Ongoing In Vivo Immunoglobulin Class Switch DNA Recombination in Chronic Lymphocytic Leukemia B Cells

Abstract

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 IH-CH and mature VHDJH-CH 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.

Richter's and prolymphocytic transformation of chronic lymphocytic leukemia are associated with high mRNA expression of activation-induced cytidine deaminase and aberrant somatic hypermutation

“…Our results provide evidences that high expression of AID mRNA and ASHM are associated with the development of RT and PLT in CLL patients. In line with previous reports, 17,18 all CLLs analyzed in this study expressed AID mRNA at various levels but the expression levels remained well below that of normal GC B-cells. Chronic lymphocytic leukemia cases that showed no transformation and cases that eventually transformed to higher-grade lymphoma expressed AID at similar levels.…”

High expression of activation-induced cytidine deaminase (AID) mRNA is associated with unmutated IGVH gene status and unfavourable cytogenetic aberrations in patients with chronic lymphocytic leukaemia

“…26,29 One could speculate that the association of AID with deletion in 11q (frequently observed in nodal CLL) and CD38 expression is somehow related to the pattern of CD19 þ / CD38 þ /IgD À germinal centre cells. 34 The majority of unmutated cases were AID-positive, but even 47% of mutated cases still expressed AID.…”

“…[20][21][22][23][24] Activation-induced cytidine deaminase (AID) is normally expressed in CD19 þ /CD38 þ germinal centre B-cells and essential for somatic hypermutation, suggesting its potential association with IGV H mutational status. [25][26][27][28][29] Indeed, two reports have recently described the expression of AID in unmutated B-CLL cases. 28,30 However, a third study found no association between AID expression and IGV H mutation status.…”

Divergence in CD19-Mediated Signaling Unfolds Intraclonal Diversity in Chronic Lymphocytic Leukemia, Which Correlates with Disease Progression

“…Growing evidence supports the concept that intraclonal CLL cell diversity plays a role in the disease biology and eventual clinical outcome (11)(12)(13). Intraclonal heterogeneity is acquired through spontaneous genetic alterations or driven by microenvironmental stimuli (14,15). Recently, CLL cells expressing CD38 were shown to be a proliferating subset, as they strongly express Ki-67 and have a faster proliferation rate in vivo than do CD38 2 CLL cells of the same clone (13).…”