CD4⁺CD56⁺ leukemic cells expressing CD33 present morphologic, phenotypic, and functional characteristics of pDCs. (A) Cells from patient no. 1 stained with standard MGG solution show the typical morphology of CD4⁺CD56⁺ malignancies with a blastic aspect containing microvacuoles near the plasma membrane and cytoplasmic expansions. After differentiation into mature APCs (stimulation with IL-3/CD40L for 48 hours), cells acquire many dendrites (original magnification × 1000). In contrast, in cultures with IL-3 alone (survival condition¹⁷ ), tumor cells acquire a plasmacytoid morphology. (B) Expression of APC-related molecules on fresh and IL-3/CD40L cultured leukemic cells is shown. An increased expression of HLA-ABC, HLA-DR, CD40, CD80, and CD86 was observed after culture with IL-3/CD40L. (C) Leukemic cells stimulate naive allogeneic CD4⁺ T cells. Cord blood-purified CD45RA⁺CD4⁺ T-lymphocyte proliferation to increasing numbers of irradiated mature (IL-3/CD40L activated) leukemic cells was measured after 6-day mixed leucocyte reaction (MLR) by [H³]-thymidine incorporation. T cells from 2 different cord blood samples (CB no. 705 and 706) were used. (D) Leukemic cells induce Th2 polarization. Production of Th1 or Th2 cytokines was directly analyzed in supernatant after MLR (left panel) or after restimulation of cocultured cells with PMA plus ionomycin (right panel) as described in “Patients, materials, and methods.” No significant levels of IFN-γ or TNF-α can be detected, whereas production of Th2 cytokine IL-5 is found. IL-4 was only detected after restimulation of cocultured T cells with PMA plus ionomycin, suggesting a consumption during MLR. Error bars represent SD.