18. Innate defence at mucosal surfaces Flashcards
what are mucosal surfaces not?
inert
what makes up the largest organ in the body?
the skin and mucosal membranes
what is the general structure of the epithelial surfaces?
outer epithelial cells how many layers depends on function
basement membrane with collagen and lamina
then connective tissue with ECM proteins
what do you need to cross to cause an infection?
an external barrier
what are the skin defences?
unfavourable conditions, desquamation, antimicrobial molecules, SALT, normal microbiota
what unfavourable conditions for bacterial growth does the skin have?
dry, acidic pH 5, high salt, 34-35oC
what is desquamation?
the shedding of dead keratinised cells that takes attached bacteria with it
what antimicrobial peptides are on the skin?
present at areas with natural breakages like glands
Lysozymes
lipids
what does SALT stand for?
skin-associated lymphoid tissue
Langerhans cells
what is the function of SALT?
immature dendritic (Langerhans) cells phagocytose microbes then travel to the lymph nodes and acts as an APC to recruit active immunity
What is the function of the normal skin microbiota?
Competition for nutrients and space
produce bactericidal products
what defences are the same in mucosal surfaces as in the skin?
normal microbiota
MALT
sloughing
why are mucosal membrane ideal places for bacterial growth?
one layer to penetrate
37oC
covered in fluid
warm and moist
Neutral pH
what maintains the barrier integrity of the epithelium?
intracellular junctional complexes or tight junctions
what 4 transmembrane proteins make up tight junctions?
Occludin, claudins, tricellulin and junction-adhesion molecules (JAMs)
where are tight junctions located?
at the apical surface of the epithelium
forms a belt around the top keeping the cells very close together to prevent bacterial entry
what does Occludin, claudins, and tricellulin do in tight junctions?
loop through the cell membrane 4 times to make interacting loops that hold the junctions together
what do JAMs do in tight junctions?
form extracellular immunoglobulin like domains to hold the cells together
what junctions other than tight junctions keep the cells together?
adherence junctions and desomosomes
how do bacteria overcome tight junctions?
use tight junction proteins as receptors for attachment and invasion
use toxins and T3SS to move the tight junctions
disrupt the cytoskeleton the junctions are attached to
what is mucus?
made up of mucins made by goblet cells
lubricant
mechanical barrier
protective as it traps antimicrobial compounds
what are mucins?
glycoprotein with >50% of mass sugar residues
7 common sugars
22 human mucin genes
different depending on the site of production
what is the function of mucins?
gel forming mucins leads to the viscous property important for external surfaces
what interaction make gel forming mucins viscous?
ionic crosslinks and disulphide bonds align both chains to be adjacent
sialic acid residues are very negative so Calcium ions form interactions and stop the chains from collapsing in on each other. This makes space for water molecules to move between the chains
how are pathogens trapped in mucus removed?
shed in blobs that are expelled by:
cilia movement in lungs
peristalsis
rapid urine flow
coughing/sneezing
vomiting/diarrhoea
what are the isoforms of DMBT1 that are in the mucus and where are they produced?
saliveryglutinin
glycoprotein 340
produced in lymphoid organs, epithelial cells and gland secretions
what does glycoprotein 340 do?
non specific trapping of viruses and bacteria in the mucus
acts similar to a PRR
mediated agglutination for easier phagocytosis
what are collectins?
collagen-like lectins
collagen-like = triple helix
lectins = protein that binds sugars
what do collectins do?
bind to PAMPs leading to agglutination and enhanced phagocytosis
what is heterotypic complexing?
components working together for a common goal
in case the goal is to clump and expel bacteria
synergistic effects increasing the efficacy of agglutination
what comprises the mucus heterotypic complex?
mucins
gp340
collectins
IgA
what are lysozymes?
found mucus, tears, saliva and plasma
high levels in new borns before they develop an immune system
digests bacterial cell walls
activate autolysins
bacteria aggregation and clearance
what do lysozymes target in bacterial cell walls?
break ß(1,4) bonds in the peptidoglycan cell wall
more effective in gram positives as there is no outer membrane to breach and more peptidoglycan to target
what is lactoferrin?
iron-binding glycoprotein
found in mucosal surfaces
secreted in tears, milk, saliva and nose
found in very high concentrations in early breast milk
what does lactoferrin do?
sequesters Fe2+ ions which inhibits bacteria growth
what is lactoferricin?
a short peptide cleaved off lactoferrin
functions as a cationic antimicrobial peptide
Bactericidal as it target nucleic acids
how do bacteria subvert lysozymes?
capsules mean lysozymes cannot get to the peptidoglycan
how do bacteria subvert lactoferrin?
siderophores
proteases
what do siderophores do?
binds the lactoferrin iron complex and takes them into bacteria
make lactoferrin release iron
some have higher affinity for iron then lactoferrin so it steals the iron
what are cationic antimicrobial peptides (AMPs)?
evolutionarily conserved and produced by all classes of life
typically 20-50 amino acids
positive charge
broad spectrum
active against antibiotic resistant bacteria
what are the 4 secondary structures of AMPs?
alpha helix
ß-stranded sheet
ß-loop/hairpin
extended
only when they are performing function do they take conformation
what do AMPs do?
positive residue attaches to the outer leaflet of the bacterial membrane causing stress and making the membrane rupture
then attaches and does the same to the inner leaflet of the membrane
forms a pore
interferes with DNA and protein synthesis
cell lysis
what property is crucial for AMP function?
amphipathic
folds positive residues on one side and negative on the other
how do AMP know to attack bacterial cells and not our cells?
different properties in the membranes
bacterial membranes have:
negative acidic phospholipids that attract positive AMPs
Our membranes have:
zwitterionic phospholipids
Cholesterol to prevent insertion into the membrane
what are the 3 classes of the human AMPs?
alpha-defensins, beta-defensins and cathelicidins
what are alpha-defensins?
6 known (HNP1-4, HD5/6)
made by polymorphonuclear neutrophils
beta sheet structure
what are beta-defensins?
4 known (HBD1-4)
made by epithelial cells
beta sheet structure
what are cathelicidins?
1 gene (hCAP18) making 3 peptides
1 studied
made by leukocytes and epithelial cells
alpha helical strucutre
what are Histatin-5?
family of histidine-rich peptides
abundant in saliva
mostly active against candida albicans
what is histatin-5 mechanism of action?
bind to candida cell surface receptors
enters cell
induces loss of K+ making osmotic imbalance
loss of cell volume and cell death
how do bacteria subvert AMP action?
cleavage by binding proteins and proteases
anionic capsules
altering the cell surface charge
alter membrane fluidity using acylation or pigmentation
expulsion using multidrug efflux pumps
