Terminal deoxynucleotidyl transferase (TdT)-containing peripheral blood mononuclear cells during remission of acute lymphoblastic leukemia: low sensitivity and specificity prevent accurate prediction of relapse

Immunophenotypic markers in adult acute lymphoblastic leukemia: the prognostic significance of CD20 and TdT expression

BACKGROUND

Efforts to overcome poor outcomes in patients with adult acute lymphoblastic leukemia (ALL) have focused on combining new therapeutic agents targeting immunophenotypic markers (IPMs) with classical cytotoxic agents; therefore, it is important to evaluate the clinical significance of IPMs.

METHODS

Baseline characteristics and clinical outcomes of patients with adult ALL were retrospectively analyzed. The percentage of blasts expressing IPMs at diagnosis was measured by multicolor flow cytometry analysis. Samples in which ≥20% of blasts expressed an IPM were considered positive.

RESULTS

Among the total patient population (N=230), almost all (92%) were in first or second hematological complete remission (HCR) and 54% received allogeneic hematopoietic cell transplant (allo-HCT). Five-year hematologic relapse-free survival (HRFS) and overall survival (OS) rates were 36% and 39%, respectively, and 45.6% and 80.5% of patients were positive for the IPMs CD20 and terminal deoxynucleotidyl transferase (TdT), respectively. Expression of CD20, CD13, CD34, and TdT was associated with HRFS rate, and expression of CD20 and CD13 was associated with OS rate, as was the performance of allo-HCT. In multivariate analysis, positivity for CD20 (HRFS: hazard ratio [HR], 2.21, P<0.001; OS: HR, 1.63, P=0.015) and negativity for TdT (HRFS: HR, 2.30, P=0.001) were both significantly associated with outcomes. When patients were categorized into three subgroups according to positivity for CD20 and TdT, there were significant differences in HRFS and OS among the subgroups.

CONCLUSION

Positivity for CD20 and TdT expression and clinical risk group were prognostic factors in adult ALL.

Detection of minimal residual disease in acute lymphoblastic leukemia by flow cytometry with monoclonal antibodies

To detect more precisely the minimal residual disease in acute lymphoblastic leukemia (ALL), two-color flow cytometric analysis for the detection of cell-surface antigen (CD10; CALLA) and nuclear terminal deoxynucleotidyl transferase (TdT) was performed in the six patients with CALLA-positive ALL coexpressing TdT. In all patients, the leukemic blasts coexpressed Ia (HLA-DR), CD9, CD19, CD20, CD24, and CD10. Five of six patients achieved complete remission, but one has so far relapsed. No leukemic blasts (CD10+, TdT+) were detected at the time of complete remission. During maintenance chemotherapy, leukemic blasts coexpressed C10 and TdT were found 2.32% in the patient's peripheral blood by two-color analysis, whereas no obvious leukemic cells were recognized morphologically. The patient relapsed leukemia with the same phenotype 4 weeks after the examination. On the basis of our findings, we suggest that two-color flow cytometric analysis with the use of these antibodies is quite valuable to detect the minimal residual leukemic cells in a patient with ALL. The reduction of leukemic cells below the threshold of detection of methods currently available appears to be necessary to achieve a cure in ALL. Hence accurate diagnosis of ALLs with monoclonal antibodies (MAbs) should contribute substantially to the development of an effective form of therapy for their cure.

Combined flow cytometric assessment of cell surface antigens and nuclear TdT for the detection of minimal residual disease in acute leukaemia

To define more precisely the immunophenotype of lymphoid blast cells, a new flow cytometric technique for the simultaneous detection of surface antigens and nuclear terminal deoxynucleotidyl transferase (TdT) was established. After staining for the cell surface marker, mononuclear cells were treated with paraformaldehyde (1%) and methanol to permeabilize the cell membrane. Then the cells were stained by indirect immunofluorescence using a rabbit anti-human TdT antibody. Dilution experiments were performed to reveal the sensitivity of the described flow cytometric assay: 0.02% leukaemic cells could reliably be detected above background level among normal peripheral blood lymphocytes. It is concluded that the double-staining procedure described here is a sensitive tool that contributes to the detection of minimal residual disease in a substantial proportion of acute leukaemias.

Detection of minimal residual T cell acute lymphoblastic leukemia by flow cytometry

We have developed a flow cytometric assay for the determination of cellular expression of terminal deoxynucleotidyl transferase (TdT) and applied this to the detection of minimal residual T cell acute lymphoblastic leukemia (T-ALL). The flow cytometric assay for TdT demonstrated requisite specificity: TdT was localized to the nucleus, and was detected in MOLT3 T lymphoblasts, clinical T-ALL samples, and normal bone marrow B lymphoid precursors, but in neither the KG1a myeloid leukemia cell line nor normal myeloid cells. Co-expression of TdT and the pan T cell marker CD5 was used to quantify T lymphoblasts. 0.25 +/- 0.13% of normal adult bone marrow CD5+ cells were TdT+; these may represent early T lymphoid precursors. When admixed with normal bone marrow, CD5+TdT+ leukemic cells could be detected above background levels at an added concentration of 0.035% (95% confidence interval 0.028-0.43%). Long term follow-up of a large number of patients will be required to determine the clinical significance of a minimal burden of leukemic cells.