Dabrafenib Alters MDSC Differentiation and Function by Activation of GCN2

The effect of targeted therapeutics on anticancer immune responses is poorly understood. The BRAF inhibitor dabrafenib has been reported to activate the integrated stress response (ISR) kinase GCN2, and the therapeutic effect has been partially attributed to GCN2 activation. Because ISR signaling is a key component of myeloid-derived suppressor cell (MDSC) development and function, we measured the effect of dabrafenib on MDSC differentiation and suppressive activity. Our data showed that dabrafenib attenuated MDSC ability to suppress T-cell activity, which was associated with a GCN2-dependent block of the transition from monocytic progenitor to polymorphonuclear (PMN)-MDSCs and proliferative arrest resulting in PMN-MDSC loss. Transcriptional profiling revealed that dabrafenib-driven GCN2 activation altered metabolic features in MDSCs enhancing oxidative respiration, and attenuated transcriptional programs required for PMN development. Moreover, we observed a broad downregulation of transcriptional networks associated with PMN developmental pathways, and increased activity of transcriptional regulons driven by Atf5, Mafg, and Zbtb7a. This transcriptional program alteration underlies the basis for PMN-MDSC developmental arrest, skewing immature MDSC development toward monocytic lineage cells. In vivo, we observed a pronounced reduction in PMN-MDSCs in dabrafenib-treated tumor-bearing mice suggesting that dabrafenib impacts MDSC populations systemically and locally, in the tumor immune infiltrate. Thus, our data reveal transcriptional networks that govern MDSC developmental programs, and the impact of GCN2 stress signaling on the innate immune landscape in tumors, providing novel insight into potentially beneficial off-target effects of dabrafenib.

SIGNIFICANCE

An important, but poorly understood, aspect of targeted therapeutics for cancer is the effect on antitumor immune responses. This article shows that off-target effects of dabrafenib activating the kinase GCN2 impact MDSC development and function reducing PMN-MDSCs in vitro and in vivo. This has important implications for our understanding of how this BRAF inhibitor impacts tumor growth and provides novel therapeutic target and combination possibilities.

Dabrafenib alters MDSC differentiation and function by activation of GCN2

The effect of targeted therapeutics on anti-cancer immune responses is poorly understood. The BRAF inhibitor dabrafenib has been reported to activate the integrated stress response (ISR) kinase GCN2, and the therapeutic effect has been partially attributed to GCN2 activation. Since ISR signaling is a key component of myeloid-derived suppressor cell (MDSC) development and function, we measured the effect of dabrafenib on MDSC differentiation and suppressive activity. Our data showed that dabrafenib attenuated MDSC ability to suppress T cell activity, which was associated with a GCN2-dependent block of the transition from monocytic progenitor to polymorphonuclear (PMN)-MDSCs and proliferative arrest resulting in PMN-MDSC loss. Transcriptional profiling revealed that dabrafenib-driven GCN2 activation altered metabolic features in MDSCs enhancing oxidative respiration, and attenuated transcriptional programs required for PMN development. Moreover, we observed a broad downregulation of transcriptional networks associated with PMN developmental pathways, and increased activity of transcriptional regulons driven by Atf5 , Mafg , and Zbtb7a . This transcriptional program alteration underlies the basis for PMN-MDSC developmental arrest, skewing immature MDSC development towards monocytic lineage cells. In vivo , we observed a pronounced reduction in PMN-MDSCs in dabrafenib-treated tumor-bearing mice suggesting that dabrafenib impacts MDSC populations systemically and locally, in the tumor immune infiltrate. Thus, our data reveals transcriptional networks that govern MDSC developmental programs, and the impact of GCN2 stress signaling on the innate immune landscape in tumors, providing novel insight into potentially beneficial off target effects of dabrafenib.

The transcription factor IRF2 drives interferon-mediated CD8+ T cell exhaustion to restrict anti-tumor immunity

Type I and II interferons (IFNs) stimulate pro-inflammatory programs that are critical for immune activation, but also induce immune-suppressive feedback circuits that impede control of cancer growth. Here, we sought to determine how these opposing programs are differentially induced. We demonstrated that the transcription factor interferon regulatory factor 2 (IRF2) was expressed by many immune cells in the tumor in response to sustained IFN signaling. CD8+ T cell-specific deletion of IRF2 prevented acquisition of the T cell exhaustion program within the tumor and instead enabled sustained effector functions that promoted long-term tumor control and increased responsiveness to immune checkpoint and adoptive cell therapies. The long-term tumor control by IRF2-deficient CD8+ T cells required continuous integration of both IFN-I and IFN-II signals. Thus, IRF2 is a foundational feedback molecule that redirects IFN signals to suppress T cell responses and represents a potential target to enhance cancer control.

Tryptophan-derived microbial metabolites activate the aryl hydrocarbon receptor in tumor-associated macrophages to suppress anti-tumor immunity

The aryl hydrocarbon receptor (AhR) is a sensor of products of tryptophan metabolism and a potent modulator of immunity. Here, we examined the impact of AhR in tumor-associated macrophage (TAM) function in pancreatic ductal adenocarcinoma (PDAC). TAMs exhibited high AhR activity and Ahr-deficient macrophages developed an inflammatory phenotype. Deletion of Ahr in myeloid cells or pharmacologic inhibition of AhR reduced PDAC growth, improved efficacy of immune checkpoint blockade, and increased intra-tumoral frequencies of IFNγ⁺CD8⁺ T cells. Macrophage tryptophan metabolism was not required for this effect. Rather, macrophage AhR activity was dependent on Lactobacillus metabolization of dietary tryptophan to indoles. Removal of dietary tryptophan reduced TAM AhR activity and promoted intra-tumoral accumulation of TNFα⁺IFNγ⁺CD8⁺ T cells; provision of dietary indoles blocked this effect. In patients with PDAC, high AHR expression associated with rapid disease progression and mortality, as well as with an immune-suppressive TAM phenotype, suggesting conservation of this regulatory axis in human disease.

Abstract 3153: Targeting the stress response kinase GCN2 to restore immunity in the tumor microenvironment

Recent advances in cancer metabolism suggest that targeting amino acid metabolism represents a promising strategy for the development of novel therapeutic agents. Tumor, stromal and myeloid-derived suppressor cells (MDSC) within the tumor microenvironment (TME) create a nutrient-poor environment that inhibit immune function and support tumor growth. GCN2 (general control nonderepressible 2), a stress response kinase, plays a key role in maintaining cellular homeostasis under a wide range of stressors. Phosphorylation of GCN2 (pGCN2) in response to stress leads to inhibition of global protein synthesis and subsequently leads to 1) T cell anergy and apoptosis, 2) enhanced MDSC-dependent immune suppression and 3) tumor cell survival. Collectively, these roles suggest that GCN2 inhibition could have both a direct anticancer and an immune-activating effect. Treating nutrient-deprived T cells in vitro with a RAPT GCN2 inhibitor (RPT-GCN2i) rescued CD4+ and CD8+ T cell proliferation and effector functions. The RPT-GCN2i also reversed T cell suppression mediated by MDSCs derived from healthy donors or cancer patients. Using syngeneic mouse tumor models, we demonstrated that translational induction of activating transcription factor 4 (ATF4) is a strong marker of GCN2 pathway activation in vivo. Oral administration of an RPT-GCN2i exhibited notable drug-target occupancy and potently inhibited GCN2 kinase and ATF4 in the TME. RPT-GCN2i as a single agent and in combination with checkpoint blockade or angiogenesis inhibitor (anti-VEGFR) led to delays in tumor growth rate in various syngeneic tumor models. In addition, GCN2 inhibition redirected MDSC within the TME from a suppressive to inflammatory phenotype through downregulation of Arg1 and iNOS. Our results show that inhibition of GCN2 is an attractive approach for enhancing antitumor immune response and therefore GCN2 is a promising therapeutic target for the treatment of cancer.

Citation Format: Lisa Marshall, Buvana Ravishankar, Urvi Kolhatkar, Mengshu Xu, Lavanya Adusumilli, Deepa Pookot, Thant Zaw, Chandru Ramana, Raashi Sreenivasan, Mikhail Zibinsky, Jeffrey Jackson, Grant Shibuya, Paul Leger, Omar Robles, Anqi Ma, Andrew Ng, Anton Shakhmin, Justy Guagua, Scott Jacobson, Steve Wong, Delia Bradford, Tracy L. McGaha, M Teresa Ciudad, James E. Talmadge, Holly C. Britton, George Katibah, Gene Cutler, David Wustrow, Paul D. Kassner, Dirk G. Brockstedt. Targeting the stress response kinase GCN2 to restore immunity in the tumor microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3153.

GCN2 drives macrophage and MDSC function and immunosuppression in the tumor microenvironment

General control nonderepressible 2 (GCN2) is an environmental sensor controlling transcription and translation in response to nutrient availability. Although GCN2 is a putative therapeutic target for immuno-oncology, its role in shaping the immune response to tumors is poorly understood. Here, we used mass cytometry, transcriptomics, and transcription factor-binding analysis to determine the functional impact of GCN2 on the myeloid phenotype and immune responses in melanoma. We found that myeloid-lineage deletion of GCN2 drives a shift in the phenotype of tumor-associated macrophages and myeloid-derived suppressor cells (MDSCs) that promotes antitumor immunity. Time-of-flight mass cytometry (CyTOF) and single-cell RNA sequencing showed that this was due to changes in the immune microenvironment with increased proinflammatory activation of macrophages and MDSCs and interferon-γ expression in intratumoral CD8⁺ T cells. Mechanistically, GCN2 altered myeloid function by promoting increased translation of the transcription factor CREB-2/ATF4, which was required for maturation and polarization of macrophages and MDSCs in both mice and humans, whereas targeting Atf4 by small interfering RNA knockdown reduced tumor growth. Last, analysis of patients with cutaneous melanoma showed that GCN2-dependent transcriptional signatures correlated with macrophage polarization, T cell infiltrates, and overall survival. Thus, these data reveal a previously unknown dependence of tumors on myeloid GCN2 signals for protection from immune attack.

Analysis of the HLA-DR peptidome from human dendritic cells reveals high affinity repertoires and nonconventional pathways of peptide generation

Dendritic cells (DCs) are the major professional APCs of the immune system; however, their MHC-II-associated peptide repertoires have been hard to analyze, mostly because of their scarce presence in blood and tissues. In vitro matured human monocyte-derived DCs (MoDCs) are widely used as professional APCs in experimental systems. In this work, we have applied mass spectrometry to identify the HLA-DR-associated self-peptide repertoires from small numbers of mature MoDCs (∼5 × 10⁶ cells), derived from 7 different donors. Repertoires of 9 different HLA-DR alleles were defined from analysis of 1319 peptides, showing the expected characteristics of MHC-II-associated peptides. Most peptides identified were predicted high binders for their respective allele, formed nested sets, and belonged to endo-lysosomal pathway-degraded proteins. Approximately 20% of the peptides were derived from cytosolic and nuclear proteins, a recurrent finding in HLA-DR peptide repertoires. Of interest, most of these peptides corresponded to single sequences, did not form nested sets, and were located at the C terminus of the parental protein, which suggested alternative processing. Analysis of cleavage patterns for terminal peptides predominantly showed aspartic acid before the cleavage site of both C- and N-terminal peptides and proline immediately after the cleavage site in C-terminal peptides. Proline was also frequent next to the cut sites of internal peptides. These data provide new insights into the Ag processing capabilities of DCs. The relevance of these processing pathways and their contribution to response to infection, tolerance induction, or autoimmunity deserve further analysis.

Central T cell tolerance: Identification of tissue-restricted autoantigens in the thymus HLA-DR peptidome

Promiscuous gene expression (pGE) of tissue-restricted self-antigens (TRA) in medullary thymic epithelial cells (mTECs) is in part driven by the Autoimmune Regulator gene (AIRE) and essential for self-tolerance. The link between AIRE functional mutations and multi-organ autoimmunity in human and mouse supports the role of pGE. Deep sequencing of the transcriptome revealed that mouse mTECs potentially transcribe an unprecedented range of >90% of all genes. Yet, it remains unclear to which extent these low-level transcripts are actually translated into proteins, processed and presented by thymic APCs to induce tolerance. To address this, we analyzed the HLA-DR-associated thymus peptidome. Within a large panel of peptides from abundant proteins, two TRA peptides were identified: prostate-specific semenogelin-1 (an autoantigen in autoimmune chronic prostatitis/chronic pelvic pain syndrome) and central nervous system-specific contactin-2 (an autoantigen in multiple sclerosis). Thymus expression of both genes was restricted to mTECs. SEMG1 expression was confined to mature HLA-DR(hi) mTECs of male and female donors and was AIRE-dependent, whereas CNTN2 was apparently AIRE-independent and was expressed by both populations of mTECs. Our findings establish a link between pGE, MHC-II peptide presentation and autoimmunity for bona fide human TRAs.

Divergent paths for the selection of immunodominant epitopes from distinct antigenic sources

Immunodominant epitopes are few selected epitopes from complex antigens that initiate T cell responses. Here, to provide further insights into this process, we use a reductionist cell-free antigen processing system composed of defined components. We use the system to characterize steps in antigen processing of pathogen-derived proteins or autoantigens and we find distinct paths for peptide processing and selection. Autoantigen-derived immunodominant epitopes are resistant to digestion by cathepsins, whereas pathogen-derived epitopes are sensitive. Sensitivity to cathepsins enforces capture of pathogen-derived epitopes by Major Histocompatibility Complex class II (MHC class II) prior to processing, and resistance to HLA-DM-mediated-dissociation preserves the longevity of those epitopes. We show that immunodominance is established by higher relative abundance of the selected epitopes, which survive cathepsin digestion either by binding to MHC class II and resisting DM-mediated-dissociation, or being chemically resistant to cathepsins degradation. Non-dominant epitopes are sensitive to both DM and cathepsins and are destroyed.

The Repertoires of Peptides Presented by MHC-II in the Thymus and in Peripheral Tissue: A Clue for Autoimmunity?

T-cell tolerance to self-antigens is established in the thymus through the recognition by developing thymocytes of self-peptide-MHC complexes and induced and maintained in the periphery. Efficient negative selection of auto-reactive T cells in the thymus is dependent on the in situ expression of both ubiquitous and tissue-restricted self-antigens and on the presentation of derived peptides. Weak or inadequate intrathymic expression of self-antigens increases the risk to generate an autoimmune-prone T-cell repertoire. Indeed, even small changes of self-antigen expression in the thymus affect negative selection and increase the predisposition to autoimmunity. Together with other mechanisms, tolerance is maintained in the peripheral lymphoid organs via the recognition by mature T cells of a similar set of self-peptides in homeostatic conditions. However, non-lymphoid peripheral tissue, where organ-specific autoimmunity takes place, often have differential functional processes that may lead to the generation of epitopes that are absent or non-presented in the thymus. These putative differences between peptides presented by MHC molecules in the thymus and in peripheral tissues might be a major key to the initiation and maintenance of autoimmune conditions.

Composition of the HLA-DR-associated human thymus peptidome

Major histocompatibility complex class II (MHC-II) molecules bind to and display antigenic peptides on the surface of antigen-presenting cells (APCs). In the absence of infection, MHC-II molecules on APCs present self-peptides and interact with CD4(+) T cells to maintain tolerance and homeostasis. In the thymus, self-peptides bind to MHC-II molecules expressed by defined populations of APCs specialised for the different steps of T-cell selection. Cortical epithelial cells present peptides for positive selection, whereas medullary epithelial cells and dendritic cells are responsible for peptide presentation for negative selection. However, few data are available on the peptides presented by MHC molecules in the thymus. Here, we apply mass spectrometry to analyse and identify MHC-II-associated peptides from five fresh human thymus samples. The data show a diverse self-peptide repertoire, mostly consisting of predicted MHC-II high binders. Despite technical limitations preventing single cell population analyses of peptides, these data constitute the first direct assessment of the HLA-II-bound peptidome and provide insight into how this peptidome is generated and how it drives T-cell repertoire formation.