Immunology Chapter 4 Part 2

In all cells (except mature B cells), what does the germline configuration look like for the variable region genes of immunoglobulins? Be able to understand this concept in reference to mature T-cells; what does the germline configuration of the TCR variable region genes look like?

- Ig genes are in a fragmented form that cannot be expressed
- Contains multiple gene segments encoding portions of a single Ig heavy or light chain
- Has a lambda OR kappa light chain and a heavy chain locus
- T cells will not have germline configuration for TCR genes

Which gene segments provide diversity to CDR1 and CDR2 in the light chain and in the heavy chain?

V segments

For the light chain and the heavy chain, which gene segment provides diversity forCDR3?

V/J segments

somatic recombination

- Maturation of B or T cell, making the BCR or TCR
- Involved a DNA change
- It takes place in the bone marrow for B cells and in the thymus for T cells.

How many rearrangement events take place in the light chain?

1 recombination

How many rearrangement events take place in the heavy chain?

2 recombinations

Is the light chain rearranged first or the heavy chain?

rearrangement of DJ in heavy chain is first

What happens during the initial rearrangement and what are the events that follow?

Partial HC rearrangement->complete HC rearrangement->LC rearrangement->change in RNA processing->Ag stimulation->differentiation->isotype/class switching

What is the role of RSSs during somatic recombination?

- signals for the recombination process that rearranges the gene
- The landing pads for VDJ recombinase

what is the 12/23 rule

- 12 or 23 base pair sequences separate conserved heptamer/nonamer sequences
- ensures that gene segments are joined in the correct order
- aka 1-2 turn rule

Why are RAG genes (RAG-1 & RAG-2) important for somatic recombination?

RAGs are components of V(D)J recombinase, which is needed to recombine V, D, and J segments

Are they expressed in developing B cells, T cells, or both?

only made by maturing B and T cells

When does P and N nucleotide addition occur (before, during or after recombination) in junctional diversity?

occurs during recombination

What enzyme carries out in junctional diversity?

Carried out by terminal deoxynucleotidyl transferase (TdT)

Where does junctional diversity occur

Occurs at each junction (V-J, D-J, and V-DJ)

immature B cell

has only IgM on surface

mature B cell

has both IgM and IgD on surface

when is a B cell naive

B cell has never been activated, no isotype switch yet (but may still have both IgM and IgD on surface)

allelic exclusion

- Only one heavy chain and one light chain are produced in each B-cell.
- B cells are diploid and contain both maternal and paternal chromosomes; Rearrangedheavy chain or light is expressed from only one chromosome; This is called allelicexclusion.
- stops after we get a productive rearrangement; if we continued->decreased avidity because binding site is no longer the same.

Which constant region genes do NOT havea switch sequence, and why is this important?

the delta gene does not have a switch sequence; this is important because it allows both IgM and IgD to be expressed on surface o naive B cells

If you switched from IgM to IgG, would it be possible to ever go back to IgM or to IgD? Why or why not?

No, because going from IgM to IgG would splice out mu and delta, so you could not go back to IgM or IgD

What is the B-cell receptor complex composed of

- Composed of Igalpha and Igbeta
- Igalpha and beta are associated w every BCR (GAMED); both have longer cytoplasmic tails than Ig and interact w intracellular signaling molecules.

What must a B cell do to change a membrane BCR into a secreted antibody?

- The B cell must migrate to the Lymph node where it is going through the final maturation phase, and it is going to get IgM and IgD. Now that it is a mature B cell, it is ready to encounterantigen.
- Now, T cells are going to get activated and present antigen to B cells in the follicle.
- Some B cells will go to the medulla chords, become plasma cells, and secrete IgM.
- Some B cells will become a germinal center and undergo Somatic hypermutation to change the CDR, and Isotype switching to change Heavy chain. All these by using AID enzymes.

Is the antigen binding sites the same for IgM and IgD? If so, what characteristics make these two immunoglobulins different from one another?

- Yes, IgM and IgD have the exact same binding sites
- IgM can form a pentamer; IgD is always a monomerIgM is released first
- M->D is an RNA change of constant genes.

When is IgM and IgD both on the surface of the B cell at the same time? If they cut the DNA to change isotype, could they both be expressed at the same time?

- on a mature B cell.
- having both IgM and IgD is an RNA choice, so DNA has not been cut yet.

what part is permanent in somatic recombination

(DNA):
- V region assembly from gene fragments, junctional diversity (These two is somatic recombination)
- somatic hypermutation, isotype switch

what part is reversible in somatic recombination

(RNA):
- transcription activated w coexpression of surface IgM and IgD
- synthesis changes from membrane Ig to secreted Ab

where are the switch regions located

on 5' side of each C gene (except delta genes)

what is being switched, specifically

- recombines within a cluster of C genes, excising the previous C genes and joins a new C gene w previously assembled V region

what enzyme is involved in isotype switching

AID

what benefit does isotype switching provide

can produce Abs with other effector fncs

Do immature B cells, mature B cells, or both undergo isotype switching

mature B cells

would B cells be considered naive in isotype switching

No, because isotype switching occurs after a B cell has been activated

What can the antibody isotype be switched to?

IgA, IgE, IgG

What isotypes cannot be switched to?

mature B cells already express M and D

what is somatic hypermutation

- after a B cell has been activated, further diversification of V-domain coding occurs
- introduces point mutations throughout rearranged V regions at a high rate
- involved AID converting cytosine->uracil, excising them, and replacing them w non-templated nucelotides

where does somatic hypermutation occur

- Takes place in the lymph node.
- V regions of both heavy and light chains

when does somatic hypermutation take place

It takes place after activation

what type of mutation is introduced in somatic hypermutation

point

what enzyme is involved in somatic hypermutation

AID (activation-induced cytidine deaminase (AID)

Know what region of the immunoglobulin gene is affected and which is not affected

Variable region is affected (V and J for light chain, V, D, J for heavy chain); constant region is unaffected

How is affinity maturation achieved as a consequence

- somatic hypermutation results in mutant Igs on surface of B cells--some have higher affinity for the Ag, are more likely to bind to Ag, and the B cells w these Igs will be preferentially selected to mature into Ab-secreting cells
- as immune response proceeds, Abs of higher affinity are produced