Selection and Characterization of Antibody Clones Are Critical for Accurate Flow Cytometry-Based Monitoring of CD123 in Acute Myeloid Leukemia

Background: CD123, the trans-membrane alpha chain of the interleukin-3 receptor (IL-3RA) is overexpressed in acute myeloid leukemia (AML) and distinguishes leukemia stem cells from their normal counterparts. There are several novel therapeutics under development to target CD123 in AML, including CD123 fused to Diphtheria toxin, a recombinant chimeric anti-CD123 MoAb, CD3/CD123 bi-specific T cell engagers, and engineered T cells that express chimeric antigen receptors (CARs). Thus, accurate detection and quantification of CD123 is critical for newly diagnosed and relapsed patients, and to follow minimal residual disease for patients in remission. Our data suggest that the evaluation of CD123 by flow cytometry varies significantly with different antibody clones.

Objective: To identify the most accurate flow cytometry method for evaluation of CD123 expression in patients with AML to evaluate CD123 targeting therapies.

Methods: 51 AML patient samples and 7 normal cord blood or bone marrow samples were stained with five different commercially available monoclonal antibodies to detect CD123 (7G3, 6H6, 9F5, AC145 and FAB301P), as well as CD45 and CD5, for evaluation by multiparameter flow cytometry. CD123 gene expression was also compared between these primary AML samples and bone marrow samples from healthy donors. Cell surface expression (by percentage and MFI) was evaluated relative to transcriptional expression and sensitivity to known therapeutics (cytarabine, parthenolide, and HSP90 inhibitors).

Results: We observed CD123 surface expression patterns varied between the antibody clones tested. For the 9F5 and 6H6 clones, 93% and 82% of the samples, respectively, showed >60% CD123+ cells whereas for the 7G3, FAB 301P and AC145 clones, 71 to 76% of the samples showed >60% positivity. Also, surface expression of CD123 using 7G3, AC 145 and FAB 301P did not correlate with transcript levels for IL3RA assessed using qPCR, while surface expression of CD123 using 9F5 and 6H6 did correlate with transcript levels of IL3RA, using both mean fluorescence intensity (MFI) and percentage. For example, the correlation between CD123 surface expression as measured by percentage and IL3RA transcripts was most significant using the 9F5 and 6H6 clone (R²=0.1084, p=0.0183, R²=0.1588, p=0.0038 respectively) whereas the correlation for 7G3 (R²=0.0004, p=0.8945), FAB301P (R²=0.0027, p=0.7151) and AC145 (R²=0.0392, p=0.1638) were not significant. Surface expression of CD123 evaluated with 7G3 antibody did not correlate with overall sensitivity to in vitro treatment with cytarabine (R²=0.03767, p= 0.6451). However, using the 9F5 antibody, we found that higher levels of surface CD123 were associated with resistance to cytarabine in vitro (R²= 0.5502, p= 0.0351). Differences were noted for other experimental therapeutics including parthenolide and PU-H71. Most importantly, when we tested the ability of a novel allogeneic anti-CD123 CAR T-cell therapy (UCART123) to eliminate CD123+ AML cells, we found that CD123 positivity as measured by the 7G3 clone was not predictive of sensitivity to UCART123 in vitro or in vivo AML patient derived xenotransplants.

Conclusions: Several novel therapeutic modalities targeting CD123 in AML are under development, including allogeneic anti-CD123 CAR T-cell therapy. Accurate, quantitative assessment of CD123 expression is thus of utmost importance for patient selection in clinical trials as well as disease monitoring. We found discrepancies between antibody clones, and such discrepancies may alter patient selection and data interpretation regarding patient response to CD123 based therapies. For therapies targeting CD123, protocol design and antibody selection should be done considering the results in this study. Based on our findings we recommend 9F5 or 6H6 antibody clones as well as the utilization of qPCR along side flow cytometry for adequate detection. Flow cytometry findings should be reported as percent positive cells. If utilizing the 9F5 clone, samples with > 60% CD123+ should be considered positive for CD123. A comparison in a large cohort may be warranted to determine the impact of multiple CD123 measurements on disease outcome.