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W. Wang et al.
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Figure 2. The location of blastic plasmacytoid dendritic cell neoplasm cells on CD45/SSC plots. (A): Most (92%) cases of blastic plasmacytoid dendritic cell neo- plasm (BPDCN) show CD45 expression at a level similar to or slightly higher than that detected in granulocytes. (B) A small subset (8%) of cases shows lower CD45 expression. The red population represents BPDCN cells.
year old girl who otherwise had a typical immunopheno- type of BPDCN. CD303, a marker which showed a high diagnostic value for BPDCN in some previous studies,14,15 was expressed in 44% (7/16) of cases. Additionally, cells were positive for CD7 in 64% (21/33) of cases, with uni- form expression in 11 and partial expression in ten cases. CD38 was variably expressed in 88% (30/34) of cases. Cells were positive for CD2 in 5/27 (19%) cases and all positive cases showed bright expression. CD33 was expressed in 48% (16/33) of cases. CD36 was expressed in 57% (17/30) of cases, with the expression being uni- form in seven (41%) and partial in ten (59%) cases. CD117 was partially expressed in three of 34 (9%) cases. CD5 expression was uncommon, only being observed in one of 30 (3%) cases. Partial CD14 without CD64 expres- sion was detected in one (3%, 1/34) case and TdT expres- sion was detected in 25% (4/16) of cases. All cases were negative for CD3 (surface and cytoplasmic), CD13, CD15, CD19, CD22, CD25, CD34, CD41, CD64 and myeloperoxidase.
Immunophenotype of normal/reactive plasmacytoid dendritic cells
The immunophenotype of normal/reactive PDC was studied in BM samples from 22 patients without BPDCN, including 11 samples that were submitted for lymphoma staging, four cases of B-lymphoblastic leukemia in remis- sion and seven post-transplant samples from patients with B-lymphoblastic leukemia or acute myeloid leukemia. In these cases, normal/reactive PDC represented 0.11%
(median) of total nucleated cells (range, 0.01% to 0.43%) as determined by flow cytometry.
Similar to BPDCN cells, normal/reactive PDC were con- sistently positive for CD123 and HLA-DR and negative for CD64. They were all positive for CD4, CD45, and CD303. Likewise, they were all positive for CD38, which was brightly expressed in all cases, and CD33, which was expressed uniformly in 12 of 21 (57%) cases and partially in the remaining (43%, 9/21). Of note, although positive, CD33 expression in normal/reactive PDC was lower than that of monocytes and basophils (Figure 3D). CD2 expres- sion by normal/reactive PDC showed a bimodal pattern with a spectrum from completely negative cells to positive cells in all cases (Figure 4A). CD7 expression was consis- tently positive in a small subset of normal/reactive PDC with a median of 13% (range, 0.3% to 21%). Of note, these CD7+ PDC were negative for CD56 (Figure 4A).
The immunophenotype of CD56+ normal/reactive plasmacytoid dendritic cells
CD56 expression was observed in a subset of normal/reactive PDC in all 22 non-BPDCN cases described above, with a median of 4.5% (range, 1.3% to 20%) of total PDC. This CD56+ subset of PDC showed substantial immunophenotypic overlap with BPDCN in PDC-defining markers, including being positive for CD4, CD123, HLA- DR, and CD303; and the panel (panel #2, Table 1) initially designed for BPDCN was incapable of distinguishing these cells from BPDCN.
This population of CD56+ PDC was further studied with
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