Low-grade inflammatory polarization of monocytes impairs wound healing

Epigenetic regulation of innate immune dynamics during inflammation

Innate immune cells play essential roles in modulating both immune defense and inflammation by expressing a diverse array of cytokines and inflammatory mediators, phagocytizing pathogens to promote immune clearance, and assisting with the adaptive immune processes through antigen presentation. Rudimentary innate immune "memory" states such as training, tolerance, and exhaustion develop based on the nature, strength, and duration of immune challenge, thereby enabling dynamic transcriptional reprogramming to alter present and future cell behavior. Underlying transcriptional reprogramming are broad changes to the epigenome, or chromatin alterations above the level of DNA sequence. These changes include direct modification of DNA through cytosine methylation as well as indirect modifications through alterations to histones that comprise the protein core of nucleosomes. In this review, we will discuss recent advances in our understanding of how these epigenetic changes influence the dynamic behavior of the innate immune system during both acute and chronic inflammation, as well as how stable changes to the epigenome result in long-term alterations of innate cell behavior related to pathophysiology.

A Robust In Vitro Culture Model and Generation of Memory Monocytes

Innate monocytes can be trained or reprogrammed to adopt distinct memory states, such as low-grade inflammation and immune exhaustion, bearing fundamental relevance to the pathogenesis of both acute diseases such as sepsis as well as chronic diseases such as atherosclerosis. Therefore, it is critically important to develop a regimen for generating memory monocytes in vitro in order to better define key monocyte memory states with diverse potentials for proliferation, differentiation, and activation, as well as underlying mechanisms. Here, we describe an efficient in vitro system to propagate a large number of highly purified murine memory monocytes through sustaining bone marrow-derived monocytes with macrophage colony-stimulating factor (M-CSF, 10 ng/mL)-containing medium, together with other polarization agents such as lipopolysaccharide (LPS) for a 5-day period. This method can yield high-purity monocytes, capable of exhibiting dynamic memory behaviors upon training with various polarizing agents.

Immune Homeostasis Maintenance Through Advanced Immune Therapeutics to Target Atherosclerosis

Atherosclerosis remains the leading cause of coronary heart disease (CHD) with enormous health and societal tolls. Traditional drug development approaches have been focused on small molecule-based compounds that aim to lower plasma lipids and reduce systemic inflammation, two primary causes of atherosclerosis. However, despite the widely available lipid-lowering and anti-inflammatory small compounds and biologic agents, CHD prevalence still remains high. Based on recent advances revealing disrupted immune homeostasis during atherosclerosis pathogenesis, novel strategies aimed at rejuvenating immune homeostasis with engineered immune leukocytes are being developed. This chapter aims to assess basic and translational efforts on these emerging strategies for the effective development of atherosclerosis treatment, as well as key challenges in this important translational field.

Modulation of Innate Immune Memory Dynamics by Subcellular Reactive Oxygen Species

Significance: Innate immune cells adopt distinct memory states during the pathogenesis of acute and chronic inflammatory diseases. Intracellular generations of reactive oxygen species (ROS) play key roles during the programming dynamics of innate immune cells such as monocytes and macrophages. Recent Advances: ROS modulate the adaptation of innate leukocytes to varying intensities and durations of inflammatory signals, facilitate fundamental reprogramming dynamics such as priming, tolerance, and exhaustion, in addition to fundamental processes of proliferation, differentiation, phagocytosis, chemotaxis, as well as expression of pro- and anti-inflammatory mediators. ROS can be generated at distinct subcellular compartments including cellular membrane, mitochondria, and peroxisome. Complex inflammatory signals may finely regulate ROS generation within distinct subcellular compartments, which in turn may differentially facilitate innate memory dynamics. Critical Issues: Complex inflammatory signals with varying strengths and durations may differentially trigger ROS generation at peroxisome, mitochondria, and other subcellular organelles. Peroxisomal or mitochondrial ROS may facilitate the assembly of distinct signaling platforms involved in the programming of memory innate leukocytes. Despite the emerging connection of subcellular ROS with innate immune memory, underlying mechanisms are still not well defined. Future Directions: Recent important discoveries linking subcellular ROS and innate memory as critically reviewed here hold novel translational relevance related to acute and chronic inflammatory diseases. Capitalizing on these novel findings, future systems studies that use next-generation single-cell dynamic analyses in response to complex inflammatory environments are urgently needed to comprehensively decipher the programming dynamics of innate immune memory, finely modulated by subcellular ROS. Antioxid. Redox Signal. 39, 1027-1038.

Quercetin and its derivatives for wound healing in rats/mice: Evidence from animal studies and insight into molecular mechanisms

Aimed to clarify the effect of quercetin and its derivatives on wound healing in animal experiments. PubMed, Embase, Science Direct, Web of Science, SinoMed, Vip Journal Integration Platform, China National Knowledge Infrastructure and WanFang databases were searched for animal experiments investigating the effect of quercetin and its derivatives on wound healing to April 2023. The Review Manager 5.4 software was used to conduct meta-analysis. Eighteen studies were enrolled in this article. According to the SYRCLE's RoB tool assessment, these studies exposed relatively low methodological quality. It was shown that animals with cutaneous wound receiving quercetin had faster wound healing in wound closure (%) than the control group. Moreover, the difference in efficacy gradually emerged after third day (WMD = 7.13 [5.52, 8.74]), with a peak reached on the tenth day after wounding (WMD = 19.78 [17.82, 21.74]). Subgroup analysis revealed that quercetin for wound closure (%) was independent of the types of rats and mice, wound area and with or without diabetes. Clear conclusion was also shown regarding the external application of quercetin for wound healing (WMD = 17.77 [11.11, 24.43]). A significant reduction in the distribution of inflammatory cells occurred in the quercetin group. Quercetin could increase blood vessel density (WMD = 1.85 [0.68, -3.02]), fibroblast distribution and collagen fraction. Biochemical indicators, including IL-1β, IL-10, TNF-α, TGF-β, vascular endothelial growth factor (VEGF), hydroxyproline and alpha-smooth muscle actin (α-SMA), had the consistent results. Quercetin and its derivatives could promote the recovery of cutaneous wound in animals, through inhibiting inflammatory response and accelerating angiogenesis, proliferation of fibroblast and collagen deposition.

Tumor-associated macrophages-educated reparative macrophages promote diabetic wound healing

Nonhealing diabetic wounds, with persistent inflammation and damaged vasculature, have failed conventional treatments and require comprehensive interference. Here, inspired by tumor-associated macrophages (TAMs) that produce abundant immunosuppressive and proliferative factors in tumor development, we generate macrophages to recapitulate TAMs' reparative functions, by culturing normal macrophages with TAMs' conditional medium (TAMs-CM). These TAMs-educated macrophages (TAMEMs) outperform major macrophage phenotypes (M0, M1, or M2) in suppressing inflammation, stimulating angiogenesis, and activating fibroblasts in vitro. When delivered to skin wounds in diabetic mice, TAMEMs efficiently promote healing. Based on TAMs-CM's composition, we further reconstitute a nine-factor cocktail to train human primary monocytes into TAMEMs^C-h , which fully resemble TAMEMs' functions without using tumor components, thereby having increased safety and enabling the preparation of autologous cells. Our study demonstrates that recapitulating TAMs' unique reparative activities in nontumor cells can lead to an effective cell therapeutic approach with high translational potential for regenerative medicine.

Accelerating cutaneous healing in a rodent model of type II diabetes utilizing non-invasive focused ultrasound targeted at the spleen

Healing of wounds is delayed in Type 2 Diabetes Mellitus (T2DM), and new treatment approaches are urgently needed. Our earlier work showed that splenic pulsed focused ultrasound (pFUS) alters inflammatory cytokines in models of acute endotoxemia and pneumonia via modulation of the cholinergic anti-inflammatory pathway (CAP) (ref below). Based on these earlier results, we hypothesized that daily splenic exposure to pFUS during wound healing would accelerate closure rate via altered systemic cytokine titers. In this study, we applied non-invasive ultrasound directed to the spleen of a rodent model [Zucker Diabetic Sprague Dawley (ZDSD) rats] of T2DM with full thickness cutaneous excisional wounds in an attempt to accelerate wound healing via normalization of T2DM-driven aberrant cytokine expression. Daily (1x/day, Monday-Friday) pFUS pulses were targeted externally to the spleen area for 3 min over the course of 15 days. Wound diameter was measured daily, and levels of cytokines were evaluated in spleen and wound bed lysates. Non-invasive splenic pFUS accelerated wound closure by up to 4.5 days vs. sham controls. The time to heal in all treated groups was comparable to that of healthy rats from previously published studies (ref below), suggesting that the pFUS treatment restored a normal wound healing phenotype to the ZDSD rats. IL-6 was lower in stimulated spleen (-2.24 ± 0.81 Log2FC, p = 0.02) while L-selectin was higher in the wound bed of stimulated rodents (2.53 ± 0.72 Log2FC, p = 0.003). In summary, splenic pFUS accelerates healing in a T2DM rat model, demonstrating the potential of the method to provide a novel, non-invasive approach for wound care in diabetes.

Evaluation of the red & blue LED effects on cutaneous refractory wound healing in male Sprague-Dawley rat using 3 different multi-drug resistant bacteria

OBJECTIVES

Photobiomodulation (PBM) is widely used in clinical therapy, and is an effective approach to resist the bacterial infection of the cutaneous wound and modulate the wound healing process. Due to the several detriments of lasers, Red & Blue LED light (RBLL) may be a more viable light source. This study is aimed to evaluate and compare the therapeutic effect of RBLL light on different multi-drug resistant (MDR) bacteria in vitro and male Sprague-Dawley (SD) rat refractory MDR infection wound model in vivo.

MATERIALS AND METHODS

Methicillin-resistant Staphylococcus aureus (MRSA), Extended-spectrum β-lactamases -producing Escherichia coli (ESBLs-Eco), and the MDR Pseudomonas aeruginosa (MDR-Pae) were employed to evaluate the antibacterial effects of the Blue LED light in vitro. Effects of RBLL on in vivo wound healing were evaluated by analyzing time to closure, wound score, semi-quantitative test for bacterial culture, histopathological examination and Masson staining of skin tissue, immunohistochemical (IHC) staining, and western blot analysis (WB) of wound tissue.

RESULTS

Blue LED light inhibited MRSA, ESBLs-Eco, and MDR-Pae in vitro study. In vivo, RBLL accelerated wound healing, reduced levels of pathogenic bacteria on the wound surface while increasing the blood supply to the wound surface and inhibiting the excessive inflammatory response.

CONCLUSION

RBLL showed a great potential gain for the treatment of MDR bacterial infected wounds, suggesting PBM therapy is an inexpensive, convenient, pain-free, and safe therapeutic intervention for refractory MDR infection wounds.

Implications of endotoxins in wound healing: a narrative review

Bacterial toxins are thought to play a role in delayed wound healing in critically colonised and infected wounds. Endotoxins are released from Gram-negative bacteria when they are lysed by host phagocytic cells during an immune response, or by antimicrobial agents, potentially leading to a detrimental effect on the host tissues. Endotoxins can affect all aspects of the wound healing process, leading to delayed healing and contributing to wound chronicity. Release of endotoxins by bacteria can also have serious systemic effects (for example, septic shock) that can lead to high levels of patient mortality. This review summarises the role and implications on wound healing of bacterial endotoxins, describing the impact of endotoxins on the various phases of the wound healing response. There is a paucity of in vivo/clinical evidence linking endotoxins attributed to a wound (via antibiotic treatment) or their release from infecting bacteria with parameters of delayed wound healing. Future work should investigate if this link is apparent and determine the mechanism(s) by which such detrimental effects occur, offering an opportunity to identify possible treatment pathways. This paper describes the phenomenon of antimicrobial-induced endotoxin release and summarises the use of wound dressings to reduce wound bioburden without inducing microbial death and subsequent release of endotoxins, thus limiting their detrimental effects.

Epigenomic and transcriptomic analyses reveal differences between low-grade inflammation and severe exhaustion in LPS-challenged murine monocytes

Emerging studies suggest that monocytes can be trained by bacterial endotoxin to adopt distinct memory states ranging from low-grade inflammation to immune exhaustion. While low-grade inflammation may contribute to the pathogenesis of chronic diseases, exhausted monocytes with pathogenic and immune-suppressive characteristics may underlie the pathogenesis of polymicrobial sepsis including COVID-19. However, detailed processes by which the dynamic adaption of monocytes occur remain poorly understood. Here we exposed murine bone-marrow derived monocytes to chronic lipopolysaccharide (LPS) stimulation at low-dose or high-dose, as well as a PBS control. The cells were profiled for genome-wide H3K27ac modification and gene expression. The gene expression of TRAM-deficient and IRAK-M-deficient monocytes with LPS exposure was also analyzed. We discover that low-grade inflammation preferentially utilizes the TRAM-dependent pathway of TLR4 signaling, and induces the expression of interferon response genes. In contrast, high dose LPS uniquely upregulates exhaustion signatures with metabolic and proliferative pathways. The extensive differences in the epigenomic landscape between low-dose and high-dose conditions suggest the importance of epigenetic regulations in driving differential responses. Our data provide potential targets for future mechanistic or therapeutic studies.

Signal-Strength and History-Dependent Innate Immune Memory Dynamics in Health and Disease

Innate immunity exhibits memory characteristics, reflected not only in selective recognition of external microbial or internal damage signals, but more importantly in history and signal-strength dependent reprogramming of innate leukocytes characterized by priming, tolerance, and exhaustion. Key innate immune cells such as monocytes and neutrophils can finely discern and attune to the duration and intensity of external signals through rewiring of internal signaling circuitries, giving rise to a vast array of discreet memory phenotypes critically relevant to managing tissue homeostasis as well as diverse repertoires of inflammatory conditions. This review will highlight recent advances in this rapidly expanding field of innate immune programming and memory, as well as its translational implication in the pathophysiology of selected inflammatory diseases.

Towards an ecological definition of sepsis: a viewpoint

In critically ill patients with sepsis, there is a grave lack of effective treatment options to address the illness-defining inappropriate host response. Currently, treatment is limited to source control and supportive care, albeit with imminent approval of immune modulating drugs for COVID-19-associated lung failure the potential of host-directed strategies appears on the horizon. We suggest expanding the concept of sepsis by incorporating infectious stress within the general stress response of the cell to define sepsis as an illness state characterized by allostatic overload and failing adaptive responses along with biotic (pathogen) and abiotic (e.g., malnutrition) environmental stress factors. This would allow conceptualizing the failing organismic responses to pathogens in sepsis with an ancient response pattern depending on the energy state of cells and organs towards other environmental stressors in general. Hence, the present review aims to decipher the heuristic value of a biological definition of sepsis as a failing stress response. These considerations may motivate a better understanding of the processes underlying "host defense failure" on the organismic, organ, cell and molecular levels.

Shift Work Predicts Increases in Lipopolysaccharide-Binding Protein, Interleukin-10, and Leukocyte Counts in a Cross-Sectional Study of Healthy Volunteers Carrying Low-Grade Systemic Inflammation

The disruption of inflammatory responses is a potential mechanism behind the harmful effects of shift work and is associated with increased risk of hypertension, stroke, obesity, diabetes, and cancer. These responses are linked to the proliferation of leukocytes in shift workers, suggesting a systemic signal as a potential mediator. The purpose of this study was to assess the relationship between systemic inflammation, leukocyte counts, and systemic endotoxemia in samples from a diverse cohort of day workers and shift workers. Participants (normothermic and normotensive) were healthy volunteers, non-smoking, and drug- and medication-free. The following outcomes were measured: C-reactive protein, TNF-α, IL-6, IL-1β, IL-10, leukocyte counts (monocytes, lymphocytes, and neutrophils), and lipopolysaccharide-binding protein (LBP). Risk factors that increase systemic inflammation, such as blood pressure, sleep loss, and cortisol, were also assessed. The results indicated that shift workers slept significantly less than day workers and had significantly increased concentrations of all of the cytokines measured as well as plasma cortisol. Regression models found that after controlling for covariates, shift-work exposure predicted the significant increase observed in IL-10, leukocyte counts, and LBP. Our results suggest that acute increases in low-grade systemic endotoxemia are unresolved during chronic shift-work exposure. This ongoing immune challenge may underlie the disrupted inflammatory responses characteristic of shift-work-related pathologies. Systemic endotoxemia may represent a novel target to investigate the early effects of exposure to shift-work schedules.

Resolving monocytes generated through TRAM deletion attenuate atherosclerosis

Polarization of low-grade inflammatory monocytes facilitates the pathogenesis of atherosclerosis. However, underlying mechanisms as well as approaches for resolving monocyte polarization conducive to the regression of atherosclerosis are not well established. In this report, we demonstrate that TRIF-related adaptor molecule (TRAM) mediated monocyte polarization in vivo and in vitro. TRAM controlled monocyte polarization through activating Src family kinase c-SRC, which not only induces STAT1/STAT5-regulated inflammatory mediators CCR2 and SIRP-α but also suppresses PPARγ-regulated resolving mediator CD200R. Enhanced PPARγ and Pex5 due to TRAM deficiency facilitated peroxisome homeostasis and reduction of cellular reactive oxygen species, further contributing to the establishment of a resolving monocyte phenotype. TRAM-deficient monocytes propagated the resolving phenotype to neighboring monocytes through CD200R-mediated intercellular communication. At the translational level, we show that TRAM-deficient mice were resistant to high-fat diet-induced pathogenesis of atherosclerosis. We further document that intravenous transfusion of TRAM-deficient resolving monocytes into atherosclerotic mice potently reduced the progression of atherosclerosis. Together, our data reveal that targeting TRAM may facilitate the effective generation of resolving monocytes conducive for the treatment of atherosclerosis.

Long Term Response to Circulating Angiogenic Cells, Unstimulated or Atherosclerotic Pre-Conditioned, in Critical Limb Ischemic Mice

Critical limb ischemia (CLI), the most severe form of peripheral artery disease, results from the blockade of peripheral vessels, usually correlated to atherosclerosis. Currently, endovascular and surgical revascularization strategies cannot be applied to all patients due to related comorbidities, and even so, most patients require re-intervention or amputation within a year. Circulating angiogenic cells (CACs) constitute a good alternative as CLI cell therapy due to their vascular regenerative potential, although the mechanisms of action of these cells, as well as their response to pathological conditions, remain unclear. Previously, we have shown that CACs enhance angiogenesis/arteriogenesis from the first days of administration in CLI mice. Also, the incubation ex vivo of these cells with factors secreted by atherosclerotic plaques promotes their activation and mobilization. Herein, we have evaluated the long-term effect of CACs administration in CLI mice, whether pre-stimulated or not with atherosclerotic factors. Remarkably, mice receiving CACs and moreover, pre-stimulated CACs, presented the highest blood flow recovery, lower progression of ischemic symptoms, and decrease of immune cells recruitment. In addition, many proteins potentially involved, like CD44 or matrix metalloproteinase 9 (MMP9), up-regulated in response to ischemia and decreased after CACs administration, were identified by a quantitative proteomics approach. Overall, our data suggest that pre-stimulation of CACs with atherosclerotic factors might potentiate the regenerative properties of these cells in vivo.

Peptide-coated polyurethane material reduces wound infection and inflammation

There is an urgent need for treatments that not only reduce bacterial infection that occurs during wounding but that also target the accompanying excessive inflammatory response. TCP-25, a thrombin-derived antibacterial peptide, scavenges toll-like receptor agonists such as endotoxins and lipoteichoic acid and prevents toll-like receptor-4 dimerization to reduce infection-related inflammation in vivo. Using a combination of biophysical, cellular, and microbiological assays followed by experimental studies in mouse and pig models, we show that TCP-25, when delivered from a polyurethane (PU) material, exerts anti-infective and anti-inflammatory effects in vitro and in vivo. Specifically, TCP-25 killed the common wound pathogens, Pseudomonas aeruginosa and Staphylococcus aureus, in both in vitro and in vivo assays. Furthermore, after its release from the PU material, the peptide retained its capacity to induce its helical conformation upon endotoxin interaction, yielding reduced activation of NF-κB in THP-1 reporter cells, and diminished accumulation of inflammatory cells and subsequent release of IL-6 and TNF-α in subcutaneous implant models in vivo. Moreover, in a porcine partial thickness wound infection model, TCP-25 treated infection with S. aureus, and reduced the concomitant inflammatory response. Taken together, these findings demonstrate a combined antibacterial and anti-inflammatory effect of TCP-25 delivered from PU in vitro, and in mouse and porcine in vivo models of localized infection-inflammation. STATEMENT OF SIGNIFICANCE: Local wound infections may result in systemic complications and can be difficult to treat due to increasing antimicrobial resistance. Surgical site infections and biomaterial-related infections present a major challenge for hospitals. In recent years, various antimicrobial coatings have been developed for infection prevention and current concepts focus on various matrices with added anti-infective components, including various antibiotics and antiseptics. We have developed a dual action wound dressing concept where the host defense peptide TCP-25, when delivered from a PU material, targets both bacterial infection and the accompanying inflammation. TCP-25 PU showed efficacy in in vitro and experimental wound models in mouse and minipigs.

Single Cell RNA-Seq and Machine Learning Reveal Novel Subpopulations in Low-Grade Inflammatory Monocytes With Unique Regulatory Circuits

Subclinical doses of LPS (SD-LPS) are known to cause low-grade inflammatory activation of monocytes, which could lead to inflammatory diseases including atherosclerosis and metabolic syndrome. Sodium 4-phenylbutyrate is a potential therapeutic compound which can reduce the inflammation caused by SD-LPS. To understand the gene regulatory networks of these processes, we have generated scRNA-seq data from mouse monocytes treated with these compounds and identified 11 novel cell clusters. We have developed a machine learning method to integrate scRNA-seq, ATAC-seq, and binding motifs to characterize gene regulatory networks underlying these cell clusters. Using guided regularized random forest and feature selection, our method achieved high performance and outperformed a traditional enrichment-based method in selecting candidate regulatory genes. Our method is particularly efficient in selecting a few candidate genes to explain observed expression pattern. In particular, among 531 candidate TFs, our method achieves an auROC of 0.961 with only 10 motifs. Finally, we found two novel subpopulations of monocyte cells in response to SD-LPS and we confirmed our analysis using independent flow cytometry experiments. Our results suggest that our new machine learning method can select candidate regulatory genes as potential targets for developing new therapeutics against low grade inflammation.

Bistable Mathematical Model of Neutrophil Migratory Patterns After LPS-Induced Epigenetic Reprogramming

The highly controlled migration of neutrophils toward the site of an infection can be altered when they are trained with lipopolysaccharides (LPS), with high dose LPS enhancing neutrophil migratory pattern toward the bacterial derived source signal and super-low dose LPS inducing either migration toward an intermediary signal or dysregulation and oscillatory movement. Empirical studies that use microfluidic chemotaxis-chip devices with two opposing chemoattractants showed differential neutrophil migration after challenge with different LPS doses. The epigenetic alterations responsible for changes in neutrophil migratory behavior are unknown. We developed two mathematical models that evaluate the mechanistic interactions responsible for neutrophil migratory decision-making when exposed to competing chemoattractants and challenged with LPS. The first model, which considers the interactions between the receptor densities of two competing chemoattractants, their kinases, and LPS, displayed bistability between high and low ratios of primary to intermediary chemoattractant receptor densities. In particular, at equilibrium, we observe equal receptor densities for low LPS (< 15ng/mL); and dominance of receptors for the primary chemoattractant for high LPS (> 15ng/mL). The second model, which included additional interactions with an extracellular signal-regulated kinase in both phosphorylated and non-phosphorylated forms, has an additional dynamic outcome, oscillatory dynamics for both receptors, as seen in the data. In particular, it found equal receptor densities in the absence of oscillation for super-low and high LPS challenge (< 0.4 and 1.1 376 ng/mL). Predicting the mechanisms and the type of external LPS challenge responsible for neutrophils migration toward pro-inflammatory chemoattractants, migration toward pro-tolerant chemoattractants, or oscillatory movement is necessary knowledge in designing interventions against immune diseases, such as sepsis.

Polarization of Low-Grade Inflammatory Monocytes Through TRAM-Mediated Up-Regulation of Keap1 by Super-Low Dose Endotoxin

Subclinical endotoxemia [low levels of bacterial endotoxin (LPS) in the blood stream] has been correlated with chronic inflammatory diseases, with less-understood mechanisms. We have previously shown that chronic exposure to super low doses of LPS polarizes monocytes/macrophages to a pro-inflammatory state characterized by up-regulation of pro-inflammatory regulators such as p62 and simultaneous down-regulation of anti-inflammatory/resolving regulators such as Nrf2. Building upon this observation, here we show that chronic exposure to super-low doses of LPS leads to accumulation of the Nrf2-inhibitory protein Keap1 in murine monocytes. This is accompanied by increases of p62 and MLKL, consistent with a disruption of autolysosome function in polarized monocytes challenged by super-low dose LPS. Monocytes subjected to persistent super-low dose LPS challenge also accumulate higher levels of IKKβ. As a consequence, SLD-LPS challenge leads to an inflammatory monocyte state represented by higher expression of the inflammatory marker Ly6C as well as lower expression of the anti-inflammatory marker CD200R. Further analysis revealed that Keap1 levels are significantly enriched in the Ly6C^hi pro-inflammatory monocyte population. Finally, we show that the TLR4 signaling adaptor TRAM is essential for these effects. Together our study provides novel insight into signaling mechanisms behind low-grade inflammatory monocyte polarization unique to chronic super-low dose LPS exposure.

Bafilomycin A1 Accelerates Chronic Refractory Wound Healing in db/db Mice

Numerous studies have reported that autophagy plays an important role in chronic wound healing, and enhancement of autophagic activity impairs cutaneous wound healing. The autophagy inhibitor Bafilomycin A1 (Baf A1) inhibits autophagy by preventing the formation of autophagosomes. This study aimed at elucidating the effect of Bafilomycin A1 on chronic refractory wound healing in diabetic mice. A total of 40 diabetic (db/db) mice and 20 nondiabetic (db/m) mice were used in this study. Full-thickness skin defects were generated in the db/db mice models, which were then divided into the following two groups: the nontreated (db/db group) and Baf A1-treated groups (Baf A1 group). The same skin defects were generated in db/m mice (db/m group) to serve as a control. We demonstrated that Baf A1 treatment significantly accelerated wound healing in db/db mice and exerted good healing effects. Moreover, Baf A1 inhibited autophagy in the newly generated epidermis and had minor effects on metabolism in db/db mice. PCNA expression, as detected by immunohistochemistry, and collagen thickness, as detected by Masson's trichrome staining on the 14th day, were higher in the db/m and Baf A1 groups than in the db/db group. In addition, the expression of the proinflammatory cytokine TNF-α in the db/m and Baf A1 groups increased significantly on day 6, and the expression of the anti-inflammatory cytokine IL-10 also increased significantly on day 9. However, there were no significant changes in the expression levels of TNF-α and IL-10 in the db/db group. Therefore, Baf A1 may accelerate diabetic chronic refractory wound healing by promoting cell proliferation, collagen production, and regulating the inflammatory balance.

Memory-Like Inflammatory Responses of Microglia to Rising Doses of LPS: Key Role of PI3Kγ

Trained immunity and immune tolerance have been identified as long-term response patterns of the innate immune system. The causes of these opposing reactions remain elusive. Here, we report about differential inflammatory responses of microglial cells derived from neonatal mouse brain to increasing doses of the endotoxin LPS. Prolonged priming with ultra-low LPS doses provokes trained immunity, i.e., increased production of pro-inflammatory mediators in comparison to the unprimed control. In contrast, priming with high doses of LPS induces immune tolerance, implying decreased production of inflammatory mediators and pronounced release of anti-inflammatory cytokines. Investigation of the signaling processes and cell functions involved in these memory-like immune responses reveals the essential role of phosphoinositide 3-kinase γ (PI3Kγ), one of the phosphoinositide 3-kinase species highly expressed in innate immune cells. Together, our data suggest profound influence of preceding contacts with pathogens on the immune response of microglia. The impact of these interactions—trained immunity or immune tolerance—appears to be shaped by pathogen dose.

Phenylbutyrate facilitates homeostasis of non-resolving inflammatory macrophages

Non-resolving inflammatory monocytes/macrophages are critically involved in the pathogenesis of chronic inflammatory diseases. However, mechanisms of macrophage polarization are not well understood, thus hindering the development of effective strategies to promote inflammation resolution. In this study, we report that macrophages polarized by subclinical super-low dose LPS preferentially expressed pro-inflammatory mediators such as ccl2 (which encodes the protein monocyte chemo attractant protein-1) with reduced expression of anti-inflammatory/homeostatic mediators such as slc40a1 (which encodes the protein ferroportin-1). We observed significantly elevated levels of the autophagy-associated and pro-inflammatory protein p62 in polarized macrophages, closely correlated with the inflammatory activation of ccl2 gene expression. In contrast, we noted a significant increase of ubiquitinated/inactive nuclear-erythroid-related factor 2 (NRF2), consistent with reduced slc40a1 gene expression in polarized macrophages. Addition of the homeostatic restorative agent phenylbutyrate (4-PBA) effectively reduced cellular levels of p62 as well as ccl2 gene induction by super-low dose LPS. On the other hand, application of 4-PBA also blocked the accumulation of ubiquitinated NRF2 and restored anti-inflammatory slc40a1 gene expression in macrophages. Together, our study provides novel insights with regard to macrophage polarization and reveals 4-PBA as a promising molecule in restoring macrophage homeostasis.

Enhanced Neutrophil Immune Homeostasis Due to Deletion of PHLPP

Neutrophils are known to adopt dynamic and distinct functional phenotypes involved in the modulation of inflammation and immune homeostasis. However, inter-cellular signaling mechanisms that govern neutrophil polarization dynamics are not well understood. Employing a novel model of PHLPP deficient mice, we examined how neutrophils deficient in PHLPP may uniquely modulate immune defense and the host response during acute colitis. We found that PHLPP^-/- mice were protected from dextran sodium sulfate (DSS)-induced septic colitis characterized by minimal body weight-loss, alleviated colon tissue destruction and reduced clinical symptoms. PHLPP^-/- neutrophils have enhanced immune homeostasis as compared to WT neutrophils, reflected in enhanced migratory capacity toward chemoattractants, and reduced expression of inflammatory mediators due to elevated phosphorylation of AKT, STAT1, and ERK. Further, adoptive transfer of PHLPP deficient neutrophils to WT mice is sufficient to potently alleviate the severity of DSS-induced colitis. Our data reveal that PHLPP deficient neutrophils can be uniquely reprogrammed to a state conducive to host inflammation resolution. As a consequence, PHLPP^-/- neutrophils can effectively transfer immune homeostasis in mice subjected to acute colitis. Our findings hold significant and novel insights into the mechanisms by which neutrophils can be effectively reprogrammed into a homeostatic state conducive for treating acute injuries such as septic colitis.

Super-Low Dose Lipopolysaccharide Dysregulates Neutrophil Migratory Decision-Making

Neutrophils are the first responders to infection and play a pivotal role in many inflammatory diseases, including sepsis. Recent studies have shown that lipopolysaccharide (LPS), a classical pattern recognition molecule, dynamically programs innate immune responses. In this study, we show that pre-treatment with super-low levels of LPS [1 ng/mL] significantly dysregulate neutrophil migratory phenotypes, including spontaneous migration and altering neutrophil decision-making. To quantify neutrophil migratory decision-making with single-cell resolution, we developed a novel microfluidic competitive chemotaxis-chip (μC³) that exposes cells in a central channel to competing chemoattractant gradients. In this reductionist approach, we use two chemoattractants: a pro-resolution (N-Formyl-Met-Leu-Phe, fMLP) and pro-inflammatory (Leukotriene B₄, LTB₄) chemoattractant to model how a neutrophil makes a decision to move toward an end target chemoattractant (e.g., bacterial infection) vs. an intermediary chemoattractant (e.g., inflammatory signal). We demonstrate that naïve neutrophils migrate toward the primary end target signal in higher percentages than toward the secondary intermediary signal. As expected, we found that training with high dose LPS [100 ng/mL] influences a higher percentage of neutrophils to migrate toward the end target signal, while reducing the percentage of neutrophils that migrate toward the intermediary signal. Surprisingly, super-low dose LPS [1 ng/mL] significantly changes the ratios of migrating cells and an increased percentage of cells migrate toward the intermediary signal. Significantly, there was also an increase in the numbers of spontaneously migrating neutrophils after treatment with super-low dose LPS. These results shed light onto the directional migratory decision-making of neutrophils exposed to inflammatory training signals. Understanding these mechanisms may lead to the development of pro-resolution therapies that correct the neutrophil compass and reduce off-target organ damage.

Metformin induces the M2 macrophage polarization to accelerate the wound healing via regulating AMPK/mTOR/NLRP3 inflammasome singling pathway

BACKGROUND

Skin wound healing is a challenging problem, especially in aging or diabetic patients, which becomes more difficult to heal, and easily results in considerable public health burden. The purpose of this study was to investigate the effects of metformin on wound healing and explore its underlying mechanism.

METHODS

Metformin was local topical application in rat skin defect models. Alterations in the wounded skin were observed, and angiogenesis in the wound also was analyzed by immunohistochemical staining. The markers associated with differentiation macrophage were analyzed by immunofluorescence staining. The roles of AMPK singling pathway and the relative protein of NLRP3 inflammasome in wound were also analyzed by western blotting. In addition, AMPK/mTOR/NLRP3 inflammasome signaling axis was investigated to further analyze the molecular mechanism of metformin treatment on inducing M2 macrophage polarization in vitro.

RESULTS

Out results showed that metformin improved wound healing and angiogenesis which was paralleled by M2 macrophage polarization. We also found that the level of relative proteins of NLRP3 inflammasome was markedly decreased after metformin treatment. Furthermore, blockage of AMPK or activation of mTOR abolished the effects of metformin treatment on depressing NLRP3 inflammasome activation, M2 polarization and improving wound healing. It suggested that the treatment effects of metformin on wound healing were through regulating AMPK/mTOR/NLRP3 inflammasome signaling axis.

CONCLUSION

Metformin regulated AMPK/mTOR singling pathway to inhibit NLRP3 inflammasome activation, which boosted M2 macrophage polarization to accelerate the wound healing. These findings provided new insights into the molecular mechanism of metformin therapy and its therapeutic potential in wound healing.

Cellular and molecular mechanisms involved in the resolution of innate leukocyte inflammation

Inflammation is a host response to infection or damage and is vital for clearing pathogens and host debris. When this resolution fails to occur, chronic inflammation ensues. Chronic inflammation is typically characterized as a low-grade, persistent inflammatory process that can last for months or even years. This differs from acute inflammation, which is typically a fast, robust response to a stimulus followed by resolution with return to homeostasis. Inflammation resolution occurs through a variety of cellular processes and signaling components that act as "brakes" to keep inflammation in check. In cases of chronic inflammation, these "brakes" are often dysfunctional. Due to its prevalent association with chronic diseases, there is growing interest in characterizing these negative regulators and their cellular effects in innate leukocytes. In this review, we aim to describe key cellular and molecular homeostatic regulators of innate leukocytes, with particular attention to the emerging regulatory processes of autophagy and lysosomal fusion during inflammation resolution.

Drug delivery and tissue engineering to promote wound healing in the immunocompromised host: Current challenges and future directions

As regenerative medicine matures as a field, more promising technologies are being translated from the benchtop to the clinic. However, many of these strategies are designed with otherwise healthy hosts in mind and validated in animal models without other co-morbidities. In reality, many of the patient populations benefiting from drug delivery and tissue engineering-based devices to enhance wound healing also have significant underlying immunodeficiency. Specifically, patients suffering from diabetes, malignancy, human immunodeficiency virus, post-organ transplantation, and other compromised states have significant pleotropic immune defects that affect wound healing. In this work, we review the role of different immune cells in the regenerative process, highlight the effect of several common immunocompromised states on wound healing, and discuss different drug delivery strategies for overcoming immunodeficiencies.

Programming and memory dynamics of innate leukocytes during tissue homeostasis and inflammation

The field of innate immunity is witnessing a paradigm shift regarding "memory" and "programming" dynamics. Past studies of innate leukocytes characterized them as first responders to danger signals with no memory. However, recent findings suggest that innate leukocytes, such as monocytes and neutrophils, are capable of "memorizing" not only the chemical nature but also the history and dosages of external stimulants. As a consequence, innate leukocytes can be dynamically programmed or reprogrammed into complex inflammatory memory states. Key examples of innate leukocyte memory dynamics include the development of primed and tolerant monocytes when "programmed" with a variety of inflammatory stimulants at varying signal strengths. The development of innate leukocyte memory may have far-reaching translational implications, as programmed innate leukocytes may affect the pathogenesis of both acute and chronic inflammatory diseases. This review intends to critically discuss some of the recent studies that address this emerging concept and its implication in the pathogenesis of inflammatory diseases.

Defective Bone Repair in C57Bl6 Mice With Acute Systemic Inflammation

BACKGROUND

Bone repair is initiated with a local inflammatory response to injury. The presence of systemic inflammation impairs bone healing and often leads to malunion, although the underlying mechanisms remain poorly defined. Our research objective was to use a mouse model of cortical bone repair to determine the effect of systemic inflammation on cells in the bone healing microenvironment. QUESTION/PURPOSES: (1) Does systemic inflammation, induced by lipopolysaccharide (LPS) administration affect the quantity and quality of regenerating bone in primary bone healing? (2) Does systemic inflammation alter vascularization and the number or activity of inflammatory cells, osteoblasts, and osteoclasts in the bone healing microenvironment?

METHODS

Cortical defects were drilled in the femoral diaphysis of female and male C57BL/6 mice aged 5 to 9 months that were treated with daily systemic injections of LPS or physiologic saline as control for 7 days. Mice were euthanized at 1 week (Control, n = 7; LPS, n = 8), 2 weeks (Control, n = 7; LPS, n = 8), and 6 weeks (Control, n = 9; LPS, n = 8) after surgery. The quantity (bone volume per tissue volume [BV/TV]) and microarchitecture (trabecular separation and thickness, porosity) of bone in the defect were quantified with time using microCT. The presence or activity of vascular endothelial cells (CD34), macrophages (F4/80), osteoblasts (alkaline phosphatase [ALP]), and osteoclasts (tartrate-resistant acid phosphatase [TRAP]) were evaluated using histochemical analyses.

RESULTS

Only one of eight defects was bridged completely 6 weeks after surgery in LPS-injected mouse bones compared with seven of nine defects in the control mouse bones (odds ratio [OR], 0.04; 95% CI, 0.003-0.560; p = 0.007). The decrease in cortical bone in LPS-treated mice was reflected in reduced BV/TV (21% ± 4% vs 39% ± 10%; p < 0.01), increased trabecular separation (240 ± 36 μm vs 171 ± 29 μm; p < 0.01), decreased trabecular thickness (81 ± 18 μm vs 110 ± 22 μm; p = 0.02), and porosity (79% ± 4% vs 60% ± 10%; p < 0.01) at 6 weeks postoperative. Defective healing was accompanied by decreased CD34 (1.1 ± 0.6 vs 3.4 ± 0.9; p < 0.01), ALP (1.9 ± 0.9 vs 6.1 ± 3.2; p = 0.03), and TRAP (3.3 ± 4.7 vs 7.2 ± 4.0; p = 0.01) activity, and increased F4/80 (13 ± 2.6 vs 6.8 ± 1.7; p < 0.01) activity at 2 weeks postoperative.

CONCLUSION

The results indicate that LPS-induced systemic inflammation reduced the amount and impaired the quality of bone regenerated in mouse femurs. The effects were associated with impaired revascularization, decreased bone turnover by osteoblasts and osteoclasts, and by increased catabolic activity by macrophages.

CLINICAL RELEVANCE

Results from this preclinical study support clinical observations of impaired primary bone healing in patients with systemic inflammation. Based on our data, local administration of VEGF in the callus to stimulate revascularization, or transplantation of stem cells to enhance bone turnover represent potentially feasible approaches to improve outcomes in clinical practice.

Inflammatory Stress Effects on Health and Function After Spinal Cord Injury

Background: Injury to the spinal cord produces immediate, adaptive inflammatory responses that can exacerbate the initial injury and lead to secondary damage. Thus far, researchers and clinicians have focused on modulating acute inflammation to preserve sensorimotor function. However, this singular approach risks overlooking how chronic inflammation negatively impacts the broader health of persons with a spinal cord injury (SCI). Objective: The aim of this monograph was to discuss interrelated processes causing persistent inflammatory stress after SCI, along with associated health risks. We review archetypal factors that contribute to a chronic inflammatory state, including response to injury, acute infection, and autonomic dysreflexia. Secondary complications producing and exacerbating inflammation are also discussed, including pain, depression, obesity, and injury to the integumentary and skeletal systems. Finally, we discuss the role of bacteria and the gut microbiome in this process and then conclude with a discussion on how a pro-inflammatory phenotype promotes an elevated risk for cardiovascular disease after injury. Conclusions: Effectively managing chronic inflammation should be a high priority for clinicians and researchers who seek to improve the health and life quality of persons with SCI. Chronic inflammation worsens secondary medical complications and amplifies the risk for cardiometabolic disorders after injury, directly impacting both the quality of life and mortality risk after SCI. Inflammation can worsen pain and depression and even hinder neurological recovery. It is, therefore, imperative that countermeasures to chronic inflammation are routinely considered from the point of initial injury and proceeding throughout the lifespan of the individual with SCI.

Molecular Mechanisms That Underlie the Dynamic Adaptation of Innate Monocyte Memory to Varying Stimulant Strength of TLR Ligands

In adaptation to rising stimulant strength, innate monocytes can be dynamically programed to preferentially express either pro- or anti-inflammatory mediators. Such dynamic innate adaptation or programing may bear profound relevance in host health and disease. However, molecular mechanisms that govern innate adaptation to varying strength of stimulants are not well understood. Using lipopolysaccharide (LPS), the model stimulant of toll-like-receptor 4 (TLR4), we reported that the expressions of pro-inflammatory mediators are preferentially sustained in monocytes adapted by lower doses of LPS, and suppressed/tolerized in monocytes adapted by higher doses of LPS. Mechanistically, monocytes adapted by super-low dose LPS exhibited higher levels of transcription factor, interferon regulatory factor 5 (IRF5), and reduced levels of transcriptional modulator B lymphocyte-induced maturation protein-1 (Blimp-1). Intriguingly, the inflammatory monocyte adaptation by super-low dose LPS is dependent upon TRAM/TRIF but not MyD88. Similar to LPS, we also observed biphasic inflammatory adaptation and tolerance in monocytes challenged with varying dosages of TLR7 agonist. In sharp contrast, rising doses of TLR3 agonist preferentially caused inflammatory adaptation without inducing tolerance. At the molecular level, the differential regulation of IRF5 and Blimp-1 coincides with unique monocyte adaptation dynamics by TLR4/7 and TLR3 agonists. Our study provides novel clue toward the understanding of monocyte adaptation and memory toward distinct TLR ligands.

The persistence of low-grade inflammatory monocytes contributes to aggravated atherosclerosis

Sustained low-grade inflammation mediated by non-resolving inflammatory monocytes has long been suspected in the pathogenesis of atherosclerosis; however, the molecular mechanisms responsible for the sustainment of non-resolving inflammatory monocytes during atherosclerosis are poorly understood. Here we observe that subclinical endotoxemia, often seen in humans with chronic inflammation, aggravates murine atherosclerosis through programming monocytes into a non-resolving inflammatory state with elevated Ly6C, CCR5, MCP-1 and reduced SR-B1. The sustainment of inflammatory monocytes is due to the disruption of homeostatic tolerance through the elevation of miR-24 and reduction of the key negative-feedback regulator IRAK-M. miR-24 reduces the levels of Smad4 required for the expression of IRAK-M and also downregulates key lipid-processing molecule SR-B1. IRAK-M deficiency in turn leads to elevated miR-24 levels, sustains disruption of monocyte homeostasis and aggravates atherosclerosis. Our data define an integrated feedback circuit in monocytes and its disruption may lead to non-resolving low-grade inflammation conducive to atherosclerosis.

Deficiency in Toll-interacting protein (Tollip) skews inflamed yet incompetent innate leukocytes in vivo during DSS-induced septic colitis

Functionally compromised neutrophils contribute to adverse clinical outcomes in patients with severe inflammation and injury such as colitis and sepsis. However, the ontogeny of dysfunctional neutrophil during septic colitis remain poorly understood. We report that the dysfunctional neutrophil may be derived by the suppression of Toll-interacting-protein (Tollip). We observed that Tollip deficient neutrophils had compromised migratory capacity toward bacterial product fMLF due to reduced activity of AKT and reduction of FPR2, reduced potential to generate bacterial-killing neutrophil extra-cellular trap (NET), and compromised bacterial killing activity. On the other hand, Tollip deficient neutrophils had elevated levels of CCR5, responsible for their homing to sterile inflamed tissues. The inflamed and incompetent neutrophil phenotype was also observed in vivo in Tollip deficient mice subjected to DSS-induced colitis. We observed that TUDCA, a compound capable of restoring Tollip cellular function, can potently alleviate the severity of DSS-induced colitis. In humans, we observed significantly reduced Tollip levels in peripheral blood collected from human colitis patients as compared to blood samples from healthy donors. Collectively, our data reveal a novel mechanism in Tollip alteration that underlies the inflamed and incompetent polarization of neutrophils leading to severe outcomes of colitis.