ROR-1 Is a Highly Discriminative Marker in Flow Cytometric Minimal Residual Disease (MRD) Detection in Chronic Lymphocytic Leukemia (CLL)

Introduction:With the advent of new potent therapies for chronic lymphocytic leukemia (CLL) minimal residual disease (MRD) detection becomes increasingly important to assess remission depth. While molecular MRD detection for CLL remains laborious and time consuming flow cytometry is a fast, economic and sensitive method in detecting low frequencies of CLL cells. The usefulness of the antigens CD81, CD5, CD20, CD43 and CD79b has been previously described for this purpose. ROR-1 has recently been identified as a signature gene in CLL and mantle cell lymphoma. The potential utility of ROR-1 in flow cytometric minimal residual cell analysis has not been evaluated yet.

Methods: 10 normal samples and 77 remnants of randomly selected samples from diagnosed patients undergoing CLL therapy were analyzed by flow cytometry. A customized dry formulation of an antibody panel was used, comprising antibodies directed against CD5, CD19, CD20, CD43, CD45, CD79b, CD81 and ROR-1 (DuraClone RE CLB). Linearity, repeatability and inter-operator variability of data analysis of the method were examined. B cell populations comprising at least 50 positive events (46 normal B cell populations, 25 CLL populations, paired and unpaired) were analyzed for their expression profile as assessed by respective mean fluorescence intensities of the antibody labels within classified populations. The expression profiles were subject to supervised discrimination analysis (DA).

Results: Between124,000 and 2,122,000 (683,000 ± 450,000) CD45+ events were acquired from the 87 samples. The background of cells with a CLL-like phenotype in the normal samples was determined as <0.001% of CD45+ events. Linearity was confirmed in the range from 1% to 0.0025%. The Repeatability analysis and the inter-operator variability showed concordance with typical Poisson distribution characteristics. The 46 populations with a typical normal B cell phenotype ranged from 0.014% to 9.592% with an average of 2.45% ± 2.75 of CD45+ events. The 25 populations with a classical or non-classical CLL phenotype ranged from 0.007% to 5.459% with an average of 1.41% ± 1.65 of CD45+ events. Posterior discrimination analysis revealed 100% correct discrimination for CLL populations and 96% correct discrimination for normal populations when relying on ROR-1 expression alone in CD19+CD45+ B cells. This result was only surpassed by the complete antibody combination (100% / 100%) but not by any other of the markers, neither in single use nor in combination

Conclusion: The 8-color dry flow cytometry panel comprising CD5, CD19, CD20, CD43, CD45, CD79b, CD81 and ROR-1 demonstrated sensitive, linear and specific detection of residual CLL cells in a relevant low range of frequency. ROR-1 revealed to be a highly discriminative marker in the analysis of residual CLL cells by flow cytometry. Utilizing this flow cytometry approach, MRD detection showing sensitivity comparable to molecular techniques can be achieved in CLL.