Chronic Inflammation
Chronic inflammation differs from acute inflammation in timing, duration, type of tissue, type of cell infiltrates, and
type of tissue response. Chronic inflammation occurs over a period of days to years, often following prolonged or
repeated exposure to stimuli that cause acute inflammation and to both infectious and non-infectious pathogens.
What are the major classes of disease that can cause chronic inflammation?
- Persistent or resistant infections
 These pathogens have special characteristics that resist phagocytosis or, if phagocytosed, resist
intracellular killing. The following are some examples of these types of pathogens:
o Bacteria – Mycobacteria, Nocardia
o Fungi – Blastomyces, Histoplasma
o Protozoa – Trypanosoma, Leishmania
o Parasites – Schistosoma
o Foreign bodies – plants, minerals, suture material
- Infections “hidden” from immune responses
 These types of pathogens may not necessarily be resistant to phagocytosis or intracellular killing, but
can be obscured within degenerate neutrophils (pus) and tissue debris associated with inflammation
o Bacteria – Streptococcus, Staphlococcus
- Presence of inert material or foreign bodies
 Plants, minerals, suture and surgical materials can be proponents of chronic inflammation
- Some autoimmune diseases
Generally speaking, how is chronic inflammation characterized?
- Mononuclear inflammation
 Inflammation is associated primarily with macrophages, lymphocytes, and plasma cells
o Acute inflammation is associated with neutrophils
 May be associated with multinucleate giant cells (merged or non-divided macrophages)
- Fibrosis and angiogenesis
 Characterized by connective tissue proliferation and granulation tissue (names for gross appearance)
o Acts as a barrier with an outer, maturing border of connective tissue and an inner, active
component of proliferating vessels and neutrophils
- Destruction or alternation of normal parenchyma
 Tissue responses to chronic inflammation typically includes fibrosis, regeneration, and repair
 Tissue responses to both acute and subacute inflammation typically include edema and vasodilation
What are the various types of chronic inflammation and their predominant cellular constituents?
Types of Chronic Inflammation
Type of Inflammation
Chronic suppurative infection
Type IV hypersensitivity
Granulomatous infection
Autoimmune-induced infection
Parasite-induced inflammation
Predominant Cell Type Involved
Neutrophils
Macrophages, lymphocytes
Macrophages, lymphocytes
Lymphocytes, immunoglobulin
Eosinophils, macrophages, lymphocytes
Example of Pathogen
Pus-forming bacteria
Poison ivy, tuberculin
Mycobacteria, fungi
Autoantibodies, molecular mimicry
Helminthes (worms)
Chronic suppurative inflammation
An individual is not typically aware of the functioning of “natural immunity” unless tissue damage is significant. In
response to tissue damage, an acute inflammatory response is induced. This acute inflammatory response is
typically successful in killing the invading pathogen. Moreover, any damage that occurred in the inflammatory
response is repaired by healing mechanisms. However, in some cases, the body’s responses are unable to clear the
pathogen. Consequently, immune responses are continuously mounted. Neutrophils and macrophages continue to
migrate to the site of infection, attempting to (but not succeeding) clear the pathogen through the release of
bactericidal factors, and ultimately causing tissue damage. Partial healing mechanisms takes place, resulting in the
deposition of granulation and scar tissue around the site of inflammation, though true healing is not achieved. This
is considered chronic suppurative inflammation, which is exudative and purulent.
Example of chronic suppurative inflammation
Examples of pathogens causing chronic suppurative infections
- Pyogenic bacteria (pus-inducing) – Staphlococcus aureus
- Many cases of bacterial pneumonia – Streptococcus pneumoniae (pneumococci)
 Alveoli and bronchi fill up with pus, depriving the patient of oxygen
 Usually characterized by minimal tissue damage and complete healing with the aid of antibiotics
- Abscess
 Local form of suppurative infection that can occur in any organ (occurs often in the subcutis)
 Characterized by a round lesion composed of central neutrophils and macrophages surrounded by an
outer wall of fibrous connective tissue
- Pyelonephritis – bacteria ascends the ureter to infect the kidney, particularly the renal pelvis
- Chronic osteomyelitis – can develop following penetrating wounds of the bone and bone marrow
What role do specific cellular components of the immune system have in causing chronic inflammation?
Antigen presenting cells (APCs)
- APCs process and present specific antigens to naïve T cells
 Professional APCs include dendritic cells, macrophages, and B cells
Lymphocyte involvement
- Both CD4+ (helper T cells) and CD8+ (CTLs) lymphocytes are tested in the thymus for self-antigen reactivity
 “CD” or “cluster of differentiation” refers to the in vitro method by which CD molecules are found
 T cells that react with self-antigens undergo apoptosis
 Some thymic self-antigen expression is controlled by a transcription factor coded by the gene, AIRE
o Mutation of the AIRE gene results in autoimmunity and an inflammatory condition known as
autoimmune-polyendocrinopathy-candidiasis-ectodermal dystrophy (affects various tissues)
 AIRE cannot account for the expression of all thymic self-antigens (there are other AIRE-like factors)
- Self reactive B cells are also removed via apoptosis in their tissue of origin (e.g. bone marrow in primates)
- Although deletion of self-reactive lymphocytes is thorough in the normal animal, self-reactive lymphocytes
normally exist in some peripheral tissues
Initiation of immune response
- To initiate an immune response, two co-stimulatory signals must be received
 First signal - binding of T cell (via TCRs) to APC (via MHC molecules with antigen)
 Second signal – binding of T cell CD28 (CTLA-4) receptor to APC ligand B7-1 or B7-2
 Third signal – binding of T cell and APC adhesion molecules, LFA-1 and ICAM-1/ICAM-2
o Strengthens binding between T cell and APC to allow for prolonged transmission of signal
transduction pathways that induce T cell division and cytokine expression
- Critical 2nd co-stimulatory signal stimulates T cells division, cytokine secretion, and memory cell differentiation
- In absence of co-stimulatory molecules, T cell activation and differentiation is compromised
 T cell may not bind to APC
 T cell may bind to APC, but remain non-responsive or inactivated (anergy)
 T cell may undergo apoptosis
- Lymphocytes can also be activated by chemokines and cytokines that are produced by various immune cells
 These activation factors are also implicated in auto-immunity, where their activation of neighboring
lymphocytes can result in immune response towards self-antigen (if one of the neighboring
lymphocytes reacts to self-antigen)
- Presence of T cells with low affinity to antigen may help mitigate the potential for auto-reactivity
Differentiation of CD4+ cells
- The differentiation of T helper cells is important for understanding activation of specific immune responses
 Th1 response – Th1 cells
o Cell-mediated immune response
o Effective pathway against intracellular pathogens (e.g. mycobacteria)
o Involves lymphocytes, macrophages, dendritic cells, and fibroblasts
o Cytokines involved
 IL-2
- Induces proliferation of T cells
- Up-regulates IL-2 receptors, further promoting Th1 cell response
 INFγ – activates macrophages; suppresses Th2 response
 IL-12 – secreted by macrophages; induces Th1 response
 TNFα – potent molecule that aids in bacterial and cell killing
 Th2 response – Th2 cells
o Humoral immune response
o Effective pathway against extracellular pathogens and their toxins (e.g. helminthes)
o Involves lymphocytes, macrophages, eosinophils, mast cells, dendritic cells, and fibroblasts
o Cytokines involves


IL-4 (predominantly) and IL-5
- Stimulate B cells
- Promote class switching from IgM to IgG and IgE
- Promote Th2 cell differentiation
IL-10, IL-4, IL-13, TGFβ
- Induce B cells and eosinophils
- Suppress Th2 response
Macrophage activation
- Macrophages are derived from blood monocytes and are present in all tissues (named for resident tissue)
 Tissue macrophages (or dendritic cells) are generally referred to as “histiocytes”
 Kupffer cells (liver), microgliacyte (brain)
- Activation can occur via phagocytosis
 Phagocytosis must occur for antigen processing and presentation to T cells
 Commonly phagocytose microbes that were opsonized with C3b complement
 Phagocytosis does not always result in complete macrophage activation
(i.e. phagocytosis of dead cells or cell machinery elicits minimal macrophage activation)
o Partial macrophage activation occurs early in infection, before the adaptive immune response
- Greatest macrophage activation occurs with cytokine stimulation, after T cell activation (can take 1-3 weeks)
 Due to amplification of immune response by cytokine expression
- Fully-activated macrophages secrete numerous powerful inflammatory substances
 NO, O2 radicals, acid hydrolases, complement, clotting and healing factors (fibrinogenesis factors)
 Two very important products of macrophage activation are IL-1 and TNFα
o Involved in tumor cytotoxicity, cachexia (loss of body mass), anti-viral and anti-parasitic
activity, activation of phagocytes and NK cells, and induction of other inflammatory cytokines
Note that macrophages are involved in many physiological processes of the body. Macrophage involvement in a
disease (i.e. Alzheimer’s, atherosclerosis, type II diabetes) does not imply that the condition is an “inflammatory
disease”, especially because macrophages are involved in the normal digestion of cellular waste products.
Regulators of inflammation
- The presence of antigen is the most prominent regulator of inflammation
 If antigens are still present, inflammation continues
 If antigens are removed, inflammation stops (or is supposed to)
- Cessation of inflammation is aided by the short half-lives of cytokines and inflammatory cell activation
- IL-10 (secreted by Th2 cells, B cells, and macrophages) has potent anti-inflammatory effects
 Inhibits INFγ (potent macrophage activator)
 Suppresses secretion of IL-1 and TNFα by macrophages
 Suppresses Th1 cell response
- IL-4 suppresses inflammation by suppressing Th1 cells
- T regulatory cells (Tregs)
 Recently-discovered CD4+ cells that express CD25 (α-chain of the IL-2 receptor)
o Expression of FoxP3 is essential for differentiation of CD4+ to CD25 regulatory cells
o FoxP3 mutation is implicated in immuno-dysregulation
 Activity is though to be mediated by production of IL-10 and TNFβ
 Exhibit immune-suppressor activity (autoimmunity develops in their absence)
 Shown to suppress experimental allergic encephalitis (EAE), infantile insulin-dependent diabetes
mellitus (IIDM), colitis, and graft vs. host disease (GVHD)
- Th17 cells
 Essential to proper function of the immune system
 Considered a third CD4+ differentiation pathway, in addition to Th1 and Th2
o Predominant cytokine produced is IL-17
o TGFβ, IL-6, and IL-1 are cytokines involved in Th17 cell differentiation
o IL-23 has a key role in expansion and survival of Th17 cells
What is Type IV, or delayed-type, hypersensitivity (DTH) and its mechanism of action?
Generally speaking, types I, II, and III hypersensitivity are humorally-mediated (B cell) and involved in both acute
and chronic inflammatory responses. Type IV hypersensitivity is T cell-mediated and chronic.
Type I hypersensitivity
- Histamine and serotonin-induced
- Involves IgE and mast cell activation
- Associated with drop in blood pressure, bronchoconstriction, and edema
- Acute (systemic anaphylaxis) and chronic (allergic rhinitis, asthma) forms
Type II hypersensitivity
- IgE is directed against cell surface or matrix antigens
- Involves complement activation, neutrophils, and macrophages
- Acute (drug reactions) and chronic (Grave’s and Addison’s disease, myasthenia gravis) forms
Type III hypersensitivity
- IgG binds to free-floating, soluble antigens to form immune complexes
- Immune complexes deposit in basement membranes and stimulate complement activation and phagocytosis
- Associated with fever, rash, edema, joint pain, leukopenia, albuminuria
- Acute (serum sickness) and chronic (SLE, glomerulonephritis) forms
Types of Type IV Hypersensitivity
Syndrome
Delayed type hypersensitivity
Contact hypersensitivity
Gluten-sensitive enteropathy
Antigen
Proteins – insect venom, mycobacterial
proteins (tuberculin)
Haptens – poison ivy, small metal ions
Gliadin
Immune Response
Skin redness, swelling, induration (firm),
cellular infiltrates, dermatitis
Blistering erythema, contact dermatitis
Intestinal villus atrophy, malabsorption
Type IV hypersensitivity
- Cell-mediated (T cells), chronic immune response
- A DTH response involves two phases
 Induction (1-2 weeks)
o Initiated by first exposure to tuberculin-like antigen
o Mediated by naïve Th1 cells (type of CD4+ lymphocytes)
o Takes 1-2 weeks for T cells to differentiate from naïve to “primed”
 Secondary phase (1-3 days)
o Initiated by second exposure to same pathogen and can occur many years after induction
o Faster response than induction phase; slower than acute hypersensitivity (hence “DTH”)
o Mediated by memory T cells
o Initially, the secondary response is similar to acute inflammation
 Stimulated Th1 cells produce cytokines that increase vascular permeability, bring fluid
and fibrin into tissue, and recruit inflammatory cells (macrophages, neutrophils) to site
o Unlike pure acute inflammation, lymphocytes traffic to the site of the lesion
o The response is self limiting and ceases when antigen is phagocytosed and destroyed
o Some DTH responses, like those to Mycobacterium tuberculosis, produce an amplified
immune and inflammatory response that results in severe tissue damage and even death
Contact hypersensitivity
- Specific type of DTH response (e.g. poison ivy, insect bites)
- Mediated by hapten antigens bound to protein carriers (hapten-carrier complex)
 Haptens, or lipophilic chemical antigens (e.g. pentadecadatechol in poison ivy), are able to penetrate
the skin and bind covalently to host skin proteins to form hapten-carrier complexes
 APCs of the skin, Langerhans cells, present complexes to Th1 cells to elicit a DTH response
What is granulomatous inflammation? A granuloma and its microscopic components?
Granulomatous Inflammation
- Broad category of chronic inflammation characterized by the predominant presence of macrophages
- Do not confuse “granulomatous inflammation” with “granulation” (healing response)
- Most lesions are discrete, nodular masses, known as granulomas
- Because of the limited immune systems of reptiles, amphibians, fish, and birds compared to most mammals,
these species more commonly produce granulomatous inflammation
- Histological components of a granuloma
 Centrally located is a layer of macrophages attempting to phagocytose antigen
o Epithelioid macrophages – squished macrophages that appear as epithelial cells
o Multinucleated giant cells (syncytial) – fused macrophages (one cytoplasm, many nuclei)
 Neutrophils will be present if antigen is or contains bacteria
 Lymphocytes and plasma cells will be present if antigen is immunogenic (normally is)
 Long-standing granulomas typically exhibit a layer of fibrous tissue around the outside of the lesion
 Granuloma will persist until antigen dissolves, is completely destroyed by macrophages, or becomes
encapsulated in fibrous tissue (“walled-off” by a layer of scar tissue)
What are some pathogens that cause granulomatous inflammation and/or granulomas?
Granulomatous Disease
Associated Disease
Etiological Agent
Bacterial Antigens
Tuberculosis
Mycobacterium tuberculosis
Leprosy
Mycobacterium leprae
Syphilis
Treponema pallidum
Brucellosis
Brucella spp.
Lymphogranuloma venereum
Chlamidia trachomatis
Cat scratch disease
Bartonella henselaea
Parasitic Antigens
Schistosomiasis
Schistosoma spp.
Fungal Antigens
Cryptococcosis
Cryptococcus neoformans
Aspergillosis
Aspergillus spp.
Histoplasmosis
Histoplasma spp.
Inorganic metals and dusts (from mining)
Pneumoconiosis (black lung disease) Coal dust
Silicosis
Silica
Berylliosis
Beryllium
Asbestosis
Asbestos
Foreign bodies
Foreign body granuloma
Sutures, splinters, etc.
Endogenous material
Gout
Urate crystals
Quasi neoplastic
Sarcoidosis
Unknown
Diagnostic Features
Acid fast bacteria in lesion
Acid fast bacteria in lesion
Granuloma is called “gumma”
Systemic disease
Affects genitalia and lymph nodes
Granuloma contains neutrophils
Associated with liver disease
Exhibits 5-10μm yeast with capsule
Exhibits fungal hyphae (branching)
Systemic disease
Coal-laden macrophages, fibrosis
Inflammation, fibrosis
Inflammation, fibrosis
Inflammation, mesothelioma
Contains multinucleated giant cells
Chronic joint inflammation
Macrophage proliferation
Tuberculosis
- Four types of tuberculosis are known in animals
 Mycobacterium tuberculosis
 Mycobacterium bovis
 Mycobacterium avium
 Mycobacterium avium ssp paratuberculosis (formerly known as M. johnei)
o Affects ruminants only, causing an intestinal disease called Johne’s disease
o Affected cattle develop severe, diffuse infiltrates of macrophages in lamina propria of gut
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-
Primary tuberculosis
 Initiated by inhalation of bacteria
 Non-specific inflammatory response involving neutrophils and non-activated macrophages
 Innate immunity does not kill mycobacteria, and infection migrates to LNs (and sometimes organs)
 Inflammatory reactions continue to be ineffective while T cell response develops over 2-3 weeks
o Lag phase for T cell response is equivalent to induction phase of DTH reaction to tuberculin
 T cell response is mounted after lag phase
o Macrophages secreting TNFα are the major effectors in this kind of disease
o CTL killing is also a mechanisms for antigen destruction (perforin and granzyme)
 Antigen killing causes caseous necrosis at center of granuloma (caseous areas are acidic and anoxic)
Disseminated (recurrent) tuberculosis
 Occurs in patients that fail to heal from a primary infection or that become re-infected
 Infection can be widespread or affect particular organs (bone marrow, kidney, adrenal gland)
Leprosy
- Transmitted via inhalation of causative agent, Mycobacterium leprae
- Bacteria spreads from lungs to extremities, particularly infecting dermis and nerves
- Mycobacterium does not secrete toxins, but is virulent due to an indigestible cell wall
- Tuberculoid leprosy
 Mildest form (not life-threatening)
 Th1 response predominates
o Macrophages are active and partially effective
 Patients develop debilitating focal granulomas of the skin and nerve
- Lepromatous leprosy
 Often fatal
 Th2 response predominates (down regulation of Th1 cells)
o Th2 cells are not effective at killing intracellular mycobacteria
o Macrophages are not activated
 Granulomatous inflammation is widespread in connective and nervous tissue
Inorganic metals and dusts
- Inhaled dust from ground rocks (e.g. quartz) induces tissue damage following phagocytosis by macrophages
- Chronic inflammation in the lungs leads to scarring
- Scarring in the lungs is particularly deleterious as it interferes with pulmonary compliance
What are the consequences of virus-associated immunodeficiencies (e.g. AIDS)?
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-
Immunodeficiency can be induced by retroviruses (HIV–human , SIV–simian, FI –feline, BIV–bovine )
AIDS (in any species) specifically targets CD4+ cells and macrophages
Impaired type IV hypersensitivity (from T cell dysfunction) results in infections that cause diffuse infiltrates
 Lack of granuloma formation and caseous necrosis in response to pathogenic infiltration
 Macrophages are not activated due to lack of Th1 response
Humans and monkeys, due to their innate resistance to many bacterial infections, are prone to fungal infections
What are some examples of primary (genetic, hereditary) immunodeficiencies?
-
SCID (severe combined immunodeficiency) – arabian horses, Basset hounds, mice
Common variable immunodeficiency
Agammaglobulinemia
Selective Ig deficiencies
Complement deficiency
Chediak-Higashi syndrome
Leukocyte adhesion deficiency (LAD)
Cite specific examples of immune-mediated diseases.
Autoimmune Disease of Animals
Disease
Ulcerative colitis
EAE (mice)
Allergic skin diseases
Autoimmune hemolytic anemia
Chronic active hepatitis
Systemic lupus erythematosus (SLE)
Myasthenia gravis
Autoimmune gastritis
IDDM
Addison’s disease
Pemphagus vulgaris
Rheumatoid arthritis
Autoimmune hemolytic anemia
Autoimmune thrombocytopenic purpura
Etiology
Unknown, involves T cells
Myelin basic protein, T cells
Food and other allergens
Drugs, infections, antibody-mediated
Toxins and viruses
Antibodies against nuclear machinery
Acetylcholine receptors
Maybe Helicobacter spp.
Antibodies to insulin
Adrenal cortical cells
Antibodies against desmoglein 3
Autoantibody (Rheumatoid factor)
Antibodies to blood group antigens
Platelet integrin
Clinical Aspects
Leads to colon cancer
Paralysis, important model of MS
Various clinical forms
Progresses to cirrhosis
Type III hypersensitivity
Muscle weakness
Pernicious anemia
Type IV hypersensitivity, diabetes
Adrenal insufficiency
Epidermis separates from dermis
Severe joint arthritis of all joints
Anemia
Type III hypersensitivity, bleeding disorder
The organ distribution in autoimmune diseases varies; some conditions affect a single organ while others affect
many (e.g. SLE). Moreover, the precise autoantigen involved is not always known.
SLE
- Type III hypersensitivity
- Multi-organ disease in which antinuclear antibodies are directed against autoantigens of the cell nuclear
machinery (nucleosomes, ribosomes, nucleoproteins involved in RNA processing)
- Antibodies forms antibody-antigen complexes that are deposited in organs (blood vessel walls, kidney)
- Associated with dermatitis, arthritis, glomerulonephritis, myositis, hematologic abnormalities, fever
Diabetes
- Type I diabetes (hypoinsulinomic) is associated with autoimmune destruction of pancreatic islet cells
- Type II diabetes (hyperinsulinomic) is also associated with a non-immune mediated dietary condition
Pemphigus complex
- Pemphigus autoantibody is directed against adhesion molecules (e.g. desmosomes) of epidermal keratinocytes
 Foliaceous – exfoliative dermatitis of the head and body (cat, dog, horse, goat)
 Vulgaris – affects mucocutaneous junctions (cat, dog)
 Vegetans – rare (dog)
 Erythematosus – erosive dermatitis of the head and neck (cat, dog)
- Keratinocytes lose adhesion with one another (acantholysis), leading to epidermal ulceration and inflammation
- Usually accompanied by secondary bacterial infection
Molecular mimicry and cross-reactivity are sometimes involved in autoimmunity. In this case, foreign antigen that
is structurally similar to self-antigen induces an immune response against both foreign and self-antigens.
What are the consequences of chronic inflammation?
Cellulitis
- Clinical term (non-specific) referring to inflammation of loose connective tissue (often of subcutis)
- Commonly used to denote inflammation in a specific location
 “Facial cellulitis” is inflammation of the subcutis of the face
Abscess formation
- An abscess is a collection of neutrophils within a walled-off cavity
- Bacteria and neutrophils release substances that cause tissue necrosis, forming a cavity around the lesion
- (2-3 days) A wall of granulation tissue forms, allowing neutrophil influx but preventing bacterial efflux
- Two possible fates of the lesion
 Bacteria are killed
o Sterile abscess slowly shrinks as neutrophils die and are digested
o Outer granulation tissue matures into collagenous tissue, preventing further neutrophil influx
 Bacteria persist and abscess remains (possibly expands)
o Abscess can potentially drain into body cavity, forming a draining tract called a fistula
Adhesions
- Result from inflammation of serosal surfaces, which become fibrin-covered
and “sticky”
- When two sticky adhesive surfaces meet, a tight fibrous adhesion can form
- Adhesions vary from plaques to long ropes of fibrous tissue
- Commonly result from pleuritis, peritonitis, and post-abdominal surgery
Adhesion
Ulceration
- Chronic, non-healing gap of a mucosal epithelial surface
 Gastric ulcers (due to “stress”, acid secretion, or NSAIDs)
 Cutaneous ulcers (due to pressure necrosis during debilitation)
 Corneal ulcers (due to bacterial infection, impaired wound healing)
Granulomatous inflammation
- The type of granulomatous response depends on the dominating lymphocytic response – Th1 or Th2
 Nodular (tuberculoid) granulomas
o Th1-derived response – driven by cytokines IFNγ, TNFα, IL-12
o Characterized by discrete nodular granulomas
o Most prominent histological components are aggregates of epitheliod macrophages
o Multinucleated giant cells are sometimes present
o Lymphocyte and plasma cell presence depends on the cause and duration of the granuloma
o Fibroblast rim of variable thickness surrounds periphery of granuloma
o Macrophages up-regulate pathways that produce NO and citrulline (decrease fibrosis)
o Central necrosis may be present (i.e. caseous necrosis of mycobacterial infection)
 Diffuse (lepromatous) granulomas
o Th2-derived – driven by cytokines IL-4, IL-5, IL-10, IL-13, and TGFβ
o Characterized by diffuse sheets of epitheliod macrophages
o Inflammation induced by parasites exhibit a large presence of eosinophils
Parasite-induced inflammation
- Inflammation induced by parasites is a variation of the Th2 response
 IL-5 induces bone marrow to produce more eosinophils
 IgE is the primary antibody involved (induced by IL-4)
- Eosinophils are the dominant effector cell type
 Perform minimal phagocytosis
 Granules contain major basic protein, which is highly toxic to both parasites and mammalian cells
- Unique chemoattractants are present in eosinophilic lesions
 Eotaxin, MIP-1α
Fibrosis
- Excess of fibrous connective tissue that replaces or interferes with normal tissue structure and function
- Pathologic fibrosis may be incited by inflammation, wound healing, foreign bodies, ischemia, radiation, etc.
- Often progresses slowly over time and accompanied by contraction (due to presence of myofibroblasts)
- Considered a permanent change (there may be some possibility for resolution if inciting cause is removed)
- Fibrotic tissues are white and firm (grossly)
- Molecular basis of fibrosis
 Driven by a small group of cytokines derived from leukocytes, mast cells, and platelet
o Platelet-derived growth factor (PDGF), fibroblast growth factor (FGF), IGF
o TGFβ is the most important
o Fibronectin
 Involved in collagen synthesis, decreased matrix degradation by metalloproteinases
 Signals via SMAD messenger proteins
Identify important processes of wound healing
The process of wound healing requires, at minimum, an integrated process of regeneration, repair, and fibrosis (to
some degree) and occurs under a variety of different conditions (i.e. surgery). The phases of normal wound healing
include necrosis, inflammation, granulation tissue, angiogenesis, fibrosis, and repair.
Primary union (first intention) healing
- Occurs in mostly aseptic, fresh, relatively-small wounds that undergo surgical closure
- Minutes to hours
 Wound is sealed by platelets and fibrin within minutes
 Inflammation consists initially of neutrophils (arrive at site within hours)
 When bacteria is absent, inflammation is dominated by macrophages
- Day 1
 Fibroblasts, endothelial cells, and epithelial cells begin migrating to wound, a process driven by
cytokines releases from macrophages, platelets (plug initial hemorrhage), and regional mast cells
o Fibroblasts migrate along a network of fibrin covered by fibronectin
o Epithelial cell migration occurs by flattened basal keratinocytes over fibrin-covered surface
o Endothelial cells sprout from adjacent vasculature
- Day 3
 Macrophages outnumber neutrophils
 Wound is bridged by early fibrous connective tissue (abundant type III collagen, limited elastin)
 Vascular proliferation and anastomoses results is formation of granulation tissue
- Week 1
 Would closure is mostly complete
 Gap is filled by fibroblasts producing type I collagen, blood vessels, and young lymphatics
 Macrophage presence has decreased, fibrin is absent
- Months to years
 Wound contraction, remodeling, and collagen cross-linking continues
 Repaired wound will never be as strong as the normal tissue it replaced
Secondary union (second intention) healing
- Occurs in various situations
 Aseptic wounds allowed to heal independently (considered “open”)
 Infected wounds
 Extensive tissue defects (e.g. large wounds, infarcts)
- Wound-healing process differs from primary union is two significant ways
 Wound gap is large
 Presence of bacterial contaminants results in massive influx of neutrophils
o Neutrophils attempt to control infection, but impede fibroblasts and endothelial cells migration
o Open wound contracts during ongoing neutrophilic inflammation
Hypoxia in wound healing
- Hypoxic conditions (e.g. due to disruption of vasculature) are important driving processes for wound repair
- Under normal oxygenation, a cytoplasmic molecule called hypoxia-inducible factor-α (HIF-α) is
hydroxylated (by prolyl hydroxylase) and destroyed by the ubiquitin pathway
- Under hypoxic conditions, prolyl hydroxylase activity is low, so HIF-α is not hydroxylated and destroyed
 HIF is a transcription factor that binds to a region of DNA to incite gene transcription
 Transcription factor EGR-1 (early growth response) is also activated
 Together, HIF-α and EGR-1 up-regulate processes that stimulate fibroblast and endothelial cell
proliferation and switch to anaerobic glycolytic metabolism
Disorders in wound healing
- Wound healing is impeded in a number of conditions
 Presence of persistent local infection
 Bacterial production of biofilms
 Excessive movement
 Systemic Vitamin C deficiency
 Persistent corticosteroid excess
 Impaired blood supply
 Old age of patient
- Contrastingly, excessive or inappropriate wound contraction (typically arises from genetic abnormalities in
control of fibrosis and remodeling) can result in disfigurement or functional impairment
Collagen
- Fibrillar and non-fibrillar types (different types predominate in different tissues)
- Fibrillar types (I, II, III, V, XI)
 Part of the extracellular matrix
 Consist of fibrils arranged in cross-linked triple-helixes
 Type I is the most common, comprising majority of collagen in skin and bone
- Non-fibrillar types (type IV)
 Part of basement membranes, among other things
- Collagen synthesis is a complex process
 Synthesis of numerous α-chains that are rich in glycine
 In the ER, proline and lysine components of α-chains undergo Vitamin C-dependent hydroxylation
 α-chains assemble into a triple-helix (procollagen) and are exported into the extracellular space
 Fibrils are clipped and cross-linked at lysine/hydroxylysine residues by lysyl oxidase (Cu++-dependent)
- Causes of inherited collagen defects
 Scurvy (primates, guinea pigs, bats)
o Vitamin C deficiency due to lack the hepatic enzyme (1-gulonolactone oxidase) essential for
conversion of glucose to ascorbic acid
 Cu++ deficiency (sheep)
Proteoglycans
- Produced by fibroblasts
- Consist of protein backbone with glycosaminoglycan (GAG) side chains
 GAG chains are negatively-charged, sulfated, and saccharide-rich
- Composition allows retention of copious amounts of water
 Allows tissues to remain hydrated, compressible, and flexible
 Excessive proteoglycans causes tissue to be mucinous (e.g. Shar Pei dogs)
 Proteoglycan deficiency results in tissue fragility (articular cartilage of degenerative joint disease)
- Examples are chondroitin sulfate, heparan sulfate, heparin, hyaluronate
Elastins
- Vital component of blood vessels (especially arteries), lung, uterus, skin, and tendons
- Consists of an elastin core surrounded by a network of fibrillin (glycoprotein)
 Can stretch to several times length and then recoil to original form
- Decreased, absent, or abnormal elastin can lead to aneurysm and dissection in elastic arteries
Glycoproteins
- Proteins that contribute adhesion to and communication between cells of the extracellular matrix
 Fibronectin
o Binds to collagen, fibrin, proteoglycans, and cellular adhesion molecules
o Tissue form plays role in wound healing, circulating form plays role in coagulation
 Laminin
o Dominant non-collagen component of basement membranes
o Binds to cells and ECM, assists in anchoring of epithelial cells
 Cadherins (“calcium-dependent adhesion protein”)
o Adhesion molecules that anchor cells to the extracellular matrix via catenin molecules
o Facilitate transmission of cellular information (about the environment) to the nucleus
o Components of intercellular bridges (zona adherens, demosomes)
o Disorders of these molecules and associated pathways can result in abnormal cell cycling,
unregulated cell growth, uncoupling from neighboring cells (may be involved in cancer)