Characterization of the Toll-like receptor expression profile in human multiple myeloma cells

Expression and function of Toll-like receptors (TLRs) in multiple myeloma (MM) has recently become the focus of several studies. Knowledge of expression and biology of these receptors in MM will provide us with a new insight into the role of an inflammatory environment in disease progression or pathogenesis of MM. However, to date a quite heterogeneous expression pattern of TLRs in MM particularly at gene level has been described while information on the TLR expression at the protein level is largely unavailable. In this study, we investigated the TLR expression in human myeloma cell lines (HMCLs) Fravel, L363, UM6, UM9, OPM1, OPM2, U266, RPMI 8226, XG1, and NCI H929 and primary cells from MM patients at both mRNA and protein level (western blot and flow cytometry). We found that all cell lines and primary cells expressed TLR1, TLR3, TLR4, TLR7, TLR8, and TLR9 mRNA and protein. TLR2 and TLR5 were expressed by the majority of HMCLs at mRNA but were not detectable at protein level, while primary samples showed a low level of TLR2, TLR3 and TLR5 protein expression. Our results indicate that MM cells express a broad range of TLRs with a degree of disparity between gene and protein expression pattern. The clear expression of TLRs in MM cells indicates a propensity for responding to tumor-induced inflammatory signals, which seem inevitable in the MM bone marrow environment.