Inhibitory Effect of ssRNA40, a Toll-Like Receptor-8 Agonist, on the Proliferation of Acute Myeloid Leukemia Cells.

Acute myeloid leukemia (AML) is a malignant hematopoietic disorder with considerable impairment of the immune system. We have previously reported that the T-cell-mediated immunity in AML patients could be in part damaged by the increased prevalence of CD4⁺CD25^high regulatory T cells (T-regs). Toll-like receptors (TLR), found on many immune cells, including T-regs, have been shown to be important molecules in both innate and adaptive immune responses. Recent studies demonstrated that synthetic and natural ligands for human TLR8 could completely reverse the suppressive function of T-regs and enhance antitumor immunity, which provides a possible application of TLR8 ligands in immunotherapy for a patient with AML. However, an important question is whether the TLR8 ligands could also have direct impacts on the AML cells. In this study, we first examined the expression of TLR8 on fresh leukemic cells from 11 AML bone marrow samples and a human AML cell line (U937) by using RT-PCR analysis. We found that U937 cells and the majority of the AML patients (8⁺/11) expressed TLR8. To further investigate the functional role of TLR8, the cell line U937 was stimulated with ssRNA40, a Toll-like receptor-8 agonist, and analyzed by the measurements of cell proliferation, apoptosis and co-stimulatory molecules (CD80 and CD86) expression. The proliferation of U937 was detected by cell counting kit-8 (CCK-8) and the apoptosis was examined by AnnexinV and PI. Interestingly, the proliferation of U937 cells could be inhibited in a dose-dependent manner, whereas apoptosis and co-stimulatory molecules expression were not significantly changed with ssRNA40 stimulation. In the same condition, the inhibitory effect of ssRNA40 was not revealed on the proliferation of K562 CML cell line, even though TLR8 was also expressed on the cells. This might suggest that a specific pathological pathway in AML was blocked by ssRNA40. The signaling pathway that can restrain AML cells from proliferation is currently being studied. In conclusion, the present data may offer a unique therapeutic potential for ssRNA40 as an inhibitor, by which we will be able to directly inhibit the proliferation of AML cells and simultaneously enhance the patients’ immune response against the malignant cells.