Double-Stranded RNA Acts as a Strong Danger Signal in Human Myeloid Leukemia Cells Leading to Increased Immunogenicity.

Leukemic cells exert immunosuppressive effects that interfere with dendritic cell function and hamper effective anti-leukemic immune responses. Recently, Toll-like receptor 3 (TLR3) was characterized in dendritic cells as an intracellular double-stranded (ds)RNA receptor which is triggered by viral infection or incubation with the synthetic dsRNA analogue polyriboinosinic polyribocytidylic acid [poly(I:C)], leading to maturation and activation of dendritic cells. Until now, little was known on the expression of TLR3 in leukemic cells and their responsiveness to dsRNA treatment. We assessed TLR3 expression in primary and transformed acute myeloid leukemia (AML) cells and hypothesized that the immunogenicity of AML cells could be improved by treatment with the synthetic TLR3 agonist poly(I:C), thereby mimicking viral infection of these leukemic cells. In view of this hypothesis, we electroporated or pulsed transformed and primary AML cells with poly(I:C) and analyzed the effect of poly(I:C) loading on TLR3 expression, costimulatory molecules, cytokine production and allogeneic T cell response. We also assessed the uptake of poly(I:C)-loaded leukemic cells by immature dendritic cells and the subsequent effect on dendritic cell activation and maturation status. We observed that primary and transformed AML cells respond to poly(I:C) electroporation by upregulation of TLR3 expression, apoptosis, elevated levels of costimulatory molecules CD80 and CD86 and by production of type I interferons (IFN). Furthermore, poly(I:C)-electroporated AML cells induced interferon-gamma production by allogeneic T cells. Upon phagocytosis of poly(I:C)-electroporated AML cells, dendritic cells showed an increased expression of maturation markers and marked production of proinflammatory cytokines. In contrast, this set of immune effects was absent or suboptimal when AML cells were passively pulsed with poly(I:C), indicating the superiority of transfection over pulsing with poly(I:C). These results demonstrate that poly(I:C) electroporation is a promising novel strategy to increase the immunogenicity of AML cells.