A novel STING agonist for cancer immunotherapy and a SARS-CoV-2 vaccine adjuvant

A near-infrared light-driven Janus nanomotor for deep tumor penetration and enhanced tumor immunotherapy

A near-infrared light-driven Janus nanomotor is constructed by collagenase-coated gold nanorods and chitosan-functionalized mesoporous organosilica nanoparticles with Mn2+ as the bridging ion. The nanomotors with excellent motility and collagenase activity can potently penetrate into tumors to sufficiently activate innate immune responses, significantly enhancing anti-tumor immune efficacy in vivo.

STING agonists as promising vaccine adjuvants to boost immunogenicity against SARS-related coronavirus derived infection: possible role of autophagy

As a major component of innate immunity and a positive regulator of interferons, the Stimulator of interferon gene (STING) has an immunotherapy potential to govern a variety of infectious diseases. Despite the recent advances regarding vaccines against COVID-19, nontoxic novel adjuvants with the potential to enhance vaccine efficacy are urgently desired. In this connection, it has been well-documented that STING agonists are applied to combat COVID-19. This approach is of major significance for boosting immune responses most likely through an autophagy-dependent manner in susceptible individuals against infection induced by severe acute respiratory syndrome Coronavirus (SARS‑CoV‑2). Given that STING agonists exert substantial immunomodulatory impacts under a wide array of pathologic conditions, these agents could be considered novel adjuvants for enhancing immunogenicity against the SARS-related coronavirus. Here, we intend to discuss the recent advances in STING agonists' recruitment to boost innate immune responses upon vaccination against SARS-related coronavirus infections. In light of the primordial role of autophagy modulation, the potential of being an antiviral vaccine adjuvant was also explored.

Pharmacological potential of cyclic nucleotide signaling in immunity

Cyclic nucleotides are important signaling molecules that play many critical physiological roles including controlling cell fate and development, regulation of metabolic processes, and responding to changes in the environment. Cyclic nucleotides are also pivotal regulators in immune signaling, orchestrating intricate processes that maintain homeostasis and defend against pathogenic threats. This review provides a comprehensive examination of the pharmacological potential of cyclic nucleotide signaling pathways within the realm of immunity. Beginning with an overview of the fundamental roles of cAMP and cGMP as ubiquitous second messengers, this review delves into the complexities of their involvement in immune responses. Special attention is given to the challenges associated with modulating these signaling pathways for therapeutic purposes, emphasizing the necessity for achieving cell-type specificity to avert unintended consequences. A major focus of the review is on the recent paradigm-shifting discoveries regarding specialized cyclic nucleotide signals in the innate immune system, notably the cGAS-STING pathway. The significance of cyclic dinucleotides, exemplified by 2'3'-cGAMP, in controlling immune responses against pathogens and cancer, is explored. The evolutionarily conserved nature of cyclic dinucleotides as antiviral agents, spanning across diverse organisms, underscores their potential as targets for innovative immunotherapies. Findings from the last several years have revealed a striking diversity of novel bacterial cyclic nucleotide second messengers which are involved in antiviral responses. Knowledge of the existence and precise identity of these molecules coupled with accurate descriptions of their associated immune defense pathways will be essential to the future development of novel antibacterial therapeutic strategies. The insights presented herein may help researchers navigate the evolving landscape of immunopharmacology as it pertains to cyclic nucleotides and point toward new avenues or lines of thinking about development of therapeutics against the pathways they regulate.

Stimulator of Interferon Gene Agonists Induce an Innate Antiviral Response against Influenza Viruses

The devastating effects of COVID-19 have highlighted the importance of prophylactic and therapeutic strategies to combat respiratory diseases. Stimulator of interferon gene (STING) is an essential component of the host defense mechanisms against respiratory viral infections. Although the role of the cGAS/STING signaling axis in the innate immune response to DNA viruses has been thoroughly characterized, mounting evidence shows that it also plays a key role in the prevention of RNA virus infections. In this study, we investigated the role of STING activation during Influenza virus (IFV) infection. In both mouse bone marrow-derived macrophages and monocytic cell line THP-1 differentiated with PMA, we found that dimeric amidobenzimidazole (diABZI), a STING agonist, had substantial anti-IFV activity against multiple strains of IFV, including A/H1N1, A/H3N2, B/Yamagata, and B/Victoria. On the other hand, a pharmacological antagonist of STING (H-151) or the loss of STING in human macrophages leads to enhanced viral replication but suppressed IFN expression. Furthermore, diABZI was antiviral against IFV in primary air-liquid interface cultures of nasal epithelial cells. Our data suggest that STING agonists may serve as promising therapeutic antiviral agents to combat IFV.

Multifunctional Lipidated Protein Carrier with a Built-In Adjuvant as a Universal Vaccine Platform Potently Elevates Immunogenicity of Weak Antigens

Weak antigens represented by MUC1 are poorly immunogenic, which greatly constrains the development of relevant vaccines. Herein, we developed a multifunctional lipidated protein as a carrier, in which the TLR1/2 agonist Pam3CSK4 was conjugated to the N-terminus of MUC1-loaded carrier protein BSA through pyridoxal 5'-phosphate-mediated transamination reaction. The resulting Pam3CSK4-BSA-MUC1 conjugate was subsequently incorporated into liposomes, which biomimics the membrane structure of tumor cells. The results indicated that this lipidated protein carrier significantly enhanced antigen uptake by APCs and obviously augmented the retention of the vaccine at the injection site. Compared with the BSA-MUC1 and BSA-MUC1 + Pam3CSK4 groups, Pam3CSK4-BSA-MUC1 evoked 22- and 11-fold increases in MUC1-specific IgG titers. Importantly, Pam3CSK4-BSA-MUC1 elicited robust cellular immunity and significantly inhibited tumor growth. This is the first time that lipidated protein was constructed to enhance antigen immunogenicity, and this universal carrier platform exhibits promise for utilization in various vaccines, holding the potential for further clinical application.

The cGAS-STING pathway: a therapeutic target in diabetes and its complications

Diabetic wound healing (DWH) represents a major complication of diabetes where inflammation is a key impediment to proper healing. The cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway has emerged as a central mediator of inflammatory responses to cell stress and damage. However, the contribution of cGAS-STING activation to impaired healing in DWH remains understudied. In this review, we examine the evidence that cGAS-STING-driven inflammation is a critical factor underlying defective DWH. We summarize studies revealing upregulation of the cGAS-STING pathway in diabetic wounds and discuss how this exacerbates inflammation and senescence and disrupts cellular metabolism to block healing. Partial pharmaceutical inhibition of cGAS-STING has shown promise in damping inflammation and improving DWH in preclinical models. We highlight key knowledge gaps regarding cGAS-STING in DWH, including its relationships with endoplasmic reticulum stress and metal-ion signaling. Elucidating these mechanisms may unveil new therapeutic targets within the cGAS-STING pathway to improve healing outcomes in DWH. This review synthesizes current understanding of how cGAS-STING activation contributes to DWH pathology and proposes future research directions to exploit modulation of this pathway for therapeutic benefit.

Targeting the cGAS-STING pathway as an inflammatory crossroad in coronavirus disease 2019 (COVID-19)

CONTEXT AND OBJECTIVE

The emerging pandemic of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), has imposed significant mortality and morbidity on the world. An appropriate immune response is necessary to inhibit SARS-CoV-2 spread throughout the body.

RESULTS

During the early stages of infection, the pathway of stimulators of interferon genes (STING), known as the cGAS-STING pathway, has a significant role in the induction of the antiviral immune response by regulating nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and Interferon regulatory factor 3 (IRF3), two key pathways responsible for proinflammatory cytokines and type I IFN secretion, respectively.

DISCUSSION

During the late stages of COVID-19, the uncontrolled inflammatory responses, also known as cytokine storm, lead to the progression of the disease and poor prognosis. Hyperactivity of STING, leading to elevated titers of proinflammatory cytokines, including Interleukin-I (IL-1), IL-4, IL-6, IL-18, and tissue necrosis factor-α (TNF-α), is considered one of the primary mechanisms contributing to the cytokine storm in COVID-19.

CONCLUSION

Exploring the underlying molecular processes involved in dysregulated inflammation can bring up novel anti-COVID-19 therapeutic options. In this article, we aim to discuss the role and current studies targeting the cGAS/STING signaling pathway in both early and late stages of COVID-19 and COVID-19-related complications and the therapeutic potential of STING agonists/antagonists. Furthermore, STING agonists have been discussed as a vaccine adjuvant to induce a potent and persistent immune response.

Overcoming challenges in the delivery of STING agonists for cancer immunotherapy: A comprehensive review of strategies and future perspectives

STING (Stimulator of Interferon Genes) agonists have emerged as promising agents in the field of cancer immunotherapy, owing to their excellent capacity to activate the innate immune response and combat tumor-induced immunosuppression. This review provides a comprehensive exploration of the strategies employed to develop effective formulations for STING agonists, with particular emphasis on versatile nano-delivery systems. The recent advancements in delivery systems based on lipids, natural/synthetic polymers, and proteins for STING agonists are summarized. The preparation methodologies of nanoprecipitation, self-assembly, and hydrogel, along with their advantages and disadvantages, are also discussed. Furthermore, the challenges and opportunities in developing next-generation STING agonist delivery systems are elaborated. This review aims to serve as a reference for researchers in designing novel and effective STING agonist delivery systems for cancer immunotherapy.

Identification of a Self-Assembling Small-Molecule Cancer Vaccine Adjuvant with an Improved Toxicity Profile

Protein or peptide cancer vaccines usually include immune potentiators, so-called adjuvants. However, it remains challenging to identify structurally simple, chemically accessible synthetic molecules that are effective and safe as vaccine adjuvant. Here, we present cholicamideβ (6), a self-assembling small-molecule vaccine adjuvant with an improved toxicity profile and proven efficacy in vivo. We demonstrate that cholicamideβ (6), which is less cytotoxic than its parent compound, forms virus-like particles to potently activate dendritic cells with the concomitant secretion of cytokines. When combined with a peptide antigen, cholicamideβ (6) potentiated the antigen presentation on dendritic cells to induce antigen-specific T cells. As a therapeutic cancer vaccine adjuvant in mice, a mixture of cholicamideβ (6) and a peptide antigen protected mice from the challenges of malignant cancer cells without overt toxicity. Cholicamideβ (6) may offer a translational opportunity as an unprecedented class of small-molecule cancer vaccine adjuvants.

Recent trends in STING modulators: Structures, mechanisms, and therapeutic potential

The cyclic GMP-AMP synthase stimulator (cGAS)-stimulator of interferon gene (STING) signaling pathway has an integral role in the host immune response through DNA sensing followed by inducing a robust innate immune defense program. STING has become a promising therapeutic target associated with multiple diseases, including various inflammatory diseases, cancer, and infectious diseases, among others. Thus, modulators of STING are regarded as emerging therapeutic agents. Recent progress has been made in STING research, including recently identified STING-mediated regulatory pathways, the development of a new STING modulator, and the new association of STING with disease. In this review, we focus on recent trends in the development of STING modulators, including structures, mechanisms, and clinical application.

STING agonist-loaded mesoporous manganese-silica nanoparticles for vaccine applications

Cyclic dinucleotides (CDNs), as one type of Stimulator of Interferon Genes (STING) pathway agonist, have shown promising results for eliciting immune responses against cancer and viral infection. However, the suboptimal drug-like properties of conventional CDNs, including their short in vivo half-life and poor cellular permeability, compromise their therapeutic efficacy. In this study, we have developed a manganese-silica nanoplatform (MnOx@HMSN) that enhances the adjuvant effects of CDN by achieving synergy with Mn2+ for vaccination against cancer and SARS-CoV-2. MnOx@HMSN with large mesopores were efficiently co-loaded with CDN and peptide/protein antigens. MnOx@HMSN(CDA) amplified the activation of the STING pathway and enhanced the production of type-I interferons and other proinflammatory cytokines from dendritic cells. MnOx@HMSN(CDA) carrying cancer neoantigens elicited robust antitumor T-cell immunity with therapeutic efficacy in two different murine tumor models. Furthermore, MnOx@HMSN(CDA) loaded with SARS-CoV-2 antigen achieved strong and durable (up to one year) humoral immune responses with neutralizing capability. These results demonstrate that MnOx@HMSN(CDA) is a versatile nanoplatform for vaccine applications.

Dressing Bacteria With a Hybrid Immunoactive Nanosurface to Elicit Dual Anticancer and Antiviral Immunity

Approaches capable of simultaneously treating cancer and protecting susceptible patients from lethal infections such as coronavirus disease 2019, are highly desirable but prove to be difficult. Here, dressing bacteria with a hybrid immunoactive nanosurface is reported to elicit dual anticancer and antiviral immunity. A combination of a checkpoint blocking antibody and a virus-specific antigen is covalently conjugated to polydopamine nanoparticles, which can be anchored onto bacterial surface, by a one-step in situ polymerization of dopamine under a cell-friendly condition. By virtue of the ability to colonize and penetrate deep tumor tissue, dressed bacteria enable sustained release and expanded exposure of carried immunoactivators to stimulate immune cells. In addition to a carrier role, bacteria are able to further provoke innate immunity due to the native immunogenicity of the pathogen-associated molecular patterns. Immunization with dressed bacteria promotes the maturation, and activation of antigen-presenting cells, which induces robust humoral and cellular immune responses in tumor-bearing mice. As evidenced by efficient production of viral-antigen-specific immunoglobulin G antibody in serum and significantly suppressed tumor growth in different models, dressing bacteria with a hybrid immunoactive nanosurface paves an avenue to prepare next-generation therapeutics for synergistic treatment and prevention.

Current understanding of the cGAS-STING signaling pathway: Structure, regulatory mechanisms, and related diseases

The innate immune system protects the host from external pathogens and internal damage in various ways. The cGAS-STING signaling pathway, comprised of cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING), and downstream signaling adaptors, plays an essential role in protective immune defense against microbial DNA and internal damaged-associated DNA and is responsible for various immune-related diseases. After binding with DNA, cytosolic cGAS undergoes conformational change and DNA-linked liquid-liquid phase separation to produce 2'3'-cGAMP for the activation of endoplasmic reticulum (ER)-localized STING. However, further studies revealed that cGAS is predominantly expressed in the nucleus and strictly tethered to chromatin to prevent binding with nuclear DNA, and functions differently from cytosolic-localized cGAS. Detailed delineation of this pathway, including its structure, signaling, and regulatory mechanisms, is of great significance to fully understand the diversity of cGAS-STING activation and signaling and will be of benefit for the treatment of inflammatory diseases and cancer. Here, we review recent progress on the above-mentioned perspectives of the cGAS-STING signaling pathway and discuss new avenues for further study.

Involvement of the STING signaling in COVID-19

The coronavirus disease 2019 (COVID-19) pandemic caused by the infection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has cast a notorious damage to the public health and global economy. The Stimulator of Interferon Genes (STING) is a crucial element of the host antiviral pathway and plays a pivotal but complex role in the infection and development of COVID-19. Herein, we discussed the antagonistic mechanism of viral proteins to the STING pathway as well as its activation induced by host cells. Specifically, we highlighted that the persistent activation of STING by SARS-CoV-2 led to abnormal inflammation, and STING inhibitors could reduce the excessive inflammation. In addition, we also emphasized that STING agonists possessed antiviral potency against diverse coronavirus and showed adjuvant efficacy in SARS-CoV-2 vaccines by inducing IFN responses.

Progress of cGAS-STING signaling in response to SARS-CoV-2 infection

Coronavirus disease 2019 (COVID-19) is an epidemic respiratory disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that can cause infections in millions of individuals, who can develop lung injury, organ failure, and subsequent death. As the first line of host defense, the innate immune system is involved in initiating the immune response to SARS-CoV-2 infection and the hyperinflammatory phenotype of COVID-19. However, the interplay between SARS-CoV-2 and host innate immunity is not yet well understood. It had become known that the cGAS-STING pathway is involved in the detection of cytosolic DNA, which elicits an innate immune response involving a robust type I interferon response against viral and bacterial infections. Nevertheless, several lines of evidence indicate that SARS-CoV-2, a single-stranded positive-sense RNA virus, triggered the cGAS-STING signaling pathway. Therefore, understanding the molecular and cellular details of cGAS-STING signaling upon SARS-CoV-2 infection is of considerable biomedical importance. In this review, we discuss the role of cGAS-STING signaling in SARS-CoV-2 infection and summarize the potential therapeutics of STING agonists as virus vaccine adjuvants.

STING Targeting in Lung Diseases

The cGAS-STING pathway displays important functions in the regulation of innate and adaptive immunity following the detection of microbial and host-derived DNA. Here, we briefly summarize biological functions of STING and review recent literature highlighting its important contribution in the context of respiratory diseases. Over the last years, tremendous progress has been made in our understanding of STING activation, which has favored the development of STING agonists or antagonists with potential therapeutic benefits. Antagonists might alleviate STING-associated chronic inflammation and autoimmunity. Furthermore, pharmacological activation of STING displays strong antiviral properties, as recently shown in the context of SARS-CoV-2 infection. STING agonists also elicit potent stimulatory activities when used as an adjuvant promoting antitumor responses and vaccines efficacy.

Cancer immunotherapy strategies that target the cGAS-STING pathway

Activation of the cGAS-STING pathway by cytoplasmic DNA induces the production of Type-1 interferons. Recent advances in research suggest that the cGAS-STING pathway is involved in different parts of the cancer-immunity cycle (CIC) to promote or suppress antitumor immune responses. Combination therapy of STING agonists has made certain progress in preclinical as well as clinical trials, but the selection of combination therapy regimens remains a challenge. In this review, we summarize the role of the cGAS-STING in all aspects of CIC, and focus on the combination immunotherapy strategies of STING agonists and current unsolved challenges.

Rational Design of T-Cell- and B-Cell-Based Therapeutic Cancer Vaccines

Cancer vaccines provide an efficient strategy to enhance tumor-specific immune responses by redeploying immune systems. Despite the approval of the first cancer vaccine (Sipuleucel-T) by the U.S. Food and Drug Administration in 2010, most therapeutic cancer vaccines fail in clinical trials. Basically, tumor-specific immune responses rely on not only T-cell but also B-cell immunity, which indicates that cancer vaccines should leverage both arms of the adaptive immune system. For example, CD8⁺ T cells activated by antigen-presenting cells (APCs) recognize and directly kill tumor cells via peptide-bound major histocompatibility complex (pMHC). B cells recognize antigen with no need of pMHC and require CD4⁺ T cells for sufficient activation and antibody generation, enabling antibody-mediated nondirect killing on tumor cells. Considering the different mechanisms of T-cell and B-cell activation, the rational design of therapeutic cancer vaccines should consider several factors, including antigen selection and recognition, immune activation, vaccine delivery, and repeatable vaccination, which can be advanced by chemical strategies.In this Account, we summarize our recent contributions to the development of effective T-cell- and B-cell-based therapeutic cancer vaccines. For T-cell-based vaccines, we focus on adjuvants as the key component for controllable APC activation and T-cell priming. Not only synthetic molecular agonists of pattern recognition receptors (PRRs) but also adjuvant nanomaterials were explored to satisfy diversiform vaccine designs. For example, a type of natural cyclic dinucleotide (CDN) that was chemically modified with fluorination and ipsilateral phosphorothioation to activate the stimulator of interferon gene (STING) was found to mediate antitumor responses. It retains structural similarity to the parent CDN scaffold but possesses increased stability, cellular uptake, and immune activation for antitumor treatment. It also facilitates facile conjugation with other agonists, which not only enhances APC-targeting delivery but also balances cellular and humoral antitumor responses. We also explored the intrinsic properties of nanomaterials that allow them to serve as adjuvants. A black phosphorus nanosheet-based nanovaccine was constructed and found to strongly potentiate antigen-specific T-cell antitumor immune responses through multiple immune-potentiating properties, leading to a highly integrated nanomaterial-based adjuvant design. For B-cell-based vaccines, multicomponent and multivalent strategies were applied to improve the immunogenicity. A multicomponent linear vaccine conjugate coordinates helper T (Th) cells and APCs to proliferate and differentiates B cells for enhanced antitumor immunoglobulin G antibody responses. To further improve antigen recognition, clustered designs on a multivalent epitope were applied by generating various structures, including branched lysine-based peptides, natural multivalent scaffold molecules, and self-assembled nanofibers. We also engineered nano- and microvaccine systems to optimize systemic and localized vaccination. A multilayer-assembled nanovaccine successfully integrated antigens and multiple agonists to modulate APC activation. A DNA hydrogel contributed to the control of APC's immune behaviors, including cell recruitment, activation, and migration, and induced robust antitumor responses as an all-in-one designable platform. In this Account, by summarizing strategies for both T-cell- and B-cell-based vaccine design, we not only compare the differences but also address the intrinsic uniformity between such vaccine designs and further discuss the potential of a combined T-cell- and B-cell-based vaccine, which highlights the applicability and feasibility of chemical strategies.

New opportunities for immunomodulation of the tumour microenvironment using chemical tools

Immunotherapy is recognised as an attractive method for the treatment of cancer, and numerous treatment strategies have emerged over recent years. Investigations of the tumour microenvironment (TME) have led to the identification of many potential therapeutic targets and methods. However, many recently applied immunotherapies are based on previously identified strategies, such as boosting the immune response by combining commonly used stimulators, and the release of drugs through changes in pH. Although methodological improvements such as structural optimisation and combining strategies can be undertaken, applying those novel targets and methods in immunotherapy remains an important goal. In this review, we summarise the latest research on the TME, and discuss how small molecules, immune cells, and their interactions with tumour cells can be regulated in the TME. Additionally, the techniques currently employed for delivery of these agents to the TME are also mentioned. Strategies to modulate cell phenotypes and interactions between immune cells and tumours are mainly discussed. We consider both modulatory and targeting methods aiming to bridge the gap between the TME and chemical modulation thereof.

Adjuvant-Protein Conjugate Vaccine with Built-In TLR7 Agonist on S1 Induces Potent Immunity against SARS-CoV-2 and Variants of Concern

With the global pandemic of the new coronavirus disease (COVID-19), a safe, effective, and affordable mass-produced vaccine remains the current focus of research. Herein, we designed an adjuvant-protein conjugate vaccine candidate, in which the TLR7 agonist (TLR7a) was conjugated to S1 subunit of SARS-CoV-2 spike protein, and systematically compared the effect of different numbers of built-in TLR7a on the immune activity for the first time. As the number of built-in TLR7a increased, a bell-shaped reaction was observed in three TLR7a-S1 conjugates, with TLR7a(10)-S1 (with around 10 built-in adjuvant molecules on one S1 protein) eliciting a more potent immune response than TLR7a(2)-S1 and TLR7a(18)-S1. This adjuvant-protein conjugate strategy allows the built-in adjuvant to provide cluster effects and prevents systemic toxicity and facilitates the co-delivery of adjuvant and antigen. Vaccination of mice with TLR7a(10)-S1 triggered a potent humoral and cellular immunity and a balanced Th1/Th2 immune response. Meanwhile, the vaccine induces effective neutralizing antibodies against SARS-CoV-2 and all variants of concern (B.1.1.7/alpha, B.1.351/beta, P.1/gamma, B.1.617.2/delta, and B.1.1.529/omicron). It is expected that the adjuvant-protein conjugate strategy has great potential to construct a potent recombinant protein vaccine candidate against various types of diseases.

Chemical regulation of the cGAS-STING pathway

Nucleic acids represent a major class of pathogen and damage signatures, recognized by a variety of host sensors to initiate signaling cascades and immune responses, such as mechanisms of RLR-MAVS, cGAS-STING, TLR-TRIF, and AIM2 inflammasome. Yet, an outstanding challenge is understanding how nucleic acid sensing initiates immune responses and its tethering in various infectious, cancerous, autoimmune, and inflammatory diseases. However, the discovery and application of a plethora of small molecule compounds have substantially facilitated this process. This review provides an overview and recent development of the innate DNA-sensing pathway of cGAS-STING and highlights the multiple agonists and inhibitors in fine-tuning the pathway that can be exploited to improve disease treatment, focusing primarily on crucial pathway components and regulators.

A protein vaccine with Alum/c-GAMP/poly(I:C) rapidly boosts robust immunity against SARS-CoV-2 and variants of concern

Adjuvants are important components in vaccines to increase the immunogenicity of proteins and induce optimal immunity. In this study, we designed a novel ternary adjuvant system Alum + c-GAMP + poly(I:C) with STING agonist 3,3'-c-GAMP (c-GAMP) and TLR3 agonist poly(I:C) co-adsorbed on the conventional adjuvant aluminum gel (Alum), and further constructed an S1 protein vaccine. Two doses of vaccination with the ternary adjuvant vaccine were sufficient to induce a balanced Th1/Th2 immune response and robust humoral and cellular immunity. Additionally, the ternary adjuvant group had effective neutralizing activity against live virus SARS-CoV-2 and pseudovirus of all variants of concern (alpha, beta, gamma, delta and omicron). These results indicate that the ternary adjuvants have a significant synergistic effect and can rapidly trigger potent immune responses; the combination of the ternary adjuvant system with S1 protein is a promising COVID-19 vaccine candidate.

MPLA-Adjuvanted Liposomes Encapsulating S-Trimer or RBD or S1, but Not S-ECD, Elicit Robust Neutralization Against SARS-CoV-2 and Variants of Concern

Safe and effective vaccines are the best method to defeat worldwide SARS-CoV-2 and its circulating variants. The SARS-CoV-2 S protein and its subunits are the most attractive targets for the development of protein-based vaccines. In this study, we evaluated three lipophilic adjuvants, monophosphoryl lipid A (MPLA), Toll-like receptor (TLR) 1/2 ligand Pam3CSK4, and α-galactosylceramide (α-GalCer), in liposomal and nonliposomal vaccines. The immunological results showed that the MPLA-adjuvanted liposomal vaccine induced the strongest humoral and cellular immunity. Therefore, we further performed a systematic comparison of S-trimer, S-ECD, S1, and RBD as antigens in MPLA-adjuvanted liposomes and found that, although these four vaccines all induced robust specific antibody responses, only S-trimer, S1, and RBD liposomes, but not S-ECD, elicited potent neutralizing antibody responses. Moreover, RBD, S-trimer, and S1 liposomes effectively neutralized variants (B.1.1.7/alpha, B.1.351/beta, P.1/gamma, B.1.617.2/delta, and B.1.1.529/omicron). These results provide important information for the subunit vaccine design against SARS-CoV-2 and its variants.

Bimetallic metal-organic frameworks for tumor inhibition via combined photothermal-immunotherapy

Herein, we report the design of therapeutic nanoparticles by encapsulating photosensitizers and aluminum ions into metal-organic frameworks. The nanoparticles could significantly inhibit the growth of primary and rechallenged tumors by a combination of photothermal therapy and immunotherapy. This work offers a promising strategy to design an immunologic nanoplatform for "cold" tumor therapy.

RBD conjugate vaccine with a built-in TLR1/2 agonist is highly immunogenic against SARS-CoV-2 and variants of concern

The coronavirus 2019 (COVID-19) pandemic is causing serious impacts in the world, and safe and effective vaccines and medicines are the best methods to combat the disease. The receptor-binding domain (RBD) of the SARS-CoV-2 spike protein plays a key role in interacting with the angiotensin-converting enzyme 2 (ACE2) receptor, and is regarded as an important target of vaccines. Herein, we constructed the adjuvant-protein conjugate Pam₃CSK₄-RBD as a vaccine candidate, in which the N-terminal of the RBD was site-selectively oxidized by transamination and conjugated with the TLR1/2 agonist Pam₃CSK₄. This demonstrated that the conjugation of Pam₃CSK₄ significantly enhanced the anti-RBD antibody response and cellular response. In addition, sera from the Pam₃CSK₄-RBD immunized group efficiently inhibited the binding of the RBD to ACE2 and protected cells from SARS-CoV-2 and four variants of concern (alpha, beta, gamma and delta), indicating that this adjuvant strategy could be one of the effective means for protein vaccine development.

Self-Adjuvanting Lipoprotein Conjugate αGalCer-RBD Induces Potent Immunity against SARS-CoV-2 and its Variants of Concern

Safe and effective vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and its variants are the best approach to successfully combat the COVID-19 pandemic. The receptor-binding domain (RBD) of the viral spike protein is a major target to develop candidate vaccines. α-Galactosylceramide (αGalCer), a potent invariant natural killer T cell (iNKT) agonist, was site-specifically conjugated to the N-terminus of the RBD to form an adjuvant-protein conjugate, which was anchored on the liposome surface. This is the first time that an iNKT cell agonist was conjugated to the protein antigen. Compared to the unconjugated RBD/αGalCer mixture, the αGalCer-RBD conjugate induced significantly stronger humoral and cellular responses. The conjugate vaccine also showed effective cross-neutralization to all variants of concern (B.1.1.7/alpha, B.1.351/beta, P.1/gamma, B.1.617.2/delta, and B.1.1.529/omicron). These results suggest that the self-adjuvanting αGalCer-RBD has great potential to be an effective COVID-19 vaccine candidate, and this strategy might be useful for designing various subunit vaccines.

A chitosan-mediated inhalable nanovaccine against SARS-CoV-2

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), with several antigenic variants, has grown into a global challenge, and the rapid establishment of an immune barrier is crucial to achieving long-term control of the virus. This has led to a great demand for easy preparation and scalable vaccines, especially in low-income countries. Here, we present an inhalable nanovaccine comprising chitosan and SARS-CoV-2 spike protein. The chitosan-mediated nanovaccine enabled a strong spike-specific antibody immune response and augmented local mucosal immunity in bronchoalveolar lavage and lungs, which might be capable of protecting the host from infection without systemic toxicity. In addition, the enhanced adaptive immunity stimulated by chitosan showed potential protection against SARS-CoV-2. Furthermore, inhalation of the nanovaccine induced a comparable antibody response compared to intramuscular injection. This inhalable nanovaccine against SARS-CoV-2 offers a convenient and compliant strategy to reduce the use of needles and the need for medical staff.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (the immune activation of CS-mediated nanovacccine on BMDCs, cell viability, immune responses in lungs and BALF, serum chemistry and H&E histopathological analysis.) is available in the online version of this article at 10.1007/s12274-021-4012-9.

Nanodelivery of STING agonists against cancer and infectious diseases

Vaccination is a modality that has been widely explored for the treatment of various diseases. To increase the potency of vaccine formulations, immunostimulatory adjuvants have been regularly exploited, and the stimulator of interferon genes (STING) signaling pathway has recently emerged as a remarkable therapeutic target. STING is an endogenous protein on the endoplasmic reticulum that is a downstream sensor to cytosolic DNA. Upon activation, STING initiates a series of intracellular signaling cascades that ultimately generate potent type I interferon-mediated immune responses. Both natural and synthetic agonists have been used to stimulate the STING pathway, but they are usually administered locally due to low bioavailability, instability, and difficulty in bypassing the plasma membrane. With excellent pharmacokinetic profiles and versatility, nanocarriers can address many of these challenges and broaden the application of STING vaccines. Along these lines, STING-inducing nanovaccines are being developed to address a wide range of diseases. In this review, we discuss the recent advances in STING nanovaccines for anticancer, antiviral, and antibacterial applications.

Overcoming STING Agonists Barriers: Peptide, Protein, Biomembrane-based Biocompatible Delivery Strategies

After the development for more ten years, stimulator of interferon genes (STING), a representative of pattern recognition receptors (PRRs), is now entering the stage of widespread applications. Along with evolution of STING agonists of cyclic dinucleotides (CDNs) and non-nucleotide molecules, the stability of agonists has been improved. However, their poor performance in clinical trials triggers urgent demands for highly effective delivery strategies to further improve the cellular permeability, tissue targetability and retention. In this review, we summarized the recent progress of STING agonists applications and delivery strategies with a focus on the biocompatible platforms of peptide, protein and biomembrane, providing a novel vision for STING field and future direction.

Bioengineering Strategies for Developing Vaccines against Respiratory Viral Diseases

Respiratory viral pathogens like influenza and coronaviruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have caused outbreaks leading to millions of deaths. Vaccinations are, to date, the best and most economical way to control such outbreaks and have been highly successful for several pathogens. Currently used vaccines for respiratory viral pathogens are primarily live attenuated or inactivated and can risk reversion to virulence or confer inadequate immunity. The recent trend of using potent biomolecules like DNA, RNA, and protein antigenic components to synthesize vaccines for diseases has shown promising results. Still, it remains challenging to translate due to their high susceptibility to degradation during storage and after delivery. Advances in bioengineering technology for vaccine design have made it possible to control the physicochemical properties of the vaccines for rapid synthesis, heightened antigen presentation, safer formulations, and more robust immunogenicity. Bioengineering techniques and materials have been used to synthesize several potent vaccines, approved or in trials, against coronavirus disease 2019 (COVID-19) and are being explored for influenza, SARS, and Middle East respiratory syndrome (MERS) vaccines as well. Here, we review bioengineering strategies such as the use of polymeric particles, liposomes, and virus-like particles in vaccine development against influenza and coronaviruses and the feasibility of adopting these technologies for clinical use.

STING and TLR7/8 agonists-based nanovaccines for synergistic antitumor immune activation

Immunostimulatory therapies based on pattern recognition receptors (PRRs) have emerged as an effective approach in the fight against cancer, with the ability to recruit tumor-specific lymphocytes in a low-immunogenicity tumor environment. The agonist cyclic dinucleotides (CDNs) of the stimulator of interferon gene (STING) are a group of very promising anticancer molecules that increase tumor immunogenicity by activating innate immunity. However, the tumor immune efficacy of CDNs is limited by several factors, including relatively narrow cytokine production, inefficient delivery to STING, and rapid clearance. In addition, a single adjuvant molecule is unable to elicit a broad cytokine response and thus cannot further amplify the anticancer effect. To address this problem, two or more agonist molecules are often used together to synergistically enhance immune efficacy. In this work, we found that a combination of the STING agonist CDG^SF and the Toll-like receptor 7/8 (TLR7/8) agonist 522 produced a broader cytokine response. Subsequently, we developed multicomponent nanovaccines (MCNVs) consisting of a PC7A polymer as a nanocarrier encapsulating the antigen OVA and adjuvant molecules. These MCNVs activate bone marrow-derived dendritic cells (BMDCs) to produce multiple proinflammatory factors that promote antigen cross-presentation to stimulate specific antitumor T-cell responses. In in vivo experiments, we observed that MCNVs triggered a strong T-cell response in tumor-infiltrating lymphocytes, resulting in significant tumor regression and, notably, a 100% survival rate in mice through 25 days without other partnering therapies. These data suggest that our nanovaccines have great potential to advance cancer immunotherapy with increased durability and potency.

ELECTRONIC SUPPLEMENTARY MATERIAL

Supplementary material (synthesis of CDG^SF, 522, PC7A and OVA; preparation of MCNVs; representative gating strategies for flow cytometry) is available in the online version of this article at 10.1007/s12274-022-4282-x.

The Regulatory Network of Cyclic GMP-AMP Synthase-Stimulator of Interferon Genes Pathway in Viral Evasion

Virus infection has been consistently threatening public health. The cyclic GMP-AMP synthase (cGAS)-Stimulator of Interferon Genes (STING) pathway is a critical defender to sense various pathogens and trigger innate immunity of mammalian cells. cGAS recognizes the pathogenic DNA in the cytosol and then synthesizes 2'3'-cyclic GMP-AMP (2'3'cGAMP). As the second messenger, cGAMP activates STING and induces the following cascade to produce type I interferon (IFN-I) to protect against infections. However, viruses have evolved numerous strategies to hinder the cGAS-STING signal transduction, promoting their immune evasion. Here we outline the current status of the viral evasion mechanism underlying the regulation of the cGAS-STING pathway, focusing on how post-transcriptional modifications, viral proteins, and non-coding RNAs involve innate immunity during viral infection, attempting to inspire new targets discovery and uncover potential clinical antiviral treatments.

Therapeutic Interventions Targeting Innate Immune Receptors: A Balancing Act

The innate immune system is an organism's first line of defense against an onslaught of internal and external threats. The downstream adaptive immune system has been a popular target for therapeutic intervention, while there is a relative paucity of therapeutics targeting the innate immune system. However, the innate immune system plays a critical role in many human diseases, such as microbial infection, cancer, and autoimmunity, highlighting the need for ongoing therapeutic research. In this review, we discuss the major innate immune pathways and detail the molecular strategies underpinning successful therapeutics targeting each pathway as well as previous and ongoing efforts. We will also discuss any recent discoveries that could inform the development of novel therapeutic strategies. As our understanding of the innate immune system continues to develop, we envision that therapies harnessing the power of the innate immune system will become the mainstay of treatment for a wide variety of human diseases.

Delivery of STING agonists for adjuvanting subunit vaccines

Adjuvant is an essential component in subunit vaccines. Many agonists of pathogen recognition receptors have been developed as potent adjuvants to optimize the immunogenicity and efficacy of vaccines. Recently discovered cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway has attracted much attention as it is a key mediator for modulating immune responses. Vaccines adjuvanted with STING agonists are found to mediate a robust immune defense against infections and cancer. In this review, we first discuss the mechanisms of STING agonists in the context of vaccination. Next, we present recent progress in novel STING agonist discovery and the delivery strategies. We next highlight recent work in optimizing the efficacy while minimizing toxicity of STING agonist-assisted subunit vaccines for protection against infectious diseases or treatment of cancer. Finally, we share our perspectives of current issues and future directions in further developing STING agonists for adjuvanting subunit vaccines.

The crosstalk between viral RNA- and DNA-sensing mechanisms

Viral infections pose a severe threat to humans by causing many infectious, even fatal, diseases, such as the current pandemic disease (COVID-19) since 2019, and understanding how the host innate immune system recognizes viruses has become more important. Endosomal and cytosolic sensors can detect viral nucleic acids to induce type I interferon and proinflammatory cytokines, subsequently inducing interferon-stimulated genes for restricting viral infection. Although viral RNA and DNA sensing generally rely on diverse receptors and adaptors, the crosstalk between DNA and RNA sensing is gradually appreciated. This minireview highlights the overlap between the RNA- and DNA-sensing mechanisms in antiviral innate immunity, which significantly amplifies the antiviral innate responses to restrict viral infection and might be a potential novel target for preventing and treating viral diseases.

Nucleic acid nanoparticles (NANPs) as molecular tools to direct desirable and avoid undesirable immunological effects

Nucleic acid nanoparticles (NANPs) represent a highly versatile molecular platform for the targeted delivery of various therapeutics. However, despite their promise, further clinical translation of this innovative technology can be hindered by immunological off-target effects. All human cells are equipped with an arsenal of receptors that recognize molecular patterns specific to foreign nucleic acids and understanding the rules that guide this recognition offer the key rationale for the development of therapeutic NANPs with tunable immune stimulation. Numerous recent studies have provided increasing evidence that in addition to NANPs' physicochemical properties and therapeutic effects, their interactions with cells of the immune system can be regulated through multiple independently programmable architectural parameters. The results further suggest that defined immunomodulation by NANPs can either support their immunoquiescent delivery or be used for conditional stimulation of beneficial immunological responses.