Lipid mediators in the regulation of innate and adaptive immunity.

Abstract

Life depends on lipids as the major structural components of membranes. Lipids are also crucial sources of energy in animals and humans. Fascinatingly, lipids have a diversity of additional roles in the immune system that range from acting as intercellular and intracellular signaling molecules to being modulators of membrane protein function, which influences tissue physiology in health and disease. How lipids are processed and metabolized has significant impact on cell and tissue function. In this issue of the journal, a series of reviews highlight the diverse ways lipids have been shown to serve as modulators of immunity. Murakami et al.1 provide a comprehensive overview of secreted phospholipase A2 (sPLA2) family members, which hydrolyze phospholipids at the sn-2 position to produce lysophospholipids and fatty acids. These liberated fatty acids are often further modified to produce bioactive lipids such as eicosanoids, prostaglandins (PGs) (also see the review in this issue by Honda et al.), and leukotrienes (also see the review in this issue by Yokomizo and Shimizu) that further influence immunity in diverse manners. These phospholipid substrates are not only found in plasma membranes but also include phospholipids present in extracellular vesicles, microbial membranes, surfactants, and dietary phospholipids. The review begins with general considerations of sPLA2 biology and then focuses on individual classical sPLA2 families. The authors include findings from mouse models of over-expression as well as genetically engineered deficiencies in specific sPLA2 family members, and considers the defined (and potential) physiological and pathophysiological contributions of sPLA2 family members to immunity. Importantly, the authors address the dual positive/negative nature of many sPLA2 activities. Beyond their roles in immunity, the authors highlight the participation of individual sPLA2s to pain, fertility, cardiovascular disease, cancer, and obesity, as well as providing a current account of sPLA2 inhibitors under investigation. As such, this review will be a valuable resource for an immunological audience. The importance of PGs in inflammation is widely appreciated because of the activities of nonsteroidal anti-inflammatory drugs (NSAIDs) that inhibit the cyclooxygenase (COX) enzymes needed for PG production. However, the full details of PG and PG receptor biology are still being elucidated. In their review, Honda et al.2 explore the broad functions of PGs and a related set of arachidonic acid derived mediators, leukotrienes (LTs), in tissue physiology and disease. They describe the enzymes involved in PG and LT synthesis, and the 13 GPCRs that they engage. The authors then provide an in-depth description of the multiple contributions of PGs and LTs to events associated with skin inflammation. This includes promoting dendritic cell (DC) migration, promoting neutrophil and eosinophil recruitment, augmenting effector T cell differentiation in draining lymph nodes, and contributing to skin-associated lymphoid tissue (iSALT) formation. The multi-faceted influences of PGs and LTs on the development of atopic dermatitis, a Type 2 immune hypersensitivity condition, and in psoriasis, a Th17-associated condition, are discussed. PGs and LTs are derived from omega-6 fatty acids (unsaturated bond at the sixth position in the hydrocarbon chain). Honda et al. also review skin immune functions of the less studied omega-3 fatty acid-derived metabolites. Both omega-6 and omega-3 fatty acids are essential fatty acids that mammals must acquire from their diet. The authors review the suppressive influence of certain omega-3 fatty acid metabolites on allergic dermatitis. In the case of one set of metabolites, these actions are mediated by GPR40, and for another, retinoid X receptor alpha (RXRa), a nuclear receptor that is further discussed in the review by Czimmerer and Nagy. They end on the topic of how dietary saturated fatty acids (SFAs) can influence psoriatic inflammation, discussing connections between high-fat diets (that are rich in SFAs) and elevated IL-17 responses. In summary, this review provides a rich description of current knowledge of the actions of omega-6, omega-3, and unsaturated fatty acid-derived metabolites in the immune system, helping the reader parse conditions where their actions may be inflammatory versus resolving. The next review by Yokomizo and Shimizu3 in this issue focuses on one class of LT, LTB4, and receptors BLT1 and BLT2. This review is from the investigators who first cloned BLT1 and BLT2 as high- and low-affinity LTB4 receptors, and the reader is provided with an in-depth historical perspective of these discoveries. The cloning of BLT1 using a subtractive hybridization approach was a landmark study, and BLT1 is now widely known as a prominent chemotactic receptor, especially on neutrophils. This article explains that while BLT2 was identified as a second LTB4 receptor, the low affinity for LTB4 led these investigators and their coworkers to assume there would be other higher affinity ligands for BLT2. Screening of lipid libraries led to their identification of 12-hydroxyheptadecatrienoic acid (12-HHT) as a high affinity ligand. The biosynthetic pathways of LTB4 and 12-HHT from arachidonic acid are detailed, with LTB4 production depending on 5-lipoxygenase (5-LO) while 12-HTT requires COX enzymes. This review provides a helpful summary of LTB4 actions in asthma, bone resorption, arthritis, and atherosclerosis, and more broadly in the immune and nervous systems. The actions in asthma are of disease significance in humans since a 5-lipoxygenase (5-LO) inhibitor is approved as an anti-asthma therapeutic. The authors describe platelet production of large amounts of 12-HHT, adding to the many other mediators released by activated platelets, as summarized in the review by De Giovanni et al. 12-HHT in damaged skin acts on BLT2 in keratinocytes to promote wound repair. The authors also detail their recent discovery of BLT1 as a sub-setting marker for DCs with different migration properties. They conclude with a discussion of the status of preclinical and clinical trials of LTB4 receptor antagonists for treatment of inflammatory disease, and speculation that the newly available structure of BLT1 should enable development of improved therapeutic agents. A fundamental mechanism by which lipids impact immune function is by influencing structural and metabolic properties of immune cells. A prominent lipid of this class is cholesterol, which makes up approximately 20% of the plasma membrane and undergoes interactions with many plasma membrane proteins, including T and B cell antigen receptors (TCR and BCR). Cholesterol is one of the products of the mevalonate pathway and Kennewick and Bensinger,4 provide a concise introduction to this pathway and the functions of key products, including how cholesterol abundance may be altered in different T cell states to influence the organization of signaling molecules, and to allosterically regulate TCR triggering. They explain the tight regulatory control that exists over flux through the mevalonate pathway and its branches. Crucial products of one branch of the pathway are isoprenoids, including farnesyl pyrophosphate and geranylgeranyl pyrophosphate. These hydrocarbon chains are attached to many of the small GTP-binding proteins in cells, and some other membrane-associated proteins. Their highly controlled availability is crucial for proper immune cell function. Kennewick and Bensinger detail the key activity of sterol regulatory element-binding proteins (SREBPs) in driving transcription of mevalonate pathway enzymes, and how SREBP activity is controlled by negative feedback mediated by cholesterol and cholesterol metabolites. The role of SREBPs downstream of the mTOR complex in supporting the anabolic needs of activated T cells is discussed. Fascinatingly, they detail how enzymes distal in the cholesterol synthetic branch, such as those involved in isoprenoid production, are not SREBP regulated and are controlled by other still poorly defined mechanisms. The authors present a discussion of the flux diversion hypothesis of Faust, Goldstein and Brown, exploring how immune cells can differentially adjust outputs of the mevalonate pathway depending on product requirements. In a further section, the authors describe the amplified importance of flux through the mevalonate pathway in activated compared to quiescent lymphocytes, as perhaps first revealed most clearly through the Bensinger lab's study of SCAP-deficient T cells. This review provides a great entry point for anyone interested in understanding how the mevalonate pathway impinges on T cell biology, and immunology more generally. Lipid droplets (LDs) are major lipid storage organelles present in most cells. Their presence often reflect cellular metabolic and nutrient status. Accordingly, many pathogens have evolved mechanisms to induce LD formation and exploit their lipid content for colonization. Conversely, LDs have also been shown to possess antibiotic activity and thus can serve both beneficial and detrimental roles towards diverse pathogens. The Safi et al.5 review provides a fascinating and comprehensive account of LD organelles and their dynamic remodeling and interaction with pathogens. Notably, this review emphasizes the emerging consideration that LDs are a potential strategic chokepoint for organizing a defensive line against certain pathogens. Accordingly, the authors categorize LDs into defensive LDs (d-LDs) and fasting-LDs (f-LDs) to signify LDs that act to subvert pathogens, versus LDs that serve as a source of lipid energy and contribute to cell metabolism. Pulmonary surfactants are important complexes of lipid and protein produced by Type II alveolar epithelial cells that regulate alveoli surface tension thus preventing lung collapse. The review by Numata et al.6 provides a detailed account of work by this team over the last dozen years defining anti-inflammatory and anti-viral properties by the specific pulmonary surfactant lipids, palmitoyl-oleoyl phosphatidylglycerol and phosphatidylinositol. More precisely, this review covers in vitro experiments with cell lines and primary human cells, demonstrating the ability of these surfactant phospholipids to potently inhibit TLR signaling and proinflammatory cytokine production. They also discuss in vivo mouse (and ferret) models that show the ability of these lipids to limit viral infections. Notably, the authors highlight the strong rationale for the need for novel therapeutics to address acute respiratory distress syndrome and viral infections (RSV, influenza A and SARS-CoV-2) and provide compelling evidence that these pulmonary surfactant lipids may provide one avenue to reach this goal. Another mode of lipid action in the immune system is via engagement of nuclear hormone receptors. Czimmerer and Nagy7 review this topic with a particular focus on their studies of gene regulation in macrophages mediated by retinoid X receptor (RXR) heterodimerizing receptors. They point out that while some nuclear hormone receptors are activated by high-affinity ligands such as all-trans retinoic acid for RARs and vitamin D3 for VDR, others have low affinity and more promiscuous binding of lipid classes including oxysterols for liver X receptors (LXRs), fatty acids for Peroxisome Proliferator Activated Receptors (PPARs), and bile acids for farnesoid X receptors (FXRs). Complexes bound without ligand are explained to interact with corepressor complexes and repress transcription, while ligand binding induces corepressor-coactivator exchange leading to transcription activation. Moreover, they highlight how next-generation sequencing-based methods have greatly expanded understanding of the binding sites for RXR heterodimers. They explain how their findings indicate the RXR cistrome in macrophages may not be regulated by exogenous ligands. Their review then focuses on an IL-4 orchestrated STAT6, EGR1, and PPARy containing transcription factor network that controls gene expression in macrophages, driving alternative (Type-2) polarization. The important role of PPARγ in macrophage polarization is reviewed and it is proposed that rather than acting as sensors of the extracellular or intracellular lipid microenvironment, PPARγ/RXR heterodimers may act as epigenetic bookmarking factors. This review gives the reader a framework for considering when a nuclear hormone receptor regulated cell behavior may be lipid ligand-dependent versus independent. The review by De Giovanni et al.8 focuses on the function of the receptor, GPR35, in the immune system. GPR35 has been suggested to respond to a diverse group of ligands, including lipids, though the best validated ligands are tryptophan metabolites, including kynurenic acid (KynA) and 5-hydroxyindoleacetic acid (5-HIAA). GPR35 is highly expressed by activated neutrophils and it supports migration and adhesion of these cells in response to 5-HIAA. The authors summarize their work providing evidence that 5-HIAA release by activated platelets and mast cells cooperates in promoting neutrophil transmigration from blood into inflamed tissue. Zooming out, they summarize the broader contributions of platelets and mast cells, and the wide diversity of mediators they release to neutrophil recruitment. Since 5-HIAA is a breakdown product of serotonin, De Giovanni et al. provide a concise review of ways in which serotonin from activated platelets and mast cells engages with the vasculature and likely other cell types to augment cell recruitment and inflammatory events. They also summarize the surprisingly diverse set of actions that have been reported for GPR35 in tissues ranging from the epithelium to the heart to neurons. This review also makes clear that while genetic studies have provided support for 5-HIAA as a ligand for GPR35, the receptor has a broad ligand binding profile, and there are sure to be additional physiologically relevant ligands, likely including some made by the microbiome. In summary, this review will be useful to readers interested in GPR35 and 5-HIAA biology, and in platelet and mast cell actions during cell recruitment to sites of inflammation. In the review by Yaginuma et al.,9 the authors provide an overview of bioactive lysophospholipids and their G-protein coupled receptors (GPCRs) with a particular emphasis on the described roles of lysophosphatidylserine-(LysoPS) induced GPCR signaling in immune modulation. This balanced review includes a general overview of (the often varied) routes of lysophospholipid synthesis and the phospholipases that contribute to their production. The authors nicely introduce and discuss the concept that lysophospholipid activity as a lipid mediator is typically autocoidal and is distinguished from, for example, S1P activity, which acts more hormone-like due to higher constitutive concentrations of S1P compared to the other lysophospholipids. Finally, the authors report interesting findings that include analysis of RNAseq data highlighting the LysoPS GPCRs that are expressed by different immune cell populations under homeostatic conditions. They further provide a general analysis of CD4 T cell and B cell numbers after immunization in wild-type and mouse strains deficient in either Lps1/Gpr34, Lps2/P2y10, or Lps3 Gpr174. These latter findings suggest that Lps1/GPR34 may positively regulate lymphocyte activation whereas Lps2/P2Y10 may play a negative role. In summary, this review summarizes the current understanding on lysophospholipid GPCR signaling with an emphasis on LysoPS and its Lps1, Lps2, and Lps3 cognate receptors and will be useful for an immunological audience. The review by Torres et al.10 provides an overview of the work by this group characterizing how lysophosphatidic acid (LPA) signaling via the LPA5 GPCR inhibits T and B lymphocyte function. Starting with their initial observation that LPA can suppress TCR and BCR-induced calcium release from intracellular stores, the authors review their biochemical and cell biological studies that identify the LPA5 signaling axis as not only interfering with TCR-induced IP3 receptor-mediated intracellular calcium release but also CD8 T cell—target cell immunological synapse formation. The ability of endogenous systemic levels of LPA to signal in vivo via LPA5 to impede lymphocyte function is demonstrated in several contexts including cytotoxic CD8 T cell antigen-specific killing of target cells and, importantly, melanoma tumors in a mouse model. More recent observations covered in this review include results revealing that LPA-LPA5 signaling also rewires cytotoxic CD8 T cell metabolic activity to promote fatty acid oxidation. Thus showing that LPA5 can also influence T cell function independent of regulating TCR signaling. Finally, the authors present provocative findings that implicate plasma LPA levels as prognostic for successful immune checkpoint blockade therapy. From our initial understanding almost two decades ago that S1PR1 was essential for thymocytes to exit the thymus and join the peripheral pool of mature antigen-responsive T cells, there has been great interest in better understanding how the sphingosine-1-phosphate lysophospholipid and its cognate S1P GPCRs orchestrate immune cell trafficking and migration in health and disease. In their review, Hallisey and Schwab11 provide an update on key areas in S1P biology as they relate to T cell trafficking in and out of lymphoid and non-lymphoid organs not only in homeostasis but particularly during inflammatory settings. This review includes a thoughtful consideration on the implications of altered T cell trafficking in lymphoid organs for T cell immunity. Notably, the authors highlight interesting work from their laboratory on the regulation of S1PR1 expression by T cells and formation of S1P gradients noting areas that still require a more thorough understanding. DCs are a key cell type that serves to communicate between the innate and adaptive arms of immunity. Important DC functions that encompass development, activation and homeostasis are regulated by the metabolism of lipids, and specifically fatty acid synthesis, oxidation and storage. In their contribution, You and Chi12 begin with a description of DC subset classification and activity, and proceed to provide a comprehensive review and account of fatty acid metabolism and lipid metabolites as they relate to in vitro and in vivo DC differentiation and function. Notably, this review of the literature focuses on the role of fatty acid and cholesterol metabolism in DC biology highlighting consistencies and discrepancies between different published studies and models. Areas of lipid biology that need further clarification and resolution with regard to DC function are also discussed. The authors declare no competing interests. Data sharing is not applicable to this article as no new data were created or analyzed in this study.