BCRs

Question 1

where is BCR located?

Answer 1

B cells

Question 2

What is BCR composed of?

Answer 2

• two identical heavy chains
• two identical light chains
• held together by disulphide bridges

Question 3

what forms antigen-binding site?

Answer 3

variable regions of the heavy and light chains which allows the BCR to recognise specific antigens

Question 4

what role does BCR play?

Answer 4

in adaptive immunity by allowing B cells to recognise a wide range of antigens and mount an effective immune response against them

Question 5

what happens when an antigen binds to the BCR?

Answer 5

triggers a seris of intracellular signalling events that activate the B cell resulting in proliferation, differentiation into antibody-sescreting plasma cells and the generation of memory B cells

Question 6

what does proliferation of B cells within the germinal centre lead to ?

Answer 6

• isotope switching of the antibodies from IGM and to IGG, IGA etc
• drive of affinity maturation

Question 7

what are the two forms of BCR

Answer 7

1- Signalling receptor
2- Antibody

Question 8

describe structure of membrane bound form of BCR

Answer 8

• hydrophobic residues at the carboxyl terminus of the heavy chain to anchor the BCR into plasma membrane

made of three sequentially arranged regions:
- extracellular hydrophillic region (spacer region about 26 AA)
- hydrophobic transmembrane segment (25 AA)
- short cytoplasmic tail inside of that membrane

Question 9

whats the difference in structure between membrane bound BCR and soluble circulating BCR (antibodies)

Answer 9

the hydrophobic residues in the membrane bound are exchanged for more hydrophillic AA so they can be soluble and circulate throughout the blood

Question 10

describe structure of membrane bound BCR further

Answer 10

• membrane-bound immunoglobulin non-covalently linked to a heterodimer composed of IGa and IGb
• this is in toa 1:1 stochiometry

Question 11

what does the IGa and IGb heterodimer responsible for?

Answer 11

transducing the antigen signal once bound from the outside to the inside of the cell

Question 12

describe IGa and IGb structure

Answer 12

• short n terminal Ig like domains at the top
• long cytoplasmic tail which contain immuno-receptor tyrosine based activation motifs (ITAMs)

Question 13

what happens when antigen binds to BCR?

Answer 13

induces phosphorylation of ITAMs which results in the triggering of signalling cascades (CD79)

Question 14

Whats another name for IGa and IGb heterodimer?

Answer 14

CD79

Question 15

What phosphorylayes CD79?

Answer 15

LYN (Kinase)

Question 16

describe the signalling cascade caused once CD79 is phosphorylated?

Answer 16

• CD79 phosphorylated by LYn cayses activation of other sites and kinases and signalling peptides such as BLINK, phospholipase C and BTK. forming a signalosome
• phospholipase C releases DAG which triggers RAS pathway and another pathway
• PPC also releases IP3 whihc can then activate Ca NFAT pathway
• leading to activation of B cell

Question 17

describe all IgX

Answer 17

IgG- membrane bound and secreted
IgM - membrane bound and secreted as a pentameter
IgD- mainly acts as a receptor
IgA - membrane bound and secreted as a dimer
IgE - membrane bound and secreted

Question 18

what is the antigen bindingsite made up of?

Answer 18

• 6 CDRs that protrude from the V domains
• CDRs are most variable regions of BCR responsible for specificity of antigen binding
• CDR1 and CDR2 short and located at N-terminal end of variable domamins
• CD3 is longest and most diverse

Question 19

name all the moleculat interactions between CDRs and antigens?

Answer 19

• electrostatic
• hydrogen bonds
• van der waals
• hydrophobic interactions
• cation-pi interactions

Question 20

what is the origin of cation-pi interactions?

Answer 20

non-covalent interaction between a cation and an electron cloud of a nearby aromatic group

Question 21

what four things contribute to the specificity of CDRs?

Answer 21

1) AA sequence
2) Structural conformation - flexibility
3) Structural conformation - binding pockets
4) Affinity

Question 22

how is AA sequence of CDRs generated?

Answer 22

through somatic recombination and somatic hypermutation - allowing for a wide range of possible amino acid sequences and a corresponding diversity of potential antigen binding sites

Question 23

which CDR has the most number of contacts with the antigen?

Answer 23

CDR3

Question 24

What three key things are important in flexibility of BCRs?

Answer 24

1) Adaptation to antigen shape
2) High specificity
3) High affinity

Question 25

name difference between binding pockets when:
A) Proteins
B) Peptides
C) Haptens

Answer 25

A) Plateau’s (flat surfacae)~ 800A
B) Grooves ~ 400-600A
C) Pockets ~ 200-400A

Question 26

what does low Kd mean?

Answer 26

high affinity antibody

Question 27

what is affinity determined by?

Answer 27

number and strength of bonds and surface shape and size

Question 28

what is avidity?

Answer 28

overall strength of binding

Question 29

what is SPR?

Answer 29

surface plasmon resonance

Question 30

what are 4 ways to improve antibody affinity?

Answer 30

1) Affinity maturation
2) site directed mutagenesis
3) protein display systems (phage, ribosome, yeast)
4) protein engineering

Question 31

what are 4 applications for the study of antigen recognition?

Answer 31

1) design of vaccines
2) development of therpeutics
3) understanding immune system function
4) engineering improved antibodies

Question 32

what is site directed mutagenesis?

Answer 32

this involves making specific changes to the antibody’s amino acid sequence to optimise its interaction with the target antigen

Question 33

what is protein engineering?

Answer 33

this involves using computational methods to design new antibody variants with improved affinity for the target antigen