Mitogen induced activation, proliferation and surface antigen expression patterns in unmutated and hypermutated chronic lymphocytic leukemia cells

Chronic lymphocytic leukaemia: a disease of activated monoclonal B cells

B cell-type chronic lymphocytic leukaemia (CLL) has long been considered a disease of resting lymphocytes. However, cell surface and intracellular phenotypes suggest that most CLL cells are activated cells, although only a small subset progresses beyond the G1 stage of the cell cycle. In addition, traditional teaching says that CLL cells divide rarely, and therefore the build-up of leukaemic cells is due to an inherent defect in cell death. However, in vivo labelling of CLL cells indicates a much more active rate of cell birth than originally estimated, suggesting that CLL is a dynamic disease. Here we review the observations that have led to these altered views of the activation state and proliferative capacities of CLL cells and also provide our interpretation of these observations in light of their potential impact on patients.

CD38 expression labels an activated subset within chronic lymphocytic leukemia clones enriched in proliferating B cells

Chronic lymphocytic leukemia (CLL) cells are thought to have diminished cell-cycling capacity, a view challenged by their phenotypic resemblance to activated human B lymphocytes. The present study addresses the cell-cycling status of CLL cells, focusing on those leukemic cells expressing CD38, a molecule involved in signaling and activation that also serves as a prognostic marker in this disease. CD38(+) and CD38(-) members of individual CLL clones were analyzed for coexpression of molecules associated with cellular activation (CD27, CD62L, and CD69), cell-cycle entry (Ki-67), signaling (ZAP-70), and protection from apoptosis (telomerase and Bcl-2). Regardless of the size of the CD38(+) fraction within a CLL clone, CD38(+) subclones are markedly enriched for expression of Ki-67, ZAP-70, human telomerase reverse transcriptase, and telomerase activity. Although the percentage of cells (approximately 2%) entering the cell cycle as defined by Ki-67 expression is small, the absolute number within a clone can be sizeable and is contained primarily within the CD38(+) fraction. Despite these activation/proliferation differences, both CD38(+) and CD38(-) fractions have similar telomere lengths, suggesting that CD38 expression is dynamic and transient. These findings may help explain why high percentages of CD38(+) cells within clones are associated with poor clinical outcome.