Defective Production of Myeloid Dendritic Cells Results from Skewing of Chronic Myelogenous Leukemia Progenitors.

The robustness of the immunostimulatory response generated by dendritic cells (DCs) has positioned these cells as central and determining cellular mediators of T cell activation. Their production in sufficient number from hematopoietic precursors is crucial in order to drive immune responses. To determine the capacity of CML clonogenic cells to generate dendritic cells expressing endogenous CML-specific epitopes, we have evaluated the generation of DCs from CD34+ progenitor cells and CD14+ monocytes isolated from patients with CML (n=28). We found that the kinetics of production of DCs, in GM-CSF 800U/ml, TNF-a 50U/ml and IL-4 10U/ml, from CD34+ progenitors of CML patients were significantly delayed in comparison to healthy controls (p<0.05). Even after 20 day-cultures, DC numbers remained decreased (P<0.01). A proliferative defect of CML blasts cannot explain this finding since CD34+ cells divided normally according to total cell culture count and CFSE staining. Flow cytometry analysis of the few DCs produced from CML CD34+ cells showed the same phenotypic characteristics of expression of MHC (class I and II), costimulatory (CD80, CD86) and adhesion molecules (CD11a, CD11b and CD58), as DCs generated from control CD34+ cells, except for late downregulation of CD11a. Normal maturation of DCs from CML CD14+ cells and a sustained proportion of the bcr-abl+ DCs before and after DC expansion excluded a 9:22 translocation-induced differentiation defect. However, retarded down-modulation of the CD34 antigen at the surface of CML blasts, independently of IL-4 receptor synthesis, rather suggested decreased susceptibility of CML progenitors to IL-4 and GM-CSF-mediated DC growth conditions. Indeed, CML progenitors, plated in semi-solid media, showed an important skewing toward more mature progenitors (BFU-E and late CFU-GM) with a significant decrease in immature CFU-GM (> 500 cells) (CML = 29 vs normal = 95 colonies, P<0.001) and CFU-Mix (CML = 0.6 vs normal = 5.6 colonies, P=0.0006). Moreover, the number of DCs obtained after 10 and 15 day-cultures showed a strong correlation with the number of immature CFU-GM and CFU-Mix colonies (R=0.95). Thus, our study identifies a selective impairment in DC production from CD34+ progenitors of CML patients corresponding to a skewing of the progenitor cell compartment toward mature myeloid and/or erythroid progenitors. The lack of DCs derived from leukemia progenitors may explain the immunologic escape of CML clonogenic cells and provides clues to enhance their immune recognition.