The mechanism of autoantibody production in an autoimmune MRL/lpr mouse

Rheumatoid factors (RF) and anti-DNA Abs from MRL/lpr mice have features similar to Abs directed toward foreign Ags, indicating a role of specific activation by Ags during disease. But our previous studies and analogous studies from others concentrated on a limited subset of hybridomas selected on the basis of Ag binding to well characterized target autoantigens. Thus, it has been unclear to what extent clonal expansion is restricted to identifiable autospecificities. To obtain a more complete picture of disease-associated autoantibody production, we designed the following experiment. A large number of B cell hybridomas were generated from the spleen of an MRL/lpr mouse and then analyzed for self-specificity, sequence, and clonal relationship. Surprisingly, we found that clonal expansion was limited to only a few autospecificities, implying a unique property of this response. In addition, we used Southern blotting with heavy and L chain constant region probes to screen both RF and non-RF hybridomas for membership in clones, one of which was first identified among RF hybridomas. We found no non-RF members of this clone. The size and number of mutations of this clone were sufficient for us to conclude that nonspecific (i.e., non-RF) mutant members are rapidly lost. Had an Ag other than IgG2a been driving clonal expansion, we should have seen mutants that retained spectificity for that Ag but that lost specificity for IgG2a. This observation, along with the restriction of clonal expansion to a few autospecificities, provides strong evidence that normal autoantigens themselves drive autoantibody clonal expansion.