Myeloid cells as a target for oligonucleotide therapeutics: turning obstacles into opportunities

Abstract

Immunotherapies emerged as an alternative for cancer treatment, yet their clinical efficacies are still limited, especially in case of solid tumors. Myeloid immune cells, such as macrophages and myeloid-derived suppressor cells (MDSCs), are often hijacked by tumors and become pivotal inhibitors of antitumor immunity. Immunosuppressive functions of tumor-associated myeloid cells result from the activity of Signal Transducer and Activator of Transcription 3 (STAT3), a transcription factor with well-defined tumorigenic and tolerogenic roles in human cancers. To overcome challenges in the development of pharmacological STAT3 inhibitors, we recently developed oligonucleotide-based strategies for cell-selective, in vivo STAT3 targeting. Conjugation of a STAT3siRNA or decoy STAT3 inhibitors to synthetic Toll-like Receptor 9 (TLR9) agonists, CpG oligonucleotides, allowed for selective delivery into TLR9-positive cells. Cellular target for CpG-STAT3 inhibitors include non-malignant, tumor-associated myeloid cells, such as polymorphonuclear MDSCs, as well as cancer cells in acute myeloid leukemia, B cell lymphoma and in certain solid tumors. The chemically modified CpG-STAT3 inhibitors resist serum nucleases and thus can be administered intravenously. Their potency relies on the intracellular gain-of-function effect: release of the central immune checkpoint regulator (STAT3) to unleash proinflammatory signaling (CpG/TLR9) in the same antigen-presenting cell. At the cellular level, CpG-STAT3 inhibitors exert two-pronged effect by rescuing T cells from the immune checkpoint control while decreasing survival of cancer cells. In this article, we review the preclinical data on CpG-STAT3 inhibitors and discuss perspectives of using TLR9-targeted delivery of oligonucleotide therapeutics for the generation of novel, more effective and safer cancer immunotherapies.

Keywords: CpG; MDSC; Oligonucleotides; Regulatory myeloid suppressor cells; STAT3; TLR9.

Fig. 1

The intracellular mode of action of bi-functional CpG-STAT3 decoy oligodeoxynucleotides. CpG-STAT3dODN conjugates are recognized by scavenger receptors and quickly internalized through endocytosis into target cells. While in early endosomes, some of the internalized conjugates bind to TLR9 triggering downstream immunostimulatory signaling. In addition, TLR9 activation facilitates the release of an excess of unbound CpG-STAT3dODNs from endosomes into cytoplasm. The CpG-STAT3dODN binds to the dimers of STAT3 activated by upstream growth factor/cytokine signaling. The sequestration of the decoy-bound STAT3 in the cytoplasm prevents its transcriptional activity, shifting the balance from immunosuppression to the production of pro-inflammatory cytokines/chemokines and towards the processing of tumor-associated antigens (TAA). Altogether, these effects generate systemic CD8⁺ T cell-mediated immune responses against specific tumor antigens

Fig. 2

Two-pronged therapeutic effect of CpG-STAT3 inhibitors against TLR9-positive human cancers. TLR9-targeted delivery of STAT3 inhibitors allows for targeting both TLR9-positive tumor-associated myeloid cells (imDC immature DC, MAC macrophages, PMN-MDSC polymorphonuclear myeloid-derived suppressor cells) as well as certain human cancers, such as castration-resistant prostate cancer (CRPC) and glioma (GBM). TLR9-activation/STAT3-inhibition in the tumor microenvironment reduces production of immunosuppressive mediators by myeloid cells while stimulating DC and MAC maturation and promoting presentation of tumor-specific antigens and release proinflammatory mediators. While CpG-STAT3 inhibitors alleviate immunosuppressive functions of PMN-MDSCs, it is yet unclear whether these effects are accompanied by loss of viability and/or differentiation of MDSCs. At the same time, CpG-STAT3 inhibitors can increase immunogenicity of cancer cells in AML and B cell lymphoma, while decreasing survival and tumorigenic potential of cancer stem-like cells in CRPC and GBM. The combination of breaking immune suppression in the tumor microenvironment and decreasing cancer cell survival is likely to augment the overall therapeutic efficacy against human TLR9-positive cancers