Double producers of kappa and lambda define a subset of B cells in mouse plasmacytomas

Rearrangement of the light chain locus is believed to be an ordered process in which Ig lambda rearrangements only occur if Ig kappa rearrangements are found to be non-productive or self-reactive. Secondary rearrangements of the B-cell receptor (BCR) have shown, however, that rescue of abortive Ig kappa rearrangements or autoreactive B cells can be achieved through receptor editing using upstream V-regions as the template sequences. Since secondary rearrangement can occur in the periphery, possibly in a subset of B cells maintaining constitutive Rag activity, it is conceivable that two light chains (kappa:kappa or kappa:lambda) could be expressed in these cells, apparently in violation of allelic exclusion. Previously, we have reported that silicone-induced plasmacytomas (SIPCs) exhibit dual expression and ongoing rearrangements of Ig kappa and Ig lambda. In this paper, we show by ELISA that both Ig kappa and Ig lambda are found at the protein level, but are secreted in different amounts. Furthermore, we demonstrate by micro-manipulation and RT-PCR amplification that Ig kappa and Ig lambda are simultaneously expressed in a single SIPC cell. We propose that these dual-expressing cells, found intermittently in cases of plasmacytomas (PCs), may have originally been immature B cells when transformed but now are maintained as a long-lived mature B cell found infrequently in the tumor population. (C) 2001 Published by Elsevier Science Ltd.