Bacterial vectors for imaging and cancer gene therapy: a review

Nanomaterial combined engineered bacteria for intelligent tumor immunotherapy

Cancer remains the leading cause of human death worldwide. Compared to traditional therapies, tumor immunotherapy has received a lot of attention and research focus due to its potential to activate both innate and adaptive immunity, low toxicity to normal tissue, and long-term immune activity. However, its clinical effectiveness and large-scale application are limited due to the immunosuppression microenvironment, lack of spatiotemporal control, expensive cost, and long manufacturing time. Recently, nanomaterial combined engineered bacteria have emerged as a promising solution to the challenges of tumor immunotherapy, which offers spatiotemporal control, reversal of immunosuppression, and scalable production. Therefore, we summarize the latest research on nanomaterial-assisted engineered bacteria for precise tumor immunotherapies, including the cross-talk of nanomaterials and bacteria as well as their application in different immunotherapies. In addition, we further discuss the advantages and challenges of nanomaterial-engineered bacteria and their future prospects, inspiring more novel and intelligent tumor immunotherapy.

Exploiting bacteria for cancer immunotherapy

Immunotherapy has revolutionized the treatment of cancer but continues to be constrained by limited response rates, acquired resistance, toxicities and high costs, which necessitates the development of new, innovative strategies. The discovery of a connection between the human microbiota and cancer dates back 4,000 years, when local infection was observed to result in tumour eradication in some individuals. However, the true oncological relevance of the intratumoural microbiota was not recognized until the turn of the twentieth century. The intratumoural microbiota can have pivotal roles in both the pathogenesis and treatment of cancer. In particular, intratumoural bacteria can either promote or inhibit cancer growth via remodelling of the tumour microenvironment. Over the past two decades, remarkable progress has been made preclinically in engineering bacteria as agents for cancer immunotherapy; some of these bacterial products have successfully reached the clinical stages of development. In this Review, we discuss the characteristics of intratumoural bacteria and their intricate interactions with the tumour microenvironment. We also describe the many strategies used to engineer bacteria for use in the treatment of cancer, summarizing contemporary data from completed and ongoing clinical trials. The work described herein highlights the potential of bacteria to transform the landscape of cancer therapy, bridging ancient wisdom with modern scientific innovation.

In Situ Synthesis of an Immune-Checkpoint Blocker from Engineered Bacteria Elicits a Potent Antitumor Response

Immune-checkpoint blockade (ICB) reinvigorates T cells from exhaustion and potentiates T-cell responses to tumors. However, most patients do not respond to ICB therapy, and only a limited response can be achieved in a "cold" tumor with few infiltrated lymphocytes. Synthetic biology can be used to engineer bacteria as controllable bioreactors to synthesize biotherapeutics in situ. We engineered attenuated Salmonella VNP20009 with synthetic gene circuits to produce PD-1 and Tim-3 scFv to block immunosuppressive receptors on exhausted T cells to reinvigorate their antitumor response. Secreted PD-1 and Tim-3 scFv bound PD-1+ Tim-3+ T cells through their targeting receptors in vitro and potentiated the T-cell secretion of IFN-γ. Engineered bacteria colonized the hypoxic core of the tumor and synthesized PD-1 and Tim-3 scFv in situ, reviving CD4+ T cells and CD8+ T cells to execute an antitumor response. The bacteria also triggered a strong innate immune response, which stimulated the expansion of IFN-γ+ CD4+ T cells within the tumors to induce direct and indirect antitumor immunity.

Could the tumor-associated microbiota be the new multi-faceted player in the tumor microenvironment?

Microorganisms have been identified in tumor specimens for over a century. It is only in recent years that tumor-associated microbiota has become a rapidly expanding field. Assessment techniques encompass methods at the frontiers of molecular biology, microbiology, and histology, requiring a transdisciplinary process to carefully decipher this new component of the tumor microenvironment. Due to the low biomass, the study of tumor-associated microbiota poses technical, analytical, biological, and clinical challenges and must be approached as a whole. To date, several studies have begun to shed light on the composition, functions, and clinical relevance of the tumor-associated microbiota. This new piece of the tumor microenvironment puzzle could potentially change the way we think about and treat patients with cancer.

Synergistic potential of immune checkpoint inhibitors and therapeutic cancer vaccines

Cancer vaccines and immune checkpoint inhibitors (ICIs) function at different stages of the cancer immune cycle due to their distinct mechanisms of action. Therapeutic cancer vaccines enhance the activation and infiltration of cytotoxic immune cells into the tumor microenvironment (TME), while ICIs, prevent and/or reverse the dysfunction of these immune cells. The efficacy of both classes of immunotherapy has been evaluated in monotherapy, but they have been met with several challenges. Although therapeutic cancer vaccines can activate anti-tumor immune responses, these responses are susceptible to attenuation by immunoregulatory molecules. Similarly, ICIs are ineffective in the absence of tumor-infiltrating lymphocytes (TILs). Further, ICIs are often associated with immune-related adverse effects that may limit quality of life and compliance. However, the combination of the improved immunogenicity afforded by cancer vaccines and restrained immunosuppression provided by immune checkpoint inhibitors may provide a suitable platform for therapeutic synergism. In this review, we revisit the history and various classifications of therapeutic cancer vaccines. We also provide a summary of the currently approved ICIs. Finally, we provide mechanistic insights into the synergism between ICIs and cancer vaccines.

Bioinformatic Analysis of the Genetic Mechanism of Biomineralization of Biogenic Magnetic Nanoparticles in Bacteria Capable of Tumor-Specific Accumulation

Background. Current methods of targeted cancer therapy are not always effective enough and can lead to side effects, such as an increased risk of autoimmune diseases. It is known that some bacteria are capable of specific accumulation in malignant tumors, and therefore can be used as an alternative means of targeted drug delivery. However, the genetic mechanism of tumor-specific accumulation of bacteria is not fully understood and needs to be studied in more detail. Objective. This work aims to identify, by methods of comparative genomics methods, magnetically controlled bacteria among those for which tumor-specific accumulation has already been experimentally shown. Methods. To identify magnetically controlled bacterial strains, i.e., bacteria that biomineralize biogenic magnetic nanoparticles (BMN), the method of comparative genomics was used, namely: pairwise alignment of proteomes with amino acid sequences of Mam-proteins of required for biomineralization of BMN in magneto­tactic bacteria Magnetospirillum gryphiswaldense MSR-1. Sequence alignments were performed in the BLAST program of the US National Center for Biotechnology Information (NCBI). Results. The conducted bioinformatic analysis showed that strains of bacteria in which the ability to accumulate specifically in tumors has been experimentally proven are potential producers of BMN of different types. Among them there are potential producers of intracellular crystalline BMN, potential producers of intracellular amorphous BMN, and extracellular crystalline BMN Conclusions. It is expedient to use bacteria-producing BMN as gene vectors and systems of targeted drug delivery to tumors that biomineralize BMN.

Bacterial-Based Methods for Cancer Treatment: What We Know and Where We Are

A severe disease, cancer is caused by the exponential and uncontrolled growth of cells, leading to organ dysfunction as well as disorders. This disease has been recognized as one of the significant challenges to health and medicine. Various treatment procedures for cancer are associated with diverse side effects; the most conventional cancer treatments include chemotherapy, surgery, and radiotherapy, among others. Numerous adverse and side effects, low specificity and sensitivity, narrow therapeutic windows, and, recently, the emergence of tumor cells resistant to such treatments have been documented as the shortcomings of conventional treatment strategies. As a group of prokaryotic microorganisms, bacteria have great potential for use in cancer therapy. Currently, utilizing bacteria for cancer treatment has attracted the attention of scientists. The high potential of bacteria to become non-pathogenic by genetic manipulation, their distinguished virulence factors (which can be used as weapons against tumors), their ability to proliferate in tissues, and the contingency to control their population by administrating antibiotics, etc., have made bacteria viable candidates and live micro-medication for cancer therapies. However, the possible cytotoxicity impacts of bacteria, their inability to entirely lyse cancerous cells, as well as the probability of mutations in their genomes are among the significant challenges of bacteria-based methods for cancer treatment. In this article, various available data on bacterial therapeutics, along with their pros and cons, are discussed.

Escherichiacoli Nissle 1917 as a Novel Microrobot for Tumor-Targeted Imaging and Therapy

Highly efficient drug delivery systems with excellent tumor selectivity and minimal toxicity to normal tissues remain challenging for tumor treatment. Although great effort has been made to prolong the blood circulation and improve the delivery efficiency to tumor sites, nanomedicines are rarely approved for clinical application. Bacteria have the inherent properties of homing to solid tumors, presenting themselves as promising drug delivery systems. Escherichia coli Nissle 1917 (EcN) is a commonly used probiotic in clinical practice. Its facultative anaerobic property drives it to selectively colonize in the hypoxic area of the tumor for survival and reproduction. EcN can be engineered as a bacteria-based microrobot for molecular imaging, drug delivery, and gene delivery. This review summarizes the progress in EcN-mediated tumor imaging and therapy and discusses the prospects and challenges for its clinical application. EcN provides a new idea as a delivery vehicle and will be a powerful weapon against cancer.

Bacteria-mediated cancer therapies: opportunities and challenges

In recent years, cancer therapy strategies utilizing live tumor-targeting bacteria have presented unique advantages. Engineered bacteria have the particular ability to distinguish tumors from normal tissues with less toxicity. Live bacteria are naturally capable of homing to tumors, resulting in high levels of local colonization because of insufficient oxygen and low pH in the tumor microenvironment. Bacteria initiate their antitumor effects by directly killing the tumor or by activating innate and adaptive antitumor immune responses. The bacterial vectors can be reprogrammed following advanced DNA synthesis, sophisticated genetic bioengineering, and biosensors to engineer microorganisms with complex functions, and then produce and deliver anticancer agents based on clinical needs. However, because of the lack of knowledge on the mechanisms and side effects of microbial cancer therapy, developing such smart microorganisms to treat or prevent cancer remains a significant challenge. In this review, we summarized the potential, status, opportunities and challenges of this growing field. We illustrated the mechanism of tumor regression induced by engineered bacteria and discussed the recent advances in the application of bacteria-mediated cancer therapy to improve efficacy, safety and drug delivery. Finally, we shared our insights into the future directions of tumor-targeting bacteria in cancer therapy.

Polarization of Tumor-Associated Macrophages by Nanoparticle-Loaded Escherichia coli Combined with Immunogenic Cell Death for Cancer Immunotherapy

The tumor immunosuppressive microenvironment greatly limits the efficacy of immunotherapy. Tumor-associated macrophages (TAMs) are the most abundant immunosuppressive cells in the tumor microenvironment, which can inhibit the tumor after converting it to an M1-like phenotype. In addition, immunogenic cell death (ICD) can increase the amount of T lymphocytes in tumors, activating antineoplastic immunity. Herein, tumor-associated macrophage polarization therapy supplemented with PLGA-DOX (PDOX)-induced ICD is developed for cancer treatment. The nanoparticles/bacteria complex (Ec-PR848) is fabricated for tumor targeting and TAM polarization, and PLGA-R848 (PR848) are attached to the surface of Escherichia coli (E. coli) MG1655 via electrostatic absorption. The toll-like receptor 7/8 (TLR7/8) agonist resiquimod (R848) and E. coli can greatly polarize M2 macrophages to M1 macrophages, while PDOX-induced ICD can also impair the immunosuppression of the tumor microenvironment. This strategy shows that tumor-associated macrophage polarization therapy combined with ICD induced by low-dose chemotherapeutic drugs can commendably enhance the efficacy of immunotherapy.

Bacteria-Based Cancer Immunotherapy

In the past decade, bacteria-based cancer immunotherapy has attracted much attention in the academic circle due to its unique mechanism and abundant applications in triggering the host anti-tumor immunity. One advantage of bacteria lies in their capability in targeting tumors and preferentially colonizing the core area of the tumor. Because bacteria are abundant in pathogen-associated molecular patterns that can effectively activate the immune cells even in the tumor immunosuppressive microenvironment, they are capable of enhancing the specific immune recognition and elimination of tumor cells. More attractively, during the rapid development of synthetic biology, using gene technology to enable bacteria to be an efficient producer of immunotherapeutic agents has led to many creative immunotherapy paradigms. The combination of bacteria and nanomaterials also displays infinite imagination in the multifunctional endowment for cancer immunotherapy. The current progress report summarizes the recent advances in bacteria-based cancer immunotherapy with specific foci on the applications of naive bacteria-, engineered bacteria-, and bacterial components-based cancer immunotherapy, and at the same time discusses future directions in this field of research based on the present developments.

Improvement of organisms by biomimetic mineralization: A material incorporation strategy for biological modification

Biomineralization, a bio-organism controlled mineral formation process, plays an important role in linking biological organisms and mineral materials in nature. Inspired by biomineralization, biomimetic mineralization is used as a bridge tool to integrate biological organisms and functional materials together, which can be beneficial for the development of diversified functional organism-material hybrids. In this review, recent progresses on the techniques of biomimetic mineralization for organism-material combinations are summarized and discussed. Based upon these techniques, the preparations and applications of virus-, prokaryotes-, and eukaryotes-material hybrids have been presented and they demonstrate the great potentials in the fields of vaccine improvement, cell protection, energy production, environmental and biomedical treatments, etc. We suggest that more researches about functional organism and material combination with more biocompatible techniques should be developed to improve the design and applications of specific organism-material hybrids. These rationally designed organism-material hybrids will shed light on the production of "live materials" with more advanced functions in future. STATEMENT OF SIGNIFICANCE: This review summaries the recent attempts on improving biological organisms by their integrations with functional materials, which can be achieved by biomimetic mineralization as the combination tool. The integrated materials, as the artificial shells or organelles, confer diversified functions on the enclosed organisms. The successful constructions of various virus-, prokaryotes-, and eukaryotes-material hybrids have demonstrated the great potentials of the material incorporation strategy in vaccine development, cancer treatment, biological photosynthesis and environment protection etc. The suggested challenges and perspectives indicate more inspirations for the future development of organism-material hybrids.

Exosome as a Natural Gene Delivery Vector for Cancer Treatment

Background:

Current gene therapy vectors such as viral, non-viral, and bacterial vectors, which are used for cancer treatment, but there are certain safety concerns and stability issues of these conventional vectors. Exosomes are the vesicles of size 40-100 nm secreted from multivesicular bodies into the extracellular environment by most of the cell types in-vivo and in-vitro. As a natural nanocarrier, exosomes are immunologically inert, biocompatible, and can cross biological barriers like the blood-brain barrier, intestinal barrier, and placental barrier.

Objective:

This review focusses on the role of exosome as a carrier to efficiently deliver a gene for cancer treatment and diagnosis. The methods for loading of nucleic acids onto the exosomes, advantages of exosomes as a smart intercellular shuttle for gene delivery and therapeutic applications as a gene delivery vector for siRNA, miRNA and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) and also the limitations of exosomes as a gene carrier are all reviewed in this article.

Methods:

Mostly, electroporation and chemical transfection are used to prepare gene loaded exosomes.

Results:

Exosome-mediated delivery is highly promising and advantageous in comparison to the current delivery methods for systemic gene therapy. Targeted exosomes, loaded with therapeutic nucleic acids, can efficiently promote the reduction of tumor proliferation without any adverse effects.

Conclusion:

In the near future, exosomes can become an efficient gene carrier for delivery and a biomarker for the diagnosis and treatment of cancer.

Efficient protection of microorganisms for delivery to the intestinal tract by cellulose sulphate encapsulation

BACKGROUND

Gut microbiota in humans and animals play an important role in health, aiding in digestion, regulation of the immune system and protection against pathogens. Changes or imbalances in the gut microbiota (dysbiosis) have been linked to a variety of local and systemic diseases, and there is growing evidence that restoring the balance of the microbiota by delivery of probiotic microorganisms can improve health. However, orally delivered probiotic microorganisms must survive transit through lethal highly acid conditions of the stomach and bile salts in the small intestine. Current methods to protect probiotic microorganisms are still not effective enough.

RESULTS

We have developed a cell encapsulation technology based on the natural polymer, cellulose sulphate (CS), that protects members of the microbiota from stomach acid and bile. Here we show that six commonly used probiotic strains (5 bacteria and 1 yeast) can be encapsulated within CS microspheres. These encapsulated strains survive low pH in vitro for at least 4 h without appreciable loss in viability as compared to their respective non-encapsulated counterparts. They also survive subsequent exposure to bile. The CS microspheres can be digested by cellulase at concentrations found in the human intestine, indicating one mechanism of release. Studies in mice that were fed CS encapsulated autofluorescing, commensal E. coli demonstrated release and colonization of the intestinal tract.

CONCLUSION

Taken together, the data suggests that CS microencapsulation can protect bacteria and yeasts from viability losses due to stomach acid, allowing the use of lower oral doses of probiotics and microbiota, whilst ensuring good intestinal delivery and release.

Molecular Imaging with Reporter Genes: Has Its Promise Been Delivered?

The first reporter systems were developed in the early 1980s and were based on measuring the activity of an enzyme-as a surrogate measure of promoter-driven transcriptional activity-which is now known as a reporter gene system. The initial objective and application of reporter techniques was to analyze the activity of a specific promoter (namely, the expression of a gene that is under the regulation of the specific promoter that is linked to the reporter gene). This system allows visualization of specific promoter activity with great sensitivity. In general, there are 2 classes of reporter systems: constitutively expressed (always-on) reporter constructs used for cell tracking, and inducible reporter systems sensitive to endogenous signaling molecules and transcription factors that characterize specific tissues, tumors, or signaling pathways.This review traces the development of different reporter systems, using fluorescent and bioluminescent proteins as well as radionuclide-based reporter systems. The development and application of radionuclide-based reporter systems is the focus of this review. The question at the end of the review is whether the "promise" of reporter gene imaging has been realized. What is required for moving forward with radionuclide-based reporter systems, and what is required for successful translation to clinical applications?

Experimental Study of Tumor Therapy Mediated by Multimodal Imaging Based on a Biological Targeting Synergistic Agent

Purpose

The high-intensity focused ultrasound (HIFU) ablation of tumors is inseparable from synergistic agents and image monitoring, but the existing synergistic agents have the defects of poor targeting and a single imaging mode, which limits the therapeutic effects of HIFU. The construction of a multifunctional biological targeting synergistic agent with high biosafety, multimodal imaging and targeting therapeutic performance has great significance for combating cancer.

Methods

Multifunctional biological targeting synergistic agent consisting of Bifidobacterium longum (B. longum), ICG and PFH coloaded cationic lipid nanoparticles (CL-ICG-PFH-NPs) were constructed for targeting multimode imaging, synergistic effects with HIFU and imaging-guided ablation of tumors, which was evaluated both in vitro and in vivo.

Results

Both in vitro and in vivo systematical studies validated that the biological targeting synergistic agent can simultaneously achieve tumor-biotargeted multimodal imaging, HIFU synergism and multimodal image monitoring in HIFU therapy. Importantly, the electrostatic adsorption method and the targeting of B. longum to tumor tissues allow the CL-ICG-PFH-NPs to be retained in the tumor tissue, achieve the targeting ability of synergistic agent. Multimodal imaging chose the best treatment time according to the distribution of nanoparticles in the body to guide the efficient and effective treatment of HIFU. CL-ICG-PFH-NPs could serve as a phase change agent and form microbubbles that can facilitate HIFU ablation by mechanical effects, acoustic streaming and shear stress. This lays a foundation for the imaging and treatment of tumors.

Conclusion

In this work, a biological targeting synergistic agent was successfully constructed with good stability and physicochemical properties. This biological targeting synergistic agent can not only provide information for early diagnosis of tumors but also realize multimodal imaging monitoring during HIFU ablation simultaneously with HIFU treatment, which improves the shortcomings of HIFU treatment and has broad application prospects.

Bioengineered smart bacterial carriers for combinational targeted therapy of solid tumours

Despite many endeavours for the development of new anticancer drugs, effective therapy of solid tumours remains a challenging issue. The current cancer chemotherapies may associate with two important limitations, including the lack/trivial specificity of treatment modalities towards diseased cells/tissues resulting in undesired side effects, and the emergence of drug-resistance mechanisms by tumour cells causing the failure of the treatment. Much attention, therefore, has currently been paid to develop smart and highly specific anticancer agents with maximal therapeutic impacts and minimal side effects. Among various strategies used to target cancer cells, bacteria-based cancer therapies (BCTs) have been validated as potential gene/drug delivery carriers, which can also be engineered to be used in diagnosis processes. They can be devised to selectively target the tumour microenvironment (TME), within which they may preferentially proliferate in the necrotic and anaerobic parts - often inaccessible to other therapeutics. BCTs are capable to sense and respond to the environmental signals, upon which they are considered as smart microrobots applicable in the controlled delivery of therapeutic agents to the TME. In this review, we aimed to provide comprehensive insights into the potentials of the bioengineered bacteria as smart and targeted bio-carriers and discuss their applications in cancer therapy.

Sequence-Based Characterization of Intratumoral Bacteria-A Guide to Best Practice

Tumors are hospitable environments to bacteria and several recent studies on cancer patient samples have introduced the concept of an endogenous tumor microbiome. For a variety of reasons, this putative tumor microbiome is particularly challenging to investigate, and a failure to account for the various potential pitfalls will result in erroneous results and claims. Before this potentially extremely medically-significant habitat can be accurately characterized, a clear understanding of all potential confounding factors is required, and a best-practice approach should be developed and adopted. This review summarizes all of the potential issues confounding accurate bacterial DNA sequence analysis of the putative tumor microbiome, and offers solutions based on related research with the hope of assisting in the progression of research in this field.

Gold Biomineralization on Bacterial Biofilms for Leaching of Au3+ Damages Eukaryotic Cells

Heavy metals not only pollute the environment but also are health and environmental hazard. Bacteria constitute inexpensive and eco-friendly material to eliminate and recycle heavy metals via biomineralization and biosorption. However, the effect of metal biomineralization in bacterial biofilms on the ecological balance of bacteria and infectious diseases is unclear. This study aimed to explore the interaction between a eukaryotic cell line HEK293T and mineralized Escherichia coli, using a model of gold biomineralization on E. coli biofilms (E. coli-Au). In our present model, bacterial activity was not disrupted and bacterial adhesion and invasion were enhanced. E. coli-Au invaded the cytoplasm and nuclei of HEK293T cells and damaged them via intracellular growth and multiplication. The present findings indicate that metal biomineralization in bacterial biofilms for leaching of heavy metal ions is hazardous to eukaryotic cells and even human health.

Phototrophic purple bacteria as optoacoustic in vivo reporters of macrophage activity

Τhe morphology, physiology and immunology, of solid tumors exhibit spatial heterogeneity which complicates our understanding of cancer progression and therapy response. Understanding spatial heterogeneity necessitates high resolution in vivo imaging of anatomical and pathophysiological tumor information. We introduce Rhodobacter as bacterial reporter for multispectral optoacoustic (photoacoustic) tomography (MSOT). We show that endogenous bacteriochlorophyll a in Rhodobacter gives rise to strong optoacoustic signals >800 nm away from interfering endogenous absorbers. Importantly, our results suggest that changes in the spectral signature of Rhodobacter which depend on macrophage activity inside the tumor can be used to reveal heterogeneity of the tumor microenvironment. Employing non-invasive high resolution MSOT in longitudinal studies we show spatiotemporal changes of Rhodobacter spectral profiles in mice bearing 4T1 and CT26.WT tumor models. Accessibility of Rhodobacter to genetic modification and thus to sensory and therapeutic functions suggests potential for a theranostic platform organism.

Current optoacoustic probes for cancer imaging have limitations including background noise, long-term toxicity and scarce imaging depth in living tissue. Here the authors use Rhodobacter, purple bacteria rich in bacteriochlorophyll a, as an optoacoustic reporter to image tumor-associated macrophages in mice in vivo.

Therapeutic benefit of Salmonella attributed to LPS and TNF-α is exhaustible and dictated by tumor susceptibility

The potential of bacteria-mediated tumor therapy (BMTT) is highlighted by more than a century of investigation. Attenuated Salmonella has prevailed as promising therapeutic agents. For BMTT - categorized as an immune therapy - the exact contribution of particular immune reactions to the therapeutic effect remains ambiguous. In addition, one could argue for or against the requirement of bacterial viability and tumor targeting. Herein we evaluate the isolated therapeutic efficacy of purified LPS and TNF-α, which together account for a dominant immunogenic pathway of gram negative bacteria like Salmonella. We show that therapeutic efficacy against CT26 tumors does not require bacterial viability. Analogous to viable Salmonella SL7207, tumor regression by a specific CD8⁺ T cell response can be induced by purified LPS or recombinant TNF-α (rTNF-α). Conversely, therapeutic effects against RenCa tumors were abrogated upon bacterial avitalization and limited using isolated adjuvants. This argues for an alternative mechanistic explanation for SL7207 against RenCa that depends on viability and persistence. Unable to boost bacterial therapies by co-injection of rTNF-α suggested therapeutic effects along this axis are exhausted by the intrinsic adjuvanticity of bacteria alone. However, the importance of TNF-α for BMTT was highlighted by its support of tumor invasion and colonization in concert with lower infective doses of Salmonella. In consideration, bacterial therapeutic effectiveness along the axis of LPS and TNF-α appears limited, and does not offer the necessary plasticity for different tumors. This emphasizes a need for recombinant strengthening and vehicular exploitation to accommodate potency, plasticity and distinctiveness in BMTT.

Therapy of solid tumors using probiotic Symbioflor-2: restraints and potential

To date, virulent bacteria remain the basis of most bacteria mediated cancer therapies. For clinical application attenuation is required. However, this might result in a drastically lowered therapeutic capacity. Herein we argue that the E. coli probiotic Symbioflor-2, with a history of safe application may constitute a viable tumor therapeutic candidate. We demonstrate that Symbioflor-2 displays a highly specific tumor targeting ability as determined in murine CT26 and RenCa tumor models. The excellent specificity was ascribed to reduced levels of adverse colonization. A high safety standard was demonstrated in WT and Rag1^−/− mice. Thus, Symbioflor-2 may represent an ideal tumor targeting delivery system for therapeutic molecules. Moreover, Symbioflor-2 was capable of inducing CT26 tumor clearance as result of an adjuvant effect on tumor specific CD8⁺ T cells analogous to the Salmonella variant SL7207. However, lower therapeutic efficacy against RenCa tumors suggested a generally reduced therapeutic potency for probiotics. Interestingly, concurrent depletion of Gr-1⁺ or Ly6G⁺ cells installed therapeutic efficacy equal to SL7207, thus highlighting the role of innate effector cells in restraining the anti-tumor effects of Symbioflor-2. Collectively, our findings argue for a strategy of safe strain application and a more sustainable use of bacteria as a delivery system for therapeutic molecules.

Development of a Click Beetle Luciferase Reporter System for Enhanced Bioluminescence Imaging of Listeria monocytogenes: Analysis in Cell Culture and Murine Infection Models

Listeria monocytogenes is a Gram-positive facultative intracellular pathogen that is widely used as a model organism for the analysis of infection biology. In this context, there is a current need to develop improved reporters for enhanced bioluminescence imaging (BLI) of the pathogen in infection models. We have developed a click beetle red luciferase (CBR-luc) based vector (pPL2CBR^opt) expressing codon optimized CBR-luc under the control of a highly expressed Listerial promoter (P_HELP) for L. monocytogenes and have compared this to a lux-based system expressing bacterial luciferase for BLI of the pathogen using in vitro growth experiments and in vivo models. The CBR-luc plasmid stably integrates into the L. monocytogenes chromosome and can be used to label field isolates and laboratory strains of the pathogen. Growth experiments revealed that CBR-luc labeled L. monocytogenes emits a bright signal in exponential phase that is maintained during stationary phase. In contrast, lux-labeled bacteria produced a light signal that peaked during exponential phase and was significantly reduced during stationary phase. Light from CBR-luc labeled bacteria was more efficient than the signal from lux-labeled bacteria in penetrating an artificial tissue depth assay system. A cell invasion assay using C2Bbe1 cells and a systemic murine infection model revealed that CBR-luc is suited to BLI approaches and demonstrated enhanced sensitivity relative to lux in the context of Listeria infection models. Overall, we demonstrate that this novel CBR reporter system provides efficient, red-shifted light production relative to lux and may have significant applications in the analysis of L. monocytogenes pathogenesis.

Designer bacteria as intratumoural enzyme biofactories

Bacterial-directed enzyme prodrug therapy (BDEPT) is an emerging form of treatment for cancer. It is a biphasic variant of gene therapy in which a bacterium, armed with an enzyme that can convert an inert prodrug into a cytotoxic compound, induces tumour cell death following tumour-specific prodrug activation. BDEPT combines the innate ability of bacteria to selectively proliferate in tumours, with the capacity of prodrugs to undergo contained, compartmentalised conversion into active metabolites in vivo. Although BDEPT has undergone clinical testing, it has received limited clinical exposure, and has yet to achieve regulatory approval. In this article, we review BDEPT from the system designer's perspective, and provide detailed commentary on how the designer should strategize its development de novo. We report on contemporary advancements in this field which aim to enhance BDEPT in terms of safety and efficacy. Finally, we discuss clinical and regulatory barriers facing BDEPT, and propose promising approaches through which these hurdles may best be tackled.

Modulating tumor hypoxia by nanomedicine for effective cancer therapy

Hypoxia, a characteristic feature of tumors, is indispensable to tumor angiogenesis, metastasis, and multi drug resistance. Hypoxic avascular regions, deeply embedded inside the tumors significantly hinder delivery of therapeutic agents. The low oxygen tension results in resistance to the current applied anti-cancer therapeutics including radiotherapy, chemotherapy, and photodynamic therapy, the efficacy of which is firmly tied to the level of tumor oxygen supply. However, emerging data indicate that nanocarriers/nanodrugs can offer substantial benefits to improve the efficacy of current therapeutics, through modulation of tumor hypoxia. This review aims to introduce the most recent advances made in nanocarrier mediated targeting of tumor hypoxia. The first part is dedicated to the approaches by which nanocarriers could be designed to target/leverage hypoxia. These approaches include i) inhibiting Hypoxia Inducer Factor (HIF-1α); ii) hypoxia activated prodrugs/linkers; and iii) obligate anaerobe mediated targeting of tumor hypoxia. The second part, details novel nanosystems proposed to modulate tumor hypoxia through tumor oxygenation. These methods seek to lessen tumor hypoxia through vascular normalization, or reoxygenation therapy. The reoxygenation of tumor could be accomplished by: i) generation of oxygen filled nanocarriers; ii) natural/artificial oxygen nanocarriers; and iii) oxygen generators. The efficacy of each approach and their potential in cancer therapy is further discussed.

Chemically individual armoured bioreporter bacteria used for the in vivo sensing of ultra-trace toxic metal ions

A chemically engineered mesoporous silica armour is developed for simultaneously improving bioreporter bacterial vitality and shielding infectivity.

Synergistic effects of overexpression of BMP‑2 and TGF‑β3 on osteogenic differentiation of bone marrow mesenchymal stem cells

Bone morphogenetic protein 2 (BMP-2) and transforming growth factor β (TGF-β) isoforms are important in advancing bone regeneration. The aim of the present study was to investigate the positive and reciprocal effect of TGF-β3, one of the three TGF-β isoforms, on BMP-2 in promoting osteogenic differentiation. Exogenous BMP-2 and TGF-β3 genes were separately, and in combination, overexpressed in rabbit bone marrow-derived mesenchymal stem cells (rBMSCs). Expression levels of BMP-2 and TGF-β3 were evaluated using reverse-transcription-polymerase chain reaction (RT-PCR) and Western blotting assays. Furthermore, the osteogenic differentiation capacities of BMSCs were assessed by measuring Alizarin Red S staining, an alkaline phosphatase activity assay, and quantification of the osteogenic-specific genes, Runt-related transcription factor 2 (Runx2) and Osterix (Osx). Using lentiviral-mediated transfection, robust co-transfection efficiency of >90% was achieved. RT-PCR and immunoblotting results indicated a marked elevated expression of BMP-2 and TGF-β3 in rBMSCs undergoing co-transfection, compared with transfection with BMP-2 or TGF-β3 alone, indicating that BMP-2 and TGF-β3 are synergistically expressed in rBMSCs. Furthermore, enhanced osteogenic differentiation was observed in rBMSCs co-transfected with BMP-2/TGF-β3. The present study successfully delivered BMP-2 together with TGF-β3 into rBMSCs with high efficiency for the first time. Furthermore, TGF-β3 overexpression was demonstrated to enhance the osteogenic efficacy of BMP-2 in rBMSCs, and vice versa. This provides a potential clinical therapeutic approach for regenerating the function of osseous tissue, and may present a promising strategy for bone defect healing.

A Salmonella nanoparticle mimic overcomes multidrug resistance in tumours

Salmonella enterica serotype Typhimurium is a food-borne pathogen that also selectively grows in tumours and functionally decreases P-glycoprotein (P-gp), a multidrug resistance transporter. Here we report that the Salmonella type III secretion effector, SipA, is responsible for P-gp modulation through a pathway involving caspase-3. Mimicking the ability of Salmonella to reverse multidrug resistance, we constructed a gold nanoparticle system packaged with a SipA corona, and found this bacterial mimic not only accumulates in tumours but also reduces P-gp at a SipA dose significantly lower than free SipA. Moreover, the Salmonella nanoparticle mimic suppresses tumour growth with a concomitant reduction in P-gp when used with an existing chemotherapeutic drug (that is, doxorubicin). On the basis of our finding that the SipA Salmonella effector is fundamental for functionally decreasing P-gp, we engineered a nanoparticle mimic that both overcomes multidrug resistance in cancer cells and increases tumour sensitivity to conventional chemotherapeutics.

Intravenous Administration Is an Effective and Safe Route for Cancer Gene Therapy Using the Bifidobacterium-Mediated Recombinant HSV-1 Thymidine Kinase and Ganciclovir

The herpes simplex virus thymidine kinase/ganciclovir (HSV TK/GCV) system is one of the best studied cancer suicide gene therapy systems. Our previous study showed that caspase 3 expression was upregulated and bladder tumor growth was significantly reduced in rats treated with a combination of Bifidobacterium (BF) and HSV TK/GCV (BF-rTK/GCV). However, it was raised whether the BF-mediated recombinant thymidine kinase combined with ganciclovir (BF-rTK/GCV) was safe to administer via venous for cancer gene therapy. To answer this question, the antitumor effects of BF-rTK/GCV were mainly evaluated in a xenograft nude mouse model bearing MKN-45 gastric tumor cells. The immune response, including analysis of cytokine profiles, was analyzed to evaluate the safety of intramuscular and intravenous injection of BF-rTK in BALB/c mice. The results suggested that gastric tumor growth was significantly inhibited in vivo by BF-rTK/GCV. However, the BF-rTK/GCV had no effect on mouse body weight, indicating that the treatment was safe for the host. The results of cytokine profile analysis indicated that intravenous injection of a low dose of BF-rTK resulted in a weaker cytokine response than that obtained with intramuscular injection. Furthermore, immunohistochemical analysis showed that intravenous administration did not affect the expression of immune-associated TLR2 and TLR4. Finally, the BF-rTK/GCV inhibited vascular endothelial growth factor (VEGF) expression in mouse model, which is helpful for inhibiting of tumor angiogenesis. That meant intravenous administration of BF-rTK/GCV was an effective and safe way for cancer gene therapy.

Photodynamic Treatment of Tumor with Bacteria Expressing KillerRed

Photodynamic therapy (PDT) is a cancer treatment modality in which a photosensitizing dye is administered and exposed to light to kill tumor cells via the production of reactive oxygen species (ROS). A fundamental obstacle for PDT is the low specificity for staining solid tumors with dyes. Recently, a tumor targeting system guided by anaerobic bacteria was proposed for tumor imaging and treatment. Here, we explore the feasibility of the genetically encoded photosensitizer KillerRed, which is expressed in Escherichia coli, to treat tumors. Using nitroblue tetrazolium (NBT), we detected a lengthy ROS diffusion from the bodies of KillerRed-expressing bacteria in vitro, which demonstrated the feasibility of using bacteria to eradicate cells in their surroundings. In nude mice, Escherichia coli (E. coli) expressing KillerRed (KR-E. coli) were subcutaneously injected into xenografts comprising CNE2 cells, a human nasopharyngeal carcinoma cell line, and HeLa cells, a human cervical carcinoma cell line. KR-E. coli seemed to proliferate rapidly in the tumors as observed under an imaging system. When the intensity of fluorescence increased and the fluorescent area became as large as the tumor one day after KR-E. coli injection, the KR-E. coli-bearing tumor was irradiated with an orange light (λ = 540 − 580 nm). In all cases, the tumors became necrotic the next day and were completely eliminated in a few days. No necrosis was observed after the irradiation of tumors injected with a vehicle solution or a vehicle carrying the E. coli without KillerRed. In successfully treated mice, no tumor recurrence was observed for more than two months. E. coli genetically engineered for KillerRed expression are highly promising for the diagnosis and treatment of tumors when the use of bacteria in patients is cleared for infection safety.

Development of a Bioluminescent Nitroreductase Probe for Preclinical Imaging

Bacterial nitroreductases (NTRs) have been widely utilized in the development of novel antibiotics, degradation of pollutants, and gene-directed enzyme prodrug therapy (GDEPT) of cancer that reached clinical trials. In case of GDEPT, since NTR is not naturally present in mammalian cells, the prodrug is activated selectively in NTR-transformed cancer cells, allowing high efficiency treatment of tumors. Currently, no bioluminescent probes exist for sensitive, non-invasive imaging of NTR expression. We therefore developed a "NTR caged luciferin" (NCL) probe that is selectively reduced by NTR, producing light proportional to the NTR activity. Here we report successful application of this probe for imaging of NTR in vitro, in bacteria and cancer cells, as well as in vivo in mouse models of bacterial infection and NTR-expressing tumor xenografts. This novel tool should significantly accelerate the development of cancer therapy approaches based on GDEPT and other fields where NTR expression is important.

A Phytase-Based Reporter System for Identification of Functional Secretion Signals in Bifidobacteria

Health-promoting effects have been attributed to a number of Bifidobacterium sp. strains. These effects as well as the ability to colonise the host depend on secreted proteins. Moreover, rational design of protein secretion systems bears the potential for the generation of novel probiotic bifidobacteria with improved health-promoting or therapeutic properties. To date, there is only very limited data on secretion signals of bifidobacteria available. Using in silico analysis, we demonstrate that all bifidobacteria encode the major components of Sec-dependent secretion machineries but only B. longum strains harbour Tat protein translocation systems. A reporter plasmid for secretion signals in bifidobacteria was established by fusing the coding sequence of the signal peptide of a sialidase of Bifidobacterium bifidum S17 to the phytase gene appA of E. coli. The recombinant strain showed increased phytase activity in spent culture supernatants and reduced phytase levels in crude extracts compared to the control indicating efficient phytase secretion. The reporter plasmid was used to screen seven predicted signal peptides in B. bifidum S17 and B. longum E18. The tested signal peptides differed substantially in their efficacy to mediate protein secretion in different host strains. An efficient signal peptide was used for expression and secretion of a therapeutically relevant protein in B. bifidum S17. Expression of a secreted cytosine deaminase led to a 100-fold reduced sensitivity of B. bifidum S17 to 5-fluorocytosine compared to the non-secreted cytosine deaminase suggesting efficient conversion of 5-fluorocytosine to the cytotoxic cancer drug 5-fluorouracil by cytosine deaminase occurred outside the bacterial cell. Selection of appropriate signal peptides for defined protein secretion might improve therapeutic efficacy as well as probiotic properties of bifidobacteria.

Experimental determination and characterization of the gap promoter of Bifidobacterium bifidum S17

The DNA sequence upstream of the glyceraldehyde 3-phosphate dehydrogenase gene (gap) of various strains of bifidobacteria is used in a number of vector systems for homologous and heterologous expression in this group of bacteria. To date none of the bifidobacterial gap promoters (Pgap) have been verified experimentally. Here, we probe a range of putative bifidobacterial promoters hypothesized to show high constitutive transcriptional activity using a β-glucuronidase reporter system. In silico analysis revealed a predicted bacterial promoter upstream of the gap gene of Bifidobacterium bifidum S17. The corresponding DNA sequences was cloned into the promoter probe vector pMDY23 and yielded highest reporter activities among the promoter sequences tested confirming previous studies. Using rapid amplification of cDNA ends (5'-RACE), we identified the transcription start site (TSS) of Pgap of B. bifidum S17. The experimentally determined TSS and the associated -10 and -35 regions do not match with the promoter predicted in silico. Moreover, a potential ribosome-binding site (RBS) was identified upstream of the ATG start codon of the gap gene, which is complementary to the 3'-end of the 16S rRNA with only 1 mismatch suggesting efficient initiation of translation. Alignment of the Pgap sequences of a number of representative bifidobacteria showed a high level of conservation and the presence of -35 and -10 regions, which are similar but not identical to the consensus promoter sequences of house-keeping genes of Escherichia coli and Bacillus subtilis. Collectively, these results confirm the suitability of Pgap for high level, constitutive expression in bifidobacteria.

Bacterial-mediated knockdown of tumor resistance to an oncolytic virus enhances therapy

Oncolytic viruses (OVs) and bacteria share the property of tumor-selective replication following systemic administration. In the case of nonpathogenic bacteria, tumor selectivity relates to their ability to grow extracellularly within tumor stroma and is therefore ideally suited to restricting the production of bacterially produced therapeutic agents to tumors. We have previously shown the ability of the type 1 interferon antagonist B18R to enhance the replication and spread of vesicular stomatitis virus (VSV) by overcoming related cellular innate immunity. In this study, we utilized nonpathogenic bacteria (E. coli) expressing B18R to facilitate tumor-specific production of B18R, resulting in a microenvironment depleted of bioactive antiviral cytokine, thus "preconditioning" the tumor to enhance subsequent tumor destruction by the OV. Both in vitro and in vivo infection by VSVΔ51 was greatly enhanced by B18R produced from E. coli. Moreover, a significant increase in therapeutic efficacy resulted from intravenous (i.v.) injection of bacteria to tumor-bearing mice 5 days prior to i.v. VSVΔ51 administration, as evidenced by a significant reduction in tumor growth and increased survival in mice. Our strategy is the first example where two such diverse microorganisms are rationally combined and demonstrates the feasibility of combining complementary microorganisms to improve therapeutic outcome.

Bacterial systems for gene delivery to systemic tumors

Certain bacteria have emerged as biological gene vectors with natural tumor specificity, capable of specifically delivering genes or gene products to the tumor environment when intravenously (i.v.) administered to rodent models. Here, we describe procedures for studying this phenomenon in vitro and in vivo for both invasive and noninvasive bacteria suitable for exploitation as tumor-specific therapeutic delivery vehicles, due to their ability to replicate specifically within tumors and/or mediate bacterial-mediated transfer of plasmid DNA to mammalian cells (bactofection).

Gene therapy on the move

The first gene therapy clinical trials were initiated more than two decades ago. In the early days, gene therapy shared the fate of many experimental medicine approaches and was impeded by the occurrence of severe side effects in a few treated patients. The understanding of the molecular and cellular mechanisms leading to treatment- and/or vector-associated setbacks has resulted in the development of highly sophisticated gene transfer tools with improved safety and therapeutic efficacy. Employing these advanced tools, a series of Phase I/II trials were started in the past few years with excellent clinical results and no side effects reported so far. Moreover, highly efficient gene targeting strategies and site-directed gene editing technologies have been developed and applied clinically. With more than 1900 clinical trials to date, gene therapy has moved from a vision to clinical reality. This review focuses on the application of gene therapy for the correction of inherited diseases, the limitations and drawbacks encountered in some of the early clinical trials and the revival of gene therapy as a powerful treatment option for the correction of monogenic disorders.

Use of optical imaging to progress novel therapeutics to the clinic

There is an undisputed need for employment and improvement of robust technology for real-time analyses of therapeutic delivery and responses in clinical translation of gene and cell therapies. Over the past decade, optical imaging has become the in vivo imaging modality of choice for many preclinical laboratories due to its efficiency, practicality and affordability, while more recently, the clinical potential for this technology is becoming apparent. This review provides an update on the current state of the art in in vivo optical imaging and discusses this rapidly improving technology in the context of it representing a translation enabler or indeed a future clinical imaging modality in its own right.

The innovative evolution of cancer gene and cellular therapies

We provide an overview of the latest developments in cancer gene therapy--from the bench to early-stage clinical trials. We describe the most recent work of worldwide teams including experienced scientists and clinicians, reflecting the recent emergence of gene therapy from the 'Valley of Death'. The treatment efficacy of clinical gene therapy has now been shown in a number of diseases including cancer and we are observing a renewed interest by big pharmaceutical and biotechnology companies most obviously demonstrated by Amgen's acquisition of Biovex for up to USD$1 billion. There is an opportunity to be cautiously hopeful regarding the future of gene therapy in the clinic and we review here some of the most recent progress in the field.