Evaluation of Analysis Strategy for Cytoplasmic Lineage-Associated Markers in Mixed-Phenotype Acute Leukemia (MPAL) As Devised By Who Classification 2022

Background: Conventional rating of flow cytometrically determined expression of cytoplasmic lineage-associated markers MPO, CD3 and CD79a has defined positivity by ≥10% leukemic cells being positive compared to internal negative controls. WHO classification 2022 has devised a new approach defining positivity by part of leukemic population exceeding 50% of expression level of normal counterpart populations used as positive controls.

Aim: Evaluate interpretation of cytoplasmic expression of MPO, CD3 and CD79a in MPAL and other acute leukemias.

Methods: 277 acute leukemia cases were analyzed (MPAL-BM, n=18; MPAL-TM, 20; AML M0, 42; AML M1, 40; AML M2, 39; APL, 39; BCP-ALL, 40; T-ALL, 39). Following WHO 2022 positivity for cytoplasmic MPO, CD3 and CD79a was determined setting a cutoff at 50% of mean fluorescence intensity (MFI) of granulocytes, mature T-cells and mature B-cells, respectively. An alternative refined approach was evaluated calculating the MFI ratio between leukemic populations and positive controls. Results of MFI ratios are given as log differences, i.e. 0 representing identical MFIs, -1 meaning 1 log lower expression in leukemic population compared to controls. Log difference of -1.5 was set as cutoff.

Results: Applying WHO 2022 majority of cases were rated positive for expected markers, i.e. AML, MPAL-BM and MPAL-TM for MPO, T-ALL and MPAL-TM for CD3 and BCP-ALL and MPAL-BM for CD79a (figure, A). However, there were cases rated positive in other groups although in general only few leukemic cells were causing positive rating, indicating limited specificity of this approach.

In detail, MPO was positive in 69% of M0, 95% of M1, 92% of M2, 92% of APL, 61% of MPAL-BM and 70% of MPAL-TM, but also in other groups (BCP-ALL, 35%; T-ALL, 51%).Applying WHO 2022, diagnosis would have changed to BCP-ALL in 39% of MPAL-BM and to T-ALL in 30% of MPAL-TM cases.

CD3 was positive in 92% T-ALL and 95% MPAL-TM cases, but also in other groups (M0, 29%; M1, 25%; M2, 15%, APL, 49%; BCP-ALL, 13%; MPAL-BM, 56%). Applying WHO 2022, diagnosis would have changed to AML in 5% of MPAL-TM cases.

CD79a was positive in 100% of BCP-ALL and 94% of MPAL-BM, but also in other groups (M0, 55%; M1, 43%; M2, 51%, APL, 49%; T-ALL, 33%; MPAL-TM, 75%). Applying WHO 2022, diagnosis would have changed to AML in 6% of MPAL-BM cases.

In order to refine the WHO 2022 approach and to improve specificity MFI ratio between leukemic cells and positive controls was applied. Similarly to the WHO 2022 approach the MFI ratio approach resulted in expected differences but also significant overlaps (figure, B), although to a lower degree for MPO.

In detail, MPO was positive in 31% of M0, 75% of M1, 77% of M2, 85% of APL, 72% of MPAL-BM and 50% of MPAL-TM, but also in other groups (BCP-ALL, 13%; T-ALL, 23%). Applying the MFI ratio approach, diagnosis would have changed to BCP-ALL in 28% of MPAL-BM and to T-ALL in 50% of MPAL-TM cases.

CD3 was positive in 92% of T-ALL and 100% of MPAL-TM cases, but also in other groups (M0, 14%; M1, 23%; M2, 18%, APL, 85%; BCP-ALL, 13%; MPAL-BM, 72%). Applying the MFI ratio approach there would have been no changes in MPAL-TM diagnosis.

CD79a was positive in 98% of BCP-ALL and 94% of MPAL-BM, but also in other groups (M0, 50%; M1, 65%; M2, 46%, APL, 87%; T-ALL, 56%; MPAL-TM, 85%). Applying the MFI ratio approach, diagnosis would have changed to AML in 6% of MPAL-BM.

In 6 and 7 cases analyzed by WHO 2022 and MFI ratio, respectively, changes in diagnosis from MPAL to ALL were not supported by NGS yielding mutations known to be associated with myeloid neoplasms while changes were supported by NGS in a minority of cases.

Discussion: The WHO 2022 definition, as compared to conventional analysis, changed diagnosis in a significant number of MPAL cases, in particular due to rating MPO negative. At the same time, application of WHO 2022 approach to cases with AML, BCP-ALL and T-ALL suggests limited specificity with significant rates of positivity outside the lineage determined by conventional approach.

The MFI ratio approach also changed diagnosis in a significant number of MPAL cases in comparison to the conventional approach, however, data suggests a better specificity for MPO. Further, MFI ratio considers the whole leukemic population and thus may result in higher reproducibility.

Overall this data argues in favor of considering multiple lineage-associated markers instead of single markers. In addition, classification based on genetic markers should be maximized also in MPAL.

Haferlach:Munich Leukemia Laboratory: Current Employment, Other: Part ownership. Meggendorfer:MLL Munich Leukemia Laboratory: Current Employment. Haferlach:MLL Munich Leukemia Laboratory: Current Employment, Other: Ownership. Kern:MLL Munich Leukemia Laboratory: Current Employment, Other: Ownership.