The mediators of inflammation are the substances that initiate and regulate inflammatory reactions. The most important inflammation mediators include vasoactive amines, lipid products (prostaglandins and leukotrienes), cytokines (including chemokines), and products of complement activation. These mediators induce various components of the inflammatory response typically by distinct mechanisms, which is why inhibiting each has been therapeutically beneficial. However, there is also some overlap (redundancy) in the actions of the mediators.
The inflammation mediators have some common characteristics, like
- Mediators are either secreted by cells or generated from plasma proteins. Cell-derived mediators are normally sequestered in intracellular granules and can be rapidly secreted by granule exocytosis (e.g., histamine in mast cell granules) or are synthesised de novo (e.g., prostaglandins and leukotrienes, cytokines) in response to a stimulus. The major cell types that produce mediators of acute inflammation are the sentinels that detect invaders and damage in tissues, that is, macrophages, dendritic cells, and mast cells, but platelets, neutrophils, endothelial cells, and most epithelia can also be induced to elaborate some of the mediators. Plasma derived mediators (e.g., complement proteins) are produced mainly in the liver and are present in the circulation as inactive precursors that must be activated. When activated a series of proteolytic and protein-protein interactions are initiated that ultimately to acquire their biologic properties.
- Ative mediators are produced only in response to various stimuli. These stimuli include microbial products and substances released from necrotic cells. Some of the stimuli trigger well-defined receptors and signalling pathways.
- Most of the mediators are short-lived. They quickly decay, or are inactivated by enzymes, or they are otherwise scavenged or inhibited. There is thus a system of checks and balances that regulates mediator actions.
- One mediator can stimulate the release of other mediators. The secondary mediators may have the same actions as the initial mediators but may also have different and even opposing activities. Such cascades provide mechanisms for amplifying or, in certain instances, counteracting the initial action off a mediator.
Vasoactive Amines: Histamine and Serotonin
The two major vasoactive amines, so named because they have important actions on blood vessels, are histamine and serotonin. They are stored as preformed molecules in cells and are therefore among the first mediators to be released during inflammation. The richest sources off histamine are the mast cells that are normally present in the connective tissue adjacent to blood vessels. It is also found in blood basophils and platelets. Histamine is stored in mast cell granules and is released by mast cell degranulation in response to a variety of stimuli, including 1.physical injury, such as trauma, cold, or heat, by unknown mechanisms;2.binding of antibodies to mast cells, which underlies immediate hypersensitivity (allergic) reactions; and 3.products of complement called anaphylatoxins (C3a and C5a). Antibodies and complement products bind to specific receptors on mast cells and trigger signalling pathways that induce rapid degranulation. In addition, leukocytes are thought to secrete some histamine-releasing proteins but these have not been characterised. Neuropeptides (e.g., substance P) and cytokines (IL-1, IL-8) may also trigger release of histamine.
Histamine causes dilation of arterioles and increases the permeability of venules. Histamine is considered to be the principle mediator of the immediate transient phase of increased vascular permeability, producing interendothelial gaps in venules. Its vasoactive effects are mediated mainly via binding to receptors, called H1 receptors, on microvascular endothelial cells. Histamine also causes contraction of some smooth muscles.
Serotonin is a preformed vasoactive mediator present in platelets and certain neuroendocrine cells, such as in the gastrointestinal tract, and in mast cells in rodents but not humans. Its primary function is as a neurotransmitter in the gastrointestinal tract. It is also a vasoconstrictor, but the importance of this action in inflammation is unclear.
Arachidonic Acid Metabolites
The lipid mediators prostaglandins and leukotrienes are produced from arachidonic acid (AA) present in membrane phospholipids, and stimulate vascular and cellular reactions in acute inflammation. AA does not occur free in the cell but is normally esterified in membrane phospholipids. Mechanical, chemical, and physical stimuli or other mediators (e.g., C5a) release AA from membrane phospholipids through the action of cellular phospholipases, mainly phospholipase A2. The biochemical signals involved in the activation of phospholipase A2 include an increase in cytoplasmic Ca2+ and activation of various kinases in response to external stimuli. AA-derived mediators, also called eicosanoids are synthesised by two major classes of enzymes: cyclooxygenases (for prostaglandins) and lipoxygenases (for leukotrienes). Eicosanoids bind to G protein-coupled receptors on many cell types and can mediate virtually every step of inflammation, including vasodilation (PGI2, PGE1, PGE2 PGD2), vasoconstriction (TxA2/Thromboxane A2, leukotrienes C4/D4/E4), increased vascular permeability (Leukotrienes C4/D4/E4), Chemotaxis, leukocyte adhesion (Leukotrienes B4/HETE or Hydroxyeicosatetraenoic acid).
Prostaglandins (PGs) are produced by mast cells, macrophages, endothelial cells, and many other cell types, and are involved in the vascular and systemic reactions of inflammation. They are generated by the actions of two cyclooxgenases, called COX-1 and COX-2. COX-1 is produced in response to inflammatory stimuli and is also constitutively expressed in most tissues, where it may serve a homeostatic function (e.g., fluid and electrolyte balance in the kidneys, cytoprotection in the gastrointestinal tract). In contrast, COX-2 is induced by inflammatory stimuli and thus generates the prostaglandins that are involved in inflammatory reactions, but it is low or absent in most normal tissues. Prostaglandins include many subtype PGs, such as TxA2, PGI2, PGD2, PGE2, PGF2a etc. These subtype prostaglandins are derived by the action of different enzymes on an intermediate in the pathways, respectively.
TxA2, a potent platelet-aggregating agent and vasoconstrictor is derived by the enzyme thromboxane synthase which locates in the platelets. Prostacyclin synthase in vascular endothelium catalyze the production of PGI2 and PGI2 has functions as vasodilator, a potent inhibitor of platelet aggregation, and markedly potentiates the permeability-increasing and chemotactic effects of other mediators. PS: a thromboxane-prostacyclin imbalance has been implicated as an early event in thrombus formation in coronary and cerebral blood vessels. PGD2 is the major prostaglandin made by mast cells; along with PGE2 (which is more widely distributed), it causes vasodilation and increases the permeability of post capillary venules, thus potentiating edema formation. Also it has a function of chemoattractant for neutrophils. PGF2a stimulates the contraction of uterine and bronchial smooth muscle and small arterioles.
In addition to their local effects, the prostaglandins are involved in the pathogenesis of pain and fever in inflammation. PGE2 is hyperalgesic and makes the skin hypersensitive painful stimuli, such as intradermal injection of suboptimal concentrations of histamine and bradykinin. It is also involved in cytokine-induced fever during infections.
Leukotrienes are produced by leukocytes and mast cells by the action of lipoxygenase and are involved in vascular and smooth muscle reactions and leukocyte recruitment. There are three different lipoxygenases, 5-lipoxygenase being the predominant one in neutrophils. This enzyme converts AA (arachidonic acid) to 5-hydroxyeicosatetraenoic acid, which is chemotactic for neutrophils, and is the precursor of the leukotrienes. Among leukotrienes, LTB4 is a potent chemotactic agent and activator of neutrophils, causing aggregation and adhesion of the cells to ventral endothelium, generation of ROS (reactive oxygen species), and release of lysosomal enzymes. The LTC4, LTD4, and LTE4 cause intense vasoconstriction, bronchospasm (important in asthma), and increased permeability of venules. Leukotrienes are more potent than is histamine in incresing vascular permeability and causing bronchospasm.
Lipoxins are also generated from AA by the lipoxygenase pathway, but unlike prostaglandins and leukotrienes, the lipoxins suppress inflammation by inhibiting the recruitment of leukocytes. They inhibit neutrophil chemotaxis and adhesion to endothelium. They are also unusual in that two cell populations are required for the transcellular biosynthesis of these mediators. Leukocytes, particularly neutrophils, produce intermediates in lipoxin synthesis, and these are converted to lipoxins by platelets interacting with the leukocytes.
Cytokines and Chemokines
Cytokines are proteins produced by many cell types (principally activated lymphocytes, macrophages, and dendritic cells, but also endothelial, epithelial, and connective tissue cells) that mediate and regulate immune and inflammatory reactions. They include TNF (tutor necrosis factor) and Interleukin-I (IL-1). These cytokines are produced mainly by activated macrophages and dendritic cells; TNF is also produced by T lymphocytes and mast cells, and IL-1 is produced by some epithelial cells as well. The most important roles of these cytokines in inflammation are the following:
1.Endothelial activation. Both TNF and IL-1 act on endothelium to induce a spectrum of changes referred to as endothelial activation. These changes include increased expression of endothelial adhesion molecules, mostly E- and P-selectins and ligands for leukocyte integrins; increased production of various mediators, including other cytokines and cheekiness, growth factors, and eicosanoids; and increased procoagulant activity of the endothelium.
2.Activation of leukocytes and other cells. TNF augments responses of neutrophils to other stimuli such as bacterial endotoxin and stimulates the microbicidal activity of macrophages, in part by inducing production of NO. IL-1 activates fibroblasts to synthesize collagen and stimulates proliferation of synovial and other mesenchymal cells. IL-1 also stimulates TH17 responses, which in turn induce acute inflammation.
3.Systemic acute-phase response. IL-1 and TNF induce the systemic acute-phase responses associated with infection or injury, including fever. They are also implicated in the syndrome of sepsis, resulting from disseminated bacterial infection. TNF regulates energy balance by promoting lipid and protein mobilisation and by suppressing appetite. Therefore, sustained production of TNF contributes to cachexia, a pathologic state characterised by weight loss and anorexia that accompanies some chronic infections and neoplastic disease.
Cheekiness are a family of small (8 to 10 kD) proteins that act primarily as chemoattractants for specific types of leukocytes. Inflammatory chemokines stimulate leukocyte attachment to endothelium by acting on leukocytes to increase the affinity of integrins, and they stimulate migration (chemotaxis) of leukocytes in tissue to the site of infection or tissue damage. Also, some chemokines are produced constitutively in tissues and are sometimes called homeostatic chemokines. These organize various cell types in different anatomic regions of the tissues.
The complement system is a collection of soluble proteins and membrane receptors that function mainly in host defines against microbes and in pathologic inflammatory reactions. This system of complement functions in both innate and adaptive immunity for defines against microbial pathogens. In the process of complement activation, several cleavage products of complement proteins are elaborated that cause increased vascular permeability, chemotaxis, and opsonization.
Complement system acts as the bridge between innate and adaptive immune system. This concept is due to the fact that complement proteins can be activated directly by antigen-antibody complexes.
There are three main effects of complement: 1.lysis of cells such as bacteria, allografts, and tumor cells; 2.generation of mediators that participate in inflammation and attract neutrophils; and 3.opsonization – enhancement of phagocytosis.
C3b is the central molecule of the complement cascade. It has two core functions: 1.it combines with other complement components to generate C5 convertase, the enzyme that leads to the production of the membrane attack complex (first it adhere to the surface of the targets); and 2.it opsonises bacteria because phagocytes have receptors for C3b on their surface.
How to activate?
In the classic pathway, antigen-antibody complexes activate C12 to form a protease and thereafter the complement cascade starts. In the lectin pathway, MBL (mannas-binding lectin/mannose-binding protein) binds to the surface of microbes bearing mannan. This activates proteases associated with MBL that activates complement cascade. In the alternative pathway, many unrelated cll surface substances can initiate the process by binding C3 and factor B. This complex is cleaved by a protease and finally the complement cascade initiates.
- Platelet-Activating Factor (PAF)
PAF is a phospholipid-derived mediator that was discovered as a factor that caused platelet aggregation, but it is now known to have multiple inflammatory effects. A variety of cell types, including platelets themselves, basophils, mast cells, neutrophils, macrophages, and endothelial cells, can elaborate PAF, in both recreated and cell-bound forms. In addition to platelet aggregation, PAF causes vasoconstriction and bronchoconstriction, and at low concentrations it induces vasodilation and increased ventral permeability.
- Products of Coagulation
Protease-activated receptors (PARs) are activated by thrombin (converting fibrinogen to fibrin), and are expressed on platelets and leukocytes.
Kinins are vasoactive peptides derived from plasma proteins called kininogens, by the action of specific proteases called kallikreins. The enzyme kallikrein cleaves a plasma glycoprotein precursor, high-molecular-weight kininogen, to produce bradykinin, a substance that increases vascualar permeability and causes contraction of smooth muscle, dilation of blood vessels, and pain when injected into the skin. These effects are similar to those of histamine. The action of bradykinin is short-lived, because it is quickly inactivated by an enzyme called kininase.
Neuropeptides are secreted by sensory nerves and various leukocytes, and may play a role in the initiation and regulation of inflammatory responses. These small peptides, such as substance P and neurokinin A, are produced in the central and peripheral nervous systems. Substance P has many biologic functions, including the transmission of pain signals, regulation of blood pressure, stimulation of hormone secretion by endocrine cells, and increasing vascular permeability.