CD38 Expression Identifies an Active Subset of Proliferating Cells within the Clonal Population in Chronic Lymphocytic Leukemia Patients (In Vitro Analysis).

The prognostic relevance of CD38 expression by clonal B cells from chronic lymphocytic leukemia (B-CLL) cases has been extensively reported. Expression of CD38 by a B cell indicates both its activation status and differentiation stage and remains fairly constant over time in most B-CLL cases although it has been reported to change with time in some cases and this change is associated with change in clinical behavior of the patient. Due to these reasons it has achieved a prominent place among other prognostic indicators such as Ig V gene mutation and ZAP-70 expression in assessment of B-CLL patients. To assess which aspects of the immunobiology of B-CLL cells are impacted by their expression of CD38 by clonal members “within” a B-CLL case we flow-sorted B-CLL cells from peripheral blood of 16 patients (% CD38 expression pre-sort, ranging from 1–90%), into CD38⁻ and CD38⁺ subsets and quantified telomere lengths as a measure of their replicative history. As a measure of cellular activation we studied telomerase activity in these flow-sorted cells. In addition, expression of CD69 and CD62L (activation-associated), ZAP-70 (signaling-associated) and Ki-67 (cell cycle-associated) were studied on CD38⁻ and CD38⁺ cell subsets by immunofluorecence and multi-color flow cytometry in 35 cases (including those used for cell sorting experiments). Telomerase activity was markedly higher in the CD38⁺ subset compared to the CD38⁻ subset in each case (p< 0.01), although telomere lengths of the subsets did not differ significantly within individual cases. Simultaneously, the percentages of cells expressing CD69 and ZAP-70 were significantly higher (p<0.01) and those expressing CD62L (lost after cell activation) were significantly lower (p<0.05) in the CD38⁺ subset compared to those in the CD38⁻⁻ subset. Not only did cells from the CD38⁺ subset exhibit greater percentages of cells with features of “activated cells”, they also identified cells that had recently exited the G0/G1 phase as indicated by differences in Ki-67 expression (p<0.001). Unlike other hematological neoplasms, cell division, noticeable in the peripheral blood, is not a feature of B-CLL. The current findings suggest that the CD38⁺ fraction of the clone is more “active” than the CD38⁻ fraction, regardless of the initial percentage of CD38⁺ cells in the clone. Therefore, a detailed phenotypic analysis of CD38-based subpopulations within a B-CLL case could identify even the low level turnover of clonal cells and provide important supplementary information for prediction of prognosis.