Physiology of Inflammation and Innate Immunity สรช ย ศร ส มะ พ.บ., Ph.D. ภาคว ชาสร รว ทยา คณะแพทยศาสตร ศ ร ราชพยาบาล มหาว ทยาล ยมห ดล
Overview Pathogens infectious agents capable of causing pathology, are composed of 4 main groups; bacteria, virus, fungus, parasites Invasion by microorganisms are initially encountered by innate defense mechanisms that preexist in all individuals and act within minutes of infection Only when the innate host defenses are bypassed, evaded, or overwhelmed, an induced or adaptive immune response is required
Defense against Pathogens
The Epithelial Surfaces are the first defenses against infection Skin, mucosal epithelia as physical barrier Mucosal epithelial secrete a viscous fluid, mucus, which contains many glycoproteins called mucin Microorganisms coated in mucus may be prevented from adhering to the epithelium In respiratory tract, microorganisms can be expelled in the flow of mucus driven by the beating of epithelial cilia mucociliary escalator In the gut, peristalsis is an important mechanism for keeping both food and infectious agents moving through
The Epithelial Surfaces are the first defenses against infection
The Epithelial Surfaces are the first defenses against infection Surface epithelia can produce chemical substances that are microbicidal or inhibit microbial growth Examples: Antibacterial enzyme lysozyme in tears and saliva Acid ph of the stomach and the digestive enzymes of the upper GI tract Paneth cells in base of the crypts in the small intestine secrete antibacterial and antifungal peptide called cryptidins or a-defensins Surfactant protein (SP)-A and D bathing epithelial surfaces of the lung can bind to and coat the surfaces of pathogens so they are more easily phagocytosed by macrophages that have left the subepithelial tissues to enter the alveoli Coating of a particle with proteins that facilitate its phagocytosis is known as opsonization
Stages of Responses to Infection/ Inflammation Infectious disease occurs when a microorganism succeeds in evading or overwhelming innate host defenses The degree of infection/inflammation may be local initially or spread through the lymphatics or bloodstream Pathogen spread is often countered by an inflammatory response that recruits more effector molecules and cells of the innate immune system from local blood vessels Blood clotting system is induced so that pathogens cannot spread out Induced responses of innate immunity act over several days An adaptive immune response gets underway in response to pathogen antigens delivered to local lymphoid tissue
Major Events in Local Inflammatory Response
Stages of Responses to Infection/ Inflammation
Tissue Damage Initiates an Inflammatory Response Inflammation is initiated by the response of macrophages to pathogens Inflammatory responses are operationally characterized by pain, redness, heat, and swelling at the site of an infection, reflecting three types of change in the local blood vessels Inflammation is traditionally defined by the four Latin words Calor = heat Dolor = pain Rubor = redness Tumor = swelling All reflect the effects of cytokines and other inflammatory mediators on the local blood vessels
Acute Inflammatory Response: Vasodilation The first is an increase in vascular diameter, leading to increased local blood flow, hence the heat and redness and a reduction in the velocity of blood flow, especially along the surfaces of small blood vessels
Acute Inflammatory Response : Extravasation The second change is that the endothelial cells lining the blood vessel are activated to express adhesion molecules that promote the binding of circulating leukocytes The combination of slowed blood flow and induced adhesion molecules allows leukocytes to attach to the endothelium and migrate into the tissues, a process known as extravasation All these changes are initiated by the cytokines produced by activated macrophages
Acute Inflammatory Response : Extravasation Once inflammation has begun, the first cells attracted to the site of infection are generally neutrophils They are followed by monocytes, which differentiate into more tissue macrophages In the later stages of inflammation, other leukocytes such as eosinophils and lymphocytes also enter the infected site.
Extravasation of Leukocyte into Tissue
Multisteps of Leukocyte Extravasation
Multisteps of Leukocyte Extravasation 1 st step: reversible binding of leukocytes rolling to endothelium through interactions between selectins (on endothelium) and carbohydrate ligands (on leukocytes) 2 nd step: stronger binding interaction through interactions between ICAM (on endothelium) and integrin receptors (on leukocytes)
Multisteps of Leukocyte Extravasation 3 rd step: diapedesis from tight binding among these molecules allows the leukocyte to squeeze between the endothelial cells (extravasation) for migration towards chemoattractants Adhesion between molecules of CD31, expressed on both leukocyte and endothelium contributes to extravasation Matrix metalloprotease enzyme at cell surface of leukocyte aids in traversing the basement membrane 4 th step: leukocytes migrate along a concentration gradient of chemokines secreted by cells at the site of inflammation
Adhesion Molecules in Leukocyte Interactions
Phagocyte Adhesion to Vascular Endothelium Vascular endothelium, when it is activated by inflammatory mediators, expresses two adhesion molecules, ICAM-1 and 2 These are ligands for integrins expressed by phagocytes A defect in the β2 chain in the disease leukocyte adhesion deficiency leads to recurrent bacterial infections and impaired healing of wounds
Acute Inflammatory Response : Increased Vascular Permeability The third major change in the local blood vessels is an increase in vascular permeability Instead of being tightly joined together, the endothelial cells lining the blood vessel walls become separated, leading to exit of fluid and proteins from the blood and their local accumulation in the tissue This accounts for the swelling, or edema, and pain
Tissue Damage Initiates an Inflammatory Response These changes are induced by a variety of inflammatory mediators released as a consequence of the recognition of pathogens These include the lipid mediators of inflammation prostaglandins, leukotrienes and platelet-activating factor (PAF) which are rapidly produced by macrophages through enzymatic pathways that degrade membrane phospholipids Cytokines and chemokines (chemoattractant cytokines) are synthesized and secreted by macrophages in response to pathogens The cytokine tumor necrosis factor-α (TNF-α), for example, is a potent activator of endothelial cells.
Pathogens and Phagocytes A microorganism crossing an epithelial barrier is immediately recognized by the mononuclear phagocytes or macrophages Macrophages, residing in tissues, mature continuously from circulating monocytes Lung alveolar and interstitial macrophage Hepatic sinusoids Kupffer cells Neutrophils, second major family of phagocytes or polymorphonuclear leukocytes, are short-lived cells that are abundant in the blood but are not present in normal, healthy tissues Both macrophages and neutrophils can recognize, ingest and destroy many pathogens without the aid of an adaptive immune response Macrophages are the first to encounter pathogens in the tissues, but they are soon reinforced by the recruitment of large number of neutrophils to sites of infection Macrophages and neutrophils recognize pathogens by means of cell-surface receptors that can discriminate between the surface molecules displayed by pathogens and those of the host.
Phagocytes bear several different receptors that recognize microbial components and induce phagocytosis
Phagocytosis
Pathogens and Phagocytes Pathogens can also interact with macrophages and neutrophils through receptors for complement borne on these cells Ligation of many of the cell-surface receptors that recognize pathogens leads to phagocytosis of the pathogen, followed by its death inside the phagocyte Phagocytosis is an active process, in which the bound pathogen is first surrounded by the phagocyte membrane and then internalized in a membranebounded vesicle known as a phagosome, which becomes acidified Lysosomes in macrophages/neutrophils contain enzymes, proteins, and peptides that can mediate an intracellular antimicrobial response The phagosome fuses with one or more lysosomes to generate a phagolysosome in which the lysosomal contents are released to destroy the pathogen
Pathogens and Phagocytes Upon phagocytosis, macrophages and neutrophils also produce a variety of other toxic products that help kill the engulfed microorganism The most important of these are hydrogen peroxide (H 2 O 2 ), the superoxide anion (O - 2 ), and nitric oxide (NO), which are directly toxic to bacteria and are generated by lysosomal NADPH oxidases and other enzymes in a process known as the respiratory burst Neutrophils are short-lived cells, dying soon after they have accomplished a round of phagocytosis Dead and dying neutrophils are a major component of the pus that forms in some infections Macrophages are long-lived and continue to generate new lysosomes
Pathogens and Phagocytes Activation of macrophages to release cytokines and other mediators that set up a state of inflammation in the tissue and bring neutrophils and plasma proteins to the site of infection The cytokines released by macrophages make an important contribution both to local inflammation and to other induced but nonadaptive responses that occur in the first few days of a new infection.
Cytokines Secreted by Macrophages
Macrophage During Inflammation inos inducible nitric oxide synthase LPS lipopolysaccharide LTA lipotechoic acid PGN peptidoglycan TNF tumor necrosis factor MHC major histocompatibility complex
Cytokines Released by Phagocytes Activates Acute-Phase Response TNF-α, IL-1, IL-6 are endogenous pyrogens increase body temperature They can induce a shift in the proteins, acutephase proteins secreted by liver to blood plasma Acute phase response proteins are composed of C-reactive protein, mannan binding lectin, SP-A, SP-D These proteins have broad specificity for pathogen-associated molecular patterns and act as opsonin
The Acute-Phase Response Produces Molecules that Bind Pathogens but not Host Cells
Neutrophil Delivers Multiple Antimicrobial Molecules MMP matrix metalloprotease PHOX phagocyte oxidase
Neutrophil Delivers Multiple Antimicrobial Molecules ROI reactive oxygen intermediates
Cytokines Cytokines are small proteins (~25 kda) that are released by various cells in the body, usually in response to an activating stimulus, and induce responses through binding to specific receptors Cytokines can act in an autocrine, paracrine or endocrine manner The cytokines secreted by macrophages in response to pathogens are a structurally diverse group of molecules and include IL-1, IL-6, IL-12, TNF-α, and the chemokine interleukin-8
Chemokines Chemokines are a class of cytokines that have chemoattractant properties, inducing cells with the appropriate receptors to migrate toward the source of the chemokine Chemokines function mainly as chemoattractants for leukocytes, recruiting monocytes, neutrophils, and other effector cells from the blood to sites of infection
Injury to Blood Vessels Triggers Two Enzyme Cascades The injury to blood vessels immediately triggers two other protective enzyme cascades The kinin system The coagulation system
Injury to Blood Vessels Triggers Two Enzyme Cascades The kinin system is an enzymatic cascade of plasma proteins that is triggered by tissue damage to produce several inflammatory mediators, including the vasoactive peptide bradykinin This causes an increase in vascular permeability that promotes the influx of plasma proteins to the site of tissue injury It also causes pain, which, although unpleasant to the victim, draws attention to the problem and leads to immobilization of the affected part of the body, which helps to limit the spread of any infectious agents. Activation of the coagulation system leads to the formation of a clot, which prevents any microorganisms from entering the bloodstream
Innate Immunity is at the Intersection of Several Pathways TLR Toll-like receptor NOD nucleotidebinding oligomerization domain Sterile inflammation is driven by ligands derived from damage cells (heat-shock proteins, oxidized lipids, oxygen/nitration radicals; all of which might be TLR ligands)