TLR9 and STING agonists cooperatively boost the immune response to SARS-CoV-2 RBD vaccine through an increased germinal center B cell response and reshaped T helper responses

Vaccines are a powerful medical intervention for preventing epidemic diseases. Efficient inactivated or protein vaccines typically rely on an effective adjuvant to elicit an immune response and boost vaccine activity. In this study, we investigated the adjuvant activities of combinations of Toll-like receptor 9 (TLR9) and stimulator of interferon genes (STING) agonists in a SARS-CoV-2 receptor binding domain protein vaccine. Adjuvants formulated with a TLR9 agonist, CpG-2722, with various cyclic dinucleotides (CDNs) that are STING agonists increased germinal center B cell response and elicited humoral immune responses in immunized mice. An adjuvant containing CpG-2722 and 2'3'-c-di-AM(PS)2 effectively boosted the immune response to both intramuscularly and intranasally administrated vaccines. Vaccines adjuvanted with CpG-2722 or 2'3'-c-di-AM(PS)2 alone were capable of inducing an immune response, but a cooperative adjuvant effect was observed when both were combined. CpG-2722 induced antigen-dependent T helper (Th)1 and Th17 responses, while 2'3'-c-di-AM(PS)2 induced a Th2 response. The combination of CpG-2722 and 2'3'-c-di-AM(PS)2 generated a distinct antigen-dependent Th response profile characterized by higher Th1 and Th17, but lower Th2 responses. In dendritic cells, CpG-2722 and 2'3'-c-di-AM(PS)2 showed a cooperative effect on inducing expression of molecules critical for T cell activation. CpG-2722 and 2'3'-c-di-AM(PS)2 have distinct cytokine inducing profiles in different cell populations. The combination of these two agonists enhanced the expression of cytokines for Th1 and Th17 responses and suppressed the expression of cytokines for Th2 response in these cells. Thus, the antigen-dependent Th responses observed in the animals immunized with different vaccines were shaped by the antigen-independent cytokine-inducing profiles of their adjuvant. The expanded targeting cell populations, the increased germinal center B cell response, and reshaped T helper responses are the molecular bases for the cooperative adjuvant effect of the combination of TLR9 and STING agonists.

Global proteomics of fibroblast cells treated with bacterial cyclic dinucleotides, c-di-GMP and c-di-AMP

BACKGROUND

Constant exposure of human gingival fibroblasts (HGFs) to oral pathogens trigger selective immune responses. Recently, the activation of immune response to cyclic dinucleotides (CDNs) via STING has come to the forefront. Reports show that other proteins outside the STING-TBK1-IRF3 axis respond to CDNs but a global view of impacted proteome in diverse cells is lacking. HGFs are constantly exposed to bacterial-derived cyclic-di-adenosine monophosphate (c-di-AMP) and cyclic-di-guanosine monophosphate (c-di-GMP).

AIM

To understand the response of HGFs to bacterial-derived CDNs, we carried out a global proteomics analysis of HGFs treated with c-di-AMP or c-di-GMP.

METHODS

The expression levels of several proteins modulated by CDNs were examined.

RESULTS

Interferon signaling proteins such as Ubiquitin-like protein ISG15 (ISG15), Interferon-induced GTP-binding protein Mx1 (MX1), Interferon-induced protein with tetratricopeptide repeats (IFIT) 1 (IFIT1), and (IFIT3) were significantly upregulated. Interestingly, other pathways not fully characterized to be regulated by CDNs, such as necroptosis signaling, iron homeostasis signaling, protein ubiquitination, EIF2 signaling, sumoylation and nucleotide excision repair pathways were also modulated by the bacterial-derived CDNs.

CONCLUSION

This study has added to the increasing appreciation that beyond the regulation of cytokine production via STING, cyclic dinucleotides also broadly affect many critical processes in human cells.

Single-dose combination nanovaccine induces both rapid and durable humoral immunity and toxin neutralizing antibody responses against Bacillus anthracis

Bacillus anthracis, the causative agent of anthrax, continues to be a prominent biological warfare and bioterrorism threat. Vaccination is likely to remain the most effective and user-friendly public health measure to counter this threat in the foreseeable future. The commercially available AVA BioThrax vaccine has a number of shortcomings where improvement would lead to a more practical and effective vaccine for use in the case of an exposure event. Identification of more effective adjuvants and novel delivery platforms is necessary to improve not only the effectiveness of the anthrax vaccine, but also enhance its shelf stability and ease-of-use. Polyanhydride particles have proven to be an effective platform at adjuvanting the vaccine-associated adaptive immune response as well as enhancing stability of encapsulated antigens. Another class of adjuvants, the STING pathway-targeting cyclic dinucleotides, have proven to be uniquely effective at inducing a beneficial inflammatory response that leads to the rapid induction of high titer antibodies post-vaccination capable of providing protection against bacterial pathogens. In this work, we evaluate the individual contributions of cyclic di-GMP (CDG), polyanhydride nanoparticles, and a combination thereof towards inducing neutralizing antibody (nAb) against the secreted protective antigen (PA) from B. anthracis. Our results show that the combination nanovaccine elicited rapid, high titer, and neutralizing IgG anti-PA antibody following single dose immunization that persisted for at least 108 DPI.

Live Cell Imaging Using Riboswitch-Spinach tRNA Fusions as Metabolite-Sensing Fluorescent Biosensors

The development of fluorescent biosensors is motivated by the desire to monitor cellular metabolite levels in real time. Most genetically encodable fluorescent biosensors are based on receptor proteins fused to fluorescent protein domains. More recently, small molecule-binding riboswitches have been adapted for use as fluorescent biosensors through fusion to the in vitro selected Spinach aptamer, which binds a profluorescent, cell-permeable small molecule mimic of the GFP chromophore, DFHBI. Here we describe methods to prepare and analyze riboswitch-Spinach tRNA fusions for ligand-dependent activation of fluorescence in vivo. Example procedures describe the use of the Vc2-Spinach tRNA biosensor to monitor perturbations in cellular levels of cyclic di-GMP using either fluorescence microscopy or flow cytometry. In this updated chapter, we have added procedures on using biosensors in flow cytometry to detect exogenously added compounds. The relative ease of cloning and imaging of these biosensors, as well as their modular nature, should make this method appealing to other researchers interested in utilizing riboswitch-based biosensors for metabolite sensing.

Amine skeleton-based c-di-GMP derivatives as biofilm formation inhibitors

Bacteria can form a biofilm composed of diverse bacterial microorganism, which work as a barrier to protect from threats, such as antibiotics and host immunity system. The formation of biofilms significantly impairs the efficacy of antibiotics against pathogenic bacteria. It is also a serious problem to be solved that the emergence of multidrug-resistant bacteria (such as methicillin-resistant Staphylococcus aureus, MRSA) accelerated by the overuse of antibiotics. Therefore, the usage of biofilm inhibition agents has attracted immense interest as a novel strategy for treatment of diseases related to bacterial infection. From the difference of mode of action against bacterial cells, biofilm inhibition agents are expected to circumvent the emergence of multidrug-resistant bacteria. In this study, we have developed the derivatives of c-di-GMP, a kind of cyclic dinucleotide that is expected to have the effect of inhibiting bacterial biofilm formation. Some of the synthesized derivatives were found to inhibit biofilm formation of Gram-positive bacteria.

cGAS-STING pathway in cancer biotherapy

The activation of the cGAS-STING pathway has tremendous potential to improve anti-tumor immunity by generating type I interferons. In recent decades, we have witnessed that producing dsDNA upon various stimuli is an initiative factor, triggering the cGAS-SING pathway for a defensive host. The understanding of both intracellular cascade reaction and the changes of molecular components gains insight into type I IFNs and adaptive immunity. Based on the immunological study, the STING-cGAS pathway is coupled to cancer biotherapy. The most challenging problem is the limited therapeutic effect. Therefore, people view 5, 6-dimethylxanthenone-4-acetic acid, cyclic dinucleotides and various derivative as cGAS-STING pathway agonists. Even so, these agonists have flaws in decreasing biotherapeutic efficacy. Subsequently, we exploited agonist delivery systems (nanocarriers, microparticles and hydrogels). The article will discuss the activation of the cGAS-STING pathway and underlying mechanisms, with an introduction of cGAS-STING agonists, related clinical trials and agonist delivery systems.

Single-dose combination nanovaccine induces both rapid and long-lived protection against pneumonic plague

Yersinia pestis, the causative agent of pneumonic plague, induces a highly lethal infection if left untreated. Currently, there is no FDA-approved vaccine against this pathogen; however, USAMRIID has developed a recombinant fusion protein, F1-V, that has been shown to induce protection against pneumonic plague. Many F1-V-based vaccine formulations require prime-boost immunization to achieve protective immunity, and there are limited reports of rapid induction of protective immunity (≤ 14 days post-immunization (DPI)). The STimulator of INterferon Genes agonists cyclic dinucleotides (CDNs) have been shown to be promising vaccine adjuvants. Polyanhydride nanoparticle-based vaccines (i.e., nanovaccines) have also shown to enhance immune responses due to their dual functionality as adjuvants and delivery vehicles. In this work, a combination nanovaccine was designed that comprised F1-V-loaded nanoparticles combined with the CDN, dithio-RP,RP-cyclic di-guanosine monophosphate, to induce rapid and long-lived protective immunity against pneumonic plague. All mice immunized with a single dose combination nanovaccine were protected from Y. pestis lethal challenge within 14 DPI and demonstrated enhanced protection over F1-V adjuvanted with CDNs alone at challenge doses ≥7000 CFU Y. pestis CO92. In addition, 75% of mice receiving the single dose of the combination nanovaccine were protected from challenge at 182 DPI, while maintaining high levels of antigen-specific serum IgG. ELISPOT analysis of vaccinated animals at 218 DPI revealed F1-V-specific long-lived plasma cells in bone marrow in mice vaccinated with CDN adjuvanted F1-V or the combination nanovaccine. Microarray analysis of serum from these vaccinated mice revealed the presence of serum antibody that bound to a broad range of F1 and V linear epitopes. These results demonstrate that combining the adjuvanticity of CDNs with a nanovaccine delivery system enables induction of both rapid and long-lived protective immunity against Y. pestis. STATEMENT OF SIGNIFICANCE: • Yersinia pestis, the causative agent of pneumonic plague, induces a highly lethal infection if left untreated. Currently, there is no FDA-approved vaccine against this biodefense pathogen. • We designed a combination nanovaccine comprising of F1-V antigen-loaded polyanhydride nanoparticles and a cyclic dinucleotide adjuvant to induce both rapid and long-lived protective immunity against pneumonic plague. • Animals immunized with the combination nanovaccine maintained high levels of antigen-specific serum IgG and long-lived plasma cells in bone marrow and the serum antibody showed a high affinity for a broad range of F1 and V linear epitopes. • The combination nanovaccine is a promising next-generation vaccine platform against weaponized Y. pestis based on its ability to induce both rapid and long-lived protective immunity.

Discovery of IACS-8803 and IACS-8779, potent agonists of stimulator of interferon genes (STING) with robust systemic antitumor efficacy

Activation of the stimulator of interferon genes (STING) pathway by both exogenous and endogenous cytosolic DNA results in the production of interferon beta (IFN-β) and is required for the generation of cytotoxic T-cell priming against tumor antigens. In the clinical setting, pharmacological stimulation of the STING pathway has the potential to synergize with immunotherapy antibodies by boosting anti-tumor immune responses. We report the discovery of two highly potent cyclic dinucleotide STING agonists, IACS-8803 and IACS-8779, which show robust activation of the STING pathway in vitro and a superior systemic anti-tumor response in the B16 murine model of melanoma when compared to one of the clinical benchmark compounds.

STING pathway stimulation results in a differentially activated innate immune phenotype associated with low nitric oxide and enhanced antibody titers in young and aged mice

BACKGROUND

One of the most concerning public health issues, related to vaccination and disease prevention, is the inability to induce durable immune responses following a single-dose immunization. In this regard, the nature of the inflammatory environment induced by vaccine adjuvants can negatively impact the resulting immune response. To address these concerns, new strategies to vaccine design are needed in order to improve the outcomes of immune responses, particularly in immunologically disadvantaged populations.

METHODS

Comparisons of the scope of innate immune activation induced by TLR agonists versus cyclic dinucleotides (CDNs) was performed. Their effects on the activation characteristics (e.g., metabolism, cytokine secretion) of bone marrow derived dendritic cells (BMDCs) were studied. In addition, the differential effects on in vivo induction of antibody responses were measured.

RESULTS

As compared to TLR ligands, the stimulation of BMDCs with CDNs induced distinctly different metabolic outcomes. Marked differences were observed in the production of nitric oxide (NO) and the cytokine BAFF. These distinct differences were correlated with improved (i.e., more rapid and persistent) vaccine antibody responses in both aged and young mice.

CONCLUSIONS

Our results illustrate that the innate immune pathway targeted by adjuvants can critically impact the outcome of the immune response post-vaccination. Specifically, CDN stimulation of APCs induced an activation phenotype that was characterized by decreased innate effector molecule production (e.g., NO) and increased BAFF. This was attributed to the induction of an innate inflammatory environment that enabled the host to make the most of the existing B lymphocyte potential. The use of adjuvants that differentially engage mechanisms of innate immune activation would be particularly advantageous for the generation of robust, single dose vaccines. The results of this study demonstrated that CDNs induced differential innate activation and enhanced vaccine induced antibody responses in both young and aged mice.

Discovery of the Second Messenger Cyclic di-GMP

The nearly ubiquitous bacterial second messenger cyclic di-GMP is involved in a multitude of fundamental physiological processes such as sessility/motility transition and the switch between the acute and chronic infection status, combined with cell cycle control. The discovery of cyclic di-GMP, though, has been an example par excellence of scientific serendipity. We recapitulate here its years-long discovery process as an activator of the cellulose synthase of the environmental bacterium Komagataeibacter xylinus and its consequences for follow-up research. Indeed, the discovery of cyclic di-GMP as a ubiquitous second messenger contributed to the change in perception of bacteria as simple unicellular organisms just randomly building-up multicellular communities. Subsequently, cyclic di-GMP also paved the way to the identification of other pro- and eukaryotic cyclic dinucleotide second messengers.

Regulating STING in health and disease

The presence of cytosolic double-stranded DNA molecules can trigger multiple innate immune signalling pathways which converge on the activation of an ER-resident innate immune adaptor named "STimulator of INterferon Genes (STING)". STING has been found to mediate type I interferon response downstream of cyclic dinucleotides and a number of DNA and RNA inducing signalling pathway. In addition to its physiological function, a rapidly increasing body of literature highlights the role for STING in human disease where variants of the STING proteins, as well as dysregulated STING signalling, have been implicated in a number of inflammatory diseases. This review will summarise the recent structural and functional findings of STING, and discuss how STING research has promoted the development of novel therapeutic approaches and experimental tools to improve treatment of tumour and autoimmune diseases.

Live Flow Cytometry Analysis of c-di-GMP Levels in Single Cell Populations

Second-generation RNA-based fluorescent biosensors have been developed that enable flow cytometry experiments to monitor the population dynamics of c-di-GMP signaling in live bacteria. These experiments are high-throughput, provide information at the single-cell level, and can be performed on cells grown in complex media and/or under anaerobic conditions. Here, we describe flow cytometry methods for three applications: (1) high-throughput screening for diguanylate cyclase activity, (2) analyzing c-di-GMP levels under anaerobic conditions, and (3) monitoring cell population dynamics of c-di-GMP levels upon environmental changes. These methods showcase RNA-based fluorescent biosensors as versatile tools for studying c-di-GMP signaling in bacteria.

In vitro analysis of riboswitch-Spinach aptamer fusions as metabolite-sensing fluorescent biosensors

The development of fluorescent biosensors has been motivated by the interest to monitor and measure the levels of specific metabolites in live cells in real time. Common approaches include fusing a protein-based receptor to fluorescent proteins or synthesizing a small molecule reactive probe. Natural metabolite-sensing riboswitches also have been used in reporter-based systems that take advantage of ligand-dependent regulation of downstream gene expression. More recently, it has been shown that RNA-based fluorescent biosensors can be generated by fusing a riboswitch aptamer to the in vitro selected Spinach aptamer, which binds a cell-permeable and conditionally fluorescent molecule. Here, we describe methods to design, prepare, and analyze riboswitch-Spinach aptamer fusion RNAs for ligand-dependent activation of fluorescence in vitro. Examples of procedures to measure fluorescence activation, ligand binding selectivity and affinity, and binding kinetics are given for a cyclic di-GMP-responsive biosensor. The relative ease of in vitro RNA synthesis and purification should make this method accessible to other researchers interested in developing riboswitch-based fluorescent biosensors.