2- Innate Immunity: Immediate Response

What is the 1st line of immunity?

prevents mo getting in, barriers & molecular mechanisms

Barriers

biggest is skin, epithelial & endothelial cells of BV

ex → skin, mucosal surfaces, secretions (tears, sebum, sweat)

What is the 2nd line of immunity?

induced intrinsic mechanisms

ex → pattern-recognition receptors, bind to pathogen-associated molecular patterns (PAMPs)

What is the 3rd line of immunity?

adaptive immunity, only if innate was overrun or not successful

• some mechanisms of innate still going on

When does the 2nd line of defense get activated?

mobilized once cells of immune system detects presence of infection

How are innate immune proteins made?

gene expression changes in certain subset of cells = proteins

What are mechanical barriers?

prevent access

• epithelial cells joint by tight junctions

• skin & gut → air/fluid flow to expel mo, peristalsis in gut

• lungs → mucociliary escalator, movement of mucus by cilia to move up & out, cough up, trap & target bacteria

• eve/nose/oral cavity → tears, nasal cilia

What mechanical barrier does the skin have?

epithelial cells joined by tight junctions

longitudinal flow of air or fluid

What chemical barriers does the skin have?

fatty acids → sebum

antimicrobial peptides → disrupt microbial membrane

What microbiological barrier does the skin, gut, lungs, ENT have?

normal microbiota

What mechanical barrier does the gut have?

epithelial cells joined by tight junctions

longitudinal flow of air or fluid

peristalsis in gut

What chemical barriers does the gut have?

low pH

antimicrobial enzymes → lysozyme, disrupt cell wall of bacteria

antimicrobial peptides

What mechanical barriers do the lungs have?

epithelial cells joined by tight junctions

• mucociliary escalator

• movement of mucus by cilia to move up & out

• cough up, trap & target bacteria

What chemical barriers do the lungs have?

pulmonary surfactant

antimicrobial peptides

What mechanical barriers do the eyes/nose/oral cavity have?

epithelial cells joined by tight junctions

tears, nasal cilia

What chemical barriers do the eyes/nose/oral cavity have?

antimicrobial enzymes → tears & saliva

antimicrobial peptides

Antimicrobial Peptides & Mucus

anywhere there’s mucus, will be antimicrobial peptides secreted into mucus

• most bind to & disrupt membranes

Lysozyme

targets & disrupts cell wall of bacteria

Where do we want/not want microbiota to be?

mucosal surfaces, tell foreign ones to get out

don’t want microbes on mucosal surfaces of lungs

How does the immune system organize itself?

launches multiple plans, never want plan A to fail, other wise too late for plan B

Microbiota

commensal relationship

• nutrition

• help w/ metabolism, use theirs to break thing down

• general health (keep pathogens in check)

Cells in Human Body

3 × 10^13 human cells

3.8 × 10^13 bacterial cells, more bacteria than human cells

• >1000 species of bacteria in gut

• move down GI tract = bacteria increases/concentrates → due to peristalsis

• mainly from mom, given in pregnancy & vaginal birth

Can the innate immune response stop?

No, once it starts won’t stop

• continues even after subsequent steps

• continues throughout battling pathogen

What does the immune system recognize?

intracellular & extracellular pathogens

Extracellular Pathogens

• grow outside of cells → replicate in space in human cells

• bacteria, viruses, parasites, fungi

  • viruses recognized extracellularly during movement w/in body not during replication

  • fungi only extracellular

• brought in → phagosome/phagolysosome do endocytosis, engulf & bring in w/ vesicle

  • still extracellular → top equiv EC & vesicle

• replicate outside

• viruses not extracellular pathogen, sometides exist otuside of/traveling to or in host cells

Intracellular Pathogens

• replicate & live inside cells

• viruses, intracell bacteria & pathogens,

  • viruses replicate & assemble new virions intracellularly

• pathogen can live otuside or in vesicle = extracell still

What replicates inside macrophages?

mycobacteria

Why are viruses never extracellular pathogens?

must be inside living cell to replicate to be an extracellular pathogen

What is the complement pathway?

.series of protein in blood working to enhance immune response

• lead to destroying pathogens → opsonization, MAC forming

What are the 3 ways of complement activation?

alternative, lectin, clasical

• waterfall → get to certain point, going & can’t be stopped

• 3 streams converging on waterfall

Which proteins are involved in the complement pathway?

proteins C1-C9

Alternative Pathway

1st complement pathway to be activated, start of infection, happening in proximity to microbe, spontaneous

Lectin Pathway

2nd to be activated, binds to sugars on microbes

Classical Pathway

last to be activated, needs antibodies to be activated

• induced, antibodies bound to microbe’s surface

What do lectin & classical pathways share?

started by specific binding events, binding of receptors to microbes

How long do antibodies take to be made?

2 weeks

What do the 3 complement pathways converge on?

C3 protein, have 3 outcomes/effector functions

• C3 always cleabed into C3a & b

• fixed when C3b attaches to surface of protein/bacteria

Which pathway involves C3 & C5?

recruiting inflammatory cells to clear microbe/phagocytes

Which pathway is more unique for C3?

opsonizing pathogens, facilitating uptake & killing by phagocytosis

What is mediated by C5?

perforation of pathogen cell membranes

make complex poking holes in PM, induce lysis

Which proteins are exclusive to lectin & classical?

C1, 2, 4

T/F: phagocytosis & perforation aren’t either/or effector functions

True, processes start at same time

Where do pathways culminate?

in elimination of infecting pathogen

How many proteins are involved in the complement paths?

9, C1-9

What is the alternative complement pathway?

A part of the immune system that enhances the ability to clear pathogens from an organism, activating complement proteins without antibody involvement and promoting opsonization and cell lysis.

serum proteins in blood, not only active against blood infections

• go out into tissue w/ vasodilation from inflammatory response

circulating in blood

functionally inactive

initially gets triggered & cleaved, become active

What does an infection trigger?

complement activation

• cascade of enzymatic react ions

• each protease cleaves & activates next protease, pull e/o trigger

What is upstream of C3 in alternative pathway?

C1, C2, C4

What is the general process of alternative pathway?

1. C3 in bloodstream

2. C3 interacts w/ H2O in bloodstream to form iC3

3. iC3 recruits factor B → makes iC3B

4. iC3B complex recruits factor D to cleave factor B & form iC3Bb

5. iC3Bb recruits 2nd C3 floating around in blood, cleaves C3 into C3a & C3b

6. C3b does 3 things: opsonize, inflammation, pores in membrane

How does C3 interact with H2O?

hydrolyzed in bloodstream

• forms iC3 (induced C3)

• undergoes conformational change

• happens in absence/presence of infection, random bacteria or infection

How does iC3 recruit factor B?

iC3 has a binding site for factor B that allows factor B to attach and initiate the cleavage process by factor D, forming iC3B

What happens when iC3B recruits factor D?

Factor D cleaves factor B into Ba and Bb.

• B more susceptible to proteases when bound to iC3, gets cleaved by factor D

• Ba released, Bb stays w/ iC3 = iC3Bb = C3 convertase

What happens whe iC3Bb recruits a 2nd C3?

C3 cleaved into C3a & C3b

• C3b covalently binds to pathogen surface

• C3a diffuses away, causes inflammation, recruits immune cells

What is the only thing bound to pathogen surface?

C3b, only thing deposited on cell surface

What acts as an opsonizer?

C3b, receptors bind to it & it acts as opsonizer/eat me signal → induces phagocytosis

How does C3b make a C3 convertase?

C3b binds Bb to become C3bBb = C3 convertase

• Factor B & D implied

• C3bBb has same function as iC3Bb but can make easier, prefer C3bBb

• C3Bb can cleave C3 to C3a & C3b

How does C3b make a C5 convertase?

C3b binds Bb & another C3b to make C3b2Bb/C5 convertase

• C3b2Bb cleaves C5 → C5a & C5b

Why does C3Bb cleave C3 to C3a & C3b?

want a lot of opsonizing tags on pathogen (C3a & need C3b to make lots of pores in membrane

Why does C3b2Bb cleave C5 to C5a & C5b?

C5a → diffuses away & causes inflammation

C5b → stays w/ pathogen, not connected to membrane, used for further MAC formation

What initiates formation of the MAC?

C5b, a reason why C5 convertase is needed for optimal pathogen elimination

B cells develop in the ____ & T cells develop in the ____

bone marrow, thymus

Which region of the antibody & the TCR is variable?

red, yellow

What is this cell & what is its function?

NK cell, kills virus-infected cells

What is this cell & what is its function?

dendritic cell, phagocytosis

What is this cell & what is its function?

mast cell, expulsion of parasites

What is this cell & what is its function?

neutrophil, phagocytosis

Which of the following is NOT a primary lymphoid organ?

A) Spleen

B) Lymph node

C) Lymph vessels

D) Thymus

A/B/C (spleen, lymph node, lymph vessels) are secondary lymphoid organs

How is the MAC complex formed?

1. C5b made from C5 convertase/C3b2Bb cleaving C5

2. C5b binds C6

3. C5b & C6 recruit C7, C7 binds to membrane, peripheral protein

4. C8 recruited, inserts into pathogen membrane

5. C9 recruited & oligomerizes (many C9s), make pore in pathogen membrane

When C5 binds to C6, are they membrane bound?

no, only become membrane-bound when C7 & C8 bind to the complex

Which protein is transmembrane in the alternative complement pathway?

C8 & C9

Which protein is peripheral in the alternative complement pathway?

C5b & C6

What proteins make up the MAC?

C5b, C6, C7, C8, C9

What are the microbe targets of complement pathways?

bacteria, fungi, parasites, archaea, viruses

T/F: Archaea are microbial targets of complement pathways & infect us.

False, are targeted but don’t infect us

T/F: Viruses are always microbial targets of complement pathways.

False, only if they have a membrane, can’t target non-enveloped viruses.

Classical Pathway

C1q binding to antibody on surface, leads to breakdown of C2 & C4

Lectin Pathway

mannose-binding lectin (MBL) bids to carb, leads to breakdown C2 & C4

Where do classical & lectin paths converge?

converge at formation of C4b2a, C3 convertase, diff version of iC3Bb

What are complement regulatory proteins?

regulate & protect host cells from complement activation, they prevent damage to host tissues by inhibiting complement pathways.

Soluble Factors - CRP

complement regulatory proteins, secreted in bloodstream, activate or inhibit complement

Membrane Bound Factors - CRP

complement regulatory protein, don’t eat me signals on our cells, inhibit complement

Properdin

complement regulatory protein, soluble factor activating complement

• binds C3bBb/C3 convertase, stabilizes it

• activate C3bBb, keeps breaking C3 → C3a&b

• lock in place to work a bunch

What is the function of factor H & I?

soluble factors that inactivate complement

1. C3b on surface

2. Factor H binds C3b

3. Factor H recruits factor I

4. Factor I cleaves C3b into iC3b(inactivated)

What do factor H & I do in an active infection?

no factor H or I to inhibit complement

Where should the complement pathway not occur?

health host cells & tissues, prevent damage

• gut especially → don’t want inflammation

  • microbiota already there

What happens if the complement occurs in places where the microbiota is?

inflammation, IBS/autoimmune, want to inhibit to stop/limit inflammation

Where are DAF & MCP?

membrane bound on surface of OUR cells

What is the function of DAF & MCP?

inhibit complement early, prevent actively killing out ells

1. DAF binds C3bBb & displaces Bb

2. mCAP binds C3bBb & displaces Bb

How does DAF work?

binds C3bBb & displaces Bb, unlikely that phagocyte will find C3b leftover before degraded by ells

• disrupt C3 covnertase

How does MCP work?

binds C3bBb, displacing Bb and preventing complement activation on our cells.

• MCP-C3b recruits factor I, shove out & bring in/cleave C3b

• factor I cleaves C3b to iC3b/inactivates, prevents acting as opsonin, X attack healthy cells

How does MAC work?

destruction of target cells occurs w/ influx of water & loss of electrolytes

What happens if DAF/MCP don’t do their job?

Complement activation leads to uncontrolled inflammatory response, causing damage to healthy cells.

Why do we need DAF/MCP?

extra measures to save our cells if complement isn’t stopped at C3 level

• don’t want inflammatory death/lysis of our cells

• if need to kill → want to induce apoptosis

What prevents recruitment of C9?

CD59, binds to C5b678, no pore formation

T/F: MAC formation can be stopped at C5.

False, can’t stop initiation of MAC from C5

How does opsonization occur?

1. C3b on pathogen binds CRI (complement receptor 1) on phagocyte

2. interacts w/ complement receptors on macrophage, induce phagocytosis

3. macrophage forms phagosome

4. macrophage membranes fuse to create phagosome

5. lysosomes fuse w/ phagosome to form a phagolysosome, destroy microbe

What leads to phagocytosis?

C3b on pathogen engaging/binding w/ CR1, initiates signaling that leads to phagocytosis & breakdown of microbe

How do macrophages interact w/ pathogen’s bound C3b?

macrophages have receptors for C3b fragments, complement receptor CR1/3/4, bacteria w/ C3b coat = better phagocytosis = opsonization

What are anaphylotoxins?

• promote inflammation, C3a & C5a

• recruit immune cells in bloodstream

• increase vascular permeability

How do anaphylotoxins increase vascular permeability?

break tight junctions between endothelial cells

• smaller things exit BV 1st (ex → complement, send out/enhance C3/5

• cells can exit layer

• positive feedback loop

Is C3a or C5a more potent?

C5a more potent than C3a, later in pathway committing to finishing pathway, start w/ less in case complement needs to be stopped