Identification of BDCA-2 and High Levels of CD123 Expression as Useful Markers for the Diagnosis of Plasmacytoid Dendritic Cell Leukemia.
Authors: Garnache-Ottou Francine , Jacob Marie-Christine , Biichle Sabeha , Chaperot Laurence , Trimoreau Franck , Plumas Joel , Tiberghien Pierre , Saas Philippe , Feuillard Jean 
Published: April 2020

Abstract
A new entity of acute leukemia co-expressing CD4 and CD56 markers without any other lineage-specific markers has recently been identified as arising from plasmacytoid dendritic cells (pDC). We proposed to call it plasmacytoid dendritic cell leukemia (pDCL) (Chaperot et al., 2001; Feuillard et al., 2002). We previously reported that pDCL may express the CD33 myeloid associated marker (Garnache-Ottou et al., 2005). This expression of CD33 on CD4+CD56+ lineage negative cells should not exclude the diagnosis of pDCL and underlines that pDCL specific markers should be identified. In order to better characterize pDCL, we organized a French pDCL network to collect cells and data on this rare leukemia. The aim of this study was to identify specific markers for the routine diagnosis. We focused on previously described pDC specific markers: CD123, BDCA-2, BDCA-4 (neuropilin-1). Fourteen pDCL and 69 other acute leukemias (10 B-ALL, 3 T-ALL, 3 BAL, 2 unclassified AML, 4 AML M0, 12 AML M1, 11 AML M2, 1 AML M3, 11 AML M4, 9 AML M5 according to FAB classification and 3 blastic transformations of myeloproliferative or myelodysplastic syndromes) were analyzed by flow cytometry. Leukemic cells were identified on their low expression of CD45 and using lineage specific markers. pDCL have been functionally characterized by their capacities to activate naïve cord blood CD4+ T cells, to induce a Th2 polarization and to produce IFN-α in response to viral supernatant. BDCA-4 was not specific for pDCL since 10% of acute leukemias (7/69) expressed this marker at the same levels as pDCL. This was not surprising since BDCA-4 has been reported to be expressed by myeloid cells as well as by some B and T cell subsets. Unlike BDCA-4, BDCA-2 was never expressed in the 69 tested acute leukemias. However, BDCA-2 expression was not systemically detected in all the pDCL tested (4 negative cases out of the 14 tested). CD123, also know as IL-3 receptor α-chain, was expressed on nearly all the acute leukemias tested. However, the levels of expression discriminated pDCL from other acute leukemias. These latter acute leukemias expressed lower levels of CD123 (fluorescence intensity ratio: 22 [mean] +/- 8 [SEM]; range: 2–71) when compared to pDCL (fluorescence intensity ratio: 171 [mean] +/− 36 [SEM]; range: 71–353). Overall, these results show that BDCA-4 is not a useful marker to identify pDCL. When BDCA-2 is expressed on leukemic cells, this is a strong argument in favor of pDCL. Finally, the level of CD123 expression is an interesting tool to characterize pDCL. We plan to analyze other acute leukemias, and in particular, acute leukemias aberrantly expressing CD4 and/or CD56 markers. This will allow us to confirm that pDCL can be identified on the basis of BDCA-2 and CD123 expression. Since the prognosis of pDCL was poor, an early diagnosis is needed to treat patients with specific therapeutic options (e.g., allogeneic hematopoietic cell transplantation). This can be achieved in the future by using anti-BDCA-2 and CD123 antibodies in addition to classical lineage specific markers. supported by the Goelam and the GEIL