The desmoplastic stroma in solid tumors presents a formidable challenge to immunotherapies that rely on endogenous or adoptively transferred T cells, however, the mechanisms are poorly understood. To define mechanisms involved, here we treat established desmoplastic pancreatic tumors with CAR T cells directed to fibroblast activation protein (FAP), an enzyme highly overexpressed on a subset of cancer-associated fibroblasts (CAFs). Depletion of FAP⁺ CAFs results in loss of the structural integrity of desmoplastic matrix. This renders these highly treatment-resistant cancers susceptible to subsequent treatment with a tumor antigen (mesothelin)-targeted CAR T cells and to anti-PD-1 antibody therapy. Mechanisms include overcoming stroma-dependent restriction of T cell extravasation and/or perivascular invasion, reversing immune exclusion, relieving T cell suppression, and altering the immune landscape by reducing myeloid cell accumulation and increasing endogenous CD8⁺ T cell and NK cell infiltration. These data provide strong rationale for combining tumor stroma- and malignant cell-targeted therapies to be tested in clinical trials.

Clinical trials repurposing lysosomotropic chloroquine (CQ) derivatives as autophagy inhibitors in cancer demonstrate encouraging results, but the underlying mechanism of action remains unknown. Here, we report a novel dimeric CQ (DC661) capable of deacidifying the lysosome and inhibiting autophagy significantly better than hydroxychloroquine (HCQ). Using an in situ photoaffinity pulldown strategy, we identified palmitoyl-protein thioesterase 1 (PPT1) as a molecular target shared across monomeric and dimeric CQ derivatives. HCQ and Lys05 also bound to and inhibited PPT1 activity, but only DC661 maintained activity in acidic media. Knockout of PPT1 in cancer cells using CRISPR/Cas9 editing abrogates autophagy modulation and cytotoxicity of CQ derivatives, and results in significant impairment of tumor growth similar to that observed with DC661. Elevated expression of PPT1 in tumors correlates with poor survival in patients in a variety of cancers. Thus, PPT1 represents a new target in cancer that can be inhibited with CQ derivatives. Significance: This study identifies PPT1 as the previously unknown lysosomal molecular target of monomeric and dimeric CQ derivatives. Genetic suppression of PPT1 impairs tumor growth, and PPT1 levels are elevated in cancer and associated with poor survival. These findings provide a strong rationale for targeting PPT1 in cancer. This article is highlighted in the In This Issue feature, p. 151

Resistance to BRAF and MEK inhibitors (BRAFi + MEKi) in BRAF-mutant tumors occurs through heterogeneous mechanisms, including ERK reactivation and autophagy. Little is known about the mechanisms by which ERK reactivation or autophagy is induced by BRAFi + MEKi. Here, we report that in BRAF-mutant melanoma cells, BRAFi + MEKi induced SEC61-dependent endoplasmic reticulum (ER) translocation of the MAPK pathway via GRP78 and KSR2. Inhibition of ER translocation prevented ERK reactivation and autophagy. Following ER translocation, ERK exited the ER and was rephosphorylated by PERK. Reactivated ERK phosphorylated ATF4, which activated cytoprotective autophagy. Upregulation of GRP78 and phosphorylation of ATF4 were detected in tumors of patients resistant to BRAFi + MEKi. ER translocation of the MAPK pathway was demonstrated in therapy-resistant patient-derived xenografts. Expression of a dominant-negative ATF4 mutant conferred sensitivity to BRAFi + MEKi in vivo. This mechanism reconciles two major targeted therapy resistance pathways and identifies druggable targets, whose inhibition would likely enhance the response to BRAFi + MEKi. Significance: ERK reactivation and autophagy are considered distinct resistance pathways to BRAF + MEK inhibition (BRAFi + MEKi) in BRAF^V600E cancers. Here, we report BRAFi + MEKi–induced ER translocation of the MAPK pathway is necessary for ERK reactivation, which drives autophagy. The ER translocation mechanism is a major druggable driver of resistance to targeted therapy. This article is highlighted in the In This Issue feature, p. 305

New strategies are needed to enhance the efficacy of anti–programmed cell death protein antibody (anti–PD-1 Ab) in cancer. Here, we report that inhibiting palmitoyl-protein thioesterase 1 (PPT1), a target of chloroquine derivatives like hydroxychloroquine (HCQ), enhances the antitumor efficacy of anti–PD-1 Ab in melanoma. The combination resulted in tumor growth impairment and improved survival in mouse models. Genetic suppression of core autophagy genes, but not Ppt1 , in cancer cells reduced priming and cytotoxic capacity of primed T cells. Exposure of antigen-primed T cells to macrophage-conditioned medium derived from macrophages treated with PPT1 inhibitors enhanced melanoma-specific killing. Genetic or chemical Ppt1 inhibition resulted in M2 to M1 phenotype switching in macrophages. The combination was associated with a reduction in myeloid-derived suppressor cells in the tumor. Ppt1 inhibition by HCQ, or DC661, induced cyclic GMP-AMP synthase/stimulator of interferon genes/TANK binding kinase 1 pathway activation and the secretion of interferon-β in macrophages, the latter being a key component for augmented T cell–mediated cytotoxicity. Genetic Ppt1 inhibition produced similar findings. These data provide the rationale for this combination in melanoma clinical trials and further investigation in other cancers.

The rough morphotypes of non-tuberculous mycobacteria have been associated with the most severe illnesses in humans. This idea is consistent with the fact that Mycobacterium tuberculosis presents a stable rough morphotype. Unlike smooth morphotypes, the bacilli of rough morphotypes grow close together, leaving no spaces among them and forming large aggregates (clumps). Currently, the initial interaction of macrophages with clumps remains unclear. Thus, we infected J774 macrophages with bacterial suspensions of rough morphotypes of Mycobacterium abscessus containing clumps and suspensions of smooth morphotypes, primarily containing isolated bacilli. Using confocal laser scanning microscopy and electron microscopy, we observed clumps of at least 5 rough-morphotype bacilli inside the phagocytic vesicles of macrophages at 3 hours post-infection. These clumps grew within the phagocytic vesicles, killing 100% of the macrophages at 72 hours post-infection, whereas the proliferation of macrophages infected with smooth morphotypes remained unaltered at 96 hours post-infection. Thus, macrophages phagocytose large clumps, exceeding the bactericidal capacities of these cells. Furthermore, proinflammatory cytokines and granuloma-like structures were only produced by macrophages infected with rough morphotypes. Thus, the present study provides a foundation for further studies that consider mycobacterial clumps as virulence factors.

Purpose: Despite the success of chimeric antigen receptor (CAR) T-cell therapy against hematologic malignancies, successful targeting of solid tumors with CAR T cells has been limited by a lack of durable responses and reports of toxicities. Our understanding of the limited therapeutic efficacy in solid tumors could be improved with quantitative tools that allow characterization of CAR T–targeted antigens in tumors and accurate monitoring of response. Experimental Design: We used a radiolabeled FAP inhibitor (FAPI) [18F]AlF-FAPI-74 probe to complement ongoing efforts to develop and optimize FAP CAR T cells. The selectivity of the radiotracer for FAP was characterized in vitro, and its ability to monitor changes in FAP expression was evaluated using rodent models of lung cancer. Results: [18F]AlF-FAPI-74 showed selective retention in FAP+ cells in vitro, with effective blocking of the uptake in presence of unlabeled FAPI. In vivo, [18F]AlF-FAPI-74 was able to detect FAP expression on tumor cells as well as FAP+ stromal cells in the tumor microenvironment with a high target-to-background ratio. We further demonstrated the utility of the tracer to monitor changes in FAP expression following FAP CAR T-cell therapy, and the PET imaging findings showed a robust correlation with ex vivo analyses. Conclusions: This noninvasive imaging approach to interrogate the tumor microenvironment represents an innovative pairing of a diagnostic PET probe with solid tumor CAR T-cell therapy and has the potential to serve as a predictive and pharmacodynamic response biomarker for FAP as well as other stroma-targeted therapies. A PET imaging approach targeting FAP expressed on activated fibroblasts of the tumor stroma has the potential to predict and monitor therapeutic response to FAP-targeted CAR T-cell therapy. See related commentary by Weber et al., p. 5241