determine the mechanisms of CpG-DNA binding to RBCs and the effects of RBC-mediated DNA scavenging on lung inflammation. METHODS: mtDNA on murine RBCs was measured under basal conditions and after systemic inflammation. mtDNA content on human RBCs from healthy control subjects and trauma patients was measured. Toll-like receptor 9 (TLR9) expression on RBCs and TLR9-dependent binding of CpG-DNA to RBCs were determined. A murine model of RBC transfusion after CpG-DNA-induced lung injury was used to investigate the role of RBC-mediated DNA scavenging in mitigating lung injury in vivo. MEASUREMENTS AND MAIN RESULTS: Under basal conditions, RBCs bind CpG-DNA. The plasma-to-RBC mtDNA ratio is low in naive mice and in healthy volunteers but increases after systemic inflammation, demonstrating that the majority of cf-mtDNA is RBC-bound under homeostatic conditions and that the unbound fraction increases during inflammation. RBCs express TLR9 and bind CpG-DNA through TLR9. Loss of TLR9-dependent RBC-mediated CpG-DNA scavenging increased lung injury in vivo. CONCLUSIONS: RBCs homeostatically bind mtDNA, and RBC-mediated DNA scavenging is essential in mitigating lung injury after CpG-DNA. Our data suggest a role for RBCs in regulating lung inflammation during disease states where cf-mtDNA is elevated, such as sepsis and trauma.

Red blood cells induce necroptosis of lung endothelial cells and increase susceptibility to lung inflammation.

RATIONALE: Red blood cell (RBC) transfusions are associated with increased risk of acute respiratory distress syndrome (ARDS) in the critically ill, yet the mechanisms for enhanced susceptibility to ARDS conferred by RBC transfusions remain unknown. OBJECTIVES: To determine the mechanisms of lung endothelial cell (EC) High Mobility Group Box 1 (HMGB1) release following exposure to RBCs and to determine whether RBC transfusion increases susceptibility to lung inflammation in vivo through release of the danger signal HMGB1. METHODS: In vitro studies examining human lung EC viability and HMGB1 release following exposure to allogenic RBCs were conducted under static conditions and using a microengineered model of RBC perfusion. The plasma from transfused and nontransfused patients with severe sepsis was examined for markers of cellular injury. A murine model of RBC transfusion followed by LPS administration was used to determine the effects of RBC transfusion and HMGB1 release on LPS-induced lung inflammation. MEASUREMENTS AND MAIN RESULTS: After incubation with RBCs, lung ECs underwent regulated necrotic cell death (necroptosis) and released the essential mediator of necroptosis, receptor-interacting serine/threonine-protein kinase 3 (RIP3), and HMGB1. RIP3 was detectable in the plasma of patients with severe sepsis, and was increased with blood transfusion and among nonsurvivors of sepsis. RBC transfusion sensitized mice to LPS-induced lung inflammation through release of the danger signal HMGB1. CONCLUSIONS: RBC transfusion enhances susceptibility to lung inflammation through release of HMGB1 and induces necroptosis of lung EC. Necroptosis and subsequent danger signal release is a novel mechanism of injury following transfusion that may account for the increased risk of ARDS in critically ill transfused patients.

Human red blood cells express the RNA sensor TLR7.

Red blood cells (RBCs) express the nucleic acid-binding toll-like receptor 9 (TLR9) and bind CpG-containing DNA. However, whether human RBCs express other nucleic acid-binding TLRs is unknown. Here we show that human RBCs express the RNA sensor TLR7. TLR7 is present on the red cell membrane and is associated with the RBC membrane protein Band 3. In patients with SARS-CoV2-associated sepsis, TLR7-Band 3 interactions in the RBC membrane are increased when compared with healthy controls. In vitro, RBCs bind synthetic ssRNA and RNA from ssRNA viruses. Thus, RBCs may serve as a previously unrecognized sink for exogenous RNA, expanding the repertoire of non-gas exchanging functions performed by RBCs.

Inflammatory and tissue injury marker dynamics in pediatric acute respiratory distress syndrome.

BACKGROUNDThe molecular signature of pediatric acute respiratory distress syndrome (ARDS) is poorly described, and the degree to which hyperinflammation or specific tissue injury contributes to outcomes is unknown. Therefore, we profiled inflammation and tissue injury dynamics over the first 7 days of ARDS, and associated specific biomarkers with mortality, persistent ARDS, and persistent multiple organ dysfunction syndrome (MODS).METHODSIn a single-center prospective cohort of intubated pediatric patients with ARDS, we collected plasma on days 0, 3, and 7. Nineteen biomarkers reflecting inflammation, tissue injury, and damage-associated molecular patterns (DAMPs) were measured. We assessed the relationship between biomarkers and trajectories with mortality, persistent ARDS, or persistent MODS using multivariable mixed effect models.RESULTSIn 279 patients (64 [23%] nonsurvivors), hyperinflammatory cytokines, tissue injury markers, and DAMPs were higher in nonsurvivors. Survivors and nonsurvivors showed different biomarker trajectories. IL-1α, soluble tumor necrosis factor receptor 1, angiopoietin 2 (ANG2), and surfactant protein D increased in nonsurvivors, while DAMPs remained persistently elevated. ANG2 and procollagen type III N-terminal peptide were associated with persistent ARDS, whereas multiple cytokines, tissue injury markers, and DAMPs were associated with persistent MODS. Corticosteroid use did not impact the association of biomarker levels or trajectory with mortality.CONCLUSIONSPediatric ARDS survivors and nonsurvivors had distinct biomarker trajectories, with cytokines, endothelial and alveolar epithelial injury, and DAMPs elevated in nonsurvivors. Mortality markers overlapped with markers associated with persistent MODS, rather than persistent ARDS.FUNDINGNIH (K23HL-136688, R01-HL148054).

The receptor for advanced glycation end products mediates lung endothelial activation by RBCs.

The receptor for advanced glycation end products (RAGE) is a multiligand pattern recognition receptor implicated in multiple disease states. Although RAGE is expressed on systemic vascular endothelium, the expression and function of RAGE on lung endothelium has not been studied. Utilizing in vitro (human) and in vivo (mouse) models, we established the presence of RAGE on lung endothelium. Because RAGE ligands can induce the expression of RAGE and stored red blood cells express the RAGE ligand N(ε)-carboxymethyl lysine, we investigated whether red blood cell (RBC) transfusion would augment RAGE expression on endothelium utilizing a syngeneic model of RBC transfusion. RBC transfusion not only increased lung endothelial RAGE expression but enhanced lung inflammation and endothelial activation, since lung high mobility group box 1 and vascular cell adhesion molecule 1 expression was elevated following transfusion. These effects were mediated by RAGE, since endothelial activation was absent in RBC-transfused RAGE knockout mice. Thus, RAGE is inducibly expressed on lung endothelium, and one functional consequence of RBC transfusion is increased RAGE expression and endothelial activation.

Neutrophil extracellular trap stabilization by platelet factor 4 reduces thrombogenicity and endothelial cell injury.

Neutrophil extracellular traps (NETs) are abundant in sepsis, and proposed NET-directed therapies in sepsis prevent their formation or accelerate degradation. Yet NETs are important for microbial entrapment, as NET digestion liberates pathogens and NET degradation products (NDPs) that deleteriously promote thrombosis and endothelial cell injury. We proposed an alternative strategy of NET-stabilization with the chemokine, platelet factor 4 (PF4, CXCL4), which we have shown enhances NET-mediated microbial entrapment. We now show that NET compaction by PF4 reduces their thrombogenicity. In vitro, we quantified plasma thrombin and fibrin generation by intact or degraded NETs and cell-free (cf) DNA fragments, and found that digested NETs and short DNA fragments were more thrombogenic than intact NETs and high molecular weight genomic DNA, respectively. PF4 reduced the thrombogenicity of digested NETs and DNA by interfering, in part, with contact pathway activation. In endothelial cell culture studies, short DNA fragments promoted von Willebrand factor release and tissue factor expression via a toll-like receptor 9-dependent mechanism. PF4 blocked these effects. Cxcl4-/- mice infused with cfDNA exhibited higher plasma thrombin anti-thrombin (TAT) levels compared to wild-type controls. Following challenge with bacterial lipopolysaccharide, Cxcl4-/- mice had similar elevations in plasma TAT and cfDNA, effects prevented by PF4 infusion. Thus, NET-stabilization by PF4 prevents the release of short fragments of cfDNA, limiting the activation of the contact coagulation pathway and reducing endothelial injury. These results support our hypothesis that NET-stabilization reduces pathologic sequelae in sepsis, an observation of potential clinical benefit.

Platelet factor 4 limits neutrophil extracellular trap- and cell-free DNA-induced thrombogenicity and endothelial injury.

Plasma cell-free DNA (cfDNA), a marker of disease severity in sepsis, is a recognized driver of thromboinflammation and a potential therapeutic target. In sepsis, plasma cfDNA is mostly derived from neutrophil extracellular trap (NET) degradation. Proposed NET-directed therapeutic strategies include preventing NET formation or accelerating NET degradation. However, NET digestion liberates pathogens and releases cfDNA that promote thrombosis and endothelial cell injury. We propose an alternative strategy of cfDNA and NET stabilization with chemokine platelet factor 4 (PF4, CXCL4). We previously showed that human PF4 (hPF4) enhances NET-mediated microbial entrapment. We now show that hPF4 interferes with thrombogenicity of cfDNA and NETs by preventing their cleavage to short-fragment and single-stranded cfDNA that more effectively activates the contact pathway of coagulation. In vitro, hPF4 also inhibits cfDNA-induced endothelial tissue factor surface expression and von Willebrand factor release. In vivo, hPF4 expression reduced plasma thrombin-antithrombin (TAT) levels in animals infused with exogenous cfDNA. Following lipopolysaccharide challenge, Cxcl4-/- mice had significant elevation in plasma TAT, cfDNA, and cystatin C levels, effects prevented by hPF4 infusion. These results show that hPF4 interacts with cfDNA and NETs to limit thrombosis and endothelial injury, an observation of potential clinical benefit in the treatment of sepsis.

Bat Red Blood Cells Express Nucleic Acid-Sensing Receptors and Bind RNA and DNA.

RBCs demonstrate immunomodulatory capabilities through the expression of nucleic acid sensors. However, little is known about bat RBCs, and no studies have examined the immune function of bat erythrocytes. In this study, we show that bat RBCs express the nucleic acid-sensing TLRs TLR7 and TLR9 and bind the nucleic acid ligands, ssRNA, and CpG DNA. Collectively, these data suggest that, like human RBCs, bat erythrocytes possess immune function and may be reservoirs for nucleic acids. These findings provide unique insight into bat immunity and may uncover potential mechanisms by which virulent pathogens of humans are concealed in bats.