For most of the twentieth century, inflammation was treated as a process that simply burned out on its own once the initiating insult was removed. That assumption collapsed in 2000 when Charles Serhan’s lab at Harvard identified a new class of lipid mediators—biosynthesized from omega-3 fatty acids—that actively terminate inflammation by signaling neutrophils to stop arriving, macrophages to clear debris, and tissues to return to homeostasis. Resolution, it turns out, is not the absence of inflammation. It is its own enzymatic program, and chronic inflammatory disease may be less a problem of excess ignition than of failed resolution.
What Are Specialized Pro-Resolving Mediators?
Specialized pro-resolving mediators (SPMs) are a family of endogenous lipid mediators enzymatically synthesized from omega-3 polyunsaturated fatty acids—primarily eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)—and, in smaller quantities, from the omega-6 fatty acid arachidonic acid (which yields lipoxins). The SPM family includes four major classes: lipoxins (from arachidonic acid), E-series resolvins (from EPA), D-series resolvins, protectins, and maresins (all from DHA).[1]
Unlike anti-inflammatory drugs that block prostaglandin or cytokine synthesis, SPMs do not suppress inflammation. They actively orchestrate its resolution—a distinction that has reshaped how immunologists and lipid biochemists conceptualize the inflammatory response. Resolution is now understood as a biosynthetically coordinated program with defined onset, agonists, and receptors, not as the passive dissipation of pro-inflammatory signals.[2]
How SPMs Are Biosynthesized
Class Switching at the Membrane: During the early phase of acute inflammation, neutrophils and tissue cells produce prostaglandins and leukotrienes from arachidonic acid via cyclooxygenase (COX) and 5-lipoxygenase (5-LOX). As the response progresses, a temporal lipid mediator class switch occurs: prostaglandins E2 and D2 induce 15-LOX expression in neutrophils, shifting the substrate preference toward EPA and DHA and initiating SPM biosynthesis.[2]
E-Series Resolvins from EPA: EPA is converted by aspirin-acetylated COX-2 or cytochrome P450 enzymes into 18R-hydroxyeicosapentaenoic acid (18R-HEPE), which is then transformed by 5-LOX in neutrophils into resolvin E1 (RvE1) and resolvin E2 (RvE2). RvE1 binds the ChemR23 receptor on monocytes and neutrophils, limiting neutrophil infiltration and promoting macrophage phagocytosis of apoptotic cells.[3]
D-Series Resolvins and Protectins from DHA: DHA is converted by 15-LOX to 17-hydroxydocosahexaenoic acid (17-HDHA), the common precursor for D-series resolvins (RvD1 through RvD6) and protectin D1 (PD1, also called neuroprotectin D1 when produced in neural tissue). RvD1 signals through the GPR32 and ALX/FPR2 receptors, while PD1 has shown particular activity in retinal and neural tissue, attenuating leukocyte infiltration and oxidative injury.[4]
Maresins from DHA via 12-LOX: Macrophages convert DHA via 12-lipoxygenase into 13S,14S-epoxy-maresin, which is hydrolyzed to maresin 1 (MaR1). Maresins are notable for promoting macrophage phenotype switching from the pro-inflammatory M1 state to the pro-resolving M2 state, and for stimulating tissue regeneration in addition to resolving inflammation.[5]
What SPMs Actually Do at Tissue Sites
Stop Signals for Neutrophils: SPMs limit further neutrophil recruitment to inflamed tissue without immunosuppression. They reduce neutrophil transmigration across endothelium and downregulate adhesion molecule expression, effectively closing the gates on continued infiltration once the initiating threat is contained.[2]
Efferocytosis Enhancement: Perhaps the defining action of SPMs is stimulation of efferocytosis—the phagocytic clearance of apoptotic neutrophils by macrophages. Apoptotic neutrophils that are not cleared undergo secondary necrosis and release damage-associated molecular patterns that perpetuate inflammation. RvD1, RvE1, PD1, and MaR1 all enhance macrophage efferocytosis in vitro and in vivo.[1]
Macrophage Reprogramming: SPMs promote a phenotypic shift in macrophages from the classically activated M1 state (TNF-α, IL-6, iNOS) toward the alternatively activated M2 state (IL-10, arginase, TGF-β), facilitating tissue repair. MaR1 in particular has been shown to drive this transition.[5]
Pain Modulation: Multiple SPMs reduce inflammatory and neuropathic pain by acting on TRPV1, TRPA1, and other nociceptor channels, as well as by reducing central sensitization in dorsal horn neurons. These effects occur at concentrations far below those required for classical analgesics.[3]
Clinical Evidence
SPM Levels in Human Disease: Plasma SPM concentrations are reduced in patients with chronic inflammatory conditions including rheumatoid arthritis, cardiovascular disease, and obesity-related metabolic dysfunction. The ratio of SPMs to pro-inflammatory eicosanoids appears to be a more informative biomarker than either measured in isolation, suggesting that failed resolution—not excess initiation—drives chronicity.[4]
Omega-3 Supplementation and SPM Production: A randomized trial published in Circulation demonstrated that high-dose EPA and DHA supplementation in patients with chronic cardiovascular disease increased circulating concentrations of 18-HEPE, 17-HDHA, RvE1, RvD1, and PD1 precursors in a dose-dependent fashion, providing the first clear human evidence that dietary omega-3 intake translates into measurable SPM biosynthesis.[6]
Cardiovascular and Metabolic Endpoints: The biosynthesis of SPMs from EPA has been proposed as one of the mechanisms underlying the cardiovascular benefit observed with high-dose icosapent ethyl in the REDUCE-IT trial, although the trial itself did not measure SPMs directly. Preclinical work has consistently shown that SPM administration reduces atherosclerotic lesion size and stabilizes plaques in murine models.[1]
Resolution in Surgical and Periodontal Models: RvE1 has been studied in periodontal disease, where topical application reduced bone loss and tissue inflammation in animal models. Several clinical observations have linked higher SPM precursor levels with faster postoperative resolution of inflammation in cardiac surgery patients.[3]
Safety and Practical Considerations
Endogenous SPMs are produced at nanomolar concentrations and act on G-protein coupled receptors—they are not consumed in stoichiometric reactions with reactive species, as antioxidants are. This means very small quantities exert significant biological effects, and the body tightly regulates their production and degradation through specific dehydrogenases.
For most clinical contexts, the practical route to enhancing SPM activity is providing adequate substrate (EPA and DHA), ensuring sufficient cofactors for the lipoxygenase enzymes, and minimizing competition from excess omega-6 substrate. Aspirin acetylates COX-2 in a way that redirects its activity toward producing 18R-HEPE and 17R-HDHA—the precursors of aspirin-triggered resolvins, which are more stable than their native counterparts. This represents one of several mechanisms thought to contribute to aspirin’s resolution-promoting effects beyond simple prostaglandin inhibition.[2]
SPM precursors (17-HDHA, 18-HEPE) and partially purified SPM mixtures are available as research-grade supplements but are not pharmaceutical products. Trials of standardized SPM mixtures in inflammatory conditions are ongoing.
SPMs vs Classical Anti-Inflammatory Approaches
NSAIDs: Nonsteroidal anti-inflammatory drugs inhibit COX-1 and COX-2, blocking prostaglandin synthesis. However, prostaglandins E2 and D2 are required to initiate the lipid mediator class switch toward SPM production. Chronic NSAID use may therefore impair endogenous resolution while suppressing the inflammatory signal, which may help explain why long-term NSAID therapy is not always associated with improved tissue outcomes.[2]
Corticosteroids: Glucocorticoids broadly suppress inflammatory transcription but also affect resolution pathways and tissue repair. Their utility in acute flare control is established, but they do not constitute resolution programs.
Omega-3 Supplementation: Providing EPA and DHA substrate is the most direct way to support endogenous SPM biosynthesis, and human data confirm dose-dependent increases in SPM precursors with supplementation.[6] This is mechanistically distinct from NSAID-based suppression: rather than blocking initiation, it supports the active resolution arm of the inflammatory response.
The conceptual shift is significant. Treating chronic inflammation as a problem of incomplete resolution—rather than excess initiation—opens therapeutic strategies that work with rather than against the body’s homeostatic machinery. Resolvins, protectins, and maresins are the molecular evidence that inflammation is meant to end, and that the body has a dedicated biochemical program to ensure it does.
References
- Serhan CN. “Pro-resolving lipid mediators are leads for resolution physiology.” Nature. 2014;510(7503):92-101.
- Serhan CN, Chiang N, Van Dyke TE. “Resolving inflammation: dual anti-inflammatory and pro-resolution lipid mediators.” Nature Reviews Immunology. 2008;8(5):349-361.
- Arita M, et al. “Stereochemical assignment, antiinflammatory properties, and receptor for the omega-3 lipid mediator resolvin E1.” Journal of Experimental Medicine. 2005;201(5):713-722.
- Serhan CN, Levy BD. “Resolvins in inflammation: emergence of the pro-resolving superfamily of mediators.” Journal of Clinical Investigation. 2018;128(7):2657-2669.
- Serhan CN, et al. “Maresins: novel macrophage mediators with potent antiinflammatory and proresolving actions.” Journal of Experimental Medicine. 2009;206(1):15-23.
- Mas E, Croft KD, Zahra P, Barden A, Mori TA. “Resolvins D1, D2, and other mediators of self-limited resolution of inflammation in human blood following n-3 fatty acid supplementation.” Clinical Chemistry. 2012;58(10):1476-1484.
