细胞与分子免疫学课程课件培训讲义

细胞与分子免疫学课程课件细胞与分子免疫学课程课件细细胞与分子免疫学胞与分子免疫学课课程程课课件件1（优选）细胞与分子免疫学课程课件（优选优选）细细胞与分子免疫学胞与分子免疫学课课程程课课件件23Chapter7LymphocyteMaturationandExpressionofAntigenReceptorGenes3Chapter734nGeneralFeaturesofLymphocyteMaturation ThematurationofBandTlymphocytesconsistsof:1.Lineagecommitmentandproliferation2.Expressionofantigenreceptorgenes3.Selectionofthematurerepertories4GeneralFeaturesofLymphocyt45pEarlymaturation:1.Pluripotentstemcellsgiverisetoalllineagesofbloodcells1)Itisdifficulttopreciselydefinethemechanismsbywhichstemcellsbecometothelymphoidlineage.2)Bcell:boneborrowTcell:thymus3)Bythegenemutationsinexperimentalanimalsorhumanpatients.i.Transcriptionfactorsii.Growthfactorsiii.Growthfactorsreceptor5Earlymaturation:Pluripotent56Example1Bubbleboydisease1.MutatedinIL-7geneandIL-7receptorgenehaveprofounddeficienciesinmatureTandBcells.2.MutationsinachainoftheIL-2receptorg gchain,leadstoanimmunodeficiencydisease:X-linkedseverecombinedimmunodeficiencydisease,bubbleboy.6Example1Bubbleboydisease167pAntigenReceptorGeneRecombinationandExpression1.Theremaybe107ormoredifferentTandBlymphocyteclonesinoneindividual,eachcloneproduceonespecificantigenreceptor.2.Bysomaticrecombination,eachindividualdoesnotneedtohavesuchenormouslygenes.3.Duringthepre-Bcellorpre-Tcellstage,animmatureformofantigenreceptorisformedwhichtotransducesignalstoinducefurthermaturation4.Inmorematurelymphocytes,completeantigenreceptorexpressed,whichpromotecellmaturation.7AntigenReceptorGeneRecomb7V:like Ig V geneUse a limited set of V gene segmentsDNA是怎样重新组合的？In terms of G to C and T to A pairing,the new nucleotides are palindromic.2）一个V区可以与不同类的C区结合，产生出不同类型的抗体。Recombination activating gene products,(RAG1&RAG 2)and high mobility group proteins bind to the RSSV:like Ig V genePre-B cell receptors are expressed on the cell surface at low levels.TCR a,d chain:chromosome 14T cells in thymus.membrane-bound antigen receptor is not expressed;1 Generation of the palindromic sequenceii)Two types of molecules produced by the nonlymphoidHost strain and treatmentTCR a,d chain:chromosome 14General Features of Lymphocyte Maturationchromosome 14:H chain locusDendritic cells;Blot with a V region probe8pSelectionProcessThatShapetheBandTLymphocyteRepertoires1.Thepreservationofusefulspecificitiesiscalledpositiveselection:Tcellsinthymus.Bcellsmaturingarenotwellknown.2.Negativeselectionistheprocessthateliminatesdevelopinglymphocyteswithstrongbindingtoself-antigen.V:likeIgVgene8SelectionPr89pAcquisitionofFunctionalCompetence1.Bcellscansecreteantibodies.2.TcellscandifferentiateintodistinctsubsetsofTcells.3.Expressionofavarietyofcellsurfaceandintracellularmoleculesthatparticipateinlymphocyteactivationandeffectorfunctions.9AcquisitionofFunctionalCom9Growth factorsRelationship to B-2 cells not understoodOrganization of TCR Gene LociHeptamer ligation-signal joint formationBlot with a V region probeImmature B cell2)Multiple copies of at least three different types of gene segments:How does recombination occur when a V gene is in opposite orientation to the DJ region?Single positiveJunctional DiversitySignals from the pre-TCR stimulate proliferation of the pre-T cells,recombination at the a chain locus.A irradiatedThe coexpression of IgM and IgD make the B cell acquire the functional competence.其他 切开发夹结构的内切酶，参与修复DNA双链断段的DNA外切酶、DNA合成酶等，如DNA连接酶IV，DNA依赖的蛋白激酶。1)For the productive rearrangements:allelic exclusion exsits.V:like Ig V geneV and C probes detect the same fragmentThe rearrangement of the TCR a chain genes and the expression of TCR ab heterodimers occur in the double-positive population.the 12-23 rule)Bone marrow derived macrophages;One or a few peptides induced the positive selection of a large repertoire of CD8+cells.D:in TCR b chain and d chain.The C fragment has got largerThymocytes can not recognize self MHC-restricted will die by apoptosis.V:like Ig V geneA irradiated and thymectomized3、B 细胞的成熟：祖B细胞，前B细胞，非成熟B细胞，成熟B细胞Single positive互补链切断，发夹结构形成。3)By the gene mutations in experimental animals or human patients.Organization of Ig Gene LociD:in TCR b chain and d chain.J:in all TCR loci,between V and C genes.What are the problems with this theory?T cells in thymus.A irradiatedB cell:bone borrowTranscription factors10nFormationofFunctionalAntigenReceptorGenesinBandTLymphocytesElucidationofthemechanismsofantigenreceptorgeneexpressionisoneofthelandmarkachievementsofmordernimmunology1胚系理论胚系理论(GermlineTheory)在胚系基因组中包含了免疫基因库，可对外界的众多抗在胚系基因组中包含了免疫基因库，可对外界的众多抗原发生应答，并能遗传。原发生应答，并能遗传。Whataretheproblemswiththistheory?2体细胞突变理论（体细胞突变理论（SomaticMutationModel）基因组中的免疫球蛋白基因相对较少，抗体的多样性主基因组中的免疫球蛋白基因相对较少，抗体的多样性主要是由体细胞基因突变引起的。要是由体细胞基因突变引起的。Whataretheproblemswiththistheory?Growthfactorsthe12-23rule)11011胚系理论与体细胞突变理论所遇到的问题胚系理论与体细胞突变理论所遇到的问题1胚系理论胚系理论1）基因的种类少于抗体的多样性）基因的种类少于抗体的多样性Somanyspecificitiessofewgenes2体细胞突变理论（体细胞突变理论（SomaticMutationModel）1）部分编码）部分编码V区基因有众多的变化但部分编码区基因有众多的变化但部分编码C区的区的基因却相对稳定。基因却相对稳定。2）不同类的同型的免疫球蛋白具有相同的）不同类的同型的免疫球蛋白具有相同的V区。区。Samevariableregionsondifferentisotypes11胚系理胚系理论论与体与体细细胞突胞突变变理理论论所遇到的所遇到的问题问题1胚系理胚系理论论11123DreyerBennett假说：假说：免疫球蛋白单一的肽链是由两个分隔的基因编码：免疫球蛋白单一的肽链是由两个分隔的基因编码：每个免疫球蛋白类基因可能只有单个每个免疫球蛋白类基因可能只有单个C区基因，在胚区基因，在胚系基因组中与系基因组中与V区基因是分隔开的。区基因是分隔开的。在抗体产生细胞的发育过程中，其中一个独立的在抗体产生细胞的发育过程中，其中一个独立的V区区基因序列将与基因序列将与C区序列结合成为完整的区序列结合成为完整的VC基因，然基因，然后在细胞内表达。后在细胞内表达。两个基因两个基因一条多肽链：一条多肽链：此模型可以解释如下问题：此模型可以解释如下问题：1）一个基因的部分片断是多变的，而另一部分相对不）一个基因的部分片断是多变的，而另一部分相对不变。变。2）一个）一个V区可以与不同类的区可以与不同类的C区结合，产生出不同类区结合，产生出不同类型的抗体。型的抗体。123DreyerBennett假假说说：免疫球蛋白：免疫球蛋白单单12Growth factorsPositive selectionchromosome 2:k chainPre-B cell receptors are expressed on the cell surface at low levels.1、Recombination Signal Sequences(RSS),重排信号序列：Proof of the Dreyer-Bennett hypothesis1、Recombination Signal Sequences(RSS),重排信号序列：The pre-T cell receptor was formed.1 Generation of the palindromic sequenceEndonuclease cleaves single strand at random sites in V and D segmentThymus donorJunctional diversity:P nucleotide additionsWithout the MHC-I peptides complex,mature T cells decreased.Self antigens deliver strong signals to IgM expressing immature B lymphocytes that happen to express receptors specific for these self antigens.Pre-B cell receptors are expressed on the cell surface at low levels.Organization of TCR Gene LociTolerance induced in immature lymphocytes by recognition of self antigen in the generative lymphoid organs is also called central tolerance.Proof of the Dreyer-Bennett hypothesischromosome 2:k chainV:like Ig V geneis different from the T cells selection,perhaps serve to preserve all the cells with the capacity to recognize antigens,regardless of specificity.This rule can explain why in heavy chain,the V can not directly bind to J.1)not produce Ig134 The genetic foundation of antibody diversityThe Nobel Prize in Physiology or Medicine 1987for his discovery of the genetic principle for generation of antibody diversity“in1976.Susumu Tonegawa Japan Massachusetts Institute of Technology(MIT)Cambridge,MA,USA b.1939Growthfactors134Thegenetic1314ProofoftheDreyer-BennetthypothesisVVVVVVVVVVVVVA mechanism to rearrange V and C genes in the genome so that they can fuse to form a complete Immunoglobulin gene.CVCA single C region gene encoded in the germline and separate from the multiple V region genesFind a way to show the existence of multiple V genes and rearrangement to the C gene14ProofoftheDreyer-Bennet1415ProofoftheDreyer-BennetthypothesisVVVCVVVVVVSize fractionate by gel electrophoresisCVVVVVVVVVCVVVVVVVVVCut germline DNA with restriction enzymesVVVVVVVVVCArangeoffragmentsizesisgeneratedBlot with a V region probeBlot with a C region probeThefollowingexampledescribeseventsononlyONEofthechromosomes15ProofoftheDreyer-Bennet1516CVVVVVCVVVVVSize fractionate by gel electrophoresisVVVVCVBlot with a V region probeBlot with a C region probeCut mature B cell DNA with restriction enzymesVVVBlot with a V region probeBlot with a C region probeCVVVVVVSize fractionate by gel electrophoresis-compare the pattern of bandswith germline DNAV and C probes detect the same fragmentSome V regions are missingThe C fragment has got largerVVVVCVEvidenceforgenerecombination16CVVVVVCVVVVVSizefractionate1617OrganizationofIgandTCRGenesintheGermline1.OrganizationofIgGeneLoci1)ThreeseparatelociencodetheIgchains.chromosome14:Hchainlocuschromosome2:k kchainchromosome22:l l chain2)Multiplecopiesofatleastthreedifferenttypesofgenesegments:V:300base,separatebynoncodingDNA,atthe5endofeachVregionisanucleotidesequencewhichiscalledleaderpeptide.17OrganizationofIgandTCRG1718C,eachIglocushasadistinctarrangementandnumberofCgene.Inhuman:Igk k lightchain:asingleCgeneIg l l lightchain:fourfunctionalCgenesIgHheavychain:ninedifferentIgisotypesandsubtypes.Jsegments,30-50basepairslong.Dsegments,inthehumanIgheavychainlocus.1.OrganizationofIgGeneLoci18C,eachIglocushasadist18192.OrganizationofTCRGeneLociInthreeseparateloci:TCRa,da,dchain:chromosome14TCRb b chain:chromosome7TCRg gchain:chromosome7V:likeIgVgeneC:twoCgenesineachofthehumanTCRb b andTCRg g chain,oneCgeneineachofTCRa,da,dchain.J:inallTCRloci,betweenVandCgenes.D:inTCR b bchainandd d chain.192.OrganizationofTCRGene1920AntigenReceptorGeneRecombinationDiversityofantigenreceptorgenes20AntigenReceptorGeneRecomb2021MechanismsofSomaticRecombinationofAnitgenReceptorGenes1.有限的有限的DNA是怎样产生无限的专一是怎样产生无限的专一性的性的?2.V区是怎样找到区是怎样找到J区的？为什么区的？为什么V区不与区不与V区相连？区相连？3.DNA是怎样断裂的？是怎样断裂的？4.DNA是怎样重新组合的？是怎样重新组合的？21MechanismsofSomaticRecomb2122RSS(重排信号序列重排信号序列)，recombinase(重组酶重组酶)，12-23 rule(一个一个规则规则)。1、Recombination Signal Sequences(RSS),重排信号序列：重排信号序列：v基因片断两端存在两个特基因片断两端存在两个特异的保守序列：异的保守序列：7聚体与聚体与9聚体。聚体。v 7聚体与聚体与9聚体之间间隔聚体之间间隔是是23bp或者是或者是12bp。vRSS:7聚体聚体-间隔间隔-九聚九聚体体(The heptamer The heptamer spacer spacer nonamer)nonamer)MechanismsofSomaticRecombinationofAnitgenReceptorGenes22RSS(重排信号序列重排信号序列)，recombinase(重重组组2223 RSS（重排信号序列（重排信号序列）23RSS（重排信号序列）（重排信号序列）23242、重组酶：、重组酶：一组参与一组参与V、（、（D）、）、J基因片断重组的酶，包括以下几种：基因片断重组的酶，包括以下几种：RAG-1与与RAG-2(recombination-activatinggenes):一种内切酶，只一种内切酶，只表达在表达在T和和B淋巴细胞淋巴细胞不成熟阶段。不成熟阶段。末端脱氧核苷酸转移酶末端脱氧核苷酸转移酶(terminaldeoxynucleotidyltransferase,TdT)：表达于：表达于T、B细胞前体，此酶可将数个核苷酸通过不细胞前体，此酶可将数个核苷酸通过不需要模板的方式加到需要模板的方式加到DNA的断段。的断段。其他其他切开发夹结构的内切酶，参与修复切开发夹结构的内切酶，参与修复DNA双链断段的双链断段的DNA外切酶、外切酶、DNA合成酶等，如合成酶等，如DNA连接酶连接酶IV，DNA依赖的蛋白依赖的蛋白激酶。激酶。242、重、重组组酶酶：2425 重组酶所催化的重排步骤1.RAG蛋白复合物结合蛋白复合物结合到到RSS。2.蛋白复合物拉近将要蛋白复合物拉近将要相连的相连的DNA片断。片断。3.互补链切断，发夹结互补链切断，发夹结构形成。构形成。4.其他其他DNA修饰蛋白结修饰蛋白结合到发夹结构剪切合到发夹结构剪切RSS末端。末端。5.发夹结构被随机剪切，发夹结构被随机剪切，碱基或者增加或者减碱基或者增加或者减少。少。6.DNA连接酶连接产生连接酶连接产生编码连接点和信号连编码连接点和信号连接。接。25重重组组酶酶所催化的重排步所催化的重排步骤骤RAG蛋白复合物蛋白复合物结结合到合到RSS。252623-mer=two turns12-mer=one turn3、Molecularexplanationofthe12-23ruleIntervening DNAof any length23V9712D J792623-mer=twoturns12-mer=o262723-mer12-merLoop of interveningDNA is excisedTheshapegeneratedbytheRSSsactsasatargetforrecombinases7997V1V2V3V4V8V7V6V5V9D JV1D JV2V3V4V8V7V6V5V9Anappropriateshapecannotbeformediftwo23-merflankedelementsattemptedtojoin(i.e.the12-23rule)3.Molecularexplanationofthe12-23rule2723-mer12-merLoopofinterven2728Thisrulecanexplainwhyinheavychain,theVcannotdirectlybindtoJ.28Thisrulecanexplainwhyin2829V7239D7129JV723972397129D7129J72397129VDJRecombinationactivatinggeneproducts,(RAG1&RAG2)andhighmobilitygroupproteinsbindtotheRSSThetwoRAG1/RAG2complexesbindtoeachotherandbringtheVregionadjacenttotheDJregion互补链断裂，发夹结构形成互补链断裂，发夹结构形成4.StepsofIggenerecombination29V7239D7129JV723972397129D71229The nucleotides that flip out,become part of the complementary DNA strandm protein in pre-B cells are translated;The hairpins at the end of the V and D regions are opened,and exonucleases and transferases remove or add random nucleotides to the gap between the V and D regionC,each Ig locus has a distinct arrangement and number of C gene.J:in all TCR loci,between V and C genes.D segments,in the human Ig heavy chain locus.其他 切开发夹结构的内切酶，参与修复DNA双链断段的DNA外切酶、DNA合成酶等，如DNA连接酶IV，DNA依赖的蛋白激酶。The Nobel Prize in Physiology or Medicine 1987Blot with a C region probeMutated in IL-7 gene and IL-7 receptor gene have profound deficiencies in mature T and B cells.1 Ig与TCR基因在染色体基因座位上的排列特点CD4+cells acquire the ability to produce cytokinesCD4+cells acquire the ability to produce cytokinesPositive selectionCan response to antigens.B-1 cells spontaneously secrete IgM antibodies that often react with microbial polysaccharides and lipids.V:300 base,separate by noncoding DNA,at the 5 end of each V region is a nucleotide sequence which is called leader peptide.A second population of B cells exists,called B-1 cells30VDJ72397129Anumberofotherproteins,(Ku70:Ku80,XRCC4andDNAdependentproteinkinases)bindtothehairpinsandtheheptamerends.VDJThehairpinsattheendoftheVandDregionsareopened,andexonucleasesandtransferasesremoveoraddrandomnucleotidestothegapbetweentheVandDregionVDJ7 2397129DNAligaseIVjoinstheendsoftheVandDregiontoformthecodingjointandthetwoheptamerstoformthesignaljoint.4.StepsofIggenerecombinationThenucleotidesthatflipout,3031V1V2V3V4V9D JLooping out works if all V genes are in the same transcriptional orientationV1V2V3V9D J缺失性重排缺失性重排D J7129V47239V17239D7129JHow does recombination occur when a V gene is in opposite orientation to the DJ region?V45.重排方式以重链重排为例非非缺失性重排缺失性重排LL31V1V2V3V4V9DJLoopingoutwork3132D J7129V47239V4andDJinoppositetranscriptionalorientationsD J7129V472391.D J7129V472393.D J7129V472392.D J7129V472394.Non-deletionalrecombination32DJ7129V47239V4andDJinopp3233D J7129V472391.D JV4712972393.V to DJ ligation-coding joint formationD J7129V472392.Heptamer ligation-signal joint formationD JV471297239Fully recombined VDJ regions in same transcriptional orientationNo DNA is deleted4.33DJ7129V472391.DJV4712972393.33347D129J6.GenerationofDiversityoftheBandTCellRepertoiresJunctionaldiversity:Pnucleotideadditions7V239D7129JV7239TC CACAGTGAG GTGTCACAT GTGACACTA CACTGTGThe recombinase complex makes single stranded nicks at random sites close to the ends of the V and D region DNA.7D129J7V239CACAGTGGTGTCACGTGACACCACTGTGTCAGATTADJVTCAGATTAUUThe 2nd strand is cleaved and hairpins form between the complementary bases at ends of the V and D region.347D129J6.GenerationofDivers3435V2V3V4V8V7V6V5V97239CACAGTGGTGTCAC7129GTGACACCACTGTGVTCAGUDJATTAUHeptamers are ligated by DNA ligase IVV and D regions juxtaposedVTCAGUD JATTAU35V2V3V4V8V7V6V5V97239CACAGTG73536VTCAGUD JATTAUEndonuclease cleaves single strand at random sites in V and D segmentVTCGAAGD JATTATAThe nucleotides that flip out,become part of the complementary DNA strand6.1GenerationofthepalindromicsequenceIn terms of G to C and T to A pairing,the new nucleotides are palindromic.The nucleotides GA and TA were not in the genomic sequence and introduce diversity of sequence at the V to D join.VTCAGUD JATTAURegions to be joined are juxtaposedThe nicked strand flips out 36VTCUDJATUEndonucleasecleave36376.2JunctionalDiversityNnucleotideadditionsVTCGAAGD JATTATATerminal deoxynucleotidyl transferase(TdT)adds nucleotides randomly to the P nucleotide ends of the single-stranded V and D segment DNACACTCCTTATTCTTGCAAVTCGAAGD JATTATACACACCTTATTCTTGCAAComplementary bases annealVD JDNA polymerases fill in the gaps with complementary nucleotides and DNA ligase IV joins the strandsTCGAAGATTATACACACCTTATTCTTGCAAD JTATAExonucleases nibble back free endsVTCGACACACCTTATTCTTGCAAVTCDTAGTTATATAGC376.2JunctionalDiversityN3738VDJTCGACGTTATATAGCTGCAATATAJunctionalDiversityNNNNNNNNNNNNNNNGermline-encoded nucleotidesPalindromic(P)nucleotides-not in the germlineNon-template(N)encoded nucleotides-not in the germlineCreates an essentially random sequence between the V region,D region and J region in heavy chains and the V region and J region in light chains.38VDJTCGACGTTATATAGCTGCAATATAJ3839JunctionalDiversity39JunctionalDiversity39Fully recombined VDJ regions in same transcriptional orientationV and C probes detect the same fragmentA irradiated and thymectomizedThe C fragment has got largerPre-B cell receptors are expressed on the cell surface at low levels.D:in TCR b chain and d chain.The coexpression of IgM and IgD make the B cell acquire the functional competence.V and C probes detect the same fragmentWhat are the problems with this theory?互补链切断，发夹结构形成。Organization of Ig Gene LociWhat are the problems with this theory?2）一个规则(12-23 rule)：可以解释为什么V区基因在重组过程中，对于V重链来说是：VDJ。chromosome 14:H chain locusThe nucleotides that flip out,become part of the complementary DNA strandV and C probes detect the same fragmentSingle positiveRAG-1与RAG-2(recombination-activating genes):一种内切酶，只表达在T和B淋巴细胞不成熟阶段。40MaturationofBLymphocytes1.Pro-Bcell2.Pre-Bcell3.ImmatureBcell4.MatureBcellFullyrecombinedVDJregionsi4041MaturationofBLymphocytes41MaturationofBLymphocytes4142StagesofBLymphocyteMaturation:1.Pro-Bcell:1)notproduceIg2)Maker:CD19andCD103)RAGproteinsarefirstexpressed4)TdTenzymeisexpressedmostabundantly5)Iga aandIg b b of BCRcomplexstartstoexpress.6)FirstrecombinationofIggenesoccursinheavychains42StagesofBLymphocyteMatur4243RecombinationinPro-Bcell:43RecombinationinPro-Bcel43442.Pre-BcellAprimarytranscriptthatincludestherearrangedVDJcomplexandtheproximalCgenesisproduced;FunctionalmRNAforthem mheavychainisproduced;m m proteininpre-Bcellsaretranslated;membrane-boundantigenreceptorisnotexpressed;Pre-Bcellreceptorsareexpressedonthecellsurfaceatlowlevels.Onlyinhematopoietictissues;442.Pre-BcellAprimarytrans44V:like Ig V geneAcquisition of Functional CompetenceDouble positive thymocytes are produced spontaneouslyJunctional DiversityA number of other proteins,(Ku70:Ku80,XRCC4 and DNA dependent protein kinases)bind to the hairpins and the heptamer ends.The rearrangement of the TCR a chain genes and the expression of TCR ab heterodimers occur in the double-positive population.4、B细胞表面受体的重排：还有一个问题是TdT随机增加几个碱基从而增加了基因的多态性，那么在重排机制中如何保证随机增加碱基后使得重排的基因的碱基数依旧是3的倍数而不发生编码错误？如果编码的不是3的倍数，是否是在转录水平上随机添加碱基U来使编码恢复正常。Recent arrivals from the bone borrow.Find a way to show the existence of multiple V genes and rearrangement to the C geneis different from the T cells selection,perhaps serve to preserve all the cells with the capacity to recognize antigens,regardless of specificity.Evidence for gene recombinationMolecular explanation of the 12-23 ruleRSS(重排信号序列)，recombinase(重组酶)，12-23 rule(一个规则)。4 The genetic foundation of antibody diversity2)Maker:CD19 and CD101)For the productive rearrangements:allelic exclusion exsits.B cells described to-date:B-2 cellsTCR a,d chain:chromosome 14Recombination of TCRBlot with a C region probeThymocytes can not recognize self MHC-restricted will die by apoptosis.45V:likeIgVgene4545468)Thepre-BCRregulatesfurthersomaticrecombinationofIggenesintwoways:8.1)Fortheproductiverearrangements:allelicexclusionexsits.Forthenonproductiveone,thesecondallelicchromosomescanrecombine.468)Thepre-BCRregulatesfur4647刘老师：刘老师：您好！上节课讲到内切酶在您好！上节课讲到内切酶在V和和D上随机切开增加了上随机切开增加了VD间连接间连接的多样性。那么这个区域将来编码成蛋白质即抗体后，并不位的多样性。那么这个区域将来编码成蛋白质即抗体后，并不位于于V片段的片段的CDR区，如何能够增加抗体的多态性？区，如何能够增加抗体的多态性？还有一个问题是还有一个问题是TdT随机增加几个碱基从而增加了基因的多态随机增加几个碱基从而增加了基因的多态性，那么在重排机制中如何保证随机增加碱基后使得重排的基性，那么在重排机制中如何保证随机增加碱基后使得重排的基因的碱基数依旧是因的碱基数依旧是3的倍数而不发生编码错误？如果编码的不是的倍数而不发生编码错误？如果编码的不是3的倍数，是否是在转录水平上随机添加碱基的倍数，是否是在转录水平上随机添加碱基U来使编码恢复正来使编码恢复正常。常。谢谢老师！来自崔雪晶谢谢老师！来自崔雪晶47刘老刘老师师：4748C,eachIglocushasadistinctarrangementandnumberofCgene.Inhuman:Igk k lightchain:asingleCgeneIg l l lightchain:fourfunctionalCgenesIgHheavychain:ninedifferentIgisotypesandsubtypes.Jsegments,30-50basepairslong.Dsegments,inthehumanIgheavychainlocus.1.OrganizationofIgGeneLoci48C,eachIglocushasadist48492.OrganizationofTCRGeneLociInthreeseparateloci:TCRa,da,dchain:chromosome14TCRb b chain:chromosome7TCRg gchain:chromosome7V:likeIgVgeneC:twoCgenesineachofthehumanTCRb b andTCRg g chain,oneCgeneineachofTCRa,da,dchain.J:inallTCRloci,betweenVandCgenes.D:inTCR b bchainandd d chain.492.OrganizationofTCRGene49508)Thepre-BCRregulatesfurthersomaticrecombinationofIggenesintwoways:8.1)Fortheproductiverearrangements:allelicexclusionexsits.Forthenonproductiveone,thesecondallelicchromosomescanrecombine.508)Thepre-BCRregulatesfur50互补链切断，发夹结构形成。with germline DNAWhat are the problems with this theory?基因组中的免疫球蛋白基因相对较少，抗体的多样性主要是由体细胞基因突变引起的。B cells described to-date:B-2 cells1 Generation of the palindromic seq