Administration of poly[di(sodium carboxylatoethylphenoxy)]phosphazene (PCEP) as adjuvant activated mixed Th1/Th2 immune responses in pigs

Adjuvants: Engineering Protective Immune Responses in Human and Veterinary Vaccines

Adjuvants are key components of many vaccines, used to enhance the level and breadth of the immune response to a target antigen, thereby enhancing protection from the associated disease. In recent years, advances in our understanding of the innate and adaptive immune systems have allowed for the development of a number of novel adjuvants with differing mechanisms of action. Herein, we review adjuvants currently approved for human and veterinary use, describing their use and proposed mechanisms of action. In addition, we will discuss additional promising adjuvants currently undergoing preclinical and/or clinical testing.

Next generation polyphosphazene immunoadjuvant: Synthesis, self-assembly and in vivo potency with human papillomavirus VLPs-based vaccine

Poly[di(carboxylatomethylphenoxy)phosphazene] (PCMP), a new member of polyphosphazene immunoadjuvant family, is synthesized. In vitro assessment of a new macromolecule revealed hydrolytic degradation profile and immunostimulatory activity comparable to its clinical stage homologue PCPP; however, PCMP was characterized by a beneficial reduced sensitivity to the ionic environment. In vivo evaluation of PCMP potency was conducted with human papillomavirus (HPV) virus-like particles (VLPs) based RG1-VLPs vaccine. In contrast with previously reported self-assembly of polyphosphazene adjuvants with proteins, which typically results in the formation of complexes with multimeric display of antigens, PCMP surface modified VLPs in a composition dependent pattern, which at a high polymer-to VLPs ratio led to stabilization of antigenic particles. Immunization experiments in mice demonstrated that PCMP adjuvanted RG1-VLPs vaccine induced potent humoral immune responses, in particular, on the level of highly desirable protective cross-neutralizing antibodies, and outperformed PCPP and Alhydrogel adjuvanted formulations.

Polyphosphazenes as Adjuvants for Animal Vaccines and Other Medical Applications

Polyphosphazenes are a class of experimental adjuvants that have shown great versatility as vaccine adjuvants in many animal species ranging from laboratory rodents to large animal species. Their adjuvant activity has shown promising results with numerous viral and bacterial antigens, as well as with crude and purified antigens. Vaccines adjuvanted with polyphosphazenes can be delivered via systemic and mucosal administration including respiratory, oral, rectal, and intravaginal routes. Polyphosphazenes can be used in combination with other adjuvants, further enhancing immune responses to antigens. The mechanisms of action of polyphosphazenes have not fully been defined, but several systematic studies have suggested that they act primarily by activating innate immunity. In the present review, we will highlight progress in the development of polyphosphazenes as adjuvants in animals and their other medical applications.

Polyphosphazene immunoadjuvants: Historical perspective and recent advances

The development of successful vaccines has been increasingly reliant on the use of immunoadjuvants - additives, which can enhance and modulate immune responses to vaccine antigens. Immunoadjuvants of the polyphosphazene family encompass synthetic biodegradable macromolecules, which attain in vivo activity via antigen delivery and immunostimulation mechanisms. Over the last decades, the technology has witnessed evolvement of next generation members, expansion to include various antigens and routes of administration, and progression to clinical phase. This was accompanied by gaining important insights into the mechanism of action and the development of a novel class of virus-mimicking nano-assemblies for antigen delivery. The present review evaluates in vitro and in vivo data generated to date in the context of latest advances in understanding the primary function and biophysical behavior of these macromolecules. It also provides an overview of relevant synthetic and characterization methods, macromolecular biodegradation pathways, and polyphosphazene-based multi-component, nanoparticulate, and microfabricated formulations.

Experimental PCEP-Adjuvanted Swine Influenza H1N1 Vaccine Induced Strong Immune Responses but Did Not Protect Piglets against Heterologous H3N2 Virus Challenge

Vaccination is the most efficient method of protection against influenza infections. However, the rapidly mutating viruses and development of new strains make it necessary to develop new influenza vaccines annually. Hence, vaccines that stimulate cross-protection against multiple influenza subtypes are highly sought. Recent evidence suggests that adjuvants such as PCEP that promote Th1-type T cell and Th2-type T cell immune responses and broad-spectrum immune responses may confer cross-protection against heterologous influenza strains. In this study, we evaluated whether the immunogenic and protective potential of PCEP-adjuvanted inactivated swine influenza virus H1N1 vaccine can protect pigs immunized against live H3N2 virus. Piglets were vaccinated via the intradermal route with PCEP-adjuvanted inactivated swine influenza virus (SIV) H1N1 vaccine, boosted at day 21 with the same vaccines then challenged with infectious SIV H3N2 virus at day 35 via the tracheobronchial route. The pigs showed significant anti-H1N1 SIV specific antibody titres and H1N1 SIV neutralizing antibody titres, and these serum titres remained after the challenge with the H3N2 virus. In contrast, vaccination with anti-H1N1 SIV did not trigger anti-H3N2 SIV antibody titres or neutralizing antibody titres and these titres remained low until pigs were challenged with H3N2 SIV. At necropsy (six days after challenge), we collected prescapular lymph nodes and tracheobronchial draining the vaccination sites and challenge site, respectively. ELISPOTs from lymph node cells restimulated ex vivo with inactivated SIV H1N1 showed significant production of IFN-γ in the tracheobronchial cells, but not the prescapular lymph nodes. In contrast, lymph node cells restimulated ex vivo with inactivated SIV H1N1 showed significantly higher IL-13 and IL-17A in the prescapular lymph nodes draining the vaccination sites relative to unchallenged animals. Lung lesion scores show that intradermal vaccination with H1N1 SIV plus PCEP did not prevent lesions when the animals were challenged with H3N2. These results confirm previous findings that PCEP is effective as a vaccine adjuvant in that it induces strong immune responses and protects against homologous swine influenza H1N1 virus, but the experimental H1N1 vaccine failed to cross-protect against heterologous H3N2 virus.

Main-Chain Phosphorus-Containing Polymers for Therapeutic Applications

Polymers in which phosphorus is an integral part of the main chain, including polyphosphazenes and polyphosphoesters, have been widely investigated in recent years for their potential in a number of therapeutic applications. Phosphorus, as the central feature of these polymers, endears the chemical functionalization, and in some cases (bio)degradability, to facilitate their use in such therapeutic formulations. Recent advances in the synthetic polymer chemistry have allowed for controlled synthesis methods in order to prepare the complex macromolecular structures required, alongside the control and reproducibility desired for such medical applications. While the main polymer families described herein, polyphosphazenes and polyphosphoesters and their analogues, as well as phosphorus-based dendrimers, have hitherto predominantly been investigated in isolation from one another, this review aims to highlight and bring together some of this research. In doing so, the focus is placed on the essential, and often mutual, design features and structure-property relationships that allow the preparation of such functional materials. The first part of the review details the relevant features of phosphorus-containing polymers in respect to their use in therapeutic applications, while the second part highlights some recent and innovative applications, offering insights into the most state-of-the-art research on phosphorus-based polymers in a therapeutic context.

Recent advances in experimental polyphosphazene adjuvants and their mechanisms of action

Vaccination continues to be a very important public health intervention to control infectious diseases in the world. Subunit vaccines are generally poorly immunogenic and require the addition of adjuvants to induce protective immune responses. Despite their critical role in vaccines, adjuvant mechanism of action remains poorly understood, which is a barrier to the development of new, safe and effective vaccines. In the present review, we focus on recent progress in understanding the mechanisms of action of the experimental adjuvants poly[di(carboxylatophenoxy)phosphazene] (PCPP) and poly[di(sodiumcarboxylatoethyl-phenoxy)phosphazene] (PCEP) (in this review, adjuvants PCPP and PCEP are collectively referred to as PZ denoting polyphosphazenes). PZs are high molecular weight, water-soluble, synthetic polymers that have been shown to regulate innate immune response genes, induce cytokines and chemokines secretion at the site of injection and, also, induce immune cell recruitment to the site of injection to create a local immune-competent environment. There is an evidence that as well as its role as an immunoadjuvant (that activate innate immune responses), PZ can also act as a vaccine carrier. The mechanism of action that explains how PZ leads to these effects is not known and is a barrier to the development of designer vaccines.

Intradermal immunization with inactivated swine influenza virus and adjuvant polydi(sodium carboxylatoethylphenoxy)phosphazene (PCEP) induced humoral and cell-mediated immunity and reduced lung viral titres in pigs

Swine influenza virus is endemic worldwide and it is responsible for significant economic losses to the swine industry. A vaccine that stimulates a rapid and long-lasting protective immune response to prevent this infection is highly sought. Poly[di(sodium carboxylatoethylphenoxy)-phosphazene (PCEP) has demonstrated adjuvant activity when formulated as part of multiple vaccines in mice and pigs. In this study we examined the magnitude and type of immune response induced in pigs vaccinated via the intramuscular or intradermal routes with inactivated swine influenza virus (SIV) H1N1 vaccine formulated with PCEP. Intradermal administration of PCEP-adjuvanted inactivated SIV vaccine stimulated significant anti-SIV antibody titres, increased neutralizing antibodies, and significantly reduced lung virus load with limited reduction of gross lung lesions after challenge with virulent H1N1 relative to control animals. These results indicate that PCEP may be effective as a vaccine adjuvant against swine influenza viruses in pigs and should be considered a potential candidate adjuvant for future swine intradermal influenza vaccines.

New insights about functional and cross-reactive properties of antibodies generated against recombinant TbpBs of Haemophilus parasuis

Vaccines have become fundamental in the control and elimination of Glässer Disease, a systemic disease of pigs caused by Haemophilus parasuis. The classic vaccines available for prevention of this infection were developed without a robust knowledge about host immunological mechanisms. In this study, we demonstrated the presence of cross-reactive epitopes on both the N-lobe and C-lobe of variants of transferrin binding protein B (TbpBs) expressed on the surface of 6 virulent serovars of H. parasuis. Antibodies against TbpB-derived antigens were capable of increasing the phagocytic capacity of neutrophils and were also capable of blocking porcine transferrin from binding to TbpB. Surprisingly, none of the pig or mice antisera from animals immunized with TbpB-derived antigens mixed with Montanide IMS 2215 VG PR adjuvant were able to activate the classical complement pathway (CCP). In contrast, antisera from mice immunized with TbpB-derived antigens adjuvanted with Freund’s adjuvants or Montanide Gel 01 were able to activate the CCP and kill H. parasuis. Our results demonstrate that the type of adjuvant can modulate the functional response induced by TbpB-derived antigens. Based on these results, we propose that a properly formulated TbpB-based vaccine may elicit a functional protective antibody response with broad cross-reactivity against heterologous strains of H. parasuis.

Adjuvants for Animal Vaccines

Vaccines are essential tools for the prevention and control of infectious diseases in animals. One of the most important steps in vaccine development is the selection of a suitable adjuvant. The focus of this review is the adjuvants used in vaccines for animals. We will discuss current commercial adjuvants and experimental formulations with attention to mineral salts, emulsions, bacterial-derived components, saponins, and several other immunoactive compounds. In addition, we will also examine the mechanisms of action for different adjuvants, examples of adjuvant combinations in one vaccine formulation, and challenges in the research and development of veterinary vaccine adjuvants.

A novel combination adjuvant platform for human and animal vaccines

Adjuvants are crucial components of many vaccines. They are used to improve the immunogenicity of vaccines with the aim of conferring long-term protection, to enhance the efficacy of vaccines in newborns, elderly or immunocompromised persons, and to reduce the amount of antigen or the number of doses required to elicit effective immunity. Novel combination adjuvants have been tested in both candidate animals and humans vaccines and have generated encouraging results. Recently, we developed a combination adjuvant platform (TriAdj) comprising of three components, namely a TLR agonist, either polyI:C or CpG oligodeoxynucleotides (ODN), host defense peptide and polyphosphazene. This adjuvant platform is stable and highly effective in a wide range of animal and human vaccines tested in mice, cotton rats, pigs, sheep, and koalas. TriAdj with various vaccines antigens induced effective long-term humoral and cellular immunity. Moreover, the adjuvant platform is suitable for maternal immunization and highly effective in neonates even in the presence of maternal antibodies. This novel vaccine platform, offers excellent opportunity for use in present and future generations of vaccines against multiple infectious agents and targets challenging populations.

Intranasal vaccination with an adjuvanted polyphosphazenes nanoparticle-based vaccine formulation stimulates protective immune responses in mice

The most promising strategy to sustainably prevent infectious diseases is vaccination. However, emerging as well as re-emerging diseases still constitute a considerable threat. Furthermore, lack of compliance and logistic constrains often result in the failure of vaccination campaigns. To overcome these hurdles, novel vaccination strategies need to be developed, which fulfill maximal safety requirements, show maximal efficiency and are easy to administer. Mucosal vaccines constitute promising non-invasive approaches able to match these demands. Here we demonstrate that nanoparticle (polyphosphazenes)-based vaccine formulations including c-di-AMP as adjuvant, cationic innate defense regulator peptides (IDR) and ovalbumin (OVA) as model antigen were able to stimulate strong humoral and cellular immune responses, which conferred protection against the OVA expressing influenza strain A/WSN/OVA_I (H1N1). The presented results confirm the potency of nanoparticle-based vaccine formulations to deliver antigens across the mucosal barrier, but also demonstrate the necessity to include adjuvants to stimulate efficient antigen-specific immune responses.

Immune responses to in ovo vaccine formulations containing inactivated fowl adenovirus 8b with poly[di(sodium carboxylatoethylphenoxy)]phosphazene (PCEP) and avian beta defensin as adjuvants in chickens

Inclusion body hepatitis (IBH) is one of the major viral infections causing substantial economic loss to the global poultry industry. The disease is characterized by a sudden onset of mortality (2-30%) and high morbidity (60-70%). IBH is caused by a number of serotypes of fowl adenovirus with substantially low levels of serotype cross protection. Thus far, there is no effective and safe vaccine commercially available in the North America for the control of IBH in chickens. Poly[di(sodium carboxylatoethylphenoxy)]phosphazene (PCEP) is a high molecular weight, biodegradable water soluble polymer that has been well characterized as a safe and effective adjuvant for a number of experimental veterinary vaccines. Similarly, host defence peptides, including β-defensins, have also been shown to exhibit strong adjuvant potential. In this study, we evaluated the adjuvant activity of PCEP and avian beta defensin (ABD) in a vaccine formulation containing inactivated fowl adenovirus (FAdV) serotype 8b administered in ovo. Our data showed that a combination of PCEP and inactivated virus is capable of inducing a robust and long lasting antibody response. Moreover, significant enhancement of IFN-γ, IFN-α, IL-12(p40) and IL-6 gene expression under the influence of PCEP suggests that as an in ovo adjuvant PCEP has the ability to activate a substantial balanced immune response in chickens. To our knowledge, these are the first studies in which PCEP and ABD have been characterized as adjuvants for the development of an in ovo poultry vaccine. It is expected that these preliminary studies will be helpful in the development of safer and more effective in ovo vaccine against IBH and other infectious diseases affecting chickens.

Antibody and Cytokine Responses of Koalas (Phascolarctos cinereus) Vaccinated with Recombinant Chlamydial Major Outer Membrane Protein (MOMP) with Two Different Adjuvants

Developing a vaccine against Chlamydia is key to combating widespread mortalities and morbidities associated with this infection in koalas (Phascolarctos cinereus). In previous studies, we have shown that two or three doses of a Recombinant Major Outer Membrane Protein (rMOMP) antigen-based vaccine, combined with immune stimulating complex (ISC) adjuvant, results in strong cellular and humoral immune responses in koalas. We have also separately evaluated a single dose vaccine, utilising a tri-adjuvant formula that comprises polyphosphazine based poly I: C and host defense peptides, with the same antigen. This formulation also produced strong cellular and humoral immune responses in captive koalas. In this current study, we directly compared the host immune responses of two sub-groups of wild Chlamydia negative koalas in one population vaccinated with the rMOMP protein antigen and adjuvanted with either the ISC or tri-adjuvant formula. Overall, both adjuvants produced strong Chlamydia-specific cellular (IFN-γ and IL-17A) responses in circulating PBMCs as well as MOMP-specific and functional, in vitro neutralising antibodies. While the immune responses were similar, there were adjuvant-specific immune differences between the two adjuvants, particularly in relation to the specificity of the MOMP epitope antibody responses.

Administration of Poly[di(sodium carboxylatoethylphenoxy)phosphazene] (PCEP) and Avian Beta Defensin as Adjuvants in Inactivated Inclusion Body Hepatitis Virus and its Hexon Protein-Based Experimental Vaccine Formulations in Chickens

Inclusion body hepatitis (IBH) is one of the major infectious diseases adversely affecting the poultry industry of the United States and Canada. Currently, no effective and safe vaccine is available for the control of IBH virus (IBHV) infection in chickens. However, based on the excellent safety and immunogenic profiles of experimental veterinary vaccines developed with the use of new generation adjuvants, we hypothesized that characterization of vaccine formulations containing inactivated IBHV or its capsid protein hexon as antigens, along with poly[di(sodium carboxylatoethylphenoxy)phosphazene] (PCEP) and avian beta defensin 2 (ABD2) as vaccine adjuvants, will be helpful in development of an effective and safe vaccine formulation for IBH. Our data demonstrated that experimental administration of vaccine formulations containing inactivated IBHV and a mixture of PCEP with or without ABD2 as an adjuvant induced significantly higher antibody responses compared with other vaccine formulations, while hexon protein-based vaccine formulations showed relatively lower levels of antibody responses. Thus, a vaccine formulation containing inactivated IBHV with PCEP or a mixture of PCEP and ABD2 (with a reduced dosage of PCEP) as an adjuvant may serve as a potential vaccine candidate. However, in order to overcome the risks associated with whole virus inactivated vaccines, characterization of additional viral capsid proteins, including fiber protein and penton of IBHV along with hexon protein in combination with more new generation adjuvants, will be helpful in further improvements of vaccines against IBHV infection.

Adjuvant-dependent immunogenicity of Staphylococcus aureus Efb and Map proteins in chickens

The avian IgY antibodies generated in hens and isolated from egg yolk have gained in popularity as they present an alternative source of antibodies for diagnostic as well as therapeutic applications. One of the advantages of IgY technology are the large amounts of produced antibodies from a single animal combined with their high reactivity representing an attractive alternative for mammalian antibodies. Despite many known protocols for the immunization of chickens, the administration of new antigens often requires additional modification such as antigen dose or use of an adjuvant in order to elicit a significant immune response. We investigated the immunogenicity of three Staphylococcus aureus antigens including two extracellular proteins Map and Efb and one selected Efb105-124 epitope conjugated to KLH that were administered to the animals. Additionally, the immunization protocol included two adjuvant systems: Freund's complete adjuvant and Emulsigen-D. The results demonstrated a high immunostimulatory potency of Freund's complete adjuvant, especially in case of Efb compared to the immune response elicited by Emulsigen-D. However, after immunization with the KLH-Efb105-124 conjugate, the obtained antibodies showed similar reactivity regardless of adjuvant system used with the only exception being their avidity.

Vaccination of koalas (Phascolarctos cinereus) with a recombinant chlamydial major outer membrane protein adjuvanted with poly I:C, a host defense peptide and polyphosphazine, elicits strong and long lasting cellular and humoral immune responses

Chlamydial infections are wide spread in koalas across their range and a solution to this debilitating disease has been sought for over a decade. Antibiotics are the currently accepted therapeutic measure, but are not an effective treatment due to the asymptomatic nature of some infections and a low efficacy rate. Thus, a vaccine would be an ideal way to address this infectious disease threat in the wild. Previous vaccine trials have used a three-dose regimen; however this is very difficult to apply in the field as it would require multiple capture events, which are stressful and invasive processes for the koala. In addition, it requires skilled koala handlers and a significant monetary investment. To overcome these challenges, in this study we utilized a polyphosphazine based poly I:C and a host defense peptide adjuvant combined with recombinant chlamydial major outer membrane protein (rMOMP) antigen to induce long lasting (54 weeks) cellular and humoral immunity in female koalas with a novel single immunizing dose. Immunized koalas produced a strong IgG response in plasma, as well as at mucosal sites. Moreover, they showed high levels of C. pecorum specific neutralizing antibodies in the plasma as well as vaginal and conjunctival secretions. Lastly, Chlamydia-specific lymphocyte proliferation responses were produced against both whole chlamydial elementary bodies and rMOMP protein, over the 12-month period. The results of this study suggest that a single dose rMOMP vaccine incorporating a poly I:C, host defense peptide and polyphosphazine adjuvant is able to stimulate both arms of the immune system in koalas, thereby providing an alternative to antibiotic treatment and/or a three-dose vaccine regime.

Caspase-1 Dependent IL-1β Secretion and Antigen-Specific T-Cell Activation by the Novel Adjuvant, PCEP

The potent adjuvant activity of the novel adjuvant, poly[di(sodiumcarboxylatoethylphenoxy)phosphazene] (PCEP), with various antigens has been reported previously. However, very little is known about its mechanisms of action. We have recently reported that intramuscular injection of PCEP induces NLRP3, an inflammasome receptor gene, and inflammatory cytokines, including IL-1β and IL-18, in mouse muscle tissue. Caspase-1 is required for the processing of pro-forms of IL-1β and IL-18 into mature forms and is a critical constituent of the NLRP3 inflammasome. Hence, in the present study, we investigated the role of caspase-1 in the secretion of IL-1β and IL-18 in PCEP-stimulated splenic dendritic cells (DCs). Caspase inhibitor YVAD-fmk-treated splenic DCs showed significantly reduced IL-1β and IL-18 secretion in response to PCEP stimulation. Further, PCEP had no effect on the expression of MHC class II or co-stimulatory molecules, CD86 and CD40, suggesting that PCEP does not induce DC maturation. However, PCEP directly activated B-cells to induce significant production of IgM. In addition, PCEP+ovalbumin (OVA) immunized mice showed significantly increased production of antigen-specific IFN-γ by CD4⁺ and CD8⁺ T-cells. We conclude that PCEP activates innate immunity, leading to increased antigen-specific T-cell responses.

Polyphosphazenes: Multifunctional, Biodegradable Vehicles for Drug and Gene Delivery

Poly[(organo)phosphazenes] are a unique class of extremely versatile polymers with a range of applications including tissue engineering and drug delivery, as hydrogels, shape memory polymers and as stimuli responsive materials. This review aims to divulge the basic principles of designing polyphosphazenes for drug and gene delivery and portray the huge potential of these extremely versatile materials for such applications. Polyphosphazenes offer a number of distinct advantages as carriers for bioconjugates; alongside their completely degradable backbone, to non-toxic degradation products, they possess an inherently and uniquely high functionality and, thanks to recent advances in their polymer chemistry, can be prepared with controlled molecular weights and narrow polydispersities, as well as self-assembled supra-molecular structures. Importantly, the rate of degradation/hydrolysis of the polymers can be carefully tuned to suit the desired application. In this review we detail the recent developments in the chemistry of polyphosphazenes, relevant to drug and gene delivery and describe recent investigations into their application in this field.