Antibody Genetics II

Question 1

What happens if B cells do not encounter antigens in the secondary lymph organs?

Answer 1

They die within a few weeks

Question 2

How are B cells activated?

Answer 2

Rearrangement of genes and expression of IgM in BM
Enter circulation and may encounter Ag in secondary lymph organs

Question 3

What are T-independent Ag?

Answer 3

Do not need T cells
Particularly resistant to degradation

Question 4

What are the 2 groups of TI Ag?

Answer 4

TI-1 and TI-2

Question 5

What are TI-1?

Answer 5

Mainly bacterial cell wall components eg. LPS
In high conc they can polyclonally activate B cells

Question 6

What are TI-2?

Answer 6

Predominantly large polysaccharides with repeating antigenic determinants
Thought to x-link B cells, causing clustering
Require help from cytokines

Question 7

What do TI Ag generate?

Answer 7

IgM and do not induce memory

Question 8

What do TI Ag activate?

Answer 8

CD5+ B cells

Question 9

What are some examples of T-independent antigens?

Answer 9

lipopolysaccharide (LPS)
Ficoll
Dextran
Levan
Poly-D amino acids
Polymeric bacterial flagellin

Question 10

What is the T dependent response?

Answer 10

T and B cells recognise diff parts of the antigen
B cells see epitope, T cells see processed peptide fragment
T cells need peptide presenting on APC
T helper cells involved

Question 11

What is the role of T helper cells?

Answer 11

Interact with B cells and aid in activation and division

Question 12

How do Th cells interact with B cells?

Answer 12

Surface IgM binds Ag and internalises it
Peptide presents on MHCII
TCR and MHC polarise on cell surfaces
Co-stimulatory molecules bind
T cell produces cytokines allowing proliferation and differentiation of B cells into memory cells or AFC (antibody forming cells)

Question 13

What do T follicular cells do?

Answer 13

Localise to germinal centres and produce IL-21 and direct Ig class switching

Question 14

What does IL-4 do?

Answer 14

Induces activation and differentiation in B cells

Question 15

What does IL-5 do?

Answer 15

Similar effect to IL-4 but additional effects on eosinophils

Question 16

What does IL-6 do?

Answer 16

Induces B cells to become AFC

Question 17

What does IL-10 do?

Answer 17

Growth and differentiation of B cells, blocks Th1

Question 18

What does IL-13 do?

Answer 18

Directs response to IgE

Question 19

What are the 2 outcomes of B cell activation?

Answer 19

Prod of antibody forming cells - secrete antibodies to clear antigens, mostly die within 2 weeks
Prod of memory B cells - long lived and responsible for long term Ab production

Question 20

Where does B cell activation occur?

Answer 20

In the germinal centres of secondary lymphoid tissue, spleen (PALS - periarteriolar lymphatic sheath) and lymph nodes

Question 21

What is the sequence of events in B cell activation?

Answer 21

• Ag taken up by DCs (TLR etc)
• DCs activate Th in Ln (DC only cell to activate naïve T cells)
• B cells in the lymphoid tissue activated by soluble Ag
• B cells present to T cells and get some Ig production-extrafollicular activation, low
  level somatic mutation.
• Some T cells develop into TFH and these cells move to follicles
• B cells move to follicles
• T and B cells cooperate to form germinal centres where extensive somatic hypermutation, affinity maturation and Ig class switching takes place

Question 22

When does Ig class switching occur?

Answer 22

In the secondary response

Question 23

Where does Ig class switching occur?

Answer 23

The germinal centre

Question 24

What does class switching do?

Answer 24

Adds plasticity to the response
Determines the functionality of the antibody
Determined by cytokines

Question 25

What do cytokines do in the immune response?

Answer 25

Direct the way in which the class switch goes

Question 26

Give examples of how cytokines direct class switching

Answer 26

• Th1 cells are activated by viruses and bacteria to
  produce interferon gamma. This causes switch to IgG - main complement fixing isotype.
• Helminths produce IL-4 from Th2, IL-4 directs
  IgE production which target eosinophils to
  helminths
  – Mucosal tissue results in the switch to IgA under
  the influence of TGFb

Question 27

What does CD40 L on T cells do?

Answer 27

Ligates to CD40 on B cell

Question 28

What does the CD40L-CD40 interaction do?

Answer 28

Induces expression of activation-induced deaminase (AID)
Involved in class switching and somatic hypermutation

Question 29

What happens in switch recombination?

Answer 29

• Rearranged VDJ exon encoding the heavy chain V domain recombines with new C region
• Intervening DNA is deleted
• Switch regions are in the introns between J and each CH region
• Upstream of the switch (S) region there is an initiation (I) sequence
• Transcription occurs along the strand but strand breaks occur at the switch regions
• S mu strand break recombines with selected downstream switch region determined by cytokines
• Translation of the protein composed of original VDJ and new C

Question 30

What does AID do?

Answer 30

Converts C to U in single strand DNA switch region
Enzyme removes U, causing abasic DNA
Endonucleases cleave DNA at this site, causing nicks
Nicks on both strands = double stranded breaks
Complementary DSBs in switch regions combine leading to new heavy chain isotype produced by B cell

Question 31

What do follicular dendritic cells do?

Answer 31

Express C receptors and FcR
They hold Ag bound to these structures and present them to B cells

Question 32

What doe Tfh cells do?

Answer 32

Produce IL-21, driving B cells into apoptosis unless they are rescued by Ag recognition

Question 33

What happens as maturation progresses?

Answer 33

Ag conc drops so select for higher affinity B cells

Question 34

What contributes the long life span of memory B cells?

Answer 34

Bcl-2

Question 35

How are so many B cell specificities generated from 35000 genes?

Answer 35

2 chains per receptor
Multiple VDJ segments
Junctional diversity
Somatic hypermutation
C region switching