The productively rearranged immunoglobulin mu chain gene and the translocated cellular oncogene c-myc are transcribed at high levels both in human Burkitt lymphoma cells carrying the t(8;14) chromosome translocation and in mouse plasmacytoma X Burkitt lymphoma cell hybrids. In the experiments reported here these genes were found to be repressed in mouse 3T3 fibroblast X Burkitt lymphoma cell hybrids. Such repression probably occurs at the transcriptional level since no human mu- and c-myc messenger RNA's are detectable in hybrid clones carrying the corresponding genes. It is therefore concluded that the ability to express these genes requires a differential B cell environment. The results suggest that the 3T3 cell assay may not be suitable to detect oncogenes directly involved in human B cell oncogenesis, since 3T3 cells apparently are incapable of transcribing an oncogene that is highly active in malignant B cells with specific chromosomal translocations.

We studied mice expressing one of two H chain transgenes. Both transgenes expressed the same 3H9 anti-DNA VDJ, but differed in their constant domains. The IgM transgene efficiently induced tolerance and selected for a subset of endogenous L chains that prevented dsDNA binding. In contrast, the IgG2b secreted-only H chain allowed expression of a broad range of L chains, most of which yielded anti-dsDNA Ab. To deduce the features of L chains that affect DNA binding, we derived hybridomas from LPS-stimulated splenic B cells from the two transgene lines and compared the V kappa sequences of Ab they secreted. Identification of L chains with related sequences but different binding to ssDNA, dsDNA, and cardiolipin allowed us to pinpoint L chain residues that correlate with enhanced or reduced binding. Arginines at the junction of V kappa 1 or V kappa 8 regions and J kappa 1, and arginines or asparagines in CDR1 or CDR2 enhanced DNA binding. Negatively charged residues at the same positions were found to interfere with binding. Thus, we predict that appropriate amino acids at these positions may form contacts with DNA. The likely locations of contact residues in the combining site were evaluated by inspection of previously determined Ab structures. Our results indicate that L chains in anti-DNA Ab are able to modulate DNA binding and contribute contact sites for additional determinants on a complex autoantigen.

We have used an Ig transgene (VH3H9) that increases the frequency of anti-DNA autoantibodies to address whether the production of antinuclear Abs in systemic lupus erythematosus is the consequence of a breakdown of B cell tolerance. We have shown that nonautoimmune mice regulate anti-DNA B cells, and that lupus-prone MRL-lpr/lpr mice are defective in this regulation. Here we show that a subset of anti-DNA B cells, namely those that stain nuclei in a homogeneous fashion, not only fail to be deleted in MRL-lpr/lpr mice, but undergo preferential clonal expansion. In addition, we describe a surprising finding: the VH3H9 transgene is less efficient at inhibiting endogenous heavy chain gene rearrangement on the autoimmune-prone MRL-lpr/lpr genetic background than on the nonautoimmune BALB/c background.

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