FLT3-ITD-Associated Accumulation of Dendritic Cell Precursors, Depletion of Mature Dendritic Cells and Abnormal Cytokine Production in Acute Myeloid Leukemia: Analyses At Diagnosis, Remission and Relapse

Abstract 2525

FLT3-ITD is a frequent (30%) mutation in AML, conferring proliferative advantage, self-renewability and blocking of myeloid differentiation. We had previously shown that ITD+ AML blasts show an accumulation of cells with characteristics of mixed-lineage dendritic cell (DC) precursors, but the effects of FLT3-ITD on the mature dendritic cell compartment and cytokine profile had not been previously described.

The frequency of different DC types in peripheral blood has been established for healthy donors (HD) such as Pan myeloid DCs (Lin−/DR+/CD11c+: 0.5 ± 0.3% frequency in HD), Pan plasmacytoid DCs (Lin−/DR+/CD123+: 0.15 ± 0.08% in HD), mature myeloid DCs type-1 (CD19−/CD14−/BDCA-1+: 0.35 ± 0.17% in HD), mature myeloid DCs type-2 (CD19−/CD14−/BDCA-3+: 0.04 ± 0.02% in HD), and mature plasmacytoid DCs (CD19−/CD14−/BDCA-2: 0.23 ± 0.2% in HD). In this study, we analyzed the frequency of DCs in diagnostic samples of 12 ITD+ compared with 22 ITD- AML patients, and in sub-cohorts of ITD+ patients in remission and relapse.

We demonstrate that ITD+ patients at diagnosis showed a significantly higher frequency of mixed-lineage DC precursors (Lin−/DR+/CD11c+/CD123+: 13 ± 12%) than ITD- patients (5 ± 4.8%) whereas the frequency of all types of mature DCs was lower for ITD+ patients compared with ITD- patients. Analyses of remission samples (collected at 3–15 months after first diagnosis) revealed a regression of the mixed-lineage DC precursors, but the frequency of mature DCs in ITD+ patients remained consistently lower than for the ITD- or HD cohorts.

Remarkably, for one ITD+ patients who eventually relapsed (at 15 months), a gradual reduction in the frequency of BDCA-1 and BDCA-2 positive DCs was clearly noticeable at 9 months after diagnosis, at a time point when the ITD mutation was not yet detectable by RT-Q-PCR, whereas relapse was correlated with the resurgence of the mixed lineage CD11c+/CD123+ DC population (17% of PBMCs) and reduction in the frequency of BDCA-1 (0.07%) and BDCA-2 positive DCs (0.1%).

Luminex analyses of several cytokines secreted by the PBMCs from 6 ITD+ patients showed that samples collected at diagnosis expressed lower levels of inflammatory and homeostatic cytokines than in remission, such as TNF-alpha (36.67 vs. 101.22 pg/ml, p-value: 0.22), IL-6 (35.27 vs. 119.15 pg/ml, p-value: 0.19), IL-7 (2.84 vs. 16.27 pg/ml, p-value: 0.07) and IL-8 (5682.59 vs. 10108.98 pg/ml, p-value: 0.07).

This is the first clinical evidence that FLT3-ITD undetectable minimal residual disease may affect the differentiation of DCs, potentially subverting the normal inflammatory and homeostatic immune reconstitution and thus hampering the anti-leukemia immune surveillance.