The Immunomodulatory Small Molecule Imiquimod Induces Apoptosis in Devil Facial Tumour Cell Lines

Beyond inhibition against the PD-1/PD-L1 pathway: development of PD-L1 inhibitors targeting internalization and degradation of PD-L1

Tumor cells hijack the programmed cell death protein-1 (PD-1)/programmed cell death ligand-1 (PD-L1) pathway to suppress the immune response through overexpressing PD-L1 to interact with PD-1 of T cells. With in-depth ongoing research, tumor-intrinsic PD-L1 is found to play important roles in tumor progression without interaction with PD-1 expressed on T cells, which provides an additional important target and therapeutic approach for development of PD-L1 inhibitors. Existing monoclonal antibody (mAb) drugs against the PD-1/PD-L1 pathway generally behave by conformationally blocking the interactions of PD-1 with PD-L1 on the cell surface. Beyond general inhibition of the protein-protein interaction (PPI), inhibitors targeting PD-L1 currently focus on the functional inhibition of the interaction between PD-1/PD-L1 and degradation of tumor-intrinsic PD-L1. This perspective will clarify the evolution of PD-L1 inhibitors and provide insights into the current development of PD-L1 inhibitors, especially targeting internalization and degradation of PD-L1.

Early redox homeostasis disruption contributes to the differential cytotoxicity of imiquimod on transformed and normal endothelial cells

The off-label use of imiquimod (IQ) for hemangioma treatment has shown clinical benefits. We have previously reported a selective direct IQ-cytotoxic effect on transformed (H5V) vs. normal (1G11) endothelial cells (EC). In the present study, we investigated the mechanism underlying this selective cytotoxicity in terms of TLR7/8 receptor expression, NF-κB signalling and time-dependent modifications of oxidative stress parameters (ROS: reactive oxygen species, catalase and superoxide dismutase activities, GSH/GSSG and lipid peroxidation). TLR7/8 level was extremely low in both cell lines, and IQ did not upregulate TLR7/8 expression or activate NF-κB signalling. IQ significantly induced ROS in H5V after 2 h and strongly affected antioxidant defenses. After 12 h, enzyme activities were restored to baseline levels but a robust drop in GSH/GSSG persisted together with increased lipid peroxidation levels and a marked mitochondrial dysfunction. Although in normal IQ-treated EC some oxidative stress parameters were affected after 4 h, mitochondrial health and GSH/GSSG ratio remained notably unaffected after 12 h. Therefore, the early alterations (0-2 h) in transformed EC breached redox homeostasis as strongly as to enhance their susceptibility to IQ. This interesting facet of IQ as redox disruptor could broaden its therapeutic potential for other skin malignancies, alone or in adjuvant schemes.

LXR stimulates a metabolic switch and reveals cholesterol homeostasis as a statin target in Tasmanian devil facial tumor disease

Devil facial tumor disease (DFTD) and its lack of available therapies are propelling the Tasmanian devil population toward extinction. This study demonstrates that cholesterol homeostasis and carbohydrate energy metabolism sustain the proliferation of DFTD cells in a cell-type-dependent manner. In addition, we show that the liver-X nuclear receptor-β (LXRβ), a major cholesterol cellular sensor, and its natural ligand 24S-hydroxycholesterol promote the proliferation of DFTD cells via a metabolic switch toward aerobic glycolysis. As a proof of concept of the role of cholesterol homeostasis on DFTD proliferation, we show that atorvastatin, an FDA-approved statin-drug subtype used against human cardiovascular diseases that inhibits cholesterol synthesis, shuts down DFTD energy metabolism and prevents tumor growth in an in vivo DFTD-xenograft model. In conclusion, we show that intervention against cholesterol homeostasis and carbohydrate-dependent energy metabolism by atorvastatin constitutes a feasible biochemical treatment against DFTD, which may assist in the conservation of the Tasmanian devil.

Recent Advances in the Development of PD-L1 Modulators: Degraders, Downregulators, and Covalent Inhibitors

Therapeutic interference of the programmed cell death protein 1(PD-1)/immunosuppressive programmed cell death ligand 1 (PD-L1) signaling pathway by monoclonal antibodies has achieved spectacular success for treating various tumors. However, the development of small molecule inhibitors of PD-1/PD-L1 has lagged far behind due to the challenge of targeting the highly hydrophobic and relatively flat binding interface, despite the benefits small molecule can bring over therapeutic antibodies. This technical challenge provokes the adoption of different strategies in searching for small, medium-sized, and large molecule modulators (e.g., degraders, downregulators, and covalent inhibitors) of the PD-1/PD-L1 protein-protein interaction. In this review article, we discuss latest advances in the development of PD-L1 modulators, with a focus on degraders, downregulators, and covalent inhibitors.

Curse of the devil: molecular insights into the emergence of transmissible cancers in the Tasmanian devil (Sarcophilus harrisii)

The Tasmanian devil (Sarcophilus harrisii) is the only mammalian species known to be affected by multiple transmissible cancers. Devil facial tumours 1 and 2 (DFT1 and DFT2) are independent neoplastic cell lineages that produce large, disfiguring cancers known as devil facial tumour disease (DFTD). The long-term persistence of wild Tasmanian devils is threatened due to the ability of DFTD cells to propagate as contagious allografts and the high mortality rate of DFTD. Recent studies have demonstrated that both DFT1 and DFT2 cancers originated from founder cells of the Schwann cell lineage, an uncommon origin of malignant cancer in humans. This unprecedented finding has revealed a potential predisposition of Tasmanian devils to transmissible cancers of the Schwann cell lineage. In this review, we compare the molecular nature of human Schwann cells and nerve sheath tumours with DFT1 and DFT2 to gain insights into the emergence of transmissible cancers in the Tasmanian devil. We discuss a potential mechanism, whereby Schwann cell plasticity and frequent wounding in Tasmanian devils combine with an inherent cancer predisposition and low genetic diversity to give rise to transmissible Schwann cell cancers in devils on rare occasions.

An oral bait vaccination approach for the Tasmanian devil facial tumor diseases

Introduction: The Tasmanian devil (Sarcophilus harrisii) is the largest extant carnivorous marsupial. Since 1996, its population has declined by 77% primarily due to a clonal transmissible tumor, known as devil facial tumor (DFT1) disease. In 2014, a second transmissible devil facial tumor (DFT2) was discovered. DFT1 and DFT2 are nearly 100% fatal.Areas covered: We review DFT control approaches and propose a rabies-style oral bait vaccine (OBV) platform for DFTs. This approach has an extensive safety record and was a primary tool in large-scale rabies virus elimination from wild carnivores across diverse landscapes. Like rabies virus, DFTs are transmitted by oral contact, so immunizing the oral cavity and stimulating resident memory cells could be advantageous. Additionally, exposing infected devils that already have tumors to OBVs could serve as an oncolytic virus immunotherapy. The primary challenges may be identifying appropriate DFT-specific antigens and optimization of field delivery methods.Expert opinion: DFT2 is currently found on a peninsula in southern Tasmania, so an OBV that could eliminate DFT2 should be the priority for this vaccine approach. Translation of an OBV approach to control DFTs will be challenging, but the approach is feasible for combatting ongoing and future disease threats.

Transmissible Cancers and Immune Downregulation in Tasmanian Devil (Sacrophilus harrisii) and Canine Populations

Known as devil facial tumor disease (DFTD) and canine transmissible venereal tumor (CTVT), transmissible cancer occurs in both Tasmanian devil and canine populations, respectively. Both malignancies show remarkable ability to be transmitted as allografts into subsequent hosts. How DFTD and CTVT avoid detection by immunocompetent hosts is of particular interest, given that these malignancies are rarely seen in other species in nature. Both of these transmissible cancers can downregulate the host immune system, enabling proliferation. DFTD is characterized by epigenetic modifications to the DNA promoter regions of β₂microglobulin, transporters associated with antigen processing 1 and 2, MHC I, and MHC II-crucial proteins required in the detection and surveillance of foreign material. Downregulation during DFTD may be achieved by altering the activity of histone deacetylases. DFTD has caused widespread destruction of devil populations, placing the species on the brink of extinction. CTVT demonstrates a proliferative phase, during which the tumor evades immune detection, allowing it to proliferate, and a regressive phase when hosts mount an effective immune response. Alteration of TGFβ signaling in CTVT likely impedes the antigen-processing capabilities of canine hosts in addition to hindering the ability of natural killer cells to detect immune system downregulation. Immunosuppressive cytokines such as CXCL7 may contribute to a favorable microenvironment that supports the proliferation of CTVT. When viewed from an evolutionary paradigm, both DFTD and CTVT may conform to a model of host-parasite coevolution. Furthermore, various genetic features, such as genetically active transposons in CTVT and chromosomal rearrangements in DFTD, play important roles in promoting the survival of these disease agents. Understanding the mode of transmission for these transmissible cancers may shed light on mechanisms for human malignancies and reveal opportunities for treatment in the future.

Recent advances in small molecule based cancer immunotherapy

Immunotherapy has been increasingly utilized for the treatment of cancer. Currently available cancer immunotherapies mainly involve the use of antibodies, which have advantages in terms of pharmacodynamics such as efficacy and specificity, however, they exhibit disadvantages in regard to the pharmacokinetics including but not limited to poor tissue and tumor penetration, very long half-life, and the lack of oral bioavailability. Also they are immunogenic and may cause undesired side effects. In addition, they are difficult and expensive to produce. In contrast to therapeutic antibodies, small molecule immuno-oncology agents generally have favorable pharmacokinetics, for example, better oral bioavailability, higher tissue and tumor penetration, reasonable half-lives etc. Furthermore, some small molecules are highly selective and efficacious with benign toxicity profiles. Therefore, small molecule immuno-oncology agents have the potential to overcome the drawbacks of therapeutic antibodies, and they can complement existing therapeutic antibodies and may also be used in combination with antibodies to achieve synergistic effects. In this article, we summarize the current advances in the field of small molecule approaches in tumor immunology which include the small molecules in clinical trials and preclinical studies, and the reported crystal structures of small molecules and their target proteins as well as the binding interactions between small molecules and the targets. The tumorigenesis mechanism of different targets (the programmed cell death 1/programmed cell death ligand 1(PD1/PD-L1), retinoic acid-related orphan receptor-gamma t (RORγt), Chemokine receptor, Stimulator of Interferon Genes (Sting), Indoleamine 2,3-dioxygenase (IDO), toll-like receptors (TLR) etc.) are also elucidated.

Transcriptome and proteome profiling reveals stress-induced expression signatures of imiquimod-treated Tasmanian devil facial tumor disease (DFTD) cells

As a topical cancer immunotherapy, the toll-like receptor 7 ligand imiquimod activates tumor regression via stimulation of immune cell infiltration and cytotoxic responses. Imiquimod also exerts direct pro-apoptotic effects on tumor cells in vitro, but a role for these effects in imiquimod-induced tumor regression remains undefined. We previously demonstrated that cell lines derived from devil facial tumor disease (DFTD), a transmissible cancer threatening the survival of the Tasmanian devil (Sarcophilus harrisii), are sensitive to imiquimod-induced apoptosis. In this study, the pro-apoptotic effects of imiquimod in DFTD have been investigated using RNA-sequencing and label-free quantitative proteomics. This analysis revealed that changes to gene and protein expression in imiquimod treated DFTD cells are consistent with the onset of oxidative and endoplasmic reticulum stress responses, and subsequent activation of the unfolded protein response, autophagy, cell cycle arrest and apoptosis. Imiquimod also regulates the expression of oncogenic pathways, providing a direct mechanism by which this drug may increase tumor susceptibility to immune cytotoxicity in vivo. Our study has provided the first global analysis of imiquimod-induced effects in any tumor cell line. These findings have highlighted the potential of cell stress pathways as therapeutic targets in DFTD, and will allow for improved mechanistic use of imiquimod as a therapy in both the Tasmanian devil and human cancers.

Gomesin peptides prevent proliferation and lead to the cell death of devil facial tumour disease cells

The Tasmanian devil faces extinction due to devil facial tumour disease (DFTD), a highly transmittable clonal form of cancer without available treatment. In this study, we report the cell-autonomous antiproliferative and cytotoxic activities exhibited by the spider peptide gomesin (AgGom) and gomesin-like homologue (HiGom) in DFTD cells. Mechanistically, both peptides caused a significant reduction at G0/G1 phase, in correlation with an augmented expression of the cell cycle inhibitory proteins p53, p27, p21, necrosis, exacerbated generation of reactive oxygen species and diminished mitochondrial membrane potential, all hallmarks of cellular stress. The screening of a novel panel of AgGom-analogues revealed that, unlike changes in the hydrophobicity and electrostatic surface, the cytotoxic potential of the gomesin analogues in DFTD cells lies on specific arginine substitutions in the eight and nine positions and alanine replacement in three, five and 12 positions. In conclusion, the evidence supports gomesin as a potential antiproliferative compound against DFTD disease.

The toll-like receptor ligands Hiltonol® (polyICLC) and imiquimod effectively activate antigen-specific immune responses in Tasmanian devils (Sarcophilus harrisii)

Devil facial tumour disease (DFTD) describes two genetically distinct transmissible tumours that pose a significant threat to the survival of the Tasmanian devil. A prophylactic vaccine could protect devils from DFTD transmission. For this vaccine to be effective, potent immune adjuvants will be required. Toll-like receptors (TLRs) promote robust immune responses in human cancer studies and are highly conserved across mammalian species. In this study, we investigated the proficiency of TLR ligands for immune activation in the Tasmanian devil using in vitro mononuclear cell stimulations and in vivo immunisation trials with a model antigen. We identified two such TLR ligands, polyICLC (Hiltonol^®) (TLR3) and imiquimod (TLR7), that in combination induced significant IFNγ production from Tasmanian devil lymphocytes in vitro. Immunisation with these ligands and the model antigen keyhole limpet haemocyanin activated robust antigen-specific primary, secondary and long-term memory IgG responses. Our results support the conserved nature of TLR signaling across mammalian species. PolyICLC and imiquimod will be trialed as immune adjuvants in future DFTD vaccine formulations.