Increased Expression of CD56 on Plasmacytoid Dendritic Cells in Myeloid Leukemia Associated with Down Syndrome (ML-DS) Is Normal Regeneration and Not Measurable Residual Disease
Authors: Hudson Chad A , Pardo Laura , Lott Loren L. , Loken Michael R. , Eidenschink Brodersen Lisa 
Published: November 2021

Abstract
Introduction: It has long been known that Down syndrome is associated with an increased risk for hematologic malignancies. One such disease is myeloid leukemia associated with Down syndrome (ML-DS), a disease that almost always occurs during the first 5 years of life. As research on ML-DS has progressed, understanding has grown that after the initiation of therapy, the non-neoplastic myeloid progenitor cells in ML-DS patients have a characteristic immunophenotype with the expression of CD56 on a subset of the CD34+ myeloid progenitor cells being one of the most notable features. The discovery that this immunophenotype is normal in ML-DS patients post-therapy has been of the utmost importance as it has led to such patients being properly classified as being negative for measurable residual disease. Plasmacytoid dendritic cells (pDCs) are another cell type in which CD56 expression is often part of the neoplastic immunophenotype, and we hypothesized that CD56 may also be differentially expressed in ML-DS pDCs post-therapy. Herein, we investigated the immunophenotype of pDCs in ML-DS patients found to be negative for measurable residual disease.
Methods: A total of 10 bone marrow specimens from ML-DS patients post-treatment initiation and 7 bone marrow specimens from patients that did not have DS, were aged 0-4 years (matching the age range of ML-DS), had a myeloid neoplasm and were post-treatment initiation (non-DS) were included in this study. All specimens were found to be negative for measurable residual disease by difference from normal (ΔN) flow cytometry (the gold standard for the determination of residual disease in the Children's Oncology Group 1531 study on ML-DS) and were evaluated for CD56 and CD303 expression on pDCs. pDCs were defined as HLA-DR+/CD123++ (high intensity).
Results: As expected, the ML-DS patients had a significantly greater percentage of CD34+CD56+ myeloid progenitor cells than the non-DS group, both in terms of percent total non-erythroid cells (0.9% vs 0.006%, P&lt;0.001) and percent total myeloid progenitors (38% vs 0.57%, P&lt;0.001). There was not a significant difference between groups in terms of pDC percentage (ML-DS 0.63% of total non-erythroid cells vs non-DS 0.53%, P=0.9%). There were also no significant differences in CD303 expression between the groups, both in terms of percent positive (ML-DS 89% vs non-DS 92%, P=0.5) and mean fluorescence intensity (MFI, in PE) (ML-DS 190 vs non-DS 246, P=0.3). On the other hand, the ML-DS group had significantly greater CD56 expression than the non-DS group, both in percent positive (74% vs 25%, P=0.005) and MFI (PE) (122 vs 5.9, P=0.005). Nine of the 10 ML-DS specimens had CD56 expression on greater than 50% of pDCs, and 3 showed a CD56 MFI of over 200.
Conclusions: The data from this preliminary study indicate that much like the myeloid progenitor cells, the pDCs in ML-DS patients after the initiation of therapy have an immunophenotype that could be mistaken as abnormal. Importantly, they show that the setting of cutoff values for the determination of abnormal pDC CD56 expression, even relatively high ones, could lead to false positive results in ML-DS specimens post-treatment initiation. The dissemination of this knowledge is of increased importance as more flow cytometry laboratories begin to increase their investigation of pDCs. Further studies are needed to delineate common mechanisms between the expression of CD56 in myeloid progenitor cells and pDCs in ML-DS patients post-treatment initiation.

Disclosures
Pardo: Hematologics, Inc.: Current Employment. Lott: Hematologics, Inc.: Current Employment. Loken: Hematologics, Inc.: Current Employment, Other: current equity holder in a privately owned company. Eidenschink Brodersen: Hematologics, Inc.: Current Employment, Other: Equity Ownership.