Inflammatory Mediators Explained: The Molecules That Start, Steer and Stop Inflammation

Inflammatory mediators are the chemical signals that turn an injury or infection into inflammation and, when things go right, switch it off again. Histamine from mast cells, bradykinin cut from a plasma protein, prostaglandins made from membrane lipids, cytokines such as interleukin-6: each has a source, a receptor and a job, and together they are the vocabulary inflammation research is written in.

This guide covers the mediators of inflammation class by class, the order in which they act, the cardinal signs they cause, acute versus chronic inflammation, resolution and measurement, then the research peptides studied as modulators, with the evidence level stated for each.

What inflammatory mediators are

An inflammatory mediator is any molecule that carries the inflammatory message from where damage or infection is detected to the vessels, white blood cells and nerves that respond. Some are stored ready-made, like histamine in mast-cell granules. Some are cut from inactive plasma proteins, like bradykinin and the complement fragments C3a and C5a. Others are synthesized only after a cell is activated, like prostaglandins and cytokines. Signalling inside the responding cell, including the NF-κB pathway, is covered in our NF-κB guide.

The main classes of mediators

Vasoactive amines

Mast cells store histamine in prepackaged granules and release it within seconds of activation, for instance when antigen cross-links IgE on their FcεRI receptors. Through the H1 receptor, histamine contracts smooth muscle and increases capillary permeability through contraction of terminal venules. Most peripheral serotonin is made in the gut and stored in platelets; unlike rodent mast cells, human mast cells normally contain little or none, a difference worth remembering when reading mouse studies. Experimental work links serotonin to early edema and neutrophil recruitment.

Lipid mediators

Phospholipases release arachidonic acid from membranes, and cyclooxygenases turn it into prostaglandins. COX-1 is constitutive in most cells and COX-2 is induced by inflammatory stimuli. Prostaglandins take part in redness, swelling and pain: prostacyclin (PGI2) is a potent vasodilator, and prostaglandin E2 acting on sensory neurons produces pain. NSAIDs inhibit both COX enzymes at the active site, which is how they cut prostaglandin output. Lipoxygenase products, the leukotrienes, split two ways: leukotriene B4 promotes leukocyte adhesion, chemotaxis and superoxide generation, while leukotrienes C4, D4 and E4 constrict airways and increase vascular permeability. Platelet-activating factor (PAF), an ether phospholipid released by activated mast cells, completes the group. A 2008 study led from St. Michael’s Hospital in Toronto found serum PAF rising with the severity of anaphylaxis, and activity of PAF acetylhydrolase, the enzyme that inactivates it, inversely related to severity.

Plasma cascades

Complement activation cleaves C3 and C5 into the anaphylatoxins C3a and C5a, peptides of 77 and 74 residues. C5a increases vascular permeability, triggers histamine release from mast cells and basophils and directs neutrophils to the site; C3a also activates mast cells and basophils. The kinin bradykinin is a nine-residue peptide (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) that plasma kallikrein releases from high-molecular-weight kininogen. In contact activation, coagulation factor XII activates on negatively charged surfaces and converts prekallikrein to kallikrein, with C1-inhibitor as the main brake. Bradykinin dilates vessels, makes small vessels leaky and mediates pain, acting mainly at the B2 receptor, which is widespread in normal tissue. Most people with hereditary angioedema carry mutations in SERPING1, the C1-inhibitor gene, and bradykinin is a key suspect in their swelling attacks; ACE inhibitors, which slow bradykinin breakdown, are associated with angioedema in a small fraction of users.

Cytokines and chemokines

IL-1, IL-6 and TNF, released by macrophages and dendritic cells, are the classic pyrogenic cytokines. In the brain they raise COX-2 synthesis, and prostaglandin E2 acting on EP3 receptors in the median preoptic nucleus produces fever of roughly 1 to 4 °C. In mice, fever after bacterial lipopolysaccharide fails to develop without IL-6. IL-6 also drives the liver’s acute-phase response, including C-reactive protein, and tocilizumab, an antibody against its receptor, is approved for rheumatoid arthritis. The chemokine CXCL8 (interleukin-8) recruits and activates neutrophils through CXCR1 and CXCR2.

Reactive species

Endothelial nitric oxide relaxes vascular smooth muscle through cGMP. In inflammation the inducible enzyme iNOS (NOS2) matters more: interferon-gamma acting with lipopolysaccharide, TNF or IL-1β switches it on, and macrophage nitric oxide is bactericidal and tumoricidal. Phagocytes also make superoxide through the NOX2 NADPH oxidase; inherited NOX2 defects cause X-linked chronic granulomatous disease, with recurrent bacterial and fungal infections. See our guide to Nrf2 and oxidative stress.

Neuropeptides

Stimulated sensory nerve endings release substance P, proposed in 1979 as the neurogenic mediator of vasodilation and plasma extravasation, and CGRP, a 37-residue neuropeptide and highly potent vasodilator acting through the CALCRL–RAMP1 receptor. CGRP antagonists have been developed for migraine. This nerve-driven component is called neurogenic inflammation; see our neuropeptides guide.

Mediators act in waves

Stored and plasma-derived signals need no new synthesis, so they act first; lipid mediators and cytokines must be made, and they appear as neutrophils arrive. In a 2001 study of inflammatory exudates, leukotrienes and prostaglandins appeared early with neutrophil recruitment, and lipoxin biosynthesis followed, concurrent with spontaneous resolution.

The five cardinal signs and their mediators

The five cardinal signs, still known by their Latin names, are the visible ends of vascular and nervous events that trace to particular mediators. Not every sign can be seen when the inflamed tissue is an internal organ.

Acute vs chronic inflammation

Acute inflammation begins immediately after injury and typically lasts a few days. Inflammation persisting beyond about six weeks is chronic and can last months or years when the acute phase fails to resolve. A 2019 review linked a low-grade, non-infective systemic form to cardiovascular disease, cancer, diabetes, chronic kidney disease and non-alcoholic fatty liver disease, noting that no standard biomarkers for it yet exist.

Resolution: switching inflammation off

Resolution is not simply a fade-out. A healthy response is self-limited, and its resolution phase is now understood as biochemically active; one influential model holds that it is driven by specialized pro-resolving mediators (SPMs): lipoxins, made from arachidonic acid, and resolvins, protectins and maresins, made from the omega-3 fatty acids EPA and DHA. Reported actions include promoting resolution, helping clear microbes and reducing pain. The switch shows up in exudates, and in cell experiments human neutrophils exposed to prostaglandin E2 shifted their output from leukotriene B4 to lipoxin A4, so the prostaglandins that help drive the early phase also help set up its end.

How mediators are measured in research

Cytokines and chemokines are measured by immunoassay, one at a time by enzyme-linked immunosorbent assay (ELISA) or many at once on multiplex bead platforms that can read up to about 100 mediators from a small sample. Lipid mediators are mostly measured by LC–MS/MS, which separates isomers better than immunoassays; see our LC-MS explainer. The bigger problems arise before the assay:

• Short lives. Bradykinin survives only a few seconds in circulation, which makes it hard to detect.
• Made in the tube. Clotting blood for serum causes massive ex vivo formation of thromboxane B2 and 12-HETE, and prostaglandin E2 can form in whole blood left at room temperature, so lipid samples should be processed fast and kept at −80 °C.
• Drift. In one evaluation, only 2 of 15 cytokines stayed stable through several freeze–thaw cycles, and its authors note that processing delays change profiles and that commercial kits disagree widely, so values are most meaningful within one platform.
• Contamination. Lipopolysaccharide (endotoxin) stimulates pyrogenic cytokine synthesis through TLR4, so a contaminated reagent can manufacture the readout; see endotoxin in research peptides.

Where research peptides come in

Some mediators are peptides themselves, from bradykinin to the anaphylatoxins and CGRP. Several research peptides are also studied as modulators of these signals, at very different evidence levels.

LL-37

LL-37, the 37-residue C-terminal peptide of hCAP18 and the only known human cathelicidin, is made by neutrophils and epithelial cells. Besides killing microbes, it chemoattracts human neutrophils, monocytes and T cells through formyl peptide receptor-like 1 in cell experiments; that receptor, now FPR2/ALX, is also the lipoxin A4 receptor and binds annexin A1. LL-37 can amplify inflammation too: in human cells and psoriatic skin samples it turned self-DNA into a potent trigger of interferon production by plasmacytoid dendritic cells. See our LL-37 guide and antimicrobial peptides explained.

VIP

VIP, a 28-residue neuropeptide whose receptors are described as mainly neuroprotective and immunomodulatory, significantly reduced the incidence and severity of collagen-induced arthritis in mice. That is animal evidence; see our VIP overview.

KPV

KPV (Lys-Pro-Val) is residues 11 to 13 of alpha-MSH. In mouse colitis models, mice given KPV recovered sooner with fewer inflammatory infiltrates, partly independently of the melanocortin-1 receptor. This is mouse work; see our KPV guide.

ARA-290

ARA-290 (cibinetide) is described in its trials as an innate repair receptor agonist. In a 28-day phase 2b trial in 64 people with sarcoidosis-associated small nerve fibre loss, the primary endpoint, change in corneal nerve fibre area, was significant only in the middle of three active arms, and pain fell in all groups: an early signal on a surrogate measure. See our ARA-290 guide.

Thymosin alpha-1

Thymosin alpha-1, a 28-residue N-acetylated peptide from prothymosin alpha, is studied as an immunomodulator. In the phase 3 TESTS trial of 1,106 adults with sepsis, 28-day mortality was 23.4% with it and 24.1% with placebo (hazard ratio 0.99), and the authors found no clear evidence that it lowered mortality. See our thymosin alpha-1 guide.

What’s established and what isn’t

Sourcing inflammation-research peptides in Canada

Inflammation assays are sensitive to what is in the vial, because their readouts are exactly what contaminants trigger. When buying research peptides in Canada for this work:

• Ask for a batch-matched certificate showing identity by mass spectrometry and purity by HPLC, and check the batch number against the vial; see our COA guide and HPLC testing guide.
• Ask about endotoxin before using a peptide on immune cells.
• Plan controls, including a vehicle and, where possible, a scrambled-sequence peptide.
• Store lyophilized material cold, dry and dark; see the peptide storage guide.

LL-37Human cathelicidin; chemoattractant through FPR2View product VIP28-residue neuropeptide; mouse arthritis studiesView product ARA-290Innate repair receptor agonist used in inflammation researchView product Thymosin Alpha-128-residue peptide studied as an immunomodulatorView product

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