Abstract Human dendritic cell (DCs) precursors are commonly divided into two distinct subsets: myeloid DCs (mDC) and Plasmacytoid DCs (pDCs). The pDCs, which show plasma cell like morphology, have been defined as the population that produce a large amount of type I interferon in response to viruses. The surface phenotypes of human pDCs are defined as CD4+, DC11c−, CD45RA+, IL3Rα (CD123)+ and lineage negative. But the developmental pathways and the regulations of pDCs have not been fully understood. On the other hand, CD4+CD56+ malignant cells in leukemia/lymphoma have been proposed to be of pDC lineage. CD4+CD56+ pDC leukemia/lymphoma are a rare hematological malignancy, totally only about 100 cases in the world by the literatures. In the recent report, these newly described CD4+CD56+ leukemic pDCs share common phenotypic and functional features with their normal counterparts. We encountered a patient with CD4+CD56+ acute leukemia in December 2004. The leukemia cells have been cultured in IMDM with 10 % FBS and a leukemia cell-derived cell line (PMDC05) was established. The effects of various cytokines on the differentiation and the function of PMDC05 were assayed by using IL-3, IL-4, IL-6, GM-CSF and CD40L alone or in combination. To evaluate the effects of CD40L, PMDC05 were cultured over adherent cell layer of 90 Gy irradiated CD40L cDNA-transfected NIH-3T3 cells at cell ration of 5:1 for 2 days. For investigation of the response of PMDC05 against the danger signals through toll like receptors, inactivated influenza viruses (A/H1N1 and A/H3N2) and GpG ODN 2006 were used. Antigen presenting ability of PMDC05 was evaluated by mixed leukocyte culture consisting of 50 Gy irradiated-PMDC05 cultured with various cytokines as stimulator cells and normal peripheral blood non-adherent cells and naïve cells as responder cells. PMDC05 maintained plasma cell like morphology with abundant cytoplasm and some cells showed small dendrites. PMDC05 showed a complex hypoploid chromosomal abnormalities (44, XY) including add(5)(q22), add(15)(q26) and del(15)(q11q15), which are identical to original leukemia cells. Abnormalities including 5q and 15q are reported as the frequent aberrations in CD4+CD56+ pDC leukemia/lymphoma. The surface phenotypes of PMDC05 were negative for CD3, CD14, CD16, CD19 and CD11c and highly positive for CD4, CD45RA, CD56, CD123, CD86, and HLA-DR. Moreover BDCA4 that is specific antigen of human blood pDC is markedly expressed on PMDC05. No TCR or IgH gene rearrangement was detected. Stimulation of PMDC05 with IL-3/CD40L, virus RNA or CpG, which are known as the potent exogenous signals for maturation of normal pDCs, showed to induce the high expression of the maturation markers such as CD83/CD40 and the production of INF-α. PMDC05 were demonstrated to possess a potent antigen presenting ability to allogeneic CD4+ cells in mixed leukocyte culture. The antigen presenting ability was remarkably enhanced in PMDC05 cultured with IL-3/CD40L for 2 days. These data demonstrated that newly established leukemia cell line PMDC05 is involved in pDC lineage and PMDC05 provides invaluable tools not only for the elucidation of pathophysiology and innovation of therapy in CD4+CD56+ leukemia/lymphoma but for the investigation of human pDCs.