Identification of a germ line transcript from the unrearranged kappa gene in human B cells

The proximal J kappa germline-transcript promoter facilitates receptor editing through control of ordered recombination

V(D)J recombination creates antibody light chain diversity by joining a Vκ gene segment with one of four Jκ segments. Two Jκ germline-transcript (GT) promoters control Vκ-Jκ joining, but the mechanisms that govern Jκ choice are unclear. Here, we show in gene-targeted mice that the proximal GT promoter helps targeting rearrangements to Jκ1 by preventing premature DNA breaks at Jκ2. Consequently, cells lacking the proximal GT promoter show a biased utilization of downstream Jκ segments, resulting in a diminished potential for receptor editing. Surprisingly, the proximal—in contrast to the distal—GT promoter is transcriptionally inactive prior to Igκ recombination, indicating that its role in Jκ choice is independent of classical promoter function. Removal of the proximal GT promoter increases H3K4me3 levels at Jκ segments, suggesting that this promoter could act as a suppressor of recombination by limiting chromatin accessibility to RAG. Our findings identify the first cis-element critical for Jκ choice and demonstrate that ordered Igκ recombination facilitates receptor editing.

Immunoglobulin kappa light chain germ-line transcripts in human precursor B lymphocytes

B lymphoblastoid cell lines (BLCL), established from bone marrow and peripheral blood mononuclear cells from two severe combined immunodeficiency (SCID) patients, manifested a complete absence of genomic rearrangements of the immunoglobulin (Ig) heavy (H) and light (L) chain loci. The BLCL contained germ-line transcripts of the Ig kappa region locus of approximately 1.2 kilobase (kb). By cDNA cloning and sequence analysis the transcripts were shown to consist of a C kappa segment, a J kappa 1 gene segment, 160 base pairs (bp) of J kappa 1 5' intervening sequence, containing the heptamer/nonamer recombination recognition sequences and at the 5' end a 523-bp segment designated human kappa zero, The first 206 bp of this 5' segment were homologous to the reported murine kappa zero region. Genomic restriction mapping and DNA sequence analysis demonstrated that the human kappa zero segment is located approximately 4 kb upstream of J kappa 1. The kappa zero segment contains a putative promoter region with an OCT2 binding site, and has a splice donor site to accomplish splicing to an acceptor site 160 bp upstream of J kappa 1. Expression of the kappa zero gene segment was found in BLCL derived from normal fetal bone marrow, in which both Ig kappa loci were in the germ-line configuration. These findings indicate that the described transcripts are not only present in SCID, but also in normal developing pre-B lymphocytes. The expression of germ-line Ig kappa L chain transcripts may be associated with the locus becoming accessible to gene rearrangement.

Re-organization of the immunoglobulin kappa gene on both alleles is not an obligatory prerequisite for Ig lambda gene expression in human cells

Re-arrangement and expression of the immunoglobulin (Ig) genes in B cells occurs under an ordered developmental control. The sequential model of Ig light chain exclusion predicts that only after non-productive re-organization or deletion of both Ig kappa alleles would re-arrangement of Ig lambda gene segments occur. To prove this model, we asked whether expression of Ig lambda light chains is always associated with rearrangement and/or deletion of both Ig kappa alleles in human cells. Therefore, we established human diploid B-cell clones in vitro that produce Ig lambda light chains. Southern blot analysis of the Ig kappa alleles revealed that three Ig lambda expressing cell lines (out of six Ig lambda+ cell lines tested) harbour one Ig kappa allele in germline configuration. Furthermore, a 1.5 kb RNA derived from the germline Ig kappa locus was detected by Northern blot hybridizations. The results implicate that the mechanism of Ig light chain exclusion is not precisely sequential and that it does not necessarily need re-arrangement or deletion of both Ig kappa alleles as a prerequisite for Ig lambda light chain expression in human cells.

Mutation of immunoglobulin J chi splice sites does not affect the rearrangement frequency of J chi segments

The rabbit b9 kappa 1 locus contains 5 joining (J chi) gene segments, only two of which, J chi 1 and J chi 2, are utilized in assembly of a complete variable region gene (as shown by Akimenko, M.-A., Mariamé, B. and Rougeon, F. (1986) Proc. natl. Acad. Sci. (Wash.), 83, 5180-5183). J chi 4 and J chi 5 do not rearrange because of deficient recombination signal sequences. The J chi 3 gene segment is also not rearranged and has a non-functional splice site. In view of the proposed relationship between transcription and rearrangement of immunoglobulin genes, we sought to determine whether splicing of the germline transcript is implicated in the rearrangement process. We addressed this question by introducing mutations in the splice sites of the J chi segments of an unrearranged kappa light chain immunoglobulin transgene. Rearrangement was analysed by polymerase chain reaction on transgenic spleen DNA. We observed that mutation of the splice sites had no effect on the utilization of the J chi gene segments, demonstrating that there is no relationship between splicing and rearrangement.

Molecular heterogeneity of the 1.0-kb T beta transcript in natural killer and gamma/delta lymphocytes

The T cell receptor (TcR) beta chain is encoded by a 1.3-kb mRNA which contains variable (V), diversity (D), joining (J) and constant (C) regions. A shorter 1.0-kb transcript with C but no V sequences is found in thymocytes, alpha/beta and gamma/delta T lymphocytes and natural killer cells. The origin, clonal variability and function of this transcript is unknown. We isolated cDNA clones representative of the 1.0-kb mRNA from cDNA libraries of either polyclonal or clonal natural killer and gamma/delta lymphocytes. Ten different types of cDNA clones belonging to three separate groups were identified: (a) "J-C clones" consisting of one of five J beta regions and the corresponding C beta 1 or C beta 2 regions; (b) "D-J-C clones" composed of D beta 1/or D beta 2/J beta rearranged sequences and C beta 1 or C beta 2 sequences; (c) "5'C clones" made up of C beta 1 or C beta 2 regions preceded by the corresponding genomic 5' flanking region. The presence of recombination signal sequences in J-C and D-J-C clones suggests that the first derive from mRNA transcribed from promoters located in the 5' J region and the second from mRNA transcribed from promoters situated in the 5' D region. Nucleotide sequence analysis demonstrated that only three of the ten types of clones isolated had the potential to code a short cytoplasmic T beta protein with a variable D-J beta N terminus. These findings have implications for the mechanisms of T beta germ-line transcription and the function of the T beta transcripts.