Learning Outcomes

▪Review basis, types and mechanisms of immune response (innate and adaptive)

▪ Explain defects in mechanisms of defense and review common infections

▪ Explain various mechanisms of alterations of immune response and provide examples

▪ Define inflammation and describe in details its mechanisms including the roles of blood vessels, blood cells, coagulation system, complement system, inflammatory mediators, platelets and endothelium

▪ Explain pathology of acute vs chronic inflammation

▪ Describe mechanisms and steps of wound healing and explain disorders of these mechanisms

▪ How do we defend against the 9 ways of cellular destruction? For example:

– Oxygen deprivation leads to impaired organelles

– Free radicals are generated all the time from processes such as catabolism and inflammation

Innate immunity (natural; native)

First line

▪ Physical barriers

– Skin integrity

– Mucous membranes lining the GI and resp tracts

– Cilia

▪ Chemical barriers

– Lysozymes in Saliva, tears, sweat, ear wax and low pH

– Natural antimicrobial substances

▪ Normal flora

Second line:

▪ Cellular response of innate immunity

– Natural killer (NK) cells (from lymphocyte)

– Phagocytes: Cells that engulf and digest (phagocytosis) other debris, cells and organisms

o Macrophages and neutrophils are phagocytes

o Monocytes are precursors for macrophages found in tissue

▪ Fever

▪ Inflammation

Inflammation

▪ The first response to injury

▪ A biological response of vascular tissue (blood vessels, cells in blood, endothelium) to harmful stimuli

▪ Causes

– Any of the 9 causes of injury previously discussed

– Ischemia, infection, mechanical damage, nutrient deprivation, extremes of temperature, radiation, etc

▪ Goals

– Prevent and limit injury and further damage

– Control the response the inflammatory response

– Initiate adaptive immune response (preparation for 3rd line of defense)

– Initiate healing

Pathophysiology of Inflammation

▪ Blood vessel dilation:

– Increases vascular permeability/leakage

– This is the role of the endothelium which acts to leak mediators to tissue

▪ White blood cell (WBC) response:

– WBCs leave the blood vessel and migrate to the site of injury and release mediators of inflammation

– WBCs exit the blood vessel through the processes of:

1. Margination (“Pavementing”): WBCs align along the walls of the blood vessels

2. Adherence of WBCs to the inner wall of the blood vessel (endothelium)

3. Emigration of cells from vessel through endothelial junctions (diapedesis)

• At sites of endothelial cell retraction (opening from dilation), the neutrophils exit the vessel by means of diapedesis (blood to tissue)

– Chemotaxis:

o Chemotaxis is when the

neutrophils detect chemotactic factors (released in response to injury) through cell surface receptors.

o This process guides neutrophils to migrate toward the site of injury, after which they become immobile

– Opsonization (by Antibodies and C3b from complement system)

o Increase adherence of phagocyte to the target cell, making it more readily and more efficiently engulfed by phagocytes

o Once bound, the target cell is digested by phagocytosis

▪ Release of mediators from inflammatory cells (mast cell = tissue bound basophil).

▪ Plasma protein release

Plasma Protein Systems

1. Complement system

2. Coagulation system

3. Kinin system

▪ Each system is composed of a group of proteins with a specific task

▪ Each system has a different task within inflammation

Common to all 3 systems

▪ Inactive enzymes (proenzymes or zymogen)

▪ Sequential activation

– First proenzyme is converted to an active enzyme

– Substrate of the activated enzyme becomes the next component in the cascade.

Complement System

▪ A group of circulating proteins that when activated, promotes phagocytosis, activates chemical mediators

▪ Ultimately forms a membrane attack complex (MAC – C5b, 6-9) that will bind to and kill bacteria

▪ The MAC can also activate other inflammatory components

▪ A major component of inflammation

▪ Can be activated by 3 pathways:

I. Classical:

– Activated by antibodies. They make C1 which can eventually make C3 and C5

II. Lectin:

– Activated by plasma proteins, especially MBL (mannose-

binding lectin = recognizes a sugar on bacteria and activates complement system)

III. Alternative:

– Activated by substances on the surface of infectious organisms. Uses factor B, factor D and properdin to form a complex that makes C3

▪ All 3 pathways lead to C3 activation and fragmentation

– C3a: Potent anaphylatoxin (induces degranulation of mast cells)

– C3b: Serve as opsonin for phagocytosis or proteolytically activate C5.

– C5a: Potent anaphylatoxin and chemotactic factor

– C5b: Activates MAC, which damages bacteria

▪ Complement proteins can be measured in serum. We usually look for C3a and C5a (active forms)

▪ **ANTIBODIES ARE NOT NEEDED TO START THE COMPLEMENT SYSTEM. SO, IT IS A PART OF BOTH INNATE AND ADAPTIVE IMMUNITY.

Coagulation (Clotting) System

▪ Forms fibrin meshwork at an injured or inflamed site.

▪ Fibrin strands contain platelets and trap RBCs and other cells

▪ Goal of fibrin: To stop bleeding and stop spread of pathogens (trap microbes)

– Fibrin provides a framework for repair/ healing

Formation of Fibrin

▪ A series of sequential events leading eventually to two main reactions to build fibrin:

1. Prothrombin (inactive) → thrombin

2. Thrombin activated fibrinogen → fibrin

Activation of the Coagulation System

▪ Two methods of activation:

1. Classical System, and

2. Cell Model Theory

Classical system

▪ Extrinsic and intrinsic activation

– Extrinsic system activated by reaction of Factor VII with tissue factor release from a damaged blood vessel

– Intrinsic system activated when Factor XII reacts with negatively charged subendothelial substances (exposed from irritation and damage within the blood vessel

▪ Both extrinsic and intrinsic systems converge at Factor X activating the common pathway, leading to fibrin formation (as explained above)

Diagnostic testing for classical system:

▪ Prothrombin (PT; INR): Tests EXTRINSIC system activated by TISSUE DAMAGE (tissue factor)

▪ Thromboplastic times (PTT): Tests INTRINSIC system activated by CONTACT (endothelial

damage)

▪ Clotting factor assays

Cell model theory

▪ Initiation, activation, amplification taking place on the surface of any TF expressing cell

Kinin-Kallikrein System

▪ Roles of kinin-kallikrein system:

– Inflammation, blood pressure control, coagulation, and pain

▪ Kininogens are activated by kallikrein

Bradykinin

▪ Primary kinin

– Causes vasodilation

– Acts with prostaglandins to induce pain

– Causes smooth muscle contraction

– Increases vascular permeability

▪ Bradykinin functions similarly to histamine and increases vascular permeability, it also stimulates nerve endings to cause pain

▪ Effects:

– Mediates pain

– Increases permeability (vasodilator)

– Smooth muscle contractions

– Stimulates leukocyte chemotaxis

▪ Relationship of Kinin-Kallikrein system to coagulation (clotting) system:

– Factor XII (of clotting system) stimulates Prekallikrein and kallikrein

o Hence, another name for Factor XIIa is Prekallikrein activator

– The XIIa produced by the clotting system can also be activated by Kallikrein of kinin system

– Prekallikrein is enzymatically converted to kininogen, which activates bradykinin

Cellular Mediators of Inflammation

Cell types

▪ Granulocytes: Contain proteins – some are mediators of inflammation

– Neutrophils: phagocytic

WBCs and the predominant cell which kill bacteria in acute inflammation

– Eosinophils: help regulate the inflammatory response (limit and control)

– Basophils: release histamine in allergy and parasitic infestation

▪ Monocytes/macrophages

– Monocytes: phagocytic WBCs

o Monocytes are produced in the bone marrow, enter the circulation

o Can migrate to the inflammatory site where they phagocytose microbes

o Can develop into tissue macrophages

– Macrophages: mature form in the tissue

– Both monocytes and macrophages can:

o Promote angiogenesis

o Release cytokines and growth factors that promote epithelial cell division

o Activate fibroblasts

o Promote the synthesis of extracellular matrix and collagen formation (healing)

▪ Lymphocytes:

– T cells mediate cell mediated immunity and B mediate antibody mediated immunity during inflmamatin. T cells support the proliferative phase of wound healing

▪ Mast cells (located in connective tissue):

– A basophil in the tissue

▪ Platelets: Coagulation cascade takes place on its surface

Cell Products (cytokines)

▪ Cytokines are small proteins (5-20kDa)

– Known as primary inflammatory mediators

– Released by inflammatory cells and affect the behaviour of other cells

– Important in cell signalling

– Can be proinflammatory and anti-inflammatory

▪ Lymphokines = cytokines from lymphocytes

▪ Monokines = cytokines from monocytes

▪ Chemokines = chemotactic and primarily attract leukocytes to sites of inflammation

– Can be made from macrophages, fibroblasts and endothelial cells

– Most act locally over a short distance, but some such as the IL-1 has systemic induction (long distance), form an endogenous pyrogen

Types of Cytokines

1. Interleukins (IL)

2. Interferons (IFN)

3. Tumor necrosis factor alpha (TNFa)

4. Chemokines (much smaller; 8-10 kDa)

1. Interleukins

▪ Produced by macrophages and lymphocytes

▪ Produced in response to a pathogen or stimulation by other products of inflammation.

▪ Many types (1-36), and the list keeps growing

▪ They manage inflammation in different ways:

a) IL-1: pro-inflammatory

– Produced in 2 forms: alpha and beta (mainly from macrophages)

– Activates monocytes, macrophages and lymphocytes, which enhances both innate and acquired immunity, and acts as a growth factor for many cells

– Increases:

o The number of neutrophils in circulation (proliferation)

o Neutrophils attraction to inflammation site (chemotaxis)

o Cellular respiration and lysosomal enzyme activity (cellular killing)

– An endogenous pyrogen (causes fever) that reacts with hypothalamus to raise temp

b) IL-10: anti-inflammatory

– Primarily produced by lymphocytes

– Suppresses the growth of other lymphocytes and the production of proinflammatory cytokines by macrophages, leading to anti-inflammatory effects

2. Interferons

▪ Related to interference with viral infections

▪ Produced by:

– Virally infected cells

– Other cells in response to bacterial, parasites and tumors

▪ Activate immune cells; NK and macrophages

▪ Types:

a) IFN alpha/beta:

– Type 1 interferon

– Induce production of antiviral proteins

– These are produced and released by virally infected cells

– DO NOT kill the virus directly but instead and induce antiviral proteins and protect neighboring healthy cells

b) IFN gamma:

– Type 2 interferon

– Produced by lymphocytes and activates macrophages

– Kills infectious agents (viruses and bacteria) and enhances the development of acquired immune response against viruses

3. Tumor Necrosis Factor-Alpha

– Secreted by macrophages (mainly) in response to microbe recognition

– Secreted by other cells:

o CD4+ lymphocytes

o NK cells

o Neutrophils

o Mast cells

o Endothelium

o Synoviocytes

o Hepatocytes

– Plays a key role in the regulation of inflammatory/immune cell function

– Major effects:

o Induces fever by acting as endogenous pyrogens

o Regulates gene expression

o Stimulates synthesis of inflammatory mediators

o Inhibits tumorigenesis and viral replication

o Induces apoptosis

o Causes muscle wasting (cachexia)

o Stimulates intravascular thrombosis

▪ Synthesised and released by:

– Macrophages

– Fibroblasts

– Endothelial cells

▪ Function:

– Guide the migration to the worst site of inflammation

– Recruit inflammatory cell (e.g. Leukocytes) to the site of inflammation

– Direct lymphocytes to lymph node

▪ Roles:

– Mast cells are the most important cellular activator of the inflammatory response

– Filled with granules and located in the loose connective tissues close to blood vessels near the body’s outer surface

▪ Major Effects:

– Cause temporary, rapid constriction of smooth muscle and dilation of the postcapillary venules → Increased blood flow into the microcirculation

– Also causes increased vascular permeability resulting from retraction of endothelial cells lining the capillaries and increased adherence of leukocytes to the endothelium