lecture 10

Question 1

why do we rely on our innate immune system?

Answer 1

bacteria grow way to rapidly and so the adaptive immune response would be too slow
we rely on our innate immune system for protection in the first few critical hours after pathogen detection

Question 2

what are the three lines of innate immune system defence?

Answer 2

1. barriers - physical and chemical
   thick layer of keratinized dead cells on the skin
   tight junctions between epithelial cells
   acidic stomach pH
   mucus layers
2. cell intrinsic responses
   pathogen induced phagocytosis
   degradation of dsrna
3. specialised proteins and specialised cells
   professional phagocytes like neutrophils and macrophages
   natural killer cells
   the complement system

–> these are not specific to particular pathogens/ non-specific rapid innate responses

Question 3

define the role of mucus layers

Answer 3

the mucus layer on moist epithelial surfaces protects against microbial mechanical and chemical insults

(hagfish are all shaped marine jawless animals that produce mucus layers on their skin when triggered)

Question 4

structure of the mucous layer

Answer 4

it is made from secreted mucins and other glycoproteins
it’s slippery which makes it hard for pathogens to attach to mucus coated epithelia
ethereal cells have beating cilia which can facilitate clearing of pathogens
mucous is slippery because the proteins are covered with soluble carbohydrates which means that there is a water shell around each one of the mucin molecules

Question 5

define the defensins of mucus layers

Answer 5

they are small positively charged antimicrobial peptides
they have hydrophobic or amphipathic helical domains
they are very simple structures of coils or flattened beta sheets
coils are amphipathic so they have different charges on different sides of the coil
beta sheets are hydrophobic and interact with cell membranes

Question 6

role of defensins

Answer 6

defensins have wide-antimicrobial activity and can kill or inactivate:
gram positive and gram negative bacteria
fungi (like yeast)
parasites (like protozoa and nemaotdes)
enveloped viruses (hiv)

there are many classes of defensins so there is a wide repertoire of targets

Question 7

define an enveloped virus

Answer 7

Enveloped virus is a virus that makes new viral particles and borrows part of the cell membrane of the host cell so that it has its own lipid membrane around it

Question 8

how defensins kill

Answer 8

They have hydrophobic patches so can interact with membranes and enter it to destabilize that membrane, leading to cell lysis
Since the coils in the defensins have the separation of charges, so the positive charge can interact with the backbone of dna and rna
→ we’re not sure how they work, this is just a guess

Question 9

how do defensins lyse pathogens but not ouor own epithelial surfaces?

Answer 9

they are much more active on membrane that do not contain cholesterol (our membranes contain cholesterol)

More cholesterol, the more resistant the cell is to defensins
it is hard for an organism to mutate to get defensins

Question 10

define PAMPs (pathogen associated molecular patterns

Answer 10

pathogens do occasionally breach the epithelial barriers.
the innate immune system recognises molecules (pathogen associated or microbe associated immunostimulants) that are common to many pathogens, but essentially absent in the host.

an alternative name is PAMP, referring to an older view that the innate immune system recognises patterns in pathogens

Question 11

examples of PAMPs

Answer 11

things that are completely foreign to us are recognised as PAMPs by our innate system

fMet
peptidoglycans
bacterial flagellae
lipopolysaccharide LPS
mannans, glucans, chitin from fungi
motifs in bacterial / viral dna

Question 12

pattern recognition receptors found in? (2)

Answer 12

toll-like receptors recognise these PAMPs (LPS, CpG motifs, flagellae) and stimulate an nflammation as a cry for help

Receptors in the blood recognise the peptidoglycans

Question 13

define complement

Answer 13

about 20 soluble proteins that are activated sequentially upon infection

Question 14

how does complement activation target pathogens for lysis?

Answer 14

there are 2 pathways:

• lectin pathways (Lectin is any protein that can bind to a sugar. Lectin receptors will bind any fungi that carries sugar on their surface)
• alternative pathway (pathogen surfaces)

Question 15

describe the complement system

Answer 15

it is a set of pro enzymes where the activation of one leads to the activation of another etc. They are proteolytic

C2 is cleaved, when activated will cleave C4, cleaves C3)
C3b targets the pathogen and binds to its membrane and C3a calls for help

Question 16

what does the binding of C3b to the pathogen membrane cause?

Answer 16

pathogen bound C3b stimulates a local cascade of reactions (C5 to C8) at the marked membrane

C9 is inserted into the membrane

a c9 pore breaches the membrane c9 multimers form a membrane-attack complex

pathogen lysis

Question 17

define toll-like receptors

Answer 17

toll is a trans-membrane protein with a large extracellular domain with repeating motifs (leucine-rich repeats)
These can bind many other proteins
binding to pathogenic fungi sends a signal to the nucleus
antifungal defensins are expressed

toll-like receptors have the same overall structure and do a similar job

Question 18

where are toll-like receptors found?

Answer 18

most TLRs are o the cell membrane of epithelial cells and macrophages, dendtritic cells and neutrophils

Question 19

toll-like receptors as an alarm system

Answer 19

TLRs bind PAMPs
signal the nucleus
result in transcription of hundred of genes (especially inflammation genes)

Question 20

explain how defensins and TLRs are ancient immune system components

Answer 20

proteins related to TLRs and defensins are apparently involved in innate immunity in all multicellular organisms

for resistance to fungal, viral and bacterial pathogens, plants require:
membrane receptors, leucine rich repeats and domain homologous to the cytosolic domains of tlrs

this suggests that rlrs and defensins pre-date the ancestral split between animals/plants/fungi

Question 21

gonorrhoeae

why is it a a facultatively intracellular organism

Answer 21

causative agent of the sexually transmitted infection gonorrheae

• the drip (refers to the dripping purulent discharge from the urethra)
• the clap (derived dfrom clapie meaning brothel)

It’s a facultatively intracellular organism - important because it can hide in our cells and infect our body

Question 22

gonorrhea symptoms

Answer 22

most are asymptopmatic

purulent foul smelling discharge
inflammation
painful urination
urethritis

males (10% asymptomatic) get prostatitis and orchitis
females (80% are asymptomatic) get pelvic inflammatory disease, probably infertility

Question 23

gonorrhoeae prevalence

Answer 23

us data up to 2016, giving a healthy people target for 19 cases per 100 000 people in 2010
this was not reached

Question 24

transmission rates

Answer 24

a female with an infected man has a 50% chance of infection

a male with an infected woman has a 20% chance of infection

Question 25

evasion of the innate immune system: gonorrhoeae

Answer 25

the capsule lacks LPS, instead has LOS lipooligosaccharide
it can utilise host derived sialic acid to sialylate its LOS
with sialylated LOS, they are less invasive than those with nonsialylated LOK, but are more resistant to the bactericidal effects of serum

this is because human cells also display sialylated glycoproteins, so they can masquerade as us and evade our immune system

Question 26

summary

Answer 26

If a pathogen interact with a TLR, we get expression of interferons and ring alarm bells to cause inflammation
The pathogen surface can be recognised by the complement surface, where C3 is cleaved and a part of it is bound to the surface, another part causes inflammation
In our toxic slime, we have defensins - direct membrane attack