Roles of Aluminum Hydroxide and Monophosphoryl Lipid A Adjuvants in Overcoming CD4+ T Cell Deficiency To Induce Isotype-Switched IgG Antibody Responses and Protection by T-Dependent Influenza Vaccine

Silicon or Calcium Doping Coordinates the Immunostimulatory Effects of Aluminum Oxyhydroxide Nanoadjuvants in Prophylactic Vaccines

Aluminum salts still remain as the most popular adjuvants in marketed human prophylactic vaccines due to their capability to trigger humoral immune responses with a good safety record. However, insufficient induction of cellular immune responses limits their further applications. In this study, we prepare a library of silicon (Si)- or calcium (Ca)-doped aluminum oxyhydroxide (AlOOH) nanoadjuvants. They exhibit well-controlled physicochemical properties, and the dopants are homogeneously distributed in nanoadjuvants. By using Hepatitis B surface antigen (HBsAg) as the model antigen, doped AlOOH nanoadjuvants mediate higher antigen uptake and promote lysosome escape of HBsAg through lysosomal rupture induced by the dissolution of the dopant in the lysosomes in bone marrow-derived dendritic cells (BMDCs). Additionally, doped nanoadjuvants trigger higher antigen accumulation and immune cell activation in draining lymph nodes. In HBsAg and varicella-zoster virus glycoprotein E (gE) vaccination models, doped nanoadjuvants induce high IgG titer, activations of CD4+ and CD8+ T cells, cytotoxic T lymphocytes, and generations of effector memory T cells. Doping of aluminum salt-based adjuvants with biological safety profiles and immunostimulating capability is a potential strategy to mediate robust humoral and cellular immunity. It potentiates the applications of engineered adjuvants in the development of vaccines with coordinated immune responses.

Monophosphoryl lipid A and poly I:C combination enhances immune responses of equine influenza virus vaccine

Equine influenza is a contagious respiratory disease caused by H3N8 type A influenza virus. Vaccination against equine influenza is conducted regularly; however, infection still occurs globally because of the short immunity duration and suboptimal efficacy of current vaccines. Hence the objective of this study was to investigate whether an adjuvant combination can improve immune responses to equine influenza virus (EIV) vaccines. Seventy-two mice were immunized with an EIV vaccine only or with monophosphoryl lipid A (MPL), polyinosinic-polycytidylic acid (Poly I:C), or MPL + Poly I:C. Prime immunization was followed by boost immunization after 2 weeks. Mice were euthanized at 4, 8, and 32 weeks post-prime immunization, respectively. Sera were collected to determine humoral response. Bone marrow, spleen, and lung samples were harvested to determine memory cell responses, antigen-specific T-cell proliferation, and lung viral titers. MPL + Poly I:C resulted in the highest IgG, IgG1, and IgG2a antibodies and hemagglutination inhibition titers among the groups and sustained their levels until 32 weeks post-prime immunization. The combination enhanced memory B cell responses in the bone marrow and spleen. At 8 weeks post-prime immunization, the combination induced higher CD8+ central memory T cell frequencies in the lungs and CD8+ central memory T cells in the spleen. In addition, the combination group exhibited enhanced antigen-specific T cell proliferation, except for CD4+ T cells in the lungs. Our results demonstrated improved immune responses when using MPL + Poly I:C in EIV vaccines by inducing enhanced humoral responses, memory cell responses, and antigen-specific T cell proliferation.

Protective antigen of Bacillus anthracis in combination with TLR4 or TLR5 agonist confers superior protection against lethal challenge in mouse model

The immunogenicity and protective ability of recombinant PA (rPA) with two innate immune system modulators, i.e., monophosphoryl lipid A (MPLA), a TLR4 agonist, and recombinant flagellin C (FliC), a TLR5 agonist, were studied in the mouse model. BALB/c mice were inoculated with three doses of rPA + alum (Alum group), rPA + FliC + alum (FliC group), rPA + MPLA + alum (MPLA group), or only alum adjuvant (Alum alone group). Significant increases in anti-PA IgG titers were observed in the Alum, FliC and MPLA groups when compared to control Alum alone group. Similarly, a significant enhancement of proinflammatory (TNF-α, IL-1β), Th1 (IFN-γ, IL-12(p70), IL-2) and Th2 (IL-10, IL-4) cytokines were also noticed in Alum, FliC and MPLA groups compared to Alum alone group. The rPA-specific IgG and cytokine responses in MPLA and FliC groups were significantly higher than the Alum group, suggesting enhancement of immune response by these TLR agonists. MPLA was also found to skew the IgG1:IgG2a ratio towards IgG2a. At a challenge dose of 25 LD50, complete protection was observed in mice of MPLA group whereas lesser protection was observed in FliC (87%) and Alum (50%) groups. Therefore, we suggest the use of MPLA in further development of rPA based anthrax vaccines.

Comparative tissue proteomics reveals unique action mechanisms of vaccine adjuvants

Radiofrequency adjuvant (RFA) was recently developed to boost influenza vaccination without the safety concerns of chemical adjuvants due to their physical nature. Yet, the action mechanisms of RFA remain largely unknown. Omics techniques offer new opportunities to identify molecular mechanisms of RFA. This study utilized comparative tissue proteomics to explore molecular mechanisms of the physical RFA. Comparison of RFA and chemical adjuvant (Alum, AddaVax, MPL, MPL/Alum)-induced tissue proteome changes identified 14 exclusively induced proteins by RFA, among which heat shock protein (HSP) 70 was selected for further analysis due to its known immune-modulating functions. RFA showed much weakened ability to boost ovalbumin and pandemic influenza vaccination in HSP70 knockout than wild-type mice, hinting crucial roles of HSP70 in RFA effects. This study supports comparative tissue proteomics to be an effective tool to study molecular mechanisms of vaccine adjuvants.

Antibody class-switching as a strategy to improve HIV-1 neutralization

Broadly neutralizing antibodies (bNAbs), when administered through passive immunization, are protective against HIV-1 infection. Current HIV-1 vaccine strategies are aimed at guiding the immune system to make bNAbs by mimicking their development during infection. Somatic hypermutation of the variable region is known to be crucial for the development of bNAbs. More recently, however, studies have shown how class-switch recombination (CSR) resulting in the generation of different antibody isotypes may serve as an additional mechanism through which antibodies can gain neutralization breadth and potency. In this review, we discuss the importance of different antibody isotypes for HIV-1 neutralization breadth and potency and how this information can be leveraged to improve passive and active immunization against HIV-1.

Vaccine-associated enhanced respiratory pathology in COVID-19 hamsters after TH2-biased immunization

Vaccine-associated enhanced respiratory disease (VAERD) is a severe complication for some respiratory infections. To investigate the potential for VAERD induction in coronavirus disease 2019 (COVID-19), we evaluate two vaccine leads utilizing a severe hamster infection model: a T helper type 1 (T_H1)-biased measles vaccine-derived candidate and a T_H2-biased alum-adjuvanted, non-stabilized spike protein. The measles virus (MeV)-derived vaccine protects the animals, but the protein lead induces VAERD, which can be alleviated by dexamethasone treatment. Bulk transcriptomic analysis reveals that our protein vaccine prepares enhanced host gene dysregulation in the lung, exclusively up-regulating mRNAs encoding the eosinophil attractant CCL-11, T_H2-driving interleukin (IL)-19, or T_H2 cytokines IL-4, IL-5, and IL-13. Single-cell RNA sequencing (scRNA-seq) identifies lung macrophages or lymphoid cells as sources, respectively. Our findings imply that VAERD is caused by the concerted action of hyperstimulated macrophages and T_H2 cytokine-secreting lymphoid cells and potentially links VAERD to antibody-dependent enhancement (ADE). In summary, we identify the cytokine drivers and cellular contributors that mediate VAERD after T_H2-biased vaccination.

A chimeric thermostable M2e and H3 stalk-based universal influenza A virus vaccine

We developed a new chimeric M2e and H3 hemagglutinin (HA) stalk protein vaccine (M2e-H3 stalk) by genetic engineering of modified H3 stalk domain conjugated with conserved M2e epitopes to overcome the drawbacks of low efficacy by monomeric domain-based universal vaccines. M2e-H3 stalk protein expressed and purified from Escherichia coli was thermostable, displaying native-like antigenic epitopes recognized by antisera of different HA subtype proteins and influenza A virus infections. Adjuvanted M2e-H3 stalk vaccination induced M2e and stalk-specific IgG antibodies recognizing viral antigens on virus particles and on the infected cell surface, CD4⁺ and CD8⁺ T-cell responses, and antibody-dependent cytotoxic cell surrogate activity in mice. M2e-H3 stalk was found to confer protection against heterologous and heterosubtypic cross-group subtype viruses (H1N1, H5N1, H9N2, H3N2, H7N9) at similar levels in adult and aged mice. These results provide evidence that M2e-H3 stalk chimeric proteins can be developed as a universal influenza A virus vaccine candidate for young and aged populations.

Production of Neuraminidase Virus Like Particles by Stably Transformed Insect Cells: A Simple Process for NA-Based Influenza Vaccine Development

Neuraminidase (NA) is a second major surface protein of the influenza virus and has recently been suggested as a supplemental antigen to the major immunodominant hemagglutinin (HA) antigen in the influenza vaccine. NA is less affected by antigenic drift compared to the HA, induces strong anti-neuraminidase immune responses, and provides broader protection against many influenza strains. However, the NA amount in currently licensed influenza virus vaccines is much lower than that of HA, and not standardized. A platform to produce NA antigen, in the form of virus-like particles (VLPs), was thus developed, to facilitate supplementation of NA antigen in the influenza vaccine formula. Stably transformed Sf9 insect cells had been engineered to express the influenza A virus (H5N1) NA gene under a baculovirus OpMNPV IE2 promoter. Recombinant NA protein was synthesized and assembled into VLPs, in the intact cellular environment provided by insect cells. Approximately 150 µg/ml of NA-VLPs was obtained in the culture medium. Purification of the NA-VLPs was achieved by a sucrose density gradient ultracentrifugation. The purified NA-VLPs effectively induced anti-NA antibodies with neuraminidase inhibition activities in mice. This work demonstrates a simple process to produce an immunocompetent NA-VLPs antigen, exclusively made of only neuraminidase, by insect cells.

Thermostable H1 hemagglutinin stem with M2e epitopes provides broad cross-protection against group1 and 2 influenza A viruses

Hemagglutinin (HA) stem-based vaccines have limitations in providing broad and effective protection against cross-group influenza viruses, despite being a promising universal vaccine target. To overcome the limited cross-protection and low efficacy by HA stem vaccination, we genetically engineered a chimeric conjugate of thermostable H1 HA stem and highly conserved M2e repeat (M2e-H1stem), which was expressed at high yields in Escherichia coli. M2e-H1stem protein presented native-like epitopes reactive to antisera of live virus infection. M2e-H1stem protein vaccination of mice induced strong M2e- and HA stem-specific immune responses, conferring broadly effective cross-protection against both antigenically distinct group 1 (H1N1, H5N1, and H9N2 subtypes) and group 2 (H3N2 and H7N9 subtypes) seasonal and pandemic potential influenza viruses. M2e-H1stem vaccination generated CD4⁺ and CD8⁺ T cell responses and antibody-dependent cytotoxic cellular and humoral immunity, which contributed to enhancing cross-protection. Furthermore, comparable broad cross-group protection was observed in older aged mice after M2e-H1stem vaccination. This study provides evidence warranting further development of chimeric M2e-stem proteins as a promising universal influenza vaccine candidate in adult and aged populations.

Evaluation of concurrent vaccinations with recombinant canarypox equine influenza virus and inactivated equine herpesvirus vaccines

Despite vaccination, equine influenza virus (EIV) and equine herpesvirus (EHV) infections still cause highly contagious respiratory diseases in horses. Recently, concurrent vaccination with EIV and EHV was suggested as a new approach; however, there have been no reports of concurrent vaccination with recombinant canarypox EIV and inactivated EHV vaccines. In this study, we aimed to compare the EIV-specific immune responses induced by concurrent administrations of a recombinant canarypox EIV vaccine and an inactivated bivalent EHV vaccine with those induced by a single recombinant canarypox EIV vaccine in experimental horse and mouse models. Serum and peripheral blood mononuclear cells (PBMCs) were collected from immunized animals after vaccination. EIV-specific serum antibody levels, serum hemagglutinin inhibition (HI) titers, and interferon-gamma (IFN-γ) levels were measured by enzyme-linked immunosorbent assay, HI assay, and quantitative polymerase chain reaction, respectively. Concurrent EIV and EHV vaccine administration significantly increased IFN-γ production, without compromising humoral responses. Our data demonstrate that concurrent vaccination with EIV and EHV vaccines can enhance EIV-specific cellular responses in horses.

Vaccine Adjuvants Induce Formation of Intraperitoneal Extracellular Traps in Flounder (Paralichthys olivaceus)

Adjuvants are used to increase the strength, quality, and duration of the immune response of vaccines. Neutrophils are the first immune cells that arrive at the injection site and can release DNA fibers together with granular proteins, so-called neutrophil extracellular traps (NETs), to entrap microbes in a sticky matrix of extracellular chromatin and microbicidal agents. Similar extracellular structures were also released by macrophages, mast cells, and eosinophils and are now generalized as “ETs.” Here we demonstrated that Alum adjuvant stimulation led to peritoneal cells swarming and ET release in vitro. Moreover, compared to antigen stimulation alone, ET release was significantly increased after stimulation with antigen-mixed adjuvants and in a time- and dose-dependent manner. In vivo, we were able to monitor and quantify the continuous changes of the ET release in the same fish by using the small animal in vivo imaging instrument at different times during the early stages after intraperitoneal immunization. The results showed that the fluorescence signal of ETs in the peritoneum increased from 0 to 12 h after injection and then gradually decreased. The fluorescence signals came from extracellular DNA fibers, which are sensitive to DNase I and confirmed by microscopy of peritoneal fluid ex vivo. In summary, this study introduced a new method for detecting ETs in the peritoneum of fish in vivo and indicated that ET formation is involved in the immune response at the early stage after intraperitoneal immunization to vaccines.

Immunological effects of adjuvanted low-dose allergoid allergen-specific immunotherapy in experimental murine house dust mite allergy

BACKGROUND

Native allergen extracts or chemically modified allergoids are routinely used to induce allergen tolerance in allergen-specific immunotherapy (AIT), although mechanistic side-by-side studies are rare. It is paramount to balance optimal dose and allergenicity to achieve efficacy warranting safety. AIT safety and efficacy could be addressed by allergen dose reduction and/or use of allergoids and immunostimulatory adjuvants, respectively. In this study, immunological effects of experimental house dust mite (HDM) AIT were investigated applying high-dose HDM extract and low-dose HDM allergoids with and without the adjuvants microcrystalline tyrosine (MCT) and monophosphoryl lipid A (MPL) in a murine model of HDM allergy.

METHODS

Cellular, humoral, and clinical effects of the different AIT strategies were assessed applying a new experimental AIT model of murine allergic asthma based on physiological, adjuvant-free intranasal sensitization followed by subcutaneous AIT.

RESULTS

While low-dose allergoid and high-dose extract AIT demonstrated comparable potency to suppress allergic airway inflammation and Th2-type cytokine secretion of lung-resident lymphocytes and draining lymph node cells, low-dose allergoid AIT was less effective in inducing a potentially protective IgG1 response. Combining low-dose allergoid AIT with MCT or MCT and dose-adjusted MPL promoted Th1-inducing mechanisms and robust B-cell activation counterbalancing the allergic Th2 immune response.

CONCLUSION

Low allergen doses induce cellular and humoral mechanisms counteracting Th2-driven inflammation by using allergoids and dose-adjusted adjuvants. In light of safety and efficacy improvement, future therapeutic approaches may use low-dose allergoid strategies to drive cellular tolerance and adjuvants to modulate humoral responses.

Comparison of the effects of different potent adjuvants on enhancing the immunogenicity and cross-protection by influenza virus vaccination in young and aged mice

Vaccination against influenza viruses suffers from low efficacy in conferring homologous and cross-protection, particularly in older adults. Here, we compared the effects of three different adjuvant types (QS-21+MPL, CpG+MPL and bacterial cell wall CWS) on enhancing the immunogenicity and homologous and heterosubtypic protection of influenza vaccination in young adult and aged mouse models. A combination of saponin QS-21 and monophosphoryl lipid A (QS-21+MPL) was most effective in inducing T helper type 1 (Th1) T cell and cross-reactive IgG as well as hemagglutination inhibiting antibody responses to influenza vaccination. Both combination adjuvants (QS-21+MPL and CpG+MPL) exhibited high potency by preventing weight loss and reducing viral loads and enhanced homologous and cross-protection by influenza vaccination in adult and aged mouse models. Bacillus Calmette-Guerin cell-wall skeleton (CWS) displayed substantial adjuvant effects on immune responses to influenza vaccination but lower adjuvant efficacy in inducing Th1 IgG responses, cross-protection in adult mice, and in conferring homologous protection in aged mice. This study has significance in comparing the effects of potent adjuvants on enhancing humoral and cellular immune responses to influenza virus vaccination, inducing homologous and cross-protection in adult and aged populations.

Protective efficacy of Hla-MntC-SACOL0723 fusion protein adjuvanted in alum and MPL against Staphylococcus aureus sepsis infection in mice

To develop a suitable and effective vaccine against Staphylococcus aureus (S. aureus), we selected the Hla-MntC-SACOL0723 (HMS) recombinant protein with two different formulations of alum and Monophosphoryl lipid A (MPL) adjuvants. In this study, we aimed to evaluate the potentials of alum and MPL adjuvants in stimulating the immune response of HMS vaccine candidate against S. aureus. To evaluate the type of induced immune response, anti-HMS total IgG, IgG1, IgG2a, and IFN-γ, IL-2, IL-4, and IL-17 cytokines were determined after vaccination of mice with HMS-alum, HMS-MPL candidates. Mice were challenged with Methicillin-resistant Staphylococcus aureus (MRSA) was isolated from pressure sores and evaluated for bacterial load in the kidney homogenates and survival rate. It was observed that total IgG and isotypes (IgG1 and IgG2a), IL-4, and IL-17 were significantly increased in the group that received HMS-alum vaccine compared with the group that received HMS-MPL formulation. On the other hand, the levels of IFN-γ and IL-2 cytokines in the group that received HMS-MPL were higher than the group that received HMS-alum formulation. Bacterial load in the mice who received HMS protein formulated with alum adjuvant was reduced more than the mice who received HMS protein formulated with MPL adjuvant. Histopathological analysis showed more pathological changes in kidney tissues of the group received of HMS-MPL compared with the HMS-alum formulation. The survival rate was equal in both groups of immunized with HMS-alum and HMS-MPL formulations. Finally, it could be concluded that both adjuvants of alum and MPL are suitable immune response enhancers to HMS vaccine candidate.

Design and application of nanoparticles as vaccine adjuvants against human corona virus infection

In recent years, some viruses have caused a grave crisis to global public health, especially the human coronavirus. A truly effective vaccine is therefore urgently needed. Vaccines should generally have two features: delivering antigens and modulating immunity. Adjuvants have an unshakable position in the battle against the virus. In addition to the perennial use of aluminium adjuvant, nanoparticles have become the developing adjuvant candidates due to their unique properties. Here we introduce several typical nanoparticles and their antivirus vaccine adjuvant applications. Finally, for the combating of the coronavirus, we propose several design points, hoping to provide ideas for the development of personalized vaccines and adjuvants and accelerate the clinical application of adjuvants.

Lost in Translation: Lack of CD4 Expression due to a Novel Genetic Defect

CD4 expression identifies a subset of mature T cells primarily assisting the germinal center reaction and contributing to CD8+ T-cell and B-cell activation, functions, and longevity. Herein, we present a family in which a novel variant disrupting the translation-initiation codon of the CD4 gene resulted in complete loss of membrane and plasma soluble CD4 in peripheral blood, lymph node, bone marrow, skin, and ileum of a homozygous proband. This inherited CD4 knockout disease illustrates the clinical and immunological features of a complete deficiency of any functional component of CD4 and its similarities and differences with other clinical models of primary or acquired loss of CD4+ T cells. The first inherited loss of any functional component of CD4, including soluble CD4, is clinically distinct from any other congenital or acquired CD4 T-cell defect and characterized by compensatory changes in T-cell subsets and functional impairment of B cells, monocytes, and natural killer cells.

Soil-Transmitted Helminth Vaccines: Are We Getting Closer?

Parasitic helminths infect over one-fourth of the human population resulting in significant morbidity, and in some cases, death in endemic countries. Despite mass drug administration (MDA) to school-aged children and other control measures, helminth infections are spreading into new areas. Thus, there is a strong rationale for developing anthelminthic vaccines as cost-effective, long-term immunological control strategies, which, unlike MDA, are not haunted by the threat of emerging drug-resistant helminths nor limited by reinfection risk. Advances in vaccinology, immunology, and immunomics include the development of new tools that improve the safety, immunogenicity, and efficacy of vaccines; and some of these tools have been used in the development of helminth vaccines. The development of anthelminthic vaccines is fraught with difficulty. Multiple lifecycle stages exist each presenting stage-specific antigens. Further, helminth parasites are notorious for their ability to dampen down and regulate host immunity. One of the first significant challenges in developing any vaccine is identifying suitable candidate protective antigens. This review explores our current knowledge in lead antigen identification and reports on recent pre-clinical and clinical trials in the context of the soil-transmitted helminths Trichuris, the hookworms and Ascaris. Ultimately, a multivalent anthelminthic vaccine could become an essential tool for achieving the medium-to long-term goal of controlling, or even eliminating helminth infections.

Adjuvant effects of killed Lactobacillus casei DK128 on enhancing T helper type 1 immune responses and the efficacy of influenza vaccination in normal and CD4-deficient mice

Lactic acid bacteria Lactobacillus casei DK128 isolated from fermented vegetable foods was suggested to stimulate innate immune responses. Here, we investigated whether heat-killed DK128 would exhibit adjuvant effects on enhancing the efficacy of influenza vaccination. Immunization of mice with split influenza virus vaccine in the presence of heat-killed DK128 induced significantly higher levels of both IgG1 and IgG2c isotype antibodies than those by vaccine only. A single dose DK128-adjuvanted influenza vaccination conferred higher efficacy of protection, as evidenced by intact lung function, less weight loss, enhanced clearance of lung viral loads, and lower levels of inflammatory cytokines and infiltrates. Immunization of CD4 T cell-knockout (CD4KO) mice with influenza vaccine and DK128, but not with vaccine alone, induced isotype-switched IgG antibodies and protection against lethal challenge in CD4KO mice. The results in this study suggest heat-killed DK128 as a potential vaccine adjuvant, promoting the induction of IgG isotype switching in CD4-deficient condition and enhancing protective efficacy of split influenza vaccination in immunocompromised and immune-competent subjects.

Rational Design of Zika Virus Subunit Vaccine with Enhanced Efficacy

Zika virus (ZIKV) infection in pregnant women can lead to fetal deaths and malformations. We have previously reported that ZIKV envelope protein domain III (EDIII) is a subunit vaccine candidate with cross-neutralization activity; however, like many other subunit vaccines, its efficacy is limited. To improve the efficacy of this subunit vaccine, we identified a nonneutralizing epitope on ZIKV EDIII surrounding residue 375, which is buried in the full-length envelope protein but becomes exposed in recombinant EDIII. We then shielded this epitope with an engineered glycan probe. Compared to the wild-type EDIII, the mutant EDIII induced significantly stronger neutralizing antibodies in three mouse strains and also demonstrated significantly improved efficacy by fully protecting mice, particularly pregnant mice and their fetuses, against high-dose lethal ZIKV challenge. Moreover, the mutant EDIII immune sera significantly enhanced the passive protective efficacy by fully protecting mice against lethal ZIKV challenge; this passive protection was positively associated with neutralizing antibody titers. We further showed that the enhanced efficacy of the mutant EDIII was due to the shielding of the immunodominant nonneutralizing epitope surrounding residue 375, which led to immune refocusing on the neutralizing epitopes. Taken together, the results of this study reveal that an intrinsic limitation of subunit vaccines is their artificially exposed immunodominant nonneutralizing epitopes, which can be overcome through glycan shielding. Additionally, the mutant ZIKV protein generated in this study is a promising subunit vaccine candidate with high efficacy in preventing ZIKV infections in mice.IMPORTANCE Viral subunit vaccines generally show low efficacy. In this study, we revealed an intrinsic limitation of subunit vaccine designs: artificially exposed surfaces of subunit vaccines contain epitopes unfavorable for vaccine efficacy. More specifically, we identified an epitope on Zika virus (ZIKV) envelope protein domain III (EDIII) that is buried in the full-length envelope protein but becomes exposed in recombinant EDIII. We further shielded this epitope with a glycan, and the resulting mutant EDIII vaccine demonstrated significantly enhanced efficacy over the wild-type EDIII vaccine in protecting animal models from ZIKV infections. Therefore, the intrinsic limitation of subunit vaccines can be overcome through shielding these artificially exposed unfavorable epitopes. The engineered EDIII vaccine generated in this study is a promising vaccine candidate that can be further developed to battle ZIKV infections.

Neuraminidase expressing virus-like particle vaccine provides effective cross protection against influenza virus

Neuraminidase is the second major surface antigen on influenza virus. We investigated the immunogenicity and cross protective efficacy of virus-like particle containing neuraminidase derived from 2009 pandemic H1N1 influenza virus (N1 VLP) in comparison with inactivated split influenza vaccine. Immunization of mice with N1 VLP induced antibody responses specific for virus and cross-reactive neuraminidase inhibition activity whereas an inactivated split vaccine induced strain-specific hemagglutination inhibition activity. N1 VLP-immunized mice developed cross protective immunity against antigenically different influenza viruses, as determined by body weight changes, lung viral titers, infiltrating innate immune cells, and cytokines, and antibody secreting cells, and germinal center B cells. Also, N1 VLP-immune sera provided cross-protection in naïve mice. Immunity by N1 VLP vaccination was not compromised in Fc receptor γ-chain deficient mice. These results suggest that neuraminidase-presenting VLP can be developed as an effective cross-protective vaccine candidate along with current influenza vaccination.

Respiratory Syncytial Virus Fusion Protein-encoding DNA Vaccine Is Less Effective in Conferring Protection against Inflammatory Disease than a Virus-like Particle Platform

Formalin-inactivated respiratory syncytial virus (RSV) vaccination causes vaccine-enhanced disease (VED) after RSV infection. It is considered that vaccine platforms enabling endogenous synthesis of RSV immunogens would induce favorable immune responses than non-replicating subunit vaccines in avoiding VED. Here, we investigated the immunogenicity, protection, and disease in mice after vaccination with RSV fusion protein (F) encoding plasmid DNA (F-DNA) or virus-like particles presenting RSV F (F-VLP). F-DNA vaccination induced CD8 T cells and RSV neutralizing Abs, whereas F-VLP elicited higher levels of IgG2a isotype and neutralizing Abs, and germinal center B cells, contributing to protection by controlling lung viral loads after RSV challenge. However, mice that were immunized with F-DNA displayed weight loss and pulmonary histopathology, and induced F specific CD8 T cell responses and recruitment of monocytes and plasmacytoid dendritic cells into the lungs. These innate immune parameters, RSV disease, and pulmonary histopathology were lower in mice that were immunized with F-VLP after challenge. This study provides important insight into developing effective and safe RSV vaccines.

Flagellin-expressing virus-like particles exhibit adjuvant effects on promoting IgG isotype-switched long-lasting antibody induction and protection of influenza vaccines in CD4-deficient mice

Incorporation of membrane-anchored flagellin molecules into the surfaces of influenza virus-like particles (VLP) was previously reported to promote T helper (Th) 1-biased IgG antibody production and protective efficacy of co-presented vaccine antigens. Herein, we investigated the potential adjuvant effects and mechanisms of flagellin-expressing VLP (FliC-VLP) as an independent component on influenza vaccination in wild-type and mutant mouse models. FliC-VLP adjuvanted influenza vaccination was highly effective in promoting the induction of Th1-biased IgG isotype switched antibodies, enhanced protection, and long-lasting IgG antibody responses in both wild-type and CD4-knockout mice. In contrast, the adjuvant effects of soluble flagellin were Th2-biased and required CD4 T helper cells. The adjuvant effects of FliC-VLP were less dependent on CD4 T cells and flagellin-mediated innate immune signaling pathways. The results suggest that FliC-VLP might play an effective adjuvant role in an immune competent condition as well as in a defect of CD4 T cells.

Chemical and Immunological Characteristics of Aluminum-Based, Oil-Water Emulsion, and Bacterial-Origin Adjuvants

Adjuvants are a diverse family of substances whose main objective is to increase the strength, quality, and duration of the immune response caused by vaccines. The most commonly used adjuvants are aluminum-based, oil-water emulsion, and bacterial-origin adjuvants. In this paper, we will discuss how the election of adjuvants is important for the adjuvant-mediated induction of immunity for different types of vaccines. Aluminum-based adjuvants are the most commonly used, the safest, and have the best efficacy, due to the triggering of a strong humoral response, albeit generating a weak induction of cell-mediated immune response. Freund's adjuvant is the most widely used oil-water emulsion adjuvant in animal trials; it stimulates inflammation and causes aggregation and precipitation of soluble protein antigens that facilitate the uptake by antigen-presenting cells (APCs). Adjuvants of bacterial origin, such as flagellin, E. coli membranes, and monophosphoryl lipid A (MLA), are known to potentiate immune responses, but their safety and risks are the main concern of their clinical use. This minireview summarizes the mechanisms that classic and novel adjuvants produce to stimulate immune responses.

Highly immunogenic influenza virus-like particles containing B-cell-activating factor (BAFF) for multi-subtype vaccine development

Virus-like particle (VLP) technology is an attractive platform for the development of seasonal and pandemic influenza vaccines. Influenza VLPs can be obtained by the overexpression of HA, M1, NA, and/or M2 viral proteins in insect, mammalian, or plant cells. In this study, we reported to obtain highly immunogenic influenza VLPs by molecular incorporation with B-cell-activating factor (BAFF) or proliferation-inducing ligand (APRIL). Since BAFF and APRIL act as homotrimers to interact with their receptors, we engineered the VLPs by direct fusion of BAFF or APRIL to the transmembrane anchored domain of H5HA gene. Results showed that immunizations with the HA-transmembrane anchored BAFF- or APRIL-VLPs only formulated in alum but not MPL adjuvant elicited significantly higher IgG titers in sera. However, only the BAFF-VLPs formulated in alum adjuvant elicited more broadly neutralizing antibodies against the homologous and two heterologous H5N1 clade/subclade viruses and conferred protective immunity against live virus challenges. As the multi-subtype influenza vaccines containing a variety of HA subtypes can confer broader protective immunity, we also obtained multi-subtype H5H7 BAFF-VLPs and H1H5H7 BAFF-VLPs and demonstrated that these multi-subtype BAFF-VLPs were able to induce the production of neutralizing antibodies against multiple HA subtypes. Our findings provided useful information for the development of highly immunogenic, multi-subtype influenza VLP vaccines.

Complement C3 Plays a Key Role in Inducing Humoral and Cellular Immune Responses to Influenza Virus Strain-Specific Hemagglutinin-Based or Cross-Protective M2 Extracellular Domain-Based Vaccination

The complement pathway is involved in eliminating antigen immune complexes. However, the role of the C3 complement system remains largely unknown in influenza virus M2 extracellular (M2e) domain or hemagglutinin (HA) vaccine-mediated protection after vaccination. Using a C3 knockout (C3 KO) mouse model, we found that complement protein C3 was required for effective induction of immune responses to vaccination with M2e-based or HA-based vaccines, which include isotype class-switched antibodies and effector CD4 and CD8 T cell responses. C3 KO mice after active immunization with cross-protective nonneutralizing M2e-based vaccine were not protected against influenza virus, although low levels of M2e-specific antibodies were protective after passive coadministration with virus in wild-type mice. In contrast, C3 KO mice that were immunized with strain-specific neutralizing HA-based vaccine were protected against homologous virus challenge despite lower levels of HA antibody responses. C3 KO mice showed impaired maintenance of innate immune cells and a defect in innate immune responses upon exposure to antigens. The findings in this study suggest that C3 is required for effective induction of humoral and cellular adaptive immune responses as well as protective immunity after nonneutralizing influenza M2e vaccination.IMPORTANCE Complement is the well-known innate immune defense system involved in the opsonization and lysis of pathogens but is less studied in establishing adaptive immunity after vaccination. Influenza virus HA-based vaccination confers protection via strain-specific neutralizing antibodies, whereas M2e-based vaccination induces a broad spectrum of protection by immunity against the conserved M2e epitopes. This study revealed the critical roles of C3 complement in inducing humoral and cellular immune responses after immunization with M2e or HA vaccines. C3 was found to be required for protection by M2e-based but not by HA-based active vaccination as well as for maintaining innate antigen-presenting cells. Findings in this study have insight into better understanding the roles of C3 complement in inducing effective innate and adaptive immunity as well as in conferring protection by cross-protective conserved M2e vaccination.

Virus-like particles presenting flagellin exhibit unique adjuvant effects on eliciting T helper type 1 humoral and cellular immune responses to poor immunogenic influenza virus M2e protein vaccine

Current licensed adjuvants including aluminum hydroxide (alum) bias immune responses toward T helper type 2 (Th2) immune responses. We tested whether virus-like particles presenting flagellin (Flag VLP) exhibit adjuvant effects on eliciting Th1 type immune responses and improving the efficacy of poor immunogenic tandem repeat M2e (M2e5x) protein vaccine against influenza virus. Co-immunization of mice with Flag VLP and M2e5x protein vaccine induced significantly higher levels of IgG2a isotype (Th1) antibodies in sera and mucosal sites, effector CD4⁺ T cells secreting IFN-γ and granzyme B, and more effective lung viral clearance and protection compared to alum adjuvant. Flag VLP stimulated primary macrophages and dendritic cells to secrete inflammatory cytokines, which is partially dependent on the Toll-like receptor 5. This study provides insight into developing effective vaccine adjuvants.

Immunology and efficacy of MF59-adjuvanted vaccines

Adjuvants are included in vaccine formulations to enhance the immunogenicity and efficacy of vaccines. MF59® is an oil-in-water emulsion adjuvant and licensed for use in pandemic and seasonal influenza vaccines in many countries. MF59 is safe and well tolerated in humans. MF59-adjuvanted vaccination spares vaccine dose and enhances hemagglutination inhibiting antibodies against homologous and heterologous influenza virus strains. The mechanisms of MF59 involve rapid induction of chemokines, inflammatory cytokines, recruiting multiple immune cells, uric acid and benign apoptosis of certain innate immune cells. The adjuvant effects of MF59 on generating vaccine-specific isotype-switched IgG antibodies, effector CD8 T cells, and protective immunity were retained even in a CD4-deficient condition by inducing effective immune-competent microenvironment with various innate and antigen presenting cells in a mouse model. CD4-independent adjuvant effects of MF59 might contribute to improving the vaccine efficacy in children, the elderly, and immunocompromised patients as well as in healthy adults. Further studies will be needed to broaden the use of MF59 in various vaccine antigens and populations as well as lead to better understanding of the action mechanisms of MF59 adjuvant.

Soluble F proteins exacerbate pulmonary histopathology after vaccination upon respiratory syncytial virus challenge but not when presented on virus-like particles

Respiratory syncytial virus (RSV) fusion (F) protein is suggested to be a protective vaccine target although its efficacy and safety concerns remain not well understood. We investigated immunogenicity, efficacy, and safety of F proteins in a soluble form or on virus-like particle (F-VLP). F VLP preferentially elicited IgG2a antibody and T helper type 1 (Th1) immune responses whereas F protein induced IgG1 isotype and Th2 responses. Despite lung viral clearance after prime or prime-boost and then RSV challenge, F protein immune mice displayed weight loss and lung histopathology and high mucus production and eosinophils. In contrast, prime or prime-boost vaccination of F VLP induced effective protection, prevented infiltration of eosinophils and vaccine- enhanced disease after challenge. This study provides insight into developing an effective and safe RSV vaccine candidate.

Humoral and cell-mediated immune responses to H5N1 plant-made virus-like particle vaccine are differentially impacted by alum and GLA-SE adjuvants in a Phase 2 clinical trial

The hemagglutinination inhibition (HI) response remains the gold standard used for the licensure of influenza vaccines. However, cell-mediated immunity (CMI) deserves more attention, especially when evaluating H5N1 influenza vaccines that tend to induce poor HI response. In this study, we measured the humoral response (HI) and CMI (flow cytometry) during a Phase II dose-ranging clinical trial (NCT01991561). Subjects received two intramuscular doses, 21 days apart, of plant-derived virus-like particles (VLP) presenting the A/Indonesia/05/2005 H5N1 influenza hemagglutinin protein (H5) at the surface of the VLP (H5VLP). The vaccine was co-administrated with Alhydrogel^® or with a glucopyranosyl lipid adjuvant-stable emulsion (GLA-SE). We demonstrated that low doses (3.75 or 7.5 μg H5VLP) of GLA-SE-adjuvanted vaccines induced HI responses that met criteria for licensure at both antigen doses tested. Alhydrogel adjuvanted vaccines induced readily detectable HI response that however failed to meet licensure criteria at any of three doses (10, 15 and 20 μg) tested. The H5VLP also induced a sustained (up to 6 months) polyfunctional and cross-reactive HA-specific CD4⁺ T cell response in all vaccinated groups. Interestingly, the frequency of central memory Th1-primed precursor cells before the boost significantly correlated with HI titers 21 days after the boost. The ability of the low dose GLA-SE-adjuvanted H5VLP to elicit both humoral response and a sustained cross-reactive CMI in healthy adults is very attractive and could result in significant dose-sparing in a pandemic situation.

Cross-Protective Efficacy of Influenza Virus M2e Containing Virus-Like Particles Is Superior to Hemagglutinin Vaccines and Variable Depending on the Genetic Backgrounds of Mice

Influenza virus M2 extracellular domain (M2e) has been a target for developing cross-protective vaccines. However, the efficacy and immune correlates of M2e vaccination are poorly understood in the different host genetic backgrounds in comparison with influenza vaccines. We previously reported the cross-protective efficacy of virus-like particle (M2e5x VLP) vaccines containing heterologous tandem M2e repeats (M2e5x) derived from human, swine, and avian influenza viruses. In this study to gain better understanding of cross-protective influenza vaccines, we compared immunogenicity and efficacy of M2e5x VLP, H5 hemagglutinin VLP (HA VLP), and inactivated H3N2 virus (H3N2i) in wild-type strains of BALB/c and C57BL/6 mice, and CD4 and CD8 knockout (KO) mice. M2e5x VLP was superior to HA VLP in conferring cross-protection whereas H3N2i inactivated virus vaccine provided high efficacy of homologous protection. After M2e5x VLP vaccination and challenge, BALB/c mice induced higher IgG responses, lower lung viral loads, and less body weight loss when compared with those in C57BL/6 mice. M2e5x VLP but not H3N2i immune mice after primary challenges developed strong immunity against a secondary heterosubtypic virus as a model of future pandemics. M2e5x VLP and HA VLP vaccines were able to raise IgG isotypes in CD4 KO mice. T cells were found to contribute to cross-protection by playing a role in reducing lung viral loads. In conclusion, M2e5x VLP vaccination induced better cross-protection than HA VLP, and its efficacy varied depending on the genetic backgrounds of mice, supporting the important roles of T cells.

Intranasal co-administration of 1,8-cineole with influenza vaccine provide cross-protection against influenza virus infection

BACKGROUND

Vaccination is the most efficient means for protection against influenza. However, the various vaccines have low efficacy to protect against pandemic strains because of antigenic drift and recombination of influenza virus. Adjuvant therapy is one of the attempts to improve influenza vaccine effective cross-protection against influenza virus infection. Our previous study confirmed that 1,8-cineole inhibits the NF-κB, reduces pro-inflammatory cytokines, and relieves the pathological changes of viral pneumonia in mice infected with influenza virus.

HYPOTHESIS/PURPOSE

1,8-cineole, administered via intranasal (i.n.) route, may also have the capacity to be an adjuvant of the influenza vaccine. This study was designed to investigate the potential use of i.n. co-administration of 1,8-cineole, a major component of the Eucalyptus essential oils, with influenza vaccine and whether could provide cross-protection against influenza virus infection in a mouse model.

STUDY DESIGN

I.n. co-administration of 1,8-cineole in two doses (6.25 and 12.5 mg/kg) with influenza vaccine was investigated in a mouse model in order to see whether it could provide cross-protection against influenza virus infection.

METHODS

The mice were intranasally immunized three times at the 0, 7 and 14 day with vaccine containing 0.2 µg hemagglutinin (HA) and/or without 1,8-cineole. Seven days after the 3rd immunization dose, the mice were infected with 50 µl of 15 LD₅₀ (50% mouse lethal dose) influenza virus A/FM/1/47 (H1N1). On day 6 post-infection, 10 mice per group were sacrificed to collect samples, to take the body weight and lung, and detect the viral load, pathological changes in the lungs and antibody, etc. The collected samples included blood serum and nasal lavage fluids. In addition, the survival experiments were carried out  to investigate the survival of mice.

RESULTS

Mice i.n. inoculated with influenza vaccine and 12.5 mg/kg 1,8-cineole increased the production of influenza-specific serum immunoglobulin (Ig) G2a antibodies, stimulated mucosal secretive IgA (s-IgA) responses at the nasal cavity, improved the expression of respiratory tract intraepithelial lymphocytes (IELs) in the upper respiratory tract, and promoted dendritic cell (DC) maturation and the expression of co-stimulatory molecules cluster of differentiation (CD)40, CD80 and CD86 in peripheral blood. Importantly, mice that had received 1,8-cineole-supplemented influenza vaccine showed longer survival time, milder inflammation, less weight loss and mortality rate and lower lung index and viral titers compared to that of mice immunized a non-1,8-cineole-adjuvanted split vaccine. Thus, i.n. immunization with 1,8-cineole-adjuvanted vaccine induces a superior cross-protective immunity against infection with influenza than an inactivated vaccine only.

CONCLUSION

These results suggest that 1,8-cineole (12.5 mg/kg) has a cross-protection against influenza virus, co-administered with inactivated influenza viral antigen in a mouse model.

Roles of antibodies to influenza A virus hemagglutinin, neuraminidase, and M2e in conferring cross protection

Although neuraminidase (NA) is the second major viral glycoprotein of influenza virus, its immune mechanism as a vaccine target has been less considered. Here we compared the properties of antibodies and the efficacy of cross protection by N1 and N2 NA proteins, inactivated split influenza vaccines (split), and tandem repeat extracellular domain M2 on virus-like particles (M2e5x VLP). Anti-NA immune sera could confer better cross-protection against multiple heterologous influenza viruses correlating with NA inhibition activity compared to split vaccine immune sera. Whereas split vaccine was superior to NA in conferring homologous protection. NA and M2e immune sera each showed comparable survival protection. Protective efficacy by NA immune sera was lower in Fc receptor common γ-chain deficient mice but comparable in C3 complement deficient mice compared to that in wild type mice, suggesting a role of Fc receptor in NA immunity.