Loss of plasmacytoid dendritic cell differentiation is highly predictive for post-induction measurable residual disease and inferior outcomes in acute myeloid leukemia

Abstract

Measurable residual disease is associated with inferior outcomes in patients with acute myeloid leukemia (AML). Measurable residual disease monitoring enhances risk stratification and may guide therapeutic intervention. The European LeukemiaNet working party recently came to a consensus recommendation incorporating leukemia associated immunophenotype-based different from normal approach by multi-color flow cytometry for measurable residual disease evaluation. However, the analytical approach is highly expertise-dependent and difficult to standardize. Here we demonstrate that loss of plasmacytoid dendritic cell differentiation after 7+3 induction in AML is highly specific for measurable residual disease positivity (specificity 97.4%) in a uniformly treated patient cohort. Moreover, loss of plasmacytoid dendritic cell differentiation as determined by a blast-to-plasmacytoid dendritic cell ratio >10 was strongly associated with inferior overall and relapse-free survival (RFS) [Hazard ratio 2.79, 95% confidence interval (95%CI): 0.98-7.97; P=0.077) and 3.83 (95%CI: 1.51-9.74; P=0.007), respectively), which is similar in magnitude to measurable residual disease positivity. Importantly, measurable residual disease positive patients who reconstituted plasmacytoid dendritic cell differentiation (blast/ plasmacytoid dendritic cell ratio <10) showed a higher rate of measurable residual disease clearance at later pre-transplant time points compared to patients with loss of plasmacytoid dendritic cell differentiation (blast/ plasmacytoid dendritic cell ratio <10) (6 of 12, 50% vs 2 of 18, 11%; P=0.03). Furthermore pre-transplant plasmacytoid dendritic cell recovery was associated with superior outcome in measurable residual disease positive patients. Our study provides a novel, simple, broadly applicable, and quantitative multi-color flow cytometry approach to risk stratification in AML.

Figure 1.

Loss of plasmacytoid dendritic cell (PDC) differentiation in acute myeloid leukemia (AML). PDC from a control subject (A and B) reside in the CD45 dim/low side scatter gate between blasts and monocytes, overlapping with basophils (A). PDC express high levels of CD123 and HLA-DR and can be easily separated from blasts and basophils (B). In AML with morphological disease (≥20% blasts) (C and D), PDC are markedly reduced. Red: CD34 positive blasts; blue: PDC; purple: basophils.

Figure 2.

Quantification of plasmacytoid dendritic cell (PDC) in acute myeloid leukemia (AML) and controls. (A) The PDC proportion of white blood cell count is significantly reduced in AML, residual AML and MRD-pos groups (Wilcoxon rank-sum test). (B) The blast/PDC ratio is markedly increased in AML, residual AML and MRD-pos groups. Interquartile range is shown. pos: positive; neg: negative.

Figure 3.

Kaplan-Meier survival analysis based on post-induction measurable residual disease (MRD) status and blast/plasmacytoid dendritic cell (PDC) ratio. (A) Overall survival (OS) of MRD-neg and MRD-pos groups. (B) Relapse-free survival (RFS) of MRD-neg and MRD-pos groups. (C) OS of blast/PDC ratio <10 and >10 groups. (D) RFS of blast/PDC ratio <10 and >10 groups. neg: negative; pos: positive.

Figure 4.

Examples of abnormal blast immunophenotype in measurable residual disease (MRD)-pos patients with blast/plasmacytoid dendritic cell (PDC) ratio >10 (A-D) versus <10 (E-H). (A-D) PDC are markedly decreased (B) and the blasts have abnormal expression of CD11b (partial, C) and CD25 (D). (E-H) PDC are well preserved (F) and a subset of the blasts (highlighted in yellow) showed abnormal expression of CD38 (absent, data not shown), CD56 (G), and CD25 (H). Red: CD34-positive blasts; blue: PDC; purple: basophils; yellow: abnormal blasts gated on CD34-positive and CD38-negative expression (gates not shown).

Figure 5.

Kaplan-Meier survival analysis based on pre-hematopoietic stem cell transplantation (HSCT) measurable residual disease (MRD) status and blast/plasmacytoid dendritic cell ratio (PDC). (A) Overall survival (OS) of MRD-negative (neg), MRD-positive (pos) with blast/PDC ratio <10, and MRD-pos with ratio >10 groups. (B) Relapse-free survival (RFS) of MRD-neg, MRD-pos with blast/PDC ratio <10, and MRD-pos with ratio >10 groups.