TA Reviews

Inflammation

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M2

Pathology

TA Reviews

Inflammation and Repair Lecture Handout

Jill Conway, 8/22/00

Acute inflammation = short duration, few minutes to few days, neutrophils predominate, usually occurs with protein exudate.

Chronic inflammation = duration of days to years, mainly lymphocytic and macrophage infiltrate, fewer neutrophils

Acute Inflammation processes:

  1. vasodilation (leads to calor, rubor)
  2. increased permeability of vessels (leads to tumor)
  3. emigration of leukocytes

Dolor and functio laesa are also features of acute inflammation.

Vasodilation occurs early and is considered the first response leading to redness and heat in the area (note this sometimes follows brief vasoconstriction). Alteration of normal hydrostatic pressure leads to a transudate fluid that begins edema formation. Permeability of the vessels gets triggered by many mediators, especially histamine and leukotrienes. Permeability is achieved via contraction of endothelial cells, interruption of junctions between endothelial cells, direct injury, and injury to endothelium mediated by leukocytes and transcytosis across the endothelial cell. A proteinaceous exudate forms in the interstitium leading to further edema.

  1. Endothelial cell contraction occurs only in the venules and occurs within minutes and lasts for 15-30 minutes.
  2. Junctional retraction occurs in response to cytokines and begins 4-6 hours after injury, lasting mores than 24 hours.
  3. Direct injury to EC cells can happen via infections, burns or from leukocyte mediated injury. This response begins immediately (except for leukocyte mediated form) and can persist for days.
  4. Increased transcytosis involves increased size of channels across the EC cytosol that allows for increased transport, especially in response to VEGF.
EXTRAVASATION:

Following the increase in permeability,

  1. Blood flow in vessels slows (stasis) and leukocytes marginate to the vessel walls. Leukocytes normally travel towards the vessel wall with RBCs in a central column in normal laminar blood flow.
  2. Leukocytes can cover an area of the vessel wall, called pavementing.
  3. Along the vessel wall, the leukocytes are grabbed by sticky receptors of selectins including E-selectin on endothelial cells (previously called ELAM-1) and P-selectin on platelets and endothelium. These receptors become more abundant through redistribution mediated by histamine or thrombin (previously in Weibel-Palade bodies in ECs) and/or via upregulation within minutes of local injury; E-selectin is induced by IL-1 and TNF. The selectins bind only weakly, and the leukocytes roll along the vessel edge being bound and released by subsequent receptors.
  4. Integrins (LFA-1) on the leukocyte surface eventually change conformation so that they bind more tightly to the endothelium receptors such as ICAM-1(intercellular adhesion molecule 1, VCAM = vascular), a process called adhesion. Integrins are normally expressed but only function following activation by chemotactic agents.
  5. Leukocytes then migrate through the endothelial cell layer via intercelleular junctions, secrete collagenases to degrade the BM and arrive at the site of injury via chemotaxis. This process, leukocyte diapedesis, occurs mainly in the venules. Neutrophils are always the first line of defense and can be found with 6-24 hours at the injury site. Within 24-48 hours monocyte/macrophage cells predominate at the injury.

Basic process: endothelial activation (mediators increase expression of selectins), rolling, adhesion, transmigration.

After transmigration, leukocytes will migrate towards the injury via chemotaxis (movement along a chemical gradient). Chemotactic agents for leukocytes include bacterial peptides, C5a, LTB4, IL-8. These mediators also help to activate leukocytes which causes them to produce AA metabolites, secrete lysosomal enzymes, and regulate expression of surface receptors. Leukocytes move by extending a pseudopod and moving towards that via filament contraction with actin and myosin.

Phagocytosis: basically recognize, attach, engulf, and kill. Recognition occurs with opsonin coating with Fc portion of IgG and C3b fragment of complement.

Binding triggers engulfment. Killing works through activated oxygen in the oxidative burst: 2 O2 + NADPH --> 2O2 activated + NADP + H+. They also use myeloperoxidase and halide to form bleach with HOCl radical, which is the most effective killer. Even without oxidative burst, certain enzymes can kill such as lysozyme, defensins (poke holes in membranes). Following phagocytosis, neutrophils die through apoptosis and are taken up by macrophages or lymphatics.

Note that leukocyte enzymes may be released into the ECM during inflammation, leading to local tissue damage. Chemical mediators may be released which further amplify the inflammatory response.

Some diseases that involve defective leukocyte function: Chediak-Higashi syndrome involves abnormal phagocytosis due to poor organelle release of enzymes, and chronic granulomatous disease from decreased ability to generate an oxidative burst due to low levels of NADHP oxidase.

MEDIATORS OF INFLAMMATION:

General principles: mediators come from plasma in precursor forms that are activated. Cell bound mediators are in granules and released or newly synthesized, usually from mast cells, platelets, neutrophils, or monocytes. Most are short lived and decay, often within seconds.

Histamine: preformed in mast cells in tissues, circulating basophils and platelets. It is released from any trauma, complement C3a and C5a, cytokines IL-1 and IL-8. Causes arteriolar dilation and increased vascular permeability of venules via H1 receptors.

Serotonin: Preformed, similar to histamine in effects, comes from platelet dense body granules.

Prostaglandins: Synthesized from arachidonic acid from the cyclooxygenase pathway in leukocytes, platelets, and endothelial cells in response to induction from mediators. AA is membrane bound component of phospholipids, released via activation of phospholipases or from C5a. Cyclooxygenase produces PGs and TXAs from AA. Lipo-oxygenases produce LTs from AA. PGI2 is a potent inhibitor of platelet aggregation and a vasodilator. TXA2 works contrary to PGI2 and promotes platelet aggregation and vasoconstriction. PGD2, PGE2, and PGF2 are vasodilators. PGs contribute to pain and fever in inflammation.

NOTE: platelets mostly produce TXA2 via thromboxane synthetase. ECs mostly produce PGI2 via prostacyclin synthetase.

Leukotrienes: Synthesized from arachidonic acid in leukocytes in the lipooxygenase pathway, especially in neutrophils. These play many roles, but particularly important is LTB4 for chemotaxis of neutrophils. LTC4, LTD4, and LTE4 cause vasoconstriciton, bronchospasm and increase permeability.

Lipoxins are a new discovery of AA produced products formed by platelet interaction with leukocytes.

Nitric Oxide: Synthesized by macrophages and ECs and very short-lived with many potential effects, especially vasodilation. NO can be toxic to infectious organisms and also reduces platelet aggregation and adhesion. Increased production in response to IL-1 and TNF and IFN. General effect is to reduce leukocyte migration to site of inflammation and thus serves to minimize the inflammatory response.

Platelet Activating Factor: produced by most inflammatory cell types. Causes vasoconstriction and bronchoconstriction, but at very low doses instead vasodilates and increases permeability effectively (better than histamine). Tobacco smoke may induce PAF and thus cause inflammation.

Cytokines: these are chemical mediators mainly produced by activated lymphocytes and macrophages which act locally (paracrine, autocrine) or systemically (endocrine). Some of the most important ones include:

IL-1 produced by activated macrophages in response to injury can activate ECs so that they produce different receptors on their surface. It (and IL-6) is associated with fever systemically and signs of malaise.

IFN, produced by T cells, works to cause systemic symptoms of fever and malaise. IFN-gamma activates macrophages.

TNF (tumor necrosis factor) is produced by activated macrophages (alpha) and T cells (beta). It also activates ECs and leads to fever, neutrophil aggregation and NO synthesis. It induces proliferation of fibroblasts. TNF-alpha regulates body mass and when it increases, cachexia can result.

TGF-beta (transforming growth factor) is produced by macrophages, T cells, endothelium and generally works as a growth inhibitor of epithelial cells. It stimulates collagen production and seems to be important in chronic inflammation and fibrosis.

Bradykinin causes pain, vasodilation and increases vascular permeability. Kallikrein, in the Hageman factor cascade that produces bradykinin, activates Hageman factor itself, setting up potential autocatalysis.

Hageman factor (Factor XII, active form Factor XIIa): produced by liver and found in plasma. Crucial initiator of kinin cascade, clotting cascade (producing thrombin that splits fibrinogen to fibrin), fibrinolytic system (producing plasmin that degrades fibrin) and complement cascade.

Complement: Present in plasma, activated via the Hageman factor cascade, the classic pathway of presentation of antigen-antibody complexes, or alternative pathway of microbial polysaccharides, or collectin binding to bacterial carbohydrates can activate via C1 and C3. Plasma proteins undergo cleavage to active products including C3a and C5a which cause degranulation of mast cells and anaphylaxis. C5a also plays a role in chemotaxis of leukocytes and activation of AA pathway. C3b opsonizes bacteria for phagocytosis. C5-C9 forms the membrane attack complex.

SUMMARY OF MEDIATORS BY FUNCTION:

Increase vascular permeability: histamine, C3a, C5a, PAF, bradykinin, LTC, LTD, LTE

Chemotaxis: C5a, LTB4, chemokines

Vasodilation: NO, PGI2

Systemic signs: TNF, IL-1, IL-6

Pain: bradykinin, prostaglandins

Tissue Destruction: leukocyte lysosomal enzymes, NO, reactive O2

Outcomes of Acute Inflammation:

Resolution = return to normal architecture and removal of dead cellular debris

Organization = scar formation with loss of original architecture from more significant injury. Fibrosis denotes connective tissue replacement of functional tissue and occurs with serious protein exudates, lots of fibrin exudation from plasma, areas where exudate cannot be adequately absorbed.

Abscesses may form in some bacterial infections.

Acute inflammation can continue and progress to chronic inflammation.

Chronic Inflammation: occurs where tissue destruction is too massive to be resolved, if acute inflammation continues with ongoing injury, some viral infections.

Consists of infiltration of mononuclear cells, including macrophages, plasma cells, and lymphocytes along with tissue destruction, repair with new vessels and fibrosis. Classic examples include atherosclerosis, TB, and autoimmune conditions like rheumatoid arthritis.

Major Cell Types in Chronic Inflammation:

Macrophages become activated by cytokines (especially IFN-gamma) and release proteases that cause tissue destruction. They play a role in secreting some factors in complement, NO, AA metabolites, and cytokines, especially IL-1 and TNF. Many of these recruit more monocytes or stimulate macrophage division in the tissues. They may also induce local tissue destruction in addition to attacking the source of inflammation. Half life in tissues is several months. Giant cell formation is induced by IL-4 and IFN-gamma.

Lymphocytes migrate to area via chemokines. T lymphocytes mutually stimulate macrophages and may set up an ongoing inflammation. Macrophages present antigen to T cells to stimulate them and release IL-1 and TNF that recruit lymphocytes. T cells release IFN-gamma to activate macrophages.

Eosinophils are associated with parasitic infections and IgE allergic reactions, mediated by the chemokine eotaxin.

Lymphatics: Lymphatics offer help in both defenses against infection and in responding to and resolving an inflammatory response. They are valvular, very thin channels without muscular wall layers. Local lymph flow increases in response to local inflammation. Lymph nodes may become enlarged and reactive in local inflammation, which is termed lymphadenitis. If the lymphatic channels are involved, this is called lymphangitis. The lymphatic system can help to spread disease (infection or cancer) if the local nodes fail to contain the infection and the lymph channels will eventually return to venous circulation, allowing overwhelming infection to occur.

Patterns of Inflammation:

Granulomas may form in sites of chronic inflammation caused by certain specific infections (TB, syphilis, cat-scratch disease) or foreign bodies. Granulomas consist of activated epithelioid macrophages surrounded by lymphocytic infiltrate. Giant cells, multinucleated enlarged and fused macrophages, may be present.

Tuberculous granulomas tend be caseating, meaning that there is an area of central necrosis.

Serous inflammation: fluid transudate from inflammatory cells or lining mesothelium

Fibrinous inflammation: fibrin accumulates due to vascular permeability

Suppurative or purulent inflammation: Pus forms of necrotic cells, neutrophils, edema. Abscesses are foci of purulent inflammation.

Ulceration: local area of necrotic tissue sloughs off

Systemic effects of fever, anorexia, increased sleep are mediated mainly by IL-1, IL-6, and TNF that can enter the BBB. Note that IL-6 stimulates hepatic production of fibrinogen which cause erythrocytes to agglutinate and increases ESR (erythrocyte sedimentation rate), a nonspecific test of systemic inflammation.

TNF-alpha affects appetite and can cause anorexia and cachexia when levels increase. Typically in systemic infection, prostaglandins act on the hypothalamus to increase temperature and white cell counts increase to 15-20,000 per microliter. This is usually accompanied by a "left shift" with increased numbers of immature neutrophils present. Certain infections such as typhoid fever occur with leukopenia, which also occurs when the body is overwhelmed by disease as in advanced cancer or TB.

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