Ongoing in vivo immunoglobulin class switch DNA recombination in chronic lymphocytic leukemia B cells

Chronic lymphocytic leukemia (CLL) results from the expansion of malignant CD5⁺ B cells that usually express IgD and IgM. These leukemic cells can give rise in vivo to clonally related IgG⁺ or IgA⁺ elements. The requirements and modalities of this process remain elusive. Here we show that leukemic B cells from 14 of 20 CLLs contain the hallmarks of ongoing Ig class switch DNA recombination (CSR), including extrachromosomal switch circular DNAs and circle transcripts generated by direct Sμ→Sγ, Sμ→Sα, and Sμ→Sε as well as sequential Sγ→Sα and Sγ→Sε CSR. Similar CLL B cells express transcripts for activation-induced cytidine deaminase, a critical component of the CSR machinery, and contain germline I_H-C_H and mature V_HDJ_H-C_H transcripts encoded by multiple Cγ, Cα, and Cε genes. Ongoing CSR occurs in only a fraction of the CLL clone, as only small proportions of CD5⁺CD19⁺ cells express surface IgG or IgA and lack IgM and IgD. In vivo class-switching CLL B cells down-regulate switch circles and circle transcripts in vitro unless exposed to exogenous CD40 ligand and IL-4. In addition, CLL B cells that do not class switch in vivo activate the CSR machinery and secrete IgG, IgA, or IgE upon in vitro exposure to CD40 ligand and IL-4. These findings indicate that in CLL at least some members of the malignant clone actively differentiate in vivo along a pathway that induces CSR. They also suggest that this process is elicited by external stimuli, including CD40 ligand and IL-4, provided by bystander immune cells.