Lecture 4 + 5 (Innate I, II & III) Flashcards
Through which surfaces do pathogens typically enter the body?
Pathogens enter through mucosal and epithelial surfaces (e.g., skin).
What must a pathogen do to cause an infection?
It must breach one of the anatomical barriers to enter the body.
How do airborne microorganisms enter the body?
They enter through the lining of the respiratory tract.
What is the first barrier against infection in innate immunity? What are examples of epithelial barriers?
1) Epithelial surfaces of the body.
2) Skin, gut epithelium, respiratory epithelium, and mucosal membranes.
What physical elements contribute to innate immunity besides epithelial barriers?
Saliva, hair, mucus, and tears.
What protective substances are produced by epithelial layers?
- acidic pH
- antimicrobial peptides (e.g., defensins)
What are the key cellular elements in innate immunity?
- Neutrophils and other granulocytes
- Monocytes and macrophages (e.g., Kupffer cells, microglia, intraglomerular mesangial cells)
- Dendritic cells (DCs)
- Natural Killer (NK) cells and other innate lymphoid cells (ILCs)
What are the two main types of dendritic cells (DCs)?
1) Immature DCs
2) Mature DCs
- conventional DCs
- plasmacytoid DCs
What are examples of tissue-specific macrophage variants?
1) Kupffer cells (liver)
2) Microglia (brain)
3) Intraglomerular mesangial cells (kidney)
What are three key molecules involved in innate immunity?
1) Antimicrobial enzymes (e.g., lysozyme)
2) Antimicrobial peptides (e.g., defensins)
3) Complement
What is the function of lysozyme in innate immunity?
Lysozyme digests peptidoglycan in bacterial cell walls.
How do defensins contribute to innate immunity?
Defensins disrupt microbial cell membranes.
What is the role of the complement system in innate immunity?
The complement system enhances pathogen clearance through opsonization, inflammation, and lysis.
What are the key stages of an infection and the immune response?
1) Pathogen adhesion: Pathogens adhere to the epithelium.
2) Local infection, epithelial penetration: Pathogens penetrate the epithelium and cause local infection.
3) Local tissue infection: The infection spreads to tissues.
4) Adaptive immunity: Specific immune responses clear the infection.
What mechanisms provide initial protection at the epithelium?
1) Normal flora: Competes with pathogens.
2) Local chemical factors: Inhibit pathogen growth.
3) Phagocytes: Especially active in the lungs.
How does the body respond to local infections after epithelial penetration?
1) Wound healing induces antimicrobial proteins and peptides.
2) Phagocytes and complement destroy invading organisms.
What key molecules and cells are involved in local tissue infection response?
1) Complement, cytokines, and chemokines.
2) Phagocytes and NK cells.
3) Activation of macrophages.
4) Migration of dendritic cells to lymph nodes to initiate adaptive immunity.
How is the infection cleared during adaptive immunity?
- Specific antibody production.
- T-cell-dependent macrophage activation.
- Cytotoxic T cells destroy infected cells.
What helps limit the spread of infection?
Blood clotting plays a crucial role in containing infection locally.
What is phagocytosis?
Phagocytosis is the engulfment and internalization of pathogens or their components after they bind to receptors on the surface of phagocytes.
Which cells are responsible for phagocytosis?
- macrophages
- granulocytes (neutrophils)
- dendritic cells
What are the outcomes of phagocytosis?
1) Removal and killing of pathogens.
2) Clearing debris (PRRs recognize DAMPs).
3)Generation of peptides for presentation to T cells.
What type of receptors mediate phagocytosis?
Many receptors involved in phagocytosis are pattern recognition receptors (PRRs), though not all PRRs induce phagocytosis.
How can phagocytosis occur indirectly?
- Phagocytes recognize soluble proteins (opsonins) bound to microbial surfaces, a process called opsonization.
–> Examples of opsonins: Antibodies and complement proteins.
How are opsonins recognized by phagocytes?
Opsonins are recognized by opsonin receptors on phagocytes, enhancing the efficiency of phagocytosis.
What does opsonization do to pathogens?
Opsonization makes pathogens more “tasty” to phagocytes, promoting their engulfment and removal.
What initiates phagocytosis?
Phagocytosis begins when receptors on phagocytes interact with ligands or pathogens.
What happens after receptors bind to the pathogen?
Membrane protrusions called pseudopodia extend to engulf the pathogen.
What is the phagosome?
The phagosome is a large, membrane-enclosed endocytic vesicle formed when the pathogen is internalized.
What happens after the phagosome forms?
The phagosome fuses with one or more lysosomes, forming a phagolysosome.
How does the phagolysosome kill and digest pathogens?
- The phagolysosome acidifies.
- Acquires antimicrobial peptides and enzymes.
- Kills and digests the pathogen using low pH-activated lysosomal enzymes.
What are the main steps of phagocytosis?
1) Pathogen binds to PRRs on pseudopodia.
2) Pathogen is ingested, forming a phagosome.
3) Phagosome fuses with lysosome, forming a phagolysosome.
4) Pathogen is killed and digested by lysosomal enzymes at low pH.
What types of granules do neutrophils contain?
Neutrophils contain two types of cytoplasmic granules:
- Primary granules
- Secondary granules
What happens when granules fuse with phagosomes?
- The granules fuse with phagosomes (forming phagolysosomes), releasing additional enzymes and antimicrobial peptides that attack the microorganism.
What do the enzymes and antimicrobial peptides in granules do?
They attack and help kill the internalized microorganism within the phagolysosome.
What triggers the process of pathogen killing during phagocytosis?
Phagocytosis/ligand binding triggers signaling that leads to a change in the cell, enabling pathogen destruction.
What products inside the phagolysosome contribute to pathogen killing?
- Low pH/acidification
- Hydrolytic enzymes (e.g., lysozymes and proteases)
- Oxidative attack using reactive oxygen species (ROS) and reactive nitrogen species (RNS)
- Antimicrobial peptides (e.g., defensins and cathelicidin)
How does oxidative attack kill phagocytosed pathogens?
- ROS (Reactive Oxygen Species) damage microbial membranes and intracellular components.
- ROS are generated by the NADPH oxidase enzyme complex in phagocytes (phagosome NADPH oxidase).
What happens during the “respiratory burst” in phagocytes?
ROS production by NADPH oxidase increases oxygen consumption, resulting in a respiratory burst.
What role does the phagolysosome play in innate immunity?
- Pathogen killing
- Pathogen processing
- Pathogen presentation to cytosolic PRRs (e.g., TLRs, NODs)
What role does the phagolysosome play in adaptive immunity?
- Antigen degradation
- Antigen processing
- Antigen presentation onto MHC molecules
How does phagocytosis contribute to cell clearance?
Phagocytosis clears cells that have undergone apoptosis (cell death).
What are DAMPs and how do they assist phagocytosis?
- DAMPs (Damage-Associated Molecular Patterns) are expressed by dead or dying cells.
- DAMPs bind to PRRs on phagocytes, signaling for the cell to be cleared (“eat me”).
What is the role of neutrophils in phagocytosis?
Neutrophils are capable of ingesting and killing microorganisms.
Are neutrophils tissue-resident?
No, neutrophils are not tissue-resident. They are recruited to the site of infection.
What causes pus formation?
Pus is composed of dead and dying neutrophils.
What are NETs and what do they do?
- NETs (Neutrophil Extracellular Traps) are produced by 20-60% of neutrophils.
- They trap microorganisms and help prevent their spread.
What is the role of CNS-resident microglia in homeostasis?
Microglia, also known as the “macrophages of the brain,” are responsible for:
Establishing proper neuronal connections
Debris clean-up
Brain development
Memory and learning
What is the role of microglia in multiple sclerosis (MS)?
- MS is a complex inflammatory disease with myelin breakdown and toxic debris in brain lesions.
- Microglia and peripheral infiltrating macrophages participate in debris clean-up to aid in the repair of CNS tissue.
How do microglia recognize and clear debris during tissue injury?
In animal models of tissue injury (e.g., laser-induced), microglia recognize cell debris via receptors, which initiate phagocytosis by forming pseudopodia.
Why is receptor binding important for microglial phagocytosis?
Receptor binding is crucial for initiating phagocytosis of cell debris, enabling effective clean-up and tissue repair.
What is the complement system?
A group of soluble proteins that work with the innate and adaptive immune systems to eliminate pathogens, dying cells, and immune complexes.
Where are most complement proteins produced?
In the liver.
What type of enzymes are most complement proteins?
Proteases that perform proteolysis (break down proteins).
How are most complement proteins named?
“C” followed by a number (e.g., C3a).
How are some complement proteins named differently?
As “factor” followed by a capital letter (e.g., factor B).
Are complement proteins numbered in order of function or discovery?
Based on when they were discovered.
What are the three key actions of the complement system?
(1) Increasing vascular permeability and chemotaxis (inflammation).
(2) Destroying pathogen cell membranes.
(3) Opsonization (increasing pathogen recognition and facilitating phagocytosis).
What does opsonization mean?
Increasing recognition of pathogens and making it easier for phagocytes to engulf them.
The coating of a pathogen’s surface by antibodies and/or complement proteins, making it easier for phagocytes to ingest.
What is phagocytosis?
The process where cells engulf particulate matter by surrounding it with the cell membrane, forming an intracellular vesicle called a phagosome that contains the ingested material.
What is the vesicle formed during phagocytosis called?
A phagosome.
How are complement components before activation?
They are inactive pro-proteases.
What are the three pathways of complement activation?
1) Classical pathway
2) Alternative pathway
3) Lectin pathway
What happens after activation?
Complement acts as a cascade, with proteolytic cleavage generating two fragments.
What are the two fragments generated during proteolytic cleavage?
(1) Small fragment (e.g., C5a):
Identified by the letter “a.”
Has a specific function (e.g., signaling).
(2) Large fragment (e.g., C5b):
Identified by the letter “b.”
Has proteolytic activity on a new substrate.
What is formed by complement fragments during activation?
Both fragments contribute to forming C3 convertase, a key enzyme in the complement cascade.
What is the functional state of complement proteins in the blood?
They are inactive pro-proteases that circulate in the blood until activated by proteases.
What happens to complement proteins upon activation?
They are cleaved by proteases, becoming active and participating in complement reactions.
What are the functional categories of complement proteins?
(1) Initiators:
- Examples: C1q, MBL, ficolins.
- Function: Initiate specific complement reactions.
(2) Convertase activators and enzymatic mediators:
- Examples: C3 convertase, C5 convertase.
- Function: Cleave and activate the next protein in the reaction sequence.
(3) Main outcomes of activation:
- Opsonins: C3b (enhances phagocytosis).
- Anaphylatoxins: C5a (triggers inflammation).
- Membrane attack complex (MAC): Destroys pathogen membranes.
- Complement receptors: CR1, CR3 (mediate immune responses).
What do complement regulators (e.g., factor I) do?
1) Inhibit MAC formation on host cells.
2) Prevent deposition of complement components to protect host tissues.
In all three pathways of complement activation, what enzyme is generated by all three of these pathways?
C3 convertase, which cleaves C3 → C3a + C3b.
What does C3a do? What does C3b do?
a: Acts as an anaphylatoxin, promoting inflammation.
b: Functions as an opsonin, enhancing phagocytosis.
What are the three main outcomes of complement activation?
1) C3a and C5a recruit phagocytic cells to the site of infection and promote inflammation.
2) Phagocytes with receptors for C3b engulf and destroy the pathogen.
3) Completion of the complement cascade leads to formation of a membrane attack complex (MAC), which disrupts cell membrane and causes cell lysis.
What triggers the lectin pathway?
Soluble proteins called lectins that act as pattern recognition receptors (PRRs) and circulate in the blood.
Which lectins are involved in the lectin pathway?
1) Mannose-binding lectin (MBL)
2) Ficolins
When is lectin expression increased?
During infection.
What do lectins bind to during the lectin pathway? What does this binding trigger.
The surface of pathogens.
A signaling cascade on the pathogen’s surface.
What enzyme is generated in the lectin pathway? What happens to C3 after convertase formation?
C3 convertase (C4b2a).
C3 is cleaved into C3a (inflammation) and C3b (opsonization).
What triggers the classical pathway?
C1q binding to the pathogen surface.
How can C1q bind to the pathogen surface? (classical pathway)
1) Directly to the pathogen.
2) To antibodies bound on the pathogen surface.
Why is classical pathway important?
It connects the adaptive immune system to the innate immune system.
In the classical pathway, What does C1q binding trigger? What enzyme is generated in the classical pathway? What happens to C3 after convertase formation?
1) A signalling cascade on the pathogen’s surface.
2) C3 convertase (C4b2a).
3) C3 is cleaved into C3a (inflammation) and C3b (opsonization).
What is the common convergence point of all complement pathways?
What is the C3 convertase generated by the classical and lectin pathways?
What does C3 convertase do?
C3.
C4b2a.
Cleaves C3 → C3a + C3b.
What is the role of C3a? What is the role of C3b? What are the roles of C5 fragments?
1) Enhances inflammation.
2)
- Acts in opsonization to promote phagocytosis.
- Functions as a C5 convertase, leading to cleavage of C5 → C5a + C5b.
3)
- C5a: Enhances inflammation.
- C5b: Initiates membrane attack complex (MAC) formation.
What triggers the alternative pathway?
C3b produced by the lectin or classical pathway activates an amplification loop for more C3b formation.
What factors are required for the amplification loop? What does the amplification loop lead to?
1) Factor B and Protease factor D.
2) Formation of C3 convertase (C3bBb), which cleaves C3 → C3a + C3b.
What happens when there is a high concentration of C3?
C3 undergoes spontaneous hydrolysis, involving factor B and factor D.
How is the alternative pathway C3 convertase stabilized?
Properdin (factor P), secreted by neutrophils, stabilizes the C3 convertase (C3bBb) by binding to microbial surfaces.
Why is the alternative pathway C3 convertase unstable?
It is unstable unless stabilized by properdin.
How does complement activation contribute to inflammation?
Complement activation leads to the cleavage of complement molecules like C3a and C5a, which recruit phagocytes and promote inflammation.
What happens if C3a and C5a are present in large amounts?
They can lead to anaphylactic shock.
How do complement receptors connect pathogens to effector cells?
Complement receptors (e.g., C3aR, C5aR) on granulocytes bind complement-tagged pathogens, triggering further immune responses.
What is the result of C3aR/C5aR activation on granulocytes?
It stimulates the release of proinflammatory cytokines and granule components from basophils, eosinophils, neutrophils, and mast cells.
How do phagocytes recognize complement-tagged pathogens?
Phagocytes have receptors for C3b.
What does the binding of C3b to phagocyte receptors lead to?
Opsonization of the pathogen, making it more readily taken up by phagocytosis.
What is opsonization? How can opsonization occur?
1) The coating of a pathogen’s surface by complement components (e.g., C3b) and/or antibodies, enhancing its recognition and uptake by phagocytes.
2)
- Via complement deposition (e.g., C3b).
- Via antibodies (phagocytes have receptors for antibodies as well).
What does the membrane-attack complex (MAC) do?
It forms a pore in the pathogen’s surface, leading to cell lysis.
Which complement factors are involved in MAC formation?
C5 (directly involved) and C3 (indirectly involved through C3b).
How is the MAC formed? What happens once the MAC is formed?
- A cascade of complement activation events leads to the formation of the MAC, which inserts a pore into the pathogen’s membrane.
- The pore causes the pathogen to lyse.
What prevents complement activation under normal conditions? And what do these proteins do?
1)
Complement-regulatory proteins in plasma or on cell surfaces.
2)
Prevent the formation of C3 convertase.
Promote the disappearance of C3 convertase.
How is MAC formation inhibited?
Through the action of CD59 (Protectin), which prevents the formation of the membrane-attack complex (MAC).
What are the two types of C3 convertases and what do they produce?
C3 convertases:
- C4b2a
- C3bBb
Both produce C3a and C3b.
What is the role of C3b in the complement system?
- Opsonization (tags pathogens for phagocytosis).
- Acts as a C5 convertase, indirectly involved in forming the membrane attack complex (MAC).
What is the role of C3a in the complement system?
Involved in inflammation.
What are the roles of C5a and C5b in the complement system?
1) C5a: Involved in inflammation.
2) C5b: Involved in forming the membrane attack complex (MAC).
What are Pathogen-Associated Molecular Patterns (PAMPs)?
Molecular patterns specific to pathogens that support their lifestyle
What recognizes PAMPs, and where are these receptors located?
1) Recognized by Pattern Recognition Receptors (PRRs).
2) PRRs are located:
- On host cell surfaces.
- Inside host cells.
- As host soluble proteins.
Why are PRRs located in different parts of the host?
To ensure recognition of PAMPs from virtually any pathogen.
What are Damage-Associated Molecular Patterns (DAMPs), and how are they recognized?
1) DAMPs: Molecular patterns released from damaged host cells.
2) Recognized by PRRs.
What types of cells express Pattern Recognition Receptors (PRRs)?
- Myeloid white blood cells (all types).
- Lymphoid cells (subset): T cells, B cells, NK cells.
Other cell types commonly exposed to pathogens:
- Epithelial cells (skin, mucosal tissues).
- Endothelial cells (lining blood vessels).
Which cells express cytosolic sensors of viral nucleic acids?
Most, if not all, cells in the body express cytosolic sensors of viral nucleic acids.
Where are PRRs located depending on the PAMP they recognize?
Cell surface
Intracellular
Secreted
What are the major groups of PRRs?
Toll-like receptors (TLRs)
NOD-like receptors (NLRs)
RIG-I-like receptors (RLRs)
C-type lectin receptors (CLRs)
Ficolins, MBL, C1q
Others
What happens when PRRs are activated?
Signaling pathways are triggered.
This contributes to innate and inflammatory responses
What is the key distinction between intracellular and extracellular Toll-like receptors (TLRs)?
- Intracellular TLRs: Detect PAMPs from pathogens that replicate inside cells (e.g., viral RNA, bacterial DNA).
- Extracellular TLRs: Detect PAMPs from extracellular pathogens (e.g., bacterial cell wall components, fungal molecules).
What determines the type of PAMP a Toll-like receptor binds to?
The location of the receptor:
- Extracellular TLRs bind surface or secreted components of pathogens.
- Intracellular TLRs bind pathogen-derived nucleic acids inside the cell.
Why does receptor location influence the type of PAMP recognized?
Pathogens occupy different environments (e.g., extracellular vs. intracellular), and TLRs are positioned to detect PAMPs where pathogens are most likely to release them.
What happens when TLRs bind PAMPs?
- Activates signaling pathways.
- Different TLRs recruit different adaptor proteins to form large signaling complexes.
What are the key adaptor proteins in TLR signaling?
- MyD88
- TRIF
What are the three major transcription factors activated by TLR signaling?
NF-κB
IRF (Interferon Regulatory Factors)
AP-1 (via MAP kinase pathway)
What is the role of NF-κB, IRF, and AP-1 transcription factors in TLR signaling?
They drive the transcription of innate immune and pro-inflammatory genes.
What key molecular event enables the activation of transcription factors in TLR signaling?
Phosphorylation: The addition of a phosphoryl group activates transcription factors.
What happens after transcription factors are activated in TLR signaling?
They translocate to the nucleus to activate gene expression
What are the downstream pathways activated by TLR signaling?
NF-κB pathway
IRF pathways
MAP kinase pathway (activates AP-1)
What are the general features of signal transduction?
1) Ligand-induced receptor dimerization initiates the process.
2) Recruitment and activation of kinases and adaptor proteins occur.
3) Second messengers (e.g., Ca²⁺, cAMP, DAG) propagate the signal.
4) Activation or nuclear translocation of transcription factors leads to:
- Changes in gene expression.
5) Outcomes:
- Functional responses (e.g., cellular actions).
- Post-transcriptional or post-translational modifications.
What are the key steps in transcription as part of the central dogma?
1) Central dogma: DNA → mRNA → Protein.
2) Each gene has a promoter region.
3) Transcription factors bind to specific sequences on the promoter (response elements).
4) This binding recruits RNA polymerase.
5) Transcription begins!
What are the key features and functions of C-Type Lectin Receptors (CLRs)?
1) Membrane receptors that bind carbohydrates on pathogens and some allergens (e.g., peanut and dust mite proteins).
2) Activation triggers tyrosine kinase signaling cascades, leading to:
- CARD adaptor protein activation.
- IRF5 activation.
- MAPK pathway activation, resulting in AP-1 and NF-κB activation.
3) These pathways induce the expression of inflammatory cytokines.
What are the key features and functions of RIG-I-like Receptors (RLRs)?
1) Intracellular PRRs: RLRs function as cytosolic pattern recognition receptors.
2) They recognize viral double-stranded RNAs and certain structured single-stranded RNAs.
3) MAVS (mitochondrial antiviral signaling protein) is involved in the signaling process.
4) Activation of RLRs triggers signaling pathways that activate:
- IRFs (Interferon Regulatory Factors, transcription factors).
- NF-κB (transcription factor).
What are NOD-like Receptors (NLRs)?
- NLRs are cytosolic pattern recognition receptors (PRRs).
- They are involved in immune response by recognizing pathogens in the cytosol.
What do NOD-like Receptors (NLRs) recognize?
Peptidoglycan from bacterial cell walls (which must enter the cytosol).
What signaling pathways can NOD-like Receptors (NLRs) trigger?
NF-κB, AP-1, and IRF transcription factors.
What is the role of caspase-1 in NLR activation?
Caspase-1 protease is activated, cleaving IL-1β and IL-18 into their active forms.
What happens after caspase-1 cleaves IL-1β and IL-18?
These cytokines are released as pro-inflammatory cytokines.
What type of modification occurs during NLR activation?
This process involves a post-translational modification.
What do PRRs recognize, and what factors determine this recognition?
- PRRs recognize PAMPs (Pathogen-Associated Molecular Patterns).
- The type of pathogen and its location in the body influence the PRR recognition.
What is a signaling cascade in PRR activation?
A signaling cascade is a series of proteins activated in response to PRR recognition of PAMPs.
What are common modifications in PRR signaling pathways?
Modifications like phosphorylation and ubiquitination.
What is ubiquitination, and what does it do in signaling?
- Ubiquitination is the attachment of one or more ubiquitin subunits to a target protein.
- This process can either activate the protein or mark it for degradation to regulate signaling.
How does PRR signaling lead to gene expression?
Activation of transcription factors leads to gene expression through the process:
DNA → mRNA → Protein.
What are the types of modifications that can occur post-transcriptionally and post-translationally?
Post-transcriptional and post-translational modifications can occur, affecting the protein function.
What are the outcomes of PRR signaling and gene expression?
The outcome includes the production of cytokines and other signaling molecules, which carry out immune functions.
