Mitochondrial damage-associated molecular patterns trigger arginase-dependent lymphocyte immunoregulation

Mitochondrial DNA levels in perfusate and bile during ex vivo normothermic machine correspond with donor liver quality

Ex vivo normothermic machine perfusion (NMP) preserves donor organs and permits real-time assessment of allograft health, but the most effective indicators of graft viability are uncertain. Mitochondrial DNA (mtDNA), released consequent to traumatic cell injury and death, including the ischemia-reperfusion injury inherent in transplantation, may meet the need for a biomarker in this context. We describe a real time PCR-based approach to assess cell-free mtDNA during NMP as a universal biomarker of allograft quality. Measured in the perfusate fluid of 29 livers, the quantity of mtDNA correlated with metrics of donor liver health including International Normalized Ratio (INR), lactate, and warm ischemia time, and inversely correlated with inferior vena cava (IVC) flow during perfusion. Our findings endorse mtDNA as a simple and rapidly measured feature that can inform donor liver health, opening the possibility to better assess livers acquired from extended criteria donors to improve organ supply.

Advances in metabolic reprogramming of NK cells in the tumor microenvironment on the impact of NK therapy

Natural killer (NK) cells are unique from other immune cells in that they can rapidly kill multiple neighboring cells without the need for antigenic pre-sensitization once the cells display surface markers associated with oncogenic transformation. Given the dynamic role of NK cells in tumor surveillance, NK cell-based immunotherapy is rapidly becoming a "new force" in tumor immunotherapy. However, challenges remain in the use of NK cell immunotherapy in the treatment of solid tumors. Many metabolic features of the tumor microenvironment (TME) of solid tumors, including oxygen and nutrient (e.g., glucose, amino acids) deprivation, accumulation of specific metabolites (e.g., lactate, adenosine), and limited availability of signaling molecules that allow for metabolic reorganization, multifactorial shaping of the immune-suppressing TME impairs tumor-infiltrating NK cell function. This becomes a key barrier limiting the success of NK cell immunotherapy in solid tumors. Restoration of endogenous NK cells in the TME or overt transfer of functionally improved NK cells holds great promise in cancer therapy. In this paper, we summarize the metabolic biology of NK cells, discuss the effects of TME on NK cell metabolism and effector functions, and review emerging strategies for targeting metabolism-improved NK cell immunotherapy in the TME to circumvent these barriers to achieve superior efficacy of NK cell immunotherapy.

Integrative transcriptomic and genomic analyses unveil the IFI16 variants and expression as MASLD progression markers

BACKGROUND AND AIMS

Metabolic dysfunction-associated steatotic liver disease (MASLD) encompasses a broad and continuous spectrum of liver diseases ranging from fatty liver to steatohepatitis. The intricate interactions of genetic, epigenetic, and environmental factors in the development and progression of MASLD remain elusive. Here, we aimed to achieve an integrative understanding of the genomic and transcriptomic alterations throughout the progression of MASLD.

APPROACH AND RESULTS

RNA-Seq profiling (n = 146) and whole-exome sequencing (n = 132) of MASLD liver tissue samples identified 3 transcriptomic subtypes (G1-G3) of MASLD, which were characterized by stepwise pathological and molecular progression of the disease. Macrophage-driven inflammatory activities were identified as a key feature for differentiating these subtypes. This subtype-discriminating macrophage interplay was significantly associated with both the expression and genetic variation of the dsDNA sensor IFI16 (rs6940, A>T, T779S), establishing it as a fundamental molecular factor in MASLD progression. The in vitro dsDNA-IFI16 binding experiments and structural modeling revealed that the IFI16 variant exhibited increased stability and stronger dsDNA binding affinity compared to the wild-type. Further downstream investigation suggested that the IFI16 variant exacerbated DNA sensing-mediated inflammatory signals through mitochondrial dysfunction-related signaling of the IFI16-PYCARD-CASP1 pathway.

CONCLUSIONS

This study unveils a comprehensive understanding of MASLD progression through transcriptomic classification, highlighting the crucial roles of IFI16 variants. Targeting the IFI16-PYCARD-CASP1 pathway may pave the way for the development of novel diagnostics and therapeutics for MASLD.

Metabolic adaptation of NK cell activity and behavior in tumors: challenges and therapeutic opportunities

The adaptation of natural killer (NK) cells to conditions in the microenvironment of tumors is deeply affected by their metabolic activity, itself a result of nutrient availability and the metabolism of the cancer cells themselves. Elevated rates of glycolysis and lipid metabolism in cancers not only lead to the accumulation of immunosuppressive byproducts but also contribute to an environment of elevated concentrations of extracellular metabolites. This results in altered NK cell bioenergetics through changes in transcriptional and translational profiles, ultimately affecting their pharmacology and impairing NK cell responses. However, understanding the metabolic processes that drive alterations in immunological signaling on NK cells remains both difficult and vastly underexplored. We discuss the varied and complex drivers of NK cell metabolism in homeostasis and the tumor microenvironment (TME), challenges associated with their targetability, and unexplored therapeutic opportunities.

The immune suppressive properties of damage associated molecular patterns in the setting of sterile traumatic injury

Associated with the development of hospital-acquired infections, major traumatic injury results in an immediate and persistent state of systemic immunosuppression, yet the underlying mechanisms are poorly understood. Detected in the circulation in the minutes, days and weeks following injury, damage associated molecular patterns (DAMPs) are a heterogeneous collection of proteins, lipids and DNA renowned for initiating the systemic inflammatory response syndrome. Suggesting additional immunomodulatory roles in the post-trauma immune response, data are emerging implicating DAMPs as potential mediators of post-trauma immune suppression. Discussing the results of in vitro, in vivo and ex vivo studies, the purpose of this review is to summarise the emerging immune tolerising properties of cytosolic, nuclear and mitochondrial-derived DAMPs. Direct inhibition of neutrophil antimicrobial activities, the induction of endotoxin tolerance in monocytes and macrophages, and the recruitment, activation and expansion of myeloid derived suppressor cells and regulatory T cells are examples of some of the immune suppressive properties assigned to DAMPs so far. Crucially, with studies identifying the molecular mechanisms by which DAMPs promote immune suppression, therapeutic strategies that prevent and/or reverse DAMP-induced immunosuppression have been proposed. Approaches currently under consideration include the use of synthetic polymers, or the delivery of plasma proteins, to scavenge circulating DAMPs, or to treat critically-injured patients with antagonists of DAMP receptors. However, as DAMPs share signalling pathways with pathogen associated molecular patterns, and pro-inflammatory responses are essential for tissue regeneration, these approaches need to be carefully considered in order to ensure that modulating DAMP levels and/or their interaction with immune cells does not negatively impact upon anti-microbial defence and the physiological responses of tissue repair and wound healing.

Emerging roles of mitochondria in animal regeneration

The regeneration capacity after an injury is critical to the survival of living organisms. In animals, regeneration ability can be classified into five primary types: cellular, tissue, organ, structure, and whole-body regeneration. Multiple organelles and signaling pathways are involved in the processes of initiation, progression, and completion of regeneration. Mitochondria, as intracellular signaling platforms of pleiotropic functions in animals, have recently gained attention in animal regeneration. However, most studies to date have focused on cellular and tissue regeneration. A mechanistic understanding of the mitochondrial role in large-scale regeneration is unclear. Here, we reviewed findings related to mitochondrial involvement in animal regeneration. We outlined the evidence of mitochondrial dynamics across different animal models. Moreover, we emphasized the impact of defects and perturbation in mitochondria resulting in regeneration failure. Ultimately, we discussed the regulation of aging by mitochondria in animal regeneration and recommended this for future study. We hope this review will serve as a means to advocate for more mechanistic studies of mitochondria related to animal regeneration on different scales.

Citrulline enteral administration markedly reduces immunosuppressive extrafollicular plasma cell differentiation in a preclinical model of sepsis

The sustained immunosuppression associated with severe sepsis favors an increased susceptibility to secondary infections and remains incompletely understood. Plasmablast and plasma cell subsets, whose primary function is to secrete antibodies, have emerged as important suppressive populations that expand during sepsis. In particular, sepsis supports CD39^hi plasmablast metabolic reprogramming associated with adenosine-mediated suppressive activity. Arginine deficiency has been linked to an increased risk of secondary infections in sepsis. Overcoming arginine shortage by citrulline administration efficiently improves sepsis-induced immunosuppression and secondary infections in the cecal ligation and puncture murine model. Here, we aimed to determine the impact of citrulline administration on B cell suppressive responses in sepsis. We demonstrate that restoring arginine bioavailability through citrulline administration markedly reduces the dominant extrafollicular B cell response, decreasing the immunosuppressive LAG3⁺ and CD39⁺ plasma cell populations, and restoring splenic follicles. At the molecular level, the IRF4/MYC-mediated B cell reprogramming required for extrafollicular plasma cell differentiation is shunted in the splenic B cells of mice fed with citrulline. Our study reveals a prominent impact of nutrition on B cell responses and plasma cell differentiation and further supports the development of citrulline-based clinical studies to prevent sepsis-associated immune dysfunction.

ATF5 Attenuates the Secretion of Pro-Inflammatory Cytokines in Activated Microglia

The highly dynamic changes in microglia necessary to achieve a rapid neuroinflammatory response require a supply of energy from mitochondrial respiration, which leads to the accumulation of unfolded mitochondrial proteins. We previously reported that microglial activation is correlated with the mitochondrial unfolded protein response (UPRmt) in a kaolin-induced hydrocephalus model, but we still do not know the extent to which these changes in microglia are involved in cytokine release. Here, we investigated the activation of BV-2 cells and found that treatment with lipopolysaccharide (LPS) for 48 h increased the secretion of pro-inflammatory cytokines. This increase was accompanied by a concurrent decrease in oxygen consumption rate (OCR) and mitochondrial membrane potential (MMP), in association with the up-regulation of the UPRmt. Inhibition of the UPRmt by knockdown of ATF5, a key upstream regulator of the UPRmt, using small-interfering RNA against ATF5 (siATF5) not only increased production of the pro-inflammatory cytokines, interleukin-6 (IL-6), IL-1β and tumor necrosis factor-α (TNF-α), but also decreased MMP. Our results suggest that ATF5-dependent induction of the UPRmt in microglia acts as a protective mechanism during neuroinflammation and may be a potential therapeutic target for reducing neuroinflammation.

Preparation of mitochondrial damage-associated molecular patterns from mouse liver tissue

Mitochondrial damage-associated molecular patterns (mitoDAMPs) are released from cells dying uncontrolled, non-apoptotic deaths, usually secondary to disease or trauma. Here, we describe preparation of mitoDAMPs from mouse liver, but this protocol can be adapted for preparation of mitoDAMPs from other species and tissues. Tissues are dissociated and then processed to isolate mitochondria. Mitochondria are then sonicated and mitoDAMPs are collected by ultracentrifugation. This procedure produces μg quantities of mitoDAMPs and facilitates research to understand their impacts in health and disease. For complete details on the use and execution of this protocol, please refer to Westhaver et al. (2022).