Module 2: Innate Immunity, Immediate Response

What is the process of the immediate innate immune response?

1. Pathogen invades tissue and proliferates

2. Pathogen is recognized by preformed effectors and tissue-resident effector cells

3. Pathogen is eliminated and the infection ends

4. Minor tissue damage is caused and repaired

What are the mechanical barriers that prevent pathogens from crossing in…

1) All

2) Skin + Gut

3) Lungs

4) ENT

1) Epithelial cells joined by tight junctions

2) Longitudinal flow of air or fluid

3) Movement of mucus by cilia

4) Tears + Nasal Cilia

What are the chemical barriers that prevent pathogens from crossing in…

1) All

2) Skin

3) Gut

4) Lungs

5) ENT

1) Antimicrobial peptides

2) Fatty acids

3) Low pH + Antimicrobial enzymes

4) Pulmonary surfactant

5) Antimicrobial enzymes in tears and saliva

What are the microbiological barriers that prevent pathogens from crossing in all systems?

Normal microbiota (commensal microorganisms)

What are extracellular pathogens?

Populate tissues in the spaces between cells, that’s how to get into and spread in your body

What are intracellular pathogens?

Enter cells in order to replicate and spread, similar to a parasite, it gets into the cell, hijack the machinery to make new viruses, then get released from cell to take on extracellular stage

Describe the complement system.

The first responder to pathogens when the physical/chemical barriers of the body fails. >30 proteins circulate in blood, lymph and extracellular fluids (most are proteases in inactive forms of zymogen) and activated upon pathogen exposure, C3 is most important bc it initiates entire cascade!

Difference between complement activation and fixation?

Activation: C3 (inactive) is activated by being cleaved to C3a and C3b to expose thioester bond, the C3b part with the bond can be attached to either water or a pathogen

Fixation: C3a after being cleaved recruits phagocytes

Which pathway out of the 3 is the first to be activated and describe.

The alternative pathway has the C3 activated by the exposure to the pathogen surface’s environment, which alters the C3 conformation

What are the 3 results of complement fixation?

1) Recruitment of inflammatory cells

2) Opsonization of pathogens, facilitating uptake and killing phagocytes

3) Perforation of pathogen cell membranes

Describe the pathway from C3 to iC3B.

1) C3 is secreted by liver and circulates as a zymogen

2) C3 can spon cleave and attach to nearby molecules to create intermediate iC3 (easier to activate and more responsive)

3) iC3 near pathogen surface is converted to active protease by binding to factor B creating the soluble C3 convertase iC3B, accelerating C3 hydrolysis

Describe the pathway from iC3B to C3bBb

1) Factor B on iC3B is cleaved by factor D, forming Ba (negative regulator; circulates and limits pre-activated B cell proliferation) and Bb

2) Bb remains attached to iC3 now making it iC3Bb

3) iC3Bb activates C3 cleavage through hydrolysis making C3a and C3b

4) Factor B binds to C3b, cleaved by factor D forming C3bBb (can’t diffuse thus fixes more C3b fragments to pathogen, inc of this = coating of pathogen surface aka complement fixation step)

What is one way of regulating complement fixation by plasma proteins?

(+): Plasma proteins such as properdin stabilize C3bBb and promote complement fixation on pathogen surfaces.

(-): Factors H makes a divot and I binds to divot to breakdown C3B on self/healthy cells to iC3b (non-functional)

What is one way of regulating complement fixation by membrane proteins?

(-): Decay-accelerating factor (DAF) binds to C3bBb and makes it let go of Bb thus no longer a convertase and continue on

(-): Membrane cofactor protein (MCP) also makes C3bBb to C3b except additionally it attaches to it still to be cleaved by factor I making iC3b (inactive)

Most proteins that regulate complement belong to the ___ family.

Regulators of complement activation (RCA) gene, built from CCP modules (~60 AA fold into 2 B-sheets crosslinked by disulfide bonds) ensuring that C3b is only deposited on pathogens and not healthy cells

Describe the pathway once phagocytosis is triggered.

1) Complement activation deposits C3b on bacterial surface

2) Macrophage CR1 binds C3b on bacterium

3) Macrophage endocytoses the bacterium

4) Macrophage membranes fuse to create a phagosome

5) Lysosomes fuse with the phagosome to form a phagolysosome

Besides C3b, what else can CR1 bind to and their role in doing so?

C3bBb, basically form of C3B, to make it susceptible to cleavage by Factor I to prevent fixation/phagocytosis

Role of CR- 1,3, and 4?

Bind to both C3b and iC3b, inc attachment strength and phagocytosis efficiency by identifying complement bounded pathogens

Describe the formation pathway from C5 to C9 for MAC

C5: When cleaved, C5b initiates assembly

C6: Binds to C5b, forms binding site for C7

C7: Binds to C5b6, exposes hydrophobic region for attachment to cell membrane

C8: Binds to C5b67, exposes hydrophobic region for insertion into cell membrane

C9: Polymerizes to C5b678 to form a membrane-spanning channel that disrupts cell integrity and causes death

What are the soluble reg proteins for MAC?

S protein, clusterin, and factor J; prevents C5b67 association with membrane

What are the membrane bound reg proteins for MAC?

Homologous restriction factor (HRF) and CD59 (protectin); prevents C9 recruitment to C5b678

Which fragments are considered anaphylatoxins?

C3a and C5a; mediates localized and systemic inflammatory responses and acts as ligands for receptors on phagocytes, mast cells, and endothelial cells to release histamine to inc vasodilation and capillary permeability

Describe the coagulation system.

Consists of a series of plasma proteins (enzymes) that form stable blood clots to prevent pathogens from entering damaged blood vessels and repair damaged vessels

How does the coagulation system interact with the complement system?

Connected by platelets that express CR’s and release coagulation activators (thrombin, Factor Xa [Fxa] Factor XIa [FXIa], plasmin) when degranulated (this activates C3 and C5)

Describe the kinin system.

Consists of a series of plasma proteins that lead to production of bradykinin, which mediates tissue inflammation and repair

How does the kinin system interact with the complement system?

Connected by kallikrein that activates C3 along with bradykinin release (vasodilator/smooth muscle relaxer = inflammation)

How has pathogens escape detection using proteases?

Ex: Streptococcus pyogenes uses PAM protein to bind to inactive human plasminogen in blood, holds on even when converted to plasmin by host proteins, coats itself, and camouflages

What is a protease inhibitor?

Specialized proteins that bind to and block the active site of the pathogen’s protease, which prevents it from breaking down host proteins necessary for normal function

What is an example of a protease inhibitor?

a2-macroglobulins lure pathogen proteases then sequester them and flag them for destruction by resembling C3 with thioester bond attached to “bait” molecule

when a pathogen protease cleaves “bait”, shape changes, thus neutralizing protease

What is the largest family of antimicrobial peptides?

Defensins (a and B) that are amphipathic molecules that can easily cross bacterial fungal membranes, and enveloped viruses

How does a defensin work?

Electrostatic attraction and the transmembrane electric field bring defensin into the lipid bilayer → Defensin peptides form a pore

What is a unique trait to defensins?

The only innate immune proteins known to neutralize a wide range of bacterial toxins (makes them instable) by binding to toxin, changes shape, and makes them susceptible to proteases

aka anti-chaperone bc it prevents folding

What are pentraxins?

Cyclic molecules that circulate in blood and lymph to bind to pathogens for destruction, similar functions to antibodies

How do pentraxins destroy pathogens?

Pathogen binding side coats pathogen as an opsonin while human cell binding side attaches to cell surface receptors, this crosslinks pathogen to human cell and triggers phagocytosis

Serum amyloid P component…

1) Pentraxin type

2) Source

3) Ligands

1) Short

2) Liver hepatocytes

3) Bacterial, viruses, fungi, parasites

PTX3 …

1) Pentraxin type

2) Source

3) Ligands

1) Long

2) Mono/Macrocytes, Dendritic, Endothelial, and Epithelial cells

3) Bacteria, viruses, fungi