Introduction
Type I interferon (IFN) plays a critical role in promoting anti-tumor immune responses through pleiotropic stimulatory effects on both immune and non-immune cell populations [
1]. Notably, type I IFN signaling in dendritic cell (DCs) has been shown to be critical for initiation of anti-tumor CD8+ T cell responses by promoting presentation of tumor-derived antigens and expression of pro-inflammatory cytokines and chemokines [
2,
3]. While multiple upstream signals are capable of inducing type I IFN expression, evidence suggests the cyclic GMP-AMP synthase (cGAMP)/Stimulator of interferon genes (STING) DNA sensing pathway is the key mediator of this pathway [
4,
5].
STING is activated following binding of cyclic dinucleotides produced by cGAS in response to cytosolic double-stranded DNA. STING activation results in the phosphorylation of TBK1 and production of type I IFN and downstream interferon stimulatory genes (ISGs) [
6]. In tumor cells, genomic instability may lead to accumulation of cytosolic DNA that induces cGAS-dependent cGAMP production leading to tumor-intrinsic STING activation [
7,
8]. In non-tumor cells, cGAMP released by tumor cells may activate STING in neighboring immune cells within the tumor microenvironment via the SLC19A1 transporter or gap junctions [
9‐
11]. Alternatively, DNA may also be released from dying tumor cells, activating the cGAS/STING pathway in a tumor-extrinsic fashion [
12]. Importantly, the relative contributions of tumor intrinsic and tumor-extrinsic STING activation leading to type I IFN production, and how that IFN production relates to tumor microenvironment inflammation and induction of productive anti-tumor immune responses, has not been well defined.
Here, we utilize two mouse tumor models with distinct immune phenotypes and immunotherapeutic intervention sensitivity to evaluate the varying roles of STING in regulating anti-tumor immunity. MC38 is a C57BL/6 origin colorectal cancer mouse syngeneic cell line that is characterized by robust immune infiltration and responsiveness to anti-PD1 therapy. In contrast, B16F10 is a C57BL/6 origin melanoma mouse syngeneic cell line with low immune cell infiltration and resistance to checkpoint inhibition [
13]. Using mice and tumor cell lines deficient in STING signaling, we define cell context dependent roles for STING in regulating tumor inflammation and response to immune checkpoint blockade (ICB).
Materials and methods
Cell lines
The MC38-STING KO cell line was made through gene editing (Neon Transfection System, Invitrogen). Knockout was confirmed by western blot. Antibodies (Abs): TMEM173 (CST #13647S), β-actin (CST #3700S), anti-rabbit HRP (CST #7074). TMEM173 gRNA sequence: GTACCCAATGTAGTATGACC.
Mice and tumor studies
5–8 week female C57BL/6J (000664) and C57BL/6J-TMEM173gt/J (017537) (Jackson Labs) were used. All experimental studies were conducted under protocols approved by the Institutional Animal Care and Use Committee of Amgen (IACUC). Animals were housed at Association for Assessment and Accreditation of Laboratory Animal Care (AAALAC) International-accredited facilities (at Amgen) in ventilated micro-isolator housing on corncob bedding. Animals had access ad libitum to sterile pelleted food and reverse osmosis-purified water and were maintained on a 12:12 h light:dark cycle with access to environmental enrichment opportunities. 2 × 105 B16F10 (ATCC CRL-6475) or 3 × 105 MC38 or MC38-STING KO cells were injected SC in the mice’ right flank. Tumor volume (mm3; LxWxH) was measured twice a week. Ab treatments, anti-CTLA4 (clone 9D9. 300ug/dose, mIgG2A) and anti-PD1 (clone 29F1A12 Cat# BE0273 in B16F10 studies and clone 29F1A12 mIgG1 N297G backbone in MC38 studies), and isotype antibodies (mIgG1 MOPC21, Cat#BE0083 and mIgG2a C1.18.4, Cat#BE0085 BioXCell) were given IP every 3 days for 3 doses starting at the time of randomization. Animals were randomized based on genotype and starting tumor volumes, typically 100mm3.
Tumor isolation, flow cytometry
Tumors were digested in RPMI + 1% FBS containing 0.2 mg/ml Liberase TL, 20 U/ml DNase I (Roche Diagnostics). Cell staining: Live/Dead (Molecular Probes L23105), FcR-blocking (553142), and antibody cocktail (anti-B220 (RA3-6B2), anti-CD49b (DX5), anti-Thy1.2 (53-2.1), anti-CD25 (PC61), anti-TCRb (H57-597), anti-CD45 (30-F11), anti-CD4 (GK1.5), anti-CD8a (53-6.7), anti-FoxP3 (FJK-16s), Foxp3/transcription factor set (eBioscience 00-5523-00).
Gene expression analyses
Tumors were flash-frozen in liquid nitrogen at indicated days post-implantation. Fluidigm: isolated RNA (Qiagen) were analyzed by Fluidigm (NC9872174) with a custom panel of immune lineage and interferon response gene probes. Gene expression was normalized to the geometric mean of 4 housekeeping genes using ΔCt. An interferon response gene signature was derived using the geometric mean of IFN-inducible genes (CMPK2, CXCL10, HERC5, IFIT1, IRF8, MX1, PD-L1). Nanostring: performed by nCounter XT Gene Expression Assay and the nCounter Mouse PanCancer Immune Profiling Panel gene codeset (SPRINT Profiler) and analysis (nSolver Analysis Software 4.0). RNA sequencing: cDNA library prepared in vitro cultured tumor cells. RNA sequencing reads (Illumina HiSeq platform) were aligned to mouse genome build 38 and transcript per million mapped reads were determined (Array Suite software (Omicsoft), in-house software). Ingenuity Pathway Analysis (IPA; Qiagen) was performed on genes with ≥ 4 expression in MC38 than B16 cells and top 5 canonical pathways by p value reported.
In vitro assays
ELISAs: B16F10, MC38 or MC38-STING KO cells were plated at 1 × 106 cells/ml in 100 μl. Transfected 2′3′-cGAMP (Invivogen), Poly(IC) (Invivogen), Interferon stimulatory DNA (ISD), or recombinant IFNβ (1 × 104 U/ml) for 6 h with Lipofectamine (ThermoFisher). BX795 (TBK1) inhibitor (Invivogen) added at indicated concentrations. Cell supernatants were collected for mCXCL10 ELISA (abcam). ISD sequence and preparation: 5′-TACAGATCTACTAGTGATCTATGACTGATCTGTACATGATCTACA-3′ and its antisense oligonucleotides annealed at 95 °C and cooled to RT.
Statistical analysis
Data were analyzed with GraphPad Prism7 software. P values were calculated by Mantel-Cox test for survival curves or one-way ANOVA with Bonferroni’s multiple comparisons test. Unpaired, two-tailed t-test was used for in vitro assays. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.
Discussion
We find that the cellular source of STING activation can dictate the importance of this pathway in regulating the anti-tumor immune response. The poorly immunogenic B16F10 mouse model has a strong dependence on STING expression in non-tumor cell types. In contrast, in the immunogenic MC38 model, constitutive STING activation in tumor cells can partially bypass the requirement for STING-dependent activity outside of the tumor cell compartment. Thus, a complex relationship likely exists between STING expression and downstream type I interferon responses in tumor cells and associated tumor stromal cell types, including immune cells, fibroblasts, and endothelial cells, that may dictate the baseline immune infiltration status of a tumor and its response to ICB therapy.
While the upstream signals responsible for constitutive STING activation in MC38 cells are unclear, pathways such as genomic instability [
17], activation of endogenous viral elements [
18], defects in endogenous nucleic acid degradation [
19], or deregulation of STING itself [
20] have been implicated in STING activation. Notably, neither TBK1 inhibition nor STING KO led to complete inhibition of constitutive type I IFN gene expression in MC38 cells, suggesting other pathways upstream or downstream of type I IFN signaling may also be dysregulated in these cells. Nevertheless, loss of STING activity in MC38 cells led to reduced tumor inflammation and partial loss of response to anti-PD1 treatment. Interestingly, only after STING signaling was abrogated in MC38 tumor cells did a contribution of STING signaling in non-tumor cell types become important to the anti-tumor immune response. These findings are consistent with dynamic crosstalk between tumor cells and the associated tumor microenvironment and potential negative feedback loops that could suppress STING signaling in one cellular compartment when signaling is high in another compartment.
Baseline type I IFN activity was substantially lower in the relatively non-immunogenic B16F10 model than in the MC38 model, both in in vitro cell culture and in vivo tumors. Lower overall levels of STING activation and downstream type I IFN signaling may be partially responsible for the resistance of B16F10 tumors to CPI monotherapy, and, based on our findings in the MC38 model, increasing STING activity in B16F10 tumor cells could reverse the resistance phenotype. Consistent with this hypothesis, STING agonists have shown improved efficacy in combination with PD1 blockade in multiple models, including B16F10 and KPC allografts [
21‐
23]. Clinically, reduced expression of cGAS and STING in tumor cells correlates with poor survival in gastric cancer patients [
24]. The benefit of these regimens, as indicated by data here, may depend on the compartments contributing to STING signaling and their crosstalk. Indeed, STING signaling in astrocytes was shown to promote tumor growth and metastasis [
25].
The present study reveals diverse and context dependent roles of STING in regulating anti-tumor immunity and highlights the need to further define the contributions of nucleic acid sensing pathways in both tumor and non-tumor cell compartments to baseline tumor inflammation and response to immunotherapy. Whether direct STING agonism or molecules that increase DNA damage, such as PARP inhibitors [
26,
27] or even chemotherapy [
28], and lead to STING activation, is preferred requires further preclinical and clinical exploration and may be context dependent. In addition, single cell studies of the tumor microenvironment should clarify whether STING signaling is heterogeneous and subject to evolution – previous work has shown that epigenetic silencing of STING can occur [
29] and this could be a biomarker for tumors that would respond differentially to STING agonists.
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