LIGHT-CHAIN EDITING IN KAPPA-DEFICIENT ANIMALS - A POTENTIAL MECHANISM OF B-CELL TOLERANCE

The genetic organization of the kappa and lambda light chain loci perm its multiple, successive rearrangement attempts at each allele. Multip le rearrangements allow autoreactive B cells to escape clonal deletion by editing their surface receptors. Editing may also facilitate effic ient B cell production by salvaging cells with nonproductive light cha in (L chain) rearrangements. To study receptor editing of kappa L chai ns, we have characterized B cells from mice hemizygous for the targete d inactivation of kappa (JCkD/wt) which have an anti-DNA heavy chain t ransgene, 3H9. Hybridomas from JCkD/wt mice exhibited an increased fre quency of rearrangements to downstream Jk segments (such as Jk5) compa red with most surveys from normal mice, consistent with receptor editi ng by sequential kappa locus rearrangements in JCkD/wt. We observed an even higher frequency of rearrangements to Jk5 in 3H9 JCkD/wt animals compared with nontransgenic JCkD/wt, consistent with editing of autor eactive kappa in 3H9 JCkD/wt. We also recovered a large number of 3H9 JCkD/wt lines with Vk12/13-Jk5 rearrangements and could demonstrate by PCR and Southern analysis that up to three quarters of these lines un derwent multiple kappa rearrangements. To investigate editing at the l ambda locus, we used homozygous kappa-deficient animals (JCkD/JCkD and 3H9 JCkD/JCkD). The frequencies of V lambda 1 and V lambda 2 rearrang ements among splenic hybridomas in 3H9 JCkD/JCkD were reduced by 75% w hereas V lambda X was increased 5-10-fold, compared with nontransgenic JCkD/JCkD animals. This indicates that V lambda 1 and V lambda 2 are negatively regulated in 3H9 JCkD/JCkD, consistent with earlier studies that showed that the 3H9 heavy chain, in combination with lambda 1 bi nds DNA. As successive lambda rearrangements to V lambda X do not inac tivate V lambda 1, the consequence of lambda editing in 3H9 JCkD/JCkD would be failed allelic exclusion at lambda. However, analysis of 18 3 H9 JCkD/JCkD hybridomas with V lambda 1 and V lambda X DNA rearrangeme nts revealed that most of these lines do not have productive lambda 1 rearrangements. In sum, both kappa and lambda loci undergo editing to recover from nonproductive rearrangement, but only kappa locus editing appears to play a substantial role in rescuing autoreactive B cells f rom deletion.