Antigen Receptors (Ch 4)

Question 1

Antibody Structure

Answer 1

• 2 light chains (1V 1C) & 2 heavy chains (1V 3/4Cs)
  
  • Ea light chain is attached to 1 heavy chain; heavy chains are attached to ea other
• Ig domain - characteristic 3D shape
  
  • 2 layers of beta-pleated sheet held together by disulfide bridges; beta sheets connected by protruding loops
• Loops in V domain make up 3 CDRs for antigen binding; CDR3 has greatest variability and thus contributes most to antigen binding and is closest to V/C junction
• Fab - (antigen binding) whole light chain + V and 1st C of heavy chain
  
  • Ea antibody has 2 identical Fab regions
• Fc- (crystalline) remaining heavy domains
  
  • Ea antibody has 1 Fc region
• Fab and Fc sep by hinge region that allows ea to move independently of other

Question 2

IgA

Answer 2

alpha 1 or alpha 2 heavy chain

mucosal immunity

Question 3

IgE

Answer 3

epsilon heavy chain

Defense against parasites; immediate hypersensitivity

Question 4

IgD

Answer 4

delta heavy chain

naive B cell antigen receptor

Question 5

IgG

Answer 5

gamma 1-4 heavy chains

Opsonization, complement activation, antibody-dependent cell-mediated cytotoxicity, neonatal immunity, feedback inhibition of B cells

Question 6

IgM

Answer 6

Heavy chain mu

Naive B cell antigen receptor (monomer); complement activation

Question 7

How do antibodies bind antigen?

Answer 7

via reversible, non-covalent interactions (H+ bonds, hydrophobic interactions of charge)

Question 8

Epitope

Answer 8

• part of antibody bound by receptor
• AKA determinant
• Diff epitopes of same protein can vary by stretch (linear epitopes) OR shape (conformational epitopes)
• Some epitopes hidden until physiochemical change

Question 9

Affinity v Avidity

Answer 9

• Affinity for antigen usually expressed in Kd (higher affinity = lower Kd - less conc to get half bound)
  
  • Affinity inc w/ subsequent immune responses (affinity maturation)
• Avidity - total strength of binding; depends on # antigen binding sites w/ epitopes in them
  
  • IgD, IgE, IgG - 2 antigen binding sites
  • IgA (dimer) - 4 antigen binding sites
  • IgM (pentamer when secreted) - 10 binding sites

Question 10

Cross Reaction

Answer 10

when antibody binds structurally similar antigen

Question 11

TCR Structure

Answer 11

• α chain and a β chain; ea chain contains one V region and one C region; both chains participate in the recognition of antigens
• Also form loops connected by disulfide bridges
• Both chains anchored inc cell membrane
• Exception = γδ TCRs (γδ instead of alpha and beta chains); recognize variety of protein and non-protein antigens; mainly in epithelium

Question 12

4 Steps of B Cell Recombination

Answer 12

• 1- Ig heavy chain rearranges first; only in 1 IgH allele (paternal or maternal)
  
  • AKA allelic exclusion
  • D-J rearrangement THEN V-DJ rearrangement
• 2- CHECKPOINT - successful IgH rearrangement —> pre-BCR complex (new heavy chain + surrogate light chains) —> signaling that inhibits further heavy chain rearrangement and start light chain rearrangement
  
  • **IF MATERNAL FAILS REPEAT WITH PATERNAL
• 3- Ig light chain rearranges next; only V and J (no D); light chain k used first
• 4- CHECKPOINT - successful light chain pairs w/ heavy chain to make IgM BCR —> feedback inhibition signaling that inhibits further light chain rearrangements
  
  • **IF MATERNAL LIGHT CHAIN K FAILS REPEAT WITH PATERNAL K OR MATERNAL LAMBDA

Question 13

4 Steps of T Cell Recombination

Answer 13

• 1- Beta chain rearranges first; only 1 beta alleles (maternal or paternal) allelic exclusion
• 2- CHECKPOINT- successful recombination of beta chain loci —> pre-TCR complex—> intracellular signals that proliferate successful cells, turn off other beta recombination and turn on alpha chain recombination
  
  • **IF PATERNAL FAILS TRY OTHER PARENT
• 3- alpha chain recombination
• 4- CHECKPOINT - successfully expressed alpha chain —> dbl position T cells (have alpha and beta chains + BOTH CD4+ and CD8+)

Question 14

What enzymes are involved in recombination?

Answer 14

• 1- VDJ Recombinase Enzyme Complex - includes rag1 and rag2 - cleave Ig or TCR loci at recombination signal sequence motifs (RSS)
• 2- rag1/2 nick one strand of DNA
• 3- free 3’ OH attacks 2nd strand —> dbl strand DNA break
• 4- Artemis opens hairpin
• 5- TdT - adds or deletes nucleotides
• 6- DNA ligases reconnect new ends

Question 15

5 Mechanisms of Receptor Diversification

Answer 15

• Multiplicity of V D and J gene segments- multiple segments inherited
• Combinational Diversity - use diff combination of V D and J gene segments
• Junctional Diversity - introducing changes in nucleotide sequence at junctions of V D and J segments
  
  • Exonucleases - remove nucleotides from VDJ gene segments at recombination sites
  • TdT (terminal deoxyribonucleotidyl transferase) - catalyzes random addition of nucleotides at junctions (form N regions)
  • B/f DNA breaks are repaired, overhanging DNA sequences can be generated and filled in (form P regions)
• Associational Diversity - pairing of different Ig heavy and light chains OR TCR alpha and beta chains
• **Somatic hypermutation - point mutations in re-arranged V region genes of B cells ONLY after activation by antigen

Question 16

3 Mechanisms of B Cell Negative Selection

Answer 16

• Clonal Deletion - if strong signal to self…apoptosis
• Anergy - if weak BCR signals to antigen…IgM internalized and non-responsive B cell
• Receptor Editing - if an immature B cell binds an antigen weakly, it may re­-express the VDJ recombinase enzyme (rag), undergo additional light­chain V­J recombination, generate a different light chain, and thus change the specificity of the antigen receptor

Question 17

Hybridoma

Answer 17

• take advantage of monoclonal antibody proliferation; combine myeloma tumor w/ B cells from immunized animal —> cont growth and production of desired antibody
• Use immortality of tumor AND enzymes needed from B cells to grow in presence of certain toxic drugs

Question 18

SCID

Answer 18

• Null mutation in rag, artemis or DNA repair enzymes
• Mutations in artemis and DNA repair enzymes also associated w/ radiation sensitivity
• Treat - bone marrow transplants and gene therapy

Question 19

Omenn Syndrome

Answer 19

• Leaky SCID”
• Hypomorphic rag rather than none at all; variable # of oligoclonal T cells and few if any B cells
• expansion of maternal lymphocytes in fetus chronic inflammation (eosinophilia, swollen spleen, diarrhea), erythroderma

Question 20

2 Examples of How Recombination Errors Can Cause Malignancy

Answer 20

• VDJ recombination error —> follicular lymphoma by causing Bcl/IgH translocation
  
  • Bcl is recruited as recombination target erroneously - greater survival
• VDJ recombination error —> second hit in acute lymphoblastic leukemia (ALL)
  
  • Delete good copy of gene via TdT

Question 21

What genes are involved in recombination?

Answer 21

• All hematopoietic stem cells have both Ig and TCR gene regions
• Ig heavy and light chain loci have 30-45 variable gene segments and a couple constant genes
• TCR alpha and beta chain loci have 30-45 variable gene segments and a couple constant genes
• B/n C and V genes are several short coding sequences called diversity (D) and joining (J) gene segments
• All antigen receptor gene loci contain V, J, and C genes, but only the Ig heavy­ chain and TCR β chain loci also contain D gene segments