抗体多样性是如何产生的

Howisantibodydiversitygenerated?,Twoearlytheories:GermlinehypothesisThegenomecontainsmanylociencodingantibodymolecules.Bcellsexpressoneoftheseloci.DifferentBcellsexpressdifferentloci.,SomaticmutationhypothesisThereareasmallnumberofantibodygeneswhichundergomutationastheBcellmatures-thusgivingrisetoBcellsexpressingantibodyofdifferentspecificity.,Threegeneticlociencodeimmunoglobulinmolecules:-twolociencodingthelightchains-kappalocus-lambdalocus-onelocusencodingtheheavychainThesethreelociarelocatedondifferentchromosomes.,Thelociencodingimmunoglobulinshaveauniquestructure.-composedofgenesegmentsTheheavychainlocushasmultipleV(variable)segments,multipleD(diversity)segments,multipleJ(joining)segmentsandmultipleC(constant)segments.Duringmaturation,oneofeachV,DandJsegmentisrandomly“chosen”andusedtoencodethefinalantibodymolecule.,Germlineconfigurationoftheheavychainlocus(mice),Eachgenesegmentmayhaveitsownintron-exonstructure-e.g.theCmgenesegment,KubyFigure5-3,Thekappalocushasasimilarstructure-BUT-doesnothaveDsegments.AkappachainisencodedbyoneVsegment,oneJsegmentandoneCsegment.,KubyFigure5-3,ThelambdalocusalsohasonlyV,JandCsegments.,KubyFigure5-3,ReadKubypages109-110:MultigeneOrganizationofIgGenes,KubyFigure5-3,Inheavychains,theV,DandJsegmentsencodethevariabledomainwhiletheCsegmentencodestheconstantdomain.Inlightchains,theVandJsegmentsencodethevariabledomainwhiletheCsegmentencodestheconstantdomain.,DuringBcellmaturation,immunoglobulingenesundergorearrangement.,DuringBcellmaturation,immunoglobulingenesundergorearrangement.,DuringBcellmaturation,immunoglobulingenesundergorearrangement.,DuringBcellmaturation,immunoglobulingenesundergorearrangement.,Thekappaandlambdalociundergosimilarrearrangement.SincetherearenoDsegments,thereisasingleV-Jrearrangement.Inbothheavyandlightchainrearrangement,anumberofJsegmentsmayremainbetweentheV(D)andCsegments.TheseareincludedintheRNAtranscriptbutaresplicedoutduringRNAprocessing.ThefinallightchainmRNAcontainsoneVJCunit.,KubyFigure5-4,KubyFigure5-4,InthecaseoftheheavychainInnaive,matureBcells,theprimarymRNAtranscriptcontainsVDJandBOTHCmandCd.ThisprimarytranscriptcanbedifferentiallyprocessedtogiverisetomRNAencodingeitherIgMorIgD.ThisexplainswhynaiveBcellsexpressbothIgMandIgD.ThevariableregionofbothareencodedbythesameVDJ-sotheyhavethesameantigenspecificity.,KubyFigure5-5,ReadKubypages110-112:Variable-RegionGeneRearrangements,Whatmechanismensurescorrectjoiningofgenesegmentsduringrearrangementoftheheavyandlightchainloci?,KubyFigure5-3,Recombinationsignalsequences-conservedsequencesinregionsjustupstreamordownstreamofgenesegments.Consistofaconservedheptamerandnonamerwitha12or23bpspacer.,KubyFigure5-3,Whatmechanismensurescorrectjoiningofgenesegmentsduringrearrangementoftheheavyandlightchainloci?Recombinationsignalsequences-conservedsequencesinregionsjustupstreamordownstreamofgenesegments.Consistofaconservedheptamerandnonamerwitha12or23bpspacer.Theone-turn/two-turnrule(12/23rule)-recombinationoccursonlybetweenasegmentwitha12bpspacerandasegmentwitha23bpspacer.,Theone-turn/two-turnrule(12/23rule)-recombinationoccursonlybetweenasegmentwitha12bpspacerandasegmentwitha23bpspacer.,ReadKubypage113:RecombinationSignalSequencesDirectRecombination,KubyFigure5-6,RecombinationActivatingGenes(encodeRAG-1andRAG-2)RAG-1andRAG-2mediaterecognitionofsignalsequencesandrearrangementofDNAsegments.MicedeficientinRAG-1orRAG-2areunabletorearrangeheavyorlightchaingenes.ThesemicehavenomatureBcells.-BcellswillnotmatureiftheycannotexpressaBCR.,RAG-deficientmicealsolackmatureTcells.-TheTcellreceptorisalsoencodedbylocicontaininggenesegmentsthatmustberearrangedbeforetheTCRcanbeexpressed.TcellswillnotmatureiftheycannotexpressaTCR.SCIDmicealsohaveadefectthataffectsrearrangementofBCRandTCRloci.TheyalsohavenomatureTorBcells.,Flowcytometryofnormalvs.RAG-1deficientmice:LymphnodecellsFITCanti-CD19(Bcellmarker)andPEanti-CD3(Tcellmarker),Normalmice,RAG-1-deficientmice,5,3,3,5,5,3,AGCT,TATA,Terminaldeoxynucleotidyltransferase(Tdt)Anenzymethatrandomlyaddsinnucleotidesduringjoiningofheavychain(NOTlightchain)segments.,ReadKubypages113-115:GeneSegmentsAreJoinedbyRecombinases,ProductiveandnonproductiverearrangementsJoiningofsegmentsisnotpreciseandmayresultinlossofthecorrectreadingframe.Thismayleadtointroductionofstopcodons-nonproductiverearrangements.ReadKubypage115:Ig-GeneRearrangementsMayBeProductiveorNonproductive,KubyFigure5-9,Twoallelesareavailableateachlocus(maternalandpaternal).ABcellexpressesonlyoneheavyandlightchainallele.-ALLELICEXCLUSION,First,onealleleoftheheavychainisrearranged.Iftherearrangementissuccessful,theotherallelewillnotberearranged.Iftherearrangementisnonproductive,theotherallelewillberearranged.Onceaheavychainallelerearrangementisproductive,lightchainrearrangementwillbegin.Ifrearrangementofbothheavychainallelesisnonproductive,theBcellwillnotmaturefurtherbutwilldieofapoptosiswithinthebonemarrow.,Ifaheavychainalleleissuccessfullyrearranged,lightchainrearrangementbegins.Inhumans,thekappalocusisrearrangedfirst.Rearrangementoccursatonealleleatatimeandcontinuesuntilaproductiverearrangementoccurs.Ifbothkappaallelesrearrangenonproductively,rearrangementwillbeginatthelambdalocus.Ifall4alleles(bothkappaallelesandbothlambdaalleles)rearrangementsarenonproductive,theBcellwillnotmat