Innate Immunity Flashcards
What are some early defenses of the innate immune system?
The innate immune system is critical to provide early defense against infectious agents.
It is comprised of fixed and induced defenses which include:
- barriers
- soluble factors (complement, chemokines, cytokines)
- immune system cells (neutrophils, macrophages, NK etc.)
- PRRs or antigen/antibody receptors to recognise microbes
Explain the defense mechanisms of barriers in the innate immune system.
Body surfaces provide the first line barriers to infection. Barriers to infection provided by body surfaces include:
- Physical barriers to prevent microbial entry
- Chemical barriers to control pathogen growth
- Biologic barriers in which normal commensal flora compete with pathogens
Epithelial cells are important in innate immunity. Aside from being a physical barrier, they also produce microbicidal & inhibitory molecules as well as a range of cytokines that facilitate immune system function:
- Eye: lysozyme, phospholipase A - Oral cavity: lysozyme, phospholipase A, histatins - Skin: lysozyme, RNA-ases, DNA-ases - Respiratory tract: beta-defensins - Stomach: acid pH, bile salts, fatty acids, lysopeptides - Gut: alpha-defensins (cryptdins)
How are commensal bacteria helpful in defense against infection?
Commensals/normal microbiota protect against infection directly by producing toxic metabolites and antibiotics, as well as by competing for binding sites on gastrointestinal epithelium. They also induce the production of antibacterial compounds by epithelial cells. Indirectly, they control the development and stimulation of the lymphoid tissues through interactions with PAMPs.
Explain complement.
Complement comprises many plasma proteins (10% total serum proteins), some of which are pro-enzymes (zymogens).
After a trigger provided by the pathogen the pro-enzymes are activated and react in a series of steps (cascade) to provide products that results in pathogen destruction.
Explain the three arms of the complement cascade.
- Activation (triggering) of complement proenzymes can be initiated by three pathways. All three pathways end in the common step; the proteolysis (activation) of C3 to C3a and C3b by a C3 convertase.
- C3b becomes covalently attached to microbial surface or antibodies bound to the microbial surface
- Bound C3b interacts with other molecules to form a C5convertase cleaving C5 to C5a and C5b
- Breakdown of C5 results in lysis and inflammation because the enzymes are produced at ach stage and the reaction accelerates and amplifies rapidly.
The alternative pathway: the results of spontaneous hydrolysis of C3 to C3a and C3b (tick over activation). C3b binds covalently to pathogen surfaces, but not host surfaces because of their charge and the presence of fluid phase and cell associated inhibitor molecules.
The classical pathway: Antigen bound IgM & IgG activate the C1complex. C1q is composed of six identical subunits, each with a globular head, which binds antibody and a collagen-like tail. Antigen/antibody interaction allows C1qbinding and activates C1r which becomes proteolytic activating C1s to begin the complement cascade. This then initiates a series of catalytic reactions which leads to the breakdown of C4, C2 and C3.
Large quantities of C3b now bind the pathogen surface, because one molecule of C3 convertase can cleave up to 1000 molecules of C3.
The mannan binding lectin (MBL) pathway: Mannan binding lectin is similar in structure to C1q, it attaches to repeating mannose and other sugars on microbes, captures MASP-2 and activates C4. This initiates a series of catalytic reactions which also leads to the breakdown of C2 and C3.
So….
Alternate, classical and lectin pathways all result in the organisms being coated with C3b and the production of C5 convertase.
Late steps of Complement activation - MAC: C5 is broken down by both alternate and classical pathway convertases to form C5a and C5b. (C5a is a powerful inflammatory mediator) Sequential reactions result in up to 15 molecules of C9 polymerising to form a pore, the membrane attack complex (MAC). The formation of the MAC is inhibited on host cell surfaces.
What are some of the physical and soluble components of the innate immune system?
The physical barriers and soluble components of the innate immune system can:
- prevent binding
- inhibit microbe growth
- kill the pathogen directly (MAC, microbicial compounds)
If the pathogen is able to avoid these earliest of responses, then resident macrophages and dendritic cells respond rapidly. They are found in a wide variety of locations and possess a wide variety of receptors.
Explain macrophages?
They are present in most tissues and are differentiated from blood monocytes into: Kupfer cells (liver), microglial cells (brain), histiocytes (connective tissues), osteoclasts (bone) and some dendritic cells.
Macrophages are phagocytic (not only for microbes but also apoptotic/necrotic cells)
They secrete many factors such as chemokines, inflammatory cytokines (IL-1, TNF-alpha) and can also release lysosomal contents making them cytotoxic, as well as acting as an antigen presenter to T cells.
They also act as sentinels (one of the fixed defenses) to defend against infection in tissues by multiple mechanisms.
Activated macrophages release many bioactive products:
What are some of the bioactive products that macrophages release?
Inflammatory cytokines (IL-1, TNF alpha, IL6) Antiviral cytokines (TNF alpha) Stimualtory cytokines (IL12) Suppressive cytokines (IL10, TGF beta) Prostaglandins Eicosanoids Enzymes for breaking down tissues lysozyme complement proteins Chemokines (CXCL8 to attract IL8)
Discuss some types of PRR and their associated PAMPs.
TLR1, TLR2, TLR6: peptidoglycan TLR3, RIG1: dsRNA TLR4: LPS TLR5: flagellin TLR7: ssRNA TLR9:CpGDNA NOD1: gram negative peptidoglycan MR: Mannose Inflammasomes: metabolic stress
Explain TLR activation & Signalling
Membrane bound receptors bind ligands of PRR using LRR extracellular domains.
Binding TLRs induces a diverse range of responses that regulate the production of inflammatory cytokines & chemokines as well as antimicrobial products and other molecules including costimulatory molecules & PRRs.
What are NOD like receptors and inflammasomes?
This is the largest family of cytosolic receptors, with more than 24 members in humans. They al have a ligand sensing LRR, oligomerising domains and an effector domain that mediates signal transduction.
Ligand binding (PAMPs/DAMPs) to some NLRs induce oligomerisation and the formation of high molecules weight complexes. -Oligomerisation results in the recruitment of inflammatory caspases which then become activated and cleave pro-IL1-beta to produce high levels of IL1 (also IL18, IL33). These multi-molecular IL1 activating machines are termed inflammasomes of which there are four kinds.
Explain the concept of phagocytosis.
Effective phagocytosis requires the microorganism to attach to the phagocyte via opsonic receptors (Mannose R, complement R, antibody receptor (FcR))
-opsonins are substances which coat microbes and for which receptors exist n phagocytes.
The microorganism must then be ingested and killed.
There are many mechanisms of killing including acidification, antimicrobial peptides (defensins, cations), enzymes (lysozyme), competitors (lactoferrin) or toxic nitrogen/oxygen intermediates.
What are the fixed/resident cells of the innate immune response?
Binding PAMPs leads to the activation of
- Resident tissue macrophages: binding by PAMPs, Antibody and C3b leads to chemokine and cytokine release and then phagocytosis
- Dendritic cells: Binding PAMPs leads to activation (includes chemokine and cytokine release) and initiation of the adaptive immunity through antigen presentation.
What are some cytotoxic cells of the innate immune response?
These also enter the infected site from the circulation and target infected or altered cells releasing granules whose contents are toxic. These include:
Natural Killer Cells:
Enter the tissues in response to chemotactic signals where they bind and kill any tumourous/infected/altered cells. They respond to changes in the self proteins (tumours/viruses down regulate or alter MHC class 1)
Eosinophils:
Important in parasite immunity and allergy
Macrophages:
As well as phagoctose, they release cytotoxic mediators.
In NK cells, What are the stimulatory and inhibitory signals involved?
Activation: NK receptor (may include Fc gamma R) called the FasL (ligand) binding antibody (Fas molecule; the expression of which is induced by IFN gamma) on the cell. Inhibition: MHC class 1 molecules binding the MHCR on the NK cell.
So if there is no MHC class 1 molecule, NK cells kill that cell because it is not a part of ‘self’
