Innate Immunity
Immunity has both specific and non-specific components. Nonspecific, or innate, immunity is the set of disease resistance mechanisms non- specific to a particular pathogen. It is the first line of defense against infection, is present from birth, is not more efficient on subsequent exposure to the same organism, and encompasses the notion that certain animals are naturally resistant to certain pathogens. Only if the pathogen fails to be cleared by innate mechanisms are the specific responses of adaptive immunity triggered.
Components of Innate Immunity
Innate immunity is comprised of several defensive barriers
 Anatomical barriers o Epithelial surfaces (e.g. skin and mucous membranes of the GI and respiratory tracts) o Microbes that penetrate body epithelium for the first time are met instantly by a cellular immunological barrier that can mount an innate immunological response
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Physiological barriers o Acid pH of the skin – inhibits most microbial growth o Mucous membrane secretions – mechanically and chemically clear pathogens
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Wash away potential invaders (saliva, tears)
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Entrap invaders and employ anti-microbial substances (mucus)
Phagocytes
 Primary function is to identify, ingest, and destroy microbes
 Macrophages o Mononuclear phagocytes o Mature continuously from circulating monocytes through blood and tissues o Conduct defense against microbes by means of surface receptors that recognize and bind to microbial surfaces, triggering phagocytosis and inducing secretion of biologically active molecules (e.g. cytokines, chemokines) that initiate inflammation
 Neutrophils o Polymorphonuclear leukocytes o Short-lived cells abundant in blood o Not present in normal, healthy tissues o Have granules that contain enzymes and other anti-microbial substances o Conduct defense against microbes by the same receptor-mediated process as macrophages
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Mast cells
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Particularly effective in cell recruitment because of their close association with blood vessels
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Alteration of vascular permeability provides an optimal environment for complement function
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Mediate several physiologic changes involved in early innate immune defense o Bronchoconstriction, increased GI motility, epithelial sloughing
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Involved in the production of AMPs
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After exposure to certain types of bacteria, mast cells release several molecules that enhance vascular permeability and expression of adhesion molecules through a variety of mechanisms o Histamine – up-regulates P-selectin, which mediates neutrophil adhesion and recruitment o Proteases – critical in recruitment of neutrophils and eosinophils to site of inflammation o TNF
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Natural killer (NK) cells
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Large, granular lymphocyte-like cell
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Involved in defense against intracellular microbes
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Produce immunomodulatory cytokines IFN-γ and TNF-α
 Possess both activation and inhibition receptors o Enables NK cells to distinguish healthy cells from infected ones
 Selectively kill cells that bear low levels of MHC class I molecules (trait that allows pathogens to interfere with the ability of MHC class I molecules to capture and display peptides to T cells)
 Mechanism of killing is the same as used by Tc cells o Cytotoxic granules are released onto the surface of bound target cells o Effector proteins within granules penetrate the cell membrane and induce apoptosis

Inflammation
 Reflects the effects of cytokines and other inflammatory mediators on local blood vessels
 Dilation and increased blood vessel permeability leads to increased local blood flow and fluid leakage o Accounts for traditional presence of heat, pain, redness, and swelling
 Cytokines have important effects on adhesive properties of endothelium o Chemokines attract leukocytes to sites of inflammation and infection o Cause leukocytes to stick to endothelial cells of blood vessels o Cause migration of leukocytes through endothelium into tissue
Extravasation
 Migration of immune cells from the blood stream into tissue
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Leukocytes undergo extravasation in a multi-step process mediated by adhesive interaction regulated by macrophage-derived cytokines and chemokines o Rolling
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Reversible binding of leukocytes to vascular endothelium via interactions between endothelial molecules and their carbohydrate ligands on leukocytes
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Interaction cannot anchor cells against shearing force of blood flow, so cells roll along endothelium, continually making and breaking contact o Activation
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Induction of endothelial cell adhesion molecules (CAMs) and their receptors on leukocytes by chemokine-like IL-8 results in tight bond between vessel and leukocyte o Adhesion
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Tight bonding arrests rolling of leukocytes o Transendothelial migration
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Cessation of rolling allows leukocytes to squeeze between endothelial cells
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Penetration of basement membrane occurs with aid of matrix metallo-proteases expressed at the cell surface
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After extravasation, leukocytes migrate along the concentration gradient of chemokines secreted by cells at the site of infection
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Toll pathway
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Receptors on effector cells signal the presence of pathogens
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Allows a way for innate immunity to provide early defense against pathogen attack and alert the adaptive immune system that pathogen invasion has begun
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Toll signaling alters gene expression to induce cytokine and chemokine production
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Activates co-stimulatory molecules (B7-1, B7-2) on macrophages and other APCs that are essential for the activation of CD4 T cells and initiation of adaptive immune responses o Activation of adaptive immunity depends on molecules induced by innate immune signaling
 Triggered through a family of evolutionarily conserved transmembrane receptors known as the Toll receptors or, in mammals, Toll-like receptors ( TLRs )
 Different TLR proteins respond to distinct pathogen molecules and can induce distinct signals o TLR-4 signals the presence of the LPS (constituent of gram-negative bacteria cell-wall) by associating with CD14, the macrophage receptor for LPS
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LPS is widely known to induce septic shock o TLR-2 signals the presence of a different set of microbial constituents, including the proteoglycans of gram-positive bacteria
Complement
 Set of plasma proteins that attack extracellular pathogens through a cascade of reactions o Terminal components of cascade create pores in the plasma membrane of the target
 Alternative pathway is triggered by almost any foreign substance
 Mannose-binding pathway is activated by terminal mannose residues commonly found as major components of pathogens o Complement activation by either of these pathways can lead to the formation of membrane attack complexes (MACs) at the pathogen surface, which results in pathogen lysis o Complement components can opsonize microbes to promote their phagocytosis (e.g. C3b) o Complement components can activate the inflammatory response (C3a, C4a, C5a)

Phagocyte Immune Response
Neutrophils and macrophages are the primary cell types of the inflammatory response
 Neutrophils are recruited to inflamed and infected tissue in large numbers o Have surface receptors for common microbial constituents o Principal cells that engulf and destroy invading microorganisms
 Influx of neutrophils is soon followed by influx of monocytes to site of inflammation o Monocytes rapidly differentiate into macrophages
In response to invading pathogens, local tissue macrophages can release cytokines that alert the immune system
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IL-1 and IL-6 o Act on the hypothalamus to induce fever o Act on the liver to induce rapid production of soluble proteins – acute phase proteins (APP)
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Mannose binding lectin (MBL) is an APP that activates complement
 TNF-α o Regulates and promotes both innate and adaptive immune responses o Activates vascular endothelium, making local blood vessels stickier and more permeable, promoting cell migration to the site of infection as well as the activation of phagocytes o Promotes platelet aggregation and clotting of local capillaries to limit the spread of infection
 IL-8 o Acts as a chemoattractant for neutrophils, basophils, and T cells
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Regulates, recruits, and directs neutrophil migration across blood vessel walls
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Activates and improves neutrophil killing properties
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IL-12 o Regulates and promotes both innate and adaptive immune responses o Promotes local NK cell activity o Stimulates IFN-γ release from both NK cells and antigen-specific T cells o Induces differentiation of CD4 cells into Th1 cells
Recognition of pathogens by phagocytes
 Macrophages and neutrophils recognize pathogens by means of cell surface receptors that discriminate between surface molecules of pathogens and those of the host o Macrophage mannose receptors o Scavenger receptors o Complement protein receptors o Toll-like receptors
Destruction of pathogens
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Phagocytosis is an active process that leads to death of the pathogen
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Bound pathogen is first surrounded by the phagocyte membrane then internalized in a membranebound vesicle called a phagosome, which becomes acidified
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Granules, or lysosomes, of macrophages and neutrophils contain enzymes, proteins, and peptides that mediate an intracellular antimicrobial response
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Phagosome fuses with lysosome to form a phagolysosome
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Toxic lysosomal contents are released and destroy the pathogen via two broad mechanisms
Oxygen-dependent killing
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Modification of organic molecules
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Interaction with and damage of targets within a microbe
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NADPH phagocyte oxidase (phox) pathway
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Generates superoxide anion (O
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Nitrous oxide synthase (iNOS) pathway
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) and other reactive oxygen species (ROS)
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Generates nitrous oxide radical (NO) and other reactive nitrogen species (RNS)
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Each ROS and RNS has a different reactivity, stability, compartmentalization, biological activity, and several
ROS and RNS species can be present simultaneously
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Accounts for the versatility of these molecules as antimicrobial effectors
It’s important to note that when neutrophils and macrophages are activated during inflammation, they can also injure normal host tissues by release of ROS, RNS, and other biomolecules

Oxygen-independent killing
More than 500 antimicrobial peptides (AMPs) have been discovered in recent years
 Found in all species (plants, insects, mollusks, amphibians fish, amphibians, birds, mammals)
 Tend to be found in areas of the body most likely to come in contact with pathogens o Skin, ear, eye, epithelial surfaces (tongue, trachea, lungs, gut)
 Abundant in cells and tissues involved in host defense against microbial infections
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Alteration in expression pattern of AMPs is associated with a variety of pathological processes
 Prevalent proteins within granules of phagocytes o Most prevalent protein species of neutrophils o Important constituents of paneth cells (intestine) and epithelial cells
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Consist of 12 to 100 amino acids (constitutively expressed or induced by microbes or their products)
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Participate in innate immunity by providing a rapid first-line defense against infection
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Inhibit growth of microbial pathogens (bacteria, fungi, protozoa, parasites, viruses)
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Divided into two major families o Both families of molecules act as natural antibiotics and signaling molecules that activate host cell processes involved in immune defense and tissue repair o Consist of small, cationic, amphipathic peptides
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Allows peptides to insert into cytoplasmic membrane of susceptible bacteria to form pores that damage and kill bacteria o Cathelicidins o Defensins – ‘carpet-wormhole’ model
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Have clusters of positively charged an hydrophobic amino acid side chains
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Electrostatic attractions pull peptide molecules into the membrane
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As peptide molecules accumulate in a 'carpet', the membrane is strained. The peptides transition into another arrangement that lowers the strain but results in the formation of membrane 'wormholes' (pores)
Antimicrobial activity and cytotoxicity results from this ability to make target membranes permeable
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Paneth cells
Defensin-rich cells found in the narrow pits of the small intestine
Degranulation is induced by bacterial entry into the intestinal lumen
Protease cleavage activates defensins
Neutrophil extracellular traps (NETs)
 Oxygen-independent method by which activated neutrophils entrap and kill pathogens
 Does not involve phagocytosis
 Formed when a resting neutrophil is activated, leading to the release of DNA, histones, and granule proteins. These molecules assemble into a web of extracellular fibers (NETs) that bind to both grampositive and gram-negative bacteria and display antibacterial activity o Cellular constituents of neutrophils are released via necrotic cell death or exocytosis of granule contents coupled with active extrusion of DNA o Elastase component of NETs can degrade bacterial virulence factors o Dispersal of NETs with deoxyribonuclease (DNase) eliminates their antimicrobial properties
 Thought to immobilize a high concentration of extracellular antimicrobial peptides
 By trapping and killing invading bacteria, they provide rapid control of infection