Organization and expression of immunoglobulin genes

From studies and predictions such as Dreyer and Bennett's, it shows that the light chains and heavy chains are encoded by separate multigene families on different chromosomes. They are referred to as gene segments and are separated by non-coding regions. The rearrangement and organization of these gene segments during the maturation of B cells produce functional proteins. The entire process of rearrangement and organization of these gene segments is the vital source where our body immune system gets its capabilities to recognize and respond to variety of antigens.

1. Dr. Dinesh C. Sharma,
Associate Professor and Head-Zoology
K.M. Government Girls P.G. College, Badalpur, GB Nagar, India

2. •Anantibody(Ab),alsoknownasanimmunoglobulin (Ig),isa
large,Y-shapedproteinproducedmainlybyplasmacells(B-
lymphocyte) thatisusedbytheimmune system to
neutralizepathogens(bacteria,virusesetc.).
•Theantibodyrecognizesauniquemoleculeofthepathogen,
calledanantigen,viathefragmentantigen-binding(Fab)
variableregion.Eachtipofthe"Y"ofanantibodycontains
aparatope(analogoustoalock)thatisspecificforone
particularepitope(analogoustoakey)onanantigen,allowing
thesetwostructurestobindtogetherwithprecision.
•Usingthisbindingmechanism,anantibodycantagamicrobeor
aninfectedcellforattackbyotherpartsoftheimmunesystem,
orcanneutralizeitstargetdirectly(forexample,byinhibitinga
partofamicrobethatisessentialforitsinvasionandsurvival).
Dependingontheantigen,thebindingmayimpedethebiological
processcausingthediseaseormayactivatemacrophages to
destroytheforeignsubstance.
•Theabilityofanantibodytocommunicate withtheother
components oftheimmune systemismediatedviaitsFc
region(locatedatthebaseofthe"Y"),whichcontainsa
conservedglycosylationsiteinvolvedintheseinteractions.The
productionofantibodiesisthemainfunctionofthehumoral
immunesystem.

3. 1.Fab region
2.Fc region
3.Heavy chain
4.Light chainwith one variable (V
L)
and one constant (C
L) domain
5.Fc regionwith paratope
6.Hinge regions
Paratope
•Aparatope,alsocalledanantigen-bindingsite,isa
partofanantibodywhichrecognizesandbindsto
anantigen.
•Itisasmallregion(of5to10aminoacids)ofthe
antibody'sfragmentantigen-binding(Fabregion),
andcontainspartsoftheantibody'sheavyandlight
chains.
•EacharmoftheYshapeofanantibodymonomeris
tippedwithaparatope,whichisasetof
6complementarity-determining regions (CDR
loops)-3ofeachlightandheavychainextending
fromthefoldofantiparallelbetasheets.
•Thepartoftheantigentowhichtheparatopebinds
iscalledanepitope.Thiscanbemimicked by
amimotope.
•Theengravedinnerportionsofidiotype(encircled
regionno.5inthediagram)istheparatopewhere
theepitopeoftheantigenbinds.

4. •Duringthedevelopment ofBcells,theimmunoglobulin geneundergoes
sequences ofrearrangements thatleadtoformationoftheantibody
repertoire.
•Forexample,inthelymphoidcell,apartialrearrangement oftheheavy-chain
geneoccurswhichisfollowedbycompleterearrangement ofheavy-chain
gene.Hereatthisstage,Pre-Bcell,mμheavychainandsurrogatelightchain
areformed.Thefinalrearrangement ofthelightchaingenegenerates
immatureBcellandmIgM.Theprocessexplainedhereoccursonlyinthe
absenceoftheantigen.
•ThematureBcellformedasRNAprocessingchangesleavesthebonemarrow
andisstimulatedbytheantigenthendifferentiatedintoIgM-secretedplasma
cells.Alsoatfirst,thematureBcellexpressesmembrane-boundIgDandIgM.
ThesetwoclassescouldswitchtosecretoryIgDandIgMduringthe
processingofmRNAs.
•Finally,furtherclassswitchingfollowsasthecellkeepdividingand
differentiating.Forinstance,IgMswitchestoIgGwhichswitchestoIgAthat
eventuallyswitchestoIgE
B-Cell Development

5. The multigene organization of immunoglobulin genes
FromstudiesandpredictionssuchasDreyerand
Bennett's,itshowsthatthelightchainsandheavy
chainsareencodedbyseparatemultigenefamilieson
differentchromosomes .Theyarereferredtoasgene
segmentsandareseparatedbynon-codingregions.The
rearrangement andorganizationofthesegenesegments
duringthematurationofBcellsproducefunctional
proteins.Theentireprocessofrearrangement and
organizationofthesegenesegmentsisthevitalsource
whereourbodyimmunesystemgetsitscapabilitiesto
recognizeandrespondtovarietyofantigens.

6. Thelightchaingenehasthreegenesegments.These
include:thelightchainvariableregion(V),joiningregion
(J),andconstantregion(C)genesegments.Thevariable
regionoflightistherefore encoded bythe
rearrangement ofVJsegments.Thelightchaincanbe
eitherkappa,κorlambda,λ.Thisprocesstakesplaceat
thelevelofmRNAsprocessing.Randomrearrangements
andrecombinationsofthegenesegmentsatDNAlevel
toformonekappaorlambdalightchainoccursinan
orderlyfashion.Asaresult,"afunctionalvariableregion
geneofalightchaincontainstwocodingsegmentsthat
areseparated byanon-codingDNAsequence in
unrearrangedgerm-lineDNA"(Barbaraetal.,2007).
Light chain multigene family

7. Heavychaincontainssimilargenesegmentssuchas
VH,JHandCH,butalsohasanothergenesegment
calledD(diversity).Unlikethelightchainmultigene
family,VDJgenesegments codeforthevariable
regionoftheheavychain.Therearrangement and
reorganizationofgenesegments inthismultigene
familyismorecomplex.Therearrangingandjoining
ofsegmentsproduceddifferentendproductsbecause
thesearecarriedoutbydifferentRNAprocesses.The
samereasoniswhytheIgMandIgGaregeneratesat
thetime.
Heavy-chain multigene family

8. Variable-RegionRearrangements -Thevariableregionrearrangementshappeninanorderly
sequenceinthebonemarrow.Usually,theassortmentofthesegenesegmentsoccursatBcellmaturation.
Light chain DNA
Thekappaandlambdalightchainsundergorearrangements of
theVandJgenesegments.Inthisprocess,afunctional
VlambdacancombinewithfourfunctionalJλ–Cλcombinations.
Ontheotherhands,Vkgenesegmentscanjoinwitheitherone
oftheJkfunctionalgenesegments.Theoverallrearrangements
resultinagenesegmentorderfrom5primeto3primeend.
Theseareashortleader(L)exon,anoncodingsequence
(intron),ajoinedVJsegment,asecondintron,andtheconstant
region.Thereisapromoterupstreamfromeachleadergene
segment.Theleaderexonisimportantinthetranscriptionof
lightchainbytheRNApolymerase.Toremainwithcoding
sequenceonly,theintronsareremovedduringRNA-processing
andrepairing.Insummary,

9. Heavy chain DNA
Therearrangements ofheavy-chainsaredifferent
fromthelightchainsbecauseDNAundergoes
rearrangements ofV-D-Jgenesegments inthe
heavychains.Thesereorganizations ofgene
segmentsproducegenesequencefrom5primeto
3primeendssuchasashortleaderexon,an
intron,ajoinedVDJsegment,asecondintronand
severalgenesegments.Thefinalproductofthe
rearrangement istranscribed when RNA
polymerase

10. Mechanism of variable region rearrangements
Itisunderstoodthatrearrangement occursbetweenspecific
sitesontheDNAcalledrecombinationsignalsequences(RSSs).
Thesignalsequencesarecomposed ofaconservedpalindromic
heptamer andaconserved AT-richnonamer.Thesesignal
sequencesareseparatedbynon-conservedspacersof12or23
basepairscalledone-turnandtwo-turnrespectively.Theyare
withinthelambda chain,k-chainandTheprocesses of
rearrangement intheseregionsarecatalyzed bytwo
recombination-activatinggenes:RAG-1andRAG-2andother
enzymes andproteins.Thesegments joinedduetosignals
generatedRSSsthatflankeachV,D,andJsegments.Onlygenes
flankby12-bpthatjointothegenesflankby23-bpspacerduring
therearrangements andcombinationstomaintainVL-JLandVH-
DH-JHjoining.

11. Generation of antibody diversity
Antibody diversity isproduced by genetic
rearrangement aftershufflingandrejoiningoneof
eachofthevariousgenesegmentsfortheheavyand
lightchains.Duetomixingandrandomrecombination
ofthegenesegmentserrorscanoccuratthesites
wheregenesegments joinwitheachother.These
errorsareoneofthesourcesoftheantibodydiversity
thatiscommonlyobservedinboththelightandheavy
chains.Moreover,whenBcellscontinuetoproliferate,
mutationsaccumulateatthevariableregionsthrough
aprocesscalledsomatichypermutation.Thehigh
concentrations ofthesemutationsatthevariable
regionalsoproducehighantibodydiversity.

12. Class-switching
WhentheBcellsgetactivated,classswitchingcanoccur.The
classswitchinginvolvesswitchregionsthatmadeupofmultiple
copiesofshortrepeatts(GAGCT andTGGGG).Theseswitches
occuratthelevelofrearrangements oftheDNAbecausethere
isaloopingeventthatchopsofftheconstantregionsforIgM
andIgDandformtheIgGmRNAs.Anycontinuouslooping
occurrence willproduce IgE orIgA mRNAs.In
addition,cytokinesarefactorsthathavegreateffectsonclass
switchingofdifferentclassesofantibodies.Theirinteraction
withBcellsprovidestheappropriatessignalsneededforBcells
differentiationandeventualclassswitchingoccurrence.For
example,interleukin-4inducestherearrangements ofheavy
chainimmunoglobulin genes.ThatisIL-4inducestheswitching
ofCμtoCγtoCκ

13. Cytokinesareabroadandloosecategoryofsmallproteins
thatareimportantincellsignaling.Cytokinesarepeptides,
andcannotcrossthelipidbilayerofcellstoenterthe
cytoplasm.Cytokineshavebeenshowntobeinvolvedin
autocrine,paracrineandendocrinesignalingasimmuno-
modulatingagents.Theirdefinitedistinctionfromhormonesis
stillpartofongoingresearch.Cytokinesincludechemokines,
interferons,interleukins,lymphokines,andtumournecrosis
factors,butgenerallynothormones orgrowthfactors.
Cytokinesareproducedbyabroadrangeofcells,including
immunecellslikemacrophages, Blymphocytes,Tlymphocytes
andmastcells,aswellasendothelialcells,fibroblasts,and
variousstromalcells;agivencytokinemaybeproducedby
morethanonetypeofcell.

14. https://en.wikipedia.org/wiki/Organization_and_expression_of_immunoglobulin_genes