Expression of Toll-like receptor 9 in bone marrow cells of myelodysplastic syndromes is down-regulated during transformation to overt leukemia.

Toll-like receptors (TLRs) play a crucial role in the host defense against invading microorganisms by recognizing pathogen-associated molecular patterns. Recently, a number of endogenous molecules have been reported to be ligands of TLRs. Some of these molecules are known to be expressed in cancer tissue and activate intracellular signal pathways via TLRs during cancer progression. Thus, in the present study, we analyzed the expression dynamics of TLRs in the bone marrow of myelodysplastic syndromes (MDS) during the course of transformation to overt leukemia (OL) using real-time RT-PCR. MDS bone marrow cells at the time of initial diagnosis tended to express higher levels of TLR2, TLR4 and TLR9 than control bone marrow cells. Among these TLRs, TLR9 exhibited a significant decrease of expression at the time of transformation to OL. The expression of TLR9 and TNF-alpha showed significant correlation in bone marrow cells from patients with MDS and OL. Immunohistochemically, TLR2 was mostly localized to neutrophils of the control and MDS bone marrow. TLR4 was observed in a subset of neutrophils and a few mononuclear cells in control and MDS bone marrow. In addition, TLR4 was weakly expressed in nearly half of immature myeloid cells of MDS cases. TLR9 was mainly localized to neutrophils in the control and RA bone marrow and strongly expressed in the immature myeloid cells of RAEB cases, although the blastic cells of OL cases did not express TLR9. Bone marrow cells in MDS exhibit frequent apoptosis, while OL cells are prone to be immortal. Thus, TLR9 might be associated with regulation of apoptotic/proliferative signals via TNF-alpha in the MDS bone marrow.