Antigen receptor on lymphocytes

Question 1

What is the composition of the BCR?

Answer 1

our polypeptides, two identical light chains and 2 identical heavy chains, connected by disulphide bonds

Question 2

What are the two types of light chains?

Answer 2

Lambda and Kappa

Question 3

What is the valency of the BCR?

Answer 3

2

Question 4

What is the composition of the TCR?

Answer 4

two polypeptides, also disulphide linked and spanning the membrane.

Question 5

What is the valency of the TCR?

Answer 5

1

Question 6

What are the two types of TCR?

Answer 6

alpha/beta or gamma/delta heterodimers

Question 7

What are the 2 fundamental differences between the TCR and the BCR?

Answer 7

1. T cells do not secrete TCR while B cells secrete immunoglobulin molecules by deleting the
   transmembrane-spanning exon during alternative splicing of the primary RNA transcript
2. The BCR is specific for native or intact antigen, but the TCR is not. Instead, antigen must be presented by self MHC molecules to the TCR

Question 8

What are 2 other differences between the BCR and the TCR

Answer 8

1. The BCR variable region undergoes hypermutation during affinity maturation while the TCR variable regions do not hypermutate
2. The constant regions of the BCR heavy chain can be replaced during class switching while the equivalent does not occur with the TCR

Question 9

What is a similarity between the TCR and the BCR

Answer 9

both receptors associate with additional polypeptides that don’t bind antigen but transmit a signal

• Ig alpha and beta for the BCR
• CD3 for the TCR

Question 10

What are the 2 theories explaining the variable amino terminus on the light chain but the constant carboxy terminus

Answer 10

1. Germ line theory

2. Somatic variation theory

Question 11

What does germ line theory propose?

Answer 11

Different genes for each immunoglobulin

-this would take up a huge portion (15%) of our genome

Question 12

What does somatic variation theory propose?

Answer 12

The genome contains a relatively small number of Ig genes from which a larger repertoire arises by recombination or mutation events

Question 13

What did Dreyer and Bennet propose in 1965?

Answer 13

wo genes encode a single immunoglobulin light or heavy chain, and that a recombination event explained their contributing to a single polypeptide

Also that there were a larger number of variable genes than constant region genes

Question 14

What did Tonegawa and Hozumi prove in 1976

Answer 14

Showed direct evidence that seperate genes encode the V and C regions of immunoglobulins

Question 15

What are the 3 types of gene segment regions?

Answer 15

(V)ariable, (D)iversity, (J)oining

Question 16

Where is the D region present?

Answer 16

Only in the heavy chain DNA and only in one of the heavy chains

Question 17

What is the order of recombination events?

Answer 17

D to J rearrangement followed by a V to D rearrangement

Question 18

What is the splice site it in each segment called?

Answer 18

The coding joint

Question 19

What is the RSS

Answer 19

recombination specific sequence

Question 20

What are the two forms of RSS and what is the general composition?

Answer 20

The two forms are based on the size of the variable spacer
-one is 12 bp the other is 23 bp

both forms contain conserved PALINDROMIC heptamer and nonamer sequences separated
by variable sequences

Question 21

What is the relevance of the variable spacer lengths?

Answer 21

may related to the number of bases in a turn of supercoiled DNA, where 12 bp is a single turn while 23 bases encompass two turns

Question 22

What multiprotein complex is responsible for recombinations?

Answer 22

the V(D)J recombinase

Question 23

What are the two lymphoid cell specific molecules in the V(D)J recombinase?

Answer 23

RAG1 and RAG2

Question 24

how do the RSS elements impact the order and process of recombination?

Answer 24

A 12 bp RSS matches with a 23 bp RSS to identify a site suitable for recombination

Question 25

What are the 5 steps of Variable region recombination?

Answer 25

1. RAGs bind to the RSS and catalyze synapse formation - forms a hairpin coding end and blunt signal end 2. Opening of hairpin loop by Artemis yields 3 possible options - P nucleotide additions results in a short palindrome 3. ** in heavy chain VD and DJ joint only there is exonuclease trimming at either side of the joint 4. N nucleotide additions by terminal deoxynucleotide transferase (TdT) 5. Ligation of free ends to seal coding joint by DNA ligase IV and NHEJ proteins

Question 26

What are 5 mechanisms that contribute to antibody diversity?

Answer 26

1. Multiple gene segments - permutations of V to D to J 2. P nucleotide addition 3. Exonuclease trimming (at 3' end) 4. Non templated N nucleotide additions 5. Combinatorial diversity - 2 chains combining into one antigen binding site

Question 27

How does polyadenylation allow for simultaneous expression of IgM and IgD?

Answer 27

polyadenylation signals flag splicing points in the mRNA product of the mu and delta heavy chains

Question 28

What is somatic hypermutation? when does it occur? Where does it occur?

Answer 28

Somatic hypermutation occurs in the complementarity determining regions (CDR) of the variable region DNA. After the BCRs bind antigen, slight alterations are made in the V region to improve affinity of the immunoglobulin for the antigen

Question 29

What is affinity maturation?

Answer 29

the net increase in Ig affinity within a B cell population

Question 30

What enzyme is responsible for somatic hypermutation?

Answer 30

AID (activation induced cytosine deaminase)

Question 31

Class switch recombination is also known as?

Answer 31

Isotype class switching

Question 32

What are switch regions?

Answer 32

Highly repetitive DNA stretches

Question 33

What enzyme is used for class switch recombination?

Answer 33

AID (activation induced cytosine deaminase)

Question 34

Describe the process of class switch recombination

Answer 34

1. AID makes double stranded break in S regions upstream and downstream of segment to be removed 2. Switch regions are joined and intervening DNA removed

Question 35

Where is there NOT a switch region present?

Answer 35

between the mu and delta regions

Question 36

What class switch does IL-4 induce?

Answer 36

to IgG1 and IgE | -has to go through IgG1 to get to IgE

Question 37

What class switch does IL-5 induce?

Answer 37

to IgA

Question 38

What class switch does TGFß induce?

Answer 38

to IgA and IgG2

Question 39

What class switch does INF gamma induce?

Answer 39

to IgG3 and IgG2a

Question 40

What are 6 similarities between TCR and BCR gene rearrangement

Answer 40

1. Multiple gene segments for each of V, (D), and J 2. ß and ∂ have D regions like the heavy chains of BCR 3. Like IgH, ß and ∂ chains pair with chains without D regions (alpha and gamma) 4. Order of recombination is D to J and then V to D 5. N and P nucleotide additions occur in the coding joint 6. 2 polypeptides combine their V regions to form the antigen binding site

Question 41

What is a fundamental difference between TCR and BCR gene rearrangement

Answer 41

TCR gene segments have RSSs but alternate arrangement of the 12 bp and 23 bp variable sequences allows for recombination between D regions

Question 42

How does alternate joining of D region segments affect the coding joint of delta chains?

Answer 42

Allows multiple D regions to be included in the coding joint

Question 43

Do TCR segments undergo somatic hypermutation?

Answer 43

nope

Question 44

Name the 5 somatic immunoglobulin deficiencies

Answer 44

1. X-linked agammaglobunlinemia (Bruton's hypogammaglobulinemia) 2. Hyper IgM syndrome 3. Common variable immunodeficiency (CVIDs) 4. Hyper IgE 5. Selective IgA deficiency

Question 45

X-linked agammaglobunlinemia | (Bruton's hypogammaglobulinemia) causes:

Answer 45

No peripheral B cells - B cells arrested at pro-B stage Extremely low IgG No other classes of Ig

Question 46

Hyper IgM syndrome causes:

Answer 46

Deficiency in IgG, IgA, and IgE Increased IgM

Question 47

What is hyper IgM caused by?

Answer 47

T cell defect which prevents B cells from receiving a class switch signal

Question 48

What causes X-linked agammaglobunlinemia | Bruton's hypogammaglobulinemia

Answer 48

Defect in Bruton's TK = B cells arrested at pro-B stage

Question 49

Common variable immunodeficiency (CVIDs) cause:

Answer 49

low # of antibody producing cells and antibody B cells fail to mature into plasma cells

Question 50

What causes hyper IgE and what is the result?

Answer 50

Multisystem disorder with high IgE due to mutates STAT3 resulting from abnormal Th activity

Question 51

What is selective IgA deficiency ?

Answer 51

IgA+ cells cannot differentiate into plasma cells