Specialized Pro-Resolving Mediators and the Lymphatic System

15-epi-lipoxin A5 promotes neutrophil exit from exudates for clearance by splenic macrophages

Specialized proresolving mediators (SPMs) promote local macrophage efferocytosis but excess leukocytes early in inflammation require additional leukocyte clearance mechanism for resolution. Here, neutrophil clearance mechanisms from localized acute inflammation were investigated in mouse dorsal air pouches. 15-HEPE (15-hydroxy-5Z,8Z,11Z,13E,17Z-eicosapentaenoic acid) levels were increased in the exudates. Activated human neutrophils converted 15-HEPE to lipoxin A5 (5S,6R,15S-trihydroxy-7E,9E,11Z,13E,17Z-eicosapentaenoic acid), 15-epi-lipoxin A5 (5S,6R,15R-trihydroxy-7E,9E,11Z,13E,17Z-eicosapentaenoic acid), and resolvin E4 (RvE4; 5S,15S-dihydroxy-6E,8Z,11Z,13E,17Z-eicosapentaenoic acid). Exogenous 15-epi-lipoxin A5, 15-epi-lipoxin A4 and a structural lipoxin mimetic significantly decreased exudate neutrophils and increased local tissue macrophage efferocytosis, with comparison to naproxen. 15-epi-lipoxin A5 also cleared exudate neutrophils faster than the apparent local capacity for stimulated macrophage efferocytosis, so the fate of exudate neutrophils was tracked with CD45.1 variant neutrophils. 15-epi-lipoxin A5 augmented the exit of adoptively transferred neutrophils from the pouch exudate to the spleen, and significantly increased splenic SIRPa+ and MARCO+ macrophage efferocytosis. Together, these findings demonstrate new systemic resolution mechanisms for 15-epi-lipoxin A5 and RvE4 in localized tissue inflammation, which distally engage the spleen to activate macrophage efferocytosis for the clearance of tissue exudate neutrophils.

Ontogenesis of the Mouse Ocular Surface Lymphatic Vascular Network

Purpose

Ocular lymphatic vessels play major physiological role in eye homeostasis and their dysfunction can contribute to the progression of several eye diseases. In this study, we characterized their spatiotemporal development and the cellular mechanisms occurring during their ontogenesis in the mouse eye.

Methods

Whole mount immunofluorescent staining and imaging by standard or lightsheet fluorescence microscopy were performed on late embryonic and early postnatal eye mouse samples.

Results

We observed that the ocular surface lymphatic vascular network develops at the early postnatal stages (between P0 and P5) from two nascent trunks arising at the nasal side on both sides of the nictitating membrane. These nascent vessels further branch and encircle the whole eye surface by sprouting lymphangiogenesis. In addition, we got evidence for the existence of a transient lymphvasculogenesis process generating lymphatic vessel fragments that will mostly formed the corneolimbal lymphatic vasculature which further connect to the conjunctival lymphatic network. Our results also support that CD206-positive macrophages can transdifferentiate and then integrate into the lymphatic neovessels.

Conclusions

Several complementary cellular processes participate in the development of the lymphatic ocular surface vasculature. This knowledge paves the way for the design of new therapeutic strategies to interfere with ocular lymphatic vessel formation when needed.

Lipid and glucose metabolism in senescence

Senescence is an inevitable biological process. Disturbances in glucose and lipid metabolism are essential features of cellular senescence. Given the important roles of these types of metabolism, we review the evidence for how key metabolic enzymes influence senescence and how senescence-related secretory phenotypes, autophagy, apoptosis, insulin signaling pathways, and environmental factors modulate glucose and lipid homeostasis. We also discuss the metabolic alterations in abnormal senescence diseases and anti-cancer therapies that target senescence through metabolic interventions. Our work offers insights for developing pharmacological strategies to combat senescence and cancer.

Dichotomous effects of in vivo and in vitro ionizing radiation exposure on lymphatic function

On the one hand, lymphatic dysfunction induces interstitial edema and inflammation. On the other hand, the formation of edema and inflammation induce lymphatic dysfunction. However, informed by the earlier reports of undetected apoptosis of irradiated lymphatic endothelial cells (LECs) in vivo, lymphatic vessels are commonly considered inconsequential to ionizing radiation (IR)-induced inflammatory injury to normal tissues. Primarily because of the lack of understanding of the acute effects of IR exposure on lymphatic function, acute edema and inflammation, common sequelae of IR exposure, have been ascribed solely to blood vessel damage. Therefore, in the present study, the lymphatic acute responses to IR exposure were quantified to evaluate the hypothesis that IR exposure impairs lymphatic pumping. Rat mesenteric lymphatic vessels were irradiated in vivo or in vitro, and changes in pumping were quantified in isolated vessels in vitro. Compared with sham-treated vessels, pumping was lowered in lymphatic vessels irradiated in vivo but increased in vessels irradiated in vitro. Furthermore, unlike in blood vessels, the acute effects of IR exposure in lymphatic vessels were not mediated by nitric oxide-dependent pathways in either in vivo or in vitro irradiated vessels. After cyclooxygenase blockade, pumping was partially restored in lymphatic vessels irradiated in vitro but not in vessels irradiated in vivo. Taken together, these findings demonstrated that lymphatic vessels are radiosensitive and LEC apoptosis alone may not account for all the effects of IR exposure on the lymphatic system.NEW & NOTEWORTHY Earlier studies leading to the common belief that lymphatic vessels are radioresistant either did not characterize lymphatic pumping, deemed necessary for the resolution of edema and inflammation, or did it in vivo. By characterizing pumping in vitro, the present study, for the first time, demonstrated that lymphatic pumping was impaired in vessels irradiated in vivo and enhanced in vessels irradiated in vitro. Furthermore, the pathways implicated in ionizing radiation-induced blood vessel damage did not mediate lymphatic responses.

Is depression the missing link between inflammatory mediators and cancer?

Patients with cancer are at greater risk of developing depression in comparison to the general population and this is associated with serious adverse effects, such as poorer quality of life, worse prognosis and higher mortality. Although the relationship between depression and cancer is now well established, a common underlying pathophysiological mechanism between the two conditions is yet to be elucidated. Existing theories of depression, based on monoamine neurotransmitter system dysfunction, are insufficient as explanations of the disorder. Recent advances have implicated neuroinflammatory mechanisms in the etiology of depression and it has been demonstrated that inflammation at a peripheral level may be mirrored centrally in astrocytes and microglia serving to promote chronic levels of inflammation in the brain. Three major routes to depression in cancer in which proinflammatory mediators are implicated, seem likely. Activation of the kynurenine pathway involving cytokines, increases tryptophan catabolism, resulting in diminished levels of serotonin which is widely acknowledged as being the hallmark of depression. It also results in neurotoxic effects on brain regions thought to be involved in the evolution of major depression. Proinflammatory mediators also play a crucial role in impairing regulatory glucocorticoid mediated feedback of the hypothalamic-pituitary-adrenal axis, which is activated by stress and considered to be involved in both depression and cancer. The third route is via the glutamatergic pathway, whereby glutamate excitotoxicity may lead to depression associated with cancer. A better understanding of the mechanisms underlying these dysregulated and other newly emerging pathways may provide a rationale for therapeutic targeting, serving to improve the care of cancer patients.

Lipoxins reduce obesity-induced adipose tissue inflammation in 3D-cultured human adipocytes and explant cultures

Adipose tissue inflammation drives obesity-related cardiometabolic diseases. Enhancing endogenous resolution mechanisms through administration of lipoxin A4, a specialized pro-resolving lipid mediator, was shown to reduce adipose inflammation and subsequently protects against obesity-induced systemic disease in mice. Here, we demonstrate that lipoxins reduce inflammation in 3D-cultured human adipocytes and adipose tissue explants from obese patients. Approximately 50% of patients responded particularly well to lipoxins by reducing inflammatory cytokines and promoting an anti-inflammatory M2 macrophage phenotype. Responding patients were characterized by elevated systemic levels of C-reactive protein, which causes inflammation in cultured human adipocytes. Responders appeared more prone to producing anti-inflammatory oxylipins and displayed elevated prostaglandin D2 levels, which has been interlinked with transcription of lipoxin-generating enzymes. Using explant cultures, this study provides the first proof-of-concept evidence supporting the therapeutic potential of lipoxins in reducing human adipose tissue inflammation. Our data further indicate that lipoxin treatment may require a tailored personalized-medicine approach.

Pro-resolving lipid mediators in traumatic brain injury: emerging concepts and translational approach

Despite the high mortality and disability associated with traumatic brain injury (TBI), effective pharmacologic treatments are lacking. Of emerging interest, bioactive lipids, including specialized pro-resolving lipid mediators of inflammation (SPMs), act to attenuate inflammation after injury resolution. The SPM lipidome may serve as a biomarker of disease and predictor of clinical outcomes, and the use of exogenous SPM administration represents a novel therapeutic strategy for TBI. This review article provides a comprehensive discussion of the current pre-clinical and clinical literature supporting the importance of bioactive lipids, including SPMs, in TBI recovery. We additionally propose a translational approach to answer important clinical and scientific questions to advance the study of bioactive lipids and SPMs towards clinical research. Given the morbidity and mortality associated with TBI with limited treatment options, novel approaches are needed.

Lipoxins modulate neutrophil oxidative burst, integrin expression and lymphatic transmigration differentially in human health and atherosclerosis

Dysregulated chronic inflammation plays a crucial role in the pathophysiology of atherosclerosis and may be a result of impaired resolution. Thus, restoring levels of specialized pro‐resolving mediators (SPMs) to promote the resolution of inflammation has been proposed as a therapeutic strategy for patients with atherosclerosis, in addition to standard clinical care. Herein, we evaluated the effects of the SPM lipids, lipoxin A₄ (LXA₄) and lipoxin B₄ (LXB₄), on neutrophils isolated from patients with atherosclerosis compared with healthy controls. Patients displayed altered endogenous SPM production, and we demonstrated that lipoxin treatment in whole blood from atherosclerosis patients attenuates neutrophil oxidative burst, a key contributor to atherosclerotic development. We found the opposite effect in neutrophils from healthy controls, indicating a potential mechanism whereby lipoxins aid the endogenous neutrophil function in health but reduce its excessive activation in disease. We also demonstrated that lipoxins attenuated upregulation of the high‐affinity conformation of the CD11b/CD18 integrin, which plays a central role in clot activation and atherosclerosis. Finally, LXB₄ enhanced lymphatic transmigration of human neutrophils isolated from patients with atherosclerosis. This finding is noteworthy, as impaired lymphatic function is now recognized as an important contributor to atherosclerosis. Although both lipoxins modulated neutrophil function, LXB₄ displayed more potent effects than LXA₄ in humans. This study highlights the therapeutic potential of lipoxins in atherosclerotic disease and demonstrates that the effect of these SPMs may be specifically tailored to the need of the individual.

Anti-Inflammatory Function of Fatty Acids and Involvement of Their Metabolites in the Resolution of Inflammation in Chronic Obstructive Pulmonary Disease

Lipid metabolism plays an important role in many lung functions. Disorders of lipid metabolism are part of the pathogenesis of chronic obstructive pulmonary disease (COPD). Lipids are involved in numerous cross-linkages with inflammation. Recent studies strongly support the involvement of fatty acids as participants in inflammation. They are involved in the initiation and resolution of inflammation, including acting as a substrate for the formation of lipid mediators of inflammation resolution. Specialized pro-inflammatory mediators (SPMs) belonging to the classes of lipoxins, resolvins, maresins, and protectins, which are formed enzymatically from unsaturated fatty acids, are now described. Disorders of their production and function are part of the pathogenesis of COPD. SPMs are currently the subject of active research in order to find new drugs. Short-chain fatty acids are another important participant in metabolic and immune processes, and their role in the pathogenesis of COPD is of great clinical interest.

Leukocyte Trafficking via Lymphatic Vessels in Atherosclerosis

In recent years, lymphatic vessels have received increasing attention and our understanding of their development and functional roles in health and diseases has greatly improved. It has become clear that lymphatic vessels are critically involved in acute and chronic inflammation and its resolution by supporting the transport of immune cells, fluid, and macromolecules. As we will discuss in this review, the involvement of lymphatic vessels has been uncovered in atherosclerosis, a chronic inflammatory disease of medium- and large-sized arteries causing deadly cardiovascular complications worldwide. The progression of atherosclerosis is associated with morphological and functional alterations in lymphatic vessels draining the diseased artery. These defects in the lymphatic vasculature impact the inflammatory response in atherosclerosis by affecting immune cell trafficking, lymphoid neogenesis, and clearance of macromolecules in the arterial wall. Based on these new findings, we propose that targeting lymphatic function could be considered in conjunction with existing drugs as a treatment option for atherosclerosis.