Purpose Tyrosine kinase inhibitors are effective in gastrointestinal stromal tumor (GIST), but often are of transient benefit as resistance commonly develops. Immunotherapy, particularly blockade of the inhibitory receptor programmed death 1 (PD-1) or the ligand programmed death ligand 1 (PD-L1), has shown effectiveness in a variety of cancers. The functional effects of PD-1/PD-L1 blockade are unknown in GIST. Experimental Design We analyzed tumor and matched blood samples from 85 patients with GIST and determined the expression of immune checkpoint molecules using flow cytometry. We investigated the combination of imatinib with PD-1/PD-L1 blockade in KitV558Δ/+ mice that develop GIST. Results The inhibitory receptors PD-1, lymphocyte activation gene 3 (LAG-3), and T cell immunoglobulin mucin-3 (TIM-3) were upregulated on tumor-infiltrating T cells compared to T cells from matched blood. PD-1 expression on T cells was highest in imatinib-treated human GISTs. Meanwhile, intratumoral PD-L1 expression was variable. In human GIST cell lines, treatment with imatinib abrogated the IFN-γ–induced upregulation of PD-L1 via STAT1 inhibition. In KitV558Δ/+ mice imatinib downregulated IFN-γ–related genes and reduced PD-L1 expression on tumor cells. PD-1 and PD-L1 blockade in vivo each had no efficacy alone, but enhanced the antitumor effects of imatinib by increasing T cell effector function in the presence of KIT and IDO inhibition. Conclusions PD-1/PD-L1 blockade is a promising strategy to improve the effects of targeted therapy in GIST. Collectively, our results provide the rationale to combine these agents in human GIST.

Human DNA-PK activates a STING-independent DNA sensing pathway Recognition of foreign nucleic acids is critical for antiviral defense. Detection of DNA is mediated by the cGAS-STING pathway, which activates a potent type I interferon response. This pathway is broadly required for antiviral defense across cell types and species, and its relevance in context of infection, cancer, and autoimmunity has been thoroughly established. However, we have discovered an additional, STING-independent DNA sensing pathway (SIDSP) in human cells. Using STING KO human cell lines, we find that the induction of interferon is indeed abrogated at early timepoints, but surprisingly, at later timepoints, we observe a robust DNAinduced interferon response. Here we identify DNA-PK as the sensor for the SIDSP and demonstrate that its kinase activity is required for the antiviral response. We show that a heat shock protein HSPA8/HSC70 is phosphorylated after DNA stimulation and acts as a marker for the SIDSP. Finally, we explore how DNA viruses antagonize both the cGAS-STING pathway and the SIDSP. Our work highlights the importance of nucleic acid sensing for both host and virus and has implications for modulating DNA sensing in order to improve therapies for cancer or autoimmunity.

Tyrosine kinase inhibition of gastrointestinal stromal tumors (GIST) is effective but typically culminates in resistance and is rarely curative. Immunotherapy has potential application to GIST, as we previously showed that T-cell checkpoint blockade increases the antitumor effects of imatinib. Here, we showed that ligation of CD40 using an agonistic antibody (anti-CD40) activated tumor-associated macrophages (TAMs) in vivo in a knock-in mouse model of GIST harboring a germline mutation in Kit exon 11. Activated TAMs had greater TNF production and NFκB signaling and directly inhibited tumor cells in vitro. Anti-CD40 required concomitant therapy with imatinib for efficacy and depended on TAMs, and to a lesser extent CD8+ T cells, but not on CD4+ T cells or B cells. In an analysis of 50 human GIST specimens by flow cytometry, we found that CD40 was expressed on human TAMs and tumor cells yet was downregulated after response to imatinib. CD40 ligation did not have a direct inhibitory effect on human GIST cells. Our findings provide the rationale for combining anti-CD40 and tyrosine kinase inhibition to treat human GIST.

Gastrointestinal stromal tumor (GIST) is the most common type of sarcoma and usually harbors either a KIT or PDGFRA mutation. However, the molecular basis for tumor malignancy is not well defined. While the Wnt/β-catenin signaling pathway is important in a variety of cancers, its role in GIST is uncertain. Through analysis of nearly 150 human GIST specimens, we found that some human GISTs expressed β-catenin and contained active, dephosphorylated nuclear β-catenin. Furthermore, advanced human GISTs expressed reduced levels of the Wnt antagonist DKK4. Accordingly, in human GIST T1 cells, Wnt stimulation increased β-catenin–mediated transcriptional activity in a reporter assay as well as transcription of the downstream target genes axin2 and cyclin D1. In contrast, DKK4 overexpression in GIST T1 cells reduced Wnt/β-catenin signaling. Additionally, we showed that nuclear β-catenin stability was partially regulated by the E3 ligase COP1, as demonstrated with co-immunoprecipitation and COP1 knockdown. Three molecular inhibitors of the Wnt/β-catenin pathway demonstrated anti-tumor efficacy in various GIST models, both in vitro and in vivo. Notably, the tankyrase inhibitor G007-LK alone had substantial activity against tumors of genetically engineered KitV558Δ/+ mice, and the effect was increased by the addition of the Kit inhibitor imatinib mesylate. Collectively, our findings demonstrate that Wnt/β-catenin signaling is a novel therapeutic target for selected untreated or imatinib-resistant GISTs.

Detection of intracellular DNA by the cGAS-STING pathway activates a type I interferon-mediated innate immune response that protects from virus infection and can be harnessed to promote anti-tumor immunity. Whether there are additional DNA sensing pathways, and how such pathways might function, remains controversial. We show here that humans -but not mice -have a second, potent, STINGindependent DNA sensing pathway that is blocked by the E1A viral oncogene of human adenovirus 5.We identify human DNA-PK as the sensor of this pathway and demonstrate that DNA-PK kinase activity drives a robust and broad antiviral response. We discover that the heat shock protein HSPA8/HSC70 is a unique target of DNA-PK. Finally, we demonstrate that detection of foreign DNA and DNA damage trigger distinct modalities of DNA-PK activity. These findings reveal the existence, sensor, unique target, and viral antagonists of a STING-independent DNA sensing pathway (SIDSP) in human cells. IntroductionThe cGAS-STING DNA sensing pathway has emerged as a key component of the innate immune response that is important for antiviral immunity (1), contributes to specific autoimmune diseases (2), and mediates important aspects of antitumor immunity (3). cGAS binds to double-stranded DNA and catalyzes the formation of cyclic GMP-AMP (4, 5), a diffusible cyclic dinucleotide that activates the endoplasmic adaptor protein STING (6). Activated STING then serves as a platform for the inducible recruitment of the TBK1 kinase, which phosphorylates and activates the transcription factor IRF3, leading to the induction of the type I interferon mediated antiviral response (7).Nearly all studies on the cGAS-STING pathway involve the use of mice and mouse cells.Knockouts of cGAS (Mb21d1) and STING (Tmem173) have clearly demonstrated that both are essential for the transcriptional and cell biological responses to foreign intracellular DNA, and that they mediate the pathology of specific autoimmune diseases (2,8,9). However, numerous additional sensors of intracellular DNA have been proposed, all of which are thought to act upstream of STING (10). Whether STING-independent DNA sensing exists is currently unknown.Here, we report the unexpected finding that the E1A oncogene of human adenovirus 5 blocks two distinct DNA sensing pathways in human cells: the well-known cGAS-STING pathway (11), and a second, STING-independent DNA sensing pathway (SIDSP). We identify the DNA damage response protein DNA-PK as the sensor of the SIDSP, along with the heat shock protein HSPA8 as a unique SIDSP target. We show that the SIDSP is potently activated in human and primate cells, but it is weak or absent from mouse cells. Our findings demonstrate that human cells have a second DNA sensing pathway, with implications for host defense, autoimmunity, and anti-tumor immunity. Results Human adenovirus 5 E1A blocks two DNA sensing pathways in human cellsWe previously demonstrated that the viral oncogenes of the DNA tumor viruses are potent antagonists of the cGAS-STING DNA sensing pathway ...

Medina et al. demonstrate that Kit oncogene activity in gastrointestinal stromal tumor modulates the CD103+CD11b− dendritic cell (DC) lineage. The antitumor efficacy of oncogene inhibition initially depends on preexisting immunity orchestrated by CD103+CD11b− DCs, but subsequently is limited by a decrease in DC lineage maturation.

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Introduction Murine Kupffer cells (KCs) comprise CD11b hi and F4/80 hi subsets. Tissue-resident macrophages are known to express the tyrosine kinase receptors colony-stimulating factor 1 receptor (Csf1r) and Mer. However, the expression of Csf1r and Mer on KC subsets and the importance of these tyrosine kinases during liver regeneration (LR) are unknown. Methods KCs from wild-type and Csf1r-GFP mice were characterized by flow cytometry. Partial hepatectomy (PH) was performed in mice treated with clodronate liposomes, a Csf1r small molecule inhibitor or depleting antibody, or a small molecule Mer inhibitor. Sera and livers were analyzed. The function of sorted KC subsets was tested in vitro. Results Mer was specifically expressed on tissue-resident F4/80 hi KCs, 55% of which also expressed Csf1r. Mer + Csf1r + and Mer + Csf1r - KCs had distinct expression of macrophage markers. Csf1r inhibition in mice reduced F4/80 hi KCs by approximately 50%, but did not affect CD11b hi KCs. Clodronate liposomes depleted F4/80 hi KCs, but also altered levels of other intrahepatic leukocytes. Csf1r inhibition delayed LR, as demonstrated by a 20% reduction in liver-to-body weight ratios 7 days after PH. At 36h after PH, Csf1r inhibition increased serum ALT and histological liver injury, and decreased liver cell proliferation. A small molecule inhibitor of Mer did not alter the percentage of KCs or their proliferation and just modestly delayed LR. In vitro, Csf1r or Mer inhibition did not decrease KC viability, but did attenuate their cytokine response to stimulation. Conclusions F4/80 hi KCs are Mer + and can be subdivided based on Csf1r expression. Csf1r or Mer inhibition each reduces KC cytokine production and delays LR.