We have generated a site-directed transgenic (sd-tg) mouse model in which the JH locus has been replaced with a rearranged VDJ coding for the heavy chain of an anti-DNA antibody. In these mice, B cells expressing the anti-dsDNA specificity are negatively regulated. We observe a novel mechanism for B cell tolerance, receptor editing at the heavy chain locus. In most sd-tg B cells, the inserted anti-DNA VH gene has been replaced by the upstream endogenous VH, or DH, or both genes through recombination with the heptamer embedded at the 3' end of most VH genes. Three types of recombination events have been identified. VH-to-VDJ, DH-to-VDJ, and VH-to-DH-VDJ. Analysis of the junctional sequences revealed features of classical V(D)J rearrangement, namely N sequence addition and nucleotide deletion. A conserved nonamer was found 12 bp upstream of the embedded heptamer. This nonamer may represent a novel recombination signal sequence used for VH editing. The sd-tg model thus provides direct evidence for secondary rearrangement at VH-D-JH. This process may play a role in tolerance by editing autoreactive receptors and may also serve to diversify the VH repertoire.
No abstract
We have generated site-directed transgenic mice whose transgenes code for anti-DNA antibodies. These antibodies are representative of the lupus-associated anti-DNAs seen in mouse models of autoimmunity and human SLE, and have the usual characteristics of pathogenic autoantibodies. As conventional transgenics in nonautoimmune mice, anti-DNA B cells have been shown to be deleted or inactivated. Autoreactive B cells can also escape negative regulation by a process called receptor editing. Here we describe two combined immunoglobulin H and L chain site-directed transgenic mouse models and characterize their editing phenotypes. One model, 3H9R/Vkappa4R, has a deletion-prone phenotype and undergoes editing, primarily by inactivation of the light chain by leap-frogging events. In the other model, 3H9R/Vkappa8R, B cells are susceptible to anergy and maintain most of their HR and LR chains. These studies clarify the relationship between editing and other mechanisms of tolerance.
A functional B cell antigen receptor is thought to regulate antibody gene rearrangement either by stopping further rearrangement (exclusion) or by promoting additional rearrangement (editing). We have developed a new model to study the regulation of antibody gene rearrangement. In this model, we used gene targeting to replace the J kappa region with a functional V kappa-J kappa light chain gene. Two different strains of mice were created; one, V kappa 4R, has a V kappa 4-J kappa 4 rearrangement followed by a downstream J kappa 5 segment, while the other, V kappa 8R, has a V kappa 8-J kappa 5 light chain. Here, we analyze the influence of these functional light chains on light chain rearrangement. We show that some V kappa 4R and V kappa 8R B cells only have the V kappa R light chain rearrangement, whereas others undergo additional rearrangements. Additional rearrangement can occur not only at the other kappa allele or isotype (lambda), but also at the targeted locus in both V kappa 4R and V kappa 8R. Rearrangement to the downstream J kappa 5 segment is observed in V kappa 4R, as is deletion of the targeted locus in both V kappa 4R and V kappa 8R. The V kappa R models illustrate that a productively rearranged light chain can either terminate further rearrangement or allow further rearrangement. We attribute the latter to editing of autoantibodies and to corrections of dysfunctional receptors.
Antibodies to single-stranded (ss)DNA are expressed in patients with systemic lupus erythematosus and in lupus-prone mouse models such as the MRL/Mp-lpr/lpr (MRL/lpr) strain. In nonautoimmune mice, B cells bearing immunoglobulin site-directed transgenes (sd-tgs) that code for anti-ssDNA are functionally silenced. In MRL/lpr autoimmune mice, the same sd-tgs are expressed in peripheral B cells and these autoantibodies gain the ability to bind other autoantigens such as double-stranded DNA and cell nuclei. These new specificities arise by somatic mutation of the anti-ssDNA sd-tgs and by secondary light chain rearrangement. Thus, B cells that in normal mice are anergic can be activated in MRL/lpr mice, which can lead to the generation of pathologic autoantibodies. In this paper, we provide the first direct evidence for peripheral rearrangement in vivo.
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