Inflammation
Inflammation enables fluid, plasma proteins (such as complement), and inflammatory cells to leave blood arteries and reach the interstitial space.
Inflammation is generally categorized as acute or chronic.
Acute Inflammation
Acute inflammation is associated with neutrophils and edema.
Acute inflammation is a reaction to infection (to eradicate the organism) or tissue necrosis (to clear necrotic debris).
Acute inflammation has limited specificity.
Acute inflammation is an immediate reaction and is part of innate immunity.
Innate immunity includes:
- Epithelium
- Mucous
- Neutrophils
- Mast cells
- Macrophages
- Eosinophils
- Basophils
- Complement system
Acute Inflammation Mediators
Toll-like receptors (TLRs)
Toll-like receptors (TLRs) are special components found in the innate immune system cells (for example, macrophages and dendritic cells).
CD14 (a specific TLR) on macrophages is activated by pathogen-associated molecular patterns (PAMPs) that are often shared by microorganisms.
CD14 detects lipopolysaccharide (a PAMP) on the outer membrane of gram-negative bacteria.
When TLRs are activated, NF-KB, a nuclear transcription factor that activates immune response genes and causes the synthesis of several immunological mediators, is upregulated.
TLRs are also found on cells of adaptive immunity, such as lymphocytes (which are mainly associated with chronic infection).
TLRs are crucial in the regulation of acute and chronic inflammation.
Arachidonic acid (AA) metabolites
Phospholipase A2 releases arachidonic acid (AA) from the phospholipid cell membrane, and cyclooxygenase or 5-lipoxygenase subsequently reacts with it.
Prostaglandins are produced by cyclooxygenase (PG).
PGI2, PGD2, and PGE2 mediate vasodilation at the level of the arteriole and enhance vascular permeability.
The mediator of pain and fever is PGE2.
Leukotrienes are created by 5-lipoxygenase.
Neutrophils are drawn to and stimulated by leukotriene B4 (LTB4).
Note that neutrophils are attracted by four key mediators: LTB4, bacterial products, C5a, and interleukin 8 (IL-8)
Vasoconstriction, bronchospasm, and increased vascular permeability are mediated by LTC4, LTE4, and LTD4 (slow reacting compounds of anaphylaxis).
Mast cells
Mast cells are special immune cells with abundant granules.
Mast cells are distributed widely across connective tissue.
Mast cells are activated by:
- Tissue trauma
- Complement proteins C5a and C3a
- Antigen-induced cross-linking of cell-surface lgE
Activated mast cells immediately release of prefabricated histamine granules, which facilitate arteriole vasodilation and enhance vascular permeability at the post capillary venule.
The delayed reaction is characterized by synthesizing arachidonic acid metabolites, mainly leukotrienes which maintain the acute inflammatory response.
Complement
Complement refers to serum proteins that support inflammation and are proinflammatory
Complement circulates as dormant precursors.
Complement can be activated by three systems:
- Classical pathway
- Alternative pathway
- Mannose-binding lectin pathway (MBL)
Classical pathway activation of complement
- Cl binds IgG or IgM linked to the antigen
Alternative pathway activation of complement
- Complement is directly activated by microbial compounds
Mannose-binding lectin (MBL) pathway
- Mannose-binding lectin (MBL) binds the mannose that is present on the surface of microorganisms which activates complement
Complement activation eventually leads to:
- C3 convertase generation
- C5 convertase generation
- Formation of the membrane attack complex (MAC)
C3 convertase mediates C3 conversion to C3a and C3b.
C3b joins to assist in the production of C5 convertase.
C5 convertase mediates C5 conversion to C5a and C5b.
C5b combines with C6, C7, C8, and C9 (C6-C9) to produce the membrane attack complex (MAC).
The main products produced by the complement system include:
- C3a
- C5a
- C3b
- Membrane assault complex (MAC)
C3a triggers mast cell degranulation.
C5a triggers mast cell degranulation and recruits neutrophils chemotactically.
C3b serves as an opsonin for phagocytosis.
Membrane assault complex (MAC) punches holes in microbes which causes them to lyse.
Hageman factor (Factor XII)
The liver produces an Hageman factor (factor XII) as an inactive proinflammatory protein.
When Hageman factor (factor XII) is exposed to collagen (either subendothelial collagen or tissue collagen) it becomes activated.
Activated Hageman factor (factor XII) stimulates systems of coagulation and fibrinolysis.
Note that Hageman factor (factor XII) plays an important role in disseminated intravascular coagulation (DIC) (such as DIC caused by gram-negative sepsis).
Gram negative organisms can activate Hageman factor (factor XII).
Activated Hageman factor (factor XII) activates:
- Coagulation system
- Fibrinolytic system
- Kinin system
Coagulation system
The coagulation system is responsible for making blood clots.
Fibrinolytic system
The fibrinolytic system is responsible for removing blood clots.
The final step in the fibrinolytic system is the plasmin-mediated cleavage of fibrin which creates fibrin split products.
Kinin system
The kinin enzyme breaks down high-molecular-weight kininogen (HMWK) into bradykinin.
Bradykinin mediates pain and causes vasodilation and increased vascular permeability.
Signs of Inflammation
The main signs of inflammation include:
- Rubor (redness)
- Calor (warmth)
- Tumor (swelling)
- Dolor (pain)
Rubor (redness) due to histamine, PGE2, and bradykinin mediated vasodilation caused by relaxation of arteriolar smooth muscle.
Calor (warmth) due to histamine, PGE2, and bradykinin mediated vasodilation caused by relaxation of arteriolar smooth muscle.
Tumor (tumor) due tissue damage and histamine mediated leakage of plasma into the interstitial space from the postcapillary venules.
Dolor (pain) is due sensitization of sensory nerve endings mediated by PGE2, and bradykinin.
Note that acute inflammation may be associated with fever.
In response to pyrogens, macrophages release interleukin-1 (IL-1) and tumor necrosis factor (TNF).
Interleukin-1 (IL-1) and tumor necrosis factor (TNF) increase cyclooxygenase (COX) activity in perivascular cells within the area of the hypothalamus responsible for body temperature.
The increased COX activity results in increased PGE2 in the hypothalamus.
The excess PGE2 in the hypothalamus increases the set point temperature of the body, which causes fever.
Neutrophils Role In Acute Inflammation
Neutrophils are the key cells of acute inflammation.
Medical professionals must know the exact steps neutrophils take to mediate their effects.
The steps include:
- Migration
- Rolling
- Adhesion
- Transmigration and chemotaxis
- Phagocytosis
- Disposing of phagocytosed material
- Resolution
Step 1- Margination
In post capillary venules, vasodilation decreases blood flow.
Vasodilation makes it easy for neutrophils to stick to the vessel wall (opposed to being carried along with the blood flow).
Step 2-Rolling
The neutrophils roll on the endothelial cells (endothelial cells line blood vessels).
The endothelial cells amplify selectin.
Selectin acts like “speed bumps” on the endothelial cells that cause the neutrophils to slow down.
The selectins include:
- P-selectin
- E-selectin
Histamine is a mediator of P-selectin release from Weibel-Palade bodies.
Tumor necrosis factor (TNF) and interleukin 1 (IL-1) stimulate E-selectin.
Selectins bind leukocytes’ sialyl Lewis X.
The rolling of leukocytes along the vessel wall is the consequence of interaction.
Step 3-Adhesion
TNF and IL-1 upregulate ICAM and VCAM.
ICAM and VCAM are cellular adhesion molecules on the endothelium.
C5a and LTB4 increase the expression of integrins on leukocytes.
Leukocytes firmly adhere to the vessel wall due to CAM and integrin interaction.
Step 4-Transmigration and Chemotaxis
Bacterial byproducts such as IL-8, C5a, and LTB4 attract neutrophils.
Leukocytes travel in the direction of chemical attractants after crossing the endothelium of postcapillary venules.
Step 5-Phagocytosis
Opsonins (IgG and C3b) promote phagocytosis.
Leukocytes create pseudopods that extend to form phagosomes, which are then internalized and combined with lysosomes to form phagolysosomes.
Microorganisms or necrotic tissue are consumed.
Step 6: Disposing of the Phagocytosed Material
Phagocytosed material can be destroyed in two ways:
- Oxygen dependent mechanism
- Oxygen independent mechanism
The most efficient killing method is the oxygen dependent mechanism.
Oxygen dependent mechanism
The phagocytosed microorganisms are destroyed by the HOCl produced by the oxidative burst in phagolysosomes.
NADPH oxidase utilizes the oxidative burst reaction to convert O2 into O2–.
Superoxide dismutase (SOD) converts O2– into H2O2.
Myeloperoxidase (MPO) converts H2O2 into HOCl (bleach).
HOCl (bleach) is the main product that destroys the pathogen.
HOCl (beach) is used within the phagolysosomes in order to destroy the phagocytosed pathogen(s).
Oxygen independent mechanism
The oxygen independent mechanism is not as effective as the oxygen dependent mechanism.
The oxygen dependent mechanism occurs with enzymes that are in the secondary granules of leukocytes such as:
- Lysozyme
- Major basic protein
Step 7- Resolution
After the inflammatory stimulation has subsided, neutrophils turn into pus by going through apoptosis within the tissue and are no longer visible after 24 hours.
Key disorders in acute inflammation to be aware of:
Leukocyte adhesion deficiency
Chediak-Higashi syndrome
Chronic granulomatous disease
Myeloperoxidase (MPO) deficiency
Leukocyte adhesion deficiency
Leukocyte adhesion deficiency, an autosomal recessive defect in CD18 subunit of integrins.
Clinical findings of leukocyte adhesion deficiency include:
- Delayed separation of the umbilical cord
- Recurrent bacterial infections without pus formation
- Neutrophilia
Chediak-Higashi syndrome
Chediak-Higashi syndrome, an autosomal recessive protein trafficking defect, that causes impaired phagolysosome formation because of faulty microtubules.
Clinical findings of Chediak-Higashi syndrome include:
- Albinism (because melanocytes have abnormal microtubules to pass melanin to keratinocytes)
- Peripheral neuropathy (because abnormal microtubules cannot pass nutrients to distal neurons)
- Pyogenic infections
- Neutropenia
- Neutrophils with giant granules
- Abnormal primary hemostasis
Chronic granulomatous disease
Chronic granulomatous disease (CGD) is due to autosomal recessive or X-linked NADPH oxidase deficiency that results in poor oxygen dependent destruction of pathogens.
Clinical findings of Chronic granulomatous disease (CGD) include recurrent infections by catalase-positive organisms.
Examples of catalase positive organisms includes (SNAPS organisms):
- Staphylococcus aureus
- Nocardia
- Aspergillus
- Pseudomonas cepacia
- Serratia marcescens
The nitro blue tetrazolium (NBT) test is used to screen for chronic granulomatous disease (CGD):
- If NADPH oxidase can convert O2 into O2– (normal) the nitro blue tetrazolium (NBT) test will turn blue (signifying normal result and proper function)
- If NADPH oxidase can not convert O2 into O2– (abnormal) the nitro blue tetrazolium (NBT) test will remain colorless (signifying abnormal result and improper function)
Poor O2– dependent killing is a hallmark of chronic granulomatous disease (CGD).
Myeloperoxidase (MPO) deficiency
Myeloperoxidase (MPO) deficiency, which results in increased risk of fungal infections (i.e. Candida).
Myeloperoxidase (MPO) deficiency results in defective conversion of H2O2 into HOCl.
NBT test will be normal (turn blue) in these patients with myeloperoxidase (MPO) deficiency.
Macrophages
Macrophages are “giant eaters”.
Macrophages and monocytes are the cell cells, but they are called differently based on their location:
- Monocytes: In blood
- Macrophages: In tissue
Macrophages arrives in tissue by way of the process of margination, rolling, adhesion, and transmigration of phagocytose organisms (aided by opsonins) and then degrades phagocytosed material utilizing enzymes (such as lysozyme) in secondary granules (oxygen independent killing).
Two to three days after inflammation starts, macrophages reach their peak numbers.
IL-8 produced by macrophages attracts more neutrophils during prolonged pus production during ongoing acute inflammation.
Macrophages may also create anti-inflammatory cytokines like IL-10 and TGF-beta which stop the inflammatory process.
IL-10 and TGF-beta also promote wound healing.
Macrophages deliver antigen to activate CD4+ helper T cells, which release cytokines that support chronic inflammatory response.
Fibrosis is mediated by macrophages via fibrogenic growth factors and cytokines in an abscess, an acute inflammation surrounded by fibrosis.
Chronic Inflammation
Chronic inflammation characteristically has lymphocytes and plasma cells as the dominant cell types that are present in the tissue.
Chronic inflammation is a delayed reaction.
Chronic inflammation is more targeted, specific, and considered adaptive immunity.
Stimuli that cause chronic inflammation include:
- Chronic infection (the most frequent cause)
- Viral infection
- Autoimmune illness
- Foreign body material
- Mycobacterial infection
- Parasitic infection
- Helminth infection
- Fungal infection
- Certain malignancies
T Cells
Progenitor T lymphocytes are produced in the bone marrow.
T lymphocytes continue to grow in the thymus.
In the thymus, progenitor T cells undergo T-cell receptor (TCR) rearrangement to become CD4+ helper T cells or CD8+ cytotoxic T cells.
T cells employ the TCR complex (TCR and CD3) to monitor for antigens.
An antigen is recognized by the TCR complex and is displayed on MHC molecules:
- MHC class II bearing CD4+ T cells
- MHC class I bearing CD8+ T cells
Antigen/MHC complex binding and a second signal are necessary for activating T lymphocytes.
CD4+ helper T-cell activation
Antigen-presenting cells express the MHC class phagocytose process and present extracellular antigen (such as a foreign protein) (APCs).
A second activation signal is produced when CD28 on CD4+ helper T cells binds to B7 on antigen presenting cells (APCs).
Two subgroups of activated CD4+ helper T cells produce cytokines that assist with inflammation.
The two subgroups of CD4+ helper T cells include:
- TH1 CD4+ T cells that help CD8+ T cells
- TH2 CD4+ T cells that help CD4+ B cells
TH1 CD4+ T cells that help CD8+ T cells
- TH1 CD4+ T cells secrete IL-2
- Intracellular antigen is processed
- Intracellular antigen is presented on MHC class I
- Interferon-gamma (IFN-y are secreted by the subset (macrophage activator)
TH2 CD4+ T cells that help CD4+ B cells
The TH2 subset secretes:
- IL-4 which promotes B-cell maturation into plasma cells and class switching to IgE and IgG
- IL-5 which promotes the chemotaxis and activation of eosinophils
- IL-10 which inhibits the development of TH1 phenotype
Cytotoxic T-cell activation with CD8+
All nucleated cells and platelets express MHC class I.
MHC class I processes and presents intracellular antigen (derived from proteins in the cytoplasm).
From CD4+ T cells, IL-2 is released from the cell leading to activation.
Cytotoxic T cells are activated.
CD8+ T cell killing happens via perforin and granzyme.
Perforin makes holes in the target cell so granzyme may enter and trigger apoptosis by activating caspases.
FasL expression activates apoptosis by binding to Fas on target cells.
B Cells
Bone marrow produces immature B cells, which go through immunoglobulin rearrangements to become naïve B cells expressing surface IgM and Ig.
Receptor binding triggers the activation of B-cells.
B cell activation can happen three ways, which includes:
- IgM binding
- IgD binding
- MHC class II to CD4+ helper T cell binding
IgM binding
Surface IgM binds to an antigen on immature B cells.
The immature B cell matures into a plasma cell that secretes IgM.
IgD binding
Surface IgD binds to an antigen on immature B cells.
The immature B cell matures into a plasma cell that secretes IgD.
MHC class II to CD4+ helper T cell binding
Presentation of B-cell antigen by MHC class II to CD4+ helper T cells.
The helper T cell’s CD40L attaches to the CD40 receptor on the B cell, generating a second activation signal.
The helper T cell releases IL-4 and IL-5 which mediate B-cell hypermutation, isotype class switching, and maturation of immature b cells to become mature plasma cells.
Granulomatous Inflammation
Granulomatous inflammation is a subtype of chronic inflammation.
Granulomatous inflammation is commonly referred to as granulomas.
Granulomatous inflammation is characterized by epithelioid histiocytes.
Epithelioid histiocytes are macrophages with rich pink cytoplasm.
Granulomatous inflammation is frequently associated with large cells (giant cells) and a ring of lymphocytes.
Subtypes of granulomatous inflammation are classified as noncaseating or caseating.
The steps in the creation of a granuloma include:
- Macrophages process antigen and deliver it to CD4+ helper T cells through MHC class II
- When macrophages interact, they produce IL-12, which causes CD4+ helper T cells to develop into the TH1 subtype
- TH1 cells release IFN-y, which causes macrophages to differentiate into epithelioid histiocytes and giant cells
Caseating granulomas have central necrosis.
Noncaseating granulomas do not have central necrosis.
Common causes of caseating granulomatous inflammation include:
- Tuberculosis (TB)
- Fungal infection
Common causes of non-caseating granulomatous inflammation include:
- Sarcoidosis
- Crohn’s disease
- Cat scratch illness
- Beryllium exposure
- Foreign body material response
