Autophagic homeostasis is required for the pluripotency of cancer stem cells

Pluripotency is an important feature of cancer stem cells (CSCs) that contributes to self-renewal and chemoresistance. The maintenance of pluripotency of CSCs under various pathophysiological conditions requires a complex interaction between various cellular pathways including those involved in homeostasis and energy metabolism. However, the exact mechanisms that maintain the CSC pluripotency remain poorly understood. In this report, using both human and murine models of CSCs, we demonstrate that basal levels of autophagy are required to maintain the pluripotency of CSCs, and that this process is differentially regulated by the rate-limiting enzyme in the NAD⁺ synthesis pathway NAMPT (nicotinamide phosphoribosyltransferase) and the transcription factor POU5F1/OCT4 (POU class 5 homeobox 1). First, our data show that the pharmacological inhibition and knockdown (K_D) of NAMPT or the K_D of POU5F1 in human CSCs significantly decreased the expression of pluripotency markers POU5F1, NANOG (Nanog homeobox) and SOX2 (SRY-box 2), and upregulated the differentiation markers TUBB3 (tubulin β 3 class III), CSN2 (casein β), SPP1 (secreted phosphoprotein 1), GATA6 (GATA binding protein 6), T (T brachyury transcription factor) and CDX2 (caudal type homeobox 2). Interestingly, these pluripotency-regulating effects of NAMPT and POU5F1 were accompanied by contrasting levels of autophagy, wherein NAMPT K_D promoted while POU5F1 K_D inhibited the autophagy machinery. Most importantly, any deviation from the basal level of autophagy, either increase (via rapamycin, serum starvation or Tat-beclin 1 [Tat-BECN1] peptide) or decrease (via ATG7 or ATG12 K_D), strongly decreased the pluripotency and promoted the differentiation and/or senescence of CSCs. Collectively, these results uncover the link between the NAD⁺ biosynthesis pathway, CSC transcription factor POU5F1 and pluripotency, and further identify autophagy as a novel regulator of pluripotency of CSCs.

The NAD+ Salvage Pathway Supports PHGDH-Driven Serine Biosynthesis

NAD⁺ is a key metabolic redox cofactor that is regenerated from nicotinamide through the NAD⁺ salvage pathway. Here, we find that inhibiting the NAD⁺ salvage pathway depletes serine biosynthesis from glucose by impeding the NAD⁺-dependent protein, 3-phosphoglycerate dehydrogenase (PHGDH). Importantly, we find that PHGDH^high breast cancer cell lines are exquisitely sensitive to inhibition of the NAD⁺ salvage pathway. Further, we find that PHGDH protein levels and those of the rate-limiting enzyme of NAD⁺ salvage, NAMPT, correlate in ER-negative, basal-like breast cancers. Although NAD⁺ salvage pathway inhibitors are actively being pursued in cancer treatment, their efficacy has been poor, and our findings suggest that they may be effective for PHGDH-dependent cancers.

Trial Watch: Oncolytic viro-immunotherapy of hematologic and solid tumors

Oncolytic viruses selectively target and kill cancer cells in an immunogenic fashion, thus supporting the establishment of therapeutically relevant tumor-specific immune responses. In 2015, the US Food and Drug Administration (FDA) approved the oncolytic herpes simplex virus T-VEC for use in advanced melanoma patients. Since then, a plethora of trials has been initiated to assess the safety and efficacy of multiple oncolytic viruses in patients affected with various malignancies. Here, we summarize recent preclinical and clinical progress in the field of oncolytic virotherapy.

Skin dendritic cells in melanoma are key for successful checkpoint blockade therapy

BACKGROUND

Immunotherapy with checkpoint inhibitors has shown impressive results in patients with melanoma, but still many do not benefit from this line of treatment. A lack of tumor-infiltrating T cells is a common reason for therapy failure but also a loss of intratumoral dendritic cells (DCs) has been described.

METHODS

We used the transgenic tg(Grm1)EPv melanoma mouse strain that develops spontaneous, slow-growing tumors to perform immunological analysis during tumor progression. With flow cytometry, the frequencies of DCs and T cells at different tumor stages and the expression of the inhibitory molecules programmed cell death protein-1 (PD-1) and T-cell immunoglobulin and mucin-domain containing-3 (TIM-3) on T cells were analyzed. This was complemented with RNA-sequencing (RNA-seq) and real-time quantitative PCR (RT-qPCR) analysis to investigate the immune status of the tumors. To boost DC numbers and function, we administered Fms-related tyrosine 3 ligand (Flt3L) plus an adjuvant mix of polyI:C and anti-CD40. To enhance T cell function, we tested several checkpoint blockade antibodies. Immunological alterations were characterized in tumor and tumor-draining lymph nodes (LNs) by flow cytometry, CyTOF, microarray and RT-qPCR to understand how immune cells can control tumor growth. The specific role of migratory skin DCs was investigated by coculture of sorted DC subsets with melanoma-specific CD8+ T cells.

RESULTS

Our study revealed that tumor progression is characterized by upregulation of checkpoint molecules and a gradual loss of the dermal conventional DC (cDC) 2 subset. Monotherapy with checkpoint blockade could not restore antitumor immunity, whereas boosting DC numbers and activation increased tumor immunogenicity. This was reflected by higher numbers of activated cDC1 and cDC2 as well as CD4+ and CD8+ T cells in treated tumors. At the same time, the DC boost approach reinforced migratory dermal DC subsets to prime gp100-specific CD8+ T cells in tumor-draining LNs that expressed PD-1/TIM-3 and produced interferon γ (IFNγ)/tumor necrosis factor α (TNFα). As a consequence, the combination of the DC boost with antibodies against PD-1 and TIM-3 released the brake from T cells, leading to improved function within the tumors and delayed tumor growth.

CONCLUSIONS

Our results set forth the importance of skin DC in cancer immunotherapy, and demonstrates that restoring DC function is key to enhancing tumor immunogenicity and subsequently responsiveness to checkpoint blockade therapy.

Dendritic Cells in Oncolytic Virus-Based Anti-Cancer Therapy

Dendritic cells (DCs) are specialized antigen-presenting cells that have a notable role in the initiation and regulation of innate and adaptive immune responses. In the context of cancer, appropriately activated DCs can induce anti-tumor immunity by activating innate immune cells and tumor-specific lymphocytes that target cancer cells. However, the tumor microenvironment (TME) imposes different mechanisms that facilitate the impairment of DC functions, such as inefficient antigen presentation or polarization into immunosuppressive DCs. These tumor-associated DCs thus fail to initiate tumor-specific immunity, and indirectly support tumor progression. Hence, there is increasing interest in identifying interventions that can overturn DC impairment within the TME. Many reports thus far have studied oncolytic viruses (OVs), viruses that preferentially target and kill cancer cells, for their capacity to enhance DC-mediated anti-tumor effects. Herein, we describe the general characteristics of DCs, focusing on their role in innate and adaptive immunity in the context of the TME. We also examine how DC-OV interaction affects DC recruitment, OV delivery, and anti-tumor immunity activation. Understanding these roles of DCs in the TME and OV infection is critical in devising strategies to further harness the anti-tumor effects of both DCs and OVs, ultimately enhancing the efficacy of OV-based oncotherapy.

Newly recruited CD11b+, GR-1+, Ly6C(high) myeloid cells augment tumor-associated immunosuppression immediately following the therapeutic administration of oncolytic reovirus

Tumor-associated immunosuppression aids cancer cells to escape immune-mediated attack and subsequent elimination. Recently, however, many oncolytic viruses, including reovirus, have been reported to overturn such immunosuppression and promote the development of a clinically desired antitumor immunity, which is known to promote favorable patient outcomes. Contrary to this existing paradigm, in this article we demonstrate that reovirus augments tumor-associated immunosuppression immediately following its therapeutic administration. Our data show that reovirus induces preferential differentiation of highly suppressive CD11b(+), Gr-1(+), Ly6C(high) myeloid cells from bone marrow hematopoietic progenitor cells. Furthermore, reovirus administration in tumor-bearing hosts drives time-dependent recruitment of CD11b(+), Gr-1(+), Ly6C(high) myeloid cells in the tumor milieu, which is further supported by virus-induced increased expression of numerous immune factors involved in myeloid-derived suppressor cell survival and trafficking. Most importantly, CD11b(+), Gr-1(+), Ly6C(high) myeloid cells specifically potentiate the suppression of T cell proliferation and are associated with the absence of IFN-γ response in the tumor microenvironment early during oncotherapy. Considering that the qualitative traits of a specific antitumor immunity are largely dictated by the immunological events that precede its development, our findings are of critical importance and must be considered while devising complementary interventions aimed at promoting the optimum efficacy of oncolytic virus-based anticancer immunotherapies.

Therapy-Induced MHC I Ligands Shape Neo-Antitumor CD8 T Cell Responses during Oncolytic Virus-Based Cancer Immunotherapy

Oncolytic viruses (OVs), known for their cancer-killing characteristics, also overturn tumor-associated defects in antigen presentation through the MHC class I pathway and induce protective neo-antitumor CD8 T cell responses. Nonetheless, whether OVs shape the tumor MHC-I ligandome remains unknown. Here, we investigated if an OV induces the presentation of novel MHC I-bound tumor antigens (termed tumor MHC-I ligands). Using comparative mass spectrometry (MS)-based MHC-I ligandomics, we determined differential tumor MHC-I ligand expression following treatment with oncolytic reovirus in a murine ovarian cancer model. In vitro, we found that reovirus changes the tumor ligandome of cancer cells. Concurrent multiplexed quantitative proteomics revealed that the reovirus-induced changes in tumor MHC-I ligand presentation were mostly independent of their source proteins. In an in vivo model, tumor MHC-I ligands induced by reovirus were detectable not only in tumor tissues but also the spleens (a source of antigen-presenting cells) of tumor-bearing mice. Most importantly, therapy-induced MHC-I ligands stimulated antigen-specific IFNγ responses in antitumor CD8 T cells from mice treated with reovirus. These data show that therapy-induced MHC-I ligands may shape underlying neo-antitumor CD8 T cell responses. As such, they should be considered in strategies promoting the efficacy of OV-based cancer immunotherapies.

Inhibition of Pyruvate Dehydrogenase Kinase Enhances the Antitumor Efficacy of Oncolytic Reovirus

Oncolytic viruses (OV) such as reovirus preferentially infect and kill cancer cells. Thus, the mechanisms that dictate the susceptibility of cancer cells to OV-induced cytotoxicity hold the key to their success in clinics. Here, we investigated whether cancer cell metabolism defines its susceptibility to OV and if OV-induced metabolic perturbations can be therapeutically targeted. Using mass spectrometry-based metabolomics and extracellular flux analysis on a panel of cancer cell lines with varying degrees of susceptibility to reovirus, we found that OV-induced changes in central energy metabolism, pyruvate metabolism, and oxidative stress correlate with their susceptibility to reovirus. In particular, reovirus infection accentuated Warburg-like metabolic perturbations in cell lines relatively resistant to oncolysis. These metabolic changes were facilitated by oxidative stress-induced inhibitory phosphorylation of pyruvate dehydrogenase (PDH) that impaired the routing of pyruvate into the tricarboxylic acid cycle and established a metabolic state unsupportive of OV replication. From the therapeutic perspective, reactivation of PDH in cancer cells that were weakly sensitive for reovirus, either through PDH kinase (PDK) inhibitors dichloroacetate and AZD7545 or short hairpin RNA-specific depletion of PDK1, enhanced the efficacy of reovirus-induced oncolysis in vitro and in vivo. These findings identify targeted metabolic reprogramming as a possible combination strategy to enhance the antitumor effects of OV in clinics. SIGNIFICANCE: This study proposes targeted metabolic reprogramming as a valid combinatorial strategy to enhance the translational efficacy of oncolytic virus-based cancer therapies.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/15/3824/F1.large.jpg.

Surfen, a proteoglycan binding agent, reduces inflammation but inhibits remyelination in murine models of Multiple Sclerosis

Proteoglycans are promising therapeutic targets in Multiple Sclerosis (MS), because they regulate many aspects of the immune response. This was studied using surfen, an agent that binds both heparan sulphate proteoglycans (HSPGs) and chondroitin sulphate proteoglycans (CSPGs). Initial cell culture work on bone marrow derived macrophages (BMDMs) found that surfen reduced concentrations of the chemokines CCL2, CCL4 and CCL5, with reduced messenger (m)RNA expression for Tumor Necrosis Factor, IL-6, IL-1β and inducible nitric oxide synthase. These data were further explored using Experimental Autoimmune Encephalomyelitis (EAE) in mice. Surfen reduced clinical signs during EAE when administered from disease onset, and reduced infiltration by CD4 positive T cells and macrophages into the central nervous system. These mice also showed reduced mRNA expression for the chemokines CCL3 and CCL5, with reduced concentrations of CCL2, CCL3 and CCL5. During EAE, surfen treatment induced a persistent increase in Interleukin (IL)-4 concentrations which may enhance T helper 2 responses. During EAE, surfen treatment reduced mRNA expression for HSPGs (NDST1, agrin, syndecan-4, perlecan, serglycin, syndecan-1) and the CSPG versican. By contrast, surfen increased mRNA expression for the CSPG aggrecan, with no effect on neurocan. During EAE, significant positive correlations were found between mRNA expression and clinical score for syndecan-4, serglycin and syndecan-1 and a significant negative correlation for aggrecan. These correlations were absent in surfen treated mice. Repair in the later stages of MS involves remyelination, which was modeled by injecting lysolecithin (lysophosphatidylcholine, LPC) into mouse corpus callosum to create regions of demyelination. When surfen was injected 2 days after LPC, it delayed remyelination of the lesions, but had no effect when injected 7 days after LPC. The delayed remyelination was associated with local increases in CSPG expression. Therefore surfen suppresses inflammation but inhibits remyelination in these models. A mechanism in common may be increased CSPG expression.

Quantitative Temporal in Vivo Proteomics Deciphers the Transition of Virus-Driven Myeloid Cells into M2 Macrophages

Myeloid cells play a central role in the context of viral eradication, yet precisely how these cells differentiate throughout the course of acute infections is poorly understood. In this study, we have developed a novel quantitative temporal in vivo proteomics (QTiPs) platform to capture proteomic signatures of temporally transitioning virus-driven myeloid cells directly in situ, thus taking into consideration host–virus interactions throughout the course of an infection. QTiPs, in combination with phenotypic, functional, and metabolic analyses, elucidated a pivotal role for inflammatory CD11b⁺, Ly6G^–, Ly6C^high-low cells in antiviral immune response and viral clearance. Most importantly, the time-resolved QTiPs data set showed the transition of CD11b⁺, Ly6G^–, Ly6C^high-low cells into M2-like macrophages, which displayed increased antigen-presentation capacities and bioenergetic demands late in infection. We elucidated the pivotal role of myeloid cells in virus clearance and show how these cells phenotypically, functionally, and metabolically undergo a timely transition from inflammatory to M2-like macrophages in vivo. With respect to the growing appreciation for in vivo examination of viral–host interactions and for the role of myeloid cells, this study elucidates the use of quantitative proteomics to reveal the role and response of distinct immune cell populations throughout the course of virus infection.

All that glitters is not gold: the need to consider desirable and undesirable immune aspects of oncolytic virus therapy

Oncolytic viruses (OVs), a novel class of anticancer therapeutic agents, can overturn cancer-mediated immunosuppression and initiate antitumor immunity. Contrary to this paradigm, our recent study illustrates that oncolytic reovirus transiently augments cancer-associated immunosuppression immediately following its therapeutic administration. To achieve the optimum efficacy for OV-based anticancer therapies, the pathophysiological as well as clinical implications of this phenomenon need to be considered.

Quantitative Proteome Responses to Oncolytic Reovirus in GM-CSF- and M-CSF-Differentiated Bone Marrow-Derived Cells

The efficacy of oncolytic viruses (OVs), such as reovirus, is dictated by host immune responses, including those mediated by the pro- versus anti-inflammatory macrophages. As such, a detailed understanding of the interaction between reovirus and different macrophage types is critical for therapeutic efficacy. To explore reovirus-macrophage interactions, we performed tandem mass tag (TMT)-based quantitative temporal proteomics on mouse bone marrow-derived macrophages (BMMs) generated with two cytokines, macrophage colony stimulating factor (M-CSF) and granulocytic-macrophage colony stimulating factor (GM-CSF), representing anti- and proinflammatory macrophages, respectively. We quantified 6863 proteins across five time points in duplicate, comparing M-CSF (M-BMM) and GM-CSF (GM-BMM) in response to OV. We find that GM-BMMs have lower expression of key intrinsic proteins that facilitate an antiviral immune response, express higher levels of reovirus receptor protein JAM-A, and are more susceptible to oncolytic reovirus infection compared to M-BMMs. Interestingly, although M-BMMs are less susceptible to reovirus infection and subsequent cell death, they initiate an antireovirus adaptive T cell immune response comparable to that of GM-BMMs. Taken together, these data describe distinct proteome differences between these two macrophage populations in terms of their ability to mount antiviral immune responses.

Rapid recruitment and IFN-I-mediated activation of monocytes dictate focal radiotherapy efficacy

The recruitment of monocytes and their differentiation into immunosuppressive cells is associated with the low efficacy of preclinical nonconformal radiotherapy (RT) for tumors. However, nonconformal RT (non-CRT) does not mimic clinical practice, and little is known about the role of monocytes after RT modes used in patients, such as conformal RT (CRT). Here, we investigated the acute immune response induced by after CRT. Contrary to non-CRT approaches, we found that CRT induces a rapid and robust recruitment of monocytes to the tumor that minimally differentiate into tumor-associated macrophages or dendritic cells but instead up-regulate major histocompatibility complex II and costimulatory molecules. We found that these large numbers of infiltrating monocytes are responsible for activating effector polyfunctional CD8+ tumor-infiltrating lymphocytes that reduce tumor burden. Mechanistically, we show that monocyte-derived type I interferon is pivotal in promoting monocyte accumulation and immunostimulatory function in a positive feedback loop. We also demonstrate that monocyte accumulation in the tumor microenvironment is hindered when RT inadvertently affects healthy tissues, as occurs in non-CRT. Our results unravel the immunostimulatory function of monocytes during clinically relevant modes of RT and demonstrate that limiting the exposure of healthy tissues to radiation has a positive therapeutic effect on the overall antitumor immune response.

CD301b+ monocyte-derived dendritic cells mediate resistance to radiotherapy

Monocytes infiltrating tumors acquire various states that distinctly impact cancer treatment. Here, we show that resistance of tumors to radiotherapy (RT) is controlled by the accumulation of monocyte-derived dendritic cells (moDCs). These moDCs are characterized by the expression of CD301b and have a superior capacity to generate regulatory T cells (Tregs). Accordingly, moDC depletion limits Treg generation and improves the therapeutic outcome of RT. Mechanistically, we demonstrate that granulocyte-macrophage colony-stimulating factor (GM-CSF) derived from radioresistant tumor cells following RT is necessary for the accumulation of moDCs. Our results unravel the immunosuppressive function of moDCs and identify GM-CSF as an immunotherapeutic target during RT.

Hardwiring tissue-specific AAV transduction in mice through engineered receptor expression

The development of transgenic mouse models that express genes of interest in specific cell types has transformed our understanding of basic biology and disease. However, generating these models is time- and resource-intensive. Here we describe a model system, SELective Expression and Controlled Transduction In Vivo (SELECTIV), that enables efficient and specific expression of transgenes by coupling adeno-associated virus (AAV) vectors with Cre-inducible overexpression of the multi-serotype AAV receptor, AAVR. We demonstrate that transgenic AAVR overexpression greatly increases the efficiency of transduction of many diverse cell types, including muscle stem cells, which are normally refractory to AAV transduction. Superior specificity is achieved by combining Cre-mediated AAVR overexpression with whole-body knockout of endogenous Aavr, which is demonstrated in heart cardiomyocytes, liver hepatocytes and cholinergic neurons. The enhanced efficacy and exquisite specificity of SELECTIV has broad utility in development of new mouse model systems and expands the use of AAV for gene delivery in vivo.