Download Immunogenetics 1

PART I
• Immnoglobulins are proteins
• Proteins are specified by genes
• There are too few genes to specify all
the antibodies.
– i. e., ~32,000 genes < 10,000,000,000 Ab’s
• How is Ig diversity specified genetically?
Ig proteins are specified by genetic
“cassettes”
• Light chains are specified by “variable” (V),
“joining (J), and “constant” (C) gene
segments (aka “cassettes”).
DNA rearrangement
alternative RNA spicing
and
Ig proteins are specified by genetic
“cassettes”
• Heavy chains are specified by “variable” (V),
“diversity” (D), “joining (J), and “constant” (C)
gene segments (aka “cassettes”).
DNA rearrangement
alternative RNA splicing
and
Another view….
PART II
• Cassettes rearrange…
• How does this happen?
• How do you get one “V” fusing to one “J” (in a light
chain)?
• In a heavy chain, a “D” fuses with a “J”; then the fused DJ
cassette fuses with a “V” cassette…
• The orderliness of this process implies that
there are genetic instructions. What are they?
Cassettes rearrange…
• The heptamer is a palindrome
– (i.e., it exhibits two-fold rotational symmetry.)
• The nonamer is AT-rich
– “Turns” refer to the DNA helix…
Cassettes rearrange…
One turn – two turn rule…
• one turn and two turn are “recombination
signal sequences”
• one turn only reacts with a two turn
• Recombination signal sequences are the
substrates of enzymes RAG-1 and RAG-2
(“RAG” = recombination-activating gene)
So… cassettes are marked by RSS
(i.e., they are substrates for recombination.)
Thus, cassettes can be fused.
What is the consequence?
Look at mouse:
A mouse has:
134 VH, 13 DH, 4 JH segments
85 V6,
4 J6 segments
and
2 V8,
3 J8 segments
Thus, a mouse has:
134 C 13 C 4 = 6968 heavy chains
85 C
4 = 340 kappa chain
and
2C
3=
6 lambda
chains
PART III
(the HARD part…)
2.4 C 106 < 1010
So, there must be additional mechanisms of diversity
other than “fusing” “cassettes”
How does a RAG enzyme work?
Junctional flexibility
The “hairpin loop”
Junctional flexibility, “P” nucleotides,
and “N” nucleotides are added to
CDR3
Somatic hypermutation
One turn – two turn rule…
• one turn only
reacts with a two
turn
• crossover
between direct
repeats (same
transcriptional
orientation) leads
to deletion
• crossover
between indirect
repeats leads to
inversion
PART IV
• How do immunoglobulins assemble?
• Some immunoglobulins are in the surface
membrane of immature B-cells while other
immunoglobulins of the same idiotype are
secreted by mature B-cells. What’s the
difference?
• Similarly, identical variable regions can be
shared among different isotypes. How?
• B-cells are diploids with two sets of genetic
instructions. How does just one set get
expressed?