Homeostatic Proliferation of Chronic Lymphocytic Leukemia.

Slow accumulation of chronic lymphocytic leukemia cells in vivo was considered due to defective apoptosis rather than proliferation. However, recent data, based on isotope incorporation studies, suggested significant continuous proliferation of CLL cells in vivo and similar observations has been made for human memory B cells. As previous gene expression studies revealed close relation of CLL cells to memory B cells we asked, whether the mechanisms driving homeostatic proliferation of human memory B cells were stimulating CLL cells as well. Comparison of CLL samples (n=20) and memory B cells of healthy donors showed high expression levels of Toll-like receptor (TLR) 7 and TLR9 and of IL2 and IL15 cytokine receptors compared to naive B cells. Proliferation of CLL cells compared to their normal counterparts was analyzed after FACS sorting and CFSE-labeling.Naive B cells did neither respond to TLR7 ligand resiquimod, TLR9 ligand CpG2006, IL2 and IL15 nor to their combination. In contrast, CLL cells and memory B cells showed comparable patterns of proliferation. Remarkably, under these conditions, terminal differentiation to plasma cells was observed for memory B cells only, while proliferation of CLL ceased after the 4th division without differentiation into Ig-secreting cells. To estimate in vivo turnover rates, B cells of healthy donors and CLL cells were stained for intracellular Ki67, that identifies recently divided cells, and were analyzed by flow cytometry. Memory B cells showed high in vivo turnover rates (2–5% Ki67+) while naive B cells where largely quiescent - results that are in line with previous isotope incorporation studies with healthy donors. Turnover rates of CLL ex vivo ranged from 0.025–1.4% Ki67+ cells, which in absolute numbers (per mL of blood) was comparable to memory B cells. These results are consistent with a concept of homeostatic proliferation of both memory B cells and CLL cells, the latter being arrested without terminal differentiation, which might account for their accumulation in vivo.