RIG-I & cancer immunotherapy

The development of immune checkpoint inhibitors (ICIs) has revolutionized cancer immunotherapy, although complete remission remains limited to a small panel of cancers and patients. ICIs act by relieving checkpoint restraints on antitumor T cell responses. They work best against immunogenic, T-cell inflamed or « hot » tumors. In contrast, ICIs are poorly efficient in «cold» tumor microenvironments (TMEs) that are largely devoid of T cells and infiltrated by immunosuppressive cells. In «hot» TMEs, increased expression of type I interferons (IFN-I) and IFN-stimulated genes (ISGs), such as apoptosis-inducing molecules and T-cell attracting chemokines, contribute to potent antitumor responses. Many therapeutic strategies are actively being explored to transform «cold» TMEs into «hot» ones. One emerging strategy exploits the adjuvancity of pattern recognition receptor (PRR) agonists. Indeed, combinations of ICIs with agonists of Toll-like receptor 9 (TLR9) or stimulator of interferon genes (STING) have reached clinical evaluation but have so far yielded disappointing preliminary results¹.

Retinoic acid-inducible gene I (RIG-I), a viral RNA sensor, is a promising alternative to enhance ICI efficacy^2,3. RIG-I is the best-known member of the RIG-I-like helicase receptor (RLR) family. Unlike TLR9 and STING, RIG-I is expressed in virtually all cell types, including tumor cells. Preclinical studies have shown that systemic delivery of a synthetic RIG-I agonist inhibits tumor growth through mechanisms similar to those triggered for elimination of virally-infected cells³. RIG-I engagement leads to preferential tumor cell death (via intrinsic or extrinsic apoptosis, and inflammasome-induced pyroptosis), and to IFN-I-mediated activation of the innate and adaptive immune systems⁴. RGT100, a specific RIG-I agonist, is currently in phase I/II clinical trials for treatment of advanced solid tumors and lymphomas (NCT03065023)⁴.

A growing number of studies have revealed multiple levels of complexity in the activation of the RIG-I pathway, starting from the agonist characteristics. RIG-I discriminates between viral and host RNA by recognizing the terminus of cytosolic short double-stranded RNA (dsRNA) containing 5’ di- or tri-phosphates (2p- or 3pRNA). Additional features, such as higher-order structure, sequence, and RNA modifications, confer better affinity for RIG-I, although their impact in vivo remains largely unknown⁴. RIG-I signaling cascade is often presented in a simplified manner. Activated RIG-I interacts with the mitochondrial antiviral signaling (MAVS) adaptor, promoting the coordinated activation of TBK1 (TANK-binding kinase 1)/IKKε kinases and IKKα/IKKβ kinases. In turn, these kinases induce activation of IRF (interferon regulatory factor)-3 and -7, and of NF-κB, leading to the production of IFN-I and pro-inflammatory cytokines, respectively⁴. However, RIG-I signaling engages many other proteins, including TRIM25 and Riplet ubiquitinases, TRAF adaptors, and the inflammasome ASC protein.

Recently, elaborate regulation of RIG-I signaling has been revealed, providing hints for future clinical applications. RIG-I ligands, such as self 5’ mono-phosphate dsRNA or viral-induced long non-coding RNA, exert antagonistic functions^4,5. It has also been suggested that distinct TBK1/TBK1, TBK1/IKKε, or IKKε/IKKε complexes are at play depending on the cellular and stimulus context⁸. Additional complexity may also arise from preferential cellular expression of individual TRAFs, which mediate TBK1/IKKε activation differently⁹. Finally, other PRRs contribute to a transregulation of RIG-I signaling. While the NLR (NOD-like receptors) member NLRP12 controls TRIM25-mediated RIG-I activation and RNF125-mediated RIG-I degradation⁶, RIG-I responses are potentiated through cross-talk with STING signaling⁷.

RIG-I is a promising target for cancer immunotherapy, either as a single agent, or in combination with ICIs. Its major features are its ubiquitous expression and signaling outcomes, notably IFN-I production and preferential tumor cell death, which are two keys factors in potent T cell responses. However, RIG-I-based therapeutic strategies face multiple challenges, such as designing highly specific and stable agonists, and developing efficient agonist delivery modes while avoiding uncontrolled release of pro-inflammatory cytokines. Finally, a deeper understanding of RIG-I signaling in different TME cell types will be required to reach therapeutic success.

References:

1. Iurescia S. et al., 2018. Nucleic acid sensing machinery: targeting innate immune system for cancer therapy. Recent Pat. Anticancer Drug Discov. 13:2.
2. Heidegger S. et al., 2019. RIG-I activating immunostimulatory RNA boosts the efficacy of anticancer vaccines and synergizes with immune checkpoint blockade. EBioMedicine. 41:146.
3. Poeck H., et al. 2008. 5’-triphosphate-siRNA: turning gene silencing and Rig-I activation against melanoma. Nat. Med. 14:1256.
4. Elion DL., et al. 2018. Harnessing RIG-I and intrinsic immunity in the tumor microenvironment for therapeutic cancer treatment. Oncotarget. 9:29007.
5. Ren X. et al., 2019. RIG-I selectively discriminates against 5’-monophosphate RNA. Cell Reports. 26:2019.
6. Chen ST., et al., 2019. NLRP12 regulates anti-viral RIG-I activation via interaction with TRIM25. Cell Host Microbe. 25:602.
7. Zevini A. et al., 2017. Crosstalk between cytoplasmic RIG-I and STING sensing pathways. Trends Immunol. 38:194.
8. Perry AK. et al., 2004. Differential requirement for TANK-binding kinase-1 in type I interferon responses to Toll-like receptor activation and viral infection. J. Exp. Med. 199:1651.
9. Fang R. et al., 2017. MAVS activates TBK1 and IKKe through TRAFs in NEMO dependent and independent manner. PLoS Pathog. 13(11):e1006720.