Mast Cells.docx

Mast Cells ________________________________________ Mast cells are located in connective tissue, close to a blood vessel and are prevalent along the mucosa of the lung and gastrointestinal tract and the dermis of the skin. They are derived from the same hemopoietic stem cells as basophils, but they...

Mast Cells ________________________________________ Mast cells are located in connective tissue, close to a blood vessel and are prevalent along the mucosa of the lung and gastrointestinal tract and the dermis of the skin. They are derived from the same hemopoietic stem cells as basophils, but they don’t mature until they leave circulation and settle in nearby tissues, and they are considered the most important activators in the inflammatory response. Mast cells produce lipid mediators and cytokines that induce inflammation. They contain, within their cytoplasm, some pre-formed granules. You’ll notice a button on the slide that will enable you to view the process of mast cell degranulation. Feel free to click on it anytime. Mast cells, when activated, release their granular contents (histamine, serotonin, chemotactic factors, enzymes, proteoglycans, proteases, and cytokines, such as TNF-α and IL-6) into the circulation and exert their effects immediately. They also synthesize lipid mediators from cell membrane precursors, such as prostaglandins and platelet activating factor, and stimulate cytokine and chemokine (leukotrienes) synthesis by other cells such as monocytes and macrophages. We’ll look at the function of these other cells later on. Mast cells are especially important in the inflammatory response related to hypersensitivity and allergies. They bind to one of the immune globulins, IgE to trigger the release of histamine and vasoactive substances from basophils. Finally, mast cells are also responsible for the release of eosinophil chemotactic factor-A (ECF-A) which serves to attract eosinophils to the site of inflammation. Plasma Protein Systems ________________________________________ There are three interrelated plasma protein systems that normally exist in an inactive state. They include the complement system, the clotting system or coagulation cascade and the kinin system, each having its own end goal. These plasma derived mediators of inflammation are synthesized in the liver and include coagulation factors and complement proteins. Many of these protein components are enzymes that circulate in an inactive or ‘proenzyme’ state. Activation of a proenzyme by infection or injury creates a cascade type effect. Components of these plasma protein systems are usually short-live and are rapidly deactivated. Let’s look at each system separately. Complement: Components of the complement system include 20 different proteins, labelled C1 through C9, and make up about 10% of the total circulating proteins in the body. They are capable of direct destruction of pathogens or can activate and work with other components of the inflammatory response to affect the same results. As proenzymes are activated, a cascade forms that plays an important role in immunity and inflammation. Complement fragments contribute to the inflammatory response by causing vasodilation, increasing vascular permeability and enhancing activity of phagocytes. C3 and C5 activation is the most important piece of this cascade: activation of C3 and C5 results in formation of opsonins, chemotactic factors and anaphylatoxins. Recall that opsonins coat bacteria and tag them for destruction. The most potent component is C3b. Chemotactic factors draw other important inflammatory mediators to the site of injury as needed. Anaphylatoxins cause rapid degranulation of mast cells, increasing the inflammatory response. The most potent one is C3a. C5a has both chemotactic and anaphylatoxic properties. Other important components are C2b which causes vasodilation and increased permeability by smooth muscle relaxation and C4a which also acts as an anaphylatoxin inducing mast cell degranulation with release of histamine. The final endpoint of the complement cascade is formation of the membrane attack complex (MAC) which is composed of C5 through C9. Its job is to create holes in the membranes of pathogens, allowing entry of water, causing the cells to explode. As you can see from this diagram, there are three possible avenues through which the complement cascade can be activated: The Classical pathway is activated by antibodies and requires at least two Ag-Ab complexes to initiate the cascade. Thus, the immune system can take advantage of this option for destruction of bacteria. The Lectin Pathway is activated by bacterial carbohydrates and is similar to the classical pathway, but no antibodies are required. The Alternative Pathway is activated by gram negative bacterial and fungal cell wall polysaccharides (termed endotoxins); no antibodies are required; it begins with activation of C3b and merges with the classical pathway at C5. In summary, the complement cascade is activated in three ways and has 4 major effects: Opsonization Mast cell degranulation through anaphylatoxic activity Leukocyte chemotaxis Cell lysis Clotting System: The clotting system is activated by substances released during tissue destruction and infection. As you can see, there are two pathways (intrinsic and extrinsic) that converge at the place where factor X becomes Xa. Factor xa and thrombin both act to provide the link between the coagulation system and inflammation. Inflammation is enhanced by many of the by-products or fragments that are produced during the clotting cascade. For example, during fibrin production, fibrinogen releases fibrinopeptides that are chemotactic for neutrophils, cause increase vascular permeability and enhance the effect of bradykinin from the kinin cascade. Similarly, plasmin works with the complement cascade to activate C3a and C5a causing the release of histamine, enhancing the inflammatory response. And factor XIIa is also called ‘prekallekrein activator’, an enzyme that activates prekallekrein in the kinin cascade. So, you can clearly see the interdependent relationship between the three plasma protein systems. Stimulation of one cascade leads to stimulation of some of the components of the others, designed to enhance the overall inflammatory response. The primary goal of the clotting system then, is to produce a fibrous clot. The fibrinous network that forms serves to prevent the spread of infection by ‘trapping’ the offending agent and retaining it at the site of inflammatory activity, to prevent bleeding and to provide the framework for eventual healing and repair. Kinin System: The kinin system is the third plasma protein system involved in the inflammatory response and interacts closely with the clotting system. Both the clotting system and the kinin system are initiated by activated factor XII (factor XIIa). In this cascade, vasoactive peptides are generated from plasma proteins called ‘kininogens’ by the action of proteases called kallekreins. Kallekreins can be found in tissue and body fluids including saliva, tears, sweat, urine, feces, etc. The end result is production of bradykinin which causes vasodilation, increases vascular permeability, causes smooth muscle contraction, enhances leukocyte chemotaxis and stimulates pain receptors. Effects of bradykinin are very similar to histamine, although the effects are much shorter in duration as kinins are eventually degraded by kininases, to maintain homeostasis. Bradykinin appears to be of greater importance during the latter phases of inflammation. Three Plasma Protein Systems In this diagram, you should be able to clearly see the interdependent relationship between all three plasma protein systems. Note the presence of Hageman Factor as the initiator of the kinin system. Hageman factor activates the following four components of the plasma protein systems: 1.Clotting cascade through factor XI 2.Fibrinolytic system through conversion of plasminogen 3.Kinin system by prekallekrein activator 4.C1 and the complement cascade It’s important to also consider the concept of homeostasis when considering how the plasma protein systems influence the inflammatory response. Imagine how we would feel if the plasma protein system effects were stimulated without some measure of control. We would be in a constant state of inflammation leading to much discomfort! Of course, we do have mechanisms in place to control the effects of inflammation so that they are self-limiting. This is the concept of homeostasis, defined as “the steady state an organism tries to maintain by self-regulating adjustments”. In order to temper these inflammatory effects brought on by activation of the plasma protein systems, there are a great number of circulating enzymes whose sole purpose is to inactivate the components of the plasma protein systems, thus turning off the inflammatory response when it is no longer required. This is how we are able to maintain balance and control to prevent injury to healthy tissues. Cell Derived Mediators of Inflammation ________________________________________ Inflammatory mediators are responsible for the wide variety of clinical manifestations seen with the inflammatory response. In the normal state, cell derived mediators are held within intracellular granules to be secreted at a time when the body is in need of protection from invaders or injury. We are also capable of synthesizing additional mediators based upon need. The major sources of these mediators include platelets, neutrophils, monocytes and macrophages and mast cells, endothelial cells, smooth muscle, fibroblasts and most epithelial cells. Several types of mediators exist, and they can act on one or several target cells and have very different effects on different types of cells. Most of these mediators are short-lived (think homeostasis) as they are inactivated by enzymes, scavenged or degraded. Let’s look at the properties and functions of each of these mediators in turn. As you click you click on each individual mediator from the list on this slide a text box will appear with highlights of that particular mediator’s function. Histamine: Histamine is present in preformed cells and is one of the first mediators to be released during acute inflammation. Histamine is found in many places, including well perfused connective tissue, where it is most prevalent, circulating platelets and basophils, and within mast cell granules where it is released in response to trauma and immune reactions involving IgE. Histamine binds to H1 receptors on endothelial cells, causing vasodilation which helps to increase blood flow to the microcirculation. It also contributes to increased vascular permeability by causing retraction of endothelial cells in capillaries and stimulates increased adherence of leukocytes to the endothelium. Histamine also has some negative consequences as it causes smooth muscle constriction of the bronchioles, making breathing difficult during an acute inflammatory reaction. Thankfully, antihistamine (H1 receptor antagonist) medications are capable of binding to the H1 receptor to compete with histamine to antagonize the many effects of the acute inflammatory response. Serotonin: Serotonin is very similar to histamine in that it is released mainly by mast cells, basophils and platelets and causes smooth muscle contraction, small blood vessel dilation and increased vascular permeability. Lysosomal enzymes: Are small membrane-enclosed sacs that contain very powerful enzymes. These lysozomes are capable of fusing with phagocytes for the purpose of destroying foreign invaders. Arachidonic metabolites: Are fatty acids found in phospholipids of the cell membrane. They are released from mast cells and initiate complex reactions that lead to the production of other inflammatory mediators (prostaglandins and leukotrienes) Prostaglandins: Are synthesized from arachidonic acid metabolites and serve to induce inflammation and enhance the effects of histamine and other inflammatory mediators. They promote vasodilation and bronchoconstriction and increase neutrophil chemotaxis, cause pain through direct action on nerves and fever. The use of ASA and NSAIDs counteract this effect by inhibiting prostaglandin synthesis. Thromboxane A2: Thromboxane A2 is produced primarily by platelets at the site of injury and promotes platelet aggregation, bronchoconstriction and vasoconstriction. Leukotrienes: Have a similar function to histamine and are synthesized while histamine is busy at work. They increase vascular permeability, induce smooth muscle contraction and constrict pulmonary airways, thus playing a major role in mediation of asthma and anaphylaxis. In addition, they affect the adhesion properties of endothelial cells and the extravasation and chemotaxis of neutrophils, eosinophils and monocytes. Leukotrienes, because of their ‘late start’, help to prolong the inflammatory response. Platelet Activating Factor: PAF is generated from lipids stored in the cell membrane and induce platelet aggregation. It also serves to activate neutrophils and potentially acts as a chemoattractant for eosinophils. Cytokines and chemokines: Are proteins that play a role in both acute and chronic inflammation and immunity. They are either pro-inflammatory or anti-inflammatory. Again, remember the concept of homeostasis. They are produced by many cell types, including activated macrophages and lymphocytes, endothelium, epithelium, and connective tissue and modulate the inflammatory response by many other cells by travelling and binding to and affecting the function of those target cells. There are literally hundreds of examples of cytokines and chemokines. A few examples of cytokines and their roles are shown on the diagram that you can enlarge by clicking on the magnifying glass. For more details on the many specific cytokines and chemokines, please refer to the attachment for this module entitled ‘Cytokines & Chemokines’ Nitric Oxide: NO is produced by the endothelial cells and causes smooth muscle relaxation and antagonizes platelet adhesion, aggregation and degranulation. It also plays a role in regulating recruitment of leukocytes. Blocking of NO production promotes leukocyte rolling and adhesion to capillary venules; delivery of NO reduces leukocyte recruitment; therefore, production of NO reduces the cellular phase of inflammation. Impaired production of NO is implicated in the inflammatory changes associated with atherosclerosis (more on this later) and also has some antimicrobial properties. Oxygen-free radicals: Are often released by leukocytes after exposure to microbes, cytokines and immune complexes or during the phagocytic process. Release of low levels increases expression of cytokines and endothelial adhesion molecules, enhancing the inflammatory process. At high levels, they can cause significant damage to the endothelium leading to increased permeability.