One Tube 23 Color Full Spectral Flow Cytometry Panel in Detecting Minimal Residual Disease in Pediatric B-Cell Acute Lymphoblastic Leukemia

Introduction Minimal residual disease (MRD) is a normalized tool for risk stratification in the treatment of pediatric acute lymphoblastic leukemia (ALL) and has been introduced into many standardized chemotherapy regimens. Flow cytometry (FC) plays an important role in the monitoring of MRD. However, FC-MRD can't reliably detect residual leukemic cells below level of 0.01% in real clinical scenarios and some patients with negative MRD relapsed, suggesting the current used set of markers are not sensitive enough or specific enough. Published studies has shown that more markers with more reliable combination of antibodies may help improve the sensitivity and specificity of FC-MRD. Spectral multicolor flow cytometry (SMFC) has shown an advantage over traditional FC that more fluorescent markers could be applied simultaneously and more complex marker combinations could be exploited to distinguish leukemic blasts from normal B cell precursors with high specificity. Here we designed a SMFC-based panel of 26 markers with 23 fluorescent colors in one tube to monitor B-ALL MRD.

Method The one-tube 23-color panels was designed according to previously published studies and our clinical experience based on standardized EuroFlow 8-color MRD panel. All the leukemia-associated immunophenotypes (LAIPs) were defined based on its deviation from hematogones. For CD10, CD19, CD22, CD34, CD58, CD73, CD86, CD123, CD200and CD304, the overexpression was considered as LAIP, while the under-expression was considered as LAIP for CD24, CD38, CD45 and CD81. Asynchronous co-expression of CD34 and CD20, CD34 and CD21 were also included. The aberrant cross-lineage expression was taken as LAIP for CD2, CD56, CD65, CD15, CD13, CD33, CD66c, NG2 and CD133. For CD44, under-expression or overexpression can both be considered as LAIP. To verify the effectiveness of the panel, 317 bone marrow samples from 317 patients with B-ALL diagnosed between 2009 to 2014 at the Shanghai Children's Medical Center (SCMC) were included. These samples were collected at day 19 to 283 (median: day 35), with 213 (67.19%) collected at around day 35, which was a regular time point to monitor MRD and to readjust the risk stratification. The results were compared with those of conventional 8-color FC-MRD.

Results The limit of blank (LOB), limit of detection (LOD), and lower limit of quantitation (LLOQ) were firstly determined, the results showed reproducible sensitivity up to 0.001% (1-in-10 ⁵) after acquisition of 4.8 million cells for 23-color panel. The 8-color and 23-color MRD panels showed high correlation of the quantitation of MRD (r ²=0.80, p<0.0001). Among cases of discordant between 8-color MRD negative (with cutoff <0.01%) and 23-color MRD negative (with cutoff <0.001%), relapse only occurred in patients with 23-color MRD positive and 8-color MRD negative, rather than patients with 23-color MRD negative and 8-color MRD positive, suggesting a higher sensitivity and specificity for 23-color MRD assay (See Figure). Patients with detectable MRD by either 8-color or 23-color had a significantly higher incidence of relapse (CIR, p<0.05) and shorter event free survival (EFS, p<0.05). Positive 23-color MRD seemed to be an even stronger factor impacting CIR and EFS. Compared to patients with 8-color MRD negative and 23-color MRD positive, patients with 8-color MRD positive and 23-color MRD negative had a significantly lower CIR (p<0.05) and a better EFS (p=0.068). Patients with positive 23-color MRD lower than 0.01% showed a trend towards poorer EFS and OS (p=0.083% and p=0.061%, respectively) compared to patients with 23-color MRD negative.

Conclusion Based on these evidences, SMFC-based one-tube 23-color MRD panel for B-ALL was convenient and maneuverable with higher sensitivity and specificity compared to conventional 8-color FC-MRD panel. It would be a promising method to improve the risk stratification.