Utilization of Fc receptors as a mucosal vaccine strategy against an intracellular bacterium, Francisella tularensis

Recombinant immune complexes as vaccines against infectious diseases

New vaccine technologies are needed to combat many existing infections and prepare better for those that may emerge in the future. The conventional technologies that rely on protein-based vaccines are still severely restricted by the sparsity and poor accessibility of available adjuvants. One possible solution to this problem is to enhance antigen immunogenicity by a more natural means by complexing it with antibodies in the form of immune complexes (ICs). However, natural ICs are impractical as vaccines, and significant research efforts have been made to generate them in recombinant form, with plant bioengineering being at the forefront of these efforts. Here, we describe the challenges and progress made to date to make recombinant IC vaccines applicable to humans.

Production and Evaluation of Ag85B:HspX:hFcγ1 Immunogenicity as an Fc Fusion Recombinant Multi-Stage Vaccine Candidate Against Mycobacterium tuberculosis

An urgent need is to introduce an effective vaccine against Mycobacterium tuberculosis (M.tb) infection. In the present study, a multi-stage M.tb immunodominant Fcγ1 fusion protein (Ag85B:HspX:hFcγ1) was designed and produced, and the immunogenicity of purified protein was evaluated. This recombinant fusion protein was produced in the Pichia pastoris expression system. The HiTrap-rPA column affinity chromatography purified and confirmed the fusion protein using ELISA and Western blotting methods. The co-localisation assay was used to confirm its proper folding and function. IFN-γ, IL-12, IL-4, and TGF-β expression in C57BL/6 mice then evaluated the immunogenicity of the construct in the presence and absence of BCG. After expression optimisation, medium-scale production and the Western blotting test confirmed suitable production of Ag85B:HspX:hFcγ1. The co-localisation results on antigen-presenting cells (APCs) showed that Ag85B:HspX:hFcγ1 properly folded and bound to hFcγRI. This strong co-localisation with its receptor can confirm inducing proper Th1 responses. The in vivo immunisation assay showed no difference in the expression of IL-4 but a substantial increase in the expression of IFN-γ and IL-12 (P ≤ 0.02) and a moderate increase in TGF-β (P = 0.05). In vivo immunisation assay revealed that Th1-inducing pathways have been stimulated, as IFN-γ and IL-12 strongly, and TGF-β expression moderately increased in Ag85B:HspX:hFcγ1 group and Ag85B:HspX:hFcγ1+BCG. Furthermore, the production of IFN-γ from splenocytes in the Ag85B:HspX:hFcγ1 group was enormously higher than in other treatments. Therefore, this Fc fusion protein can make a selective multi-stage delivery system for inducing appropriate Th1 responses and is used as a subunit vaccine alone or in combination with others.

Current vaccine strategies and novel approaches to combatting Francisella infection

Tularemia is caused by subspecies of Francisella tularensis and can manifest in a variety of disease states, with the pneumonic presentation resulting in the greatest mortality. Despite decades of research, there are no approved vaccines against F. tularensis in the United States. Traditional vaccination strategies, such as live-attenuated or subunit vaccines, are not favorable due to inadequate protection or safety concerns. Because of this, novel vaccination strategies are needed to combat tularemia. Here we discuss the current state of and challenges to the tularemia vaccine field and suggest novel vaccine approaches going forward that might be better suited for protecting against F. tularensis infection.

Development, Strategies, and Challenges for Tularemia Vaccine

Francisella tularensis is a facultative intracellular bacterial pathogen that affects both humans and animals. It was developed into a biological warfare weapon as a result. In this article, the current status of tularemia vaccine development is presented. A live-attenuated vaccine that was designed over 50 years ago using the less virulent F. tularensis subspecies holarctica is the only prophylactic currently available, but it has not been approved for use in humans or animals. Other promising live, killed, and subunit vaccine candidates have recently been developed and tested in animal models. This study will investigate some possible vaccines and the challenges they face during development.

Intranasal Nose-to-Brain Drug Delivery via the Olfactory Region in Mice: Two In-Depth Protocols for Region-Specific Intranasal Application of Antibodies and for Expression Analysis of Fc Receptors via In Situ Hybridization in the Nasal Mucosa

A region-specific catheter-based intranasal administration method was successfully developed, established, and validated as reported previously. By using this method, drugs can be applicated specifically to the olfactory region. Thereby, intranasally administered drugs could be delivered via neuronal connections to the central nervous system. Here, we present a detailed protocol with a step-by-step procedure for nose-to-brain delivery via the olfactory mucosa.Fc receptors such as the neonatal Fc receptor (FcRn) and potentially Fcγ receptor IIb (FcγRIIb) are involved in the uptake and transport of antibodies via the olfactory nasal mucosa. To better characterize their expression levels and their role in CNS drug delivery via the nose, an in situ hybridization (ISH) protocol was adapted for nasal mucosa samples and described in abundant details.

SARS-CoV-2: Immunity, Challenges with Current Vaccines, and a Novel Perspective on Mucosal Vaccines

The global rollout of COVID-19 vaccines has played a critical role in reducing pandemic spread, disease severity, hospitalizations, and deaths. However, the first-generation vaccines failed to block severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and transmission, partially due to the limited induction of mucosal immunity, leading to the continuous emergence of variants of concern (VOC) and breakthrough infections. To meet the challenges from VOC, limited durability, and lack of mucosal immune response of first-generation vaccines, novel approaches are being investigated. Herein, we have discussed the current knowledge pertaining to natural and vaccine-induced immunity, and the role of the mucosal immune response in controlling SARS-CoV2 infection. We have also presented the current status of the novel approaches aimed at eliciting both mucosal and systemic immunity. Finally, we have presented a novel adjuvant-free approach to elicit effective mucosal immunity against SARS-CoV-2, which lacks the safety concerns associated with live-attenuated vaccine platforms.

Francisella and Antibodies

Immune responses to intracellular pathogens depend largely upon the activation of T helper type 1-dependent mechanisms. The contribution of B cells to establishing protective immunity has long been underestimated. Francisella tularensis, including a number of subspecies, provides a suitable model for the study of immune responses against intracellular bacterial pathogens. We previously demonstrated that Francisella infects B cells and activates B-cell subtypes to produce a number of cytokines and express the activation markers. Recently, we documented the early production of natural antibodies as a consequence of Francisella infection in mice. Here, we summarize current knowledge on the innate and acquired humoral immune responses initiated by Francisella infection and their relationships with the immune defense systems.

Bimodal Targeting of Human Leukocytes by Fc- and CpG-Decorated Polymersomes to Tune Immune Induction

The use of well-defined nanovesicles composed of amphiphilic block copolymers (polymersomes) for delivery of adjuvants and antigens is a promising strategy for vaccine development. However, the potency of nanoparticle vaccines depends on efficient interaction with and activation of cells involved in antigen presentation, which can be achieved by targeting cellular receptors. Here, we showed that the Fc fragment display on the polymersome surface resulted in markedly improved interactions with granulocytes, monocytes, and NK cells, while for “naked” polymersomes, virtually no binding to leukocytes was observed. Moreover, CpG-decorated polymersomes were found to also interact with T and/or B cells. Interestingly, whole blood stimulations with Fc fragment and CpG-decorated polymersomes induced interleukin (IL)-6, IL-8, and TNF-α production, while naked polymersomes did not induce any cytokine production. In conclusion, specific immune induction by polymersomes can be controlled using bimodal targeting of different immune receptors, which is an essential feature for targeted vaccine delivery.

FcRn as a Transporter for Nasal Delivery of Biologics: A Systematic Review

FcRn plays a major role in regulating immune homeostasis, but it is also able to transport biologics across cellular barriers. The question of whether FcRn could be an efficient transporter of biologics across the nasal epithelial barrier is of particular interest, as it would allow a less invasive strategy for the administration of biologics in comparison to subcutaneous, intramuscular or intravenous administrations, which are often used in clinical practice. A focused systematic review was conducted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. It was registered on the international prospective register of systematic reviews PROSPERO, which helped in identifying articles that met the inclusion criteria. Clinical and preclinical studies involving FcRn and the nasal delivery of biologics were screened, and the risk of bias was assessed across studies using the Oral Health Assessment Tool (OHAT). Among the 12 studies finally included in this systematic review (out of the 758 studies screened), 11 demonstrated efficient transcytosis of biologics through the nasal epithelium. Only three studies evaluated the potential toxicity of biologics’ intranasal delivery, and they all showed that it was safe. This systematic review confirmed that FcRn is expressed in the nasal airway and the olfactory epithelium, and that FcRn may play a role in IgG and/or IgG-derived molecule-transcytosis across the airway epithelium. However, additional research is needed to better characterize the pharmacokinetic and pharmacodynamic properties of biologics after their intranasal delivery.

Co-Opting Host Receptors for Targeted Delivery of Bioconjugates-From Drugs to Bugs

Bioconjugation has allowed scientists to combine multiple functional elements into one biological or biochemical unit. This assembly can result in the production of constructs that are targeted to a specific site or cell type in order to enhance the response to, or activity of, the conjugated moiety. In the case of cancer treatments, selectively targeting chemotherapies to the cells of interest limit harmful side effects and enhance efficacy. Targeting through conjugation is also advantageous in delivering treatments to difficult-to-reach tissues, such as the brain or infections deep in the lung. Bacterial infections can be more selectively treated by conjugating antibiotics to microbe-specific entities; helping to avoid antibiotic resistance across commensal bacterial species. In the case of vaccine development, conjugation is used to enhance efficacy without compromising safety. In this work, we will review the previously mentioned areas in which bioconjugation has created new possibilities and advanced treatments.

Immune Complex Vaccine Strategies to Combat HIV-1 and Other Infectious Diseases

Immune complexes (ICs) made of antibody-bound antigens exhibit immunomodulatory activities exploitable in a vaccination strategy to optimize vaccine efficacy. The modulatory effects of ICs are typically attributed to the Fc fragments of the antibody components, which engage Fc receptors, complement and complement receptors on various immune cells. These Fc-mediated functions facilitate the critical interplay between innate and adaptive immune systems to impact the quality and quantity of the elicited adaptive responses. In addition to the Fc contribution, the Fab fragment also plays an immunoregulation role. The antigen-binding domains of the Fab fragment can bind their specific epitopes at high affinity to sterically occlude these antigenic sites from recognition by other antibodies. Moreover, the Fab-mediated binding has been demonstrated to induce allosteric alterations at nearby or distant antigenic sites. In this review article, we survey published studies to illuminate how the immunomodulatory functions of ICs have been investigated or utilized in a vaccination strategy to fight against an array of infectious pathogens, culminating with IC vaccine designs aimed at preventing HIV-1 infection. In particular, we highlight IC vaccine candidates that exploit Fab-mediated steric and allosteric effects to direct antibody responses away or toward the V1V2 domain, the V3 loop, and other antigenic sites on the HIV-1 envelope gp120 glycoprotein. Like other HIV-1 vaccine approaches, the path for IC-based vaccines to reach the clinic faces major hurdles yet to be overcome; however, investigations into this vaccine strategy have provided insights into the multifaceted activities of antibodies beyond their conventional roles in the host defense against HIV-1 and other microbial pathogens.

Advances in Oral Subunit Vaccine Design

Many pathogens invade the host at the intestinal surface. To protect against these enteropathogens, the induction of intestinal secretory IgA (SIgA) responses is paramount. While systemic vaccination provides strong systemic immune responses, oral vaccination is the most efficient way to trigger protective SIgA responses. However, the development of oral vaccines, especially oral subunit vaccines, is challenging due to mechanisms inherent to the gut. Oral vaccines need to survive the harsh environment in the gastrointestinal tract, characterized by low pH and intestinal proteases and need to reach the gut-associated lymphoid tissues, which are protected by chemical and physical barriers that prevent efficient uptake. Furthermore, they need to surmount default tolerogenic responses present in the gut, resulting in suppression of immunity or tolerance. Several strategies have been developed to tackle these hurdles, such as delivery systems that protect vaccine antigens from degradation, strong mucosal adjuvants that induce robust immune responses and targeting approaches that aim to selectively deliver vaccine antigens towards specific immune cell populations. In this review, we discuss recent advances in oral vaccine design to enable the induction of robust gut immunity and highlight that the development of next generation oral subunit vaccines will require approaches that combines these solutions.

A Plant-Derived Antigen-Antibody Complex Induces Anti-Cancer Immune Responses by Forming a Large Quaternary Structure

The antigen–antibody complex (AAC) has novel functions for immunomodulation, encouraging the application of diverse quaternary protein structures for vaccination. In this study, GA733 antigen and anti-GA733 antibody proteins were both co-expressed to obtain the AAC protein structures in a F1 plant obtained by crossing the plants expressing each protein. In F1 plant, the antigen and antibody assembled to form a large quaternary circular ACC structure (~30 nm). The large quaternary protein structures induced immune response to produce anticancer immunoglobulins G (IgGs) that are specific to the corresponding antigens in mouse. The serum containing the anticancer IgGs inhibited the human colorectal cancer cell growth in the xenograft nude mouse. Taken together, antigens and antibodies can be assembled to form AAC protein structures in plants. Plant crossing represents an alternative strategy for the formation of AAC vaccines that efficiently increases anticancer antibody production.

Isotype-specific outcomes in Fc gamma receptor targeting of PspA using fusion proteins as a vaccination strategy against Streptococcus pneumoniae infection

Streptococcus pneumoniae (Spn) remains a considerable threat to public health despite the availability of antibiotics and polysaccharide conjugate vaccines. The lack of mucosal immunity in addition to capsular polysaccharide diversity, has proved to be problematic in developing a universal vaccine against Spn. Targeting antigen to Fc receptors is an attractive way to augment both innate and adaptive immunity against mucosal pathogens, by promoting interactions with activating Fcγ receptors (FcγR) that mediate diverse immunomodulatory functions. The effect of targeting FcγR is highly influenced by the IgG subclass, which bares differential affinities for activating and inhibitory FcγR. In the current study we demonstrate targeting activating FcγR with fusion proteins consisting of PspA and IgG2a Fc enhance PspA-specific immune responses, and effectively protect against mucosal Spn challenge. Specifically, targeting PspA to FcγR polarized alveolar macrophage to the AM1 phenotype and increased conventional dendritic cell subsets in the lung in addition to augmenting Th1 cytokines and PspA-specific IgG and IgA. In contrast, fusion proteins consisting of PspA fused to the IgG1 Fc provided minimal benefit over administration of PspA alone, as a result of interaction with the inhibitory FcγRIIB. Protective efficacy of the IgG1 fusion protein was significantly enhanced in animals deficient for FcγRIIB accompanied by increased B cell maturation and proliferation levels in these animals. These studies demonstrate FcγR targeting is an effective strategy for inducing potent cellular and humoral responses via mucosal immunization with Fc fusion proteins, however, careful consideration of the Fc region utilized is required since Fc isotype subclass heavily influenced immunization induced effector functions and survival against lethal Spn challenge. Fc-engineering with specific attention to FcγRIIB engagement presents a valuable vaccine strategy for protecting against Spn infection.

Autotransporter-Mediated Display of Complement Receptor Ligands by Gram-Negative Bacteria Increases Antibody Responses and Limits Disease Severity

The targeting of immunogens/vaccines to specific immune cells is a promising approach for amplifying immune responses in the absence of exogenous adjuvants. However, the targeting approaches reported thus far require novel, labor-intensive reagents for each vaccine and have primarily been shown as proof-of-concept with isolated proteins and/or inactivated bacteria. We have engineered a plasmid-based, complement receptor-targeting platform that is readily applicable to live forms of multiple gram-negative bacteria, including, but not limited to, Escherichia coli, Klebsiella pneumoniae, and Francisella tularensis. Using F. tularensis as a model, we find that targeted bacteria show increased binding and uptake by macrophages, which coincides with increased p38 and p65 phosphorylation. Mice vaccinated with targeted bacteria produce higher titers of specific antibody that recognizes a greater diversity of bacterial antigens. Following challenge with homologous or heterologous isolates, these mice exhibited less weight loss and/or accelerated weight recovery as compared to counterparts vaccinated with non-targeted immunogens. Collectively, these findings provide proof-of-concept for plasmid-based, complement receptor-targeting of live gram-negative bacteria.

Interleukin 34 Serves as a Novel Molecular Adjuvant against Nocardia Seriolae Infection in Largemouth Bass (Micropterus Salmoides)

DNA vaccines have been widely employed in controlling viral and bacterial infections in mammals and teleost fish. Co-injection of molecular adjuvants, including chemokines, cytokines, and immune co-stimulatory molecules, is one of the potential strategies used to improve DNA vaccine efficacy. In mammals and teleost fish, interleukin-34 (IL-34) had been described as a multifunctional cytokine and its immunological role had been confirmed; however, the adjuvant capacity of IL-34 remains to be elucidated. In this study, IL-34 was identified in largemouth bass. A recombinant plasmid of IL-34 (pcIL-34) was constructed and co-administered with a DNA vaccine encoding hypoxic response protein 1 (Hrp1; pcHrp1) to evaluate the adjuvant capacity of pcIL-34 against Nocardia seriolae infection. Our results indicated that pcIL-34 co-injected with pcHrp1 not only triggered innate immunity and a specific antibody response, but also enhanced the mRNA expression level of immune-related genes encoding for cytokines, chemokines, and humoral and cell-mediated immunity. Moreover, pcIL-34 enhanced the protection of pcHrp1 against N. seriolae challenge and conferred the relative percent survival of 82.14%. Collectively, IL-34 is a promising adjuvant in a DNA vaccine against nocardiosis in fish.

The protective efficacy of recombinant hypoxic response protein 1 of Nocardia seriolae in largemouth bass (Micropterus salmoides)

Nocardia seriolae has become one of the major pathogens affecting the aquaculture industry and causes Nocardiosis, a highly devastating disease of marine and freshwater fish that leads to severe economic losses. Therefore, research efforts towards developing efficacious vaccines to control this disease are of high importance. In this study, the hypoxic response protein 1 (HRP1) cloned into pET32a vector was expressed, and produced in Escherichia coli strain BL21 (DE3). The antigenicity of purified recombinant TRX-tagged HRP (rHRP1) was analysed by western blotting using largemouth bass anti-N. seriolae sera. The results showed that largemouth bass anti-N. seriolae sera could specifically detect a 33 kDa rHRP1 protein. Further, the vaccine efficacy of rHRP1 was evaluated in a largemouth bass fish model by calculating the relative percent survival (RPS). rHRP1 incurred an RPS of 73.33% as compared to the control group. Immunological analysis showed that rHRP1 could produce significantly higher serum concentrations of anti-N. seriolae antibodies and serum lysozyme activity as compared to the control groups. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis showed that rHRP1 significantly enhanced the expression of immune-related genes, such as IL-12p40, IL-8, IL-1β, TNFα, IFNγ, NKEF, MHCIα, MHCIIα, CD4-1, CD8α, IgM, NF-κβ, STAT3, IRF4, RORα, and CCL20. These results indicate that rHRP1 may be a promising vaccine candidate against nocardiosis.

Fcγ receptors and toll-like receptor 9 synergize to drive immune complex-induced dendritic cell maturation

Previous in vivo studies established that inactivated Francisella tularensis immune complexes (mAb-iFt) are a more protective vaccine against lethal tularemia than iFt alone. Subsequent in vitro studies revealed enhanced DC maturation marker expression with mAb-iFt stimulation. The goal of this study was to determine the mechanism of enhanced DC maturation. Multiparameter analysis of surface marker expression and cytokine secretion demonstrates a requirement for FcγR signaling in enhanced DC maturation. MyD88 was also found to be essential for heightened DC maturation, implicating MyD88-dependent TLRs in DC maturation. Upon further study, we discovered that TLRs 2 & 4 drive cytokine secretion, but surprisingly TLR9 is required for DC maturation marker upregulation. These studies reveal a separation of DC cytokine and maturation marker induction pathways and demonstrate that FcγR-TLR/MyD88 synergy underlies the enhanced dendritic cell maturation in response to the mAb-iFt vaccine.

Evaluation of an outbred mouse model for Francisella tularensis vaccine development and testing

Francisella tularensis (Ft) is a biothreat agent for which there is no FDA-approved human vaccine. Currently, there are substantial efforts underway to develop both vaccines and the tools to assess these vaccines. Tularemia laboratory research has historically relied primarily upon a small number of inbred mouse strains, but the utility of such findings to outbred animals may be limited. Specifically, C57BL/6 mice are more susceptible than BALB/c mice to Ft infection and less easily protected against challenge with highly virulent type A Ft. Thus, depending on the inbred mouse strain used, one could be misled as to which immunogen(s)/vaccine will ultimately be effective in an outbred human population. Accordingly, we evaluated an outbred Swiss Webster (SW) mouse model in direct comparison to a well-established, inbred C57BL/6 mouse model. Mucosal vaccination with the live, attenuated Ft LVS superoxide dismutase (sodB) mutant demonstrated significantly higher protection in outbred SW mice compared to inbred C57BL/6 mice against Ft SchuS4 respiratory challenge. The protection observed in vaccinated outbred mice correlated with lower bacterial density, reduced tissue inflammation, and reduced levels of pro-inflammatory cytokine production. This protection was CD4⁺ and CD8⁺ T cell-dependent and characterized by lower titers of serum antibody (Ab) that qualitatively differed from vaccinated inbred mice. Enhanced protection of vaccinated outbred mice correlated with early and robust production of IFN-γ and IL-17A. Neutralizing Ab administered at the time of challenge revealed that IFN-γ was central to this protection, while IL-17A neutralization did not alter bacterial burden or survival. The present study demonstrates the utility of the outbred mouse as an alternative vaccination model for testing tularemia vaccines. Given the limited MHC repertoire in inbred mice, this outbred model is more analogous to the human in terms of immunological diversity.

The phagosome and redox control of antigen processing

In addition to debris clearance and antimicrobial function, versatile organelles known as phagosomes play an essential role in the processing of exogenous antigen in antigen presenting cells. While there has been much attention on human leukocyte antigen haplotypes in the determination of antigenic peptide repertoires, the lumenal biochemistries within phagosomes and endosomes are emerging as equally-important determinants of peptide epitope composition and immunodominance. Recently, the lumenal redox microenvironment within these degradative compartments has been shown to impact two key antigenic processing chemistries: proteolysis by lysosomal cysteine proteases and disulfide reduction of protein antigens. Through manipulation of the balance between oxidative and reductive capacities in the phagosome-principally by modulating NADPH oxidase (NOX2) and γ-interferon-inducible lysosomal thiol reductase (GILT) activities-studies have demonstrated changes to antigen processing patterns leading to modified repertoires of antigenic peptides available for presentation, and subsequently, altered disease progression in T cell-driven autoimmunity. This review focuses on the mechanisms and consequences of redox-mediated phagosomal antigen processing, and the potential downstream implications to tolerance and autoimmunity.

Differential In Vitro Cultivation of Francisella tularensis Influences Live Vaccine Protective Efficacy by Altering the Immune Response

Francisella tularensis (Ft) is a biothreat agent for which there is no FDA-approved human vaccine. Currently, there are substantial efforts underway to develop both vaccines and improved tools to assess these vaccines. Ft expresses distinct sets of antigens (Ags) in vivo as compared to those expressed in vitro. Importantly, Ft grown in brain-heart infusion medium (BHIM) more closely mimics the antigenic profile of macrophage-grown Ft when compared to Mueller-Hinton medium (MHM)-grown Ft. Thus, we predicted that when used as a live vaccine BHIM-grown Ft (BHIM-Ft) would provide better protection, as compared to MHM-Ft. We first determined if there was a difference in growth kinetics between BHIM and MHM-Ft. We found that BHIM-Ft exhibited an initial growth advantage ex vivo that manifests as slightly hastened intracellular replication as compared to MHM-Ft. We also observed that BHIM-Ft exhibited an initial growth advantage in vivo represented by rapid bacterial expansion and systemic dissemination associated with a slightly shorter mean survival time of naive animals. Next, using two distinct strains of Ft LVS (WT and sodB), we observed that mice vaccinated with live BHIM-Ft LVS exhibited significantly better protection against Ft SchuS4 respiratory challenge compared to MHM-Ft-immunized mice. This enhanced protection correlated with lower bacterial burden, reduced tissue inflammation, and reduced pro-inflammatory cytokine production late in infection. Splenocytes from BHIM-Ft sodB-immunized mice contained more CD4⁺, effector, memory T-cells, and were more effective at limiting intracellular replication of Ft LVS in vitro. Concurrent with enhanced killing of Ft LVS, BHIM-Ft sodB-immune splenocytes produced significantly higher levels of IFN-γ and IL-17A cytokines than their MHM-Ft sodB-immunized counterparts indicating development of a more effective T cell memory response when immunizing mice with BHIM-Ft.

Murine macrophage TLR2-FcγR synergy via FcγR licensing of IL-6 cytokine mRNA ribosome binding and translation

Macrophages (MØs) are sentinels of the immune system that use pattern recognition receptors such as Toll-like receptors (TLR) to detect invading pathogens and immune receptors such as FcγR to sense the host’s immune state. Crosstalk between these two signaling pathways allows the MØ to tailor the cell’s overall response to prevailing conditions. However, the molecular mechanisms underlying TLR-FcγR crosstalk are only partially understood. Therefore, we employed an immunologically-relevant MØ stimulus, an inactivated gram-negative bacterium that bears TLR2 agonists but no TLR4 agonist (iB^TLR2) opsonized with a monoclonal antibody (mAb-iB^TLR2), as a tool to study FcγR regulation of TLR2-driven production of IL-6, a key inflammatory cytokine. We chose this particular agonist as an investigational tool because MØ production of any detectable IL-6 in response to mAb-iB^TLR2 requires both TLR2 and FcγR signaling, making it an excellent system for the study of receptor synergy. Using genetic, pharmacological and immunological approaches, we demonstrate that the murine MØ IL-6 response to mAb-iB^TLR2 requires activation of both the TLR/NF-κB and FcγR/ITAM signaling pathways. mAb-iB^TLR2 engagement of TLR2 drives NF-κB activation and up-regulation of IL-6 mRNA but fails to result in IL-6 cytokine production/release. Here, Src family kinase-driven FcγR ITAM signaling is necessary to enable IL-6 mRNA incorporation into polysomes and translation. These results reveal a novel mechanism by which FcγR ITAM signaling synergizes with TLR signaling, by “licensing” cytokine mRNA ribosome binding/translation to drive a strong murine MØ cytokine response.

Longitudinal Evaluation of Humoral Immunity and Bacterial and Clinical Parameters Reveals That Antigen-Specific Antibodies Suppress Inflammatory Responses in Active Tuberculosis Patients

A novel tuberculosis vaccine to replace BCG has long been desired. However, recent vaccine trials focused on cell-mediated immunity have failed to produce promising results. It is worth noting that most commercially available successful vaccines rely on humoral immunity. To establish a basic understanding of humoral immunity against tuberculosis, we analyzed and evaluated longitudinal levels and avidity of immunoglobulin to various tuberculosis antigens compared with bacterial and clinical parameters during treatment. We found that levels of IgG antibodies against HrpA and HBHA prior to treatment exhibited a positive correlation with bacterial burden. Analysis of changes in CRP during treatment revealed an association with high levels of specific IgG and IgA antibodies against mycobacterial antigens. Levels of CRP prior to treatment were negatively associated with IgG avidity to CFP-10 and MDP1 and IgA avidity to HrpA, while IgA avidity to MDP1 and Acr exhibited a negative correlation with CRP levels after 60 days of treatment. These results may provide insight for the development of a novel tuberculosis (TB) vaccine candidate to induce protective humoral immunity against tuberculosis.

Antibody-based vaccine strategies against intracellular pathogens

Historically, antibody-mediated immunity was considered effective against toxins, extracellular pathogens and viruses, while control of intracellular pathogens was the domain of cellular immunity. However, numerous observations in recent decades have conclusively shown that antibody can protect against intracellular pathogens. This paradigmatic shift has tremendous implications for immunology and vaccine design. For immunology the observation that antibody can protect against intracellular pathogens has led to the discovery of new mechanisms of antibody action. For vaccine design the knowledge that humoral immunity can be effective in protection means that the knowledge acquired in more than a century of antibody studies can be applied to make new vaccines against this class of pathogens.

Intranasal administration of a two-dose adjuvanted multi-antigen TMV-subunit conjugate vaccine fully protects mice against Francisella tularensis LVS challenge

Tularemia is a fatal human disease caused by Francisella tularensis, a Gram-negative encapsulated coccobacillus bacterium. Due to its low infectious dose, ease of aerosolized transmission, and lethal effects, the CDC lists F. tularensis as a Category A pathogen, the highest level for a potential biothreat agent. Previous vaccine studies have been conducted with live attenuated, inactivated, and subunit vaccines, which have achieved partial or full protection from F. tularensis live vaccine strain (LVS) challenge, but no vaccine has been approved for human use. We demonstrate the improved efficacy of a multi-antigen subunit vaccine by using Tobacco Mosaic virus (TMV) as an antigen carrier for the F. tularensis SchuS4 proteins DnaK, OmpA, SucB and Tul4 (DOST). The magnitude and quality of immune responses were compared after mice were immunized by subcutaneous or intranasal routes of administration with a TMV-DOST mixture, with or without four different adjuvants. Immune responses varied in magnitude and isotype profile, by antigen, by route of administration, and by protection in an F. tularensis LVS challenge model of disease. Interestingly, our analysis demonstrates an overwhelming IgG2 response to SucB after intranasal dosing, as well as a robust cellular response, which may account for the improved two-dose survival imparted by the tetravalent vaccine, compared to a previous study that tested efficacy of TMV-DOT. Our study provides evidence that potent humoral, cellular and mucosal immunity can be achieved by optimal antigen combination, delivery, adjuvant and appropriate route of administration, to improve vaccine potency and provide protection from pathogen challenge.

Adaptive Immunity to Francisella tularensis and Considerations for Vaccine Development

Francisella tularensis is an intracellular bacterium that causes the disease tularemia. There are several subspecies of F. tularensis whose ability to cause disease varies in humans. The most virulent subspecies, tularensis, is a Tier One Select Agent and a potential bioweapon. Although considerable effort has made to generate efficacious tularemia vaccines, to date none have been licensed for use in the United States. Despite the lack of a tularemia vaccine, we have learned a great deal about the adaptive immune response the underlies protective immunity. Herein, we detail the animal models commonly used to study tularemia and their recapitulation of human disease, the field's current understanding of vaccine-mediated protection, and discuss the challenges associated with new vaccine development.

Defective anti-polysaccharide IgG vaccine responses in IgA deficient mice

We report that IgA^-/- mice exhibit specific defects in IgG antibody responses to various polysaccharide vaccines (Francisella tularensis LPS and Pneumovax), but not protein vaccines such as Fluzone. This defect further included responses to polysaccharide-protein conjugate vaccines (Prevnar and Haemophilus influenzae type b-tetanus toxoid vaccine). In agreement with these findings, IgA^-/- mice were protected from pathogen challenge with protein- but not polysaccharide-based vaccines. Interestingly, after immunization with live bacteria, IgA^+/+ and IgA^-/- mice were both resistant to lethal challenge and their IgG anti-polysaccharide antibody responses were comparable. Immunization with live bacteria, but not purified polysaccharide, induced production of serum B cell-activating factor (BAFF), a cytokine important for IgG class switching; supplementing IgA^-/- cell cultures with BAFF enhanced in vitro polyclonal IgG production. Taken together, these findings show that IgA deficiency impairs IgG class switching following vaccination with polysaccharide antigens and that live bacterial immunization can overcome this defect. Since IgA deficient patients also often show defects in antibody responses following immunization with polysaccharide vaccines, our findings could have relevance to the clinical management of this population.

Fc-based delivery system enhances immunogenicity of a tuberculosis subunit vaccine candidate consisting of the ESAT-6:CFP-10 complex

Tuberculosis (TB) remains a major global health threat despite chemotherapy and Bacilli Calmette-Guérin (BCG) vaccination. Therefore, a safer and more effective vaccine against TB is urgently needed. This study evaluated the immunogenicity of a recombinant fusion protein consisting of early secreted antigenic target protein 6 kDa (ESAT-6), culture filtrate protein 10 kDa (CFP-10) and the Fc-domain of mouse IgG2a as a novel subunit vaccine. The recombinant expression vectors (pPICZαA-ESAT-6:CFP-10:Fcγ2a and pPICZαA-ESAT-6:CFP-10:His) were transferred into Pichia pastoris. After SDS-PAGE and immunoblotting, the immunogenicity of the recombinant proteins was evaluated in mice. When both recombinant proteins (ESAT-6:CFP-10:Fcγ2a and ESAT-6:CFP-10:His) were used for vaccination, Th1-type cellular responses were induced producing high levels of IFN-γ and IL-12. However, the Fc-tagged recombinant protein induced more effective Th1-type cellular responses with a small increase in IL-4 as compared to the BCG and ESAT-6:CFP-10:His groups. Moreover, mice primed with BCG and then supplemented with ESAT-6:CFP-10:Fcγ2a produced the highest levels of IFN-γ and IL-12 in immunized groups. The findings indicate that when Fcγ2a is fused to the ESAT-6:CFP-10 complex, as a delivery vehicle, there could be an increase in the immunogenicity of this type of subunit vaccine. Therefore, additional investigations are necessary for the development of appropriate Fc-based tuberculosis vaccines.

Ex vivo antigen-pulsed PBMCs generate potent and long lasting immunity to infection when administered as a vaccine

Numerous studies have demonstrated that administration of antigen (Ag)-pulsed dendritic cells (DCs) is an effective strategy for enhancing immunity to tumors and infectious disease organisms. However, the generation and/or isolation of DCs can require substantial time and expense. Therefore, using inactivated F. tularensis (iFt) Ag as a model immunogen, we first sought to determine if DCs could be replaced with peripheral blood mononuclear cells (PBMCs) during the ex-vivo pulse phase and still provide protection against Ft infection. Follow up studies were then conducted using the S. pneumoniae (Sp) vaccine Prevnar ®13 as the Ag in the pulse phase followed by immunization and Sp challenge. In both cases, we demonstrate that PBMCs can be used in place of DCs when pulsing with iFt and/or Prevnar ®13 ex vivo and re-administering the Ag-pulsed PBMCs as a vaccine. In addition, utilization of the i.n. route for Ag-pulsed PBMC administration is superior to use of the i.v. route in the case of Sp immunization, as well as when compared to direct injection of Prevnar ®13 vaccine i.m. or i.n. Furthermore, this PBMC-based vaccine strategy provides a more marked and enduring protective immune response and is also capable of serving as a multi-organism vaccine platform. The potential for this ex-vivo vaccine strategy to provide a simpler, less time consuming, and less expensive approach to DC-based vaccines and vaccination in general is also discussed.

Differential Cultivation of Francisella tularensis Induces Changes in the Immune Response to and Protective Efficacy of Whole Cell-Based Inactivated Vaccines

Francisella tularensis (Ft) is a category A biothreat agent for which there is no Food and Drug Administration-approved vaccine. Ft can survive in a variety of habitats with a remarkable ability to adapt to changing environmental conditions. Furthermore, Ft expresses distinct sets of antigens (Ags) when inside of macrophages (its in vivo host) as compared to those grown in vitro with Mueller Hinton Broth (MHB). However, in contrast to MHB-grown Ft, Ft grown in Brain-Heart Infusion (BHI) more closely mimics the antigenic profile of macrophage-grown Ft. Thus, we anticipated that when used as a vaccine, BHI-grown Ft would provide better protection compared to MHB-grown Ft, primarily due to its greater antigenic similarity to Ft circulating inside the host (macrophages) during natural infection. Our investigation, however, revealed that inactivated Ft (iFt) grown in MHB (iFt-MHB) exhibited superior protective activity when used as a vaccine, as compared to iFt grown in BHI (iFt-BHI). The superior protection afforded by iFt-MHB compared to that of iFt-BHI was associated with significantly lower bacterial burden and inflammation in the lungs and spleens of vaccinated mice. Moreover, iFt-MHB also induced increased levels of Ft-specific IgG. Further evaluation of early immunological cues also revealed that iFt-MHB exhibits increased engagement of Ag-presenting cells including increased iFt binding to dendritic cells, increased expression of costimulatory markers, and increased secretion of pro-inflammatory cytokines. Importantly, these studies directly demonstrate that Ft growth conditions strongly impact Ft vaccine efficacy and that the growth medium used to produce whole cell vaccines to Ft must be a key consideration in the development of a tularemia vaccine.

FcγR mediates TLR2- and Syk-dependent NLRP3 inflammasome activation by inactivated Francisella tularensis LVS immune complexes

IgG (mAb)-opsonized, inactivated Francisella tularensis LVS (iFt-mAb) enhances TLR2-dependent IL-6 production by macrophages via Fcγ receptors (FcγR). In mice, vaccination with iFt-mAb provides IgA-dependent protection against lethal challenge with Ft LVS. Because inflammasome maturation of IL-1β is thought important for antibody-mediated immunity, we considered the possibility that iFt-mAb elicits an FcγR-dependent myeloid cell inflammasome response. Herein, we find that iFt-mAb enhances macrophage and dendritic cell IL-1β responses in a TLR2- and FcγR-dependent fashion. Although iFt-mAb complexes bind FcγR and are internalized, sensing of cytosolic DNA by absent in melanoma 2 (AIM2) is not required for the IL-1β response. In contrast, ASC, caspase-1, and NLR family pyrin domain-containing 3 (NLRP3) are indispensable. Further, FcγR-mediated spleen tyrosine kinase (Syk) signaling is required for this NLRP3-dependent IL-1β response, but the alternative IL-1β convertase caspase-8 is insufficient. Finally, iFt-mAb-vaccinated wild-type mice exhibit a significant delay in time to death, but IL-1R1- or Nlrp3-deficient mice vaccinated in this way are not protected and lack appreciable Francisella-specific antibodies. This study demonstrates that FcγR-mediated Syk activation leads to NLRP3 inflammasome-dependent IL-1β production in macrophages and suggests that an Nlrp3- and IL-1R-dependent process contributes to the IgA response important for protection against Ft LVS. These findings extend our understanding of cellular responses to inactivated pathogen-opsonized vaccine, establish FcγR-elicited Syk kinase-mediated NLRP3 inflammasome activation, and provide additional insight toward understanding crosstalk between TLR and FcγR signals.

Protective efficacy of Fc targeting conserved influenza virus M2e antigen expressed by Lactobacillus plantarum

The influenza A (H1N1) virus is a highly contagious acute respiratory disease affecting pigs and humans. This disease causes severe economic loss in many countries, and developing mucosal vaccines is an efficient strategy to control the influenza virus. The neonatal Fc receptor (FcRn) plays an important role in transferring IgG across polarized epithelial cells. In the present study, an oral vaccine was developed using Lactobacillus plantarum to deliver the internal influenza viral protein M2e fused to an IgG Fc fragment. Oral vaccination with recombinant L. plantarum expressing 3M2e-Fc elicited Peyer's patch (PP) DC activation, improved the number of gamma interferon (IFN-γ)-producing T cells and increased the frequency of CD8⁺IFN-γ⁺ cells in the mesenteric lymph nodes (MLNs). In addition, the recombinant L. plantarum can induce PP B220⁺IgA⁺ expression and enhance specific sIgA secretion and the shaping of growth centers (GCs) in PPs. Furthermore, the data demonstrated that immunization with recombinant L. plantarum expressing 3M2e-Fc markedly reduced the viral load in the lung and protected against H1N1 influenza virus and mouse-adapted H9N2 avian influenza virus (AIV) challenge in BALB/c mice. Collectively, the data also showed that this vaccine strategy provided effective protective immunity against infection with homologous and heterologous influenza viruses in a mouse model and may be useful for future influenza vaccine development.

Respiratory and oral vaccination improves protection conferred by the live vaccine strain against pneumonic tularemia in the rabbit model

Tularemia is a severe, zoonotic disease caused by a gram-negative bacterium, Francisella tularensis We have previously shown that rabbits are a good model of human pneumonic tularemia when exposed to aerosols containing a virulent, type A strain, SCHU S4. We further demonstrated that the live vaccine strain (LVS), an attenuated type B strain, extended time to death when given by scarification. Oral or aerosol vaccination has been previously shown in humans to offer superior protection to parenteral vaccination against respiratory tularemia challenge. Both oral and aerosol vaccination with LVS were well tolerated in the rabbit with only minimal fever and no weight loss after inoculation. Plasma antibody titers against F. tularensis were higher in rabbits that were vaccinated by either oral or aerosol routes compared to scarification. Thirty days after vaccination, all rabbits were challenged with aerosolized SCHU S4. LVS given by scarification extended time to death compared to mock-vaccinated controls. One orally vaccinated rabbit did survive aerosol challenge, however, only aerosol vaccination extended time to death significantly compared to scarification. These results further demonstrate the utility of the rabbit model of pneumonic tularemia in replicating what has been reported in humans and macaques as well as demonstrating the utility of vaccination by oral and respiratory routes against an aerosol tularemia challenge.

Construction and immunogenicity of a new Fc-based subunit vaccine candidate against Mycobacterium tuberculosis

As an ancient disease, tuberculosis (TB) is a major global health threat. Therefore, there is an urgent need for an effective and safe anti-TB vaccine. In the current study, a delivery system of Fc domain of mouse IgG2a and early secreted antigenic target protein 6 (ESAT-6) was evaluated for the selective uptake of antigens by antigen-presenting cells (APCs). Thus, it was based on the immunogenicity of a fusion protein. The study was initiated by the transfer of recombinant expression vectors of pPICZαA-ESAT-6:Fcγ2a and pPICZαA-ESAT-6: His into Pichia pastoris (P. pastoris). Recombinant proteins were assessed for immunogenicity following the immunoblotting analysis. High levels of IFN-γ and IL-12 were produced to induce Th1-type cellular responses through vaccination with both recombinant proteins [ESAT-6:Fcγ2a (EF) and ESAT-6:His (EH)]. The Fc-tagged recombinant protein induced more effective Th1-type cellular responses with a low increment in IL-4 compared to PBS, BCG, and EH groups. Although in all the immunized groups, the ratio of IFN-γ/IL-4 was in favor of Th1 responses, the highest Th1/Th2 balance was observed in EF immunized group. Fc fragment of mouse IgG2a may induce a selective uptake of APCs towards the cross-presentation and formation of Th1 responses in favor of an appropriate protective anti-tuberculosis reaction. Thus, further research on Fc-fusion proteins is required to develop Fc-based TB vaccines.

Vaccination evokes gender-dependent protection against tularemia infection in C57BL/6Tac mice

Francisella tularensis (Ft) is a Category A biothreat agent for which there currently is no FDA-approved vaccine. Thus, there is a substantial effort underway to develop an effective tularemia vaccine. While it is well established that gender can significantly impact susceptibility to primary infection, the impact of gender on vaccine efficacy is not well established. Thus, development of a successful vaccine against tularemia will require an understanding of the impact gender has on vaccine-induced protection against this organism. In this study, a role for gender in vaccine-induced protection following Ft challenge is identified for the first time. In the present study, mucosal vaccination with inactivated Ft (iFt) LVS elicited gender-based protection in C57BL/6Tac mice against respiratory challenge with Ft LVS. Specifically, vaccinated male mice were more susceptible to subsequent Ft LVS challenge. This increased susceptibility in male mice correlated with increased bacterial burden, increased tissue inflammation, and increased proinflammatory cytokine production late in post-challenge infection. In contrast, improved survival of iFt-vaccinated female mice correlated with reduced bacterial burden and enhanced levels of Ft-specific Abs in serum and broncho-alveolar lavage (BAL) fluid post-challenge. Furthermore, vaccination with a live attenuated vaccine consisting of an Ft LVS superoxide dismutase (SodB) mutant, which has proven efficacious against the highly virulent Ft SchuS4 strain, demonstrated similar gender bias in protection post-Ft SchuS4 challenge. Of particular significance is the fact that these are the first studies to demonstrate that gender differences impact disease outcome in the case of lethal respiratory tularemia following mucosal vaccination. In addition, these studies further emphasize the fact that gender differences must be a serious consideration in any future tularemia vaccine development studies.

Tularemia vaccine development: paralysis or progress?

Francisella tularensis (Ft) is a gram-negative intercellular pathogen and category A biothreat agent. However, despite 15 years of strong government investment and intense research focused on the development of a US Food and Drug Administration-approved vaccine against Ft, the primary goal remains elusive. This article reviews research efforts focused on developing an Ft vaccine, as well as a number of important factors, some only recently recognized as such, which can significantly impact the development and evaluation of Ft vaccine efficacy. Finally, an assessment is provided as to whether a US Food and Drug Administration-approved Ft vaccine is likely to be forthcoming and the potential means by which this might be achieved.

Fused Mycobacterium tuberculosis multi-stage immunogens with an Fc-delivery system as a promising approach for the development of a tuberculosis vaccine

Tuberculosis (TB) remains a major health problem worldwide. Currently, the Bacilli Calmette-Guérin (BCG) is the only available licensed TB vaccine, which has low efficacy in protection against adult pulmonary TB. Therefore, the development of a safe and effective vaccine against TB needs global attention. In the present study, a novel multi-stage subunit vaccine candidate from culture filtrate protein-10 (CFP-10) and heat shock protein X (HspX) of Mycobacterium tuberculosis fused to the Fc domain of mouse IgG2a as a selective delivery system for antigen-presenting cells (APCs) was produced and its immunogenicity assessed. The optimized gene constructs were introduced into pPICZαA expression vectors, and the resultant plasmids (pPICZαA-CFP-10:Hspx:Fcγ2a and pPICZαA-CFP-10:Hspx:His) were transferred into Pichia pastoris by electroporation. The identification of both purified recombinant fusion proteins was evaluated by SDS-PAGE and immunoblotting. Then the immunogenicity of the recombinant proteins with and without BCG was evaluated in BALB/c mice by assessing the level of IFN-γ, IL-12, IL-4, IL-17 and TGF-β cytokines. Both multi-stage vaccines (CFP-10:HspX:Fcγ2a and CFP-10:HspX:His) induced Th1-type cellular responses by producing high level of IFN-γ (272 pg/mL, p<0.001) and IL-12 (191 pg/mL, p<0.001). However, the Fc-tagged recombinant protein induced more effective Th1-type cellular responses with a low level of IL-4 (10 pg/mL) compared to the CFP-10:HspX:His group. The production of IFN-γ to CFP-10:HspX:Fcγ2a was markedly consistent and showed an increasing trend for IL-12 compared with the BCG or CFP-10:HspX:His primed and boosted groups. Findings revealed that CFP-10:Hspx:Fcγ2a fusion protein can elicit strong Th1 antigen-specific immune responses in favor of protective immunity in mice and could provide new insight for introducing an effective multi-stage subunit vaccine against TB.

Rational Design of Targeted Next-Generation Carriers for Drug and Vaccine Delivery

Pattern recognition receptors on innate immune cells play an important role in guiding how cells interact with the rest of the organism and in determining the direction of the downstream immune response. Recent advances have elucidated the structure and function of these receptors, providing new opportunities for developing targeted drugs and vaccines to treat infections, cancers, and neurological disorders. C-type lectin receptors, Toll-like receptors, and folate receptors have attracted interest for their ability to endocytose their ligands or initiate signaling pathways that influence the immune response. Several novel technologies are being developed to engage these receptors, including recombinant antibodies, adoptive immunotherapy, and chemically modified antigens and drug delivery vehicles. These active targeting technologies will help address current challenges facing drug and vaccine delivery and lead to new tools to treat human diseases.

B cells and antibodies in the defense against Mycobacterium tuberculosis infection

Better understanding of the immunological components and their interactions necessary to prevent or control Mycobacterium tuberculosis (Mtb) infection in humans is critical for tuberculosis (TB) vaccine development strategies. Although the contributory role of humoral immunity in the protection against Mtb infection and disease is less defined than the role of T cells, it has been well-established for many other intracellular pathogens. Here we update and discuss the increasing evidence and the mechanisms of B cells and antibodies in the defense against Mtb infection. We posit that B cells and antibodies have a variety of potential protective roles at each stage of Mtb infection and postulate that such roles should be considered in the development strategies for TB vaccines and other immune-based interventions.

Targeting of a Fixed Bacterial Immunogen to Fc Receptors Reverses the Anti-Inflammatory Properties of the Gram-Negative Bacterium, Francisella tularensis, during the Early Stages of Infection

Production of pro-inflammatory cytokines by innate immune cells at the early stages of bacterial infection is important for host protection against the pathogen. Many intracellular bacteria, including Francisella tularensis, the agent of tularemia, utilize the anti-inflammatory cytokine IL-10, to evade the host immune response. It is well established that IL-10 has the ability to inhibit robust antigen presentation by dendritic cells and macrophages, thus suppressing the generation of protective immunity. The pathogenesis of F. tularensis is not fully understood, and research has failed to develop an effective vaccine to this date. In the current study, we hypothesized that F. tularensis polarizes antigen presenting cells during the early stages of infection towards an anti-inflammatory status characterized by increased synthesis of IL-10 and decreased production of IL-12p70 and TNF-α in an IFN-ɣ-dependent fashion. In addition, F. tularensis drives an alternative activation of alveolar macrophages within the first 48 hours post-infection, thus allowing the bacterium to avoid protective immunity. Furthermore, we demonstrate that targeting inactivated F. tularensis (iFt) to Fcγ receptors (FcɣRs) via intranasal immunization with mAb-iFt complexes, a proven vaccine strategy in our laboratories, reverses the anti-inflammatory effects of the bacterium on macrophages by down-regulating production of IL-10. More specifically, we observed that targeting of iFt to FcγRs enhances the classical activation of macrophages not only within the respiratory mucosa, but also systemically, at the early stages of infection. These results provide important insight for further understanding the protective immune mechanisms generated when targeting immunogens to Fc receptors.

Development of a Multivalent Subunit Vaccine against Tularemia Using Tobacco Mosaic Virus (TMV) Based Delivery System

Francisella tularensis is a facultative intracellular pathogen, and is the causative agent of a fatal human disease known as tularemia. F. tularensis is classified as a Category A Biothreat agent by the CDC based on its use in bioweapon programs by several countries in the past and its potential to be used as an agent of bioterrorism. No licensed vaccine is currently available for prevention of tularemia. In this study, we used a novel approach for development of a multivalent subunit vaccine against tularemia by using an efficient tobacco mosaic virus (TMV) based delivery platform. The multivalent subunit vaccine was formulated to contain a combination of F. tularensis protective antigens: OmpA-like protein (OmpA), chaperone protein DnaK and lipoprotein Tul4 from the highly virulent F. tularensis SchuS4 strain. Two different vaccine formulations and immunization schedules were used. The immunized mice were challenged with lethal (10xLD₁₀₀) doses of F. tularensis LVS on day 28 of the primary immunization and observed daily for morbidity and mortality. Results from this study demonstrate that TMV can be used as a carrier for effective delivery of multiple F. tularensis antigens. TMV-conjugate vaccine formulations are safe and multiple doses can be administered without causing any adverse reactions in immunized mice. Immunization with TMV-conjugated F. tularensis proteins induced a strong humoral immune response and protected mice against respiratory challenges with very high doses of F. tularensis LVS. This study provides a proof-of-concept that TMV can serve as a suitable platform for simultaneous delivery of multiple protective antigens of F. tularensis. Refinement of vaccine formulations coupled with TMV-targeting strategies developed in this study will provide a platform for development of an effective tularemia subunit vaccine as well as a vaccination approach that may broadly be applicable to many other bacterial pathogens.

Preclinical testing of a vaccine candidate against tularemia

Tularemia is caused by a gram-negative, intracellular bacterial pathogen, Francisella tularensis (Ft). The history weaponization of Ft in the past has elevated concerns that it could be used as a bioweapon or an agent of bioterrorism. Since the discovery of Ft, three broad approaches adopted for tularemia vaccine development have included inactivated, live attenuated, or subunit vaccines. Shortcomings in each of these approaches have hampered the development of a suitable vaccine for prevention of tularemia. Recently, we reported an oxidant sensitive mutant of Ft LVS in putative EmrA1 (FTL_0687) secretion protein. The emrA1 mutant is highly sensitive to oxidants, attenuated for intramacrophage growth and virulence in mice. We reported that EmrA1 contributes to oxidant resistance by affecting the secretion of antioxidant enzymes SodB and KatG. This study investigated the vaccine potential of the emrA1 mutant in prevention of respiratory tularemia caused by Ft LVS and the virulent SchuS4 strain in C57BL/6 mice. We report that emrA1 mutant is safe and can be used at an intranasal (i. n.) immunization dose as high as 1x106 CFU without causing any adverse effects in immunized mice. The emrA1 mutant is cleared by vaccinated mice by day 14-21 post-immunization, induces minimal histopathological lesions in lungs, liver and spleen and a strong humoral immune response. The emrA1 mutant vaccinated mice are protected against 1000-10,000LD100 doses of i.n. Ft LVS challenge. Such a high degree of protection has not been reported earlier against respiratory challenge with Ft LVS using a single immunization dose with an attenuated mutant generated on Ft LVS background. The emrA1 mutant also provides partial protection against i.n. challenge with virulent Ft SchuS4 strain in vaccinated C57BL/6 mice. Collectively, our results further support the notion that antioxidants of Ft may serve as potential targets for development of effective vaccines for prevention of tularemia.

Downmodulation of vaccine-induced immunity and protection against the intracellular bacterium Francisella tularensis by the inhibitory receptor FcγRIIB

Fc gamma receptor IIB (FcγRIIB) is the only Fc gamma receptor (FcγR) which negatively regulates the immune response, when engaged by antigen- (Ag-) antibody (Ab) complexes. Thus, the generation of Ag-specific IgG in response to infection or immunization has the potential to downmodulate immune protection against infection. Therefore, we sought to determine the impact of FcγRIIB on immune protection against Francisella tularensis (Ft), a Category A biothreat agent. We utilized inactivated Ft (iFt) as an immunogen. Naïve and iFt-immunized FcγRIIB knockout (KO) or wildtype (WT) mice were challenged with Ft-live vaccine strain (LVS). While no significant difference in survival between naïve FcγRIIB KO versus WT mice was observed, iFt-immunized FcγRIIB KO mice were significantly better protected than iFt-immunized WT mice. Ft-specific IgA in serum and bronchial alveolar lavage, as well as IFN-γ, IL-10, and TNF-α production by splenocytes harvested from iFt-immunized FcγRIIB KO, were also significantly elevated. In addition, iFt-immunized FcγRIIB KO mice exhibited a reduction in proinflammatory cytokine levels in vivo at 5 days after challenge, which correlates with increased survival following Ft-LVS challenge in published studies. Thus, these studies demonstrate for the first time the ability of FcγRIIB to regulate vaccine-induced IgA production and downmodulate immunity and protection. The immune mechanisms behind the above observations and their potential impact on vaccine development are discussed.

Chemically synthesized molecules with the targeting and effector functions of antibodies

This article reports the design, synthesis, and evaluation of a novel class of molecules of intermediate size (approximately 7000 Da), which possess both the targeting and effector functions of antibodies. These compounds—called synthetic antibody mimics targeting prostate cancer (SyAM-Ps)—bind simultaneously to prostate-specific membrane antigen and Fc gamma receptor I, thus eliciting highly selective cancer cell phagocytosis. SyAMs have the potential to combine the advantages of both small-molecule and biologic therapies, and may address many drawbacks associated with available treatments for cancer and other diseases.

The role of B cells and humoral immunity in Mycobacterium tuberculosis infection

Mycobacterium tuberculosis remains a major public health burden. It is generally thought that while B cell- and antibody-mediated immunity plays an important role in host defense against extracellular pathogens, the primary control of intracellular microbes derives from cellular immune mechanisms. Studies on the immune regulatory mechanisms during infection with M. tuberculosis, a facultative intracellular organism, has established the importance of cell-mediated immunity in host defense during tuberculous infection. Emerging evidence suggest a role for B cell and humoral immunity in the control of intracellular pathogens, including obligatory species, through interactions with the cell-mediated immune compartment. Recent studies have shown that B cells and antibodies can significantly impact on the development of immune responses to the tubercle bacillus. In this review, we present experimental evidence supporting the notion that the importance of humoral and cellular immunity in host defense may not be entirely determined by the niche of the pathogen. A comprehensive approach that examines both humoral and cellular immunity could lead to better understanding of the immune response to M. tuberculosis.

In vivo mechanisms involved in enhanced protection utilizing an Fc receptor-targeted mucosal vaccine platform in a bacterial vaccine and challenge model

Targeting antigens (Ag) to Fcγ receptors (FcγR) intranasally (i.n.) enhances immunogenicity and protection against intracellular and extracellular pathogens. Specifically, we have demonstrated that targeting fixed (inactivated) Francisella tularensis (iFT) organisms to FcR in mice i.n., with MAb-iFT immune complexes, enhances F. tularensis-specific immune responses and protection against F. tularensis challenge. Furthermore, traditional adjuvant is not required. In addition, we have demonstrated that the increased immunogenicity following the targeting of iFT to FcR is due, in part, to enhanced dendritic cell (DC) maturation, enhanced internalization, and processing and presentation of iFT by DCs, as well as neonatal FcR (FcRn)-enhanced trafficking of iFT from the nasal passage to the nasal mucosa-associated lymphoid tissue (NALT). Using this immunization and challenge model, we expanded on these studies to identify specific in vivo immune responses impacted and enhanced by FcR targeting of iFT i.n. Specifically, the results of this study demonstrate for the first time that targeting iFT to FcR increases the frequency of activated DCs within the lungs of MAb-iFT-immunized mice subsequent to F. tularensis LVS challenge. In addition, the frequency and number of gamma interferon (IFN-γ)-secreting effector memory (EM) CD4(+) T cells elicited by F. tularensis infection (postimmunization) is increased in an interleukin 12 (IL-12)-dependent manner. In summary, these studies build significantly upon previously published work utilizing this vaccine platform. We have identified a number of additional mechanisms by which this novel, adjuvant-independent, FcR-targeted mucosal vaccine approach enhances immunity and protection against infection, while further validating its potential as a universal vaccine platform against mucosal pathogens.

Fc receptor-targeting of immunogen as a strategy for enhanced antigen loading, vaccination, and protection using intranasally administered antigen-pulsed dendritic cells

Dendritic cells (DCs) play a critical role in the generation of adaptive immunity via the efficient capture, processing, and presentation of antigen (Ag) to naïve T cells. Administration of Ag-pulsed DCs is also an effective strategy for enhancing immunity to tumors and infectious disease organisms. Studies have also demonstrated that targeting Ags to Fcγ receptors (FcγR) on Ag presenting cells can enhance humoral and cellular immunity in vitro and in vivo. Specifically, our studies using a Francisella tularensis (Ft) infectious disease vaccine model have demonstrated that targeting immunogens to FcγR via intranasal (i.n.) administration of monoclonal antibody (mAb)-inactivated Ft (iFt) immune complexes (ICs) enhances protection against Ft challenge. Ft is the causative agent of tularemia, a debilitating disease of humans and other mammals and a category A biothreat agent for which there is no approved vaccine. Therefore, using iFt Ag as a model immunogen, we sought to determine if ex vivo targeting of iFt to FcγR on DCs would enhance the potency of i.n. administered iFt-pulsed DCs. In this study, bone marrow-derived DCs (BMDCs) were pulsed ex vivo with iFt or mAb-iFt ICs. Intranasal administration of mAb-iFt-pulsed BMDCs enhanced humoral and cellular immune responses, as well as protection against Ft live vaccine strain (LVS) challenge. Increased protection correlated with increased iFt loading on the BMDC surface as a consequence of FcγR-targeting. However, the inhibitory FcγRIIB had no impact on this enhancement. In conclusion, targeting Ag ex vivo to FcγR on DCs provides a method for enhanced Ag loading of DCs ex vivo, thereby reducing the amount of Ag required, while also avoiding the inhibitory impact of FcγRIIB. Thus, this represents a simple and less invasive strategy for increasing the potency of ex vivo-pulsed DC vaccines against chronic infectious diseases and cancer.

Live attenuated mutants of Francisella tularensis protect rabbits against aerosol challenge with a virulent type A strain

Francisella tularensis, a Gram-negative bacterium, is the causative agent of tularemia. No licensed vaccine is currently available for protection against tularemia, although an attenuated strain, dubbed the live vaccine strain (LVS), is given to at-risk laboratory personnel as an investigational new drug (IND). In an effort to develop a vaccine that offers better protection, recombinant attenuated derivatives of a virulent type A strain, SCHU S4, were evaluated in New Zealand White (NZW) rabbits. Rabbits vaccinated via scarification with the three attenuated derivatives (SCHU S4 ΔguaBA, ΔaroD, and ΔfipB strains) or with LVS developed a mild fever, but no weight loss was detected. Twenty-one days after vaccination, all vaccinated rabbits were seropositive for IgG to F. tularensis lipopolysaccharide (LPS). Thirty days after vaccination, all rabbits were challenged with aerosolized SCHU S4 at doses ranging from 50 to 500 50% lethal doses (LD50). All rabbits developed fevers and weight loss after challenge, but the severity was greater for mock-vaccinated rabbits. The ΔguaBA and ΔaroD SCHU S4 derivatives provided partial protection against death (27 to 36%) and a prolonged time to death compared to results for the mock-vaccinated group. In contrast, LVS and the ΔfipB strain both prolonged the time to death, but there were no survivors from the challenge. This is the first demonstration of vaccine efficacy against aerosol challenge with virulent type A F. tularensis in a species other than a rodent since the original work with LVS in the 1960s. The ΔguaBA and ΔaroD SCHU S4 derivatives warrant further evaluation and consideration as potential vaccines for tularemia and for identification of immunological correlates of protection.

Antigen targeting to APC: from mice to veterinary species

Antigen delivery to receptors expressed on antigen presenting cells (APC) has shown to improve immunogenicity of vaccines in mice. An enhancement of cytotoxic T lymphocyte (CTL), helper T cell or humoral responses was obtained depending on the type of APC and the surface molecule targeted. Although this strategy is being also evaluated in livestock animals with promising results, some discrepancies have been found between species and pathogens. The genetic diversity of livestock animals, the different pattern of expression of some receptors among species, the use of different markers to characterize APC in large animals and sometimes the lack of reagents make difficult to compare results obtained in different species. In this review, we summarize the data available regarding antigen targeting to APC receptors in cattle, sheep and pig and discuss the results found in these animals in the context of what has been obtained in mice.