Antibody Diversity

Question 1

describe the steps in B-cell development

Answer 1

Question 2

describe the function of IL-7

Answer 2

Question 3

describe negative vs. positive selection in B-cells (the checkpoints)

Answer 3

Question 4

describe the steps in heavy chain rearrangement

Answer 4

Question 5

describe class switching

Answer 5

• during class switching, the antigenic specificity (or V protein domain) is preserved and only the constant domain changes
  
  • therefore different antibody classes will have the same antibody binding properties
• variety of diverse B-cell repertoire (as well as T cell) is generated through V-D-J somatic recombination before the B-cells meet any antigen
• once a particular B-cell clone encounters an antigen, it becomes active, proliferates and secretes first IgM
• under the influence of cytokines, the B-cell clone will undergo further gene rearrangement and be able to produce other Ig classes (G, A, E)

Question 6

describe light kappa chain rearrangement

Answer 6

• Pre-B cells go through another gene rearrangement round to generate light k (kappa) chain from chr. 2
• variable domain of light protein chains is coded by 2 gene segments: Vk (variable) and Jk (joining) genes
• the constant region has only one gene segment: Ck
• if the newly generated immature B-cell has high affinity to self-antigens, the rearrangment process may be reactivated to act upon lambda (λ) light chain genes

Question 7

describe allelic exlusion and light chain exclusion

Answer 7

• allelic exclusion: suppression of the recombination from the second chromosome therefore only one allele (maternal or paternal) will be expressed in a single clone
  
  • single antibody can combine heavy chain from maternal gene origin with a light chain from paternal gene origin
• light chain exclusion is the process of either k or λ chain synthesis, but never both of them in a single B cell

Question 8

describe the BCR associated membrane proteins

Answer 8

• cell surface Ig molecules (BCR) function is to:
  
  • recognize foreign antigens
  • send signal to intra-cellular components (tyrosine kinase) which initiates the B-cell activation cascades
• the trans-membrane region of the Ig anchors the molecule on the cell membrane
• signaling requires 2 adaptor surface glycoprotein molecules which are associated with the BCR
  
  • Iga (CD79a)
  • IgB (CD79b)

Question 9

describe the combinatorial diversity seen through random joining

Answer 9

Question 10

describe the function of TdT (terminal deoxynucleotidyl transferase)

Answer 10

• TdT adds random non-germline coded nucleotided to V-D and D-J junctions
  
  • the amount of newly added nucleotides increases with the individual’s age

Question 11

describe somatic hypermutation and AID

Answer 11

• somatic hypermutation is used in the process of affinity maturation initiated during the secondary stimulation
  
  • antibodies produced during primary exposure (immunization) have relatively low affinity compared to antibodies produced after secondary infection (boosting)
• somatic hypermutation is facilitated by activation-induced cytidine deaminase (AID)

Question 13

describe the function of AID

Answer 13

• AID is only expressed in activated B-cells and it works on:
  
  • immunoglobulin V regions hypermutation
  • the region which initiates antibody class switch
• the enzyme deaminates the cytidine to uridine only in a ssDNA
• such nucleotide conversion activates DNA mismatch and/or base excision repair mechanisms stabilizing the initial B cell mutation
• defects in AID will interfere with antibody maturation process and will prevent class switching, limiting antibody production predominantly to low variability IgM = hyper IgM syndrome

Question 14

describe the activation of B-cells by T-helper cells and cytokines

Answer 14

• B-cell activation
  
  • clustering of antigen receptors by binding of multiple IgV receptors
  • Iga and IgB signaling modulates gene expression in lymphoid follicles of lymphoid tissues
  • B-cell presents antigen to T helper cell which secretes cytokines that binds to cytokine receptors on B cells
  • CD40, CD40L and cytokines induce isotope switching in B cells and B cell proliferation

Question 15

describe the generation of TCR diversity

Answer 15

• analogous to BCR diversity
  
  • multiple germ-line segments
  • combinatorial V-D-J joining (random rearrangement)
  • junctional flexibility: imprecise joining at junctions
  • P-region nucleotide addition
  • N-region nucleotide addition
  • combinatorial association of a-chains with B-chains or γ- and δ-chains
• the B-gene rearrangement (D-J joining, then DJ-V joining) occurs first in pre-T cells
• followed by the a-chain rearrangement (V-J) joining in the immature T cell
• rearrangement of both γ- and δ- genes appear to occur simultaneously
• TCR does not exhibit somatic hypermutation

Question 16

summarize the mechanisms for generating receptor diversity between B-cells and T-cells

Answer 16