Outline of Lecture
Leukocyte cellular events
Leukocytes leave the vasculature routinely through the following sequence of events:
Margination and rolling
Adhesion and transmigration
Chemotaxis and activation
They are then free to participate in:
Phagocytosis and degranulation
Leukocyte-induced tissue injury
Phagocytosis
1. Recognition & attachment
Opsonins (IgG and C3) coat target
2. Engulfment
Pseudopods flow around the particle to be
engulfed. Particle is engulfed and fuses with lysosome
3. Killing/degradation
O2 dep: Reactive O2 species in lysosomes
O2 indep: Bactericidal permeability agents, lysozyme, MBP, lactoferrin
Recognition and Binding
Opsonized by serum complement, immunoglobulin (C3b, Fc portion of IgG)
Corresponding receptors on leukocytes (FcR, CR1, 2, 3) leads to binding
Phagocytosis and Degranulation
Triggers oxidative burst, engulfment and formation of vacuole which fuses with lysosomal granule membrane (phagolysosome)
Granules discharge within phagolysosome and extracellularly (degranulation)
Oxidative Burst
Reactive oxygen species formed through oxidative burst that includes:
increased oxygen consumption
glycogenolysis
increased glucose oxidation
formation of superoxide ion
2O2 + NADPH ® 2O2-rad + NADP+ + H+ (NADPH oxidase)
O2-rad + 2H+ H2O2 (dismutase)
Reactive Oxygen Species
Hydrogen peroxide alone insufficient
MPO (azurophilic granules) converts hydrogen peroxide to HOCl– , an oxidant/antimicrobial agent (“bleach”)
Degradation and Clean-up
Reactive end-products only active within phagolysosome
Hydrogen peroxide broken down to water and oxygen by catalase
Dead microorganisms degraded by lysosomal acid hydrolases
Leukocyte-induced Tissue Injury
There is a cost, destructive enzymes may enter extracellular space when:
premature degranulation
frustrated phagocytosis (large, flat)
membranolytic substances (urate crystals)
persistent leukocyte activation (RA, emphysema)
Defects of Leukocyte Function
Defects of adhesion:
LFA-1 and Mac-1 (integrins) subunit defects lead to impaired adhesion (LAD-1)
recurrent bacterial infections with impaired wound healing
Defects of Leukocyte Function
Defects of adhesion:
absence of sialyl-Lewis X, and defect in E- and P-selectin sugar epitopes (LAD-2)
clinically milder than LAD-1, but recurrent bacterial infections
Defects of Leukocyte Function
Defects of chemotaxis/phagocytosis:
microtubule assembly defect leads to impaired locomotion and lysosomal degranulation
neutropenia, defective neutrophil degranulation, delayed microbial killing, recurrent bacterial infections
Defects of Leukocyte Function
Defects of microbicidal activity:
deficiency of NADPH oxidase that generates superoxide, therefore no oxygen-dependent killing mechanism (Chronic Granulomatous Disease)
recurrent bacterial infections
Chemical Mediators Of Inflammation
Chemical mediators
Plasma-derived:
Complement, kinins, coagulation factors
Many in “pro-form” requiring activation (enzymatic cleavage)
Cell-derived:
Preformed, sequestered and released (mast cell histamine)
Synthesized as needed (prostaglandin)
Mediators are tightly regulated, too many or too few cause many different diseases
Chemical mediators
May or may not utilize a specific cell surface receptor for activity
May also signal target cells to release other effector molecules that either amplify or inhibit initial response (regulation)
Are tightly regulated:
Quickly decay (AA metabolites), are inactivated enzymatically (kininase), or are scavenged (antioxidants)
Specific mediators
Vasoactive amines
Histamine: vasodilation and venular endothelial cell contraction, junctional widening; released by mast cells, basophils, platelets in response to injury (trauma, heat), immune reactions (IgE-mast cell FcR), anaphylatoxins (C3a, C5a fragments), cytokines (IL-1, IL-8), neuropeptides, leukocyte-derived histamine-releasing peptides
Specific mediators
Serotonin: vasodilatory effects similar to those of histamine; platelet dense-body granules; release triggered by platelet aggregation
Plasma proteases
Clotting system
Complement
Kinins
Fibrinolytic system
Clotting cascade
Cascade of plasma proteases
Hageman factor (factor XII)
Collagen, basement membrane, activated platelets converts XII to XIIa (active form)
Ultimately converts soluble fibrinogen to insoluble fibrin clot
Factor XIIa simultaneously activates the “brakes” through the fibrinolytic system to prevent continuous clot propagation
Kinin system
Leads to formation of bradykinin from cleavage of precursor (HMWK)
Vascular permeability
Arteriolar dilation
Non-vascular smooth muscle contraction (e.g., bronchial smooth muscle)
Causes pain
Rapidly inactivated (kininases)
Complement system
Components C1-C9 present in inactive form
Activated via classic (C1) or alternative (C3) pathways to generate MAC (C5 – C9) that punch holes in microbe membranes
In acute inflammation
Vasodilation, vascular permeability, mast cell degranulation (C3a, C5a)
Leukocyte chemotaxin, increases integrin avidity (C5a)
As an opsonin, increases phagocytosis (C3b, C3bi)
Complement system
Role in immunity
Membrane Attack Complex (MAC C5-9)
Punches a hole in bacterial membrane
If you have deficiencies in the MAC
you are predisposed to infections that are encapsulated
Neisseria, Strept, Hemoph, Listeria
Specific Mediators
Arachidonic acid metabolites (eicosanoids)
Prostaglandins and thromboxane: via cyclooxygenase pathway; cause vasodilation and prolong edema; but also protective (gastric mucosa); COX blocked by aspirin and NSAIDS
Specific Mediators
Leukotrienes: via lipoxygenase pathway; are chemotaxins, vasoconstrictors, cause increased vascular permeability, and bronchospasm
PAF (platelet activating factor)
Derived also from cell membrane phospholipid, causes vasodilation, increased vascular permeability, increases leukocyte adhesion (integrin conformation)
Specific Mediators
Cytokines and Chemokines
Protein cell products that act as a message to other cells, telling them how to behave.
IL-1, TNF-a and -, IFN- are especially important in inflammation.
Increase endothelial cell adhesion molecule expression, activation and aggregation of PMNs, etc., etc., etc.
Specific mediators
Nitric Oxide
short-acting soluble free-radical gas with many functions
Produced by endothelial cells, macrophages, causes:
Vascular smooth muscle relaxation and vasodilation
Kills microbes in activated macrophages
Counteracts platelet adhesion, aggregation, and degranulation
Specific mediators
Lysosomal components
Leak from PMNs and macrophages after demise, attempts at phagocytosis, etc.
Acid proteases (only active within lysosomes).
Neutral proteases such as elastase and collagenase are destructive in ECM.
Counteracted by serum and ECM anti-proteases.
More specific mediators
Oxygen derived free radicals
Released extracellularly from leucocytes
Dependent on NADPH oxydative system e.g Super oxide ,Hydrogen per oxide & OH ion important in inflammation.
Endothelial cell adhesion molecule expression, activation and aggregation of PMNs, etc., etc., etc.
More specific mediators
Neuropeptides
Similar to vasoactive amines/ecosanoids
play role in initiation and propagation of inflammation.
1. Substance P
2. Neurokinine A
Other Mediators
Response to hypoxia
Hypoxia induced factor 1 a
Response to necrotic cells
Uric acid
