c-di-GMP is an effective immunomodulator and vaccine adjuvant against pneumococcal infection

Advances in therapeutic cancer vaccines: Harnessing immune adjuvants for enhanced efficacy and future perspectives

Preventive cancer vaccines are highly effective in preventing viral infection-induced cancer, but advances in therapeutic cancer vaccines with a focus on eliminating cancer cells through immunotherapy are limited. To develop therapeutic cancer vaccines, the integration of optimal adjuvants is a potential strategy to enhance or complement existing therapeutic approaches. However, conventional adjuvants do not satisfy the criteria of clinical trials for therapeutic cancer vaccines. To improve the effects of adjuvants in therapeutic cancer vaccines, effective vaccination strategies must be formulated and novel adjuvants must be identified. This review offers an overview of the current advancements in therapeutic cancer vaccines and highlights in situ vaccination approaches that can be synergistically combined with other immunotherapies by harnessing the adjuvant effects. Additionally, the refinement of adjuvant systems using cutting-edge technologies and the elucidation of molecular mechanisms underlying immunogenic cell death to facilitate the development of innovative adjuvants have been discussed.

EvfG is a multi-function protein located in the Type VI secretion system for ExPEC

The Type VI secretion system (T6SS) functions as a protein transport nanoweapon in several stages of bacterial life. Even though bacterial competition is the primary function of T6SS, different bacteria exhibit significant variations. Particularly in Extraintestinal pathogenic Escherichia coli (ExPEC), research into T6SS remains relatively limited. This study identified the uncharacterized gene evfG within the T6SS cluster of ExPEC RS218. Through our experiments, we showed that evfG is involved in T6SS expression in ExPEC RS218. We also found evfG can modulate T6SS activity by competitively binding to c-di-GMP, leading to a reduction in the inhibitory effect. Furthermore, we found that evfG can recruit sodA to alleviate oxidative stress. The research shown evfG controls an array of traits, both directly and indirectly, through transcriptome and additional tests. These traits include cell adhesion, invasion, motility, drug resistance, and pathogenicity of microorganisms. Overall, we contend that evfG serves as a multi-functional regulator for the T6SS and several crucial activities. This forms the basis for the advancement of T6SS function research, as well as new opportunities for vaccine and medication development.

TLR agonists as vaccine adjuvants in the prevention of viral infections: an overview

Tol-like receptor (TLR) agonists, as potent adjuvants, have gained attention in vaccine research for their ability to enhance immune responses. This study focuses on their application in improving vaccine efficacy against key viral infections, including hepatitis B virus (HBV), hepatitis C virus (HCV), human immunodeficiency virus (HIV), SARS-CoV-2, influenza virus, and flaviviruses, including West Nile virus, dengue virus, and chikungunya virus. Vaccines are crucial in preventing microbial infections, including viruses, and adjuvants play a vital role in modulating immune responses. However, there are still many diseases for which effective vaccines are lacking or have limited immune response, posing significant threats to human health. The use of TLR agonists as adjuvants in viral vaccine formulations holds promise in improving vaccine effectiveness. By tailoring adjuvants to specific pathogens, such as HBV, HCV, HIV, SARS-CoV-2, influenza virus, and flavivirus, protective immunity against chronic and emerging infectious disease can be elicited.

Adjuvant physiochemistry and advanced nanotechnology for vaccine development

Vaccines comprising innovative adjuvants are rapidly reaching advanced translational stages, such as the authorized nanotechnology adjuvants in mRNA vaccines against COVID-19 worldwide, offering new strategies to effectively combat diseases threatening human health. Adjuvants are vital ingredients in vaccines, which can augment the degree, extensiveness, and longevity of antigen specific immune response. The advances in the modulation of physicochemical properties of nanoplatforms elevate the capability of adjuvants in initiating the innate immune system and adaptive immunity, offering immense potential for developing vaccines against hard-to-target infectious diseases and cancer. In this review, we provide an essential introduction of the basic principles of prophylactic and therapeutic vaccination, key roles of adjuvants in augmenting and shaping immunity to achieve desired outcomes and effectiveness, and the physiochemical properties and action mechanisms of clinically approved adjuvants for humans. We particularly focus on the preclinical and clinical progress of highly immunogenic emerging nanotechnology adjuvants formulated in vaccines for cancer treatment or infectious disease prevention. We deliberate on how the immune system can sense and respond to the physicochemical cues (e.g., chirality, deformability, solubility, topology, and chemical structures) of nanotechnology adjuvants incorporated in the vaccines. Finally, we propose possible strategies to accelerate the clinical implementation of nanotechnology adjuvanted vaccines, such as in-depth elucidation of nano-immuno interactions, antigen identification and optimization by the deployment of high-dimensional multiomics analysis approaches, encouraging close collaborations among scientists from different scientific disciplines and aggressive exploration of novel nanotechnologies.

Development of a Universal Multi-Epitope Vaccine Candidate against Streptococcus suis Infections Using Immunoinformatics Approaches

Streptococcus suis is a significant zoonotic pathogen that is a great threat not only to the swine industry but also to human health, causing arthritis, meningitis, and even streptococcal toxic shock-like syndrome. Owing to its many serotypes and high geographic variability, an efficacious cross-protective S. suis vaccine is not readily available. Therefore, this study aimed to design a universal multi-epitope vaccine (MVHP6) that involved three highly immunogenic proteins of S. suis, namely, the surface antigen containing a glycosaminoglycan binding domain (HP0197), endopeptidase (PepO), and 6-phosphogluconate dehydrogenase (6PGD). Forecasted T-cell and B-cell epitopes with high antigenic properties and a suitable adjuvant were linked to construct a multi-epitope vaccine. In silico analysis showed that the selected epitopes were conserved in highly susceptible serotypes for humans. Thereafter, we evaluated the different parameters of MVHP6 and showed that MVHP6 was highly antigenic, non-toxic, and non-allergenic. To verify whether the vaccine could display appropriate epitopes and maintain high stability, the MVHP6 tertiary structure was modeled, refined, and validated. Molecular docking studies revealed a strong binding interaction between the vaccine and the toll-like receptor (TLR4), whereas molecular dynamics simulations demonstrated the vaccine's compatibility, binding stability, and structural compactness. Moreover, the in silico analysis showed that MVHP6 could evoke strong immune responses and enable worldwide population coverage. Moreover, MVHP6 was cloned into the pET28a (+) vector in silico to ensure the credibility, validation, and proper expression of the vaccine construct. The findings suggested that the proposed multi-epitope vaccine can provide cross-protection against S. suis infections.

Activation of Stimulation of Interferon Genes (STING) Signal and Cancer Immunotherapy

Stimulator of interferon gene (STING), an intracellular receptor in the endoplasmic reticulum, could induce the production of cytokines such as type I interferon (IFN) by activating the cGAS-STING signal pathway. In recent years, activation of STING has shown great potential to enhance anti-tumor immunity and reshape the tumor microenvironment, which is expected to be used in tumor immunotherapy. A number of STING agonists have demonstrated promising biological activity and showed excellent synergistic anti-tumor effects in combination with other cancer therapies in preclinical studies and some clinical trials. The combination of STING agonists and ICI also showed a potent effect in improving anti-tumor immunity. In this review, we introduce the cGAS-STING signaling pathway and its effect in tumor immunity and discuss the recent strategies of activation of the STING signaling pathway and its research progress in tumor immunotherapy.

Cyclic-di-GMP stimulates keratinocyte innate immune responses and attenuates methicillin-resistant Staphylococcus aureus colonization in a murine skin wound infection model

Background

Staphylococcus aureus is a leading cause for morbidity and mortality associated with skin and burn wound infections. Therapeutic options for methicillin-resistant S. aureus (MRSA) have dwindled and therefore alternative treatments are urgently needed. In this study, the immuno-stimulating and anti-MRSA effects of cyclic di-guanosine monophosphate (c-di-GMP), a uniquely bacterial second messenger and immuno-modulator, were investigated in HaCaT human epidermal keratinocytes and a murine skin wound infection model.

Results

Stimulation of HaCaT cells with 125 μM c-di-GMP for 12 h prior to MRSA challenge resulted in a 20-fold reduction in bacterial colonization compared with untreated control cells, which was not the result of a direct c-di-GMP toxic effect, since bacterial viability was not affected by this dose in the absence of HaCaT cells. C-di-GMP-stimulated or MRSA-challenged HaCaT cells displayed enhanced secretion of the antimicrobial peptides human β-defensin 1 (hBD-1), hBD-2, hBD-3 and LL-37, but for hBD1 and LL-37 the responses were additive in a c-di-GMP-dose-dependent manner. Secretion of the chemokines CXCL1 and CXCL8 was also elevated after stimulation of HaCaT cells with lower c-di-GMP doses and peaked at a dose of 5 μM. Finally, pre-treatment of mice with a 200 nmol dose of c-di-GMP 24 h before a challenge with MRSA in skin wound infection model resulted in a major reduction (up to 1,100-fold by day 2) in bacterial CFU counts recovered from challenged skin tissue sections compared PBS-treated control animals. Tissue sections displayed inflammatory cell infiltration and enhanced neutrophil influx in the c-di-GMP pre-treated animals, which might account for the reduced ability of MRSA to colonize c-di-GMP pre-treated mice.

Conclusions

These results demonstrate that c-di-GMP is a potent immuno-modulator that can stimulate anti-MRSA immune responses in vivo and might therefore be a suitable alternative prophylactic or therapeutic agent for MRSA skin or burn wound infections.

Enhanced Immune Responses in Mice Induced by the c-di-GMP Adjuvanted Inactivated Vaccine for Pseudorabies Virus

Cyclic dimeric guanosine monophosphate (c-di-GMP) is a bacterial second messenger with immunomodulatory activities in mice, suggesting potential applications as a vaccine immunopotentiator or therapeutic agent. In this study, we evaluated the efficacy of c-di-GMP as an immunopotentiator for pseudorabies virus (PRV) inactivated vaccine in a murine model. We found that c-di-GMP improved the humoral and cellular immune responses induced by PRV inactivated vaccine and its effects on immunity reached the level comparable to that of a live attenuated vaccine. Furthermore, c-di-GMP enhanced the murine antibody response against the viral glycoprotein gB up to 120 days after immunization. The c-di-GMP–adjuvanted PRV inactivated vaccine induced long-term humoral immunity by promoting a potent T follicular helper cell response, which is known to directly control the magnitude of the germinal center B cell response. Furthermore, the c-di-GMP enhanced the response of bone marrow plasma cells and upregulated the expression of Bcl-2 and Mcl-1, which have been identified as anti-apoptotic regulatory genes of germinal center and memory B cells. Our findings open a new avenue for improving the immune efficacy of PRV inactivated vaccines.

An attenuated multiple genetic mutant of Mycoplasma pneumoniae imparts good immuno-protection against M. pneumoniae pneumonia in BALB/c mice

Mycoplasma pneumoniae (M. pneumoniae) is the causative agent of both upper and lower respiratory infections that can lead to pneumonia, extrapulmonary complications and devastating sequela. With the increasing rate of macrolide-resistant strains, the severe clinical consequence of refractory mycoplasma pneumonia in children health calls for the need of vaccine research for this pathogen. In this report, the immunomodulatory effectiveness of a live attenuated M. pneumoniae vaccine was evaluated. The vaccine strain was a mutant strain of M. pneumoniae, MUT129, obtained after multiple passages of M129 strain in PPLO broth. The SNP/InDel detection results showed that mutations were present in genes encoding the adhesion organelle-associated proteins and lipoproteins of M. pneumoniae MUT129. Upon intranasal challenge of BALB/c mice with 1 × 10⁷ CFU of MUT129, there were very small amount of Mycoplasma antigens and almost no M. pneumoniae present in the lung tissues of BALB/c mice. Besides, there was almost no inflammatory cell infiltration in the lung tissue. Results of the M. pneumoniae challenge study showed that mice immunized with MUT129 presented with less inflammation, lower detectable number of M. pneumoniae in the lungs when compared with the unimmunized mice. These results indicated that the live attenuated vaccine can efficiently prevent the proliferation of M. pneumonia in the lungs, reduce but not completely prevent the pulmonary inflammatory response.

2',3'-Cyclic GMP-AMP Dinucleotides for STING-Mediated Immune Modulation: Principles, Immunotherapeutic Potential, and Synthesis

The cGAS-STING pathway discovered ten years ago is an important component of the innate immune system. Activation of cGAS-STING triggers downstream signalling, such as TBK1-IRF3, NF-κB and autophagy, which in turn leads to antipathogen responses, durable antitumour immunity or autoimmune diseases. 2',3'-Cyclic GMP-AMP dinucleotides (2',3'-cGAMP), the key second messengers produced by cGAS, play a pivotal role in cGAS-STING signalling by binding and activating STING. Thus, 2',3'-cGAMP has immunotherapeutic potential, which in turn has stimulated research on the design and synthesis of 2',3'-cGAMP analogues for clinical applications over the past ten years. This review presents the discovery, metabolism, and function of 2',3'-cGAMP in the cGAS-STING innate immune signalling axis. The enzymatic and chemical syntheses of 2',3'-cGAMP analogues as STING-targeting therapeutics are also summarized.

Yin and Yang of Biofilm Formation and Cyclic di-GMP Signaling of the Gastrointestinal Pathogen Salmonella enterica Serovar Typhimurium

Within the last 60 years, microbiological research has challenged many dogmas such as bacteria being unicellular microorganisms directed by nutrient sources; these investigations produced new dogmas such as cyclic diguanylate monophosphate (cyclic di-GMP) second messenger signaling as a ubiquitous regulator of the fundamental sessility/motility lifestyle switch on the single-cell level. Successive investigations have not yet challenged this view; however, the complexity of cyclic di-GMP as an intracellular bacterial signal, and, less explored, as an extracellular signaling molecule in combination with the conformational flexibility of the molecule, provides endless opportunities for cross-kingdom interactions. Cyclic di-GMP-directed microbial biofilms commonly stimulate the immune system on a lower level, whereas host-sensed cyclic di-GMP broadly stimulates the innate and adaptive immune responses. Furthermore, while the intracellular second messenger cyclic di-GMP signaling promotes bacterial biofilm formation and chronic infections, oppositely, Salmonella Typhimurium cellulose biofilm inside immune cells is not endorsed. These observations only touch on the complexity of the interaction of biofilm microbial cells with its host. In this review, we describe the Yin and Yang interactive concepts of biofilm formation and cyclic di-GMP signaling using S. Typhimurium as an example.

The Promise and Challenges of Cyclic Dinucleotides as Molecular Adjuvants for Vaccine Development

Cyclic dinucleotides (CDNs), originally discovered as bacterial second messengers, play critical roles in bacterial signal transduction, cellular processes, biofilm formation, and virulence. The finding that CDNs can trigger the innate immune response in eukaryotic cells through the stimulator of interferon genes (STING) signalling pathway has prompted the extensive research and development of CDNs as potential immunostimulators and novel molecular adjuvants for induction of systemic and mucosal innate and adaptive immune responses. In this review, we summarize the chemical structure, biosynthesis regulation, and the role of CDNs in enhancing the crosstalk between host innate and adaptive immune responses. We also discuss the strategies to improve the efficient delivery of CDNs and the recent advance and future challenges in the development of CDNs as potential adjuvants in prophylactic vaccines against infectious diseases and in therapeutic vaccines against cancers.

C-di-GMP with influenza vaccine showed enhanced and shifted immune responses in microneedle vaccination in the skin

A microneedle is a biomedical device which consists of multiple micron scale needles. It is widely used in various fields to deliver drugs and vaccines to the skin effectively. However, when considering improved vaccine efficacy in microneedle vaccination, it is important to find an appropriate adjuvant that is able to be used in transdermal delivery. Herein, we demonstrated the applicability of c-di-GMP, which is a stimulator of interferon genes (STING) agonist, as an adjuvant for influenza microneedle vaccination. Thus, 2 and 10 μg of GMP with the influenza vaccine were coated onto a microneedle, and then, BALB/c mice were immunized with the coated microneedle to investigate the immunogenicity and protection efficacy of the influenza microneedle vaccination. As a result, the adjuvant groups had an enhanced IgG response, IgG subtypes and HI titer compared to the vaccine only group. In addition to the humoral immunity, the use of an adjuvant has also been shown to improve the cellular immune response. In a challenge study, adjuvant groups had a 100% survival rate and rapid weight recovery. Taken together, this study confirms that GMP is an effective adjuvant for influenza microneedle vaccination. Graphical abstract.

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.

AdrA as a Potential Immunomodulatory Candidate for STING-Mediated Antiviral Therapy That Required Both Type I IFN and TNF-α Production

Several dinucleotide cyclases, including cyclic GMP-AMP synthase, and their involvement in STING-mediated immunity have been extensively studied. In this study, we tested five bacterial diguanylate cyclases from the Gram-negative bacterium Salmonella Enteritidis, identifying AdrA as the most potent inducer of a STING-mediated IFN response. AdrA wild-type (wt) or its inactive version AdrA mutant (mut) were delivered by an adenovirus (Ad) vector. Dendritic cells obtained from wt mice and infected in vitro with Ad vector containing AdrA wt, but not mut, had increased activation markers and produced large amounts of several immunostimulatory cytokines. For dendritic cells derived from STING-deficient mice, no activation was detected. The potential antiviral activity of AdrA was addressed in hepatitis B virus (HBV)-transgenic and adenovirus-associated virus (AAV)-HBV mouse models. Viremia in serum of Ad AdrA wt-treated mice was reduced significantly compared with that in Ad AdrA mut-injected mice. The viral load in the liver at sacrifice was in line with this finding. To further elucidate the molecular mechanism(s) by which AdrA confers its antiviral function, the response in mice deficient in STING or its downstream effector molecules was analyzed. wt and IFN-αR (IFNAR)^-/- animals were additionally treated with anti-TNF-α (Enbrel). Interestingly, albeit less pronounced than in wt mice, in IFNAR^-/- and Enbrel-treated wt mice, a reduction of serum viremia was achieved-an observation that was lost in anti-TNF-α-treated IFNAR^-/- animals. No effect of AdrA wt was seen in STING-deficient animals. Thus, although STING is indispensable for the antiviral activity of AdrA, type I IFN and TNF-α are both required and act synergistically.

The Age of Cyclic Dinucleotide Vaccine Adjuvants

As prophylactic vaccine adjuvants for infectious diseases, cyclic dinucleotides (CDNs) induce safe, potent, long-lasting humoral and cellular memory responses in the systemic and mucosal compartments. As therapeutic cancer vaccine adjuvants, CDNs induce potent anti-tumor immunity, including cytotoxic T cells and NK cells activation that achieve durable regression in multiple mouse models of tumors. Clinical trials are ongoing to fulfill the promise of CDNs (ClinicalTrials.gov: NCT02675439, NCT03010176, NCT03172936, and NCT03937141). However, in October 2018, the first clinical data with Merck's CDN MK-1454 showed zero activity as a monotherapy in patients with solid tumors or lymphomas (NCT03010176). Lately, the clinical trial from Aduro's CDN ADU-S100 monotherapy was also disappointing (NCT03172936). The emerging hurdle in CDN vaccine development calls for a timely re-evaluation of our understanding on CDN vaccine adjuvants. Here, we review the status of CDN vaccine adjuvant research, including their superior adjuvant activities, in vivo mode of action, and confounding factors that affect their efficacy in humans. Lastly, we discuss the strategies to overcome the hurdle and advance promising CDN adjuvants in humans.

New MoDC-Targeting TNF Fusion Proteins Enhance Cyclic Di-GMP Vaccine Adjuvanticity in Middle-Aged and Aged Mice

Cyclic dinucleotides (CDNs) are promising vaccine adjuvants inducing balanced, potent humoral, and cellular immune responses. How aging influences CDN efficacy is unclear. We examined the vaccine efficacy of 3',5'-cyclic diguanylic acid (cyclic di-GMP, CDG), the founding member of CDNs, in 1-year-old (middle-aged) and 2-year-old (aged) C57BL/6J mice. We found that 1- and 2-year-old C57BL/6J mice are defective in CDG-induced memory T helper (Th)1 and Th17 responses and high-affinity serum immunoglobulin (Ig)G, mucosal IgA production. Next, we generated two novel tumor necrosis factor (TNF) fusion proteins that target soluble TNF (solTNF) and transmembrane TNF (tmTNF) to monocyte-derived dendritic cells (moDCs) to enhance CDG vaccine efficacy in 1- and 2-year-old mice. The moDC-targeting TNF fusion proteins restored CDG-induced memory Th1, Th17, and high-affinity IgG, IgA responses in the 1- and 2-year-old mice. Together, the data suggested that aging negatively impacts CDG vaccine adjuvanticity. MoDC-targeting TNF fusion proteins enhanced CDG adjuvanticity in the aging mice.

The World of Cyclic Dinucleotides in Bacterial Behavior

The regulation of multiple bacterial phenotypes was found to depend on different cyclic dinucleotides (CDNs) that constitute intracellular signaling second messenger systems. Most notably, c-di-GMP, along with proteins related to its synthesis, sensing, and degradation, was identified as playing a central role in the switching from biofilm to planktonic modes of growth. Recently, this research topic has been under expansion, with the discoveries of new CDNs, novel classes of CDN receptors, and the numerous functions regulated by these molecules. In this review, we comprehensively describe the three main bacterial enzymes involved in the synthesis of c-di-GMP, c-di-AMP, and cGAMP focusing on description of their three-dimensional structures and their structural similarities with other protein families, as well as the essential residues for catalysis. The diversity of CDN receptors is described in detail along with the residues important for the interaction with the ligand. Interestingly, genomic data strongly suggest that there is a tendency for bacterial cells to use both c-di-AMP and c-di-GMP signaling networks simultaneously, raising the question of whether there is crosstalk between different signaling systems. In summary, the large amount of sequence and structural data available allows a broad view of the complexity and the importance of these CDNs in the regulation of different bacterial behaviors. Nevertheless, how cells coordinate the different CDN signaling networks to ensure adaptation to changing environmental conditions is still open for much further exploration.

Multiple Roles of c-di-GMP Signaling in Bacterial Pathogenesis

The intracellular signaling molecule cyclic di-GMP (c-di-GMP) regulates the lifestyle of bacteria and controls many key functions and mechanisms. In the case of bacterial pathogens, a wide variety of virulence lifestyle factors have been shown to be regulated by c-di-GMP. Evidence of the importance of this molecule for bacterial pathogenesis has become so great that new antimicrobial agents are tested for their capacity of targeting c-di-GMP signaling. This review summarizes the current knowledge on this topic and reveals its application for the development of new antivirulence intervention strategies.

STING activation in cancer immunotherapy

Cancer immunotherapy modulates and leverages the host immune system to treat cancer. The past decade has witnessed historical advancement of cancer immunotherapy. A myriad of approaches have been explored to elicit or augment anticancer innate immunity and/or adaptive immunity. Recently, activation of stimulator of interferon (IFN) genes (STING), an intracellular receptor residing in the endoplasmic reticulum, has shown great potential to enhance antitumor immunity through the induction of a variety of pro-inflammatory cytokines and chemokines, including type I IFNs. A number of natural and synthetic STING agonists have been discovered or developed, and tested in preclinical models and in the clinic for the immunotherapy of diseases such as cancer and infectious diseases. Cyclic dinucleotides (CDNs), such as cyclic dimeric guanosine monophosphate (c-di-GMP), cyclic dimeric adenosine monophosphate (c-di-AMP), and cyclic GMP-AMP (cGAMP), are a class of STING agonists that can elicit immune responses. However, natural CDNs are hydrophilic small molecules with negative charges and are susceptible to enzymatic degradation, leading to low bioavailability in target tissues yet unwanted toxicities and narrow therapeutic windows. Drug delivery systems, coupled with nucleic acid chemistry, have been exploited to address these challenges. Here, we will discuss the underlying immunological mechanisms and approaches to STING activation, with a focus on the delivery of STING agonists, for cancer immunotherapy.

STING-Activating Adjuvants Elicit a Th17 Immune Response and Protect against Mycobacterium tuberculosis Infection

There are a limited number of adjuvants that elicit effective cell-based immunity required for protection against intracellular bacterial pathogens. Here, we report that STING-activating cyclic dinucleotides (CDNs) formulated in a protein subunit vaccine elicit long-lasting protective immunity to Mycobacterium tuberculosis in the mouse model. Subcutaneous administration of this vaccine provides equivalent protection to that of the live attenuated vaccine strain Bacille Calmette-Guérin (BCG). Protection is STING dependent but type I IFN independent and correlates with an increased frequency of a recently described subset of CXCR3-expressing T cells that localize to the lung parenchyma. Intranasal delivery results in superior protection compared with BCG, significantly boosts BCG-based immunity, and elicits both Th1 and Th17 immune responses, the latter of which correlates with enhanced protection. Thus, a CDN-adjuvanted protein subunit vaccine has the capability of eliciting a multi-faceted immune response that results in protection from infection by an intracellular pathogen.

Prospects for the Use of Thermal Extracts of Brucella abortusI-206 in S-and L-Forms in the Diagnosis and Prevention of Brucellosis

Currently, one of the topical areas of research is the development of new antigen preparations for the specific diagnosis and prevention of brucellosis, since indication of the pathogen and prevention of the disease is complicated by the ability of brucella to dissociate, and live vaccines used for specific prevention of brucellosis have residual virulence. Thermal extracts (TE) obtained from Brucella abortus I-206 in the L- and S-form can be used as such promising antigens. It is known that TE in the L- and S-forms have immunogenic properties, as well as a modulating effect on the proliferation of immunocompetent cells, morphological changes in the immunocompetent organs of experimental animals.

The aimof the work is to study the effect of Brucella abortus thermal extracts in L- and S-forms on the functional state of the cells of experimental animals.

Materials  and  methods. The  study  was  performed  on  100  certified  white  mice.  As  objects  of  study,  we  used  the B. abortus I-206 TE in L- and S-forms. Evaluation of the effect of antigenic drugs on the functional state of phagocytes of laboratory animals in vitro was performed on peritoneal macrophages. The total activity of the respiratory chain enzymes in the NBT-test and superoxide dismutase was determined. Cells of intact animals served as controls. As a positive control, a commercial antigenic LPS preparation Escherichia coli was used. The content of cyclic nucleotides in homogenates of immunocompetent organs was determined using ELISA.

Results.This study presents materials on the study of the effect of TE on the bactericidal activity of phagocytes and the level of cyclic nucleotides in immunocompetent organs. It has been established that TEs activate oxygen-dependent bactericidal systems of phagocytes. When studying the effect of TE on the content of cyclic nucleotides in immunocompetent organs of white mice, an increase in their concentration was revealed, indicating an increase in the functional activity of the cells.Conclusion.The obtained data make it possible to substantiate the need for a further detailed study of the immunogenic properties of B. abortus TE in the L- or S-form on the organism of experimental animals.

Cyclic Dinucleotide-Adjuvanted Dengue Virus Nonstructural Protein 1 Induces Protective Antibody and T Cell Responses

Endothelial dysfunction and vascular leak, pathogenic hallmarks of severe dengue disease, are directly triggered by dengue virus (DENV) nonstructural protein 1 (NS1). Previous studies have shown that immunization with NS1, as well as passive transfer of NS1-immune serum or anti-NS1 mAb, prevent NS1-mediated lethality in vivo. In this study, we evaluated the immunogenicity and protective capacity of recombinant DENV NS1 administered with cyclic dinucleotides (CDNs), potent activators of innate immune pathways and highly immunogenic adjuvants. Using both wild-type C57BL/6 mice and IFN-α/β receptor-deficient mice, we show that NS1-CDN immunizations elicit serotype-specific and cross-reactive Ab and T cell responses. Furthermore, NS1-CDN vaccinations conferred significant homotypic and heterotypic protection from DENV2-induced morbidity and mortality. In addition, we demonstrate that high anti-NS1 Ab titers are associated with protection, supporting the role of humoral responses against DENV NS1 as correlates of protection. These findings highlight the potential of CDN-based adjuvants for inducing Ab and T cell responses and validate NS1 as an important candidate for dengue vaccine development.

Immature lung TNFR2- conventional DC 2 subpopulation activates moDCs to promote cyclic di-GMP mucosal adjuvant responses in vivo

Cyclic dinucleotides (CDNs), including cyclic di-GMP (CDG), are promising vaccine adjuvants in preclinical/clinical trials. The in vivo mechanisms of CDNs are not clear. Here we investigated the roles of lung DC subsets in promoting CDG mucosal adjuvant responses in vivo. Using genetically modified mice and adoptive cell transfer, we identified lung conventional DC 2 (cDC2) as the central player in CDG mucosal responses. We further identified two functionally distinct lung cDC2 subpopulations: TNFR2+pRelB+ and TNFR2-pRelB- cDC2. The TNFR2+ cDC2 were mature and migratory upon intranasal CDG administration while the TNFR2- cDC2 were activated but not mature. Adoptive cell transfer showed that TNFR2- cDC2 mediate the antibody responses of CDG, while the TNFR2+ cDC2 generate Th1/17 responses. Mechanistically, immature TNFR2- cDC2 activate monocyte-derived DCs (moDCs), which do not take up intranasally administered CDG. moDCs promote CDG-induced generation of T follicular helper- and germinal center B cells in the lungs. Our data revealed a previously undescribed in vivo mode of DCs action, whereby an immature lung TNFR2- cDC2 subpopulation directs the non-migratory moDCs to generate CDG mucosal responses in the lung.

Regulation of gingival epithelial cytokine response by bacterial cyclic dinucleotides

BACKGROUND

Cyclic dinucleotides (cyclic di-guanosine monophosphate (c-di-GMP) and cyclic di-adenosine monophosphate (c-di-AMP)) and lipopolysaccharides (LPS) are pathogen-associated molecular patterns (PAMPs). Individual impacts of PAMPs on immune system have been evaluated, but simultaneous actions of multiple PAMPs have not been studied.

OBJECTIVE

Examination the effects of cyclic dinucleotides and Porphyromonas gingivalis LPS on gingival epithelial cytokine response.

METHODS

Human gingival keratinocytes (HMK) were incubated with 1, 10, and 100 µM concentrations of c-di-GMP and c-di-AMP, either in the presence or absence of P. gingivalis LPS. Intra- and extracellular levels of interleukin (IL)-1β, IL-8, IL-1Ra, monocyte chemoattractant protein (MCP)-1, and vascular endothelial growth factor (VEGF), were measured using the Luminex technique.

RESULTS

LPS decreased extracellular IL-8 levels, while the presence of c-di-AMP inhibited this effect. Incubating HMK cells with c-di-AMP (alone or with LPS) elevated the extracellular level of MCP-1. Extracellular VEGF level increased when cells were incubated with LPS and c-di-GMP together, or with c-di-AMP alone. LPS and c-di-AMP suppressed intracellular IL-1β levels. The c-di-AMP elevated intracellular levels of IL-1Ra.

CONCLUSION

c-di-AMP and, to a lesser extent, c-di-GMP regulate keratinocyte cytokine response, either as an aggregator or as a suppressor of LPS, depending on the cytokine type.

Codon optimization and improved delivery/immunization regimen enhance the immune response against wild-type and drug-resistant HIV-1 reverse transcriptase, preserving its Th2-polarity

DNA vaccines require a considerable enhancement of immunogenicity. Here, we optimized a prototype DNA vaccine against drug-resistant HIV-1 based on a weak Th2-immunogen, HIV-1 reverse transcriptase (RT). We designed expression-optimized genes encoding inactivated wild-type and drug-resistant RTs (RT-DNAs) and introduced them into mice by intradermal injections followed by electroporation. RT-DNAs were administered as single or double primes with or without cyclic-di-GMP, or as a prime followed by boost with RT-DNA mixed with a luciferase-encoding plasmid (“surrogate challenge”). Repeated primes improved cellular responses and broadened epitope specificity. Addition of cyclic-di-GMP induced a transient increase in IFN-γ production. The strongest anti-RT immune response was achieved in a prime-boost protocol with electroporation by short 100V pulses done using penetrating electrodes. The RT-specific response, dominated by CD4+ T-cells, targeted epitopes at aa 199–220 and aa 528–543. Drug-resistance mutations disrupted the epitope at aa 205–220, while the CTL epitope at aa 202–210 was not affected. Overall, multiparametric optimization of RT strengthened its Th2- performance. A rapid loss of RT/luciferase-expressing cells in the surrogate challenge experiment revealed a lytic potential of anti-RT response. Such lytic CD4+ response would be beneficial for an HIV vaccine due to its comparative insensitivity to immune escape.

Identification and Characterization of c-di-GMP Metabolic Enzymes of Leptospira interrogans and c-di-GMP Fluctuations After Thermal Shift and Infection

Leptospirosis is a widespread zoonotic disease caused by pathogenic Leptospira species. The most common species, Leptospira interrogans, can transfer from contaminated soil or water to the human body. It is able to survive these changing environments through sensing and responding to the changes of environmental cues. Cyclic di-GMP (c-di-GMP) is a special secondary messenger in bacteria, which can respond to the environment and regulate diverse bacterial behaviors. The c-di-GMP levels in bacterial cells are regulated by diguanylatecyclases (DGC) and phosphodiesterases (PDE), which are responsible for synthesizing or hydrolyzing c-di-GMP, respectively. In this study, distribution and phylogenetics of c-di-GMP metabolic genes among 15 leptospiral species were systematically analyzed. Bioinformatics analysis revealed that leptospiral species contain a multitude of c-di-GMP metabolic genes. C-di-GMP metabolic genes in L. interrogans strain Lai 56601 were further analyzed and the results showed that these genes have very diverse expression patterns. Most of the putative DGCs and PDEs possess enzymatic activities, as determined by riboswitch-based dual-fluorescence reporters in vivo or HPLC in vitro. Furtherer analysis of subdomains from GGDEF-containing proteins revealed that the ability to synthesize c-di-GMP was lost when the GAF domain from LA1483 and PAS domain from LA2932 were deleted, while deletion of the REC domain from LA2528 did not affect its ability to synthesize c-di-GMP. Furthermore, high temperatures generally resulted in low c-di-GMP concentrations in L. interrogans and most of the c-di-GMP metabolic genes exhibited differential temperature regulation. Also, infection of murine J774A.1 cells resulted in reduced c-di-GMP levels, while no significant change of c-di-GMP metabolic genes on transcriptional levels were observed during the infection of J774A.1 cells. Taken together, these results provide a basic platform for future studies of c-di-GMP signaling pathways in Leptospira.

Identification and Characterization of Stimulator of Interferon Genes As a Robust Adjuvant Target for Early Life Immunization

Immunization is key to preventing infectious diseases, a leading cause of death early in life. However, due to age-specific immunity, vaccines often demonstrate reduced efficacy in newborns and young infants as compared to adults. Here, we combined in vitro and in vivo approaches to identify adjuvant candidates for early life immunization. We employed newborn and adult bone marrow-derived dendritic cells (BMDCs) to perform a screening of pattern recognition receptor agonists and found that the stimulator of interferon genes ligand 2′3′-cGAMP (hereafter cGAMP) induces a comparable expression of surface maturation markers in newborn and adult BMDCs. Then, we utilized the trivalent recombinant hemagglutinin (rHA) influenza vaccine, Flublok, as a model antigen to investigate the role of cGAMP in adult and early life immunization. cGAMP adjuvantation alone could increase rHA-specific antibody titers in adult but not newborn mice. Remarkably, as compared to alum or cGAMP alone, immunization with cGAMP formulated with alum (Alhydrogel) enhanced newborn rHA-specific IgG2a/c titers ~400-fold, an antibody subclass associated with the development of IFNγ-driven type 1 immunity in vivo and endowed with higher effector functions, by 42 days of life. Highlighting the amenability for successful vaccine formulation and delivery, we next confirmed that cGAMP adsorbs onto alum in vitro. Accordingly, immunization early in life with (cGAMP+alum) promoted IFNγ production by CD4⁺ T cells and increased the proportions and absolute numbers of CD4⁺ CXCR5⁺ PD-1⁺ T follicular helper and germinal center (GC) GL-7⁺ CD138⁺ B cells, suggesting an enhancement of the GC reaction. Adjuvantation effects were apparently specific for IgG2a/c isotype switching without effect on antibody affinity maturation, as there was no effect on rHA-specific IgG avidity. Overall, our studies suggest that cGAMP when formulated with alum may represent an effective adjuvantation system to foster humoral and cellular aspects of type 1 immunity for early life immunization.

Superior immunogenicity of HCV envelope glycoproteins when adjuvanted with cyclic-di-AMP, a STING activator or archaeosomes

Three decades after the discovery, hepatitis C virus (HCV) is still the leading cause of liver transplantation and poses a major threat to global health. In spite of recent advances in the development of direct acting antivirals, there is still a need for a prophylactic vaccine to limit the virus spread and protect at-risk populations, especially in developing countries, where the cost of the new treatments may severely limit access. The use of recombinant HCV glycoproteins E1E2 (rE1E2) in combination with the MF59, an oil-in-water emulsion-based adjuvant, has previously been shown to reduce the rate of chronicity in chimpanzees and to induce production of cross-neutralizing antibodies and cellular immune responses in human volunteers. To further improve neutralizing antibody responses in recipients along with robust T cell responses, we have explored the immunogenicity of different adjuvants when formulated with the HCV rE1E2 vaccine in mice. Our data show that cyclic di-adenosine monophosphate (c-di-AMP) and archaeosomes elicit strong neutralizing antibodies similar to those elicited using aluminum hydroxide/monophosphoryl lipid A (Alum/monophos. /MPLA) and MF59. However, both c-di-AMP and archaeosomes induced a more robust cellular immune response, which was confirmed by the detection of vaccine-specific poly-functional CD4⁺ T cells. We conclude that these adjuvants may substantially boost the immunogenicity of our E1E2 vaccine. In addition, our data also indicates that use of a partial or exclusive intranasal immunization regimen may also be feasible using c-di-AMP as adjuvant.

Cyclic Dinucleotides in the Scope of the Mammalian Immune System

First discovered in prokaryotes and more recently in eukaryotes, cyclic dinucleotides (CDNs) constitute a unique branch of second messenger signaling systems. Within prokaryotes CDNs regulate a wide array of different biological processes, whereas in the vertebrate system CDN signaling is largely dedicated to activation of the innate immune system. In this book chapter we summarize the occurrence and signaling pathways of these small-molecule second messengers, most importantly in the scope of the mammalian immune system. In this regard, our main focus is the role of the cGAS-STING axis in the context of microbial infection and sterile inflammation and its implications for therapeutic applications.

Natural STING Agonist as an "Ideal" Adjuvant for Cutaneous Vaccination

A potent adjuvant that induces strong protective immunity without incurring any significant skin reactogenicity is urgently needed for cutaneous vaccination. Here, we report that a natural agonist of stimulator of interferon genes (STING), 2'3'- cyclic guanosine monophosphate-adenosine monophosphate (cGAMP), robustly augmented and prolonged the cellular and humoral immune responses provoked by H5N1 and 2009 H1N1 pandemic influenza vaccines after a single dose of intradermal, but not intramuscular, immunization. The potency of cGAMP for cutaneous vaccination was ascribed to a large number of antigen-presenting cells resident in the skin and ready for immediate activation when cGAMP was injected. However, its potency was severely compromised in the muscle, because antigen-presenting cells could not be promptly recruited to the injection site before the injected cGAMP was diffused out. The superior adjuvant effect and safety of cGAMP were also confirmed in a more clinically relevant swine model of skin. The vigorous immune responses elicited by cGAMP with no overt skin irritation was attributable to its stay in the skin, which was brief but sufficient to activate dermal dendritic cells. This small and well-characterized self-molecule holds great promise as an ideal adjuvant for cutaneous vaccination.

Cyclic (di)nucleotides: the common language shared by microbe and host

Fluency in a common language allows individuals to convey information and carry out complex activities that otherwise would be difficult or even impossible without the benefit of shared communication. Cyclic (di)nucleotides have recently been recognized as such an accessible language understood by both microbe and the host, ever since remarkable progresses have revealed the molecular details of these nucleotide second messengers used in cellular communication systems. Though undergoing separate evolutionary pathways in prokaryotes and eukaryotes, cyclic (di)nucleotides enable microbes to influence host cells immediately and fiercely by modulating a variety of cellular activities. Here we highlight recent insights in cyclic (di)nucleotides and focus on the balancing of these indispensable signaling molecules by synthases and phosphodiesterases.

Nanoparticulate STING agonists are potent lymph node-targeted vaccine adjuvants

Cyclic dinucleotides (CDNs) are agonists of stimulator of IFN genes (STING) and have potential as vaccine adjuvants. However, cyclic di-GMP (cdGMP) injected s.c. shows minimal uptake into lymphatics/draining lymph nodes (dLNs) and instead is rapidly distributed to the bloodstream, leading to systemic inflammation. Here, we encapsulated cdGMP within PEGylated lipid nanoparticles (NP-cdGMP) to redirect this adjuvant to dLNs. Compared with unformulated CDNs, encapsulation blocked systemic dissemination and markedly enhanced dLN accumulation in murine models. Delivery of NP-cdGMP increased CD8+ T cell responses primed by peptide vaccines and enhanced therapeutic antitumor immunity. A combination of a poorly immunogenic liposomal HIV gp41 peptide antigen and NP-cdGMP robustly induced type I IFN in dLNs, induced a greater expansion of vaccine-specific CD4+ T cells, and greatly increased germinal center B cell differentiation in dLNs compared with a combination of liposomal HIV gp41 and soluble CDN. Further, NP-cdGMP promoted durable antibody titers that were substantially higher than those promoted by the well-studied TLR agonist monophosphoryl lipid A and comparable to a much larger dose of unformulated cdGMP, without the systemic toxicity of the latter. These results demonstrate that nanoparticulate delivery safely targets CDNs to the dLNs and enhances the efficacy of this adjuvant. Moreover, this approach can be broadly applied to other small-molecule immunomodulators of interest for vaccines and immunotherapy.

The mucosal adjuvant cyclic di-GMP enhances antigen uptake and selectively activates pinocytosis-efficient cells in vivo

Effective mucosal adjuvants enhance the magnitude and quality of the vaccine response. Cyclic di-GMP (CDG) is a promising mucosal vaccine adjuvant. However, its in vivo mechanisms are unclear. Here, we showed, in mice, that CDG elicits stronger Ab and TH responses than the mammalian 2'3'-cyclic GMP-AMP (cGAMP), and generated better protection against Streptococcus pneumoniae infection than 2'3'-cGAMP adjuvanted vaccine. We identified two in vivo mechanisms of CDG. First, intranasally administered CDG greatly enhances Ag uptake, including pinocytosis and receptor-mediated endocytosis in vivo. The enhancement depends on MPYS (STING, MITA) expression in CD11C(+) cells. Second, we found that CDG selectively activated pinocytosis-efficient-DCs, leading to T(H) polarizing cytokines IL-12p70, IFNγ, IL-5, IL-13, IL-23, and IL-6 production in vivo. Notably, CDG induces IFNλ, but not IFNβ, in vivo. Our study revealed previously unrecognized in vivo functions of MPYS and advanced our understanding of CDG as a mucosal vaccine adjuvant.

Enhanced immunostimulatory activity of cyclic dinucleotides on mouse cells when complexed with a cell-penetrating peptide or combined with CpG

Recognition of pathogen-derived nucleic acids by immune cells is critical for the activation of protective innate immune responses. Bacterial cyclic dinucleotides (CDNs) are small nucleic acids that are directly recognized by the cytosolic DNA sensor STING (stimulator of IFN genes), initiating a response characterized by proinflammatory cytokine and type I IFN production. Strategies to improve the immune stimulatory activities of CDNs can further their potential for clinical development. Here, we demonstrate that a simple complex of cylic-di-GMP with a cell-penetrating peptide enhances both cellular delivery and biological activity of the cyclic-di-GMP in murine splenocytes. Furthermore, our findings establish that activation of the TLR-dependent and TLR-independent DNA recognition pathways through combined use of CpG oligonucleotide (ODN) and CDN results in synergistic activity, augmenting cytokine production (IFN-α/β, IL-6, TNF-α, IP-10), costimulatory molecule upregulation (MHC class II, CD86), and antigen-specific humoral and cellular immunity. Results presented herein indicate that 3'3'-cGAMP, a recently identified bacterial CDN, is a superior stimulator of IFN genes ligand than cyclic-di-GMP in human PBMCs. Collectively, these findings suggest that the immune-stimulatory properties of CDNs can be augmented through peptide complexation or synergistic use with CpG oligonucleotide and may be of interest for the development of CDN-based immunotherapeutic agents.

Stimulation of innate immunity by in vivo cyclic di-GMP synthesis using adenovirus

The bacterial second messenger cyclic di-GMP (c-di-GMP) stimulates inflammation by initiating innate immune cell recruitment and triggering the release of proinflammatory cytokines and chemokines. These properties make c-di-GMP a promising candidate for use as a vaccine adjuvant, and numerous studies have demonstrated that administration of purified c-di-GMP with different antigens increases protection against infection in animal models. Here, we have developed a novel approach to produce c-di-GMP inside host cells as an adjuvant to exploit a host-pathogen interaction and initiate an innate immune response. We have demonstrated that c-di-GMP can be synthesized in vivo by transducing a diguanylate cyclase (DGC) gene into mammalian cells using an adenovirus serotype 5 (Ad5) vector. Expression of DGC led to the production of c-di-GMP in vitro and in vivo, and this was able to alter proinflammatory gene expression in murine tissues and increase the secretion of numerous cytokines and chemokines when administered to animals. Furthermore, coexpression of DGC modestly increased T-cell responses to a Clostridium difficile antigen expressed from an adenovirus vaccine, although no significant differences in antibody titers were observed. This adenovirus c-di-GMP delivery system offers a novel method to administer c-di-GMP as an adjuvant to stimulate innate immunity during vaccination.

Rationale, progress and development of vaccines utilizing STING-activating cyclic dinucleotide adjuvants

A principal barrier to the development of effective vaccines is the availability of adjuvants and formulations that can elicit both effector and long-lived memory CD4 and CD8 T cells. Cellular immunity is the presumptive immune correlate of protection against intracellular pathogens: a group composed of bacteria, viruses and protozoans that is responsible for a staggering level of morbidity and mortality on a global scale. T-cell immunity is also correlated with clinical benefit in cancer, and the development of therapeutic strategies to harness the immune system to treat diverse malignancies is currently undergoing a renaissance. Cyclic dinucleotides (CDNs) are ubiquitous small molecule second messengers synthesized by bacteria that regulate diverse processes and are a relatively new class of adjuvants that have been shown to increase vaccine potency. CDNs activate innate immunity by directly binding the endoplasmic reticulum-resident receptor STING (stimulator of interferon genes), activating a signaling pathway that induces the expression of interferon-β (IFN-β) and also nuclear factor-κB (NF-κB) dependent inflammatory cytokines. The STING signaling pathway has emerged as a central Toll-like receptor (TLR) independent mediator of host innate defense in response to sensing cytosolic nucleic acids, either through direct binding of CDNs secreted by bacteria, or, as shown recently, through binding of a structurally distinct CDN produced by a host cell receptor in response to binding cytosolic double-stranded (ds)DNA. Although this relatively new class of adjuvants has to date only been evaluated in mice, newly available CDN-STING cocrystal structures will likely intensify efforts in this field towards further development and evaluation in human trials both in preventive vaccine and immunotherapy settings.

Antibiotics in Canadian poultry productions and anticipated alternatives

The use of antibiotics in food-producing animals has significantly increased animal health by lowering mortality and the incidence of diseases. Antibiotics also have largely contributed to increase productivity of farms. However, antibiotic usage in general and relevance of non-therapeutic antibiotics (growth promoters) in feed need to be reevaluated especially because bacterial pathogens of humans and animals have developed and shared a variety of antibiotic resistance mechanisms that can easily be spread within microbial communities. In Canada, poultry production involves more than 2600 regulated chicken producers who have access to several antibiotics approved as feed additives for poultry. Feed recipes and mixtures vary greatly geographically and from one farm to another, making links between use of a specific antibiotic feed additive and production yields or selection of specific antibiotic-resistant bacteria difficult to establish. Many on-farm studies have revealed the widespread presence of antibiotic-resistant bacteria in broiler chickens. While some reports linked the presence of antibiotic-resistant organisms to the use of feed supplemented with antibiotics, no recent studies could clearly demonstrate the benefit of antimicrobial growth promoters on performance and production yields. With modern biosecurity and hygienic practices, there is a genuine concern that intensive utilization of antibiotics or use of antimicrobial growth promoters in feed might no longer be useful. Public pressure and concerns about food and environmental safety (antibiotic residues, antibiotic-resistant pathogens) have driven researchers to actively look for alternatives to antibiotics. Some of the alternatives include pre- and probiotics, organic acids and essential oils. We will describe here the properties of some bioactive molecules, like those found in cranberry, which have shown interesting polyvalent antibacterial and immuno-stimulatory activities.

The mucosal adjuvant cyclic di-AMP exerts immune stimulatory effects on dendritic cells and macrophages

The cyclic di-nucleotide bis-(3′,5′)-cyclic dimeric adenosine monophosphate (c-di-AMP) is a candidate mucosal adjuvant with proven efficacy in preclinical models. It was shown to promote specific humoral and cellular immune responses following mucosal administration. To date, there is only fragmentary knowledge on the cellular and molecular mode of action of c-di-AMP. Here, we report on the identification of dendritic cells and macrophages as target cells of c-di-AMP. We show that c-di-AMP induces the cell surface up-regulation of T cell co-stimulatory molecules as well as the production of interferon-β. Those responses were characterized by in vitro experiments with murine and human immune cells and in vivo studies in mice. Analyses of dendritic cell subsets revealed conventional dendritic cells as principal responders to stimulation by c-di-AMP. We discuss the impact of the reported antigen presenting cell activation on the previously observed adjuvant effects of c-di-AMP in mouse immunization studies.

MPYS/STING-mediated TNF-α, not type I IFN, is essential for the mucosal adjuvant activity of (3'-5')-cyclic-di-guanosine-monophosphate in vivo

The bacterial second messenger (3'-5')-cyclic-di-guanosine-monophosphate (CDG) is a promising mucosal adjuvant candidate that activates balanced Th1/Th2/Th17 responses. We showed previously that CDG activates stimulator of IFN genes (STING)-dependent IFN-I production in vitro. However, it is unknown whether STING or IFN-I is required for the CDG adjuvant activity in vivo. In this study, we show that STING(-/-) mice (Tmem173(<tm1Camb>)) do not produce Ag-specific Abs or Th1/Th2/Th17 cytokines during CDG/Ag immunization. Intranasal administration of CDG did not induce TNF-α, IL-1β, IL-6, IL-12, or MCP-1 production in STING(-/-) mice. Surprisingly, we found that the cytokine and Ab responses were unaltered in CDG/Ag-immunized IFNAR(-/-) mice. Instead, we found that CDG activates STING-dependent, IFN-I-independent TNF-α production in vivo and in vitro. Furthermore, using a TNFR1(-/-) mouse, we demonstrate that TNF-α signaling is critical for CDG-induced Ag-specific Ab and Th1/Th2 cytokine production. This is distinct from STING-mediated DNA adjuvant activity, which requires IFN-I, but not TNF-α, production. Finally, we found that CDG activates STING-dependent, but IRF3 stimulation-independent, NF-κB signaling. Our results established an essential role for STING-mediated TNF-α production in the mucosal adjuvant activity of CDG in vivo and revealed a novel IFN-I stimulation-independent STING-NF-κB-TNF-α pathway.

Cyclic di-GMP: the first 25 years of a universal bacterial second messenger

Twenty-five years have passed since the discovery of cyclic dimeric (3'→5') GMP (cyclic di-GMP or c-di-GMP). From the relative obscurity of an allosteric activator of a bacterial cellulose synthase, c-di-GMP has emerged as one of the most common and important bacterial second messengers. Cyclic di-GMP has been shown to regulate biofilm formation, motility, virulence, the cell cycle, differentiation, and other processes. Most c-di-GMP-dependent signaling pathways control the ability of bacteria to interact with abiotic surfaces or with other bacterial and eukaryotic cells. Cyclic di-GMP plays key roles in lifestyle changes of many bacteria, including transition from the motile to the sessile state, which aids in the establishment of multicellular biofilm communities, and from the virulent state in acute infections to the less virulent but more resilient state characteristic of chronic infectious diseases. From a practical standpoint, modulating c-di-GMP signaling pathways in bacteria could represent a new way of controlling formation and dispersal of biofilms in medical and industrial settings. Cyclic di-GMP participates in interkingdom signaling. It is recognized by mammalian immune systems as a uniquely bacterial molecule and therefore is considered a promising vaccine adjuvant. The purpose of this review is not to overview the whole body of data in the burgeoning field of c-di-GMP-dependent signaling. Instead, we provide a historic perspective on the development of the field, emphasize common trends, and illustrate them with the best available examples. We also identify unresolved questions and highlight new directions in c-di-GMP research that will give us a deeper understanding of this truly universal bacterial second messenger.

Impairment of pneumococcal antigen specific isotype-switched Igg memory B-cell immunity in HIV infected Malawian adults

Pneumococcal disease is associated with a particularly high morbidity and mortality amongst adults in HIV endemic countries. Our previous findings implicating a B-cell defect in HIV-infected children from the same population led us to comprehensively characterize B-cell subsets in minimally symptomatic HIV-infected Malawian adults and investigate the isotype-switched IgG memory B-cell immune response to the pneumococcus. We show that similar to vertically acquired HIV-infected Malawian children, horizontally acquired HIV infection in these adults is associated with IgM memory B-cell (CD19(+) CD27(+) IgM(+) IgD(+)) depletion, B-cell activation and impairment of specific IgG B-cell memory to a range of pneumococcal proteins. Our data suggest that HIV infection affects both T-cell independent and T-cell dependent B-cell maturation, potentially leading to impairment of humoral responses to extracellular pathogens such as the pneumococcus, and thus leaving this population susceptible to invasive disease.

Adjuvant immunotherapies as a novel approach to bacterial infections

The rapid emergence of multidrug-resistant pathogens, especially Gram-negative bacteria and mycobacteria, represents one of the major medical challenges of the 21st century. The gradual loss of effective classical antibiotics for many bacterial pathogens, combined with an increasing population density and mobility, urgently calls for the development of novel treatments. Here, we discuss the potential of adjuvant immunotherapies to selectively stimulate protective immune responses as a treatment option for bacterial infections. In order to elicit appropriate immune responses and to avoid unwanted inflammatory tissue damage, it is essential to identify ligands and receptor pathways that specifically control protective responses at the site of infection. We summarize existing data and discuss suitable candidate targets for future immunotherapies of infectious diseases.

Cyclic di-GMP mediates Mycobacterium tuberculosis dormancy and pathogenecity

Dormancy of Mycobacterium tuberculosis is likely to be a major cause of extended chemotherapeutic regimens and wide prevalence of tuberculosis. The molecular mechanisms underlying M. tuberculosis dormancy are not well understood. In this study, single-copy genes responsible for synthesis (dgc) and degradation (pde) of the ubiquitous bacterial second messenger, cyclic di-GMP (c-di-GMP), were deleted in the virulent M. tuberculosis strain H37Rv to generate dgc(mut) and Δpde, respectively. Under aerobic growth conditions, the two mutants and wild-type cells showed similar phenotypes. However, dgc(mut) and Δpde exhibited increased and reduced dormancy, respectively, in both anaerobiosis-triggered and vitamin C-triggered in vitro dormancy models, as determined by survival and growth recovery from dormancy. The transcriptomes of aerobic cultures of dgc(mut) and wild-type H37Rv exhibited no difference, whereas those of anaerobic cultures showed a significant difference with 61 genes that are not a part of the dosR regulon. Furthermore, Δpde but not dgc(mut) showed decreased infectivity with human THP-1 cells. Δpde also showed attenuated pathogenicity in a C57BL/6 mouse infection model. These findings are explained by c-di-GMP-mediated signaling negatively regulating M. tuberculosis dormancy and pathogenicity.

Biotinylation of a propargylated cyclic (3'-5') diguanylic acid and of its mono-6-thioated analog under "click" conditions

Commercial N(2)-isobutyryl-5'-O-(4,4'-dimethoxytrityl)-2'-O-(propargyl)guanosine is converted to its 3'-O-levulinyl ester in a yield of 91%. The reaction of commercial N(2)-isobutyryl-5'-O-(4,4'-dimethoxytrityl)-2'-O-tert-butyldimethylsilyl-3'-O-[(2-cyanoethyl)-N,N-diisopropylaminophosphinyl]guanosine with N(2)-isobutyryl-2'-O-propargyl-3'-O-(levulinyl)guanosine provides, after P(III) oxidation, 3'-/5'-deprotection, and purification, the 2'-O-propargylated guanylyl(3'-5')guanosine 2-cyanoethyl phosphate triester in a yield of 88%. Phosphitylation of this dinucleoside phosphate triester with 2-cyanoethyl tetraisopropylphosphordiamidite and 1H-tetrazole, followed by an in situ intramolecular cyclization, gives the propargylated cyclic dinucleoside phosphate triester, which is isolated in a yield of 40% after P(III) oxidation and purification. Complete removal of the nucleobases, phosphates, and 2'-O-tert-butyldimethylsilyl protecting groups leads to the desired propargylated c-di-GMP diester. Cycloaddition of a biotinylated azide with the propargylated c-di-GMP diester under click conditions provides the biotinylated c-di-GMP conjugate in an isolated yield of 62%. Replacement of the 6-oxo function of N(2)-isobutyryl-5'-O-(4,4'-dimethoxytrityl)-3'-O-levulinyl-2'-O-(propargyl)guanosine with a 2-cyanoethylthio group is effected by treatment with 2,4,6-triisopropybenzenesulfonyl chloride and triethylamine to give a 6-(2,4,6-triisopropylbenzenesulfonic acid) ester intermediate. Reaction of this key intermediate with 3-mercaptoproprionitrile and triethylamine, followed by 5'-dedimethoxytritylation, affords the 6-(2-cyanoethylthio)guanosine derivative in a yield of 70%. The 5'-hydroxy function of this derivative is reacted with commercial N(2)-isobutyryl-5'-O-(4,4'-dimethoxytrityl)-2'-O-tert-butyldimethylsilyl-3'-O-[(2-cyanoethyl)-N,N-diisopropylaminophosphinyl]guanosine. The reaction product is then converted to the mono-6-thioated c-di- GMP biotinylated conjugate under conditions highly similar to those described above for the preparation of the biotinylated c-di-GMP conjugate, and isolated in similar yields.

Biofilm infections, their resilience to therapy and innovative treatment strategies

Biofilm formation of microorganisms causes persistent tissue and foreign body infections resistant to treatment with antimicrobial agents. Up to 80% of human bacterial infections are biofilm associated; such infections are most frequently caused by Staphylococcus epidermidis, Pseudomonas aeruginosa, Staphylococcus aureus and Enterobacteria such as Escherichia coli. The accurate diagnosis of biofilm infections is often difficult, which prevents the appropriate choice of treatment. As biofilm infections significantly contribute to patient morbidity and substantial healthcare costs, novel strategies to treat these infections are urgently required. Nucleotide second messengers, c-di-GMP, (p)ppGpp and potentially c-di-AMP, are major regulators of biofilm formation and associated antibiotic tolerance. Consequently, different components of these signalling networks might be appropriate targets for antibiofilm therapy in combination with antibiotic treatment strategies. In addition, cyclic di-nucleotides are microbial-associated molecular patterns with an almost universal presence. Their conserved structures sensed by the eukaryotic host have a widespread effect on the immune system. Thus, cyclic di-nucleotides are also potential immunotherapeutic agents to treat antibiotic-resistant bacterial infections.

Evidence for cyclic diguanylate as a vaccine adjuvant with novel immunostimulatory activities

Cyclic diguanylate (c-di-GMP), a bacterial signaling molecule, possesses protective immunostimulatory activity in bacterial challenge models. This study explored the potential of c-di-GMP as a vaccine adjuvant comparing it with LPS, CpG oligonucleotides, and a conventional aluminum salt based adjuvant. In this evaluation, c-di-GMP was a more potent activator of both humoral and Th1-like immune responses as evidenced by the robust IgG2a antibody response it induced in mice and the strong IFN-γ, TNF-α and IP-10 responses, it elicited in mice and in vitro in non-human primate peripheral blood mononuclear cells. Further, compared to LPS or CpG, c-di-GMP demonstrated a more pronounced ability to induce germinal center formation, a hallmark of long-term memory, in immunized mice. Together, these data add to the growing body of evidence supporting the utility of c-di-GMP as an adjuvant in vaccination for sustained and robust immune responses and provide a rationale for further evaluation in appropriate models of immunization.

Identification of a novel pneumococcal vaccine antigen preferentially expressed during meningitis in mice

Streptococcus pneumoniae is the most common cause of severe bacterial meningitis in children, the elderly, and immunocompromised individuals. To identify virulence factors preferentially expressed during meningitis, we conducted niche-specific genome-wide in vivo transcriptomic analysis after intranasal infection of mice with serotype 4 or 6A pneumococci. The expression of 34 bacterial genes was substantially altered in brain tissue of mice infected with either of the 2 strains. Ten upregulated genes were common to both strains, 7 of which were evaluated for their role in the development of meningitis. One previously uncharacterized protein, α-glycerophosphate oxidase (GlpO), was cytotoxic for human brain microvascular endothelial cells (HBMECs) via generation of H(2)O(2). A glpO deletion mutant was defective in adherence to HBMECs in vitro as well as in progression from the blood to the brain in vivo. Mutant bacteria also induced markedly reduced meningeal inflammation and brain pathology compared with wild type, despite similar levels of bacteremia. Immunization of mice with GlpO protected against invasive pneumococcal disease and provided additive protection when formulated with pneumolysin toxoid. Our results provide the basis of a strategy that can be adapted to identify genes that contribute to the development of meningitis caused by other pathogens.

Innate immune pathways triggered by Listeria monocytogenes and their role in the induction of cell-mediated immunity

Acquired cell-mediated immunity to Listeria monocytogenes is induced by infection with live, replicating bacteria that grow in the host cell cytosol, whereas killed bacteria, or those trapped in a phagosome, fail to induce protective immunity. In this chapter, we focus on how L. monocytogenes is sensed by the innate immune system, with the presumption that innate immunity affects the development of acquired immunity. Infection by L. monocytogenes induces three innate immune pathways: an MyD88-dependent pathway emanating from a phagosome leading to expression of inflammatory cytokines; a STING/IRF3-dependent pathway emanating from the cytosol leading to the expression of IFN-β and coregulated genes; and very low levels of a Caspase-1-dependent, AIM2-dependent inflammasome pathway resulting in proteolytic activation and secretion of IL-1β and IL-18 and pyroptotic cell death. Using a combination of genetics and biochemistry, we identified the listerial ligand that activates the STING/IRF3 pathway as secreted cyclic diadenosine monophosphate, a newly discovered conserved bacterial signaling molecule. We also identified L. monocytogenes mutants that caused robust inflammasome activation due to bacteriolysis in the cytosol, release of DNA, and activation of the AIM2 inflammasome. A strain was constructed that ectopically expressed and secreted a fusion protein containing Legionella pneumophila flagellin that robustly activated the Nlrc4-dependent inflammasome and was highly attenuated in mice, also in an Nlrc4-dependent manner. Surprisingly, this strain was a poor inducer of adaptive immunity, suggesting that inflammasome activation is not necessary to induce cell-mediated immunity and may even be detrimental under some conditions. To the best of our knowledge, no single innate immune pathway is necessary to mount a robust acquired immune response to L. monocytogenes infection.