Fully activated innate immune cells are required for effective responses to infection, but their prompt deactivation and removal are essential for limiting tissue damage. Here, we have identified a critical role for the prolyl hydroxylase enzyme Phd2 in maintaining the balance between appropriate, predominantly neutrophil-mediated pathogen clearance and resolution of the innate immune response. We demonstrate that myeloid-specific loss of Phd2 resulted in an exaggerated inflammatory response to Streptococcus pneumonia, with increases in neutrophil motility, functional capacity, and survival. These enhanced neutrophil responses were dependent upon increases in glycolytic flux and glycogen stores. Systemic administration of a HIF–prolyl hydroxylase inhibitor replicated the Phd2-deficient phenotype of delayed inflammation resolution. Together, these data identify Phd2 as the dominant HIF-hydroxylase in neutrophils under normoxic conditions and link intrinsic regulation of glycolysis and glycogen stores to the resolution of neutrophil-mediated inflammatory responses. These results demonstrate the therapeutic potential of targeting metabolic pathways in the treatment of inflammatory disease.

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Rationale: Acute respiratory distress syndrome is defined by the presence of systemic hypoxia and consequent on disordered neutrophilic inflammation. Local mechanisms limiting the duration and magnitude of this neutrophilic response remain poorly understood. Objectives: To test the hypothesis that during acute lung inflammation tissue production of proresolution type 2 cytokines (IL-4 and IL-13) dampens the proinflammatory effects of hypoxia through suppression of HIF-1α (hypoxia-inducible factor-1α)-mediated neutrophil adaptation, resulting in resolution of lung injury. Methods: Neutrophil activation of IL4Ra (IL-4 receptor α) signaling pathways was explored ex vivo in human acute respiratory distress syndrome patient samples, in vitro after the culture of human peripheral blood neutrophils with recombinant IL-4 under conditions of hypoxia, and in vivo through the study of IL4Ra-deficient neutrophils in competitive chimera models and wild-type mice treated with IL-4. Measurements and Main Results: IL-4 was elevated in human BAL from patients with acute respiratory distress syndrome, and its receptor was identified on patient blood neutrophils. Treatment of human neutrophils with IL-4 suppressed HIF-1α–dependent hypoxic survival and limited proinflammatory transcriptional responses. Increased neutrophil apoptosis in hypoxia, also observed with IL-13, required active STAT signaling, and was dependent on expression of the oxygen-sensing prolyl hydroxylase PHD2. In vivo , IL-4Ra–deficient neutrophils had a survival advantage within a hypoxic inflamed niche; in contrast, inflamed lung treatment with IL-4 accelerated resolution through increased neutrophil apoptosis. Conclusions: We describe an important interaction whereby IL4Rα-dependent type 2 cytokine signaling can directly inhibit hypoxic neutrophil survival in tissues and promote resolution of neutrophil-mediated acute lung injury.

The paper presents the design principles of a degenerate mode resonant mass sensor in which the unloaded sensor takes the form of a cyclically symmetric structure. The simplest structure with these features is a circular diaphragm and its properties are exploited in this paper. Such structures support pairs of independent modes of vibration which share a common natural frequency and these are referred to as degenerate modes. If extra mass is added to the structure over predefined regions, then the degeneracy can be broken and this produces a separation of the previously identical frequencies. This frequency split is the output of the sensor and is proportional to the added mass. Such a sensor is self-compensating, and ambient effects which equally influence both modes, such as temperature and in-plane stress, do not add to the frequency split. A Lagrangian approach is used to derive the relationship between added mass and frequency split.

Leukocytes recruited to infected, damaged, or inflamed tissues during an immune response must adapt to oxygen levels much lower than those in the circulation. Hypoxia inducible factors (HIFs) are key mediators of cellular responses to hypoxia and, as in other cell types, HIFs are critical for the upregulation of glycolysis, which enables innate immune cells to produce adenosine triphosphate anaerobically. An increasing body of evidence demonstrates that hypoxia also regulates many other innate immunological functions, including cell migration, apoptosis, phagocytosis of pathogens, antigen presentation and production of cytokines, chemokines, and angiogenic and antimicrobial factors. Many of these functions are mediated by HIFs, which are not only stabilized posttranslationally by hypoxia, but also transcriptionally upregulated by inflammatory signals. Here, we review the role of HIFs in the responses of innate immune cells to hypoxia, both in vitro and in vivo, with a particular focus on myeloid cells, on which the majority of studies have so far been carried out.

This paper investigates laser ablation as a mechanism for fine tuning the flexural modes of vibration of a slightly imperfect suspended ring. A theoretical analysis of the effect of ablation on the natural frequencies of both in-plane and out-of-plane flexural modes is developed. Contributions made to the frequency shift from the reduced mechanical stiffness and reduced mass, as a result of ablation, are included. A specific cyclic symmetric relationship between the modes of vibration and the ablation configuration permitting multimodal tuning is developed. This relationship is expanded for the particular case of flexural modes of order 2 and 3, which are commonly used in vibrating ring gyroscope designs. For vibrating ring gyroscopes, tuning between certain flexural modes of vibration greatly increases the sensitivity of the device to applied rates of rotation. Therefore, a fine-tuning mechanism is highly desirable. An experimental examination of multimodal tuning using laser ablation is performed. In-plane modes of order 2 and out-of-plane modes of order 3 are investigated. Stiffness reduction as a possible method for modifying the natural frequencies of the out-of-plane modes of vibration is explored.

Neutrophils are unusual in their reliance on glycolysis to maintain their energy requirements 1 despite the presence of mitochondria and tricarboxylic acid (TCA) cycle intermediaries.2 This metabolic adaptation is thought in part to underpin their survival and antimicrobial function in tissues that are typically hypoxic.3-5 Despite their unique metabolism, little is known about the importance of flux between metabolic pathways in determining neutrophil survival responses. Recent work has demonstrated the importance of the hypoxia-inducible factor (HIF)/prolyl hydroxylase domain (PHD)-containing enzyme oxygen-sensing pathway in this regard, identifying both HIF-1a and PHD3 as critical regulators of neutrophil survival in hypoxia, 6,7 with the extended survival of neutrophils in hypoxia being dependent on HIF-1a expression. In parallel, an expanding body of work has addressed the role of HIF-1a in coordinating macrophage functional responses to proinflammatory mediators. [8][9][10][11] This work led to the observation that, in macrophages, lipopolysaccharide (LPS) causes an intracellular increase in succinate levels, resulting in HIF-1a stabilization and enhanced interleukin-1b signaling.11 Subsequently, the metabolic rewiring of antimicrobial (M1) and tissue repair (M2) macrophages has been elucidated, with important consequences of TCA cycle activity and integrity for regulation of nitric oxide and N-glycosylation signaling, respectively.12 Whether TCA cycle activity and succinate accumulation regulates HIF-1a and hypoxic survival in neutrophils is unknown.Patients with rare germ line mutations in genes encoding the TCA cycle enzyme succinate dehydrogenase (SDH) allow us to directly question the role of the TCA cycle and mitochondrial respiratory chain in neutrophil survival responses. SDH oxidizes succinate to fumarate in the TCA cycle and is a ubiquinone oxidoreductase, also functioning in complex II of the respiratory chain.13 SDH comprises four subunits (A-D), with inherited mutations of each of the subunits linked to the development of pheochromocytoma (PHEO) and paraganglioma (PGL) after somatic inactivation of the wild-type allele and loss of heterozygosity. [14][15][16] We questioned whether heterozygous germ line mutations in SDHB (SDHBx) would reduce SDH activity in the peripheral blood neutrophils of these patients, leading to accumulation of intracellular succinate, HIF-1a stabilization, and a pseudohypoxic survival phenotype, given the importance of the B subunit for SDH catalytic function and its high prevalence within PHEO/PGL patient populations. 13,17,18