IgM-producing chronic lymphocytic leukemia cells undergo immunoglobulin isotype-switching without acquiring somatic mutations

Abstract

The malignant B cells in chronic lymphocytic leukemia (CLL) typically express low-density membrane IgM or IgM/IgD. In vitro experiments have shown that the CLL cells can be induced to differentiate into cells that secrete immunoglobulin (Ig) and can occasionally undergo heavy (H) chain class switching. We now show that the CLL cells also undergo isotype-switching in vivo, since γ and/or α H chain transcripts with identical FW3/CDR3/FW4 regions as the μ CLL transcripts were detected in all of the 13 investigated patients with IgM+ CLL. In most cases switching had occurred to α1 and γ3, but CLL transcripts corresponding to the other γ chain isotypes were also detected. In one case both the productively and nonproductively rearranged allele were found to undergo H chain class switching. CLL γ transcripts were also present in surface IgG+ sorted B cells, demonstrating that a small subset of the CLL cells express membrane IgG. In addition, transcripts encoding secretory γ2 and γ3 H chains were detected in two cases, which suggests that some serum IgG could be produced by the leukemic clone. Analysis of sorted PBL showed that isotype-switching occurs in CLL cells that express the CD5 antigen. Finally, nucleotide sequence analysis showed that the μ, α, and γ CLL transcripts are identical, demonstrating that the CLL cells do not accumulate somatic mutations in their variable region genes after the H chain class switching. These data provide in vivo evidence that isotype-switching is a frequent phenomenon in CLL, and indicate that a subset of the CLL lymphocytes progress to later stages of B cell differentiation.