Innate immune response

Question 1

what molecules are involved in the innate immunity

Answer 1

antimicrobial peptides, antimicrobial enzymes, and complement system

Question 2

2 antimicrobial enzymes

Answer 2

lysozyme and phospholipase A2

Question 3

what is the function of lysozyme

Answer 3

enzyme which attacks chemical features specific to bacterial cell walls -> it is a glycosidase that breaks the beta(1,4) linkage between two sugars of the polymer of the peptidoglycan layer.

Question 4

killing which type of bacteria is lysozyme more effective, and why?

Answer 4

more effective killing gram-pos bacteria because they don’t have LPS (outer layer) so the peptidoglycan is more exposed

Question 5

function of phospholipase A2

Answer 5

enters bacterial cell wall and hydrolizes phospholipids in the membrane = killing bacteria

Question 6

types of antimicrobial peptides

Answer 6

defensins, cathelicidins, histatins

Question 7

which cells produce antimicrobial enzymes and where are they secreted

Answer 7

phagocytes and paneth cells (small intestine) and they are secreted in the saliva and tears (for example)

Question 8

which cells produce antimicrobial peptides

Answer 8

phagocytic immune cells = neutrophils (granules) and epithelial cells = lungs for example (secreted into the mucosal surfaces)

Question 9

what are defensins and what is their mechanism of action

Answer 9

small, cationic proteins which disrupt the cell membrane of bacteria and fungi (also some enveloped viruses). they are amphipatic and they insert their hydrophobic region into the cell membrane bilayer of bacteria. results into the loss of membrane integrity

Question 10

what are three subfamilies of defensins and which cell produce them?

Answer 10

alpha - neutrophils, beta - epithelial cells of the respiratory and urinogenital tracts) and theta-(not in humans)

Question 11

what are cathelicidins and what cells produce them?

Answer 11

similar as defensins (also disrupt microbial membrane) - but they do not have the disulphide bonds. constantly produced by neutrophils, macrophages, epithelial cells (lungs, intestine) and keratinocytes -> in response to an infection.

Question 12

what are histatins and their function?

Answer 12

small, cationic proteins constantly produced in the oral cavity. they are active against pathogens in the mouth.

Question 13

what are lectins?

Answer 13

carbohydrate-binding proteins with a bactericidal function

Question 14

what is a C-type lectin fold?

Answer 14

it’s the structure of the carbohydrate-recognition domain (CRD) -> can bind a variety of different carbohydrate structures

Question 15

what molecules are included in the C-type lectin family and what is their function?

Answer 15

selectins, collectins, proteoglycans, and endocytic receptors = some are secreted and some are transmembrane proteins
they function as adhesion and signalling molecules in many different immune functions -> for example, inflammation, immunity to tumour, and virally infected cells

Question 16

what is the complement system composed of

Answer 16

30+ plasma proteins mainly produced by the liver

Question 17

3 pathways of complement activation

Answer 17

classical -> antibody-triggered pathway
alternative -> amplification pathway
lectin pathway -> activated by lectin-type proteins that recognize and bind carbohydrates on pathogen surfaces

Question 18

what are the 3 effector functions of the complement cascade?

Answer 18

inflammation, phagocytosis, and membrane attack

Question 19

zymogen?

Answer 19

inactive pro-enzyme (proteases are synthesized as zymogens which become enzymatically active only after proteolytic cleavage -> usually by another complement protein)

Question 20

how is the lectin pathway initiated

Answer 20

direct recognition of pathogens -> initiated by soluble carbohydrate-binding proteins = MBL and ficolin

Question 21

MASPs? function?

Answer 21

MBL-associated serine proteases -> they are associated with the recognition proteins of the lectin pathway and they are the ones who activate the complement cascade

Question 22

how is the classical pathway initiated?

Answer 22

it is initiated when C1 - which comprises of C1q + 2 serine proteases = C1r and C1s -> either recognizes a microbial surface directly or binds to antibodies already bound to a pathogen

Question 23

how is the alternative pathway initiated?

Answer 23

can be initiated by a spontaneous hydrolysis and activation of C3, which can then directly bind to microbial surfaces. or more commonly -> is activated once lectin/classical pathway is activated

Question 24

what is the purpose of the alternative pathway?

Answer 24

amplification of the complement cascade -> it drives the production of more C3a and C5a to activate mast cells (inflammation), more complement to opsonize the pathogen, and more production of the membrane attack complex

Question 25

what factors are involved in the alternative pathway (exclusively)

Answer 25

factor B and D and properdin (factor P)

Question 26

what is the central step of the complement cascade

Answer 26

the initiation of the enzymatic activity of the C3 convertase

Question 27

what is the main effector molecule of the complement system and what is the function?

Answer 27

C3b - molecule that opsonizes pathogens --> phagocytic cells have receptors for C3b

Question 28

what are the complement molecules which mediate inflammation?

Answer 28

C3a and C5a

Question 29

what are the 2 functions of C3a and C5a?

Answer 29

recruitment of phagocytic cells (they act as chemokines) and the promotion of inflammation

Question 30

what is the mechnism of action of C3b opsonization of microbes?

Answer 30

C3b can form a covalent bond with the microbial surface -> due to a highly reactive thioester bond which is hidden inside the folded C3 protein --> cannot react until C3 is cleaved by C3 convertase

Question 31

what happens after the release of C3a fragment?

Answer 31

C3b undergoes conformational changes -> this allows the thioester bind to react with a hydroxyl or amino group on the nearby microbial surface

Question 32

what bacterial features are detected by the recognition proteins of the lectin pathway?

Answer 32

gram-pos bacteria -> lipoteichoic acid (cell wall) gram-neg bacteria -> LPS (outer membrane)

Question 33

what is an MBL monomer and what is it comprised of?

Answer 33

mannose binding lectin. a collagen region, an alpha-helix neck region, and a carbohydrate recognition domain -> C-type lectin like domain

Question 34

MBL structure

Answer 34

3 MBL monomers form a trimer. then between 2 - 6 trimers assemble into oligomers => mature MBL molecule

Question 35

MBL activation mechanism

Answer 35

Once MBL binds to a carbohydrate on a pathogen’s surface -> causes a conformational change in the MBL which activates the serum proteases . Then the active serine proteases activate C4 and C2 -> production of C3 convertase

Question 36

why is the MBL molecule an oligomer?

Answer 36

Because MBL has low affinity, but an MBL oligomer has a high avidity for carbohydrates

Question 37

what is an additional function of MBL?

Answer 37

MBL serves as an opsonin for monocytes which lack the macrophage mannose receptor but have receptors that can bind mannose-binding lectin coating a bacterial surface

Question 38

difference between MBL and ficolins

Answer 38

overall similar shape and function but instead of the lectin domain -> they have a fibrinogen-like domain

Question 39

the regulation of the lectin and classical pathways (2 ways)

Answer 39

1. the removal of the pathogen 2. C4-binding protein and factor I can dissociate the C3 converatse (C4b2a molecule)

Question 40

how is the membrane attack complex regulated?

Answer 40

by CD59 = protein expressed on human cells -> prevents the recruitment of C9 to form pores