In Vitro and In Vivo Stimulation of Toll-Like Receptor 9 by CpG Oligodeoxynucleotides Incorporated Into Polypod-Like DNA Nanostructures

Emerging nanoparticle platforms for CpG oligonucleotide delivery

Unmethylated cytosine-phosphate-guanine (CpG) oligodeoxynucleotides (ODNs), which were therapeutic DNA with high immunostimulatory activity, have been applied in widespread applications from basic research to clinics as therapeutic agents for cancer immunotherapy, viral infection, allergic diseases and asthma since their discovery in 1995. The major factors to consider for clinical translation using CpG motifs are the protection of CpG ODNs from DNase degradation and the delivery of CpG ODNs to the Toll-like receptor-9 expressed human B-cells and plasmacytoid dendritic cells. Therefore, great efforts have been devoted to the advances of efficient delivery systems for CpG ODNs. In this review, we outline new horizons and recent developments in this field, providing a comprehensive summary of the nanoparticle-based CpG delivery systems developed to improve the efficacy of CpG-mediated immune responses, including DNA nanostructures, inorganic nanoparticles, polymer nanoparticles, metal-organic-frameworks, lipid-based nanosystems, proteins and peptides, as well as exosomes and cell membrane nanoparticles. Moreover, future challenges in the establishment of CpG delivery systems for immunotherapeutic applications are discussed. We expect that the continuously growing interest in the development of CpG-based immunotherapy will certainly fuel the excitement and stimulation in medicine research.

Macrophage-Targeting DNA Nanomaterials: A Future Direction of Biological Therapy

DNA can be used for precise construction of complex and flexible micro-nanostructures, including DNA origami, frame nucleic acids, and DNA hydrogels. DNA nanomaterials have good biocompatibility and can enter macrophages via scavenger receptor-mediated endocytosis. DNA nanomaterials can be uniquely and flexibly designed to ensure efficient uptake by macrophages, which represents a novel strategy to regulate macrophage function. With the development of nanotechnology, major advances have been made in the design and manufacturing of DNA nanomaterials for clinical therapy. In diseases accompanied by macrophage disturbances including tumor, infectious diseases, arthritis, fibrosis, acute lung injury, and atherosclerosis, DNA nanomaterials received considerable attention as potential treatments. However, we lack sufficient information to guarantee precise targeting of macrophages by DNA nanomaterials, which precludes their therapeutic applications. In this review, we summarize recent studies of macrophage-targeting DNA nanomaterials and discuss the limitations and challenges of this approach with regard to its potential use as a biological therapy.

Addressing the in vivo delivery of nucleic-acid nanostructure therapeutics

DNA and RNA nanostructures are being investigated as therapeutics, vaccines, and drug delivery systems. These nanostructures can be functionalized with guests ranging from small molecules to proteins with precise spatial and stoichiometric control. This has enabled new strategies to manipulate drug activity and to engineer devices with novel therapeutic functionalities. Although existing studies have offered encouraging in vitro or pre-clinical proof-of-concepts, establishing mechanisms of in vivo delivery is the new frontier for nucleic-acid nanotechnologies. In this review, we first provide a summary of existing literature on the in vivo uses of DNA and RNA nanostructures. Based on their application areas, we discuss current models of nanoparticle delivery, and thereby highlight knowledge gaps on the in vivo interactions of nucleic-acid nanostructures. Finally, we describe techniques and strategies for investigating and engineering these interactions. Together, we propose a framework to establish in vivo design principles and advance the in vivo translation of nucleic-acid nanotechnologies.

Targeted Delivery of Immunostimulatory CpG Oligodeoxynucleotides to Antigen-Presenting Cells in Draining Lymph Nodes by Stearic Acid Modification and Nanostructurization

Polypod-like structured nucleic acids (polypodnas), which are nanostructured DNAs, are useful for delivering cytosine-phosphate guanine oligodeoxynucleotides (CpG ODNs) to antigen-presenting cells (APCs) expressing Toll-like receptor 9 (TLR9) for immune stimulation. Lipid modification is another approach to deliver ODNs to lymph nodes, where TLR9-positive APCs are abundant, by binding to serum albumin. The combination of these two methods can be useful for delivering CpG ODNs to lymph nodes in vivo. In the present study, CpG1668, a phosphodiester-type CpG ODN, was modified with stearic acid (SA) to obtain SA-CpG1668. Tripodna, a polypodna with three pods, was selected as the nanostructured DNA. Tripodnas loaded with CpG1668 or SA-CpG1668 were obtained in high yields. SA-CpG1668/tripodna bound more efficiently to plasma proteins than CpG1668/tripodna and was more efficiently taken up by macrophage-like RAW264.7 cells than CpG1668/tripodna, whereas the levels of tumor necrosis factor-α released from the cells were comparable between the two. After subcutaneous injection into mice, SA-CpG1668/tripodna induced significantly higher interleukin (IL)-12 p40 production in the draining lymph nodes than SA-CpG1668 or CpG1668/tripodna, with reduced IL-6 levels in plasma. These results indicate that the combination of SA modification and nanostructurization is a useful approach for the targeted delivery of CpG ODNs to lymph nodes.

Prospective Cancer Therapies Using Stimuli-Responsive DNA Nanostructures

Nanostructures based on DNA self-assembly present an innovative way to address the increasing need for target-specific delivery of therapeutic molecules. Currently, most of the chemotherapeutics being used in clinical practice have undesired and exceedingly high off-target toxicity. This is a challenge in particular for small molecules, and hence, developing robust and effective methods to lower these side effects and enhance the antitumor activity is of paramount importance. Prospectively, these issues could be tackled with the help of DNA nanotechnology, which provides a route for the fabrication of custom, biocompatible, and multimodal structures, which can, to some extent, resist nuclease degradation and survive in the cellular environment. Similar to widely employed liposomal products, the DNA nanostructures (DNs) are loaded with selected drugs, and then by employing a specific stimulus, the payload can be released at its target region. This review explores several strategies and triggers to achieve targeted delivery of DNs. Notably, different modalities are explained through which DNs can interact with their respective targets as well as how structural changes triggered by external stimuli can be used to achieve the display or release of the cargo. Furthermore, the prospects and challenges of this technology are highlighted.

Enhanced Immunostimulatory Activity of CpG Oligodeoxynucleotide by the Combination of Mannose Modification and Incorporation into Nanostructured DNA

The immunostimulatory activity of unmethylated cytosine-phosphate-guanine oligodeoxynucleotide (CpG ODN) could be improved via delivery to immune cells expressing Toll-like receptor 9 (TLR9). Previously, we showed that the polypod-like structured nucleic acid (polypodna), a nanostructured DNA comprised of three or more ODNs, was an efficient system for the delivery of CpG ODNs to immune cells. Because some TLR9-positive immune cells express mannose receptors (MR), the uptake of polypodna by immune cells can be further increased by its modification with mannose. In this study, we selected the phosphodiester CpG ODN, ODN1668, which has a sequence identical to CpG1668, and a hexapodna, a polypodna with six pods, to design a hexapodna that harbored ODN1668 or the mannosylated CpG ODN (Man-ODN1668) synthesized via modification of the 5'-terminal of ODN1668 with a synthesized mannose motif. By mixing ODN1668 or Man-ODN1668 with the hexapodna, ODN1668/hexapodna and Man-ODN1668/hexapodna were successfully formed with high yields. However, Man-ODN1668/hexapodna was found to induce a greater tumor necrosis factor-α release from TLR9- and MR-positive mouse peritoneal macrophages and macrophage-like J774.1 cells than Man-ODN1668 or ODN1668/hexapodna. These results indicate that the combination of mannose modification and incorporation into nanostructured DNA is a useful approach for enhancing the immunostimulatory activity of CpG ODN.

Construction of Monomeric and Dimeric G-Quadruplex-Structured CpG Oligodeoxynucleotides for Enhanced Uptake and Activation in TLR9-Positive Macrophages

The G-quadruplex (GQ) structure has potential applications in nucleic acid drug delivery because of its superior stability. In this study, we added one G-tract (five guanines) to an unmethylated phosphodiester-linked cytosine-phosphate-guanine oligodeoxynucleotide (CpG ODN), a potential immune adjuvant, to construct a GQ-structured CpG ODN with precise structural properties, increased biological stability, and efficient delivery to Toll-like receptor 9 (TLR9)-positive immune cells. A G-tract was added to phosphodiester-backboned CpG1668 at the 5'-end [1668(5'-G₅)], 3'-end [1668(3'-G₅)], or within the sequence [1668(mid-G₅)]. Circular dichroism analysis showed that all CpG ODNs with a G-tract formed parallel GQ structures, irrespective of its position. Electrophoresis showed that 1668(5'-G₅) formed a GQ dimer, whereas others remained GQ monomers. GQ-structured CpG ODNs induced greater tumor necrosis factor-α and interleukin-6 secretion from TLR9-positive mouse macrophage-like RAW264.7 cells than single-stranded CpG ODNs, with the highest for 1668(3'-G₅). GQ structuration increased CpG ODN uptake by RAW264.7 cells, and 1668(3'-G₅) decomposed more slowly in serum than 1668(5'-G₅). Thus, GQ formation with one G-tract is a simple and efficient strategy for CpG ODN delivery to TLR9-positive cells, and addition of a G-tract to the 3'-end is effective in obtaining monomeric GQ-structured CpG ODN with high biological stability and immunostimulatory activity.

The crosstalk between microbiome and asthma: Exploring associations and challenges

With the advancement of high‐throughput DNA/RNA sequencing and computational analysis techniques, commensal bacteria are now considered almost as important as pathological ones. Understanding the interaction between these bacterial microbiota, host and asthma is crucial to reveal their role in asthma pathophysiology. Several airway and/or gut microbiome studies have shown associations between certain bacterial taxa and asthma. However, challenges remain before gained knowledge from these studies can be implemented into clinical practice, such as inconsistency between studies in choosing sampling compartments and/or sequencing approaches, variability of results in asthma studies, and not taking into account medication intake and diet composition especially when investigating gut microbiome. Overcoming those challenges will help to better understand the complex asthma disease process. The therapeutic potential of using pro‐ and prebiotics to prevent or reduce risk of asthma exacerbations requires further investigation. This review will focus on methodological issues regarding setting up a microbiome study, recent developments in asthma bacterial microbiome studies, challenges and future therapeutic potential.

Growing prospects of DNA nanomaterials in novel biomedical applications

As an important genetic material for life, DNA has been investigated widely in recent years, especially in interdisciplinary fields crossing nanomaterials and biomedical applications. It plays an important role because of its extraordinary molecular recognition capability and novel conformational polymorphism. DNA is also a powerful and versatile building block for the fabrication of nanostructures and nanodevices. Such DNA-based nanomaterials have also been successfully applied in various aspects ranging from biosensors to biomedicine and special logic gates, as well as in emerging molecular nanomachines. In this present mini-review, we briefly overview the recent progress in these fields. Furthermore, some challenges are also discussed in the conclusions and perspectives section, which aims to stimulate broader scientific interest in DNA nanotechnology and its biomedical applications.

Recent progress in DNA-based nanomaterials is summarized, ranging from applications in biosensors, biomedicine/imaging, and molecular logic gates to emerging nanomachines, as well as future perspective discussions.

Nanoparticle-based CpG-oligonucleotide therapy for treating allergic asthma

Allergic asthma is becoming increasingly prevalent in the developed world, and many common allergens are capable of inducing allergic asthma responses, particularly in atopic individuals. Unmethylated CpG-oligonucleotide (ODN) therapy can shift the immune response to mitigate these allergic responses. Therapeutic and prophylactic delivery of soluble CpG-ODN in preclinical studies has shown promise in treating existing asthma and preventing allergic responses upon subsequent allergen exposure, respectively. However, when CpG-ODN is coupled with nanoparticles or self assembled into nanostructures, improved efficacy of CpG-ODN treatment for several common allergens is observed in preclinical studies and clinical trials. Here we discuss the role of CpG-ODN in treating allergic asthma and how nanoparticle-based delivery can further enhance its therapeutic properties.

A Dual Macrophage Targeting Nanovector for Delivery of Oligodeoxynucleotides To Overcome Cancer-Associated Immunosuppression

To overcome cancer-associated immunosuppression, we prepared a dual-targeting vector to deliver CpG oligodeoxynucleotides (ODN) to macrophages. The dual-targeting system composed of mannosylated carboxymethyl chitosan (MCMC)/hyaluronan (HA) for macrophage targeting and protamine sulfate for ODN complexation was prepared by self-assembly. The effects of ODN delivery on immune cells was studied in J774A.1 cells. Due to the enhanced delivery efficiency, the dual-targeting delivery system exhibits a higher immune stimulatory activity compared with the monotargeting delivery system containing either MCMC or HA, resulting in a dramatically enhanced secretion of proinflammatory cytokines and a successful shift to activated macrophages (M1). Besides macrophages, the influence of the delivery system on tumor cells (MCF-7) was also investigated. In MCF-7 cells, the increased expressions of nuclear transcription factor-κB (NF-κB), PIK3R3, and phosphorylated protein kinase B (p-Akt) caused by activated NF-κB and phosphoinositide 3-kinase/Akt signalings were observed. Nevertheless, upregulated Fas as well as Fas ligand (FasL) may induce Fas/FasL-mediated apoptosis, which results in the increased expressions of caspases in tumor cells.

Construction of nanostructured DNA harbouring phosphorodiamidate morpholino oligonucleotide for controlled tissue distribution in mice

Phosphorodiamidate morpholino oligonucleotides (PMOs) are a class of antisense oligonucleotides used in the treatment of neuromuscular diseases. Their major drawbacks are high blood clearance and poor cellular delivery. Previously, we demonstrated that tripod-like nanostructured DNA, or tripodna, was efficiently taken up by macrophages and dendritic cells. In this study, we used iodine-125(¹²⁵I)-labelled PMOs, designed a tripodna harbouring an ¹²⁵I-PMO (¹²⁵I-PMO/tripodna), and evaluated whether this tripodna could control the pharmacokinetic properties of PMO. Gel electrophoresis showed that ¹²⁵I-PMO was almost completely incorporated into the tripodna. Compared to ¹²⁵I-PMO, ¹²⁵I-PMO/tripodna was more efficiently taken up by macrophage-like RAW264.7 cells. Moreover, after intravenous injection into mice, the area under the plasma concentration-time curve of ¹²⁵I-PMO/tripodna was significantly larger than that of ¹²⁵I-PMO. The distribution of ¹²⁵I-PMO/tripodna in the liver and spleen at 24 h was 32- and 51-fold higher than that of ¹²⁵I-PMO, respectively. The fractionation of liver cells revealed that non-parenchymal cells were the major cells contributing to the hepatic uptake of ¹²⁵I-PMO/tripodna. These results indicate that tripodna has the potential to deliver PMO, particularly to the liver and spleen.