Flow Cytometry Profiling of Plasmacytoid Dendritic Cell Neoplasms

Abstract

In this review, we aim to provide a summary of the diverse immunophenotypic presentations of distinct entities associated with plasmacytoid dendritic cell (pDC) proliferation. These entities include the following: (1) blastic plasmacytoid dendritic cell neoplasm (BPDCN); (2) mature pDC proliferation (MPDCP), most commonly seen in chronic myelomonocytic leukemia (CMML); and (3) myeloid neoplasms with pDC differentiation, in which pDCs show a spectrum of maturation from early immature pDCs to mature forms, most commonly seen in acute myeloid leukemia (pDC-AML). Our aim is to provide a flow cytometry diagnostic approach to these distinct and sometimes challenging entities and to clarify the immunophenotypic spectrum of neoplastic pDCs in different disease presentations. In this review, we also cover the strategies in the evaluation of residual disease, as well as the challenges and pitfalls we face in the setting of immune and targeted therapy. The differential diagnosis will also be discussed, as blasts in some AML cases can have a pDC-like immunophenotype, mimicking pDCs.

Keywords: AML with plasmacytoid dendritic cell differentiation; BPDCN; MPDCP; blastic plasmacytoid dendritic cell neoplasm; flow cytometry analysis; mature plasmacytoid dendritic cell proliferation; pDC-AML; plasmacytoid dendritic cells.

Figure 1

Flow cytometry panel and the gating strategy for evaluation of pDCs. Markers and their corresponding fluorochromes are listed. The gating strategy starts with CD123/SSC, followed by CD123/HLA-DR and CD123/CD64.

Figure 2

Normal pDC maturation in the bone marrow. pDCs are highlighted in pink. Early pDCs are in the traditional blast gate with dim CD45 expression on the CD45/SSC plot. As they mature, CD45 expression increases (A). CD123 and HLA-DR are brightly positive in all stages of maturation (B). Early pDCs are positive for CD34 and negative for CD303 and CD4 expression. As they mature, they become negative for CD34 expression and positive for CD4 and CD303 expression (C–E). Most pDCs in all maturation stages are positive for CD2 expression (F), including the CD56+ subset. A small subset of pDCs expresses CD7, and this subset is largely negative for CD56 expression (G). The CD56+ subset expresses bright CD38 (H).

Figure 3

A representative case of BPDCN in bone marrow. Neoplastic pDCs (highlighted in pink) are positive for CD45 expression (A) and are characterized by a uniform expression of CD123, HLA-DR (B), CD56 (C) and CD4 (D). They are negative for CD34, CD117 (E), CD13 (F), CD14, CD64 (G) and CD15 (H) expression. CD33 (F), CD7 (I) and CD36 (J) are positive in a subset of cells, respectively.

Figure 4

The immunophenotypic difference between CD56+ reactive and neoplastic pDCs. Bone marrow aspirates are analyzed. The upper panel shows reactive CD56+ pDCs (pink) that are brightly positive for CD38 (A) and CD2 (B). They are partially positive for CD303 (C). CD7 is negative (D). CD123 and HLA-DR are brightly positive (E). The lower panel shows neoplastic CD56+ pDCs (black) in post-treatment evaluation for minimal residual disease in a case of BPDCN. Cells show uniform CD56 with decreased CD38 expression (F). In contrast to reactive pDCs, they are negative for CD2 (G) and positive for CD7 expression (I). CD303 is completely negative (H). CD123 level (J) is decreased after treatment.

Figure 5

Neoplastic pDCs show decreased CD123 expression after targeted therapy. This is a post-treatment specimen from a BPDCN patient who was treated with IMGN632, a CD123-targeting antibody–drug conjugate. When only CD123 bright cells were gated (pink, (A)) in this bone marrow, they were 0.1% of cells. The majority of these cells are negative for CD56 (B), positive for CD2 (C) and CD303 (D), consistent with reactive pDCs. However, when both CD123+ moderate and bright cells are gated (1.2% of total cells, blue and pink, (E)), many are CD56-positive (F) with negativity for CD2 expression (G). CD303 is positive (H). The overall findings are consistent with aberrant pDCs and residual bPDCN.

Figure 6

The immunophenotype of pDCs in mature pDC proliferations. This is a patient with a recent diagnosis of chronic myelomonocytic leukemia who presented with lymphadenopathy. A lymph node biopsy showed myeloid sarcoma with a pDC population ((A), pink), accounting for 50% of total cells. They are brightly positive for CD123 and HLA-DR (B). CD303 (C) and CD4 (D) are positive. They are negative for the immature marker CD34. CD56 (F) and CD64 (D) are negative. CD2 is partially positive. When compared to normal pDCs, they show an aberrant immunophenotype with increased CD7 (E) and decreased CD38 expression (F).

Figure 7

A representative case of pDC-AML. This is a case of acute myelomonocytic leukemia with pDC differentiation in the bone marrow. In (A), three populations are highlighted: red: myeloblasts; pink: pDCs; blue: monocytes. pDCs are brightly positive for CD123 and HLA-DR expression (B). They show a spectrum of maturation, with early immature cells positive for CD117 and CD34 while negative for CD303 and CD4 (C–E). Mature cells with an opposite immunophenotype are also present. pDCs are negative for CD56 (F). They show partial expression of CD7 (G), CD2 and CD64 (H).

Figure 8

A representative case of AML with a pDC-like immunophenotype. Blasts ((A), red) in this bone marrow show strong CD123 and HLA-DR expression (B). CD4 (C) and CD56 (D) are strongly positive. This immunophenotypic feature mimics BPDCN. However, they are completely negative for CD303 expression (D), with bright CD64 expression (E), which is not consistent with pDCs. The bright CD64 coupled with bright CD33 (F) and partial CD13 expression are consistent monocytic cells. Blue population in D is reactive pDCs in the background with CD303 expression.