Ch 6 - Organization and expression of lymphocyte receptor genes

how can a limited amt of DNA produce extensive variety of Ab?

mechanisms used by BC, each set of gene families are encoded on separate chromosomes (allows variation at germline level), BC use recombination of segments of genes (random event = diversity)

chromosomes in human/mice

humans - lambda light chain 22, kappa light chain 2, heavy chain 14

mice - lambda light chain 16, kappa light chain 6, heavy chain 12

segments in immunoglobulin gene

variable (V), diversity (D only in heavy chain), joining (J), constant (C)

light chain vs heavy chain recombination

light = VJ recombination

heavy = VDJ recombination

organization of Ig genes

kappa chain - V, J, C

lambda chain - V, J and C alternate

heavy chain - V, D, J, C

lambda gene family

4 C genes, 30 V genes, leader region before V gene region, the L V J C exons are separated by introns

kappa gene family

1 C gene region, many V region genes, several J exons, all exons separated by introns

mechanism of VJ recombination

one of V genes is randomly brought next to one of J regions (all VJ combos eventually possible), recombination event removes intron between V and J, transcription of entire gene, mRNA introns are spliced in nucleus (LVJC are contiguous), mRNA translated in cytoplasm, then assembly with heavy chain

mechanism of VDJ recombination

first one of D regions is brought next to one of J regions, then one of V genes is brought next to rearranged DJ region (V gene chosen is random), only mu or delta is possible for C region (BC only IgM or IgD, can have IgM and IgD with same V region=same specificity), transcription of entire gene, mRNA processed so segments are contiguous, translated in cytoplasm, heavy chain assembled with light chain in ER

recombination signal sequences (RSS)

tells cell how to do recombination event properly, flanks the V,J,D exons, there are 2 turn (23 bp) and 1 turn (12 bp) RSS (a 2 turn and 1 turn must pair up), in VJ 1 has each (lambda V has 2 turn, kappa V has 1 turn), in VDJ 1 turn is on either side of D and V and J have 2 turn

Rag-1 and Rag-2 (recombination activating gene)

removes introns, Rag-1 is more important but both are essential, mutations may result in SCID

order of Ig gene expression

each BC has a maternal and paternal chromosome, there is an orderly way to make sure only 1 type of heavy chain and light chain are produced; first is heavy chain, random selection, 2 chances to get it right (if not it doesn't move to light chain), next it tries light chain, 4 chances, 2 for kappa and 2 for lambda (tries kappa first); this is why a ind BC can only make one type of Ab

allelic exclusion

process where BC ensure that only one heavy and one light chain are transcribed/translated, for BCR, mechanism still under investigation

receptor editing

double checks receptor to make sure it only recognizes nonself and not self, test for autoreactivity, if it is autoreactive it can be swapped for a different receptor

class switch recombination

process by which Ab other than IgM and IgD are generated, involves additional DNA recombination to get C region switched

Ab diversity theories

germ line theory - we have different V regions for each Ab possible

somatic mutation theory - we only have a few V regions but somatic mutations in BCs generates diversity

Ab diversity mechanisms

large number of V genes (30 lambda, 300 kappa, 1000 heavy chain), VJ and VDJ joining, junctional diversity (lesser mechanism, inaccuracies in VJ and VDJ recombination), N region insertion (at DJ junction, insertion of nucleotides by terminal transferase)

TCR genes and expression (similarities to BCR)

the alpha chain in TC is similar to Ab light chain, the beta chain in TC is similar to Ab heavy chain, random recombination (rearrangement is very similar to Ab rearrangement, TCR don't have functional C regions, multiple D regions possible in gamma chain, no somatic mutations)