Flow cytometric analysis of normal B cell differentiation: a frame of reference for the detection of minimal residual disease in precursor-B-ALL

The Evolving Landscape of Flowcytometric Minimal Residual Disease Monitoring in B-Cell Precursor Acute Lymphoblastic Leukemia

Detection of minimal residual disease (MRD) is a major independent prognostic marker in the clinical management of pediatric and adult B-cell precursor Acute Lymphoblastic Leukemia (BCP-ALL), and risk stratification nowadays heavily relies on MRD diagnostics. MRD can be detected using flow cytometry based on aberrant expression of markers (antigens) during malignant B-cell maturation. Recent advances highlight the significance of novel markers (e.g., CD58, CD81, CD304, CD73, CD66c, and CD123), improving MRD identification. Second and next-generation flow cytometry, such as the EuroFlow consortium's eight-color protocol, can achieve sensitivities down to 10-5 (comparable with the PCR-based method) if sufficient cells are acquired. The introduction of targeted therapies (especially those targeting CD19, such as blinatumomab or CAR-T19) introduces several challenges for flow cytometric MRD analysis, such as the occurrence of CD19-negative relapses. Therefore, innovative flow cytometry panels, including alternative B-cell markers (e.g., CD22 and CD24), have been designed. (Semi-)automated MRD assessment, employing machine learning algorithms and clustering tools, shows promise but does not yet allow robust and sensitive automated analysis of MRD. Future directions involve integrating artificial intelligence, further automation, and exploring multicolor spectral flow cytometry to standardize MRD assessment and enhance diagnostic and prognostic robustness of MRD diagnostics in BCP-ALL.

Diagnostic and therapeutic advances in adults with acute lymphoblastic leukemia in the era of gene analysis and targeted immunotherapy

Acute lymphoblastic leukemia (ALL) is one of the most rapidly changing hematological malignancies with advanced understanding of the genetic landscape, detection methods of minimal residual disease (MRD), and the development of immunotherapeutic agents with good clinical outcomes. The annual incidence of adult ALL in Korea is 300-350 patients per year. The WHO classification of ALL was revised in 2022 to reflect the molecular cytogenetic features and suggest new adverse- risk subgroups, such as Ph-like ALL and ETP-ALL. We continue to use traditional adverse-risk features and cytogenetics, with MRD-directed post-remission therapy including allogeneic hematopoietic cell transplantation. However, with the introduction of novel agents, such as ponatinib, blinatumomab, and inotuzumab ozogamicin incorporated into frontline therapy, good MRD responses have been achieved, and overall survival outcomes are improving. Accordingly, some clinical trials have suggested a possible era of chemotherapy-free or transplantation-free approaches in the near future. Nevertheless, relapse of refractory ALL still occurs, and some poor ALL subtypes, such as Ph-like ALL and ETP-ALL, are unsolved problems for which novel agents and treatment strategies are needed. In this review, we summarize the currently applied diagnostic and therapeutic practices in the era of advanced genetic analysis and targeted immunotherapies in United States and Europe and introduce real-world Korean data.

Analytical Appraisal of Hematogones in B-ALL MRD Assessment Using Multidimensional Dot-Plots by Multiparametric Flow Cytometry: A Critical Review and Update

The spectrum of benign B-cell precursors, known as hematogones (HGs), shows a significant morphological and immunophenotypic overlap with their malignant counterpart i.e. B-lymphoid blasts (BLBs). This results in a diagnostic dilemma in assessment of cases wherein there is a physiological preponderance of HGs and also poses a significant challenge in measurable residual disease assessment in B-cell acute lymphoblastic leukaemia. Consequently, expression patterns of various immunophenotypic markers are considered the most important tool in identification and delineation of HGs from BLBs. However, certain aspects of B-cell compartment evaluation by flow cytometric immunophenotyping and its relevance in clinical scenarios is yet to be defined precisely. This review summarizes current flowcytometric data on HGs and its discrimination from BLBs based on thorough review of literature and evaluation of in-house data. Furthermore, it focuses on the utility of an additional analytical tool i.e., radar plot for a comprehensive representation of various subsets of the B-cell compartment and their differentiation from BLBs.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12288-023-01696-5.

Association of Minimal Residual Disease by a Single-Tube 8-Color Flow Cytometric Analysis With Clinical Outcome in Adult B-Cell Acute Lymphoblastic Leukemia: A Real-World Study Based on 486 Patients

CONTEXT.—

Minimal/measurable residual disease (MRD) measured by molecular and multiparametric flow cytometry (MFC) has been proven to be predictive of relapse and survival in patients with B-cell acute lymphoblastic leukemia (B-ALL). A universally applicable antibody panel at a low cost but without compromising sensitivity and power of prognosis prediction in adult B-ALL remains unestablished.

OBJECTIVE.—

To report our experience of using a single-tube 8-color MFC panel to measure the MRD status as a prognostic indicator in adult B-ALL patients.

DESIGN.—

We retrospectively analyzed the characteristics, MRD status, and prognosis of adult B-ALL based on a large real-world cohort of 486 patients during a 10-year period.

RESULTS.—

MRD assessed by MFC and polymerase chain reaction (PCR) assays for BCR-ABL+ patients showed concordant results in 74.2% of cases. MRD- status by our MFC panel could clearly predict a favorable relapse-free survival (RFS) and overall survival (OS) both at the end of induction and at the end of 1 consolidation course. Patients with continuous MRD- and with at least 1 MRD- result showed a favorable RFS and OS compared with those with at least 1 MRD+ result and continuous MRD+, respectively.

CONCLUSIONS.—

The single-tube 8-color MFC panel demonstrated a low cost, decent sensitivity, and comparability with polymerase chain reaction-MRD but an excellent performance in predicting RFS and OS, and thus could potentially be taken as a routine indicator in the evaluation of the treatment response for adult patients with B-ALL.

Defining the Basal and Immunomodulatory Mediator-Induced Phosphoprotein Signature in Pediatric B Cell Acute Lymphoblastic Leukemia (B-ALL) Diagnostic Samples

It is theorized that dysregulated immune responses to infectious insults contribute to the development of pediatric B-ALL. In this context, our understanding of the immunomodulatory-mediator-induced signaling responses of leukemic blasts in pediatric B-ALL diagnostic samples is rather limited. Hence, in this study, we defined the signaling landscape of leukemic blasts, as well as normal mature B cells and T cells residing in diagnostic samples from 63 pediatric B-ALL patients. These samples were interrogated with a range of immunomodulatory-mediators within 24 h of collection, and phosflow analyses of downstream proximal signaling nodes were performed. Our data reveal evidence of basal hyperphosphorylation across a broad swath of these signaling nodes in leukemic blasts in contrast to normal mature B cells and T cells in the same sample. We also detected similarities in the phosphoprotein signature between blasts and mature B cells in response to IFNγ and IL-2 treatment, but significant divergence in the phosphoprotein signature was observed between blasts and mature B cells in response to IL-4, IL-7, IL-10, IL-21 and CD40 ligand treatment. Our results demonstrate the existence of both symmetry and asymmetry in the phosphoprotein signature between leukemic and non-leukemic cells in pediatric B-ALL diagnostic samples.

The shape of cancer relapse: Topological data analysis predicts recurrence in paediatric acute lymphoblastic leukaemia

Although children and adolescents with acute lymphoblastic leukaemia (ALL) have high survival rates, approximately 15-20% of patients relapse. Risk of relapse is routinely estimated at diagnosis by biological factors, including flow cytometry data. This high-dimensional data is typically manually assessed by projecting it onto a subset of biomarkers. Cell density and "empty spaces" in 2D projections of the data, i.e. regions devoid of cells, are then used for qualitative assessment. Here, we use topological data analysis (TDA), which quantifies shapes, including empty spaces, in data, to analyse pre-treatment ALL datasets with known patient outcomes. We combine these fully unsupervised analyses with Machine Learning (ML) to identify significant shape characteristics and demonstrate that they accurately predict risk of relapse, particularly for patients previously classified as 'low risk'. We independently confirm the predictive power of CD10, CD20, CD38, and CD45 as biomarkers for ALL diagnosis. Based on our analyses, we propose three increasingly detailed prognostic pipelines for analysing flow cytometry data from ALL patients depending on technical and technological availability: 1. Visual inspection of specific biological features in biparametric projections of the data; 2. Computation of quantitative topological descriptors of such projections; 3. A combined analysis, using TDA and ML, in the four-parameter space defined by CD10, CD20, CD38 and CD45. Our analyses readily extend to other haematological malignancies.

Laboratory Aspects of Minimal / Measurable Residual Disease Testing in B-Lymphoblastic Leukemia

Minimal residual disease detection provides critical prognostic predictor of treatment outcome and is the standard of care for B lymphoblastic leukemia. Flow cytometry-based minimal residual disease detection is the most common test modality and has high sensitivity (0.01%) and a rapid turnaround time (24 hours). This article details the leukemia associated immunophenotype analysis approach for flow cytometry-based minimal residual disease detection used at St. Jude Children's Research Hospital and importance of using guide gates and back-gating.

The prognostic significance of hematogones in childhood B-cell acute lymphoblastic leukemia

BACKGROUND

Recent studies have demonstrated hematogones (HGs) expansion to be associated with favorable outcomes in hematological diseases, especially in patients with acute myeloid leukemia and patients undergoing hematopoietic stem cell transplantation. Acute lymphoblastic leukemia (ALL) is the most common form of cancer in children. As of now, minimal residual disease (MRD) remains the most compelling independent prognostic factor in childhood ALL. There is need for more prognostic tools for evaluating relapse risk.

PROCEDURE

The goal of this study was to assess the prognostic value of HGs on relapse-free survival (RFS) and overall survival (OS) in childhood ALL. In this prospective cohort study, a total of 122 subjects with definitive diagnosis of precursor B lymphoblastic leukemia were evaluated. Flow cytometric HG detection was performed in bone marrow aspirates after induction and consolidation therapy.

RESULTS

The median follow-up period of patients was 35.5 ± 9.4 (SD) months. Patients who had at least 1.0% HGs had a significantly better RFS (p = .023). Moreover, univariate and multivariate analyses confirmed that positive HGs were independently associated with longer RFS (unadjusted model: hazard ratio = 0.33, 95% CI = 0.12-0.91, p = .031; adjusted model: hazard ratio = 0.30, 95% CI = 0.11-0.82, p = .020).

CONCLUSIONS

Along with the role of MRD, our study shows the significance of HGs as an independent prognostic factor. The results indicate the independent prognostic value of HGs on RFS after adjustment for other prognostic factors, and can be beneficial for risk stratification and treatment modifications amongst pediatric B-cell ALL patients.

Antigen Receptors Gene Analysis for Minimal Residual Disease Detection in Acute Lymphoblastic Leukemia: The Role of High Throughput Sequencing

The prognosis of adult acute lymphoblastic leukemia (ALL) is variable but more often dismal. Indeed, its clinical management is challenging, current therapies inducing complete remission in 65–90% of cases, but only 30–40% of patients being cured. The major determinant of treatment failure is relapse; consequently, measurement of residual leukemic blast (minimal residual disease, MRD) has become a powerful independent prognostic indicator in adults. Numerous evidences have also supported the clinical relevance of MRD assessment for risk class assignment and treatment selection. MRD can be virtually evaluated in all ALL patients using different technologies, such as polymerase chain reaction amplification of fusion transcripts and clonal rearrangements of antigen receptor genes, flow cytometric study of leukemic immunophenotypes and, the most recent, high throughput sequencing (HTS). In this review, the authors focused on the latest developments on MRD monitoring with emphasis on the use of HTS, as well as on the clinical impact of MRD monitoring.

Hematogones: The Supreme Mimicker and a Cytomorphological Confounder in Acute Lymphoblastic Leukemia

Objective B-lymphocyte progenitors, namely the hematogones (HGs), may pose problems in morphological assessment of bone marrow, not only during the diagnostic workup but also while evaluating bone marrow for remission status following chemotherapy. Here, we describe a series of 12 cases of acute lymphoblastic leukemia (ALL) that included both B-ALL and T-ALL cases, which were evaluated for remission status and revealed blast-like mononuclear cells in bone marrow in the range of 6 to 26%, which on immunophenotypic analysis turned out to be HGs.

Materials and Methods This is a case series of 12 ALL cases who were undergoing treatment at the Army Hospital (Referral and Research), New Delhi. All these cases were under workup for post-induction status (day 28) and to check for suspected ALL relapse. Bone marrow aspirate (BMA), biopsy, and immunophenotyping were performed. Multicolored flow cytometry was performed using CD10, CD20, CD22, CD34, CD19, and CD38 antibodies panel.

Results BMA assessment of 12 cases revealed a maximum of 26% blastoid cells and a minimum of up to 6%, raising the suspicion of hematological relapse. However, on clinical assessment, these patients were well preserved, with preserved peripheral counts. Hence, marrow aspirates were subjected to flow cytometry using the CD markers panel, as discussed above, which revealed HGs. These cases were followed by minimal residual disease (MRD) analysis that revealed MRD-negative status, further confirming our findings.

Conclusion This case series highlights the importance of morphology and bone marrow immunophenotyping in unveiling the diagnostic dilemma in post-induction ALL patients.

Bone Marrow Grafts From Pediatric Donors May Contain A Considerable Number of Hematogones

During CD34 + stem cell count to determine the number of stem cells in the allografts from pediatric donors, we noticed a considerable amount of early hematogones (eHGs) within the stem cell gate in flow cytometry. Since the number of hematogones causes a decrease in the total number of stem cells counted within the graft, we planned a retrospective study to analyze the effect of eHGs on transplant outcomes. We also wanted to show how allografts containing high amounts of early HGs affect transplant outcomes. Quantification of CD34 numbers and the number of eHGs were determined by flow cytometry. Patients were divided into 2 groups according to the number of CD 34+ cells calculated after subtracting the number of hematogones within the allograft. Those who received < 2 × 106/kg CD34+ cells and ≥ 2 × 106/kg were defined as group 1 and 2, respectively. Twenty-six patients and their 26 donors were included in the study. The median age of patients was 6.5 years and 5.4 years in Group 1 and 2, respectively. The median donor age was 9 years in Group 1 and 7 years in Group 2. The ages and genders were similar in the two groups (p > 0.05). The number of nucleated cells given to both groups was not different. The number of early hematogones given to both groups was similar (p = 0.93). The mean times to myeloid and platelet engraftments were also similar in the two groups. In this study, we provided trilineage engraftment to all patients in two groups. We could not find a considerable effect of these eHGs in myeloid and platelet engraftments. However, the number of patients included in our study is low, therefore we suggest a study including a large number of donors in order to confirm our findings.

Towards a Better Characterisation of Leukemic Cells in Chronic Lymphocytic Leukaemia: Cell-Size Heterogeneity Reflects Their Activation Status and Migratory Abilities

Simple Summary

Chronic lymphocytic leukaemia (CLL) is a heterogeneous chronic disease characterised by the clonal expansion of mature CD19⁺CD23⁺CD5⁺ B-cells in blood, bone marrow and lymphoid tissue. Despite the CLL tumour cell population showing considerable heterogeneity in cell size, the functional characteristics of leukemic cells that differ in size have not been explored. The results of our study demonstrate that differences in CLL cell size reflect their activation state, polarisation and migratory capacity, with large CLL cells being more activated, polarised and motile than the small CLL cells present in the CLL cell pool. Our data provide evidence of the importance of cell-size heterogeneity within the CLL cell pool and the dynamics of cell-size changes for disease pathogenesis.

Abstract

Chronic lymphocytic leukaemia (CLL) is a genetically, morphologically and phenotypically heterogeneous chronic disease with clinical variability between patients. Whether the significant heterogeneity of cell size within the CLL population contributes to the heterogeneous features of this disease has not been investigated. The present study aimed to characterise the phenotypic and functional properties of two subpopulations of typical CLL cells that differ in cell size: small (s-CLL) and large (l-CLL) CLL cells delineated by forward scatter cytometry. The s-CLL cells were characterised by the CD5^lowCXCR4^hi phenotype, while the l-CLL cells were characterised by the CD5^hiCXCR4^dim phenotype and indicated a higher expression of CXCR3, CD20, CD38 and HLA-DR. The l-CLL cells displayed higher migration activity towards CXCL12, a tendency towards a higher proliferation rate and an increased capacity to produce IgM in the presence of CpG compared with s-CLL cells. When stimulated with CpG and CXCL12, l-CLL cells were characterised by a higher polarisation phenotype and motility than s-CLL cells. Our study revealed that the differences in CLL cell size reflected their activation status, polarisation and migratory abilities. Our data provide evidence of the importance of cell-size heterogeneity within a CLL pool and the dynamics of cell-size changes for disease pathogenesis, thus deserving further investigation.

Time point-dependent concordance and prognostic significance of flow cytometry and real time quantitative PCR for measurable/minimal residual disease detection in acute myeloid leukemia with t(8;21)(q22;q22.1)

BACKGROUND

Flow cytometry (FCM) and PCR are reliable methods for assessing minimal residual disease (MRD) in acute myeloid leukemia with t(8;21)(q22;q22.1). The aim of this study was to analyze the concordant rate of these two methods and their prognostic significance.

METHODS

PCR and FCM were simultaneously used for MRD analysis at four different time points on 450 BM samples from 124 patients with AML with t(8;21)(q22;q22.1). The four monitoring time points included post-induction (first), after the first consolidation (second) and the second consolidation (third), and at the end of chemotherapy or before Allo/Auto stem cell transplantation (fourth).

RESULTS

The concordant rates of the two methods were 33.06%, 25.81%, 49.59%, and 75.31%, respectively, and the main discordant cases were FCM-/PCR+ cases. At all monitoring time points, the MRD level ≥ 10^-4 by FCM indicated a poor 3-year Relapse-Free Survival (RFS) (p < 0.001). More than 2-log MRD reduction by PCR after induction and more than 3-log reduction by PCR after the first consolidation remained the significant predictors of better RFS (p < 0.001). After the second consolidation, the negative MRD by PCR (<10-5) was also associated with improved RFS (p = 0.002). A > 1-log increase in PCR can effectively predict recurrence after molecular remission (p < 0.001). In the multivariate analysis, MRD≥0.01% by. FCM and less than 2-log MRD reduction by PCR after induction remained the significant predictors of poor RFS (p < 0.05).

CONCLUSIONS

FCM+ always indicates a poor prognosis. Sequential monitoring by PCR is of significance for evaluating prognosis. Our findings suggest a complementary role of two analyses in optimizing risk stratification in clinical practice.

Laboratory Aspects of Minimal / Measurable Residual Disease Testing in B-Lymphoblastic Leukemia

Minimal residual disease detection provides critical prognostic predictor of treatment outcome and is the standard of care for B lymphoblastic leukemia. Flow cytometry-based minimal residual disease detection is the most common test modality and has high sensitivity (0.01%) and a rapid turnaround time (24 hours). This article details the leukemia associated immunophenotype analysis approach for flow cytometry-based minimal residual disease detection used at St. Jude Children's Research Hospital and importance of using guide gates and back-gating.

A Mathematical Description of the Bone Marrow Dynamics during CAR T-Cell Therapy in B-Cell Childhood Acute Lymphoblastic Leukemia

Chimeric Antigen Receptor (CAR) T-cell therapy has demonstrated high rates of response in recurrent B-cell Acute Lymphoblastic Leukemia in children and young adults. Despite this success, a fraction of patients' experience relapse after treatment. Relapse is often preceded by recovery of healthy B cells, which suggests loss or dysfunction of CAR T-cells in bone marrow. This site is harder to access, and thus is not monitored as frequently as peripheral blood. Understanding the interplay between B cells, leukemic cells, and CAR T-cells in bone marrow is paramount in ascertaining the causes of lack of response. In this paper, we put forward a mathematical model representing the interaction between constantly renewing B cells, CAR T-cells, and leukemic cells in the bone marrow. Our model accounts for the maturation dynamics of B cells and incorporates effector and memory CAR T-cells. The model provides a plausible description of the dynamics of the various cellular compartments in bone marrow after CAR T infusion. After exploration of the parameter space, we found that the dynamics of CAR T product and disease were independent of the dose injected, initial B-cell load, and leukemia burden. We also show theoretically the importance of CAR T product attributes in determining therapy outcome, and have studied a variety of possible response scenarios, including second dosage schemes. We conclude by setting out ideas for the refinement of the model.

Prognostic significance of hematogone presence after autologous stem cell transplantation in patients with multiple myeloma

BACKGROUND

Morphologically and immunophenotypically, the number of B-lymphocyte progenitor cells, so-called hematogones increases after chemotherapy and allogeneic stem cell transplantions. It is thought that hematogones can be used as a prognostic marker in these patients. It is aimed to determine the prognostic significance and factors affecting the development of hematogones, which can be seen after autologous stem cell transplantation in multiple myeloma (MM) patients.

METHODS

This retrospective and single center study includes 80 patients who underwent autologous stem cell transplantation with the diagnosis of MM in our clinic between January 2013 and December 2019. The primary endpoint of the study was the relationship between the presence and rate of hematogone (HG) and progression free survival (PFS) and overall survival (OS). The secondary endpoint was to identify the factors affecting the development of HG.

RESULTS

HG was detected in 61.2% of the patients. There was a moderate and positive linear correlation between the amount of stem cells given and HG ratio (r = 0.387, p = 0.000). PFS and OS were significantly shorter in the group with HG (p = 0.000 and p = 0.012).

CONCLUSIONS

HG positivity after autologous stem cell transplantation was found to be an independent prognostic marker for PFS and OS in patients with MM. There is a positive relationship between the amount of stem cells used during transplantation and the ratio of HG. As the amount of stem cells increases, the ratio of HG increases and when the ratio of HG increases, PFS and OS become shorter.

Dynamical properties of feedback signalling in B lymphopoiesis: A mathematical modelling approach

Haematopoiesis is the process of generation of blood cells. Lymphopoiesis generates lymphocytes, the cells in charge of the adaptive immune response. Disruptions of this process are associated with diseases like leukaemia, which is especially incident in children. The characteristics of self-regulation of this process make them suitable for a mathematical study. In this paper we develop mathematical models of lymphopoiesis using currently available data. We do this by drawing inspiration from existing structured models of cell lineage development and integrating them with paediatric bone marrow data, with special focus on regulatory mechanisms. A formal analysis of the models is carried out, giving steady states and their stability conditions. We use this analysis to obtain biologically relevant regions of the parameter space and to understand the dynamical behaviour of B-cell renovation. Finally, we use numerical simulations to obtain further insight into the influence of proliferation and maturation rates on the reconstitution of the cells in the B line. We conclude that a model including feedback regulation of cell proliferation represents a biologically plausible depiction for B-cell reconstitution in bone marrow. Research into haematological disorders could benefit from a precise dynamical description of B lymphopoiesis.

Monitoring minimal/measurable residual disease in B-cell acute lymphoblastic leukemia by flow cytometry during targeted therapy

B-cell acute lymphoblastic leukemia (B-ALL) is a hematologic malignancy of B-type lymphoid precursor cells. Minimal/measurable residual disease (MRD) is an important prognostic factor for B-ALL relapse. Traditional flow cytometry detection mainly relies on CD19-based gating strategies. However, relapse of CD19-negative B-ALL frequently occurs in patients who receive cellular and targeted therapy. This review will summarize the technical aspects of standard MRD assessment in B-ALL by flow cytometry, and then discuss the challenges of MRD strategies to deal with the scenario of CD19 negative or dim B-ALL relapse.

Immunophenotypic shift in the B-cell precursors from regenerating bone marrow samples: A critical consideration for measurable residual disease assessment in B-lymphoblastic leukemia

Accurate knowledge of expression patterns/levels of commonly used MRD markers in regenerative normal-B-cell-precursors (BCP) is highly desirable to distinguish leukemic-blasts from regenerative-BCP for multicolor flow cytometry (MFC)-based measurable residual disease (MRD) assessment in B-lymphoblastic leukemia (B-ALL). However, the data highlighting therapy-related immunophenotypic-shift in regenerative-BCPs is scarce and limited to small cohort. Herein, we report the in-depth evaluation of immunophenotypic shift in regenerative-BCPs from a large cohort of BALL-MRD samples. Ten-color MFC-MRD analysis was performed in pediatric-BALL at the end-of-induction (EOI), end-of-consolidation (EOC), and subsequent-follow-up (SFU) time-points. We studied normalized-mean fluorescent intensity (nMFI) and coefficient-of-variation of immunofluorescence (CVIF) of CD10, CD19, CD20, CD34, CD38, and CD45 expression in regenerative-BCP (early, BCP1 and late, BCP2) from 200 BALL-MRD samples, and compared them with BCP from 15 regenerating control (RC) TALL-MRD samples and 20 treatment-naïve bone-marrow control (TNSC) samples. Regenerative-BCP1 showed downregulation in CD10 and CD34 expression with increased CVIF and reduced nMFI (p < 0.001), upregulation of CD20 with increased nMFI (p = 0.014) and heterogeneous CD45 expression with increased CVIF (p < 0.001). Immunophenotypic shift was less pronounced in the BCP2 compared to BCP1 compartment with increased CVIF in all but CD45 (p < 0.05) and reduced nMFI only in CD45 expression (p = 0.005). Downregulation of CD10/CD34 and upregulation of CD20 was higher at EOI than EOC and SFU time-points (p < 0.001). Regenerative-BCPs are characterized by the significant immunophenotypic shift in commonly used B-ALL-MRD markers, especially CD10 and CD34 expression, as compared to treatment-naïve BCPs. Therefore, the templates/database for BMRD analysis must be developed using regenerative-BCP.

Improved Recognition of Hematogones From Precursor B-Lymphoblastic Leukemia by a Single Tube Flow Cytometric Analysis

OBJECTIVES

To improve diagnostic accuracy in differentiating hematogones from leukemic blasts in cases of precursor B-lymphoblastic leukemia/lymphoma (B-ALL), particularly those that are posttreatment or after bone marrow transplant, and to provide an algorithmic approach to this diagnostic challenge.

METHODS

A seven-color antibody panel including CD10, CD19, CD45, CD38, CD34, CD58, and CD81 was generated to assess the feasibility of a single tube panel and provide an algorithmic approach to distinguish hematogones from B-ALL. Fifty-three cases were analyzed, and results were correlated with histology and ancillary studies.

RESULTS

There was a significant difference in mean fluorescent intensity (MFI) for CD81 and CD58 when comparing hematogones and B-ALL populations (P < .001). B-ALL cases had a mean (SD) MFI of 24.6 (27.5; range, 2-125) for CD81 and 135.6 (72.6; range, 48-328) for CD58. Hematogones cases had a mean (SD) MFI of 70.2 (19.2; range, 42-123) for CD81 and 38.8 (9.4; range, 23-58) for CD58.

CONCLUSIONS

The flow cytometry panel with the above markers and utilization of the proposed algorithmic approach provide differentiation of hematogones from B-ALL. This includes rare cases of hematogones and B-ALL overlap where additional ancillary studies are necessary.

The influence of a dosage regimen of dexamethasone on detection of normal B-cell precursors in the bone marrow of children with BCP-ALL at the end of induction therapy

Minimal residual disease (MRD) monitoring by flow cytometry at the end of induction therapy is one of the key ways of a prognosis assessment in patients with acute lymphoblastic leukemia (ALL). In B-cell precursor ALL (BCP–ALL), this method of MRD detection is complicated due to the immunophenotypic similarity between leukemic cells and normal B-cell precursors (BCPs). A decrease in intensity of induction therapy can lead to a more frequent appearance of normal BCPs in the bone marrow, which significantly complicates the MRD monitoring. Aim: to assess the incidence of normal BCPs in bone marrow on the 36th day of induction therapy with two different regimens of glucocorticoid (GC) administration according to ALL-MB 2015 protocol. This study was approved by the Independent Ethical Committee and the Academic Council of Dmitriy Rogachev National Medical Research Center of Pediatric Hematology, Oncology, Immunology Ministry of Healthcare of Russian Federation. The study included 220 patients with BCP-ALL who were randomized to two types of GC-based induction therapy: a continuous administration of dexamethasone (n = 139) and an intermittent regimen with a 1-week dexamethasone therapy stop (n = 81). On the 36th day of induction therapy, MRD and normal BCPs were quantified in bone marrow samples by flow cytometry. On the 36th day of treatment, 43.2% of BCP(+) samples were established in the intermittent-therapy group, and 27.3% in the continuous-therapy group (p = 0.016). Comparison of the BCP level in BCP(+) samples revealed the more equitable distribution of BCPs at different developmental stages in the intermittent-therapy group, meanwhile mainly the immature BCPs in a quantity of less than 0.01% were found in the continuous-therapy group. Reduced-intensity induction therapy for patients with BCP-ALL leads to a noticeable increase of normal BCPs in bone marrow at the end of this treatment stage. A higher rate of BCP(+) bone marrow samples hinder the MRD detection due to the immunophenotypic similarity of BCPs and leukemic cells.

Minimal Residual Disease Detection in Acute Lymphoblastic Leukemia

Minimal residual disease (MRD) refers to a chemotherapy/radiotherapy-surviving leukemia cell population that gives rise to relapse of the disease. The detection of MRD is critical for predicting the outcome and for selecting the intensity of further treatment strategies. The development of various new diagnostic platforms, including next-generation sequencing (NGS), has introduced significant advances in the sensitivity of MRD diagnostics. Here, we review current methods to diagnose MRD through phenotypic marker patterns or differential gene patterns through analysis by flow cytometry (FCM), polymerase chain reaction (PCR), real-time quantitative polymerase chain reaction (RQ-PCR), reverse transcription polymerase chain reaction (RT-PCR) or NGS. Future advances in clinical procedures will be molded by practical feasibility and patient needs regarding greater diagnostic sensitivity.

Tumor-stroma interaction increases CD147 expression in neoplastic B lymphocytes in chronic lymphocytic leukemia

BACKGROUND

Chronic lymphocytic leukemia (CLL) microenvironment plays a critical role in disease pathogenesis. Matrix metalloproteinases (MMPs) are involved in CLL-B cell migration and survival. CD147 is associated with MMPs production by tumor and stromal cells.

AIM

To analyze CD147, MMP2 and MMP9 expression in CLL-B cells and its modulation by fibroblasts (Fb)-CLL-B cell interaction.

METHODS

CLL-B cells were co-cultured with Fb, as a simulation of CLL microenvironment. CD147 was evaluated in healthy donor (HD)-B cells and CLL-B cells by flow cytometry. MMP2 and MMP9 activity in CLL-plasma samples and conditioned media (CMs) was studied by zymography.

RESULTS

MMP9/MMP2 plasma levels were significantly higher in CLL patients than in HD. CD147 expression (median fluorescence intensity) in CLL patients characterized 3 groups: low- (19.1 ± 3.2; n=3), middle- (42.7 ± 12.8; n=18) and high- (76.5 ± 9.6; n=5) related to CD147 expression in HD-B cells. CD147 expression significantly increased in CLL-B cells after Fb-CLL-B cell co-culture. A significant increase in proMMP2 activity was observed in CMs obtained from Fb-CLL-B cell co-cultures in comparison with isolated CLL-B cells.

CONCLUSIONS

CD147 expression in CLL-B cells and MMPs secretion was induced by Fb-CLL-B cell contact, suggesting CD147 participation in the CLL pathophysiology.

EuroFlow Lymphoid Screening Tube (LST) data base for automated identification of blood lymphocyte subsets

In recent years the volume and complexity of flow cytometry data has increased substantially. This has led to a greater number of identifiable cell populations in a single measurement. Consequently, new gating strategies and new approaches for cell population definition are required. Here we describe how the EuroFlow Lymphoid Screening Tube (LST) reference data base for peripheral blood (PB) samples was designed, constructed and validated for automated gating of the distinct lymphoid (and myeloid) subsets in PB of patients with chronic lymphoproliferative disorders (CLPD). A total of 46 healthy/reactive PB samples which fulfilled pre-defined technical requirements, were used to construct the LST-PB reference data base. In addition, another set of 92 PB samples (corresponding to 10 healthy subjects, 51 B-cell CLPD and 31 T/NK-cell CLPD patients), were used to validate the automated gating and cell-population labeling tools with the Infinicyt software. An overall high performance of the LST-PB data base was observed with a median percentage of alarmed cellular events of 0.8% in 10 healthy donor samples and of 44.4% in CLPD data files containing 49.8% (range: 1.3-96%) tumor cells. The higher percent of alarmed cellular events in every CLPD sample was due to aberrant phenotypes (75.6% cases) and/or to abnormally increased cell counts (86.6% samples). All 18 (22%) data files that only displayed numerical alterations, corresponded to T/NK-cell CLPD cases which showed a lower incidence of aberrant phenotypes (41%) vs B-cell CLPD cases (100%). Comparison between automated vs expert-bases manual classification of normal (r² = 0.96) and tumor cell populations (rho = 0.99) showed a high degree of correlation. In summary, our results show that automated gating of cell populations based on the EuroFlow LST-PB data base provides an innovative, reliable and reproducible tool for fast and simplified identification of normal vs pathological B and T/NK lymphocytes in PB of CLPD patients.

Differential expression of CD73, CD86 and CD304 in normal vs. leukemic B-cell precursors and their utility as stable minimal residual disease markers in childhood B-cell precursor acute lymphoblastic leukemia

BACKGROUND

Optimal discrimination between leukemic blasts and normal B-cell precursors (BCP) is critical for treatment monitoring in BCP acute lymphoblastic leukemia (ALL); thus identification of markers differentially expressed on normal BCP and leukemic blasts is required.

METHODS

Multicenter analysis of CD73, CD86 and CD304 expression levels was performed in 282 pediatric BCP-ALL patients vs. normal bone marrow BCP, using normalized median fluorescence intensity (nMFI) values.

RESULTS

CD73 was expressed at abnormally higher levels (vs. pooled normal BCP) at diagnosis in 71/108 BCP-ALL patients (66%), whereas CD304 and CD86 in 119/202 (59%) and 58/100 (58%) patients, respectively. Expression of CD304 was detected at similar percentages in common-ALL and pre-B-ALL, while found at significantly lower frequencies in pro-B-ALL. A significant association (p = 0.009) was found between CD304 expression and the presence of the ETV6-RUNX1 fusion gene. In contrast, CD304 showed an inverse association with MLL gene rearrangements (p = 0.01). The expression levels of CD73, CD86 and CD304 at day 15 after starting therapy (MRD15) were stable or higher than at diagnosis in 35/37 (95%), 40/56 (71%) and 19/41 (46%) cases investigated, respectively. This was also associated with an increased mean nMFI at MRD15 vs. diagnosis of +24 and +3 nMFI units for CD73 and CD86, respectively. In addition, gain of expression of CD73 and CD86 at MRD15 for cases that were originally negative for these markers at diagnosis was observed in 16% and 18% of cases, respectively. Of note, CD304 remained aberrantly positive in 63% of patients, despite its levels of expression decreased at follow-up in 54% of cases.

CONCLUSIONS

Here we show that CD73, CD86 and CD304 are aberrantly (over)expressed in a substantial percentage of BCP-ALL patients and that their expression profile remains relatively stable early after starting therapy, supporting their potential contribution to improved MRD analysis by flow cytometry.

NY-ESO-1 Vaccination in Combination with Decitabine Induces Antigen-Specific T-lymphocyte Responses in Patients with Myelodysplastic Syndrome

Purpose: Treatment options are limited for patients with high-risk myelodysplastic syndrome (MDS). The azanucleosides, azacitidine and decitabine, are first-line therapy for MDS that induce promoter demethylation and gene expression of the highly immunogenic tumor antigen NY-ESO-1. We demonstrated that patients with acute myeloid leukemia (AML) receiving decitabine exhibit induction of NY-ESO-1 expression in circulating blasts. We hypothesized that vaccinating against NY-ESO-1 in patients with MDS receiving decitabine would capitalize upon induced NY-ESO-1 expression in malignant myeloid cells to provoke an NY-ESO-1-specific MDS-directed cytotoxic T-cell immune response.Experimental Design: In a phase I study, 9 patients with MDS received an HLA-unrestricted NY-ESO-1 vaccine (CDX-1401 + poly-ICLC) in a nonoverlapping schedule every four weeks with standard-dose decitabine.Results: Analysis of samples serially obtained from the 7 patients who reached the end of the study demonstrated induction of NY-ESO-1 expression in 7 of 7 patients and NY-ESO-1-specific CD4⁺ and CD8⁺ T-lymphocyte responses in 6 of 7 and 4 of 7 of the vaccinated patients, respectively. Myeloid cells expressing NY-ESO-1, isolated from a patient at different time points during decitabine therapy, were capable of activating a cytotoxic response from autologous NY-ESO-1-specific T lymphocytes. Vaccine responses were associated with a detectable population of CD141^Hi conventional dendritic cells, which are critical for the uptake of NY-ESO-1 vaccine and have a recognized role in antitumor immune responses.Conclusions: These data indicate that vaccination against induced NY-ESO-1 expression can produce an antigen-specific immune response in a relatively nonimmunogenic myeloid cancer and highlight the potential for induced antigen-directed immunotherapy in a group of patients with limited options. Clin Cancer Res; 24(5); 1019-29. ©2017 AACRSee related commentary by Fuchs, p. 991.

When Less Is Good, Is None Better? The Prognostic and Therapeutic Significance of Peri-Transplant Minimal Residual Disease Assessment in Pediatric Acute Lymphoblastic Leukemia

The measurement of minimal residual disease (MRD) in pediatric acute lymphoblastic leukemia (ALL) has become the most important prognostic tool of, and the backbone to, upfront risk stratification. While MRD assessment is the standard of care for assessing response and predicting outcomes for pediatric patients with ALL receiving chemotherapy, its use in allogeneic hematopoietic stem cell transplant (HSCT) has been less clearly defined. Herein, we discuss the importance of MRD assessment during the peri-HSCT period and its role in prognostication and management.

Significant Haematogone Proliferation Mimicking Relapse in Acute Lymphoblastic Leukaemia on Therapy

Haematogones are benign B lymphoid precursors which may mimic neoplastic lymphoblasts and pose diagnostic difficulty especially when the percentage of haematogones exceeds 10% in the bone marrow. Flow cytometric analysis with combination of CD19/CD10/CD20/CD34/CD38/CD58 can be used to differentiate the two depending upon the difference in the fluorescence intensity between blasts and haematogones. We hereby present a case of Common Acute Lymphoblastic Leukaemia Associated Antigen (CALLA) positive Acute Lymphoblastic Leukaemia (ALL), in which patient presented with haematogone proliferation in bone marrow after 6 months of chemotherapy mimicking relapse. The distinction was made on flow cytometric immunophenotyping by using optimal antibody combination. Distinction of benign haematogones from neoplastic lymphoblasts is essential for disease management in cases of post chemotherapy or post marrow transplant, especially in patients of ALL. Flow cytometric immunophenotyping is reliable to distinguish haematogones from residual lymphoblasts in almost all cases when optimal antibody combinations are used.

Standardized flow cytometry for highly sensitive MRD measurements in B-cell acute lymphoblastic leukemia

A fully-standardized EuroFlow 8-color antibody panel and laboratory procedure was stepwise designed to measure minimal residual disease (MRD) in B-cell precursor (BCP) acute lymphoblastic leukemia (ALL) patients with a sensitivity of ≤10^-5, comparable to real-time quantitative polymerase chain reaction (RQ-PCR)-based MRD detection via antigen-receptor rearrangements. Leukocyte markers and the corresponding antibodies and fluorochromes were selected based on their contribution in separating BCP-ALL cells from normal/regenerating BCP cells in multidimensional principal component analyses. After 5 multicenter design-test-evaluate-redesign phases with a total of 319 BCP-ALL patients at diagnosis, two 8-color antibody tubes were selected, which allowed separation between normal and malignant BCP cells in 99% of studied patients. These 2 tubes were tested with a new erythrocyte bulk-lysis protocol allowing acquisition of high cell numbers in 377 bone marrow follow-up samples of 178 BCP-ALL patients. Comparison with RQ-PCR-based MRD data showed a clear positive relation between the percentage concordant cases and the number of cells acquired. For those samples with >4 million cells acquired, concordant results were obtained in 93% of samples. Most discordances were clarified upon high-throughput sequencing of antigen-receptor rearrangements and blind multicenter reanalysis of flow cytometric data, resulting in an unprecedented concordance of 98% (97% for samples with MRD < 0.01%). In conclusion, the fully standardized EuroFlow BCP-ALL MRD strategy is applicable in >98% of patients with sensitivities at least similar to RQ-PCR (≤10^-5), if sufficient cells (>4 × 10⁶, preferably more) are evaluated.

Current Strategies for the Detection of Minimal Residual Disease in Childhood Acute Lymphoblastic Leukemia

Acute lymphoblastic leukemia (ALL) is the most common cancer in children. Current treatment strategies for childhood ALL result in long-term remission for approximately 90% of patients. However, the therapeutic response is worse among those who relapse. Several risk stratification approaches based on clinical and biological aspects have been proposed to intensify treatment in patients with high risk of relapse and reduce toxicity on those with a greater probability of cure.

The detection of residual leukemic cells (minimal residual disease, MRD) is the most important prognostic factor to identify high-risk patients, allowing redefinition of chemotherapy. In the last decades, several standardized research protocols evaluated MRD using immunophenotyping by flow cytometry and/or real-time quantitative polymerase chain reaction at different time points during treatment. Both methods are highly sensitive (10⁻³ a 10⁻⁵), but expensive, complex, and, because of that, require qualified staff and frequently are restricted to reference centers.

The aim of this article was to review technical aspects of immunophenotyping by flow cytometry and real-time quantitative polymerase chain reaction to evaluate MRD in ALL.

Clinical impact of hematogones on outcomes of allogeneic hematopoietic stem cell transplantation

Increased levels of normal B cell precursors, termed hematogones (HGs), are observed in regenerating bone marrow after chemotherapy or hematopoietic stem cell transplantation (HSCT). Recent reports suggest that emergence of HGs is associated with better outcomes following allogeneic HSCT (allo-HSCT). We reviewed the emergence of HGs and the clinical features of 192 patients after allo-BMT. Patients undergoing allo-BMT from related donors were more likely to develop HGs at day 30 compared to unrelated donors. Furthermore, patients undergoing allo-BMT from HLA-mismatched donors were less likely to develop HGs at day 30. The emergence of HGs at day 30 was an independent prognostic factor among patients who underwent BMT. We found no difference in the relapse rate between HG-positive (+) and HG-negative (-) patients undergoing BMT. HG (-) patients had high non-relapse mortality, grade II to IV acute graft-versus-host-disease (GVHD), fungal infection, and lower IgG levels compared to HG (+) patients. The emergence of HGs at day 30 among patients undergoing BMT may be a very useful indicator of subsequent survival outcomes or acute GVHD in common clinical practice.

Increased μ-Calpain Activity in Blasts of Common B-Precursor Childhood Acute Lymphoblastic Leukemia Correlates with Their Lower Susceptibility to Apoptosis

Childhood acute lymphoblastic leukemia (ALL) blasts are characterized by inhibited apoptosis promoting fast disease progress. It is known that in chronic lymphocytic and acute myeloid leukemias the reduced apoptosis is strongly related with the activity of calpain-calpastatin system (CCS) composed of cytoplasmic proteases—calpains—performing the modulatory proteolysis of key proteins involved in cell proliferation and apoptosis, and of their endogenous inhibitor—calpastatin. Here, the CCS protein abundance and activity was for the first time studied in childhood ALL blasts and in control bone marrow CD19⁺ B cells by semi-quantitative flow cytometry and western blotting of calpastatin fragments resulting from endogenous calpain activity. Significantly higher μ-calpain (CAPN1) gene transcription, protein amounts and activity (but not those of m-calpain), with calpastatin amount and transcription of its gene (CAST) greatly varying were observed in CD19⁺ ALL blasts compared to control cells. Significant inverse relation between the amount/activity of calpain and spontaneous apoptosis was noted. Patients older than 10 years (considered at higher risk) displayed increased amounts and activities of blast calpain. Finally, treatment of blasts with the tripeptide calpain inhibitors II and IV significantly and in dose-dependent fashion increased the percentage of blasts entering apoptosis. Together, these findings make the CCS a potential new predictive tool and therapeutic target in childhood ALL.

Minimal residual disease analysis by eight-color flow cytometry in relapsed childhood acute lymphoblastic leukemia

Multiparametric flow cytometry is an alternative approach to the polymerase chain reaction method for evaluating minimal residual disease in treatment protocols for primary acute lymphoblastic leukemia. Given considerable differences between primary and relapsed acute lymphoblastic leukemia treatment regimens, flow cytometric assessment of minimal residual disease in relapsed leukemia requires an independent comprehensive investigation. In the present study we addressed evaluation of minimal residual disease by flow cytometry in the clinical trial for childhood relapsed acute lymphoblastic leukemia using eight-color flow cytometry. The major challenge of the study was to reliably identify low amounts of residual leukemic cells against the complex background of regeneration, characteristic of follow-up samples during relapse treatment. In a prospective study of 263 follow-up bone marrow samples from 122 patients with B-cell precursor acute lymphoblastic leukemia, we tested various B-cell markers, adapted the antibody panel to the treatment protocol, and evaluated its performance by a blinded parallel comparison with the polymerase chain reaction data. The resulting eight-color single-tube panel showed a consistently high overall concordance (P<0.001) and, under optimal conditions, sensitivity similar to that of the reference polymerase chain reaction method. Overall, evaluation of minimal residual disease by flow cytometry can be successfully integrated into the clinical management of relapsed childhood acute lymphoblastic leukemia either as complementary to the polymerase chain reaction or as an independent risk stratification tool. ALL-REZ BFM 2002 clinical trial information: NCT00114348.

Minimal residual disease diagnostics in acute lymphoblastic leukemia: need for sensitive, fast, and standardized technologies

Monitoring of minimal residual disease (MRD) has become routine clinical practice in frontline treatment of virtually all childhood acute lymphoblastic leukemia (ALL) and in many adult ALL patients. MRD diagnostics has proven to be the strongest prognostic factor, allowing for risk group assignment into different treatment arms, ranging from significant treatment reduction to mild or strong intensification. Also in relapsed ALL patients and patients undergoing stem cell transplantation, MRD diagnostics is guiding treatment decisions. This is also why the efficacy of innovative drugs, such as antibodies and small molecules, are currently being evaluated with MRD diagnostics within clinical trials. In fact, MRD measurements might well be used as a surrogate end point, thereby significantly shortening the follow-up. The MRD techniques need to be sensitive (≤10(-4)), broadly applicable, accurate, reliable, fast, and affordable. Thus far, flow cytometry and polymerase chain reaction (PCR) analysis of rearranged immunoglobulin and T-cell receptor genes (allele-specific oligonucleotide [ASO]-PCR) are claimed to meet these criteria, but classical flow cytometry does not reach a solid 10(-4), whereas classical ASO-PCR is time-consuming and labor intensive. Therefore, 2 high-throughput technologies are being explored, ie, high-throughput sequencing and next-generation (multidimensional) flow cytometry, both evaluating millions of sequences or cells, respectively. Each of them has specific advantages and disadvantages.

B-ALL minimal residual disease flow cytometry: an application of a novel method for optimization of a single-tube model

OBJECTIVES

Optimizing a clinical flow cytometry panel can be a subjective process dependent on experience. We develop a quantitative method to make this process more rigorous and apply it to B lymphoblastic leukemia/lymphoma (B-ALL) minimal residual disease (MRD) testing.

METHODS

We retrospectively analyzed our existing three-tube, seven-color B-ALL MRD panel and used our novel method to develop an optimized one-tube, eight-color panel, which was tested prospectively.

RESULTS

The optimized one-tube, eight-color panel resulted in greater efficiency of time and resources with no loss in diagnostic power.

CONCLUSIONS

Constructing a flow cytometry panel using a rigorous, objective, quantitative method permits optimization and avoids problems of interdependence and redundancy in a large, multiantigen panel.

Hematogones in the peripheral blood of a 5½-month-old boy with cyclic neutropenia due to heterozygous, novel ELANE gene mutation p.Q97P, c.290 A>C

We have identified a novel point mutation in the ELANE gene of a 5.5-month-old boy with severe cyclic neutropenia, and we are reporting for the first time, to our knowledge, the presence of hematogones in the peripheral blood of an infant. The novel point mutation occurred at base number 290 in codon 97, where adenine was replaced with cytosine. The mutation caused the replacement of amino acid glutamine with amino acid proline in the activation domain of the elastase 2 enzyme. The heterozygous mutation generated severe cyclic neutropenia, granulocytic maturation arrest, an increased number of hematogones (26% of marrow cells) in the bone marrow, an absence of neutrophils, and the presence of stage 3 (mature) hematogones in the peripheral blood. The percentage of hematogones in the peripheral blood was inversely proportional to the absolute number of neutrophils. Leukemic number of blast-like cells (hematogones) in the bone marrow, blast-like cells in the peripheral blood, marked neutropenia, and the arrest of granulopoiesis might suggest an acute leukemia. However, the finding of characteristic flow cytometric features of hematogones should help to avoid a wrong diagnosis.

Advances and issues in flow cytometric detection of immunophenotypic changes and genomic rearrangements in acute pediatric leukemia

Flow cytometry with its rapidly increasing applications has been using to aid the diagnosis of hematological disorders for more than two decades. It is also the most commonly used technology in childhood leukaemia diagnosis, characterization, prognosis prediction and even in the decision making of targeted therapy. Leukemia cells can be recognized by virtue of unique cell marker combinations, visualized with monoclonal antibodies conjugated and detected by flow cytometry. Currently, such instruments allow the detection of eight or more markers by providing a comprehensive description of the leukemic cell phenotype to facilitate their identification, especially in detecting and monitoring of minimal residual disease (MRD) during treatment. Additionally, the flow cytometric DNA index (DI) can identify biclonality at diagnosis and distinguish persistent aneuploid leukemia during induction therapy, when the standard cytogenetic and morphologic techniques fail to do so. This review focuses on the latest advances and application issues about some of flow cytometric diagnostic and prognostic applications for acute pediatric leukemia.

Transitional B cell subsets in human bone marrow

B cells originate from precursors in the bone marrow, and the first cells which migrate to the peripheral blood have been classified as 'transitional B cells'. Transitional B cells have been characterized in human blood with stage 1 (T1) and stage 2 (T2) subsets being proposed. In the present study, 27 normal human bone marrow samples were analysed for transitional B cell markers by eight-colour flow cytometry. T1 transitional B cells (CD45(+)CD19(+)CD10(+)IgM(+)IgD(lo)) and T2 transitional B cells (CD45(+)CD19(+)CD10(+)IgM(+)IgD(+)) were identified in normal bone marrow samples at a mean frequency of 3·2 and 3·1% of total B lineage cells, respectively. A majority of the bone marrow transitional B cells were CD24(hi)CD38(hi) , the phenotype of blood transitional B cells. Consistent with recent peripheral blood data, T2 B cells had a significantly higher CD21 expression compared with T1 B cells (72·4 versus 40·9%) in the bone marrow. These data raise the possibility that transitional B cells are capable of differentiating from T1 to T2 B cells within the bone marrow. Furthermore, transitional cells at either stages 1 or 2 might be capable of migrating out of the bone marrow.