Repair and Resolution of Inflammation PDF

Resolution and Repair Debra Hazen-Martin, PhD Office: 792-2906 Email: [email protected] Outcomes of Acute Inflammation: Resolution, Chronic Inflammation and Repair Outline: I. Outcomes of Acute Inflammation A. Overview B. Serous inflammation C. Fibrinous inflammation D. Suppurative / purulent infla...

Resolution and Repair Debra Hazen-Martin, PhD Office: 792-2906 Email: [email protected] Outcomes of Acute Inflammation: Resolution, Chronic Inflammation and Repair Outline: I. Outcomes of Acute Inflammation A. Overview B. Serous inflammation C. Fibrinous inflammation D. Suppurative / purulent inflammation E. Ulceration II. Chronic Inflammation A. Overview B. Causes C. Mechanisms and features D. Patterns 1. Granuloma III. Repair A. Overview B. Parenchymal Regeneration 1. Capacity 2. Growth factors 3. Signaling C. Extracellular Matrix 1. Components 2. Contributions D. Wound Healing 1. Sequence of events 2. Granulation tissue a) Angiogenesis b) Scar formation c) Scar remodeling 3. Healing a) First intention b) Second intention 4. Wound Strength E. Aberrant Outcomes 1. Inhibitors of healing 2. Proud flesh 3. Keloid Suggested Reading: Robbins Basic Pathology by Kumar, Abbas and Aster, Chapter 3 (78-93) 1 Resolution and Repair Objectives: 1) Describe the possible outcomes of acute inflammation. 2) Describe the factors that determine each of the 3 outcomes. 3) Describe the causes and morphologic characteristics of chronic inflammation. 4) List some of the types of agents that may result in granulomatous inflammation. 5) Describe the characteristics of the granuloma. 6) Compare/contrast a granuloma and an abscess. 7) Describe the composition of a fibrinous exudate and the consequences of organization of this type of exudate in the pleural, pericardial, or peritoneal cavities. 8) Define the terms pyogenic, suppurative, pus, and purulent. 9) Identify the two factors that will determine whether normal tissue structure and function will be restored in an area of injury and necrosis. 10) Describe which cell types are categorized as stable, labile, and permanent. 11) List the 3 major components of the extracellular matrix and describe the role of each in repair. 12) Describe the components of granulation tissue. 13) Describe steps in angiogenesis and the factors that regulate it. 14) Describe the enzymes and growth factors involved in scar formation and remodeling. 15) Describe the differences between 1st intention healing and 2nd intention healing. 16) Correlate the duration of healing time with improvements in wound strength. 17) Describe proud flesh and keloid, the components, and mechanisms for that may drive development. 2 Resolution and Repair I. Outcomes of Acute Inflammation: The products of the acute inflammatory response in injured tissue include a cellular infiltration of leukocytes and edema (exudates/transudate). The successful removal of these components and replacement of damaged parenchyma determine the success of the healing/repair process. A. Overview - One of 3 outcomes is possible: 1) Resolution - If injury is short-lived with minimal tissue damage and remaining cells are capable of replacing dead cells, the tissue will regain normal histology and function. This requires clearance of all inflammatory cells and excess tissue fluid (edema). This is accomplished by macrophage phagocytosis and normal lymphatic channel drainage. 2) Scarring or Fibrosis - When inflammation occurs in tissues that do not regenerate or injury is long-standing with considerable tissue damage, scarring may be the outcome. Scarring may be due to the persistence of fibrinous exudates that organize or persistent neutrophilic infiltrates that form pyogenic abscesses. The only outcome of an abscess is scarring. 3) Progression to Chronic Inflammation- Chronic inflammation may follow acute inflammation of long duration or be present from the onset in cases of viral infection or certain immune responses. As in acute inflammation, chronic inflammation may resolve leading to tissue regeneration or may persist leading to scarring. B. Serous inflammation and resolution When necrosis is minimal and the exudate of acute inflammation is successfully removed complete resolution is possible and normal tissue structure may be restored. In this case there is a watery, protein poor transudate or effusion. 3 Resolution and Repair An example of perfect resolution is healing that follows formation of a blister in the skin. A watery effusion forms between the epidermis and dermis. The minimal transudate and neutrophil infiltrate is quickly resolved in the absence of secondary infection and perfect healing occurs with growth of a new epidermal lining. C. Fibrinous inflammation If injury is more severe and increased vascular permeability is more prominent, a protein-rich exudate will contain fibrinogen and other plasma proteins. Once in the extravascular space, the fibrinogen is coagulated into an amorphous or threadlike mass of insoluble fibrin. If this material is not successfully removed by macrophages it will organize with the help of fibroblasts and form a scar. Depending on the location this may result in loss of function. Fibrinous Inflammation is frequently found in body cavities (pleural, pericardial, and peritoneal) during specific diseases. In the adjacent slide a pleural effusion has collected between the visceral and parietal layers of pleura. Fibrinogen is cleaved to form insoluble fibrin which leads to organization and scarring with loss of lung function due to restricted movement. 4 Resolution and Repair Fibrinous inflammation in the pericardial cavity has similar outcomes and loss of function. In this histologic section of the serosal surface of an organ, there is a thick layer of eosinophilic fibrin. If the fibrinous exudate is not cleared promptly it may become organized by ingrowth of fibroblasts and new vessels. Fibrinous inflammation may be further complicated by the persistence of cellular debris, and neutrophils within the fibrin field. This is referred to as a fibrinopurulent exudate as seen in the adjacent tissue section of an inflamed appendix. Factors 1. persistence CELLULAR DEBRIS 2. neutrophils 5 Resolution and Repair D. Suppurative / purulent inflammation Large amounts of pus (neutrophils, edema, necrotic cells) are typical of suppurative inflammation. Purulent means pus forming. Certain organisms typical cause suppurative inflammation (Staphylococci) and are termed pyogenic. Organism? Xter(3) All three terms are used to indicate the presence of many neutrophils and debris which makes up pus. Abscesses are focal sites of suppurative inflammation around an invading organism and are found deep within a tissue. Abscesses frequently exhibit central necrotic foci of cellular debris with a loss of parenchymal structure. A rim of neutrophils surrounds the abscess. Fibrosis and new vessel formation may also occur surrounding the abscess to “wall it off” from surrounding tissues. In the adjacent slide, the abscess is associated with diverticulitis of colon - actinomycosis infection. You will see many other sites of abscess as you study pathology in other systems. 1. Central necrotic foci with cellular debris 2. Loss of parenchymal structure 3. Rim of neutrophils surround abscess E. Ulceration Ulceration is erosion and necrosis of an area of surface epithelium with associated acute and chronic inflammation beneath the epithelial surface. Neutrophilic infiltrates are found at the base of the eroded pit. In long standing or repeated ulceration components of chronic inflammation are seen and include vascular dilation, fibroblasts, scarring and chronic inflammatory cells. 6 Resolution and Repair The adjacent slide illustrates a gastric ulcer with grossly visible pus at the base of ulcer. The companion slide illustrates the surface collection of neutrophils at the base of the ulcer. Underlying this acutely inflamed area is the submucosa with dilated vessels and cells typical of chronic inflammation. II. Chronic Inflammation A. Overview Def “Inflammation of prolonged duration in which active inflammation, tissue injury, and healing proceed simultaneously.” (Basic Pathology) The cellular infiltrate of chronic inflammation is one of mononuclear cells: macrophages, lymphocytes, and plasma cells. There is tissue destruction induced by inflammatory cells and evidence of repair with angiogenesis and fibrosis. B. Causes There may be progression from acute inflammation to chronic inflammation. In this case, you may see cellular profiles that are characteristics of both acute and chronic events in the same tissue. For example, in a long standing peptic ulcer you may see inflammatory cell infiltrates and fluid, characteristic of acute inflammation and at the same time see signs of chronic infiltrating cells (monocytes) and attempted repair (scarring and angiogenesis). Chronic inflammation is present from the onset in response to specific types of infection or immune reactions. 7 Resolution and Repair C. Mechanisms and histologic features This is a section from a pannus of rheumatoid arthritis. The pannus is an overgrowth of synovial cells lining the synovial membrane. The pannus may encroach on the hyaline cartilage of the articulating surfaces of the joint. The focus of inflammation includes a very different collection of cells than that seen in acute inflammation. The field is flooded with macrophages (histiocytes) and lymphocytes as well as occasional plasma cells. Also note the presence of small vessels. In this slide from your textbook review the histology of acute vs. chronic inflammation in the lung. Chronic Inflammatory Cells and Mediators Macrophages are tissue cells derived from circulating monocytes. When found in certain organs they are given specific names (liver- Kupffer cells, spleen or lymph nodes - histiocytes, lungs - alveolar macrophages). The half-life of the circulating monocyte is 1 day. Within 24 to 48 hours after the onset of acute inflammation, the monocyte will immigrate to the extravascular space and there increase in size and become capable of phagocytosis. The maturation process is termed activation and is regulated by cytokines. Once in the tissue space the half-life of the macrophage is several months to years. 8 Resolution and Repair Monocytes emigrate from the vascular space by the same mechanisms by which leukocytes emigrate in acute inflammation. Macrophages activation occurs in response to receptor mediated binding of cytokines (some from Tcells), endotoxins, and other chemical mediators. The activated macrophage is increased in size, metabolic activity, and amount of lysosomal enzymes. Macrophages may secrete: acid and neutral proteases, some complement and coagulation factors, reactive oxygen species and NO, eicosanoids, IL-1 and TNF, cytokines that promote inflammation and can lead to tissue injury. Macrophages may be activated by alternate pathways to produce enzymes and growth factors contributing to repair, initiating angiogenesis and/or fibrosis. Macrophages clear up debris from acute inflammation and once the insult is resolved the macrophages will eventually die or leave. If the insult persists, as in chronic inflammation, macrophage numbers continue to increase. Activated macrophages appear large and pink by H&E, they are sometimes called epithelioid macrophages. This is purely descriptive; it does not imply a functional change of cell type. Lymphocytes – Steps(4) Both T and B lymphocytes migrate to sites of inflammation similar to the monocyte. Activated macrophages present T-cells with processed antigens and cytokines (IL12) which prompts T-cell activation. The activated T-cell then produces a mediator (IFN-γ) that activates maturation/activation of macrophages creating a cycle of activation between the two cell types. Lymphocyte derived factors may stimulate fusion of macrophages into multinucleated giant cells in sites of chronic inflammation. 9 Resolution and Repair D. Patterns of Chronic Inflammation 1. Granulomatous Inflammation is a distinct histologic pattern of chronic inflammation involves formation of granulomas. Definition: A granuloma is a collection of epithelioid macrophages surrounded by a rim of lymphocytes. A granuloma may have a border of multinucleated giant cells, central necrosis, and/or peripheral fibroblasts and connective tissue. Many different agents cause granuloma formation and the unique histological characteristics of any granuloma are determined by the specific agent. 10 Resolution and Repair The granuloma in the adjacent slide is formed in response to mycobacterium tuberculosis. This granuloma exhibits central caseous necrosis and peripheral Langerhans-type giant cells. There is also a rim of lymphocytes surrounding granuloma. 1. Caseous necrosis 2.Peripheral Langerhands-type giant cells 3. Rim of lymphocytes III. Repair A. Overview The previous lectures have focused on the morphology of tissue injury with inflammation. What determines whether injury with substantial necrosis will result in restitution of normal structure or a permanent scar? Two factors are: 1) the extent of damage to the tissue framework and 2) the regenerative capacity of the cells in the injured tissue. B. Parenchymal Regeneration 1. Capacity - Normal tissues exhibit a stable baseline number of parenchymal cells. Normally the proliferative rate is equivalent to the normal rate of cell death and/or differentiation into another cell population. Anything that sets these two sides off balance will alter tissue cell numbers. Stimuli that may induce cell proliferation include cell injury, cell death, intrinsic growth factor stimulation and most importantly, any stimulus that induces a resting cell population to enter the cell cycle. 11 Resolution and Repair Cells of the body have differing ability to regenerate. Usually, the most differentiated a cells have the least capacity for regeneration (ex: cardiac myocytes or neurons). Cells are categorized as follows: a. Labile cells - As a population, have a steady rate of proliferation and death (turnover). So the cell cycle is continuous with stem cells entering the cell cycle at the same rate as cell death occurs. Example: surface epithelial cells (epidermal and mucosal linings of GI and UG tracts) epithelial duct cells and hematopoietic cells in bone marrow. b. Stable cells- These cells are normally quiescent but may return to the cell cycle quickly when injury and/or cell loss occurs. Examples include parenchymal cells of most organs (liver, kidney), endothelial cells of vessels and mesenchymal cells (fibroblasts, smooth muscle). c. Permanent cells- These are cells that have terminally differentiated and lost the capacity to reenter the cycle. Cell death in these populations will not result in regeneration but instead, scar formation. Cells of this type include cardiac myocytes and neurons. 2. Growth factors The majority of mediators that influence cell proliferation and differentiation are proteins called growth factors. Growth factors also mediate cell migration among other activities. Growth factors act by influencing gene expression of protooncogenes which normally regulate proliferation and repair events. Protooncogene expression or structure may be altered to the extent that they are converted to oncogenes that allow uncontrolled growth as in cancer. Growth factors rely on extra cellular signaling. 12 Resolution and Repair 3. Signaling Autocrine- A mediator produced by a cell may act on the same cell. Cytokines are common mediators that act in this way. The compensatory hyperplasia one observes in the liver after partial hepatectomy is a consequence of autocrine signaling via growth factors secreted from the remaining liver tissue. Paracrine signaling- A mediator acts on cells in the immediate vicinity of its cell of origin to provide a local response. There is only minimal diffusion of the mediator. An example is leukocyte recruitment by chemokines and growth factors produced by cells to influence growth in wound repair. C. Extracellular Matrix 1. Components - ECM is found in two forms, the interstitial matrix and the basement membrane. The interstitial matrix fills the spaces between the cells of connective tissues. It is a 3 dimensional amorphous gel whose components are synthesized by fibroblasts and include fibrillar and nonfibrillar collagens, proteoglycans, and glycoproteins. The interstitial matrix is more organized adjacent endothelial cells, epithelial cells, and smooth muscle cells to form a discrete basement membrane. Basement membrane components are synthesized by both epithelial and mesenchymal cells and include nonfibrillar collagen type IV and glycoproteins. 13 Resolution and Repair 2. Contributions All elements of the ECM contribute to the process of wound repair. Some create an environment that supports the correct growth factors and conditions for growth. Others directly influence the cell to provide the infrastructure and structural support for growth and differentiation. D. Wound Healing 1. Sequence of events When tissue injury continues and repair is not possible by simple regeneration of the parenchyma, the areas once occupied by parenchymal cells will be replaced by connective tissues. Repair begins within the first 24 hours by emigration of fibroblasts and endothelial cell proliferation. These events result in the formation of granulation tissue. 2. Granulation tissue is composed of new vessels in a loose extracellular matrix with proliferating fibroblasts. With time the granulation tissue accumulates more and more dense fibrous connective tissue resulting in formation of a scar. 14 Resolution and Repair a) Angiogenesis - The process for vessel formation in granulation tissue is a consequence of angiogenesis, a process where new vessels form from existing vessels. The steps in angiogenesis include the following: 1. Proteolytic degradation of the parent vessel basement membraneThis step is essential if a new sprout is to form. Basement membrane is digested by proteases secreted by the endothelial cell in response to VEGF and FGF-2 released by stromal cells at the site of injury. NO also induces vessel permeability. 2. Migration of endothelial cells from the original capillary to the site of the angiogenic stimulus- Migration is initiated by VEGF and bFGF as well. 3. Proliferation of endothelial cells behind the migrating endothelial cells- This increase in mitotic rate is also due to growth factor stimulation and is essential to growth of the sprout. 4. Maturation of the endothelium into effective tubes with recruitment of necessary supporting cells. In a previous lecture we discussed that the new vessels were leaky due to imperfect cell junctions and increased transcytosis. This accounts for the edema found in granulation tissue. b) Scar formation (fibrosis) The process of scar formation includes the emigration of fibroblasts into the site of injury and their secretion of extracellular matrix components. Fibroblasts recruitment and proliferation is stimulated by growth factors: PDGF, FGF-2, and TGF -beta which are secreted by endothelial cells. Inflammatory cells (mast cells, lymphocytes, and macrophages) also secrete mediators to contribute to this process. With time, new vessel formation slows and fibroblast proliferation will decline while their synthetic activity will increase. Synthesis is stimulated by many of the growth factors that also stimulated fibroblast recruitment and also IL-1 and TNF. Fibroblasts will secrete collagen from day 3-5 through a period of weeks. Collagen build up is due to combined secretion without compensatory degradation. 15 Resolution and Repair c) Scar remodeling - After initial collagens and other extracellular matrix proteins are laid down in the scar, these components are remodeled by a process of degradation coupled with synthesis. The enzymes that will digest the initial collagen are produced by fibroblasts, macrophages, epithelial cells, neutrophils and others. Enzyme production is stimulated by growth factors (PDGF, EGF, and cytokines IL-1 and TNF). These enzymes are produced in an inactive zymogen form which is activated by substances that occur during injury: HClO free radical, and activated plasmin. These enzymes produced include the metalloproteinases which include collagenase I, II, III, IV, (for digestion of specific collagen types) and stromelysins (that degrade proteoglycans). To protect against unregulated digestion of matrix TIMPs (tissue inhibitors of metalloproteinase) are produced by mesenchymal cells. Summary: A number of growth factors and cytokines are involved in recruitment of cells to the site of injury, proliferation of cells, angiogenesis, and collagen synthesis/degradation. The adjacent chart summarizes the factors that participate in each activity previously described. 16 Resolution and Repair 3. Healing a) First intention - In this type of cutaneous wound, the injury is a clean cut such as a surgical incision. There is little disruption to the epithelium, basement membrane or connective tissues. Regeneration predominates over fibrosis. The following sequence of events occurs: Within 24 hours: A fibrin clot has filled the incision and neutrophils migrate into the clot. Basal epidermal cells increase mitotic rate and grow together from each side of the interruption, laying down new basement membrane as they go. 3-7 days: Neutrophils are replaced by macrophages and granulation tissue replaces the fibrin clot with new vessels and proliferating fibroblasts. Collagen fibrils are secreted and begin to cross the incision. 2nd week and beyond: Edema resolves, macrophages and vessels retreat. Fibroblasts continue to secrete collagen and tensile strength improves with time. Dermal appendages do not return. b) Second intention - In this type of wound the amount of tissue damage is much greater as in infarct, ulceration, abscess formation, etc. Regeneration of parenchymal cells cannot restore the original architecture and there is a greater role for scarring. Within 24 hours: The response is the same as with 1st intention but the area to be filled with fibrin is much more expansive and the inflammatory response is greater leading to inflammatory-mediated injury. There is an increased amount of necrotic debris, edema, and fibrin deposition. 3-7 days: Epidermal cells have not yet bridged the gap and the expanse of granulation tissue that forms is substantial. 2nd week and beyond: Edema, macrophages and vessels retreat but fibroblasts secrete much more collagen. Myofibroblasts grow in and contribute to wound contraction to make the scar much smaller than the original site of injury. 17 Resolution and Repair 4. Wound Strength The secretion of collagen at the site of the wound accounts for the tensile strength of the repaired area. 1 week after a clean wound is carefully sutured it will exhibit only 10% of the strength of the original tissue at that site. By 3 months, 70-80% of the strength is regained. Improvement is not added beyond this time point. E. Aberrant Outcomes 1. Inhibitors of healing A number of factors are involved in the outcome of wound healing and are summarized in the adjacent slide. Infection, poor nutrition (protein deficiency, vitamin C deficiency), glucocorticoid administration, dehiscence due to mechanical pressure or torsion, poor oxygen perfusion, and/or the presence of foreign bodies may all contribute to compromised wound healing. 2. Proud flesh The persistence of granulation tissue at the site of a wound without resolution can interfere with normal healing. 1.Over-production of granulation tissue 2. Interferes with normal healing 18 Resolution and Repair 3. Keloid formation involves an excessive growth production of collagen during wound healing. In patients who are predisposed to keloid formation even surgical wounds may become disfiguring during the process of healing. 19