Immunoglobulin Expression and the Role of Antigen in Immunoglobulin Diversity March 20, 2009 10:00-11:00

REVIEW OF ANTIGEN INDEPENDENT GENERATION OF DIVERSITY

Genomic Ig Loci Figure 2-14

Figure 2-18 V(D)J SEGMENT JOINING GENERATES VARIABLE REGION NB—Joining deletes intervening gene segments

ANTIGEN BINDING SITES ARE HYPERVARIABLE

THE HEAVY CHAIN LOCUS AFTER VDJ JOINING NB---the complete VH (VDJ segments) is juxtaposed to the C region genes

H and L Proteins are Expressed from Rearranged Loci

MEMBRANE Ig IS ASSOCIATED WITH OTHER PROTEINS Ig expression requires the presence of other proteins Iga and Igb are invariant proteins that are necessary for mIg expression Iga and Igb are also required for signal transduction

ALLELIC EXCLUSION MAKES B CELLS MONOSPECIFIC ALL THE Ig MOLECULES ON A B CELL ARE THE SAME Every cell has 2 Alleles for each gene B cells express only one heavy chain allele: ALLELIC EXCLUSION B cells express only one allele of either k or l LIGHT CHAIN ISOTYPE EXCLUSION ALLELIC EXCLUSION MAKES B CELLS MONOSPECIFIC

CO-EXPRESSION OF IgM AND IgD Naïve B cells exit the bone marrow with both mIgM and mIgD The same VDJ is juxtaposed to Cm and Cd VDJ becomes associated with Cm or Cd by differential RNA processing IgM and IgD EXPRESSION ARE ANTIGEN INDEPENDENT

THE EFFECT OF ANTIGEN ON IMMUNOGLOBULIN EXPRESSION

ANTIGEN CAUSES B CELLS TO BECOME PLASMA CELLS The form of immunoglobulin on the membrane of a B cell is different from secreted form of immunoglobulin

ALL HEAVY CHAIN GENES ARE ORGANIZED SIMILARLY Ce Ca Cm Cg 4 3 2 1 s M1 M2 All isotypes have membrane and secreted forms

MEMBRANE AND SECRETED FORMS ARE GENERATED BY DIFFERENTIAL RNA PROCESSING Membrane form is longer: it has extracellular, transmembrane, and cytoplasmic regions Secreted form is shorter : it only has an short tail

ANTIGEN INCREASES THE DIVERSITY OF V REGIONS The process that increases diversity is called SOMATIC HYPERMUTATION The result is AFFINITY MATURATION

ANTIGEN CAN CAUSE VDJ TO ASSOCIATE WITH A DIFFERENT CH

ISOTYPE SWITCHING OCCURS BY DNA REARRANGEMENT S regions are repetitive sequence 5’ to every C except Cd Recombination occurs between S regions Intervening DNA is deleted and VDJ becomes associated with a new CH

THE EFFECTS OF ANTIGEN ON Ig EXPRESSION Somatic hypermutation…affinity maturation Alterations in RNA processing to yield secreted form of Ig Isotype switching

The Effect of Isotype (Heavy Chain Class) Switching on Ig Function

GENOMIC ORGANIZATION VS PROTEIN STRUCTURE Cg 3 2 s M1 M2 1 Ca 3 2 1 s M1 M1 M2 Cm 1 2 3 4 s Ce 4 3 2 1 s M1 M2 Structural differences are responsible for functional differences

ISOTYPES HAVE DIFFERENT PHYSICAL PROPERTIES

IgA AND IgM FORM MULTIMERS

DIFFERENT ISOTYPES HAVE DIFFERENT FUNCTIONS

The importance of multiple isotypes Dennis Fawcett 5 year old presenting with an acute sinus infection Recurrent sinus infections since he was 1 Pneumocystis carinii at 3 years of age Testing revealed: IgG 25 mg/dl (normal 600-1500) IgA not detected IgM 210 mg/dl (normal 75-150) Point: Absence of specific isotypes increases susceptibility to infection Many bacteria are effectively cleared by opsonization