Viswa Teja Colluru

DNA vaccines for prostate cancer

DNA vaccines offer many advantages over other anti-tumor vaccine approaches due to their simplicity, ease of manufacturing, and safety. Results from several clinical trials in patients with cancer have demonstrated that DNA vaccines are safe and can elicit immune responses. However, to date few DNA vaccines have progressed beyond phase I clinical trial evaluation. Studies into the mechanism of action of DNA vaccines in terms of antigen-presenting cell types able to directly present or cross-present DNA-encoded antigens, and the activation of innate immune responses due to DNA itself, have suggested opportunities to increase the immunogenicity of these vaccines. In addition, studies into the mechanisms of tumor resistance to anti-tumor vaccination have suggested combination approaches that can increase the anti-tumor effect of DNA vaccines. This review focuses on these mechanisms of action and mechanisms of resistance using DNA vaccines, and how this information is being used to improve the anti-tumor effect of DNA vaccines. These approaches are then specifically discussed in the context of human prostate cancer, a disease for which DNA vaccines have been and continue to be explored as treatments.

Vaccination with High-Affinity Epitopes Impairs Antitumor Efficacy by Increasing PD-1 Expression on CD8 + T Cells

T cells activated by high-affinity epitopes are not guaranteed strong antitumor activity. High-affinity epitopes prolonged APC: T-cell contact time and led to elevated, persistent PD-1 expression and decreased antitumor efficacy in the absence of PD-1 blockade. Antitumor vaccines encoding self-antigens generally have low immunogenicity in clinical trials. Several approaches are aimed at improving vaccine immunogenicity, including efforts to alter encoded epitopes. Immunization with epitopes altered for increased affinity for the major histocompatibility complex (MHC) or T-cell receptor (TCR) elicits greater numbers of CD8 T cells but inferior antitumor responses. Our previous results suggested that programmed death 1 (PD-1) and its ligand (PD-L1) increased on antigen-specific CD8 T cells and tumor cells, respectively, after high-affinity vaccination. In this report, we use two murine models to investigate whether the dose, MHC affinity, or TCR affinity of an epitope affected the antitumor response via the PD-1/PD-L1 axis. T cells activated with high-affinity epitopes resulted in prolonged APC:T-cell contact time that led to elevated, persistent PD-1 expression, and expression of other checkpoint molecules, in vitro and in vivo. Immunization with high-affinity epitopes also decreased antitumor efficacy in the absence of PD-1 blockade. Thus, APC:T-cell contact time can be altered by epitope affinity and lead to therapeutically relevant changes in vaccine efficacy mediated by changes in PD-1 expression. These findings have implications for the use of agents targeting PD-1 expression or function whenever high-affinity CD8 T cells are elicited or supplied by means of vaccination or adoptive transfer. Cancer Immunol Res; 5(8); 630–41. ©2017 AACR.

B lymphocytes as direct antigen-presenting cells for anti-tumor DNA vaccines

In spite of remarkable preclinical efficacy, DNA vaccination has demonstrated low immunogenicity in humans. While efforts have focused on increasing cross-presentation of DNA-encoded antigens, efforts to increase DNA vaccine immunogenicity by targeting direct presentation have remained mostly unexplored. In these studies, we compared the ability of different APCs to present antigen to T cells after simple co-culture with plasmid DNA. We found that human primary peripheral B lymphocytes, and not monocytes or in vitro derived dendritic cells (DCs), were able to efficiently encode antigen mRNA and expand cognate tumor antigen-specific CD8 T cells ex vivo. Similarly, murine B lymphocytes co-cultured with plasmid DNA, and not DCs, were able to prime antigen-specific T cells in vivo. Moreover, B lymphocyte-mediated presentation of plasmid antigen led to greater Th1-biased immunity and was sufficient to elicit an anti-tumor effect in vivo. Surprisingly, increasing plasmid presentation by B cells, and not cross presentation of peptides by DCs, further augmented traditional plasmid vaccination. Together, these data suggest that targeting plasmid DNA to B lymphocytes, for example through transfer of ex vivo plasmidloaded B cells, may be novel means to achieve greater T cell immunity from DNA vaccines.

Mini-intronic plasmid vaccination elicits tolerant LAG3+ CD8+ T cells and inferior antitumor responses

ABSTRACT Increasing transgene expression has been a major focus of attempts to improve DNA vaccine-induced immunity in both preclinical studies and clinical trials. Novel mini-intronic plasmids (MIPs) have been shown to cause elevated and sustained transgene expression in vivo. We sought to test the antitumor activity of a MIP, compared to standard DNA plasmid immunization, using the tumor-specific antigen SSX2 in an HLA-A2-restricted tumor model. We found that MIP vaccination elicited a greater frequency of antigen-specific CD8+ T cells when compared to conventional plasmid, and protected animals from subsequent tumor challenge. However, therapeutic vaccination with the MIP resulted in an inferior antitumor effect, and CD8+ tumor-infiltrating lymphocytes from these mice expressed higher levels of surface LAG3. Antitumor efficacy of MIP vaccination could be recovered upon antibody blockade of LAG3. In non-tumor bearing mice, MIP immunization led to a loss of epitope dominance, attenuated CD8+ cytokine responses to the dominant p103 epitope, and increased LAG3 expression on p103-specific CD8+ T cells. Further, LAG3 expression on CD8+ T cells was associated with antigen dose and persistence in spite of DNA-induced innate immunity. These data suggest that for antitumor immunization, approaches leading to increased antigen expression following vaccination might optimally be combined with LAG3 inhibition in human trials. On the other hand, mini-intronic vector approaches may be a superior means to elicit LAG3-dependent tolerance in the treatment of autoimmune diseases.

In-silico characterization of ECE-1 inhibitors

Atherosclerosis is the primary cause of CAD and cerebrovascular disease. Endothelin (ET)-1 is a vasoconstrictive peptide implicated in Atherosclerosis pathology. Endothelin-converting enzyme (ECE) is a membrane metalloprotease that generates endothelin. Reported inhibitors of ECE-1 and their IC(50) values were retrieved from literature and their structures were docked with the parent protein using the Molegro virtual docker. The obtained MolDock scores of each of the compounds are hereby reported and are subject to graphical analysis in conjunction with their respective IC(50) values to characterize potent inhibitors. A search was then run in the ZINC database for compounds with similar properties. Potent inhibitors with higher Dock scores and better Ranking were isolated and are reported.

Safety and Immunological Efficacy of a DNA Vaccine Encoding the Androgen Receptor Ligand-Binding Domain (AR-LBD)

The androgen receptor (AR) is a key oncogenic driver of prostate cancer, and has been the primary focus of prostate cancer treatment for several decades. We have previously demonstrated that the AR is also an immunological target antigen, recognized in patients with prostate cancer, and targetable by means of vaccines in rodent models with delays in prostate tumor growth. The current study was performed to determine the safety and immunological efficacy of a GMP‐grade plasmid DNA vaccine encoding the ligand‐binding domain (LBD) of the AR, pTVG‐AR.

B lymphocyte mediated antigen presentation of plasmid DNA

Meeting abstracts DNA vaccination is a safe and economical therapeutic modality that has demonstrated robust elicitation of (cellular and humoral) immunity and remarkable pre-clinical efficacy in over 30 disease models, including those of breast, prostate and colon malignancies, multiple myeloma,

Abstract 2360: GP350 containing exosomes as DNA vaccine carriers

Background: Plasmid DNA vaccination is a safe and economical vaccine modality that has been demonstrated to elicit cellular and humoral immunity in preclinical models and human clinical trials. We have previously shown that enriched mature naive B cells, and not myeloid/DC lineages, are capable of expanding Ag specific CD8 T cells in vivo in mice, and in vitro with human cells. In this study, we sought a means to directly deliver plasmid DNA to B cells as a means to specifically activate this antigen-presenting cell type. Methods: Exosomes were isolated from EBV+ lymphoblastic cell lines (LCL) and from HEK293T cells that had been transfected to express the EBV protein GP350. Plasmid DNA was labeled with a fluorescent peptide nucleic acid (PNA) probe for detection by imaging and conventional cytometry. Exosomes were transfected with fluorescently labeled plasmid DNA and incubated with either human CD21 transgenic mouse splenocytes or human PBMC and assayed for uptake and activation by FACS. Plasmid DNA transfected exosomes were coincubated with human PBMC and evaluated for their ability to expand antigen-specific CD8+ T cells by different methods. Results: EBV LCL and HEK293T derived exosomes were successfully transfected with plasmid DNA as verified by image assisted flow cytometry. Transient transfection of HEK293T cells with plasmids encoding a transgene led to release of exosomes containing the transgene product. EBV LCL and HEK 293T-GP350 derived exosomes increased delivery of plasmid DNA to both human B lymphocytes and transgenic hCD21 mouse B lymphocytes, when compared to naked DNA alone, in an hCD21 dependent fashion. At 24h, plasmid DNA containing B lymphocytes exhibited increased activation and upregulation of CD80/86 costimulatory molecules. Further, human B lymphocytes treated with exosome-plasmid DNA complexes led to antigen mRNA production and expansion of Ag specific CD8 T cells without the need for APC subset enrichment. Studies comparing the immunogenicity of GP350-exosome mediated delivery versus naked DNA delivery are underway in different mouse models. Conclusion: GP350 containing exosomes can successfully increase delivery of plasmid DNA to B lymphocytes even in the presence of other phagocytic cells like macrophages or dendritic cells. This led to detectable antigen production and expansion of cognate Ag specific CD8 T cell expansion without the need for B cell enrichment. Studies examining in vivo immunogenicity of exosomal plasmid DNA delivery are underway, but results to date suggest this approach may be a simple method to boost vaccine efficacy through increased delivery of DNA directly to relevant antigen-presenting B cells. Citation Format: Viswa Teja Colluru, Douglas G. McNeel. GP350 containing exosomes as DNA vaccine carriers. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2360.

Abstract CN04-03: DNA vaccines as treatment for prostate cancer - understanding mechanisms of resistance

Anti-tumor vaccines have demonstrated clinical benefit in patients with advanced prostate cancer, and the first vaccine approved by FDA as a treatment for human cancer was a vaccine targeting the prostate tumor antigen prostatic acid phosphatase (PAP). Plasmid DNA, as another method of antigen delivery, offers significant potential safety and manufacturing advantages over other immunization approaches. DNA vaccines have been approved by USDA as treatments for diseases in fish, horses, dogs, and non-human primates. However, DNA vaccines have generally been perceived as less immunologically potent than other methods of antigen delivery in human clinical trials, leading to efforts to increase their immunogenicity. Most of these efforts have focused on increasing the delivery of vaccine to antigen-presenting cells using different carriers or by electroporation. Our group has evaluated repetitive immunization with plasmid DNA vaccines in rodent models, and has translated this approach to early clinical trials in men with low-volume recurrent prostate cancer with a DNA vaccine encoding PAP. We have demonstrated that persistent Th1-biased PAP-specific T-cell immune responses can be elicited in men with prostate cancer. Moreover, the development of persistent immunity appeared to be associated with favorable changes in tumor growth rate. A randomized phase II trial using this approach is currently ongoing in men with minimal residual prostate cancer to determine whether immunization can delay the time to metastatic disease progression (NCT01341652). In an effort to further improve the immunogenicity of DNA vaccines, we have explored methods to increase antigen presentation by encoding epitopes with increased MHC class I affinity or by increasing the duration of target antigen expression. While such “optimized” vaccines elicited increased numbers of antigen-specific CD8+ T cells, these T cells had less anti-tumor activity in vivo. We determined that immunization with plasmid DNA elicited antigen-specific IFNγ-secreting T cells that led to increased expression of PD-L1 on antigen-expressing tumor cells. However, immunization with the vaccine encoding high affinity epitopes resulted in antigen-specific CD8+ T cells with higher PD-1 expression. In the presence of PD-L1-expressing tumors these CD8+ were less effective. The anti-tumor activity was restored in the presence of PD-1 or PD-L1 blockade. Conversely, immunization with plasmid DNA engineered to increase the duration of antigen expression led to CD8+ T cells with normal PD-1 expression but higher LAG-3 expression. The anti-tumor activity was similarly restored in the presence of LAG-3 blockade. We are currently studying the mechanisms by which different T cell regulatory ligands are expressed following immunization. Notwithstanding, such findings suggest that subtle differences in the method of immunization can affect these different regulatory mechanisms, and hence the choice of optimal combination strategies may depend on identifying the specific mechanisms of resistance to immunization. In the case of patients immunized with a DNA vaccine encoding PAP, we have recently found that PD-L1 expression on circulating tumor (CD45-EpCAM+) cells was increased following immunization, analogous to our murine studies. In addition, PAP-specific secretion of IFNγ and granzyme B by T cells obtained from patients after immunization was increased in the presence of PD-1/PD-L1 blockade. Finally, using a trans vivo delayed type hypersensitivity (tvDTH) assay, we observed that PAP-specific immune responses in T cells obtained from patients after immunization with a DNA vaccine encoding PAP could be “uncovered” with PD-1 blockade but not blockade with anti-LAG-3 or anti-TIM-3. Collectively, these findings suggest that the PD-1/PD-L1 pathway and mechanism of resistance is most relevant to a DNA vaccine encoding PAP, and the efficacy of this vaccine might be augmented with PD-1 blockade. A clinical trial testing this approach in men with advanced prostate cancer, using this DNA vaccine and pembrolizumab, delivered either concurrently or sequentially, is currently underway (NCT02499835). Citation Format: Brian T. Rekoske, Viswa T. Colluru, Douglas G. McNeel. DNA vaccines as treatment for prostate cancer - understanding mechanisms of resistance. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr CN04-03.

Abstract 2502: Immunization with minicircle and mini-intronic DNA vectors induce LAG-3 expressing CD8+ T cells and inferior anti-tumor responses

BACKGROUND: Plasmid DNA-based gene delivery is a safe and economical vaccine modality that has demonstrated elicitation of robust cellular and humoral immunity and remarkable pre-clinical efficacy in multiple tumor models. In spite of this, DNA vaccines have been relatively unsuccessful in several human trials so far. One explanation for the difference in preclinical and clinical studies has been low or insufficient transgene expression from the plasmid vector, effectively reducing the antigen/body mass ratio in relatively large human subjects. Recent work in the gene therapy field has led to the discovery of a novel class of DNA vectors devoid of bacterial backbone sequences that leads to enhanced and sustained transgene expression in vitro and in vivo. In the following study we examined the potential of these vectors to induce enhanced immune responses compared to conventional plasmid vectors. METHODS: Minicircle (DMC) and mini-intronic (MIP) plasmid vectors encoding the cancer testis antigen SSX-2 (synovial sarcoma breakpoint protein 2) were constructed from a conventional plasmid (pTVG-SSX2) and produced using standard methods. Sustained antigen expression was assessed following direct transfection of reporter cells after 2 or 7 days. The efficacy of these vectors as immunization vehicles was evaluated in HHDII-DRI transgenic mice (expressing human HLA-A2 and HLA-DR) by CD8 tetramer staining, IFNγ ELISPOT, intracellular cytokine staining, and in vivo anti-tumor response against syngeneic tumors expressing SSX2. RESULTS: As expected, DMC and MIP vectors led to sustained and elevated levels of SSX2 protein in vitro when compared to standard plasmid DNA. Immunization with DMC-SSX2 led to a 3-fold increase in antigen-specific CD8+ T cells as measured by tetramer staining and with IFNγ release as measured by ELISPOT. However, these CD8+ T cells were less multifunctional as measured by IL2 and TNFα release using intracellular cytokine staining. This was associated with higher expression of LAG3 on SSX2-specific CD8 T cells. No significant changes in the expression of other regulatory markers (TIM3, PD1 or CD160) were noted. Immunization with a MIP-SSX2 construct similarly elicited higher numbers of antigen-specific LAG3+ CD8+ T cells and an inferior anti-tumor response compared to plasmid DNA. CONCLUSION: Elevated and sustained antigen expression in vivo as mediated by minicircle and mini-intronic vectors led to the generation of greater frequencies of antigen-specific CD8+ T cells as compared to a standard plasmid vector. These T cells were, however, not multifunctional, expressed LAG3, and provided an inferior anti-tumor response compared to plasmid DNA. We suspect induction of tolerance to a DNA-encoded antigen might be mediated by sustained antigen presentation in a non-inflammatory context. Studies to evaluate this mechanism further are underway. Citation Format: Viswa Teja Colluru, Douglas G. McNeel. Immunization with minicircle and mini-intronic DNA vectors induce LAG-3 expressing CD8+ T cells and inferior anti-tumor responses. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2502. doi:10.1158/1538-7445.AM2015-2502