Depletion of Circulating Plasmacytoid Dendritic Cells in Chronic Myelogenous Leukemia and Their Restoration after Interferon-alpha or Imatinib Mesylate.

It is currently established that the immune system plays a critical role in the control of chronic myelogenous leukemia (CML). Several cell populations including T cells, dendritic cells (DC) and NK cells have been shown to exhibit potential anti-leukemic activities. The possible role of plasmacytoid dendritic cells (pDC) in this anti-leukemic response has not been explored so far. As pDC are the major source of interferon(IFN)-α in vivo, the frequency and the function of circulating pDC were compared in CML patients and in healthy subjects. Three groups of CML patients were studied:

1. chronic phase patients (CP) (i.e. at diagnosis or unresponsive to treatment);

2. patients in complete/major cytogenetic remission induced by IFN-α and

3. patients in complete/major cytogenetic remission induced by imatinib mesylate (IM).

Flow cytometric analysis of pDC (assessed by the strong co-expression of CD123 and CD303, formerly BDCA2) showed a dramatic depletion of pDC compartment in CP patients (n = 11) as compared to healthy donors (n = 9) (0.99 versus 5.71 pDC/μL of blood, respectively). Moreover, their functional capacity to produce IFN-α after peripheral blood mononuclear cells (PBMC) stimulation with influenza virus was abolished (883 versus 8201 pg/ml of supernatant, respectively). Patients treated with IFN-α exhibited a complete and long-termed restoration of pDC compartment (4.88 pDC/μL of blood) and of IFN-α production (6932 pg/ml), which was sustained even after the treatment was stopped. In contrast, although IM therapy led to the complete restoration of IFN-α production (6662 pg/ml), its effect on circulating pDC number remained solely partial (2.82 pDC/μL of blood). This last observation and the absence of any linear relationship between pDC number and IFN-α production in patients in complete remission suggested that treatments, and more particularly IM, were able to induce “pDC-independent” pathways of in vivo IFN-α production. Consistently with this hypothesis, we found that pDC-depleted PBMC from patients in cytogenetic remission were able to produce IFN-α, whereas those from healthy donors were not. As a conclusion, our results show for the first time that IFN-α and IM treatments reverse the quantitative and functional impairment of pDC compartment in CP CML patients. They also suggest that these treatments are able to induce differentiation mechanisms leading to the generation of new IFN-α producing cell populations.