Antigen Recognition in the Adaptive Immune System

Question 1

describe B-cells receptors (BCRs)

Answer 1

• BCRs are antibodies
• BCRs recognize a broad range of chemical structures: proteins, lipid, carbohydrates, and nucleic acids
• B cells can recognize microbes and toxins in their native form
• antibodies exist in 2 forms:
  
  • membrane-bound antigen receptors on B-cells
  • secreted proteins
• both forms of antibodies recognize antigen by their variable domains
• the constant region of some secreted antibodies can also participate in antigen elimination

Question 2

describe the B-cell receptor: immunoglobulin

Answer 2

• composed of 4 polypeptide chains:
  
  • 2 identical heavy chains (H)
  • 2 identical light chains (L)
• end of the fork formed by light and heavy chains contain a wide range of variable antigen binding sites
• V = variable regions
• C = constant regions

Question 3

describe the light chain (red)

Answer 3

• comprises 2 domains:
  
  • the amino (N) terminal domain is variable and the site of antigen binding
  • the constant domain at the carboxy (C) terminal; it can be kappa or lambda

Question 4

describe the heavy chain (blue)

Answer 4

• it has a variable domain attached to several constant domains
  
  • the heavy chain determines the 5 classes of Ig: G, A, M, E or D
• constant region (C): nearly invariant; not involved in antigen binding
  
  • AA sequence defines the five heavy chain classes

Question 5

describe the variability in different regions of the Ig

Answer 5

Question 6

name the 5 classes of immunoglobulins

Answer 6

• IgG: monomer, produced by plasma cells (primary response) and memory cells (secondary), most prevalent
• IgA: monomer circulates in blood, dimer in mucous and serous secretions
• IgM: five monomers, first class synthesized following Ag encounter
• IgD: monomer, serves as a receptor for antigen on B cells
• IgE: involved in allergic responses and parasitic worm infections

Question 7

describe the antigen/antibody interaction for the B-cell receptor

Answer 7

• the antigen binds to the antibody by non-covalent interactions
  
  • strengths of the interaction: affinity
  • with repeat stimulation, the affinity increases: affinity maturation
  • avidity refers to the accumulated strength of multiple affinities of individual non-covalent binding interactions (10 for IgM compared to 2 for IgG)
  • cross-reactivity: one antibody, different epitopes

Question 8

BCR (B-cell receptor) recognizes ___ but can not transmit signals on its own

signaling functions are mediated by __ and __ associated with membrane Ig

Answer 8

BCR (B-cell receptor) recognizes antigen but can not transmit signals on its own

signaling functions are mediated by Igα and Igβ associated with membrane Ig

Question 9

describe T-cell receptor (TCR)

Answer 9

• TCR heterodimer recognizes peptides displayed by MHC molecules
• composed of 2 chains: α and β, each with a variable (V) and a constant (C) region
  
  • there are 3 hypervariable or complementary determining regions with V
  • like in antibodies CDR3 is the most variable
• TCRs are not produced in a secreted form
• TCRs do not undergo class switching

Question 10

describe lymphopoiesis

Answer 10

• B-cell production occurs throughout life; does not wane as does T cell production
• about 5 million produced per day
• about 10% of B cells mature
• naive B cells survive about 1 week
  
  • undergo negative selection

Question 11

describe B-cell development (3 stages)

Answer 11

• goal: production of plasma and memory cells
• process if divided in 3 stages:
  
  • generation of mature immunocompetent cells (antigen-independent phase)
  • activation of mature cells (antigen-dependent phase)
  • differentiation of active B cells into plasma and memory cells

Question 12

describe the pro B cells (antigen-independent phase of B cell maturation)

Answer 12

• Pro B cells
  
  • expression of CD45R and CD19
  • requires the bone marrow stromal cells to develop into a pre-B cells
    
    • cytokines (IL-7) support the process
• rearrangement of Ig heavy chain genes
  
  • successful rearrangement = pre-B cells
  • unsuccessful = apoptosis
• the enzyme terminal deoxyribonucleotidyl transferase (TdT) is expressed: catalyzes insertino of N-nucleotides at the heavy chain coding (only)

Question 13

describe the development of pre-B cells (antigen-independent phase of B-cell maturation)

Answer 13

• Pre-B cells
  
  • presence of Igμ heavy chain in cytoplasm
  • few expressed on cell surface associate with a surrogate light chain and Igα/β
  • signal:
    
    • induce recombination of Igk light chain locus (failure will bring about recombination of λ light chain) (V to J joining)
    • RAG1 and RAG2 are expressed in Pro and Pre-cell development
    • allelic exclusion (no Ig heavy chain recombination on second allele)

Question 14

describe the development of immature B-cells (antigen-independent phase of B cell maturation)

Answer 14

• immature B cells
  
  • IgM expressed on surface
  • functional B cell receptor appears
  • negative selection (clonal deletion) occurs

Question 15

describe further maturation and development of naive B cells (antigen-independent phase of B cell maturation)

Answer 15

• further maturation: bone marrow or spleen
  
  • expression of IgM and IgD = mature B-cell (follicular B cells)
  • this process involves a change in RNA processing on the heavy chain
• naive B cells
  
  • migrate out of bone marrow
  • quiescent; no dividing; G0
  • has not encountered antigen

Question 16

summarize B-cell development

Answer 16

Question 17

describe targets for treatment during B-cell maturation

Answer 17

• dysregulated B-cell receptor (BCR) signaling is a key driver of B-cell malignancies
  
  • kinases downstream of the B-cell receptor, such as SYK, PI3K and Bruton Tyrosine Kinase (BTK) are clinical targets for treatment of B-cell malignancies
• CD20 is expressed on all stages of B-cell development except the first and last; it is present from pre-B cells through memory cells, but not on either early pro-B cells or plasma cells
  
  • CD20 is the target of the monoclonal antibodies: rituximab, obinutuzumab, ibritumomab, and tositumomab, which are all active agents in the treatment of all B cell lymphomas and leukemias
  • function: enable optimal B-cell immune resposne, specifically against T-independent antigens

Question 18

give an overview of thymocyte development

Answer 18

• hematopoietic stem cells (HSC)
  
  • bone marrow
• common lymphoid progenitor
  
  • migrates to thymus
  • thymus is the maturation site
  • proliferation: IL-7 (common lymphoid only)
• T-cell precursor
  
  • pro-T cell
  • no TCR, no CD3 expressed

Question 19

describe the double negative cells (pre-T cells) in thymocyte development

Answer 19

• double negative cells: Pre-T cells
  
  • no expression of CD4 or CD8
  • rearrangement of TCR beta genes
  • loss of stem cell markers
    
    • c-kit, CD44
  • expression of pre-TCR (if TCR beta genes rearrangement is successful)
    
    • beta chain plus pre alpha chain
  • signals:
    
    • proliferation
    • allelic exclusion
    • alpha chain rearrangements CD4, CD8 expression

Question 20

describe positive selection during thymocye development

Answer 20

double positive cells (CD4+, CD8+)

• positive selection:
  
  • DP thymocytes interact with cortical epithelial cells
  • cells that do bind moderately to MHC molecules become single positive (CD4 or CD8)
    
    • TCRs recognize class I MHC-peptide complexes: preserve the expression of CD8
      
      • capable of becoming CTLs on activation
    • TCRs recognize class II MHC-peptide complexes: preserve the expression of CD4
      
      • T cell/thymocyte or T helper cells
  • ensures self-MHC restriction

Question 21

describe negative selection during thymocyte development

Answer 21

• negative selection
  
  • elimination of any CD4 or CD8 cells that have high affinity to self-MHC alone or to self MHC-self epitope complexes
  • these cells are programmed to die (apoptosis)
  • ensures self-tolerance

Question 22

describe AIRE

Answer 22

• autoimmune regulator (AIRE): transcription factor expressed in the medulla of the thymus
  
  • controls a mechanism that prevents the immune system from attacking the body
  • AIRE drives negative selection of self-recognizing T cells
  • through the action of AIRE, medullary thymic epithelial cells (mTEC) express major proteins from elsewhere in the body (“tissue-restricted antigens” - TRA) and T cells that respond to those proteins are eliminated through apoptosis
• when AIRE is defective, this can result in a variety of autoimmune diseases
  
  • hypoparathyroidism, primary adrenocortical failure, chronic mucocutaneous candidiasis