Intratumoral DNA electroporation induces anti-tumor immunity and tumor regression

Acute Effects of Intratumor DNA Electrotransfer

Intratumor therapeutic DNA electroporation or electrotransfer is in clinical trials in the United States and is under development in many other countries. Acute changes in endogenous gene expression in response to DNA or to pulse application may significantly modulate the therapeutic efficacy of the expressed proteins. Oligonucleotide arrays were used in this study to quantify changes in mRNA expression in B16-F10 mouse melanoma tumors four hours after DNA electrotransfer. The data were subjected to the DAVID v6.8 web server for functional annotation to reveal regulated genes and genetic pathways. Gene ontology analysis revealed several molecular functions related to cytoskeletal remodeling and inflammatory signaling. In B16-F10 cells, F-actin remodeling was confirmed by phalloidin staining in cells that received pulse application alone or in the presence of DNA. Chemokine secretion was confirmed in cells receiving DNA electrotransfer. These results indicate that pulse application alone or in the presence of DNA may modulate the therapeutic efficacy of therapeutic DNA electrotransfer.

Novel Mouse Models for Cancer Immunology

Mouse models for the study of cancer immunology provide excellent systems in which to test biological mechanisms of the immune response against cancer. Historically, these models have been designed to have different strengths based on the current major research questions at the time. As such, many mouse models of immunology used today were not originally developed to study questions currently plaguing the relatively new field of cancer immunology, but instead have been adapted for such purposes. In this review, we discuss various mouse model of cancer immunology in a historical context as a means to provide a fuller perspective of each model's strengths. From this outlook, we discuss the current state of the art and strategies for tackling future modeling challenges.

Intratumoral electroporation of plasmid encoded IL-12 and membrane-anchored anti-CD3 increases systemic tumor immunity

Intratumoral delivery of plasmid IL 12 via electroporation (IT tavo EP) induces localized expression of IL 12 leading to regression of treated and distant tumors with durable responses and minimal toxicity. A key driver in amplifying this local therapy into a systemic response is the magnitude and composition of immune infiltrate in the treated tumor. While intratumoral IL 12 typically increases the density of CD3+ tumor infiltrating lymphocytes (TIL), this infiltrate is composed of a broad range of T cell subsets, including activated tumor specific T cells, less functional bystander T cells, as well as suppressive T regulatory cells. To encourage a more favorable on treatment tumor microenvironment, we explored combining this IL 12 therapy with an intratumoral polyclonal T cell stimulator membrane anchored anti CD3 to productively engage a diverse subset of lymphocytes including the non reactive and suppressive T cells. This study highlighted that combined intratumoral electroporation of IL 12 and membrane anchored anti CD3 plasmids can enhance cytokine production, T cell cytotoxicity, and proliferation while limiting the suppressive capacity within the TME. These collective anti tumor effects not only improve regression of treated tumors but drive systemic immunity with control of non treated contralateral tumors in vivo. Moreover, combination of IL 12 and anti CD3 restored the function of TIL isolated from a melanoma patient actively progressing on PD 1 checkpoint inhibitor therapy. This DNA encodable polyclonal T cell stimulator (membrane anchored anti CD3 plasmid) may represent a key addition to intratumoral IL-12 therapies in the clinic.

A mouse model for the study of anti-tumor T cell responses in Kras-driven lung adenocarcinoma

Kras-driven lung adenocarcinoma (LUAD) is the most common lung cancer. A significant fraction of patients with Kras-driven LUAD respond to immunotherapy, but mechanistic studies of immune responses against LUAD have been limited because of a lack of immunotherapy-responsive models. We report the development of the immunogenic KP × NINJA (inversion inducible joined neoantigen) (KP-NINJA) LUAD model. This model allows temporal uncoupling of antigen and tumor induction, which allows one to wait until after infection-induced inflammation has subsided to induce neoantigen expression by tumors. Neoantigen expression is restricted to EPCAM+ cells in the lung and expression of neoantigen was more consistent between tumors than when neoantigens were encoded on lentiviruses. Moreover, tumors were infiltrated by tumor-specific CD8 T cells. Finally, LUAD cell lines derived from KP-NINJA mice were immunogenic and responded to immune checkpoint therapy (anti-PD1 and anti-CTLA4), providing means for future studies into the immunobiology of therapeutic responses in LUAD.

Bioluminescence as a sensitive electroporation indicator in sub-microsecond and microsecond range of electrical pulses

The cell membrane permeabilization in electroporation studies is usually quantified using fluorescent markers such as propidium iodide (PI) or YO-PRO, while Chinese Hamster Ovary cell line frequently serves as a model. In this work, as an alternative, we propose a sensitive methodology for detection and analysis of electroporation phenomenon based on bioluminescence. Luminescent mice myeloma SP2/0 cells (transfected using Luciferase-pcDNA3 plasmid) were used as a cell model. Electroporation has been studied using the 0.1-5 μs × 250 and 100 μs × 1-8 pulsing protocols in 1-2.5 kV/cm PEF range. It was shown that the bioluminescence response is dependent on the cell permeabilization state and can be effectively used to detect even weak permeabilization. During saturated permeabilization the methodology accurately predicts the losses of cell viability due to irreversible electroporation. The results have been superpositioned with permeabilization and pore resealing (1 h post-treatment) data using PI. Also, the viability of the cells was evaluated. Lastly, the SP2/0 tumors have been developed in BALB/C mice and the methodology has been tested in vivo using electrochemotherapy with bleomycin.

Intratumoral Gene Electrotransfer of Plasmid DNA Encoding shRNA against Melanoma Cell Adhesion Molecule Radiosensitizes Tumors by Antivascular Effects and Activation of an Immune Response

In this study, radiotherapy was combined with the gene electrotransfer (GET) of plasmid encoding shRNA against melanoma cell adhesion molecule (pMCAM) with dual action, which was a vascular-targeted effect mediated by the silencing of MCAM and an immunological effect mediated by the presence of plasmid DNA in the cytosol-activating DNA sensors. The effects and underlying mechanisms of therapy were evaluated in more immunogenic B16F10 melanoma and less immunogenic TS/A carcinoma. The silencing of MCAM potentiated the effect of irradiation (IR) in both tumor models. Combined therapy resulted in 81% complete responses (CR) in melanoma and 27% CR in carcinoma. Moreover, after the secondary challenge of cured mice, 59% of mice were resistant to challenge with melanoma cells, and none were resistant to carcinoma. Combined therapy reduced the number of blood vessels; induced hypoxia, apoptosis, and necrosis; and reduced cell proliferation in both tumor models. In addition, the significant increase of infiltrating immune cells was observed in both tumor models but more so in melanoma, where the expression of IL-12 and TNF-α was determined as well. Our results indicate that the combined therapy exerts both antiangiogenic and immune responses that contribute to the antitumor effect. However, tumor immunological status is crucial for a sufficient immune system contribution to the overall antitumor effect.

An HER2 DNA vaccine with evolution-selected amino acid substitutions reveals a fundamental principle for cancer vaccine formulation in HER2 transgenic mice

Enhancement of endogenous immunity to tumor-associated self-antigens and neoantigens is the goal of preventive vaccination. Toward this goal, we compared the efficacy of the following HER2 DNA vaccine constructs: vaccines encoding wild-type HER2, hybrid HER2 vaccines consisting of human HER2 and rat Neu, HER2 vaccines with single residue substitutions and a novel human HER2 DNA vaccine, ph(es)E2TM. ph(es)E2TM was designed to contain five evolution-selected substitutions: M198V, Q398R, F425L, H473R and A622T that occur frequently in 12 primate HER2 sequences. These ph(es)E2TM substitutions score 0 to 1 in blocks substitutions matrix (BLOSUM), indicating minimal biochemical alterations. h(es)E2TM recombinant protein is recognized by a panel of anti-HER2 mAbs, demonstrating the preservation of HER2 protein structure. Compared to native human HER2, electrovaccination of HER2 transgenic mice with ph(es)E2TM induced a threefold increase in HER2-binding antibody (Ab) and elevated levels of IFNγ-producing T cells. ph(es)E2TM, but not pE2TM immune serum, recognized HER2 peptide p95 ₃₅₅LPESFDGDPASNTAP₃₆₉, suggesting a broadening of epitope recognition induced by the minimally modified HER2 vaccine. ph(es)E2TM vaccination reduced tumor growth more effectively than wild-type HER2 or HER2 vaccines with more extensive modifications. The elevation of tumor immunity by ph(es)E2TM vaccination would create a favorable tumor microenvironment for neoantigen priming, further enhancing the protective immunity. The fundamental principle of exploiting evolution-selected amino acid substitutions is novel, effective and applicable to vaccine development in general.

Melanoma treatment with intratumoral electroporation of tavokinogene telseplasmid (pIL-12, tavokinogene telseplasmid)

Tumors evade detection and/or clearance by the immune system via multiple mechanisms. IL-12 is a potent immunomodulatory cytokine that plays a central role in immune priming. However, systemic delivery of IL-12 can result in life-threatening toxicity and therefore has shown limited efficacy at doses that can be safely administered. We developed an electroporation technique to produce highly localized IL-12 expression within tumors leading to regression of both treated and untreated lesions in animal models and in patients with a favorable safety profile. Furthermore, intratumoral tavokinogene telseplasmid electroporation can drive cellular immune responses, converting 'cold' tumors into 'hot' tumors. Clinical trials are ongoing to determine whether intratumoral tavokinogene telseplasmid electroporation synergizes with checkpoint blockade therapy in immunologically cold tumors predicted not to respond to PD-1 antibody monotherapy.

APOBEC3A intratumoral DNA electroporation in mice

Human APOBEC3A (A3A) cytidine deaminase shows pro-apoptotic properties resulting from hypermutation of genomic DNA, induction of double-stranded DNA breaks (DSBs) and G1 cell cycle arrest. Given this, we evaluated the antitumor efficacy of A3A by intratumoral electroporation of an A3A expression plasmid. DNA was repeatedly electroporated into B16OVA, B16Luc tumors of C57BL/6J mice as well as the aggressive fibrosarcoma Sarc2 tumor of HLA-A*0201/DRB1*0101 transgenic mice using noninvasive plate electrodes. Intratumoral electroporation of A3A plasmid DNA resulted in regression of ~50% of small B16OVA melanoma tumors that did not rebound in the following 2 months without treatment. Larger or more aggressive tumors escaped regression when so treated. As APOBEC3A was much less efficient in provoking hypermutation and DSBs in B16OVA cells compared with human or quail cells, it is likely that APOBEC3A would be more efficient in a human setting than in a mouse model.

Cytosolic DNA Sensor Upregulation Accompanies DNA Electrotransfer in B16.F10 Melanoma Cells

In several preclinical tumor models, antitumor effects occur after intratumoral electroporation, also known as electrotransfer, of plasmid DNA devoid of a therapeutic gene. In mouse melanomas, these effects are preceded by significant elevation of several proinflammatory cytokines. These observations implicate the binding and activation of intracellular DNA-specific pattern recognition receptors or DNA sensors in response to DNA electrotransfer. In tumors, IFNβ mRNA and protein levels significantly increased. The mRNAs of several DNA sensors were detected, and DAI, DDX60, and p204 tended to be upregulated. These effects were accompanied with reduced tumor growth and increased tumor necrosis. In B16.F10 cells in culture, IFNβ mRNA and protein levels were significantly upregulated. The mRNAs for several DNA sensors were present in these cells; DNA-dependent activator of interferon regulatory factor (DAI), DEAD (Asp-Glu-Ala-Asp) box polypeptide 60 (DDX60), and p204 were significantly upregulated while DDX60 protein levels were coordinately upregulated. Upregulation of DNA sensors in tumors could be masked by the lower transfection efficiency compared to in vitro or to dilution by other tumor cell types. Mirroring the observation of tumor necrosis, cells underwent a significant DNA concentration-dependent decrease in proliferation and survival. Taken together, these results indicate that DNA electrotransfer may cause the upregulation of several intracellular DNA sensors in B16.F10 cells, inducing effects in vitro and potentially in vivo.

Evolution of animal models in cancer vaccine development

Advances in cancer vaccine development are facilitated by animal models reflecting key features of human cancer and its interface with host immunity. Several series of transplantable preneoplastic and neoplastic mouse mammary lesions have been used to delineate mechanisms of anti-tumor immunity. Mimicking immune tolerance to tumor-associated antigens (TAA) such as HER2/neu, transgenic mice developing spontaneous mammary tumors are strong model systems for pre-clinical vaccine testing. In these models, HER2 DNA vaccines are easily administered, well-tolerated, and induce both humoral and cellular immunity. Although engineered mouse strains have advanced cancer immunotherapy, basic shortcomings remain. For example, multiple mouse strains have to be tested to recapitulate genetic regulation of immune tolerance in humans. Outbred domestic felines more closely parallel humans in the natural development of HER2 positive breast cancer and their varying genetic background. Electrovaccination with heterologous HER2 DNA induces robust adaptive immune responses in cats. Importantly, homologous feline HER2 DNA with a single amino acid substitution elicits unique antibodies to feline mammary tumor cells, unlocking a new vaccine principle. As an alternative approach to targeted vaccination, non-surgical tumor ablation such as cryoablation induces anti-tumor immunity via in situ immunization, particularly when combined with toll-like receptor (TLR) agonist. As strategies for vaccination advance, non-invasive monitoring of host response becomes imperative. As an example, magnetic resonance imaging (MRI) and positron emission tomography (PET) scanning following administration of tryptophan metabolism tracer [11C]-alpha-methyl-tryptophan (AMT) provides non-invasive imaging of both tumor growth and metabolic activities. Because AMT is a substrate of indoleamine-pyrrole 2,3-dioxygenase (IDO), an enzyme that produces the immune regulatory molecule kynurenine, AMT imaging can provide novel insight of host response. In conclusion, new feline models improve the predictive power of cancer immunotherapy and real-time PET imaging enables mechanistic monitoring of host immunity. Strategic utilization of these new tools will expedite cancer vaccine development.

Cancer vaccines

Cancer vaccines are designed to promote tumor specific immune responses, particularly cytotoxic CD8 positive T cells that are specific to tumor antigens. The earliest vaccines, which were developed in 1994-95, tested non-mutated, shared tumor associated antigens that had been shown to be immunogenic and capable of inducing clinical responses in a minority of people with late stage cancer. Technological developments in the past few years have enabled the investigation of vaccines that target mutated antigens that are patient specific. Several platforms for cancer vaccination are being tested, including peptides, proteins, antigen presenting cells, tumor cells, and viral vectors. Standard of care treatments, such as surgery and ablation, chemotherapy, and radiotherapy, can also induce antitumor immunity, thereby having cancer vaccine effects. The monitoring of patients' immune responses at baseline and after standard of care treatment is shedding light on immune biomarkers. Combination therapies are being tested in clinical trials and are likely to be the best approach to improving patient outcomes.

In situ immunization via non-surgical ablation to prevent local and distant tumor recurrence

Host immunities are induced during cryoablation or oncolytic adenovirus therapy when the entire repertoire of tumor associated antigens (TAA) is released. Local and systemic protection is enhanced by the combined treatment with toll-like receptor agonist or immune stimulating cytokines. Non-surgical tumor ablation is an effective platform for in situ immunization.

Immunotherapeutic intervention with oncolytic adenovirus in mouse mammary tumors

The goal is to elucidate the immune modulating activity of an adenovirus (Adv) vector which showed therapeutic activity in human clinical trials. The oncolytic adenovirus (Adv/CD-TK) expressing two suicide genes was tested in two HER2/neu positive BALB/c mouse mammary tumor systems: rat neu-induced TUBO and human HER2-transfected D2F2/E2. Intra-tumoral (i.t.) Adv/CD-TK injection of TUBO tumor plus systemic prodrug therapy showed limited antitumor activity, not exceeding that by the virus itself. Antibody (Ab) to the virus was induced in Adv-/Luc-treated mice, to coincide with the loss of transgene expression. Low replication activity of adenoviruses in rodent cells may limit viral persistence. Host immunity against Adv or Adv-infected cells further mutes suicide gene activity. Treatment of TUBO tumors with Adv/CD-TK alone, however, induced neu-specific Ab responses. Treatment with Adv/CD-TK/GM (Adv/GM) that also expressed mouse granulocyte macrophage colony stimulating factor (GM-CSF), but without prodrug treatment, delayed tumor growth, enhanced anti-neu Ab production and conferred complete protection against secondary tumor challenge. D2F2/E2 tumor-bearing mice showed decreased tumor growth following i.t. Adv/GM treatment and they generated greater HER2-specific T-cell responses. These data suggest that i.t. injection of Adv itself induces immune reactivity to tumor-associated antigens and the encoded cytokine, GM-CSF, amplifies that immune response, resulting in tumor growth inhibition. Incorporation of suicide gene therapy did not improve the efficacy of Adv therapy in this mouse mammary tumor system. Oncolytic adenoviral therapy may be streamlined and improved by substituting the suicide genes with immune modulating genes to exploit tumor immunity for therapeutic benefit.

Tapping the Potential of DNA Delivery with Electroporation for Cancer Immunotherapy

Cancer is a worldwide leading cause of death, and current conventional therapies are limited. The search for alternative preventive or therapeutic solutions is critical if we are going to improve outcomes for patients. The potential for DNA vaccines in the treatment and prevention of cancer has gained great momentum since initial findings almost 2 decades ago that revealed that genetically engineered DNA can elicit an immune response. The combination of adjuvants and an effective delivery method such as electroporation is overcoming past setbacks for naked plasmid DNA (pDNA) as a potential preventive or therapeutic approach to cancer in large animals and humans. In this chapter, we aim to focus on the novel advances in recent years for DNA cancer vaccines, current preclinical data, and the importance of adjuvants and electroporation with emphasis on prostate, melanoma, and cervical cancer.

Electrotransfer of single-stranded or double-stranded DNA induces complete regression of palpable B16.F10 mouse melanomas

Enhanced tumor delivery of plasmid DNA with electric pulses in vivo has been confirmed in many preclinical models. Intratumor electrotransfer of plasmids encoding therapeutic molecules has reached Phase II clinical trials. In multiple preclinical studies, a reduction in tumor growth, increased survival or complete tumor regression have been observed in control groups in which vector or backbone plasmid DNA electrotransfer was performed. This study explores factors that could produce this antitumor effect. The specific electrotransfer pulse protocol employed significantly potentiated the regression. Tumor regression was observed after delivery of single-stranded or double-stranded DNA with or without CpG motifs in both immunocompetent and immunodeficient mice, indicating the involvement of the innate immune system in response to DNA. In conclusion, this study demonstrated that the observed antitumor effects are not due to a single factor, but to a combination of factors.

Induction of proapoptotic antibodies to triple-negative breast cancer by vaccination with TRAIL death receptor DR5 DNA

TNF-related apoptosis-inducing ligand receptor 2 [TRAIL-R2 or death receptor 5 (DR5)] is expressed at elevated levels in a broad range of solid tumors to mediate apoptotic signals from TRAIL or agonist antibodies. We tested the hypothesis that DR5 DNA vaccination will induce proapoptotic antibody to trigger apoptosis of tumor cells. BALB/c mice were electrovaccinated with DNA-encoding wild-type human DR5 (phDR5) or its derivatives. Resulting immune serum or purified immune IgG induced apoptosis in triple-negative breast cancer (TNBC) cells, which were also TRAIL sensitive. The proapoptotic activity of immune serum at dilutions of 0.5-2% was comparable to that of 1-2 μg/ml of TRAIL. Apoptotic activity of immune serum was enhanced by antibody crosslinking. Apoptotic cell death induced by anti-DR5 antibody was shown by the cleavage of PARP and caspase-3. In contrast, immune serum had no effect on the proliferation of activated human T cells, which expressed low levels of DR5. In vivo, hDR5 reactive immune serum prevented growth of SUM159 TNBC cells in severe combined immune-deficient mice. DR5-specific IFN-γ-secreting T cells were also induced by DNA vaccination. Furthermore, the feasibility to overcome immune tolerance to self DR5 was shown by the induction of mouse DR5-binding antibody after electrovaccination of BALB/c mice with pmDR5ectm-Td1 encoding a fusion protein of mouse DR5 and an immunogenic fragment of tetanus toxin. These findings support DR5 as a promising vaccine target for controlling TNBC and other DR5-positive cancers.