Specialized pro-resolving mediators

Through most of its early period of study, acute inflammatory responses were regarded as self-limiting innate immune system reactions to invading foreign organisms, tissue injuries, and other insults. These reactions were orchestrated by various soluble signaling agents such as a) foreign organism-derived N-formylated oligopeptide chemotactic factors (e.g. N-formylmethionine-leucyl-phenylalanine); b) complement components C5a and C3a which are chemotactic factors formed during the activation of the host's blood complement system by invading organisms or injured tissues; and c) host cell-derived pro-inflammatory cytokines (e.g. interleukin 1s), host-derived pro-inflammatory chemokines (e.g. CXCL8, CCL2, CCL3, CCL4, CCL5, CCL11, CXCL10), platelet-activating factor, and PUFA metabolites including in particular leukotrienes (e.g. LTB4), hydroxyeicosatetraenoic acids (e.g., 5-HETE, 12-HETE), the hydroxylated heptadecatrienoic acid, 12-HHT, and oxoeicosanoids (e.g. 5-oxo-ETE). These agents functioned as pro-inflammatory signals by increasing the permeability of local blood vessels; activating tissue-bound pro-inflammatory cells such as mast cells, and macrophages; and attracting to nascent inflammatory sites and activating circulating neutrophils, monocytes, eosinophils, gamma delta T cells, and natural killer T cells. The cited cells then proceeded to neutralize invading organisms, limit tissue injury, and initiate tissue repair. Hence, the classic inflammatory response was viewed as fully regulated by the soluble signaling agents. That is, the agents formed, orchestrated an inflammatory cell response, but then dissipated to allow resolution of the response. In 1974, however, Charles N. Serhan, Mats Hamberg and Bengt Samuelsson, discovered that human neutrophils metabolize arachidonic acid to two novel products that contain 3 hydroxyl residues and 4 double bonds viz., 5,6,15-trihydroxy-7,9,11,13-icosatetraenoic acid and 5,14,15-trihydroxy-6,8,10,12-icosatetraenoic acid. These products are now termed lipoxin A4 and B4, respectively. While initially found to have in vitro activity suggesting that they might act as pro-inflammatory agents, Serhan and colleagues and other groups found that the lipoxins as well as a large number of newly discovered metabolites of other PUFA possess primarily if not exclusively anti-inflammatory activities and therefore may be crucial for causing the resolution of inflammation. In this view, inflammatory responses are not self-limiting but rather limited by the formation of a particular group of PUFA metabolites that counteract the actions of pro-inflammatory signals. Later, these PUFA metabolites were classified together and termed specialized pro-resolving mediators (i.e. SPM). ==Inflammation==

The production and activities of the SPM suggest a new view of inflammation wherein the initial response to foreign organisms, tissue injury, or other insults involves numerous soluble cell signaling molecules that not only recruit various cell types to promote inflammation but concurrently cause these cells to produce SPM which feed back on their parent and other cells to dampen their pro-inflammatory activity and to promote repair. Resolution of an inflammatory response is thus an active rather than self-limiting process which is set into motion at least in part by the initiating pro-inflammatory mediators (e.g. prostaglandin E2 and prostaglandin D2) which instruct relevant cells to produce SPM and to assume a more anti-inflammatory phenotype. Resolution of the normal inflammatory response, then, may involve switching production of pro-inflammatory to anti-inflammatory PUFA metabolites. Excessive inflammatory responses to insult as well as many pathological inflammatory responses that contribute to diverse diseases such as atherosclerosis, obesity, diabetes, Alzheimer's disease, inflammatory bowel disease, etc. (see ) may reflect, in part, a failure in this class switching. Diseases caused or worsened by non-adaptive inflammatory responses may by amenable to treatment with SPM or synthetic SPM which, unlike natural SPM, resist in vivo metabolic inactivation. The SPM possess overlapping activities which work to resolve inflammation. SPMs (typically more than one for each listed action) have the following anti-inflammatory activities on the indicated cell types as defined in animal and human model studies: • Neutrophils: inhibit their migration from the blood circulation into inflamed tissues and their release of tissue-injuring reactive oxygen species and granule-bound enzymes; stimulates their expression the chemokine receptor, CCR5, to inhibit chemokine signaling, enhances their phagocyte activity, and promotes their death by apoptosis. • Eosinophils: inhibit their migration from the blood circulation into inflamed tissues. • Monocytes: inhibit their migration response to chemotactic factors and release of pro-inflammatory mediators. • Lymphocytes: Inhibit the infiltration of CD4+ and CD8+ lymphocytes into inflamed sites and inhibits production of the pro-inflammatory signals, interleukin-4 and interferon gamma by CD4+ lymphocytes; promotes the apoptosis of Th-17 pro-inflammatory lymphocytes; promotes B cell lymphocytes to differentiate into antibody secreting cells; inhibits innate lymphoid cells from releasing pro-inflammatory cytokines such as interleukin-13 while stimulating them to secrete amphiregulin, a product which acts to restore mucosal integrity; Inhibits the production of the pro-inflammatory cytokines, interleukin-17 and interleukin-23, thereby contributing to the dampening adaptive immune responses in T helper 17 cells; stimulates natural killer T cell lymphocytes to induce apoptosis in the neutrophils and eosinophil of inflamed tissues; and increases the cytotoxicity of the natural killer cell type of lymphocytes by, e.g. promoting their ability to induce apoptosis in neutrophils and eosinophils in inflamed tissues. • Platelets: inhibit their aggregation and possibly thereby their contribution to blood clotting. • Macrophages: inhibit their infiltration into inflamed tissues and release of pro-inflammatory cytokines; stimulate their conversion from a pro-inflammatory M1 phenotype to an anti-inflammatory M2 phenotype (see ) that are more active in secreting the anti-inflammatory cytokine, Interleukin-10, more resistant to become apoptotic, and more active in leaving sites of inflammation. • Microglia cells: inhibit the release of pro-inflammatory cytokines by this central nervous system type of macrophage. • Mast cells: inhibit their infiltration into inflamed tissues and, in lung mast cells, the release of histamine. • Dendritic cells: suppresses their migration to lymph nodes as well as their release of pro-inflammatory cytokines and expression of MHC class II proteins. • Neurons: act through their target G protein–coupled receptors to inhibit pain receptors (i.e. TRPV1, TRPV3, TRPV4, TRPA1, TNFR, NMDAR, and/or mGluR) on neurons in the peripheral nervous system, dorsal root ganglia, and/or spinal cord thereby suppressing pain perception. SPMs also stimulate anti-inflammatory and tissue reparative types of responses in epithelium cells, endothelium cells, fibroblasts, smooth muscle cells, osteoclasts, osteoblasts, goblet cells, and kidney podocytes ==Biochemistry==

SPM are metabolites of arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), or n−3 DPA (i.e. 7Z,10Z,13Z,16Z,19Z-docosapentaenoic acid or clupanodonic acid); these metabolites are termed lipoxins (Lx), resolvins (Rv), protectins (PD) (also termed neuroprotectins [NP]), and maresins (MaR). EPA, DHA, and n−3 DPA are n−3 fatty acids; their conversions to SPM are proposed to be one mechanism by which n−3 fatty acids may ameliorate inflammatory diseases (see ). SPM act, at least in part, by either activating or inhibiting cells through binding to and thereby activating or inhibiting the activation of specific cellular receptors. Lipoxins Human cells synthesize LxA4 and LxB4 by serially metabolizing arachidonic acid (5Z,8Z,11Z,14Z-eicosatetraenoic acid) with a) ALOX15 (or possibly ALOX15B) followed by ALOX5; b) ALOX5 followed by ALOX15 (or possibly ALOX15B); or c) ALOX5 followed by ALOX12. Cells and, indeed, humans treated with aspirin form the 15R-hydroxy epimer lipoxins of these two 15S-lipoxins viz., 15-epi-LXA4 and 15-epi-LXB4, through a pathway that involves ALOX5 followed by aspirin-treated cyclooxygenase-2 (COX-2). Aspirin-treated COX-2, while inactive in metabolizing arachidonic acid to prostanoids, metabolizes this PUFA to 15R-hydroperoxy-eicosatetraenoic acid whereas the ALOX15 (or ALOX15B) pathway metabolizes arachidonic acid to 15S-hydroperoxy-eicosatetraenoic acid. The two aspirin-triggered lipoxins (AT-lipoxins) or epi-lipoxins differ structurally from LxA4 and LxB4 only in the S versus R chirality of their 15-hydroxyl residue. Numerous studies have found that these metabolites have potent anti-inflammatory activity in vitro and in animal models and in humans may stimulate cells by binding to certain receptors on these cells. The following table lists the structural formulae (ETE stands for eicosatetraenoic acid), major activities, and cellular receptor targets (where known). • The FPL2 receptor (also termed the ALX, ALX/FPR2 receptor) is expressed on human neutrophils, eosinophils, monocytes, macrophages, T cells, synovial fibroblasts, and intestinal and airway epithelium as well as on astrocytes in the spinal cord of mice; GPR32 (also termed the RvD1 receptor or DRV1)is expressed on human neutrophils, lymphocytes, monocytes, macrophages, and vascular tissue. Both of these receptors are involved in regulating inflammation. The AHR (i.e. the aryl hydrocarbon receptor) is a ligand-activated transcription factor that regulates xenobiotic-metabolizing enzymes such as cytochrome P450 enzymes. Resolvins Resolvins are metabolites of omega−3 fatty acids, EPA, DHA, and 7Z,10Z,13Z,16Z,19Z-docosapentaenoic acid (n−3 DPA). All three of these omega−3 fatty acids are abundant in salt water fish, fish oils, and other seafood. In vitro studies find that ALOX5 can convert 18S-hydroperoxy-EPA to the 18S-hydroxy analog of RvE2 termed 18S-RvE2. 18S-RvE2, however has little or no SPM activity The following table lists the structural formulae, major activities with citations and cellular receptor targets of D series resolvins. • The distribution and major functions of GPR32, FPR2, TRPV1, and TRPV3 are given in the above EPA-derived resolvins section; TRPA1 is a chemosensor ion channel located on the plasma membrane of many human cell types; TRPV4, also termed the vanilloid-receptor related osmotically activated channel (VR-OAC) and OSM9-like transient receptor potential channel member 4 (OTRPC4)2], is involved in multiple physiologic functions and dysfunctions. With respect to the SPMS, both receptors mediate the perception of various forms of inflammation-triggered pain. Similarly, 14S,20R-dihyrdoxy-4Z,7Z,10Z,12E,16Z,18E-docosahexaenoic acid, while not yet assigned a RvD number, qualifies as a RvD-related SPM. It is a DHA metabolite made by mouse eosinophils, detected in the peritoneal fluid of mice undergoing experimental peritonitis, and possessing the ability to inhibit the influx of leukocytes into the peritoneum of the mice. Finally, two resolvin sulfido-conjugates (8-glutathionyl,7,17-dihydroxy-4Z,9,11,13Z,15E,19Z-docosahexaenoic acid and 8-cysteinylglycinyl,7,17-dihydroxy-4Z,9,11,13Z,15E,19Z-docosahexaenoic acid) have been shown to be formed from their 7,17-dihydroxy precursor by cells in vitro, to accelerate regeneration of experimental injuries in planaria worms, and to have potent anti-inflammatory activity in various in vitro model systems. n−3 DPA-derived resolvins n−3 DPA (i.e. 7Z,10Z,13Z,16Z,19Z-docosapentaenoic acid)-derived resolvins are recently identified SPM. In the model system used to identify them, human platelets pretreated with aspirin to form acetylated COX-2 or with the statin, atorvastatin, to form S-nitrosylated COX-2, thereby modify this enzyme's activity. The modified enzyme metabolizes n−3 DPA to a 13R-hydroperoxy-n−3 DPA intermediate which is passed over to nearby human neutrophils; these cells then metabolize the intermediate to four poly-hydroxyl metabolites termed resolvin T1 (RvT1), RvT2, RvT3, and RvT4. These T series resolvins also form in mice undergoing experimental inflammatory responses and have potent in vitro and in vivo anti-inflammatory activity; they are particularly effective in reducing the systemic inflammation as well as increasing the survival of mice injected with lethal doses of E. coli bacteria. Another set of newly described n−3 DPA resolvins, RvD1n−3, RvD2n−3, and RvD5n−3, have been named based on their presumed structural analogies to the DHA-derived resolvins RvD1, RvD2, and RvD5, respectively. These three n−3 DPA-derived resolvins have not been defined with respect to the chirality of their hydroxyl residues or the cis–trans isomerism of their double bonds but do possess potent anti-inflammatory activity in animal models and human cells; they also have protective actions in increasing the survival of mice subjected to E. coli sepsis. The following table lists the structural formulae (DHA stands for docosahexaenoic acid), major activities, cellular receptor targets (where known), and Wikipedia pages giving further information on the activity and syntheses. • The TRPV1 receptor is discussed in the EPA-derived resolvin section. • While not yet given trivial names, certain isomers of the protectins also prove to have SPM activity: the 13Z cis–trans isomer of 10-epi-PD1, 10S,17S-dihydroxy-4Z,7Z,11E,13Z,15E,19Z-DHA, is a relatively abundant metabolite compared to PD1 detected in the peritoneal fluid from a mouse model of peritonitis (although not detected in stimulated leukocytes) and has moderately potent anti-inflammatory activity in this model; 10R,17S-dihydroxy-4Z,7Z,11E,13E,15E,19Z-DHA, is a prominent metabolite detected in stimulated leukocytes, not detected the mouse peritonitis model, and has modest anti-inflammatory activity in the latter model; and 10S,17S-dihydroxy-4Z,7Z,11E,13E,15Z,19Z-DHA, while not detected by in the mouse model of peritonitis or stimulated leukocytes, is more potent than even PD1 in inhibiting peritonitis in the mouse model. In addition to these compounds, two protectin sulfido-conjugates () form in vitro, accelerate regeneration of injured planaria worms, and have potent anti-inflammatory activity in in vitro model systems. The following table lists the structural formulae (DPA stands for docosapentaenoic acid), major activities and cellular receptor targets (where known). Maresins DHA-derived maresins Cells metabolize DHA by ALOX12, other lipoxygenase, (12/15-lipoxygenase in mice), or an unidentified pathway to a 13S,14S-epoxide-4Z,7Z,9E,11E,16Z,19Z-DHA intermediate (13S,14S-epoxy-maresin MaR) and then hydrolyze this intermediate by an epoxide hydrolase activity (which ALOX 12 and mouse 12/15-lipoxygenase possess) to MaR1 and MaR2. During this metabolism, cells also form 7-epi-Mar1, i.e. the 7S-12E isomer of Mar1, as well as the 14S-hydroxy and 14R-hydroxy metabolites of DHA. The latter hydroxy metabolites can be converted by an unidentified cytochrome P450 enzyme to maresin like-1 (Mar-L1) and Mar-L2 by omega oxidation; alternatively, DHA may be first metabolized to 22-hydroxy-DHA by CYP1A2, CYP2C8, CYP2C9, CYP2D6, CYP2E1, or CYP3A4 and then metabolized through the cited epoxide-forming pathways to Mar-L1 and MaR-L2. Studies have found that these metabolites have potent anti-inflammatory activity in vitro and in animal models. • Mouse eosinophils metabolize DHA to a maresin-like product, 14S,20R-dihydroxy-4Z,7Z,10Z,12E,16Z,18Z-docosahexaenoic acid. This product, as well as its 14,S,20S isomer possesses potent anti-inflammatory activity in mice. Prostaglandins and isoprostanes PUFA derivatives containing a cyclopentenone structure are chemically reactive and can form adducts with various tissue targets, particularly proteins. Certain of these PUFA-cyclopentenones bind to the sulfur residues in the KEAP1 component of the KEAP1-NFE2L2 protein complex in the cytosol of cells. This negates KEAP1's ability to bind NFE2L2; in consequence, NFE2L2 becomes free to translocate to the nuclease and stimulate the transcription of genes that encode proteins active in detoxifying reactive oxygen species; this effect tends to reduce inflammatory reactions. PUFA-cyclopentenones may likewise react with the IKK2 component of the cytosolic IKK2-NFκB protein complex thereby inhibiting NFκB from stimulating the transcription of genes that encode various pro-inflammatory proteins. One or both of these mechanisms appears to contribute to the ability of certain highly reactive PUFA-cyclopenetenones to exhibit SPM activity. The PUFA-cyclopentenones include two prostaglandins, (PG) Δ12-PGJ2 and 15-deoxy-Δ12,14-PGJ2, and two isoprostanes, 5,6-epoxyisoprostane E2 and 5,6-epoxyisoprostane A2. Both PGJ2's are arachidonic acid-derived metabolites made by cyclooxygenases, primarily COX-2, which is induced in many cell types during inflammation. Both isoprostanes form non-enzymatically as a result the attack on the arachidonic acid bond to cellular phospholipids by reactive oxygen species; they are then release from the phospholipids to become free in attacking their target proteins. All four products have been shown to form and possess SPM activity in various in vitro studies of human and animal tissue as well as in in vivo studies of animal models of inflammation; they have been termed pro-resolving mediators of inflammation ==Gene manipulation studies==

Mice made deficient in their 12/15-lipoxygenase gene (Alox15) exhibit a prolonged inflammatory response along with various other aspects of a pathologically enhanced inflammatory response in experimental models of cornea injury, airway inflammation, and peritonitis. These mice also show an accelerated rate of progression of atherosclerosis whereas mice made to overexpress 12/15-lipoxygenase exhibit a delayed rate of atherosclerosis development. Alox15 overexpressing rabbits exhibited reduced tissue destruction and bone loss in a model of periodontitis. ==Clinical studies==

In a randomized controlled trial, AT-LXA4 and a comparatively stable analog of LXB4, 15R/S-methyl-LXB4, reduced the severity of eczema in a study of 60 infants. A synthetic analog of ReV1 is in clinical phase III testing (see Phases of clinical research) for the treatment of the inflammation-based dry eye syndrome; along with this study, other clinical trials (NCT01639846, NCT01675570, NCT00799552 and NCT02329743) using an RvE1 analogue to treat various ocular conditions are underway. RvE1, Mar1, and NPD1 are in clinical development studies for the treatment of neurodegenerative diseases and hearing loss. And, in a single study, inhaled LXA4 decreased LTC4-initiated bronchoprovocation in patients with asthma. ==See also==