Single-cell transcriptomics reveal a hyperacute cytokine and immune checkpoint axis after cardiac arrest in patients with poor neurological outcome

Mitochondrial DNA-Mediated Immune Activation After Resuscitation from Cardiac Arrest

Background

Post-cardiac arrest syndrome (PCAS) is characterized by a robust inflammatory response that contributes to significant morbidity and mortality among patients resuscitated from sudden cardiac arrest (SCA). Mitochondrial DNA (mtDNA), with its bacterial-like genomic motifs, has been implicated as a damage-associated molecular pattern in other inflammatory contexts, but its role as a pro-inflammatory stimulus in PCAS has not been studied. Accordingly, the present study was designed to determine if PCAS is characterized by a rise in circulating mtDNA and, if so, whether mtDNA is selectively released, how it activates immune cells, and if targeting mtDNA-sensing pathways attenuates leukocyte activation.

Methods

Plasma mtDNA and nuclear DNA (nucDNA) levels were measured in peripheral blood samples collected ∼4-hours post-ROSC from swine with PCAS (n=8) and patients hospitalized after resuscitation from out-of-hospital cardiac arrest (OHCA; n= 57). Additionally, in vitro studies were performed where porcine peripheral blood mononuclear cells (PBMCs) were treated with mtDNA or extracellular vesicles (EVs) isolated from post-ROSC plasma. Pharmacological inhibitors were utilized to inhibit toll-like receptor 9 (TLR9)- and cyclic GMP-AMP synthase (cGAS)-mediated mtDNA sensing.

Results

A significant ∼250-fold elevation in circulating mtDNA was observed shortly after ROSC in swine despite negligible changes in circulating nucDNA, suggesting selective release of mtDNA in PCAS. This finding was corroborated in human OHCA survivors, in which circulating mtDNA was similarly elevated during the early post-ROSC period. Circulating mtDNA was largely encapsulated within EVs in swine and humans, suggesting a conserved mechanism of release across species. In vitro studies demonstrated that PBMC internalization of mtDNA-containing-EVs was required for immune activation and promoted development of a pro-inflammatory leukocyte phenotype characterized by altered surface marker expression and increased release of TNFα, IL-1β, and IL-6. Disrupting EVs or degrading enclosed DNA attenuated these responses, which were partially restored upon reintroduction of mtDNA. Pharmacological blockade of TLR9 or cGAS pathways significantly reduced mtDNA-induced inflammation, providing insight regarding signaling pathways that may be targeted to modulate mtDNA-mediated immune activation in PCAS.

Conclusions

These novel findings demonstrate that brief whole-body ischemia and reperfusion in the context of resuscitation from SCA triggers selective mtDNA release, primarily within EVs, that acts as a potent driver of immune activation in PCAS. By linking EV-encapsulated mtDNA to TLR9 and cGAS activation, this study provides a foundation for the development of novel therapeutic interventions aimed at limiting mtDNA release or disrupting its downstream sensing pathways to enhance survival and improve outcomes after SCA.

Clinical Perspective

What is new?: Our study reveals that circulating mitochondrial DNA (mtDNA), primarily encapsulated in extracellular vesicles (EV), is selectively released into the bloodstream after resuscitation from sudden cardiac arrest.EV-encapsulated mtDNA triggers immune cell activation, evidenced by phenotypic shifts toward inflammatory dendritic cells and macrophages, as well as increased pro-inflammatory cytokine secretion.Pharmacological inhibition of TLR9 and cGAS pathways significantly attenuates the mtDNA-induced inflammatory response, pointing to novel therapeutic avenues for modulating post-resuscitation immune activation in patients with post-cardiac arrest syndrome (PCAS).What are the clinical implications?: Identification of mtDNA as a key driver of sterile inflammation in PCAS highlights a potential target for interventions aimed at reducing multi-organ damage and improving neurological outcomes.Therapeutic strategies to block mtDNA release or downstream signaling (e.g., TLR9/cGAS inhibition) may limit harmful pro-inflammatory cascades and bolster long-term survival following resuscitation from cardiac arrest.Early clinical screening for elevated EV-encapsulated mtDNA could help refine prognostic evaluations, complement current biomarkers, and guide personalized therapy to lessen the inflammatory burden of PCAS.

Targeting Cytokine-Mediated Inflammation in Brain Disorders: Developing New Treatment Strategies

Cytokine-mediated inflammation is increasingly recognized for playing a vital role in the pathophysiology of a wide range of brain disorders, including neurodegenerative, psychiatric, and neurodevelopmental problems. Pro-inflammatory cytokines such as interleukin-1 (IL-1), tumor necrosis factor-alpha (TNF-α), and interleukin-6 (IL-6) cause neuroinflammation, alter brain function, and accelerate disease development. Despite progress in understanding these pathways, effective medicines targeting brain inflammation are still limited. Traditional anti-inflammatory and immunomodulatory drugs are effective in peripheral inflammatory illnesses. Still, they face substantial hurdles when applied to the central nervous system (CNS), such as the blood-brain barrier (BBB) and unwanted systemic effects. This review highlights the developing treatment techniques for modifying cytokine-driven neuroinflammation, focusing on advances that selectively target critical cytokines involved in brain pathology. Novel approaches, including cytokine-specific inhibitors, antibody-based therapeutics, gene- and RNA-based interventions, and sophisticated drug delivery systems like nanoparticles, show promise with respect to lowering neuroinflammation with greater specificity and safety. Furthermore, developments in biomarker discoveries and neuroimaging techniques are improving our ability to monitor inflammatory responses, allowing for more accurate and personalized treatment regimens. Preclinical and clinical trial data demonstrate the therapeutic potential of these tailored techniques. However, significant challenges remain, such as improving delivery across the BBB and reducing off-target effects. As research advances, the creation of personalized, cytokine-centered therapeutics has the potential to alter the therapy landscape for brain illnesses, giving patients hope for better results and a higher quality of life.

Diverse NKT cells regulate early inflammation and neurological outcomes after cardiac arrest and resuscitation

Neurological injury drives most deaths and morbidity among patients hospitalized for out-of-hospital cardiac arrest (OHCA). Despite its clinical importance, there are no effective pharmacological therapies targeting post-cardiac arrest (CA) neurological injury. Here, we analyzed circulating immune cells from a large cohort of patients with OHCA, finding that lymphopenia independently associated with poor neurological outcomes. Single-cell RNA sequencing of immune cells showed that T cells with features of both innate T cells and natural killer (NK) cells were increased in patients with favorable neurological outcomes. We more specifically identified an early increase in circulating diverse NKT (dNKT) cells in a separate cohort of patients with OHCA who had good neurological outcomes. These cells harbored a diverse T cell receptor repertoire but were consistently specific for sulfatide antigen. In mice, we found that sulfatide-specific dNKT cells trafficked to the brain after CA and resuscitation. In the brains of mice lacking NKT cells (Cd1d-/-), we observed increased inflammatory chemokine and cytokine expression and accumulation of macrophages when compared with wild-type mice. Cd1d-/- mice also had increased neuronal injury, neurological dysfunction, and worse mortality after CA. To therapeutically enhance dNKT cell activity, we treated mice with sulfatide lipid after CA, showing that it improved neurological function. Together, these data show that sulfatide-specific dNKT cells are associated with good neurological outcomes after clinical OHCA and are neuroprotective in mice after CA. Strategies to enhance the number or function of dNKT cells may thus represent a treatment approach for CA.

Combination of Hydrogen Inhalation and Hypothermic Temperature Control After Out-of-Hospital Cardiac Arrest: A Post hoc Analysis of the Efficacy of Inhaled Hydrogen on Neurologic Outcome Following Brain Ischemia During PostCardiac Arrest Care II Trial

OBJECTIVE

The Efficacy of Inhaled Hydrogen on Neurologic Outcome Following Brain Ischemia During Post-Cardiac Arrest Care (HYBRID) II trial (jRCTs031180352) suggested that hydrogen inhalation may reduce post-cardiac arrest brain injury (PCABI). However, the combination of hypothermic target temperature management (TTM) and hydrogen inhalation on outcomes is unclear. The aim of this study was to investigate the combined effect of hydrogen inhalation and hypothermic TTM on outcomes after out-of-hospital cardiac arrest (OHCA).

DESIGN

Post hoc analysis of a multicenter, randomized, controlled trial.

SETTING

Fifteen Japanese ICUs.

PATIENTS

Cardiogenic OHCA enrolled in the HYBRID II trial.

INTERVENTIONS

Hydrogen mixed oxygen (hydrogen group) versus oxygen alone (control group).

MEASUREMENTS AND MAIN RESULTS

TTM was performed at a target temperature of 32-34°C (TTM32-TTM34) or 35-36°C (TTM35-TTM36) per the institutional protocol. The association between hydrogen + TTM32-TTM34 and 90-day good neurologic outcomes was analyzed using generalized estimating equations. The 90-day survival was compared between the hydrogen and control groups under TTM32-TTM34 and TTM35-TTM36, respectively. The analysis included 72 patients (hydrogen [ n = 39] and control [ n = 33] groups) with outcome data. TTM32-TTM34 was implemented in 25 (64%) and 24 (73%) patients in the hydrogen and control groups, respectively ( p = 0.46). Under TTM32-TTM34, 17 (68%) and 9 (38%) patients achieved good neurologic outcomes in the hydrogen and control groups, respectively (relative risk: 1.81 [95% CI, 1.05-3.66], p < 0.05). Hydrogen + TTM32-TTM34 was independently associated with good neurologic outcomes (adjusted odds ratio 16.10 [95% CI, 1.88-138.17], p = 0.01). However, hydrogen + TTM32-TTM34 did not improve survival compared with TTM32-TTM34 alone (adjusted hazard ratio: 0.22 [95% CI, 0.05-1.06], p = 0.06).

CONCLUSIONS

Hydrogen + TTM32-TTM34 was associated with improved neurologic outcomes after cardiogenic OHCA compared with TTM32-TTM34 monotherapy. Hydrogen inhalation is a promising treatment option for reducing PCABI when combined with TTM32-TTM34.

Protocol for immunophenotyping out-of-hospital cardiac arrest patients

Immunophenotyping of out-of-hospital cardiac arrest (OHCA) patients is of increasing interest but has challenges. Here, we describe steps for the design of the clinical cohort, planning patient enrollment and sample collection, and ethical review of the study protocol. We detail procedures for blood sample collection and cryopreservation of peripheral blood mononuclear cells (PBMCs). We detail steps to modulate immune checkpoints in OHCA PBMC ex vivo. This protocol also has relevance for immunophenotyping other types of critical illness. For complete details on the use and execution of this protocol, please refer to Tamura et al. (2023).1.