Platelet-induced myeloid Socs3 expression in humans and mice contributes to unresolved atherosclerosis inflammation and sustained plaque growth.

Critical conditions such as sepsis following infection or traumatic injury disturb the complex state of homeostasis that may lead to uncontrolled inflammation resulting in organ failure, shock and death. They are associated with endogenous mediators that control the onset of acute inflammatory response, but the central problem remains the complete resolution of inflammation. Omega-3 enriched lipid emulsions (Ω-3⁺ LEs) were used in experimental studies and clinical trials to establish homeostasis, yet with little understanding about their role on the resolution of inflammation and tissue regeneration. Here, we demonstrate that Ω-3 lipid emulsions (LEs) orchestrate inflammation-resolution/regeneration mechanism during sterile peritonitis and murine polymicrobial sepsis. Ω-3⁺ LEs recessed neutrophil infiltration, reduced pro-inflammatory mediators, reduced the classical monocyte and enhanced the non-classical monocytes/macrophages recruitment and finally increased the efferocytosis in sepsis. The actions of Ω-3⁺ LE were 5-lipoxygenase (5-LOX) and 12/15-lipoxygenase (12/15-LOX) dependent. Ω-3⁺ LEs shortened the resolution interval by 56%, stimulated the endogenous biosynthesis of resolution mediators lipoxin A4, protectin DX and maresin 1 and contributed to tissue regeneration. Ω-3⁺ LEs protected against hypothermia and weight loss and enhanced survival in murine polymicrobial sepsis. We highlighted a role of Ω-3⁺ LEs in regulating key mechanisms within the resolution terrain during murine sepsis. This might form the basis for a rational design of sepsis specific clinical nutrition.

Hepatic ischemia/reperfusion (I/R) is a major adverse reaction to liver transplantation, hemorrhagic shock, or resection. Recently, the anti‐inflammatory properties of the axonal guidance cue netrin‐1 were reported. Here, we demonstrate that netrin‐1 also impacts the resolution of inflammation and promotes hepatic repair and regeneration during liver I/R injury. In initial studies, we investigated the induction of netrin‐1 and its receptors in murine liver tissues after I/R injury. Hepatic I/R injury was performed in mice with a partial genetic netrin‐1 deficiency (Ntn1^+/−) or wild‐type C57BL/6 treated with exogenous netrin‐1 to examine the endogenous and therapeutically administered impact of netrin‐1. These investigations were corroborated by studies determining the characteristics of intravascular leukocyte flow, clearance of apoptotic neutrophils (polymorphonuclear cells [PMNs]), production of specialized proresolving lipid mediators (SPMs), generation of specific growth factors contributing to the resolution of inflammation, and liver repair. Hepatic I/R was associated with a significant reduction of netrin‐1 transcript and protein in murine liver tissue. Subsequent studies in netrin‐1‐deficient mice revealed lower efficacies in reducing PMN infiltration, proinflammatory cytokine levels, and hepatic‐specific injury enzymes. Conversely, mice treated with exogenous netrin‐1 exhibited increased liver protection and repair, reducing neutrophil influx into the injury site, decreasing proinflammatory mediators, increasing efferocytosis of apoptotic PMNs, and stimulating local endogenous biosynthesis of SPMs and the generation of specific growth factors. Finally, genetic studies implicated the A2B adenosine receptor in netrin‐1‐mediated protection during hepatic I/R injury. Conclusion: The present study indicates a previously unrecognized role for netrin‐1 in liver protection and its contribution to tissue homeostasis and regeneration. (Hepatology 2016;63:1689‐1705)

The bidirectional communication between the immune and nervous system is important in regulating immune responses. Here we show that the adrenergic nerves of sympathetic nervous system orchestrate inflammation resolution and regenerative programs by modulating repulsive guidance molecule A (RGM-A). In murine peritonitis, adrenergic nerves and RGM-A show bidirectional activation by stimulating the mutual expression and exhibit a higher potency for the cessation of neutrophil infiltration; this reduction is accompanied by increased pro-resolving monocyte or macrophage recruitment, polymorphonucleocyte clearance and specialized pro-resolving lipid mediators production at sites of injury. Chemical sympathectomy results in hyperinflammation and ineffective resolution in mice, while RGM-A treatments reverse these phenotypes. Signalling network analyses imply that RGM-A and β2AR agonist regulate monocyte activation by suppressing NF-κB activity but activating RICTOR and PI3K/AKT signalling. Our results thus illustrate the function of sympathetic nervous system and RGM-A in regulating resolution and tissue repair in a murine acute peritonitis model.

Macrophages accumulate prominently in the visceral adipose tissue (VAT) of obese humans and high fat diet (HFD) fed mice, and this is linked to insulin resistance and type II diabetes. While the mechanisms regulating macrophage recruitment in obesity have been delineated, the signals directing macrophage persistence in VAT are poorly understood. We previously showed that the neuroimmune guidance cue netrin-1 is expressed in the VAT of obese mice and humans, where it promotes macrophage accumulation. To better understand the source of netrin-1 and its effects on adipose tissue macrophage (ATM) fate and function in obesity, we generated mice with myeloid-specific deletion of netrin-1 (Ntn1 ^fl/fl LysMCre ^+/–; Ntn1^Δmac). Interestingly, Ntn1mac mice showed a modest decrease in HFD-induced adiposity and adipocyte size, in the absence of changes in food intake or leptin, that was accompanied by an increase in markers of adipocyte beiging (Prdm16, UCP-1). Using single cell RNA-seq, combined with conventional histological and flow cytometry techniques, we show that myeloid-specific deletion of netrin-1 caused a 50% attrition of ATMs in HFD-fed mice, particularly of the resident macrophage subset, and altered the phenotype of residual ATMs to enhance lipid handling. Pseudotime analysis of single cell transcriptomes showed that in the absence of netrin-1, macrophages in the obese VAT underwent a phenotypic switch with the majority of ATMs activating a program of genes specialized in lipid handling, including fatty acid uptake and intracellular transport, lipid droplet formation and lipolysis, and regulation of lipid localization. Furthermore, Ntn1^Δmac macrophages had reduced expression of genes involved in arachidonic acid metabolism, and targeted LCMS/MS metabololipidomics analysis revealed decreases in proinflammatory eicosanoids (5-HETE, 6-trans LTB₄, TXB₂, PGD₂) in the obese VAT. Collectively, our data show that targeted deletion of netrin-1 in macrophages reprograms the ATM phenotype in obesity, leading to reduced adipose inflammation, and improved lipid handling and metabolic function.

Netrins belong to the family of laminin-like secreted proteins, which guide axonal migration and neuronal growth in the developing central nervous system. Over the last 20 years, it has been established that netrin-1 acts as a chemoattractive or chemorepulsive cue in diverse biological processes far beyond neuronal development. Netrin-1 has been shown to play a central role in cell adhesion, cell migration, proliferation, and cell survival in neuronal and non-neuronal tissue. In this context, netrin-1 was found to orchestrate organogenesis, angiogenesis, tumorigenesis, and inflammation. In inflammation, as in neuronal development, netrin-1 plays a dichotomous role directing the migration of leukocytes, especially monocytes in the inflamed tissue. Monocyte-derived macrophages have long been known for a similar dual role in inflammation. In response to pathogen-induced acute injury, monocytes are rapidly recruited to damaged tissue as the first line of immune defense to phagocyte pathogens, present antigens to initiate the adaptive immune response, and promote wound healing in the resolution phase. On the other hand, dysregulated macrophages with impaired phagocytosis and egress capacity accumulate in chronic inflammation sites and foster the maintenance—and even the progression—of chronic inflammation. In this review article, we will highlight the dichotomous roles of netrin-1 and its impact on acute and chronic inflammation.