16. antigen receptor genes & antibody diversity

antigen-independent phase

• maturation in bone marrow/thymus → produces mature, naive lymphocytes

• prior to antigen exposure

do progenitor B cells express antigen receptors?

no

what is the key event in lymphocyte development/maturation?

• expression of antigen receptor genes

• functional antigen receptor gene is created in immature lymphocytes

positive selection

selection for lymphocytes with useful specificities

negative selection

selection against lymphocytes that are self reactive

organization of antigen receptor gene loci

• 3 loci: one for heavy chain and two for light chain (kappa & lambda)

• spatial segregation of sequences that must be joined together to produce functional antigen receptor gene

heavy chain variable region genes

encoded for by 3 separate gene segments

• V_H (variable)

• D_H (diversity)

• J_H (joining)

light chain variable region genes

• encoded for by 2 separate gene segments

  • V_L (variable)

  • J_L (joining)

*note: no diversity gene segment (no D_L)

constant region gene

both heavy and light chains have one gene segment (C_H & C_L)

why is IgM the first class of antibody produced?

naive B cells transcribe Cμ (mu), the most proximal gene segment on the chromosome

what gene segment(s) encode CDR1 and CDR2?

V segment

what gene segment(s) encode CDR3 for heavy chains?

V, D, and J segments

what gene segment(s) encode CDR3 for light chains?

V and J segments (light chains do not have D segments)

how does gene rearrangement code for different antibodies?

• somatic recombination of separate gene fragments generates complete variable regions

• one gene segment from each group will combine randomly to form a complete antibody

where and when does gene rearrangement in B cells occur?

• occurs in the bone marrow

• prior to antigenic exposure → antigen-independent

what does the joined VDJ unit encode?

the entire variable region of the heavy chain

what enzyme mediates gene rearrangement?

VDJ recombinase → Rag-1 and Rag-2 (recombination activating gene)

• only expressed in immature B and T cells (turned off in mature lymphocytes)

• randomly binds J, D, and V segments → chops off spare DNA between segments

recombination signal sequence (RSS)

• motifs that flank the recombining gene segments

• recognized by VDJ recombinase

combinatorial diversity

the ability to create many different specificities by making many different combinations of a small number of gene segments (V-[D]-J joining)

junctional diversity

• random addition and deletion of nucleotides at the junctions between V, D, and J segments in heavy chains (V and J for light chains)

• increases diversity of hypervariable region 3 (CDR3)

terminal deoxyribonucleotidyl transferase (TdT)

• enzyme that randomly adds nucleotides to junctions/coding joints

• does not need a template to add nucleotides

what are mechanisms of antibody diversity?

• combinatorial diversity

• junctional diversity

• combinatorial association of heavy and light chains

• somatic hypermutation — occurs AFTER antigen exposure

what is monospecificity of B cells?

• a fully differentiated B cell has only one successfully rearranged heavy chain and one successfully rearranged light chain

• each B cell will produce antibodies of one specificity

allelic exclusion

expression of a gene from only one of the parental chromosomes in each individual B cell

what dermines which alleles will generate the heavy & light chains?

• both alleles start recombining → whichever completes it faster wins out and suppresses the recombination of the other allele

• if the first allele fails, the second allele will continue to recombine and generate the chain