TLR8 ligation induces apoptosis of monocytic myeloid-derived suppressor cells.

Myeloid-derived suppressor cells (MDSCs) accumulate in tumors and the peripheral blood of cancer patients and demonstrate cancer-promoting activity across multiple tumor types. A limited number of agents are known to impact MDSC activity. TLR8 is expressed in myeloid cells. We investigated expression of TLR8 on MDSC and the effect of a TLR8 agonist, motolimod, on MDSC survival and function. TLR8 was highly expressed in monocytic MDSC (mMDSC) but absent in granulocytic MDSC (gMDSC). Treatment of human PBMC with motolimod reduced the levels of mMDSC in volunteers and cancer donors versus control (P < 0.001). Motolimod did not impact levels of gMDSC. The reduction of mMDSC was due to induced cell death by TLR8 ligation. Pretreatment of PBMC with a FAS neutralizing antibody inhibited motolimod-induced reduction of mMDSC (P < 0.001). Finally, we demonstrated that mMDSC impeded IL-2 secretion by CD3/CD28-activated T cells; IL-2 secretion was partially restored when cells were cocultured with motolimod (142 ± 36 pg/ml vs. 59 ± 13 pg/ml; P = 0.03). There is increasing evidence that MDSCs contribute to the progression of cancer by inhibiting tumor-directed T cells. TLR8 agonists may synergize with cancer immunotherapeutic approaches to enhance the antitumor effects of the adaptive immune response.

Misprocessing and functional arrest of microRNAs by miR-Pirate: roles of miR-378 and miR-17.

miRNAs (microRNAs) are short non-coding RNAs that can regulate gene expression in cancer development, which makes them valuable targets for therapeutic intervention. In the present study we report on an approach that can not only arrest the functions of mature miRNAs by binding to them, but it can also induce the 'mis-processing' of the target miRNA, producing a non-functional truncated miRNA. This approach involves generating an expression construct that produces an RNA fragment with 16 repeat sequences. The construct is named miR-Pirate (miRNA-interacting RNA-producing imperfect RNA and tangling endogenous miRNA). The transcript of the construct contained mismatches to the seed region, and thus it would not target the potential targets of the miRNA under study. The homology of the construct is sufficiently high, allowing the transcript to block miRNA functions. The functions of the construct were validated in cell cultures, in tumour formation assays and in transgenic mice stably expressing this construct. To explore the possibility of adopting this approach in gene therapy, we transfected cells with synthetic miR-Pirate and obtained the results we expected. The miR-Pirate, expressed by the construct or synthesized chemically, was found to be able to specifically pirate and silence a mature miRNA through its dual roles and thus could be clinically applied for miRNA intervention.