SYK coordinates neuroprotective microglial responses in neurodegenerative disease.

Recent studies have begun to reveal critical roles for the brain's professional phagocytes, microglia, and their receptors in the control of neurotoxic amyloid beta (Aß) and myelin debris accumulation in neurodegenerative disease. However, the critical intracellular molecules that orchestrate neuroprotective functions of microglia remain poorly understood. In our studies, we find that targeted deletion of SYK in microglia leads to exacerbated Aß deposition, aggravated neuropathology, and cognitive defects in the 5xFAD mouse model of Alzheimer's disease (AD). Disruption of SYK signaling in this AD model was further shown to impede the development of disease-associated microglia (DAM), alter AKT/GSK3ß-signaling, and restrict Aß phagocytosis by microglia. Conversely, receptor-mediated activation of SYK limits Aß load. We also found that SYK critically regulates microglial phagocytosis and DAM acquisition in demyelinating disease. Collectively, these results broaden our understanding of the key innate immune signaling molecules that instruct beneficial microglial functions in response to neurotoxic material.

CARD9 attenuates Aß pathology and modifies microglial responses in an Alzheimer's disease mouse model.

Recent advances have highlighted the importance of several innate immune receptors expressed by microglia in Alzheimer's disease (AD). In particular, mounting evidence from AD patients and experimental models indicates pivotal roles for TREM2, CD33, and CD22 in neurodegenerative disease progression. While there is growing interest in targeting these microglial receptors to treat AD, we still lack knowledge of the downstream signaling molecules used by these receptors to orchestrate immune responses in AD. Notably, TREM2, CD33, and CD22 have been described to influence signaling associated with the intracellular adaptor molecule CARD9 to mount downstream immune responses outside of the brain. However, the role of CARD9 in AD remains poorly understood. Here, we show that genetic ablation of CARD9 in the 5xFAD mouse model of AD results in exacerbated amyloid beta (Aß) deposition, increased neuronal loss, worsened cognitive deficits, and alterations in microglial responses. We further show that pharmacological activation of CARD9 promotes improved clearance of Aß deposits from the brains of 5xFAD mice. These results help to establish CARD9 as a key intracellular innate immune signaling molecule that regulates Aß-mediated disease and microglial responses. Moreover, these findings suggest that targeting CARD9 might offer a strategy to improve Aß clearance in AD.

SMARCB1 regulates the hypoxic stress response in sickle cell trait.

Renal medullary carcinoma (RMC) is an aggressive kidney cancer that almost exclusively develops in individuals with sickle cell trait (SCT) and is always characterized by loss of the tumor suppressor SMARCB1. Because renal ischemia induced by red blood cell sickling exacerbates chronic renal medullary hypoxia in vivo, we investigated whether the loss of SMARCB1 confers a survival advantage under the setting of SCT. Hypoxic stress, which naturally occurs within the renal medulla, is elevated under the setting of SCT. Our findings showed that hypoxia-induced SMARCB1 degradation protected renal cells from hypoxic stress. SMARCB1 wild-type renal tumors exhibited lower levels of SMARCB1 and more aggressive growth in mice harboring the SCT mutation in human hemoglobin A (HbA) than in control mice harboring wild-type human HbA. Consistent with established clinical observations, SMARCB1-null renal tumors were refractory to hypoxia-inducing therapeutic inhibition of angiogenesis. Further, reconstitution of SMARCB1 restored renal tumor sensitivity to hypoxic stress in vitro and in vivo. Together, our results demonstrate a physiological role for SMARCB1 degradation in response to hypoxic stress, connect the renal medullary hypoxia induced by SCT with an increased risk of SMARCB1-negative RMC, and shed light into the mechanisms mediating the resistance of SMARCB1-null renal tumors against angiogenesis inhibition therapies.

Programmable synthetic biomolecular condensates for cellular control.

The formation of biomolecular condensates mediated by a coupling of associative and segregative phase transitions plays a critical role in controlling diverse cellular functions in nature. This has inspired the use of phase transitions to design synthetic systems. While design rules of phase transitions have been established for many synthetic intrinsically disordered proteins, most efforts have focused on investigating their phase behaviors in a test tube. Here, we present a rational engineering approach to program the formation and physical properties of synthetic condensates to achieve intended cellular functions. We demonstrate this approach through targeted plasmid sequestration and transcription regulation in bacteria and modulation of a protein circuit in mammalian cells. Our approach lays the foundation for engineering designer condensates for synthetic biology applications.

Thiol-catalyzed formation of NO-ferroheme regulates intravascular NO signaling.

Nitric oxide (NO) is an endogenously produced signaling molecule that regulates blood flow and platelet activation. However, intracellular and intravascular diffusion of NO are limited by scavenging reactions with several hemoproteins, raising questions as to how free NO can signal in hemoprotein-rich environments. We explore the hypothesis that NO can be stabilized as a labile ferrous heme-nitrosyl complex (Fe2+-NO, NO-ferroheme). We observe a reaction between NO, labile ferric heme (Fe3+) and reduced thiols to yield NO-ferroheme and a thiyl radical. This thiol-catalyzed reductive nitrosylation occurs when heme is solubilized in lipophilic environments such as red blood cell membranes or bound to serum albumin. The resulting NO-ferroheme resists oxidative inactivation, is soluble in cell membranes and is transported intravascularly by albumin to promote potent vasodilation. We therefore provide an alternative route for NO delivery from erythrocytes and blood via transfer of NO-ferroheme and activation of apo-soluble guanylyl cyclase.

Functional effects of an African glucose-6-phosphate dehydrogenase (G6PD) polymorphism (Val68Met) on red blood cell hemolytic propensity and post-transfusion recovery.

BACKGROUND: Donor genetic variation is associated with red blood cell (RBC) storage integrity and post-transfusion recovery. Our previous large-scale genome-wide association study demonstrated that the African G6PD deficient A- variant (rs1050828, Val68Met) is associated with higher oxidative hemolysis after cold storage. Despite a high prevalence of X-linked G6PD mutation in African American population (>10%), blood donors are not routinely screened for G6PD status and its importance in transfusion medicine is relatively understudied. STUDY DESIGN AND METHODS: To further evaluate the functional effects of the G6PD A- mutation, we created a novel mouse model carrying this genetic variant using CRISPR-Cas9. We hypothesize that this humanized G6PD A- variant is associated with reduced G6PD activity with a consequent effect on RBC hemolytic propensity and post-transfusion recovery. RESULTS: G6PD A- RBCs had reduced G6PD protein with ~5% residual enzymatic activity. Significantly increased in vitro hemolysis induced by oxidative stressors was observed in fresh and stored G6PD A- RBCs, along with a lower GSH:GSSG ratio. However, no differences were observed in storage hemolysis, osmotic fragility, mechanical fragility, reticulocytes, and post-transfusion recovery. Interestingly, a 14% reduction of 24-h survival following irradiation was observed in G6PD A- RBCs compared to WT RBCs. Metabolomic assessment of stored G6PD A- RBCs revealed an impaired pentose phosphate pathway (PPP) with increased glycolytic flux, decreasing cellular antioxidant capacity. DISCUSSION: This novel mouse model of the common G6PD A- variant has impaired antioxidant capacity like humans and low G6PD activity may reduce survival of transfused RBCs when irradiation is performed.

Harnessing the Power of Generative AI for Clinical Summaries: Perspectives From Emergency Physicians.

STUDY OBJECTIVE: The workload of clinical documentation contributes to health care costs and professional burnout. The advent of generative artificial intelligence language models presents a promising solution. The perspective of clinicians may contribute to effective and responsible implementation of such tools. This study sought to evaluate 3 uses for generative artificial intelligence for clinical documentation in pediatric emergency medicine, measuring time savings, effort reduction, and physician attitudes and identifying potential risks and barriers. METHODS: This mixed-methods study was performed with 10 pediatric emergency medicine attending physicians from a single pediatric emergency department. Participants were asked to write a supervisory note for 4 clinical scenarios, with varying levels of complexity, twice without any assistance and twice with the assistance of ChatGPT Version 4.0. Participants evaluated 2 additional ChatGPT-generated clinical summaries: a structured handoff and a visit summary for a family written at an 8th grade reading level. Finally, a semistructured interview was performed to assess physicians' perspective on the use of ChatGPT in pediatric emergency medicine. Main outcomes and measures included between subjects' comparisons of the effort and time taken to complete the supervisory note with and without ChatGPT assistance. Effort was measured using a self-reported Likert scale of 0 to 10. Physicians' scoring of and attitude toward the ChatGPT-generated summaries were measured using a 0 to 10 Likert scale and open-ended questions. Summaries were scored for completeness, accuracy, efficiency, readability, and overall satisfaction. A thematic analysis was performed to analyze the content of the open-ended questions and to identify key themes. RESULTS: ChatGPT yielded a 40% reduction in time and a 33% decrease in effort for supervisory notes in intricate cases, with no discernible effect on simpler notes. ChatGPT-generated summaries for structured handoffs and family letters were highly rated, ranging from 7.0 to 9.0 out of 10, and most participants favored their inclusion in clinical practice. However, there were several critical reservations, out of which a set of general recommendations for applying ChatGPT to clinical summaries was formulated. CONCLUSION: Pediatric emergency medicine attendings in our study perceived that ChatGPT can deliver high-quality summaries while saving time and effort in many scenarios, but not all.

The acute effects of dietary nitrate supplementation on postmenopausal endothelial resistance to ischemia reperfusion injury: A randomized, placebo-controlled, double blind, crossover clinical trial.

Postmenopausal cardiovascular health is a critical determinant of longevity. Consumption of beetroot juice (BR) and other nitrate rich foods is a safe, effective non-pharmacological intervention strategy to increase systemic bioavailability of the vasoprotective molecule, nitric oxide (NO), through the exogenous nitrate (NO3-)-nitrite (NO2-)-NO pathway. We hypothesized that a single dose of nitrate-rich beetroot juice (BRnitrate 600 mg NO3- / 140 mL, BRplacebo ~ 0 mg/ 140 mL) would improve resting endothelial function and resistance to ischemia-reperfusion (IR) injury to a greater extent in early- (1-6 years following their final menstrual period (FMP), n=12) compared to late- (6+ years after FMP, n=12) postmenopausal women. Analyses with general linear models revealed a significant (p<0.05) time*treatment interaction effect for brachial artery adjusted FMD. Pairwise comparisons revealed adjusted FMD was significantly lower following IR-injury in comparison to all other time points with BRplacebo (Early FMD 2.51 ± 1.18%, Late FMD 1.30 ± 1.10, p<0.001) and was lower than post-IR with BRnitrate (Early FMD 3.84 ± 1.21%, Late FMD 3.21 ± 1.13 %, p=0.014). Considering the postmenopausal stage-dependent variations in endothelial responsiveness to dietary nitrate at rest and post-IR, we predict differing mechanisms underpin macrovascular protection against IR injury. NCT03644472.

Management of Pediatric Pneumonia: A Decade After the Pediatric Infectious Diseases Society and Infectious Diseases Society of America Guideline.

BACKGROUND: Incomplete uptake of guidelines can lead to nonstandardized care, increased expenditures, and adverse clinical outcomes. The objective of this study was to evaluate the impact of the 2011 Pediatric Infectious Diseases Society and Infectious Diseases Society of America (PIDS/IDSA) pediatric community-acquired pneumonia (CAP) guideline that emphasized aminopenicillin use and de-emphasized the use of chest radiographs (CXRs) in certain populations. METHODS: This quasi-experimental study queried a national administrative database of children's hospitals to identify children aged 3 months-18 years with CAP who visited 1 of 28 participating hospitals from 2009 to 2021. PIDS/IDSA pediatric CAP guideline recommendations regarding antibiotic therapy, diagnostic testing, and imaging were evaluated. Segmented regression interrupted time series was used to measure guideline-concordant practices with interruptions for guideline publication and the Coronavirus Disease 2019 (COVID-19) pandemic. RESULTS: Of 315 384 children with CAP, 71 804 (22.8%) were hospitalized. Among hospitalized children, there was a decrease in blood culture performance (0.5% per quarter) and increase in aminopenicillin prescribing (1.1% per quarter). Among children discharged from the emergency department (ED), there was an increase in aminopenicillin prescription (0.45% per quarter), whereas the rate of obtaining CXRs declined (0.12% per quarter). However, use of CXRs rebounded during the COVID-19 pandemic (increase of 1.56% per quarter). Hospital length of stay, ED revisit rates, and hospital readmission rates remained stable. CONCLUSIONS: Guideline publication was associated with an increase of aminopenicillin prescribing. However, rates of diagnostic testing did not materially change, suggesting the need to consider implementation strategies to meaningfully change clinical practice for children with CAP.

SSRI treatment modifies the effects of maternal inflammation on in utero physiology and offspring neurobiology.

Perturbations to the in utero environment can dramatically change the trajectory of offspring neurodevelopment. Insults commonly encountered in modern human life such as infection, toxins, high-fat diet, prescription medications, and others are increasingly linked to behavioral alterations in prenatally-exposed offspring. While appreciation is expanding for the potential consequence that these triggers can have on embryo development, there is a paucity of information concerning how the crucial maternal-fetal interface (MFI) responds to these various insults and how it may relate to changes in offspring neurodevelopment. Here, we found that the MFI responds both to an inflammatory state and altered serotonergic tone in pregnant mice. Maternal immune activation (MIA) triggered an acute inflammatory response in the MFI dominated by interferon signaling that came at the expense of ordinary development-related transcriptional programs. The major MFI compartments, the decidua and the placenta, each responded in distinct manners to MIA. MFIs exposed to MIA were also found to have disrupted sex-specific gene expression and heightened serotonin levels. We found that offspring exposed to MIA had sex-biased behavioral changes and that microglia were not transcriptionally impacted. Moreover, the combination of maternal inflammation in the presence of pharmacologic inhibition of serotonin reuptake further transformed MFI physiology and offspring neurobiology, impacting immune and serotonin signaling pathways alike. In all, these findings highlight the complexities of evaluating diverse environmental impacts on placental physiology and neurodevelopment.

Optimizing interpretation of survival studies of fresh and aged transfused biotin-labeled RBCs.

BACKGROUND: Ex vivo labeling with 51 chromium represents the standard method to determine red blood cell (RBC) survival after transfusion. Limitations and safety concerns spurred the development of alternative methods, including biotinylated red blood cells (BioRBC). STUDY DESIGN AND METHODS: Autologous units of whole blood were divided equally into two bags and stored under standard blood bank conditions at 2 to 6°C (N = 4 healthy adult volunteers). One bag was biotinylated (15 µg/ml) on storage days 5 to 7 (fresh) and the other was biotinylated (3 µg/ml) on days 35 to 42 (aged). The proportion of circulating BioRBC was measured serially, and cell-surface biotin was quantified with reference to molecules of equivalent soluble fluorochrome. Clearance kinetics were modeled by RBC age distribution at infusion (Gaussian vs. uniform) and decay over time (constant vs. exponential). RESULTS: Data were consistent with biphasic exponential clearance of cells of uniform age. Our best estimate of BioRBC clearance (half-life [T1/2 ]) was 49.7 ± 1.2 days initially, followed by more rapid clearance 82 days after transfusion (T1/2  = 15.6 ± 0.6 days). As BioRBC aged in vivo, molecules of equivalent soluble fluorochrome declined with a T1/2 of 122 ± 9 days, suggesting gradual biotin cleavage. There were no significant differences between the clearance of fresh and aged BioRBC. CONCLUSION: Similar clearance kinetics of fresh and aged BioRBC may be due to the extensive washing required during biotinylation. Survival kinetics consistent with cells with uniform rather than Gaussian or other non-uniform age distributions suggest that washing, and potentially RBC culling, may extend the storage life of RBC products.

Association between clinician specialty and prescription of preventive medication for young adults with migraine: A retrospective cohort study.

OBJECTIVE: We aimed to compare the prescribing patterns of preventive medications between pediatric and adult neurologists for young adults with migraine. BACKGROUND: Although preventive medications are effective for adults with migraine, studies in children have failed to demonstrate similar efficacy. As a result, lifestyle modifications and non-pharmacological interventions are often emphasized in children. It is not known whether young adults are prescribed preventive medications at different rates according to whether they are cared for by an adult or pediatric neurologist. METHODS: We performed a multicenter retrospective cohort analysis of patients with migraine aged 18-25 years who were seen by a pediatric or adult neurologist at Mass General Brigham Hospital between 2017 and 2021. The primary outcome was whether the patient received a prescription for any preventive medication during the study period. RESULTS: Among the 767 included patients, 290 (37.8%) were seen by a pediatric neurologist. Preventive medications were prescribed for 131/290 (45.2%; 95% confidence interval [CI]: 39.5%, 51.0%) patients seen by a pediatric neurologist and 206/477 (43.2%; 95% CI: 39.0%, 47.7%) patients seen by an adult neurologist (p = 0.591). In the mixed effects logistic regression model, clinician specialty was not associated with preventive medication use (adjusted odds ratio [AOR] 1.20, 95% CI: 0.62, 2.31). Female sex (AOR 1.69, 95% CI: 1.07, 2.66) and number of visits during the study period (AOR 1.64, 95% CI: 1.49, 1.80) were associated with receiving preventive medication. CONCLUSION: Approximately two fifths of young adults with migraine were prescribed preventive medications, and this proportion did not differ according to clinician specialty. Although these findings suggest that pediatric and adult neurologists provide comparable care, both specialties may be underusing preventive medications in this patient population.

Non-Hermitian Hamiltonians for linear and nonlinear optical response: A model for plexcitons.

In polaritons, the properties of matter are modified by mixing the molecular transitions with light modes inside a cavity. Resultant hybrid light-matter states exhibit energy level shifts, are delocalized over many molecular units, and have a different excited-state potential energy landscape, which leads to modified exciton dynamics. Previously, non-Hermitian Hamiltonians have been derived to describe the excited states of molecules coupled to surface plasmons (i.e., plexcitons), and these operators have been successfully used in the description of linear and third order optical response. In this article, we rigorously derive non-Hermitian Hamiltonians in the response function formalism of nonlinear spectroscopy by means of Feshbach operators and apply them to explore spectroscopic signatures of plexcitons. In particular, we analyze the optical response below and above the exceptional point that arises for matching transition energies for plasmon and molecular components and study their decomposition using double-sided Feynman diagrams. We find a clear distinction between interference and Rabi splitting in linear spectroscopy and a qualitative change in the symmetry of the line shape of the nonlinear signal when crossing the exceptional point. This change corresponds to one in the symmetry of the eigenvalues of the Hamiltonian. Our work presents an approach for simulating the optical response of sublevels within an electronic system and opens new applications of nonlinear spectroscopy to examine the different regimes of the spectrum of non-Hermitian Hamiltonians.

The effects of vitrification on oocyte quality.

Vitrification, is an ultra-rapid, manual cooling process that produces glass-like (ice crystal-free) solidification. Water is prevented from forming intercellular and intracellular ice crystals during cooling as a result of oocyte dehydration and the use of highly concentrated cryoprotectant. Though oocytes can be cryopreserved without ice crystal formation through vitrification, it is still not clear whether the process of vitrification causes any negative impact (temperature change/chilling effect, osmotic stress, cryoprotectant toxicity, and/or phase transitions) on oocyte quality, which translates to diminished embryo developmental potential or subsequent clinical outcomes. In this review, we attempt to assess the technique's potential effects and the consequence of these effects on outcomes.

Tri-iodide and vanadium chloride based chemiluminescent methods for quantification of nitrogen oxides.

Nitric Oxide (NO) is an important signaling molecule that plays roles in controlling vascular tone, hemostasis, host defense, and many other physiological functions. Low NO bioavailability contributes to pathology and NO administration has therapeutic potential in a variety of diseases. Thus, accurate measurements of NO bioavailability and reactivity are critical. Due to its short lifetime in vivo and many in vitro conditions, NO bioavailability and reactivity are often best determined by measuring NO congeners and metabolites that are more stable. Chemiluminescence-based detection of NO following chemical reduction of these compounds using the tri-iodide and vanadium chloride methods have been widely used in a variety of clinical and laboratory studies. In this review, we describe these methods used to detect nitrite, nitrate, nitrosothiols and other species and discuss limitations and proper controls.

Inorganic nitrate supplementation and blood flow restricted exercise tolerance in post-menopausal women.

Exercise tolerance appears to benefit most from dietary nitrate (NO3-) supplementation when muscle oxygen (O2) availability is low. Using a double-blind, randomized cross-over design, we tested the hypothesis that acute NO3- supplementation would improve blood flow restricted exercise duration in post-menopausal women, a population with reduced endogenous nitric oxide bioavailability. Thirteen women (57-76 yr) performed rhythmic isometric handgrip contractions (10% MVC, 30 per min) during progressive forearm blood flow restriction (upper arm cuff gradually inflated 20 mmHg each min) on three study visits, with 7-10 days between visits. Approximately one week following the first (familiarization) visit, participants consumed 140 ml of NO3- concentrated (9.7 mmol, 0.6 gm NO3-) or NO3-depleted beetroot juice (placebo) on separate days (≥7 days apart), with handgrip exercise beginning 100 min post-consumption. Handgrip force recordings were analyzed to determine if NO3- supplementation enhanced force development as blood flow restriction progressed. Nitrate supplementation increased plasma NO3- (16.2-fold) and NO2- (4.2-fold) and time to volitional fatigue (61.8 ± 56.5 s longer duration vs. placebo visit; p = 0.03). Nitrate supplementation increased the rate of force development as forearm muscle ischemia progressed (p = 0.023 between 50 and 75% of time to fatigue) with non-significant effects thereafter (p = 0.052). No effects of nitrate supplementation were observed for mean duration of contraction or relaxation rates (all p > 0.150). These results suggest that acute NO3- supplementation prolongs time-to-fatigue and speeds grip force development during progressive forearm muscle ischemia in postmenopausal women.

Endoscopic Browlift in Patients With Receding Hairlines.

ABSTRACT: Patients with receding or high hairlines have traditionally been considered unfavorable candidates for endoscopic brow lift as this can further lengthen the hairline. We analyzed outcomes in patients that underwent a novel endoscopic brow lift technique with placement of incisions and anchoring Endotine Forehead Devices (CoApt Systems Inc, Palo Alto, CA, USA) directly at the natural forehead crease lines, in an effort to minimize elevation of the hairline, whereas providing well-hidden scars. We retrospectively reviewed all patients who underwent this new Endotine and incision placement between 2016 and 2020. Preoperative and postoperative photographs of all patients were analyzed to determine the postoperative changes in brow elevation and forehead length proportion (defined as length from cranium to chin).The forehead length proportion was unchanged pre- and post-operatively, with no statistically significant differences noted (P = 0.48). The average brow position elevation ranged from 2.78 mm in the medial location to 5.05 mm in the lateral location. All patients were happy with their appearance and had improved visual fields postoperatively. The forehead scars healed well and were well hidden in forehead rhytids at long term follow-up.This novel endoscopic brow lift technique provides an option to utilize a minimally invasive approach in patients with receding hairline. With this technique, visible scars were minimized, whereas still being able to achieve reasonable brow elevation. Thus, our approach enables long term maintenance of brow elevation with inconspicuous scars in the forehead.

The Seniority Swoop: Young Nurse Burnout, Violence, and Turnover Intention in an 11-Hospital Sample.

We sought to understand whether nurses aged 20 to 29 years burnout and intend to turnover in higher proportions than more senior nurses, and if so, why. Guided by Maslow's hierarchy, we used brief inventories to assess hospital-based bedside nurses at 11 hospitals in Pennsylvania and Rhode Island (n = 3549/9520) prior to the pandemic. In a second study, we compared scheduling policies, bargaining, and Magnet status to see whether these variables predicted worsened burnout rates in young nurses. In a pattern that appears like a swooping line when graphed, nurses aged 20 to 29 years reported higher burnout and intention to leave than more senior nurses. They also reported being punched, bitten, spit on, kicked, or otherwise physically struck more often, worked more long shifts, worked more nights, and reported more dehydration and poorer sleep. Notably, age alone was not a strong predictor of turnover until burnout was added to the model, indicating that there is no inherent millennial trait resulting in higher turnover. Instead, preventing and addressing burnout is key to retention. When comparing hospital characteristics, only scheduling perks for senior nurses predicted the seniority swoop pattern. We offer 9 recommendations to reduce burnout and turnover in young nurses.

Unnecessary Antibiotic Prescribing in US Ambulatory Care Settings, 2010-2015.

The proportion of antibiotic prescriptions prescribed in US physician offices and emergency departments that were unnecessary decreased slightly, from 30% in 2010-2011 to 28% in 2014-2015. However, a greater decrease occurred in children: 32% in 2010-2011 to 19% in 2014-2015. Unnecessary prescribing in adults did not change during this period.

Mechanistic insights into cell-free hemoglobin-induced injury during septic shock.

Cell-free hemoglobin (CFH) levels are elevated in septic shock and are higher in nonsurvivors. Whether CFH is only a marker of sepsis severity or is involved in pathogenesis is unknown. This study aimed to investigate whether CFH worsens sepsis-associated injuries and to determine potential mechanisms of harm. Fifty-one, 10-12 kg purpose-bred beagles were randomized to receive Staphylococcus aureus intrapulmonary challenges or saline followed by CFH infusions (oxyhemoglobin >80%) or placebo. Animals received antibiotics and intensive care support for 96 h. CFH significantly increased mean pulmonary arterial pressures and right ventricular afterload in both septic and nonseptic animals, effects that were significantly greater in nonsurvivors. These findings are consistent with CFH-associated nitric oxide (NO) scavenging and were associated with significantly depressed cardiac function, and worsened shock, lactate levels, metabolic acidosis, and multiorgan failure. In septic animals only, CFH administration significantly increased mean alveolar-arterial oxygenation gradients, also to a significantly greater degree in nonsurvivors. CFH-associated iron levels were significantly suppressed in infected animals, suggesting that bacterial iron uptake worsened pneumonia. Notably, cytokine levels were similar in survivors and nonsurvivors and were not predictive of outcome. In the absence and presence of infection, CFH infusions resulted in pulmonary hypertension, cardiogenic shock, and multiorgan failure, likely through NO scavenging. In the presence of infection alone, CFH infusions worsened oxygen exchange and lung injury, presumably by supplying iron that promoted bacterial growth. CFH elevation, a known consequence of clinical septic shock, adversely impacts sepsis outcomes through more than one mechanism, and is a biologically plausible, nonantibiotic, noncytokine target for therapeutic intervention.NEW & NOTEWORTHY Cell-free hemoglobin (CFH) elevations are a known consequence of clinical sepsis. Using a two-by-two factorial design and extensive physiological and biochemical evidence, we found a direct mechanism of injury related to nitric oxide scavenging leading to pulmonary hypertension increasing right heart afterload, depressed cardiac function, worsening circulatory failure, and death, as well as an indirect mechanism related to iron toxicity. These discoveries alter conventional thinking about septic shock pathogenesis and provide novel therapeutic approaches.