Timothée Fettrelet

Macrophage activation is tightly coupled to cellular metabolism: classically activated pro-inflammatory (M1) macrophages rely on glycolysis and a disrupted tricarboxylic acid cycle, whereas alternatively activated (M2) macrophages depend on oxidative phosphorylation (OXPHOS) and fatty acid oxidation. Although mitochondria are central to this metabolic plasticity, it remains unclear whether mitochondrial dysfunction itself can dictate macrophage polarization. Using macrophage-specific OPA1 knockout mice, we investigated how mitochondrial dysfunction influences macrophage metabolism and immune homeostasis. Loss of OPA1 caused severe impairment of OXPHOS, reduced mitochondrial membrane potential, and a compensatory glycolytic shift, driving M0 and M2 macrophages toward an M1-like bioenergetic state. Integrative metabolomic and transcriptomic analyses revealed strong priming of OPA1-deficient macrophages towards classical activation, including accumulation of M1-associated metabolites (lactate, succinate, itaconate) and upregulation of NF-κB–driven and other inflammatory gene programs, resulting in increased secretion of IL-6 and TNF even in the absence of stimulation. Functionally, this metabolic shift primed non-activated and M2 macrophages toward partial M1 polarization with enhanced bactericidal capacity, while simultaneously suppressing M2-associated processes such as proliferation, efferocytosis, and expression of Arg1, CD206, and RELMα. In vivo, OPA1 ^ΔM mice displayed reduced peritoneal macrophage abundance, impaired self-renewal after IL-4 complex stimulation, and compensatory monocyte recruitment. The remaining macrophages exhibited increased MHCII and reduced RELMα expression, consistent with partial M1 skewing and loss of alternative activation. These local alterations were mirrored systemically: blood profiling revealed enhanced T-cell activation and sex-specific remodeling of immune composition during inflammation and aging. Collectively, these findings demonstrate that mitochondrial dysfunction serves as a cell-intrinsic cue that primes macrophages toward a glycolytic, pro-inflammatory phenotype while constraining their M2 properties and proliferative capacities. This dual metabolic and functional rewiring highlights mitochondrial integrity as a pivotal determinant of macrophage immunometabolic identity and reveals how its disruption can reshape both local and systemic immune homeostasis.

Eosinophils, a type of granulocyte derived from myeloid precursors in the bone marrow, are distinguished by their cytoplasmic granules. They play crucial roles in immunoregulation, tissue homeostasis, and host defense, while also contributing to the pathogenesis of various inflammatory diseases. Although long non-coding RNAs (lncRNAs) are known to be involved in eosinophilic conditions, their specific expression and functions within eosinophils have not been thoroughly investigated, largely due to the reliance on tissue homogenates. In an effort to address this gap, we analyzed publicly available high-throughput RNA sequencing data to identify lncRNAs associated with eosinophilic conditions. Among the identified lncRNAs, ITGB2 antisense RNA 1 (ITGB2-AS1) was significantly downregulated in blood eosinophils from patients with hypereosinophilia. To further explore its role in eosinophil biology, we generated a stable ITGB2-AS1 knockdown in the HL-60 cell line. Interestingly, ITGB2-AS1 deficiency led to impaired eosinophil differentiation, as evidenced by a reduction in cytoplasmic granules and decreased expression of key eosinophil granule proteins, including eosinophil peroxidase (EPX) and major basic protein-1 (MBP-1). Additionally, ITGB2-AS1-deficient cells exhibited compromised eosinophil effector functions, with reduced degranulation and impaired production of reactive oxygen species (ROS). These findings suggest that ITGB2-AS1 plays a pivotal role in eosinophil differentiation and function, positioning it as a novel regulator in eosinophil biology.

Neutrophils are a specialized subset of white blood cells, which have the ability to store pre-formed mediators in their cytoplasmic granules. Neutrophils are well-known effector cells involved in host protection against pathogens through diverse mechanisms such as phagocytosis, degranulation, extracellular traps, and oxidative burst. In this study, we provide evidence highlighting the significance of the SNARE proteins syntaxin-4 and synaptosomal-associated protein (SNAP) 23 in the release of azurophilic granules, specific granules, and the production of reactive oxygen species in human neutrophils. In contrast, the specific blockade of either syntaxin-4 or SNAP23 did not prevent the release of mitochondrial dsDNA in the process of neutrophil extracellular trap (NET) formation. These findings imply that degranulation and the release of mitochondrial dsDNA involve at least partially distinct molecular pathways in neutrophils.

Eosinophils are bone marrow‐derived granulocytes and are found in low numbers in the peripheral blood of healthy subjects. In type 2 inflammatory diseases, eosinopoiesis in the bone marrow is increased, resulting in a rise in the number of mature eosinophils released in the circulation. From the blood, eosinophils can migrate in multiple tissues and organs under both physiological and pathological conditions. Eosinophils exert their various functions through the synthesis and release of a variety of granule proteins and pro‐inflammatory mediators. Despite being present in all species of vertebrates, the functional role of eosinophils is still a matter of debate. Eosinophils may play a role in host defense against various pathogens. In addition, eosinophils have been reported to be involved in tissue homeostasis and exhibit immunomodulatory activities. In this review, we aim to provide a broad overview of eosinophil biology and eosinophilic diseases in a lexicon‐style format using keywords starting from A until Z with cross‐references to other chapters indicated in italics in the text or specified in parentheses.

Neutrophil extracellular traps (NETs) and oxidative stress are considered to be beneficial in the innate immune defense against pathogens. However, defective clearance of NETs in the lung of acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-infected patients could lead to severe respiratory syndrome infection, the so-called coronavirus disease 2019 (COVID-19). To elucidate the pathways that are related to NETs within the pathophysiology of COVID-19, we utilized RNA sequencing (RNA-seq) as well as immunofluorescence and immunohistochemistry methods. RNA-seq analysis provided evidence for increased oxidative stress and the activation of viral-related signaling pathways in post-mortem lungs of COVID-19 patients compared to control donors. Moreover, an excess of neutrophil infiltration and NET formation were detected in the patients’ lungs, where the extracellular DNA was oxidized and co-localized with neutrophil granule protein myeloperoxidase (MPO). Interestingly, staining of the lipid peroxidation marker 4-hydroxynonenal (4-HNE) depicted high colocalization with NETs and was correlated with the neutrophil infiltration of the lung tissues, suggesting that it could serve as a suitable marker for the identification of NETs and the severity of the disease. Moreover, local inhalation therapy to reduce the excess lipid oxidation and NETs in the lungs of severely infected patients might be useful to ameliorate their clinical conditions.

Cell death plays a critical role in inflammatory responses. During pyroptosis, inflammatory caspases cleave Gasdermin D (GSDMD) to release an N-terminal fragment that generates plasma membrane pores that mediate cell lysis and IL-1 cytokine release. Terminal cell lysis and IL-1β release following caspase activation can be uncoupled in certain cell types or in response to particular stimuli, a state termed hyperactivation. However, the factors and mechanisms that regulate terminal cell lysis downstream of GSDMD cleavage remain poorly understood. In the course of studies to define regulation of pyroptosis duringYersiniainfection, we identified a line ofCard19-deficient mice (Card19^lxcn)whose macrophages were protected from cell lysis and showed reduced apoptosis and pyroptosis, yet had wild-type levels of caspase activation, IL-1 secretion, and GSDMD cleavage. Unexpectedly, CARD19, a mitochondrial CARD-containing protein, was not directly responsible for this, as an independently-generated CRISPR/Cas9Card19knockout mouse line (Card19^Null) showed no defect in macrophage cell lysis. Notably,Card19is located on chromosome 13, immediately adjacent toNinj1, which was recently found to regulate cell lysis downstream of GSDMD activation. RNA-seq and western blotting revealed thatCard19^lxcnBMDMs have significantly reduced NINJ1 expression, and reconstitution ofNinj1inCard19^lxcnimmortalized BMDMs restored their ability to undergo cell lysis in response to caspase-dependent cell death stimuli.Card19^lxcnmice exhibited increased susceptibility toYersiniainfection, whereas independently-generatedCard19^Nullmice did not, demonstrating that cell lysis itself plays a key role in protection against bacterial infection, and that the increased infection susceptibility ofCard19^lxcnmice is attributable to loss of NINJ1. Our findings identify genetic targeting ofCard19being responsible for off-target effects on the adjacent geneNinj1, disrupting the ability of macrophages to undergo plasma membrane rupture downstream of gasdermin cleavage and impacting host survival and bacterial control duringYersiniainfection.

Eosinophils are specialized white blood cells, which are involved in the pathology of diverse allergic and nonallergic inflammatory diseases. Eosinophils are traditionally known as cytotoxic effector cells but have been suggested to additionally play a role in immunomodulation and maintenance of homeostasis. The exact role of these granule-containing leukocytes in health and diseases is still a matter of debate. Degranulation is one of the key effector functions of eosinophils in response to diverse stimuli. The different degranulation patterns occurring in eosinophils (piecemeal degranulation, exocytosis and cytolysis) have been extensively studied in the last few years. However, the exact mechanism of the diverse degranulation types remains unknown and is still under investigation. In this review, we focus on recent findings and highlight the diversity of stimulation and methods used to evaluate eosinophil degranulation.

Eosinophils are a subset of granulocytes characterized by a high abundance of specific granules in their cytoplasm. To act as effector cells, eosinophils degranulate and form eosinophil extracellular traps (EETs), which contain double-stranded DNA (dsDNA) co-localized with granule proteins. The exact molecular mechanism of EET formation remains unknown. Although the term “EET release” has been used in scientific reports, it is unclear whether EETs are pre-formed in eosinophils and subsequently released. Moreover, although eosinophil degranulation has been extensively studied, a precise time-course of granule protein release has not been reported until now. In this study, we investigated the time-dependent release of eosinophil peroxidase (EPX) and mitochondrial DNA (mtDNA) following activation of both human and mouse eosinophils. Unexpectedly, maximal degranulation was already observed within 1 min with no further change upon complement factor 5 (C5a) stimulation of interleukin-5 (IL-5) or granulocyte/macrophage colony-stimulating factor (GM-CSF)-primed eosinophils. In contrast, bulk mtDNA release in the same eosinophil populations occurred much slower and reached maximal levels between 30 and 60 min. Although no single-cell analyses have been performed, these data suggest that the molecular pathways leading to degranulation and mtDNA release are at least partially different. Moreover, based on these data, it is likely that the association between the mtDNA scaffold and granule proteins in the process of EET formation occurs in the extracellular space.

Gasdermin D (GSDMD) is a pore-forming protein that promotes pyroptosis and release of proinflammatory cytokines. Recent studies revealed that apoptotic caspase-8 directly cleaves GSDMD to trigger pyroptosis. However, the molecular requirements for caspase-8–dependent GSDMD cleavage and the physiological impact of this signaling axis are unresolved. Here, we report that caspase-8–dependent GSDMD cleavage confers susceptibility to tumor necrosis factor (TNF)–induced lethality independently of caspase-1 and that GSDMD activation provides host defense against Yersinia infection. We further demonstrate that GSDMD inactivation by apoptotic caspases at aspartate 88 (D88) suppresses TNF-induced lethality but promotes anti-Yersinia defense. Last, we show that caspase-8 dimerization and autoprocessing are required for GSDMD cleavage, and provide evidence that the caspase-8 autoprocessing and activity on various complexes correlate with its ability to directly cleave GSDMD. These findings reveal GSDMD as a potential therapeutic target to reduce inflammation associated with mutations in the death receptor signaling machinery.