Survivin the battle against immunosuppression

Improving on the limited success of cancer immunotherapy requires new approaches to inhibit immunosuppressive pathways initiated by tumor cells to "escape" protective immunity. One unique approach utilizes Salmonella for systemic delivery of inhibitory RNA, targeting the immunosuppressive molecule Stat3, and a Survivin vaccine to suppress growth of aggressive murine tumors.

Salmonella-Based Therapy Targeting Indoleamine 2,3-Dioxygenase Restructures the Immune Contexture to Improve Checkpoint Blockade Efficacy

Therapeutic options for non-small cell lung cancer (NSCLC) treatment have changed dramatically in recent years with the advent of novel immunotherapeutic approaches. Among these, immune checkpoint blockade (ICB) using monoclonal antibodies has shown tremendous promise in approximately 20% of patients. In order to better predict patients that will respond to ICB treatment, biomarkers such as tumor-associated CD8⁺ T cell frequency, tumor checkpoint protein status and mutational burden have been utilized, however, with mixed success. In this study, we hypothesized that significantly altering the suppressive tumor immune landscape in NSCLC could potentially improve ICB efficacy. Using sub-therapeutic doses of our Salmonella typhimurium-based therapy targeting the suppressive molecule indoleamine 2,3-dioxygenase (shIDO-ST) in tumor-bearing mice, we observed dramatic changes in immune subset phenotypes that included increases in antigen presentation markers, decreased regulatory T cell frequency and overall reduced checkpoint protein expression. Combination shIDO-ST treatment with anti-PD-1/CTLA-4 antibodies enhanced tumor growth control, compared to either treatment alone, which was associated with significant intratumoral infiltration by CD8⁺ and CD4⁺ T cells. Ultimately, we show that increases in antigen presentation markers and infiltration by T cells is correlated with significantly increased survival in NSCLC patients. These results suggest that the success of ICB therapy may be more accurately predicted by taking into account multiple factors such as potential for antigen presentation and immune subset repertoire in addition to markers already being considered. Alternatively, combination treatment with agents such as shIDO-ST could be used to create a more conducive tumor microenvironment for improving responses to ICB.

Development of a multi-antigenic SARS-CoV-2 vaccine candidate using a synthetic poxvirus platform

Modified Vaccinia Ankara (MVA) is a highly attenuated poxvirus vector that is widely used to develop vaccines for infectious diseases and cancer. We demonstrate the construction of a vaccine platform based on a unique three-plasmid system to efficiently generate recombinant MVA vectors from chemically synthesized DNA. In response to the ongoing global pandemic caused by SARS coronavirus-2 (SARS-CoV-2), we use this vaccine platform to rapidly produce fully synthetic MVA (sMVA) vectors co-expressing SARS-CoV-2 spike and nucleocapsid antigens, two immunodominant antigens implicated in protective immunity. We show that mice immunized with these sMVA vectors develop robust SARS-CoV-2 antigen-specific humoral and cellular immune responses, including potent neutralizing antibodies. These results demonstrate the potential of a vaccine platform based on synthetic DNA to efficiently generate recombinant MVA vectors and to rapidly develop a multi-antigenic poxvirus-based SARS-CoV-2 vaccine candidate.

Targeting desmoplasia in pancreatic cancer as an essential first step to effective therapy

Pancreatic cancer is considered one of the most lethal cancers in the US. It contributes to an estimated 47,000 deaths annually and is predicted to surpass prostate, breast and colorectal cancers as the leading cause of cancer-related death. Although major advancements in cancer treatment have improved outcomes for many cancer types, survival rate for pancreatic cancer has not improved in nearly four decades despite tremendous effort. One attribute of pancreatic cancer that is considered a major barrier to effective treatment is the formation of fibrotic tissue around tumor cells known as desmoplasia. A number of promising approaches have been developed to deplete fibrotic components in pancreatic tumors to enhance drug delivery, some of which have been tested in clinical trials of advanced, unresectable pancreatic cancer. Here, we discuss previous efforts, shortcomings and new considerations for developing more effective agents to eliminate desmoplasia.

Development of a Synthetic Poxvirus-Based SARS-CoV-2 Vaccine

Modified Vaccinia Ankara (MVA) is a highly attenuated poxvirus vector that is widely used to develop vaccines for infectious diseases and cancer. We developed a novel vaccine platform based on a unique three-plasmid system to efficiently generate recombinant MVA vectors from chemically synthesized DNA. In response to the ongoing global pandemic caused by SARS coronavirus-2 (SARS-CoV-2), we used this novel vaccine platform to rapidly produce fully synthetic MVA (sMVA) vectors co-expressing SARS-CoV-2 spike and nucleocapsid antigens, two immunodominant antigens implicated in protective immunity. Mice immunized with these sMVA vectors developed robust SARS-CoV-2 antigen-specific humoral and cellular immune responses, including potent neutralizing antibodies. These results demonstrate the potential of a novel vaccine platform based on synthetic DNA to efficiently generate recombinant MVA vectors and to rapidly develop a multi-antigenic poxvirus-based SARS-CoV-2 vaccine candidate.

5-Azacytidine Potentiates Anti-tumor Immunity in a Model of Pancreatic Ductal Adenocarcinoma

Tumors evolve a variety of mechanisms to escape immune detection while expressing tumor-promoting molecules that can be immunogenic. Here, we show that transposable elements (TE) and gene encoded, tumor-associated antigens (TAA), which can be both highly immunogenic and tumor-promoting, are significantly upregulated during the transition from pre-malignancy to malignancy in an inducible model of pancreatic ductal adenocarcinoma (PDAC). Coincident with the increased presence of TEs and TAAs was the downregulation of gene transcripts associated with antigen presentation, T cell recruitment and intrinsic anti-viral responses, suggesting a unique strategy employed by PDAC to possibly augment tumorigenesis while escaping detection by the immune system. In vitro treatment of mouse and human PDAC cell lines with the DNA methyltransferase inhibitor 5-azacytidine (Aza) resulted in augmented expression of transcripts for antigen presentation machinery and T cell chemokines. When immunocompetent mice implanted with PDAC were therapeutically treated with Aza, we observed significant tumor regression that was not observed in immunocompromised mice, implicating anti-tumor immunity as the principal mechanism of tumor growth control. Analysis of PDAC tumors, immediately following Aza treatment in immunocompetent mice, revealed a significantly greater infiltration of T cells and various innate immune subsets compared to control treatment, suggesting that Aza treatment enhances tumor immunogenicity. Thus, augmenting antigen presentation and T cell chemokine expression using DNA methyltransferase inhibitors could be leveraged to potentiate adaptive anti-tumor immune responses against PDAC.

Hyaluronidase-Expressing Salmonella Effectively Targets Tumor-Associated Hyaluronic Acid in Pancreatic Ductal Adenocarcinoma

In pancreatic ductal adenocarcinoma (PDAC), the extracellular matrix (ECM) surrounding cancer cells forms a barrier that often limits the ability of chemotherapeutic drugs and cytotoxic immune subsets to penetrate and eliminate tumors. The dense stromal matrix protecting cancer cells, also known as desmoplasia, results from the overproduction of major ECM components such as collagens and hyaluronic acid (HA). Although candidate drugs targeting ECM components have shown promise in increasing penetration of chemotherapeutic agents, severe adverse effects associated with systemic depletion of ECM in peripheral healthy tissues limits their use at higher, more effective doses. Currently, few strategies exist that preferentially degrade ECM in tumor tissue over healthy tissues. In light of this, we have developed an attenuated, tumor-targeting Salmonella typhimurium (ST) expressing functional bacterial hyaluronidase (bHs-ST), capable of degrading human HA deposited within PDAC tumors. Our data show that bHs-ST (i) targets and colonizes orthotopic human PDAC tumors following systemic administration and (ii) is efficiently induced in vivo to deplete tumor-derived HA, which in turn (iii) significantly increases diffusion of Salmonella typhimurium within desmoplastic tumors. BHs-ST represents a promising new tumor ECM-targeting strategy that may be instrumental in minimizing off-tumor toxicity while maximizing drug delivery into highly desmoplastic tumors.

Desmoplasia and oncogene driven acinar-to-ductal metaplasia are concurrent events during acinar cell-derived pancreatic cancer initiation in young adult mice

The five-year survival rate of patients diagnosed with advanced pancreatic ductal adenocarcinoma (PDAC) has remained static at <5% despite decades of research. With the exception of erlotinib, clinical trials have failed to demonstrate the benefit of any targeted therapy for PDAC despite promising results in preclinical animal studies. The development of more refined mouse models of PDAC which recapitulate the carcinogenic progression from non-neoplastic, adult exocrine subsets of pancreatic cells to invasive carcinoma in humans are needed to facilitate the accurate translation of therapies to the clinic. To study acinar cell-derived PDAC initiation, we developed a genetically engineered mouse model of PDAC, called KPT, utilizing a tamoxifen-inducible Cre recombinase/estrogen receptor (ESR1) fusion protein knocked into the Ptf1a locus to activate the expression of oncogenic KrasG12D and Trp53R270H alleles in mature pancreatic acinar cells. Oncogene-expressing acinar cells underwent acinar-to-ductal metaplasia, and formed pancreatic intraepithelial neoplasia lesions following the induction of oncogene expression. After a defined latency period, oncogene-expressing acinar cells initiated the formation of highly differentiated and fibrotic tumors, which metastasized to the lungs and liver. Whole-transcriptome analysis of microdissected regions of acinar-to-ductal metaplasia and histological validation experiments demonstrated that regions of acinar-to-ductal metaplasia are characterized by the deposition of the extracellular matrix component hyaluronan. These results indicate that acinar cells expressing KrasG12D and Trp53R270H can initiate PDAC development in young adult mice and implicate hyaluronan deposition in the formation of the earliest characterized PDAC precursor lesions (and the progression of pancreatic cancer). Further studies are necessary to provide a comprehensive characterization of PDAC progression and treatment response in KPT mice and to investigate whether the KPT model could be used as a tool to study translational aspects of acinar cell-derived PDAC tumorigenesis.

Salmonella-mediated therapy targeting indoleamine 2, 3-dioxygenase 1 (IDO) activates innate immunity and mitigates colorectal cancer growth

Patients with colon cancer remain largely refractory to current immunotherapeutic strategies. This is, in part, due to the overexpression of the immune checkpoint protein indoleamine 2,3-dioxygenase 1 (IDO). IDO is an important enzyme contributing to tumor-mediated immunosuppression and also correlates with poor prognosis in colon cancer patients. The aim of this study was to assess the therapeutic efficacy of attenuated Salmonella typhimurium delivering an shRNA plasmid targeting IDO (shIDO-ST) in two mouse models of colorectal cancer. In vitro, the CT26 and MC38 murine colon cancer cell lines were shown to upregulate IDO expression following stimulation with interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α). Transfection of both cell lines with shIDO plasmid reduced IDO protein expression and function. In vivo, shIDO-ST treatment significantly delayed CT26 and MC38 tumor progression compared to mice treated with scrambled shRNA control (shScr-ST) or the clinically-tested IDO inhibitor epacadostat. Increased tumor infiltration of neutrophils was found to be the primary immune cell population associated with shIDO-ST treatment, suggesting robust activation of innate immunity. Although increased tumor expression of IDO is associated with resistance to antibody therapy against programed cell death-1 (anti-PD1), co-administration of anti-PD1 with shIDO-ST did not provide additional tumor growth control in either model of colorectal cancer. Altogether, we demonstrate that treatment with shIDO-ST markedly delays tumor growth in two immunocompetent colorectal mouse models and this appears to be a superior therapeutic strategy compared to epacadostat or blocking anti-PD1 antibody therapy in colon cancer.

Utilizing Salmonella to treat solid malignancies

Despite intensive research into novel treatment strategies for cancer, it remains the second most common cause of death in industrialized populations. Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive disease with dismal prognosis. Currently, surgical resection offers the best chance for extended survival, yet recurrence remains high and is associated with poor outcome. Systemic treatment has evolved from non-specific, cytotoxic chemotherapy to the use of cancer-targeting agents, profoundly changing treatment approaches in the metastatic and adjuvant settings. One promising approach, highlighted in this review, uses the inherent capacity of Salmonella to colonize and eliminate solid tumors.

Evaluation of innate and adaptive immunity contributing to the antitumor effects of PD1 blockade in an orthotopic murine model of pancreatic cancer

Despite the clinical success of anti-PD1 antibody (α-PD1) therapy, the immune mechanisms contributing to the antineoplastic response remain unclear. Here, we describe novel aspects of the immune response involved in α-PD1-induced antitumor effects using an orthotopic Kras (G12D)/p53(R172H)/Pdx1-Cre (KPC) model of pancreatic ductal adenocarcinoma (PDA). We found that positive therapeutic outcome involved both the innate and adaptive arms of the immune system. Adoptive transfer of total splenocytes after short-term (3 d) but not long-term (28 d) PD1 blockade significantly extended survival of non-treated tumor-bearing recipient mice. This protective effect appeared to be mostly mediated by T cells, as adoptive transfer of purified natural killer (NK) cells and/or granulocyte receptor 1 (Gr1)(+) cells or splenocytes depleted of Gr1(+) cells and NK cells did not exhibit transferrable antitumor activity following short-term PD1 blockade. Nevertheless, splenic and tumor-derived CD11b(+)Gr1(+) cells and NK cells showed significant persistence of α-PD1 bound to these cells in the treated primary recipient mice. We observed that short-term inhibition of PD1 signaling modulated the profiles of multifunctional cytokines in the tumor immune-infiltrate, including downregulation of vascular endothelial growth factor A (VEGF-A). Altogether, the data suggest that systemic blockade of PD1 results in rapid modulation of antitumor immunity that differs in the tumor microenvironment (TME) when compared to the spleen. These results demonstrate a key role for early immune-mediated events in controlling tumor progression in response to α-PD1 treatment and warrant further investigation into the mechanisms governing responses to the therapy at the innate-adaptive immune interface.

Metronomic Doses of Temozolomide Enhance the Efficacy of Carbon Nanotube CpG Immunotherapy in an Invasive Glioma Model

Even when treated with aggressive current therapies, most patients with glioblastoma survive less than two years. Rapid tumor growth, an invasive nature, and the blood-brain barrier, which limits the penetration of large molecules into the brain, all contribute to the poor tumor response associated with conventional therapies. Immunotherapy has emerged as a therapeutic approach that may overcome these challenges. We recently reported that single-walled carbon nanotubes (SWCNTs) can be used to dramatically increase the immunotherapeutic efficacy of CpG oligonucleotides in a mouse model of glioma. Following implantation in the mouse brain, the tumor cell line used in these previous studies (GL261) tends to form a spherical tumor with limited invasion into healthy brain. In order to evaluate SWCNT/CpG therapy under more clinically-relevant conditions, here we report the treatment of a more invasive mouse glioma model (K-Luc) that better recapitulates human disease. In addition, a CpG sequence previously tested in humans was used to formulate the SWCNT/CpG which was combined with temozolomide, the standard of care chemotherapy for glioblastoma patients. We found that, following two intracranial administrations, SWCNT/CpG is well-tolerated and improves the survival of mice bearing invasive gliomas. Interestingly, the efficacy of SWCNT/CpG was enhanced when combined with temozolomide. This enhanced anti-tumor efficacy was correlated to an increase of tumor-specific cytotoxic activity in splenocytes. These results reinforce the emerging understanding that immunotherapy can be enhanced by combining it with chemotherapy and support the continued development of SWCNT/CpG.

Salmonella-Based Therapy Targeting Indoleamine 2,3-Dioxygenase Coupled with Enzymatic Depletion of Tumor Hyaluronan Induces Complete Regression of Aggressive Pancreatic Tumors

Bacterial-based therapies are emerging as effective cancer treatments and hold promise for refractory neoplasms, such as pancreatic ductal adenocarcinoma (PDAC), which has not shown significant improvement in therapy for more than 25 years. Using a novel combination of shIDO-ST, a Salmonella-based therapy targeting the immunosuppressive molecule indoleamine 2,3-dioxygenase (IDO), with an enzyme, PEGPH20, which depletes extracellular matrix hyaluronan, we observed extended survival with frequent total regression of autochthonous and orthotopic PDAC tumors. This observation was associated with migration and accumulation of activated polymorphonuclear neutrophils (PMN) from spleens into tumors, which was not seen using a scrambled control (shScr-ST). Purified splenic PMNs from PEGPH20/shIDO-ST-treated mice exhibited significant IDO knockdown and were able to kill tumor targets ex vivo through mechanisms involving FasL and serine proteases. In addition, CD8(+) T cells were observed to contribute to late control of pancreatic tumors. Collectively, our data demonstrate that entry of shIDO-ST and PMNs into otherwise impermeable desmoplastic tumors is facilitated by PEGPH20-mediated HA removal, further highlighting an important component of effective treatment for PDAC.

Abstract A97: Control of PDAC tumor progression by RNA interference targeting indoleamine 2,3-dioxygenase using salmonella as a delivery vehicle facilitated by PEGPH20

The dismal prognosis of late stage, unresectable pancreatic adenocarcinoma (PDAC) patients has prompted the search for novel chemotherapeutic agents. However, PDAC tumors are encapsulated by a remarkable fibrotic extracellular matrix (ECM), termed desmoplasia that prevents efficient intratumoral delivery of therapeutics. Thus, current PDAC therapies such as gemcitabine, Abraxane, erlotinib, and folfirinox must be given at elevated doses resulting in systemic toxicity with only marginal improvement in patient survival. The increased deposition of ECM components, such as hyaluronan (HA), surrounding and internal to the tumor mass serves as the ideal target for reducing desmoplasia and improving drug delivery. PEGPH20 is a pegylated hyaluronidase developed by Halozyme Therapeutics that depletes HA and increases vascular permeability in spontaneous murine PDAC tumors. Data from a Phase 1b trial indicates a clinical benefit of using PEGPH20 with gemcitabine in patients with high levels of PDAC-associated HA. In the present work, we tested a unique combination of PEGPH20 with a Salmonella-based therapeutic, shIDO-ST, which expresses an inhibitory RNA (RNAi) that degrades indoleamine 2,3-dioxygenase (IDO) mRNA. We observed striking regression in 100% of mice (N=4/group) with our unique therapeutic combination leading to elimination of orthotopically transplanted PDAC tumors, whereas combinations of gemcitabine and Abraxane marginally stabilized tumor growth (p&lt;0.001, ANOVA). The conditions of these experiments were 90 μg PEGPH20 administered alone or combined with shIDO-ST (5 million CFU on day 14 and 18 (post-tumor implantation)), shScrambled (Scr)-ST (5 million CFU on day 14 and 18), or Abraxane (120 mg/Kg given i.v. on days 14, 18, and 22) + Gemcitabine (100 mg/Kg given i.p. on days 14, 18, and 22). This remarkable and unexpected result is the first demonstration that PEGPH20 facilitates entry of a bacterium into the intratumoral space. This response was uniquely associated with accumulation of shIDO-ST and neutrophils in the tumor which confirms the importance of the shIDO component, since a control scrambled RNAi expressed from Salmonella did not accumulate nor caused regression of PDAC tumors. Tracking experiments in shIDO-ST + PEGPH20 treated mice revealed that the neutrophils originate from the spleen and later migrate into the tumor. Surprisingly, purified splenic neutrophils from these mice were sufficient to kill PDAC tumor cells ex-vivo in cytotoxicity assays. We also observed significant IDO knockdown in purified neutrophils that corresponded to a greater activation phenotype typical of anti-tumor (N1) polarized neutrophils. Neutrophil killing of tumor targets could be inhibited in a dose-dependent manner using heparin or polysialic acid treatment, which specifically acts to neutralize the apoptosis-inducing activity of histones in the DNA matrixes formed as part of neutrophil extracellular traps (NETs). Collectively, these data support continued development of a combination therapy that overcomes desmoplasia in PDAC with remarkable anti-tumor activity.

Supported by R21 CA174306, the Tim Nesvig Foundation, and Halozyme Therapeutics

Citation Format: Edwin R. Manuel, Melanie G. Lampa, Joseph Kim, Vincent Chung, Massimo D'Apuzzo, Curtis B. Thompson, Zhongdong Huang, Daniel C. Maneval, Don J. Diamond. Control of PDAC tumor progression by RNA interference targeting indoleamine 2,3-dioxygenase using salmonella as a delivery vehicle facilitated by PEGPH20. [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Innovations in Research and Treatment; May 18-21, 2014; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2015;75(13 Suppl):Abstract nr A97.

A road less traveled paved by IDO silencing: Harnessing the antitumor activity of neutrophils

Orchestrating a cytotoxic polymorphonuclear neutrophil (PMN) response strictly focused within the tumor tissue remains a formidable challenge for the successful therapeutic use of these cells. A Salmonella vector carrying an shRNA against indoleamine 2,3-dioxygenase has been shown to recruit PMNs and enhance their activation specifically in the tumor bed, resulting in significant anticancer effects.

Effective cancer vaccine platform based on attenuated salmonella and a type III secretion system

Vaccines explored for cancer therapy have been based generally on injectable vector systems used to control foreign infectious pathogens, to which the immune system evolved to respond naturally. However, these vectors may not be effective at presenting tumor-associated antigens (TAA) to the immune system in a manner that is sufficient to engender antitumor responses. We addressed this issue with a novel orally administered Salmonella-based vector that exploits a type III secretion system to deliver selected TAA in the cytosol of professional antigen-presenting cells in situ. A systematic comparison of candidate genes from the Salmonella Pathogenicity Island 2 (SPI2) locus was conducted in the vaccine design, using model antigens and a codon-optimized form of the human TAA survivin (coSVN), an oncoprotein that is overexpressed in most human cancers. In a screen of 20 SPI2 promoter:effector combinations, a PsifB::sseJ combination exhibited maximal potency for antigen translocation into the APC cytosol, presentation to CD8 T cells, and murine immunogenicity. In the CT26 mouse model of colon carcinoma, therapeutic vaccination with a lead PsifB::sseJ-coSVN construct (p8032) produced CXCR3-dependent infiltration of tumors by CD8 T cells, reversed the CD8:Treg ratio at the tumor site, and triggered potent antitumor activity. Vaccine immunogenicity and antitumor potency were enhanced by coadministration of the natural killer T-cell ligand 7DW8-5, which heightened the production of IL12 and IFNγ. Furthermore, combined treatment with p8032 and 7DW8-5 resulted in complete tumor regression in A20 lymphoma-bearing mice, where protective memory was demonstrated. Taken together, our results demonstrate how antigen delivery using an oral Salmonella vector can provide an effective platform for the development of cancer vaccines.

Systemic delivery of Salmonella typhimurium transformed with IDO shRNA enhances intratumoral vector colonization and suppresses tumor growth

Generating antitumor responses through the inhibition of tumor-derived immune suppression represents a promising strategy in the development of cancer immunotherapeutics. Here, we present a strategy incorporating delivery of the bacterium Salmonella typhimurium (ST), naturally tropic for the hypoxic tumor environment, transformed with a small hairpin RNA (shRNA) plasmid against the immunosuppressive molecule indoleamine 2,3-dioxygenase 1 (shIDO). When systemically delivered into mice, shIDO silences host IDO expression and leads to massive intratumoral cell death that is associated with significant tumor infiltration by polymorphonuclear neutrophils (PMN). shIDO-ST treatment causes tumor cell death independently of host IDO and adaptive immunity, which may have important implications for use in immunosuppressed patients with cancer. Furthermore, shIDO-ST treatment increases reactive oxygen species (ROS) produced by infiltrating PMNs and, conversely, PMN immunodepletion abrogates tumor control. Silencing of host IDO significantly enhances S. typhimurium colonization, suggesting that IDO expression within the tumor controls the immune response to S. typhimurium. In summary, we present a novel approach to cancer treatment that involves the specific silencing of tumor-derived IDO that allows for the recruitment of ROS-producing PMNs, which may act primarily to clear S. typhimurium infection, but in the process also induces apoptosis of surrounding tumor tissue resulting in a vigorous antitumor effect.

Intracerebral CpG immunotherapy with carbon nanotubes abrogates growth of subcutaneous melanomas in mice

PURPOSE

Recently, we showed that intratumoral delivery of low-dose, immunostimulatory CpG oligodeoxynucleotides conjugated with carbon nanotubes (CNT-CpG) was more effective than free CpG and not only eradicated intracranial (i.c.) gliomas but also induced antitumor immunity that protected mice from subsequent i.c. or systemic tumor rechallenge. Here, we examined whether the same "intracerebral immunotherapy" strategy could be applied to the treatment of metastatic brain tumors.

EXPERIMENTAL DESIGN

Mice with both i.c. and s.c. melanomas were injected intratumorally with CNT-CpG into either location. Antitumor responses were assessed by flow cytometry, bioluminescent imaging, and animal survival.

RESULTS

When given s.c., CNT-CpG response was mostly local, and it only modestly inhibited the growth of i.c. melanomas. However, i.c. CNT-CpG abrogated the growth of not only brain but also s.c. tumors. Furthermore, compared with s.c. injections, i.c. CNT-CpG elicited a stronger inflammatory response that resulted in more potent antitumor cytotoxicity and improved in vivo trafficking of effector cells into both i.c. and s.c. tumors. To investigate factors that accounted for these observations, CNT-CpG biodistribution and cellular inflammatory responses were examined in both tumor locations. Intracranial melanomas retained the CNT-CpG particles longer and were infiltrated by Toll-like receptor (TLR-9)-positive microglia. In contrast, myeloid-derived suppressive cells were more abundant in s.c. tumors. Although depletion of these cells before s.c. CNT-CpG therapy enhanced its cytotoxic responses, antitumor responses to brain melanomas were unchanged.

CONCLUSIONS

These findings suggest that intracerebral CNT-CpG immunotherapy is more effective than systemic therapy in generating antitumor responses that target both brain and systemic melanomas.

Enhancement of cancer vaccine therapy by systemic delivery of a tumor-targeting Salmonella-based STAT3 shRNA suppresses the growth of established melanoma tumors

Cancer vaccine therapies have only achieved limited success when focusing on effector immunity with the goal of eliciting robust tumor-specific T-cell responses. More recently, there is an emerging understanding that effective immunity can only be achieved by coordinate disruption of tumor-derived immunosuppression. Toward that goal, we have developed a potent Salmonella-based vaccine expressing codon-optimized survivin (CO-SVN), referred to as 3342Max. When used alone as a therapeutic vaccine, 3342Max can attenuate growth of aggressive murine melanomas overexpressing SVN. However, under more immunosuppressive conditions, such as those associated with larger tumor volumes, we found that the vaccine was ineffective. Vaccine efficacy could be rescued if tumor-bearing mice were treated initially with Salmonella encoding a short hairpin RNA (shRNA) targeting the tolerogenic molecule STAT3 (YS1646-shSTAT3). In vaccinated mice, silencing STAT3 increased the proliferation and granzyme B levels of intratumoral CD4(+) and CD8(+) T cells. The combined strategy also increased apoptosis in tumors of treated mice, enhancing tumor-specific killing of tumor targets. Interestingly, mice treated with YS1646-shSTAT3 or 3342Max alone were similarly unsuccessful in rejecting established tumors, whereas the combined regimen was highly potent. Our findings establish that a combined strategy of silencing immunosuppressive molecules followed by vaccination can act synergistically to attenuate tumor growth, and they offer a novel translational direction to improve tumor immunotherapy.

Modified vaccinia Ankara expressing survivin combined with gemcitabine generates specific antitumor effects in a murine pancreatic carcinoma model

Survivin is overexpressed by 70-80% of pancreatic cancers, and is associated with resistance to chemotherapy and a poor prognosis. Gemcitabine has been a standard treatment for patients with advanced pancreatic cancer for a decade. Recent reports have demonstrated that gemcitabine treatment attenuates the tumor-suppressive environment by eliminating CD11b(+)/Gr-1(+) myeloid-derived suppressor cells (MDSCs). We hypothesize that a cancer vaccine targeting survivin can achieve enhanced efficacy when combined with gemcitabine. In this study, we tested this hypothesis using modified vaccinia Ankara (MVA) expressing full-length murine survivin. The poorly immunogenic mouse pancreas adenocarcinoma cell line, Pan02, which expresses murine survivin and is syngeneic to C57BL/6, was used for this study. Immunization with MVA-survivin resulted in a modest therapeutic antitumor effect on established Pan02 tumors. When administered with gemcitabine, MVA-survivin immunization resulted in significant tumor regression and prolonged survival. The enhanced vaccine efficacy was associated with decreased CD11b(+)/Gr-1(+) MDSCs. To analyze the survivin-specific immune response to MVA-survivin immunization, we utilized a peptide library of 15mers with 11 residues overlapping from full-length murine survivin. Splenocytes from mice immunized with MVA-survivin produced intracellular γ-interferon in response to in vitro stimulation with the overlapping peptide library. Increased survivin-specific CD8(+) T cells that specifically recognized the Pan02 tumor line were seen in mice treated with MVA-survivin and gemcitabine. These data suggest that vaccination with MVA-survivin in combination with gemcitabine represents an attractive strategy to overcome tumor-induced peripheral immune tolerance, and this effect has potential for clinical benefit in pancreatic cancer.

Intergenic region 3 of modified vaccinia ankara is a functional site for insert gene expression and allows for potent antigen-specific immune responses

Integration of exogenous DNA into modified vaccinia Ankara (MVA) is often accomplished using mapped deletion sites in the viral genome. Since MVA has a large capacity (> or =30kb) for foreign gene inserts and a limited number of unique integration sites, development of additional integration sites is needed to take full advantage of the extraordinary capacity for foreign gene insertion. In this report, we evaluate an alternative insertion site known as intergenic region 3 (IGR3). Recombinant MVA carrying the cytomegalovirus pp65 gene in IGR3 (rMVA-pp65-IGR3) demonstrated expression and genetic stability of the insert gene upon passage. Immunization of transgenic HLA-A2 mice with rMVA-pp65-IGR3 induced robust antigen-specific immune responses. Moreover, rMVA-pp65-IGR3-infected human EBV-transformed B cell lines were able to stimulate high levels of pp65-specific memory T cell responses in human PBMCs. These data support the usage of IGR3 for the development of highly immunogenic rMVA vaccines for clinical or veterinary use.

Contribution of T-cell receptor repertoire breadth to the dominance of epitope-specific CD8+ T-lymphocyte responses

Dominant epitope-specific CD8(+) T-lymphocyte responses play a central role in controlling viral spread. We explored the basis for the development of this focused immune response in simian immunodeficiency virus (SIV)- and simian-human immunodeficiency virus (SHIV)-infected rhesus monkeys through the use of two dominant (p11C and p199RY) and two subdominant (p68A and p56A) epitopes. Using real-time PCR to quantitate T-cell receptor (TCR) variable region beta (Vbeta) family usage, we show that CD8(+) T-lymphocyte populations specific for dominant epitopes are characterized by a diverse Vbeta repertoire, whereas those specific for subdominant epitopes employ a dramatically more focused Vbeta repertoire. We also demonstrate that dominant epitope-specific CD8(+) T lymphocytes employ TCRs with multiple CDR3 lengths, whereas subdominant epitope-specific cells employ TCRs with a more restricted CDR3 length. Thus, the relative dominance of an epitope-specific CD8(+) T-lymphocyte response reflects the clonal diversity of that response. These findings suggest that the limited clonal repertoire of subdominant epitope-specific CD8(+) T-lymphocyte populations may limit the ability of these epitope-specific T-lymphocyte populations to expand and therefore limit the ability of these cell populations to contribute to the control of viral replication.