L2- genetics of anitgen reognition receptors

Whta is the numebr of possibel different antibodies humans

>10^9

What allows the vast numer of possibel antibodies

A rnage of mechanisms simialr but not identical to thos that exist for the generation of TCRs → is heavily regulated and controlled

Where do you want variability

At the antibody binds site

Where are the V and C regions of antibody and TCR polypeptide chains encoded

Seperate gene segments that rearrange during lymphocyte differentiation

On the h chain and TCR beta, what encodes the V region

3 gene segments: V, D and J where V is the biggest

What encodes the L chasing and TCR alpha

V region encoded by 2 gene segments V and J → is much smaller so requires fewer segments that H and TCRb regiosn

What are Ig genes foudn in the genome as

Gene segments

What happens to DNA containing Ig segments in B cells during their development

Is deliberatly broken and gene segments rearragne through non homologous end joining recombination to form functional Ig genes that are random in each B cells

What does the production of functional imunoglobulin genes involves (3)

• Recombination of a singel V and singel J gene segments that rearrange to encode the V region of the light chain in a random process

• a singe random, V, D and J gene segments are joined together in a singel B cells to encode the V region of the heavy chain in a

• Loop out unwatned parts during each process

Fromation of polypeptide chain (11)

germline DNA → somatic recombination → D-J joined rearranged DNA → somatic recombination → V-J pr D-J joined rearranged DNA → Transcription → primary transcript RNA → splicing → mRNA → translation → polypeptide chains

Where does somatic recombination of the germline dna happen

In the bone marrow

What is the order of Ig rearrangements (3)

• H chain gene segments rearrange (D-J then V-DJ) with greater variability in H chain as V, D and J segments

• Light chain gene segments rearrange: kappa segments (V-J) first

• If kappa rerrangement is unsuccessful, then lambda gene segments (V-J) rearragne

What segments make up the light and heavy chains

• Light : kappa and lambda

• Heavy: heavy

Where are the V D and J segments encoded

• H chain→ chromosome 14

• Kappa chain → chromosme 2

• Lambda chain → chromosome 22

How ar. And H loci in the human genome organised

DNA can be looped but once this happens itself and everything downstream of it is cut out permanently

What is somethig. That changes between each antibody

The constant region

Which calss of antobody is always eh first made

IgM

What guide Ig genes segment rearrangement

Special recombination signal sequences that flank each of the V,D and J gene segments

What enzyem is invovled in gene rearrangement

Complex of enzymes → V(D)J recombinant

What do recombination activating genese encode

RAG1 and 2 encode lymphoid specific components of the recombinase enzymes → are used in B sna d T cell develoepmtn

What can mutations in RAG genes result in

Combined immunodeficiency

What is the principle of allelic exclusion

• in each individual B cell, only one rearragend H chain gene from one crhomosome is expressed

• Only one rearranged L chain from one chromosome is expressed by each individual B cell

What is light chain isotype exclusion

Each B cel expresses either a rearragne end kappa or lambda light chain NEVER BOTH

What do mechanisms of allelic exclusion ensure

That each indivudal B cell produces just one randomly generated BCR/antobody that is different frmo the BCR/ antobody made by every other b cell

What are the mechanisms for generation of antibody diversity (5)

• there are multiple gene segments for each chain as well as mutliple VH, Vkappa and Vlambda variations + multiple D and J gene segments

• Combinatorial iverity

• Combination of heat and light chains

• Junctioanl diverity

• Somatic hypermutation

Combinatorial diverity

Different V,D and J segments recombine to prduce different sequences with 8100 combinations for H chains and 300 possibel combos for the light chain

Why is the H chain more diverse tha nthe light chain

H chain has D segments

How many posible BCR/antobody combos are there thanks to the combination fo heacy and iwth chains

2.43 × 10^6

Where do errors mainyl occur in antobody diversity

Junctioanl diveristy mechanism

How does Junctioanl diveristy work/ create diveristy

• get imprecise joining → have small differences in sequences wher V-D and D-J segments join

• N regions (random addition of nuceltodies at junctions of V-D and D-J by terminal transferase)

Somatic hypermutation

Mutation frequency in antobody (H/L) V genes in orders of magnitude hgiher than that seen in all other areas of the genome

Where does somatic hypermutation occur

In teh geminal centres as B cells recongise Ag and proliferate/ become activated

How mnay antbodies/ BCRs are possibel

10^9

How does SHM work and cuase mutation

• performed by enzyme activaiton induced deaminase (AID)

• AID acts on DNA to de amintae cytosine to uracil

• Uracil is then recongised by error prone DNA repair pathways leading to mutations