Heavy chain variable (VH) region diversity generated by VH gene replacement in the progeny of a single precursor cell transformed with a temperature-sensitive mutant of Abelson murine leukemia virus

Characterization of a new V gene replacement in the absence of activation-induced cytidine deaminase and its contribution to human B-cell receptor diversity

In B cells, B-cell receptor (BCR) immunoglobulin revision is a common route for modifying unwanted antibody specificities via a mechanism called VH replacement. This in vivo process, mostly affecting heavy-chain rearrangement, involves the replacement of all or part of a previously rearranged IGHV gene with another germline IGHV gene located upstream. Two different mechanisms of IGHV replacement have been reported: type 1, involving the recombination activating genes complex and requiring a framework region 3 internal recombination signal; and type 2, involving an unidentified mechanism different from that of type 1. In the case of light-chain loci, BCR immunoglobulin editing ensures that a second V-J rearrangement occurs. This helps to maintain tolerance, by generating a novel BCR with a new antigenic specificity. We report that human B cells can, surprisingly, undergo type 2 replacement associated with κ light-chain rearrangements. The de novo IGKV-IGKJ products result from the partial replacement of a previously rearranged IGKV gene by a new germline IGKV gene, in-frame and without deletion or addition of nucleotides. There are wrcy/rgyw motifs at the 'IGKV donor-IGKV recipient chimera junction' as described for type 2 IGHV replacement, but activation-induced cytidine deaminase (AID) expression was not detected. This unusual mechanism of homologous recombination seems to be a variant of gene conversion-like recombination, which does not require AID. The recombination phenomenon described here provides new insight into immunoglobulin locus recombination and BCR immunoglobulin repertoire diversity.

V(H) replacement in rearranged immunoglobulin genes

Examples suggesting that all or part of the V(H) segment of a rearranged V(H)DJ(H) may be replaced by all or part of another V(H) have been appearing since the 1980s. Evidence has been presented of two rather different types of replacement. One of these has gained acceptance and has now been clearly demonstrated to occur. The other, proposed more recently, has not yet gained general acceptance because the same effect can be produced by polymerase chain reaction artefact. We review both types of replacement including a critical examination of evidence for the latter. The first type involves RAG proteins and recombination signal sequences (RSS) and occurs in immature B cells. The second was also thought to be brought about by RAG proteins and RSS. However, it has been reported in hypermutating cells which are not thought to express RAG proteins but in which activation-induced cytidine deaminase (AID) has recently been shown to initiate homologous recombination. Re-examination of the published sequences reveals AID target sites in V(H)-V(H) junction regions and examples that resemble gene conversion.

VH gene analysis in sporadic Burkitt's lymphoma: somatic mutation and intraclonal diversity with special reference to the tumor cells involving germinal center

We analyzed the immunoglobulin heavy chain variable region (V(H)) gene in seven cases of sporadic Burkitt's lymphoma (sBL) to elucidate their cell of origin. In particular, we focused on the V(H) gene status of tumor cells involving adjacent germinal center (GC) by microdissecting histological sections. Among the seven V(H) genes V(H)1 family was found in two, V(H)3 in four, and V(H)4 in one. All rearranged V(H) genes demonstrated somatic mutations at percentages ranging from 1.4 to 7.5% (mean, 4.2%), which is a similar level to that seen in IgM-only B cells. Three out of four V(H) genes with more than 2% sequence difference from their corresponding germline counterpart showed evidence of antigen selection in their framework region 3. Three cases demonstrated signs of intraclonal diversity with a mutational frequency of 0.47-0.98%, which was 13.5-28.8 times as great as the Taq infidelity in our experimental conditions. However the level of somatic mutation and the effect of antigen selection on V(H) gene were diverse in these three cases, and the relationship between V(H) gene somatic mutation status and intraclonal diversity was unclear in sBL. In the analysis of microdissected tissues, all 20 tumor clones in the adjacent GCs showed additional replacement mutations in complementarity determining region 3, suggesting a role of antigen in tumor progression. This finding resembles the phenomenon that memory B-cells reenter into GC to undergo further affinity maturation. In contrast, 7/11 V(H) gene sequences irrelevant to GC were identical to those of the major tumor clone. Thus our findings suggested that sBL is derived from memory B-cells rather than GC B-cells, and that antigen stimulation is involved in the clonal expansion of a proportion of sBL.

Changes in the V(H) gene repertoire of developing precursor B lymphocytes in mouse bone marrow mediated by the pre-B cell receptor

The V(H) repertoire on both H chain alleles of normal and lambda5-deficient B lineage cells were analyzed by single-cell PCR. The mu H chains were tested for their capacity to form a pre-B cell receptor. In bone marrow, D-proximal V(H) genes were found preferentially expressed in lambda5-deficient pre-B cells and in a newly identified early c-kit+ cytoplasmic mu H chain+ pre-B cell population of normal mice. Only half of the mu H chains expressed in these cells have the capacity to form a pre-B cell receptor. Representation of the D-proximal V(H) genes was found suppressed on the productive but not on the nonproductive V(H)DJ(H) rearranged alleles of c-kit preB-II cells and splenic lambda5-deficient B cells. More than 95% of the mu H chains expressed in preB-II cells can form a pre-B cell receptor. These results demonstrate that the pre-B cell receptor in normal mice and the B cell receptor in lambda5-deficient mice mediate a shift in the V(H) repertoire.

A quasi-monoclonal mouse

As a model for studying the generation of antibody diversity, a gene-targeted mouse was produced that is hemizygous for a rearranged V(D)J segment at the immunoglobulin (Ig) heavy chain locus, the other allele being nonfunctional. The mouse also has no functional kappa light chain allele. The heavy chain, when paired with any lambda light chain, is specific for the hapten (4-hydroxy-3-nitrophenyl) acetyl (NP). The primary repertoire of this quasi-monoclonal mouse is monospecific, but somatic hypermutation and secondary rearrangements change the specificity of 20 percent of the antigen receptors on B cells. The serum concentrations of the Ig isotypes are similar to those in nontransgenic littermates, but less than half of the serum IgM binds to NP, and none of the other isotypes do. Thus, neither network interactions nor random activation of a small fraction of the B cell population can account for serum Ig concentrations.

Rearrangement of upstream DH and VH genes to a rearranged immunoglobulin variable region gene inserted into the DQ52-JH region of the immunoglobulin heavy chain locus

We investigated gene rearrangements in the mutant IgH locus of a mouse strain generated by insertion of a rearranged heavy chain variable region gene (VT15) into the DQ52-JH region through gene targeting. In more than half of the B cells of heterozygous mutant mice, the mutant IgH locus was silenced by the rearrangement of an endogenous DH or DH and VH gene to the inserted VT15 gene. In these cases, a functional VHDHJH gene was present on the wild-type allele. The silencing rearrangement appeared to be mediated by recombination signal sequence (RSS)-like elements present in the "recipient" VT15 gene. Among the many such elements on the inserted VT15 gene, which apparently met the requirement for an RSS with respect to nucleotide sequence, only two were observed in the actual rearrangements. This indicates that targeting of the recombination machinery involves sequences in addition to the RSS motifs as they have been characterized so far. In homozygous mutant mice, most B cells appeared to carry the intact VT15 gene on both mutant IgH alleles, although single-cell polymerase chain reaction revealed that silencing rearrangements occurred frequently in B cell progenitors in the bone marrow. This observation indicates that once silencing rearrangements are initiated in a cell, they involve both VT15 genes in most cases, reminiscent of normal DH-JH rearrangement. B cells which did not initiate such rearrangements develop to populate the peripheral B cell compartment.

Identical VHD and DJH junctions in monoclonal antibodies derived in response to dextran B512 could be the result of developmental selection

We describe here the CDR3s of a collection of monoclonal antibodies (MoAb) with specificity for the carbohydrate dextran B512 produced in the mouse strain C57BL/6. In spite of the postulated mechanisms for variability in this region, a high proportion of these monoclonals displayed identical VHD (24/30) and DJH (21/30) junctions and 21 of them were identical in the whole CDR3. These 21 independently generated identical CDR3s could be ordered in eight groups indicating that not a particular CDR3, but instead the mechanism for generating identical junctions was preserved. Two of the CDR3s in this study were found to be identical to the CDR3 of the monoclonal B1-8 produced in C57BL/6 in response to proteins bearing the hapten (4-hydroxy-3-nitrophenyl)acetyl (NP). This and other parameters support the notion that the generation of identical junctions could be independent of antigenic selection. We also report here the association between JH usage and amino acid (aa) residues at the VHD and DJH junctions. Since these MoAb were generated in response to dextran B512, immunoglobulin conformation has to be compatible with antigen binding. Nevertheless, no aa residue of CDR3 could be directly related to antigen binding. We postulate therefore, that the observed selection of CDR3s could be directed to the production of variable regions with protein configuration most suitable with immunoglobulin folding and may occur prior to antigenic selection. Selection for junctional residues in relation to JH usage and the generation of identical CDR3s are probably different events. Possible genetic mechanisms operating for CDR3 construction and/or selection by cellular ligands are discussed.