Up-regulated recombination-activating gene expression in slg− variants of a human mature B cell line undergoing secondary Igλ, rearrangements in cell culture

To study the role of surface immunoglobulin (sIg) expression in the control of rearrangement activity at the Ig light chain loci, we established rare sIg- clones (lambda-) from a human sIg+ B cell line (mu lambda+). Upon expansion of these sIg- clones, surface immunofluorescence analysis revealed a gradual emergence of sIg+ subpopulations, differing from the original tumor cell line both in their idiotypes and C lambda isotypes. DNA analysis revealed that this sIg heterogeneity resulted from a process of ongoing Ig lambda rearrangements. That is, one of the Ig lambda rearrangements in the parental cell line was replaced by novel Ig lambda rearrangements in the sIg- clones, which in turn were replaced by yet additional Ig lambda rearrangements in the sIg+ variants. Northern analysis demonstrated that while the expression of the recombination-activating genes RAG1 and RAG2 was relatively low in the parental cell line, their expression was significantly increased in both the sIg- variants and their sIg+ progenies. We thus describe a human mature B cell line, in which differential RAG expression allows sIg heterogeneity to be generated through secondary Ig lambda gene rearrangements. Our results indicate that the induction of RAG expression may be inversely associated with sIg expression, but that sIg expression, alone, is not sufficient to down-regulate this expression.