Background Resistance to an immune checkpoint inhibitor (ICI) is a major obstacle in cancer immunotherapy. The causes of ICI resistance include major histocompatibility complex (MHC)/histocompatibility locus antigen (HLA) class I loss, neoantigen loss, and incomplete antigen presentation. Elimination by natural killer (NK) cells would be expected to be an effective strategy for the treatment of these ICI-resistant tumors. We previously demonstrated that a lipid nanoparticle containing a stimulator of an interferon gene (STING) agonist (STING-LNP) efficiently induced antitumor activity via the activation of NK cells. Thus, we evaluated the potential of reducing ICI resistance by STING-LNPs. Methods Lung metastasis of a B16-F10 mouse melanoma was used as an anti-programmed cell death 1 (anti-PD-1)-resistant mouse model. The mice were intravenously injected with the STING-LNP and the mechanism responsible for the improvement of anti-PD-1 resistance by the STING-LNPs was analyzed by RT-qPCR and flow cytometry. The dynamics of STING-LNP were also investigated. Results Although anti-PD-1 monotherapy failed to induce an antitumor effect, the combination of the STING-LNP and anti-PD-1 exerted a synergistic antitumor effect. Our results indicate that the STING-LNP treatment significantly increased the expression of CD3, CD4, NK1.1, PD-1 and interferon (IFN)-γ in lung metastases. This change appears to be initiated by the type I IFN produced by liver macrophages that contain the internalized STING-LNPs, leading to the systemic activation of NK cells that express PD-1. The activated NK cells appeared to produce IFN-γ, resulting in an increase in the expression of the PD ligand 1 (PD-L1) in cancer cells, thus leading to a synergistic antitumor effect when anti-PD-1 is administered. Conclusions We provide a demonstration to show that a STING-LNP treatment can overcome PD-1 resistance in a B16-F10 lung metastasis model. The mechanism responsible for this indicates that NK cells are activated by stimulating the STING pathway which, in turn, induced the expression of PD-L1 on cancer cells. Based on the findings reported herein, the STING-LNP represents a promising candidate for use in combination therapy with anti-PD-1-resistant tumors.

Applying small interfering RNA (siRNA) to dendritic cell (DC) based therapy represents a potential candidate for cancer immunotherapy. However, delivering siRNA to DCs is a challenging issue for non-viral vectors. To date, only viral vectors have achieved efficient gene silencing in DCs. We report herein that a novel cationic lipid, YSK12-C4, when loaded in a nanoparticle with siRNA (YSK12-C4 multifunctional envelope type nano device [YSK12-MEND]), greatly facilitated gene silencing in mouse DCs. The use of the YSK12-MEND resulted in a gene silencing efficiency in excess of 90%, with a median effective dose (ED50) of 1.5nM, whereas the maximum gene silencing efficiency of Lipofectamine RNAiMAX was less than 60% and the ED50 was 25nM. Furthermore, suppressor of cytokine signaling 1, an immune suppressive molecule in DCs, silenced in the mouse DC by the YSK12-MEND showed a drastic enhancement in cytokine production, resulting in the significant suppression of tumor growth when it was applied to DC-based therapy against a mouse lymphoma. These results clearly indicate that YSK12-MEND overcomes the obstacle associated with non-viral vectors and can be considered to be a promising non-viral vector for siRNA delivery to DCs, thus accelerating DC-based therapies with siRNA.

Cell-based therapy using dendritic cells (DC) represents a potent cancer immunotherapy. However, activated DC express indoleamine 2,3-dioxygenase 1 (IDO1), a counter-regulatory and tolerogenic molecule, leading to the inhibition of T cell activation and the promotion of T cell differentiation into regulatory T cells. Silencing the IDO1 gene in DC by small interfering RNA (siRNA) represents a potent therapeutic strategy. We report on the successful and efficient introduction of a siRNA targeting IDO1 into mouse DCs by a means of a multifunctional envelope-type nanodevice (MEND) containing a YSK12-C4 (YSK12-MEND). The YSK12-C4 has both fusogenic and cationic properties. The YSK12-MEND induced an effective level of gene silencing of IDO1 at siRNA doses in the range of 1–20 nM, a concentration that commercially available transfection reagents are not able to silence. The YSK12-MEND mediated IDO1 silencing had no effect on the characteristic determinants of DC phenotype such as CD11c, CD80 and MHC class II. The silencing of IDO1 in DC by the YSK12-MEND significantly enhanced the antitumor effect against E.G7-OVA tumor. Moreover, a decrease in the numbers of regulatory T cells in the tumor was observed in mice that were treated with the IDO1-silenced DC. The YSK12-MEND appears to be a potent delivery system for IDO1-silenced DC based cancer immunotherapy.