The bacterial second messenger cyclic diGMP exhibits potent adjuvant properties

Biofilm switch and immune response determinants at early stages of infection

Biofilm development is recognized as a major virulence factor underlying most chronic bacterial infections. When a biofilm community is established, planktonic cells growing in the surroundings of a tissue switch to a sessile lifestyle and start producing a biofilm matrix. The initial steps of in vivo biofilm development are poorly characterized and difficult to assess experimentally. A great amount of in vitro evidence has shown that accumulation of high levels of cyclic dinucleotides (c-di-NMPs) is the most prevalent hallmark governing the initiation of biofilm development by bacteria. As mentioned above, recent studies also link detection of c-di-NMPs by host cells with the activation of a type I interferon immune response against bacterial infections. We discuss here c-di-NMP signaling and the host immune response in the context of the initial steps of in vivo biofilm development.

Mucosal immunity and nasal influenza vaccination

Influenza remains a threat to public health, with immunization being a suitable method of infection prevention and control. Our understanding of the immunological regulations at the mucosa, antigen processing and presentation, and B-cell activation has improved, enabling research and targeted induction of immune responses at the site of antigen delivery. Nasal influenza immunization has distinct features compared with intramuscular vaccines, providing protection at the pathogen's entry site, higher levels of mucosal antibodies, cross-protection and needle-free application. This review summarizes our knowledge about mucosal immunity and the experience from clinical trials on the impact and safety of nasal influenza vaccination.

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.

Cyclic di-nucleotides: new era for small molecules as adjuvants

The implementation of vaccination as an empiric strategy to protect against infectious diseases was introduced even before the advent of hygiene and antimicrobials in the medical practice. Nevertheless, it was not until a few decades ago that we really started understanding the underlying mechanisms of protection triggered by vaccination. Vaccines were initially based on attenuated or inactivated organisms. Subunit vaccines were then introduced as more refined formulations, exhibiting improved safety profiles. However, purified antigens tend to be poorly immunogenic and often require the use of adjuvants to achieve adequate stimulation of the immune system. Vaccination strategies, such as mucosal administration, also require potent adjuvants to improve performance. In the 1990s, immunologists found that pathogens could be sensed as ‘danger signals’ by receptors recognizing conserved motifs. Although our knowledge is still limited, tremendous advances were made in the understanding of host defence mechanisms regulated by these evolutionary conserved receptors, and the molecular structures which are recognized by them. This opened a new era in adjuvant development. Some of the latest players arrived to this field are the cyclic di‐nucleotides, which are ubiquitous prokaryotic intracellular signalling molecules. This review is focused on their potential for the development of vaccines and immunotherapies.

Large-scale production of the immunomodulator c-di-GMP from GMP and ATP by an enzymatic cascade

(3'-5')-Cyclic diguanylate (c-di-GMP) is a bacterial second messenger with immunomodulatory activities in mice suggesting potential applications as a vaccine adjuvant and as a therapeutic agent. Clinical studies in larger animals or humans will require larger doses that are difficult and expensive to generate by currently available chemical or enzymatic synthesis and purification methods. Here we report the production of c-di-GMP at the multi-gram scale from the economical precursors guanosine monophosphate (GMP) and adenosine triphosphate by a "one-pot" three enzyme cascade consisting of GMP kinase, nucleoside diphosphate kinase, and a mutated form of diguanylate cyclase engineered to lack product inhibition. The c-di-GMP was purified to apparent homogeneity by a combination of anion exchange chromatography and solvent precipitation and was characterized by reversed phase high performance liquid chormatography and mass spectrometry, nuclear magnetic resonance spectroscopy, and further compositional analyses. The immunomodulatory activity of the c-di-GMP preparation was confirmed by its potentiating effect on the lipopolysaccharide-induced interleukin 1β, tumor necrosis factor α, and interleukin 6 messenger RNA expression in J774A.1 mouse macrophages.

Bis-(3',5')-cyclic dimeric adenosine monophosphate: strong Th1/Th2/Th17 promoting mucosal adjuvant

New effective adjuvants are required to improve the performance of subunit vaccines. Here, we showed that bis-(3',5')-cyclic dimeric adenosine monophosphate (c-di-AMP), a second messenger molecule in bacteria and archaea, exerts strong adjuvant activities when delivered by mucosal route. In vitro studies showed that c-di-AMP was able to both stimulate pre-activated murine macrophages and promote the activation and maturation of dendritic cells of murine and human origin. Co-administration of c-di-AMP with β-galactosidase (β-Gal) by intranasal route to BALB/c mice resulted in the elicitation of significantly higher serum antigen-specific IgG titres than in controls. The induction of local immune responses was shown by the production of antigen-specific secretory IgA in different mucosal territories. In addition, strong cellular immune responses were observed against both the β-Gal protein and a peptide encompassing its MHC class I-restricted epitope. The ratio of β-Gal-specific antibodies and the secreted cytokine profiles by in vitro re-stimulated splenocytes suggested that a balanced Th1/Th2/Th17 response pattern is promoted by c-di-AMP. When C57BL/6 mice were immunized with OVA and c-di-AMP, vigorous in vivo CTL responses were also observed. These results indicated that c-di-AMP exhibits a high potential as adjuvant for the development of mucosal vaccines, in particular when cellular immunity is needed.

Intranasal c-di-GMP-adjuvanted plant-derived H5 influenza vaccine induces multifunctional Th1 CD4+ cells and strong mucosal and systemic antibody responses in mice

Vaccination is the best available measure of limiting the impact of the next influenza pandemic. Ideally, a candidate pandemic influenza vaccine should be easy to administer and should elicit strong mucosal and systemic immune responses. Production of influenza subunit antigen in transient plant expression systems is an alternative to overcome the bottleneck in vaccine supply during influenza pandemic. Furthermore, a needle-free intranasal influenza vaccine is an attractive approach, which may provide immunity at the portal of virus entry. The present study investigated the detailed humoral and cellular immune responses in mice vaccinated intranasally or intramuscularly with plant-derived influenza H5N1 (A/Anhui/1/05) antigen alone or formulated with bis-(3',5')-cyclic dimeric guanosine monophosphate (c-di-GMP) as adjuvant. The use of c-di-GMP as intramuscular adjuvant did not enhance the immune response to plant-derived influenza H5 antigen. However, intranasal c-di-GMP-adjuvanted vaccine induced strong mucosal and systemic humoral immune responses. Additionally, the intranasal vaccine elicited a balanced Th1/Th2 profile and, most importantly, high frequencies of multifunctional Th1 CD4(+) cells. Our results highlight that c-di-GMP is a promising mucosal adjuvant for pandemic influenza vaccine development.

c-di-GMP protects against intranasal Acinetobacter baumannii infection in mice by chemokine induction and enhanced neutrophil recruitment

Acinetobacter baumannii has emerged as a major cause of both community-associated and nosocomial infections worldwide. A. baumannii rapidly develops resistance to multiple antibiotics; as a result, infections by this pathogen have become increasingly difficult to treat. In this study, we evaluated the effect of 3',5'-cyclic diguanylic acid (c-di-GMP), a bacterial second messenger and immunomodulator, in the host defense against A. baumannii infection in a mouse model of intranasal infection. Our results showed that 50 μg of c-di-GMP administered 18 h prior to infection provided the best protection against intranasal infection with A. baumannii. Mechanistically, administration of c-di-GMP induced the production of chemokines KC, MCP-1, MIP-1α, MIP-2 and RANTES, and enhanced neutrophil recruitment in the lung. Moreover, depletion of neutrophils abolished the protective role of c-di-GMP. Taken together, our data suggest that c-di-GMP confers resistance against intranasal A. baumannii infection in mice through a neutrophil-dependent mechanism and that c-di-GMP should be further explored as an immunomodulator for the treatment of A. baumannii infection.

The potential of 3',5'-cyclic diguanylic acid (c-di-GMP) as an effective vaccine adjuvant

3', 5'-Cyclic diguanylic acid (c-di-GMP) is a bacterial intracellular signaling molecule that plays a crucial role in the regulation of bacterial motility, adhesion, cell-to-cell communication, exopolysaccharide synthesis, biofilm formation and virulence. The recent finding that c-di-GMP can act as a danger signal on eukaryotic cells has prompted the study of the immunostimulatory and immunomodulatory properties of c-di-GMP in an effort to determine whether c-di-GMP might be further developed as a potential vaccine adjuvant. In this review, we discussed the recent in vitro and in vivo studies of the immunostimulatory properties of c-di-GMP and the progress that has been made in the preclinical development of c-di-GMP as a potential vaccine adjuvant for systemic and mucosal vaccination.

The member of the cyclic di-nucleotide family bis-(3', 5')-cyclic dimeric inosine monophosphate exerts potent activity as mucosal adjuvant

Here we demonstrated that bis-(3',5')-cyclic dimeric inosine monophosphate (c-di-IMP) exhibits potent adjuvant properties. BALB/c or C57BL/6 mice were immunized with the model antigens beta-galactosidase (beta-Gal) or Ovalbumin (OVA) alone or co-administered with c-di-IMP by the intranasal route. Animals receiving c-di-IMP showed significantly higher anti-beta-Gal or OVA immunoglobulin G titres (IgG) in sera than those vaccinated with beta-Gal or OVA alone. Furthermore, strong local immune responses were also detectable in different mucosal territories, as shown by the high levels of beta-Gal-specific secretory IgA (sIgA). The analysis of the antigen-specific IgG isotypes in sera, together with the profiles of the cytokines and chemokines secreted by lymphocytes from vaccinated animals showed that the use of c-di-IMP resulted in stimulation of a mixed T(H)1/T(H)2/T(H)17 response. Mucosal immunization of C57BL/6 mice with OVA using c-di-IMP as adjuvant also led to the stimulation of strong in vivo CTL responses (i.e., 60% of antigen-specific lysis) [corrected].Our results demonstrated that the novel compound c-di-IMP exhibits strong adjuvant properties when co-administered with an antigen by the mucosal route, thereby representing a promising candidate adjuvant for the development of mucosal vaccination strategies.

A host type I interferon response is induced by cytosolic sensing of the bacterial second messenger cyclic-di-GMP

The innate immune system responds to unique molecular signatures that are widely conserved among microbes but that are not normally present in host cells. Compounds that stimulate innate immune pathways may be valuable in the design of novel adjuvants, vaccines, and other immunotherapeutics. The cyclic dinucleotide cyclic-di–guanosine monophosphate (c-di-GMP) is a recently appreciated second messenger that plays critical regulatory roles in many species of bacteria but is not produced by eukaryotic cells. In vivo and in vitro studies have previously suggested that c-di-GMP is a potent immunostimulatory compound recognized by mouse and human cells. We provide evidence that c-di-GMP is sensed in the cytosol of mammalian cells via a novel immunosurveillance pathway. The potency of cytosolic signaling induced by c-di-GMP is comparable to that induced by cytosolic delivery of DNA, and both nucleic acids induce a similar transcriptional profile, including triggering of type I interferons and coregulated genes via induction of TBK1, IRF3, nuclear factor κB, and MAP kinases. However, the cytosolic pathway that senses c-di-GMP appears to be distinct from all known nucleic acid–sensing pathways. Our results suggest a novel mechanism by which host cells can induce an inflammatory response to a widely produced bacterial ligand.

HIV pseudovirion vaccine exposing Env "fusion intermediates"-response to immunisation in human CD4/CCR5-transgenic rats

Immune responses to a pseudovirion-based HIV vaccine enriched in Env conformations, which have been induced to an authentic intermediate fusion stage by interaction with the cellular HIV receptor complex, have been analysed in human CD4/CCR5-transgenic rats. High titre Env-binding antibodies were elicited. However, these immune sera failed to neutralise HIV-1, but rather led to an enhancement of infection in vitro. This enhancing activity appeared to be directed towards contaminating cellular proteins in the vaccine and was able to mask neutralisation of potent, mixed-in neutralising antibodies. The induced Env-specific antibodies, purified on the basis of binding to monomeric Env, retained high-binding activity, but failed to be neutralising. Thus, it remains unclear whether vaccines based on induced HIV Env fusion intermediates can elicit broadly neutralising responses.

Immune modulators with defined molecular targets: cornerstone to optimize rational vaccine design

Vaccination remains the most valuable tool for preventing infectious diseases. However, the performance of many existing vaccines should be improved and there are diseases for which vaccines are still not available. The use of well-defined antigens for the generation of subunit vaccines has led to products with an improved safety profile. However, purified antigens are usually poorly immunogenic, making essential the use of adjuvants. Despite the fact that adjuvants have been used to increase the immunogenicity of vaccines for more than 70 years, only a handful has been licensed for human use (e.g., aluminium salts, the micro-fluidized squalene-in-water emulsion MF59 and monophosphoryl lipid A). Thus, the development of new adjuvants which are able to promote broad and sustained immune responses at systemic and mucosal levels still remains as a major challenge in vaccinology. Recent advances in our understanding of the immune system have facilitated the identification of new biological targets for screening programs aimed at the discovery of novel immune stimulators. This resulted in the identification of new candidate adjuvants, which made possible the modulation of the immune responses elicited according to specific needs. A number of promising adjuvants which are currently under preclinical or clinical development will be described in this chapter.

Synthesis and immunostimulatory properties of the phosphorothioate analogues of cdiGMP

The synthesis of mono- and bisphosphorothioate analogues of 3',5'-cyclic diguanylic acid (cdiGMP) via the modified H-phosphonate chemistry is reported. The immunostimulatory properties of these analogues were compared with those of cdiGMP.

The bacterial second messenger cdiGMP exhibits promising activity as a mucosal adjuvant

The development of mucosal adjuvants is still a critical need in vaccinology. In the present work, we show that bis(3',5')-cyclic dimeric GMP (cdiGMP), a second messenger that modulates cell surface properties of several microorganisms, exerts potent activity as a mucosal adjuvant. BALB/c mice were immunized intranasally with the model antigen beta-galactosidase (beta-Gal) coadministered with cdiGMP. Animals receiving cdiGMP as an adjuvant showed significantly higher anti-beta-Gal immunoglobulin G (IgG) titers in sera than controls (i.e., 512-fold [P < 0.05]). Coadministration of cdiGMP also stimulated efficient beta-Gal-specific secretory IgA production in the lung (P < 0.016) and vagina (P < 0.036). Cellular immune responses were observed in response to both the beta-Gal protein and a peptide encompassing its major histocompatibility complex class I-restricted epitope. The IgG1-to-IgG2a ratio of anti-beta-Gal antibodies and the observed profiles of secreted cytokines suggest that a dominant Th1 response pattern is promoted by mucosal coadministration of cdiGMP. Finally, the use of cdiGMP as a mucosal adjuvant also led to the stimulation of in vivo cytotoxic T-lymphocyte responses in C57BL/6 mice intranasally immunized with ovalbumin and cdiGMP (up to 30% of specific lysis). The results obtained indicate that cdiGMP is a promising tool for the development of mucosal vaccines.