Inflammation-resolution is mediated by the balance between specialized pro-resolving mediators (SPMs) like resolvin D1 (RvD1) and pro-inflammatory factors, like leukotriene B4 (LTB4). A key cellular process of inflammation-resolution is efferocytosis. Aging is associated with defective inflammation-resolution and the accumulation of pro-inflammatory senescent cells (SCs). Therefore, understanding mechanism(s) that underpin this impairment is a critical gap. Here, using a model of hind limb ischemia-reperfusion (I/R) remote lung injury, we present evidence that aging is associated with heightened inflammation, impaired SPM:LT ratio, defective efferocytosis, and a decrease in MerTK levels in injured lungs. Treatment with RvD1 mitigated I/R lung injury in aging, promoted efferocytosis, and prevented the decrease of MerTK in injured lungs from old mice. Old MerTK cleavage-resistant mice (MerTKCR) exhibited less neutrophils or polymorpho nuclear cells infiltration and had improved efferocytosis compared with old WT controls. Mechanistically, macrophages that were treated with conditioned media (CM) from senescent cells had increased MerTK cleavage, impaired efferocytosis, and a defective RvD1:LTB4 ratio. Macrophages from MerTKCR mice were resistant to CM-induced efferocytosis defects and had an improved RvD1:LTB4 ratio. RvD1-stimulated macrophages prevented CM-induced MerTK cleavage and promoted efferocytosis. Together, these data suggest a new mechanism and a potential therapy to promote inflammation-resolution and efferocytosis in aging.

Inflammation-resolution is a protective response that is mediated by specialized pro-resolving mediators (SPMs). The clearance of dead cells or efferocytosis is a critical cellular program of inflammation-resolution. Impaired efferocytosis can lead to tissue damage in prevalent human diseases, like atherosclerosis. Therefore understanding mechanisms associated with swift clearance of dead cells is of utmost clinical importance. Recently, the accumulation of necroptotic cells (NCs) was observed in human plaques and we postulated that this is due to defective clearance programs. Here we present evidence that NCs are inefficiently taken up by macrophages because they have increased surface expression of a well-known “don’t eat me” signal called CD47. High levels of CD47 on NCs stimulated RhoA-pMLC signaling in macrophages that promoted “nibbling”, rather than whole-cell engulfment of NCs. Anti-CD47 blocking antibodies limited RhoA-p-MLC signaling and promoted whole-cell NC engulfment. Treatment with anti-CD47 blocking antibodies to Ldlr −/− mice with established atherosclerosis decreased necrotic cores, limited the accumulation of plaque NCs and increased lesional SPMs, including Resolvin D1 (RvD1) compared with IgG controls. Mechanistically, RvD1 promoted whole-cell engulfment of NCs by decreasing RhoA signaling and activating CDC42. RvD1 specifically targeted NCs for engulfment by facilitating the release of the well-known “eat me signal” called calreticulin from macrophages in a CDC42 dependent manner. Lastly, RvD1 enhanced the clearance of NCs in advanced murine plaques. Together, these results suggest new molecules and signaling associated with the clearance of NCs, provide a new paradigm for the regulation of inflammation-resolution, and offer a potential treatment strategy for diseases where NCs underpin the pathology.

Inflammation-resolution is a protective response that is mediated by specialized pro-resolving mediators (SPMs). The clearance of dead cells or efferocytosis is a critical cellular program of inflammation-resolution. Impaired efferocytosis can lead to tissue damage in prevalent human diseases, like atherosclerosis. Therefore understanding mechanisms associated with swift clearance of dead cells is of utmost clinical importance. Recently, the accumulation of necroptotic cells (NCs) has been observed in human plaques and we postulate that this is due to defective clearance of NCs. Here we present evidence that NCs are inefficiently taken up by macrophages because they have increased surface expression of a well-known “don’t eat me” signal called CD47. High levels of CD47 on NCs stimulated RhoA-pMLC signaling in macrophages that promoted “nibbling”, rather than whole-cell engulfment of NCs. Anti-CD47 blocking antibodies or RhoA inhibition limited RhoA-p-MLC signaling and promoted whole-cell NC engulfment in vitro . Treatment with anti-CD47 blocking antibodies to Ldlr -/- mice with established atherosclerosis led to decreased necrotic cores, less accumulation of plaque NCs and increased lesional SPMs, including Resolvin D1 (RvD1) compared with anti-IgG controls. Next, we observed that RvD1 acts in a feed-forward manner to further enhance the clearance of NCs. Mechanistically, RvD1 promoted whole-cell engulfment of NCs by decreasing RhoA signaling and activating CDC42. RvD1 specifically targeted NCs for engulfment by facilitating the release of the well-known “eat me signal” called calreticulin from macrophages in a CDC42 dependent manner. Lastly, RvD1 enhanced the clearance of NCs in advanced murine plaques. Together, these results suggest new molecules and signaling associated with the clearance of NCs, provide a new paradigm for the regulation of inflammation-resolution, and offer a potential treatment strategy for diseases that are associated with the accumulation of NCs.

Non-resolving inflammation is the underpinning of several prevalent diseases including atherosclerosis. Understanding new mechanisms to promote the resolution of inflammation in atherosclerosis are of interest. Resolution is mediated by resolvins, including Resolvin D1 (RvD1). We recently showed that RvD1 prevents lesional necrosis in Ldlr -/- mice. However, the mechanisms underlying RvD1’s protective actions remain unknown. In this regard, the accumulation of senescent cells (SCs) was recently shown to promote necrotic core formation in Ldlr -/- mice. SCs are harmful because they possess a pro-inflammatory and proteolytic phenotype called the senescence associated secretory phenotype (SASP). Because RvD1 decreased lesional necrosis, we questioned whether RvD1’s actions were through limiting SCs in plaques. First, we found that human symptomatic plaques had significantly less RvD1 and significantly more SCs (quantified by positive p16 INK4A immunofluorescence staining) compared with asymptomatic plaques. To prove causation, we administered RvD1 to Ldlr -/- mice during advanced atherosclerosis and observed a significant decrease in lesional SCs and necrosis compared with vehicle controls. Mechanistically, we found that RvD1 significantly decreased the SASP as well as beta-galactosidase and p16 INK4A in WI-38 senescent cells. Importantly, a major component of the SASP is ADAM17, which is an enzyme that cleaves (and thus inactivates) a critical efferocytosis receptor on macrophages called MerTK. Because SCs are not cleared in plaques we questioned whether the SASP deranges efferocytosis mechanisms via their ability to cleave MerTK on macrophages. To investigate whether MerTK cleavage deranges SC clearance in plaques, we transplanted bone marrow from wild type (WT) or MerTK cleavage resistant (Mertk CR ) mice into Ldlr -/- mice, and quantified the levels of lesional p16 INK4A after 12 weeks of Western Diet feeding. Mertk CR mice had significantly less p16 INK4A levels and smaller necrotic cores compared with WT controls. Overall, we identified new functions for RvD1 and uncovered, for the first time, an endogenous mechanism for SC clearance. Our findings may provide new therapeutic strategies to treat atherosclerosis.

PURPOSE OF REVIEW:Nonresolving inflammation is now considered the underpinning of several prevalent human diseases, including atherosclerosis. The resolution of inflammation is a highly coordinated program to counterbalance proinflammatory signals for a swift return to tissue homeostasis. This process is controlled in part by endogenous specialized proresolving lipid mediators (SPMs). Emerging evidence has revealed that the balance of SPMs and proinflammatory mediators during acute inflammation regulates the duration of the inflammatory response and the timing of tissue resolution. Moreover, an imbalance between SPMs and proinflammatory mediators has been linked to several prevalent chronic inflammatory diseases in humans, including atherosclerosis.RECENT FINDINGS:Lipid mediator imbalances have recently been linked to atherosclerotic plaque instability. Administration of key SPMs restored this imbalance and led to plaque stability. SPMs have also recently been shown to be protective in other cardiovascular disease models including myocardial infarction, stroke and neointimal hyperplasia.SUMMARY:The current review highlights recent work that supports the concept of dysregulated inflammation-resolution in atherosclerosis with a particular focus on mechanisms and therapeutic opportunities associated with SPM receptors and lipid mediator imbalances. This article is based on experimental studies.