Total Number of CD34 + Cells Per Ten High Power Fields in Bone Marrow Trephines of Patients with Chronic Myeloid Leukaemia Correlates with Probability of Complete Cytogenetic Response during the First Year of Treatment with Imatinib.

We studied expression of CD34 positive cells in the bone marrow trephines of two groups of patients with CML treated with imatinib. In the first group (n=17), patients went on to achieve sustained complete cytogenetic response during their disease course. In the second group (n =33) complete cytogenetic response was not achieved and patients later developed either accelerated or blastic phase disease. We studied paraffin embedded bone marrow trephines stained with the monoclonal antibody QBEND10 during an eighteen month period following initiation of imatinib therapy and recorded the highest CD34 positive cell counts and aspirate blast counts in three six-month intervals. CD34 positive cells were evaluated using three counting methods. The first method, established in our department for several years, expressed the percentage of CD34 positive cells vs. all nucleated cells in a minimum 500-cell count (% CD34+/500 cells). The second recorded the total number of CD34 positive cells in ten high power fields (CD34+/10hpf) using x 40 magnification /numerical aperture 0.65 and the third method utilised the highest number of CD34 positive cells expressed in a single high power field (CD34+/hpf), the value being taken from the previous CD34+/10 hpf count.

We hypothesised that the overall CD34 positive cell burden and absolute CD34 positive numbers would be more important in predicting the likelihood of complete cytogenetic response and this would be better assessed by volumetric evaluation than percentage calculation relative to other nucleated cells.

CD34 expression in endothelial cells was taken as a positive internal control. We aimed to establish which variable: % blast count, % CD34+/500 cells, CD34+/10hpf and CD34+/hpf provided better correlation with subsequent cytogenetic response to treatment. Values for each variable were dichotomised at the following levels; aspirate blast count <5 and > 5, % CD34+/500 cells < 2 and > 2%, CD34+/10hpf < 60 and > 60 cells and CD34+/hpf < 10 and > 10 cells. Univariate analysis was performed on samples within the first six months, seven to twelve months and thirteen to eighteen months. Only CD34+/10hpf showed significant discriminatory ability throughout the entire eighteen month period (Pearson’s Chi-square 2-sided sig: 0.031, 0.003 and 0.007 in the first six months, seven to twelve months and thirteen to eighteen months samples respectively). By logistic regression, we confirmed that CD34+/10hpf had the best correlation with cytogenetic response during the first year following initiation of imatinib therapy. Larger studies are required to confirm these initial findings and to establish volumetric assessment of CD34 positive cells as a predictor for cytogenetic response.