A controlled clinical trial comparing the safety and immunogenicity of a new adjuvanted hepatitis B vaccine with a standard hepatitis B vaccine

Synthesis, evaluation and molecular dynamics study of human toll-like receptor 2/6 specific monoacyl lipopeptides as candidate immunostimulants

TLR2 agonists typified by the S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-R-cysteinyl-S-serine (Pam2CS) motif have exhibited powerful immunostimulatory activities. Based on simplified monoacyl lipopeptide (Carbamate-linked N-Ac PamCS), we describe interesting SAR investigations where modifications are done to alter the size of substituents on the cysteine amine, introduce ionizable groups to the terminal and insert aromatic substitutions to the aliphatic chain. Our structural modifications have led to a highly specific human TLR2/6 agonist 14a (EC50 = 0.424 nM), which behaves like Pam2CSK4 by inducing NF-κB activation to trigger downstream signaling pathways, such as subsequent phosphorylation of related proteins (p65, p38) and production of key inflammatory cytokines (IL-6, IL-1β, TNF-α). Importantly, the ability to stimulate enhanced T cell response compared to Carbamate-linked N-Ac PamCS makes compound 14a a further potential candidate immunostimulant.

Diamino Allose Phosphates: Novel, Potent, and Highly Stable Toll-like Receptor 4 Agonists

Most known synthetic toll-like receptor 4 (TLR4) agonists are carbohydrate-based lipid-A mimetics containing several fatty acyl chains, including a labile 3-O-acyl chain linked to the C-3 position of the non-reducing sugar known to undergo cleavage impacting stability and resulting in loss of activity. To overcome this inherent instability, we rationally designed a new class of chemically more stable synthetic TLR4 ligands that elicit robust innate and adaptive immune responses. This new class utilized a diamino allose phosphate (DAP) scaffold containing a nonhydrolyzable 3-amide bond instead of the classical 3-ester. Accordingly, the DAPs have significantly improved thermostability in aqueous formulations and potency relative to other known natural and synthetic TLR4 ligands. Furthermore, the DAP analogues function as potent vaccine adjuvants to enhance influenza-specific antibodies in mice and provide protection against lethal influenza virus challenges. This novel set of TLR4 ligands show promise as next-generation vaccine adjuvants and stand-alone immunomodulators.

Adjuvant physiochemistry and advanced nanotechnology for vaccine development

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

Recent Advances in the Development of Toll-like Receptor Agonist-Based Vaccine Adjuvants for Infectious Diseases

Vaccines are powerful tools for controlling microbial infections and preventing epidemic diseases. Efficient inactive, subunit, or viral-like particle vaccines usually rely on a safe and potent adjuvant to boost the immune response to the antigen. After a slow start, over the last decade there has been increased developments on adjuvants for human vaccines. The development of adjuvants has paralleled our increased understanding of the molecular mechanisms for the pattern recognition receptor (PRR)-mediated activation of immune responses. Toll-like receptors (TLRs) are a group of PRRs that recognize microbial pathogens to initiate a host’s response to infection. Activation of TLRs triggers potent and immediate innate immune responses, which leads to subsequent adaptive immune responses. Therefore, these TLRs are ideal targets for the development of effective adjuvants. To date, TLR agonists such as monophosphoryl lipid A (MPL) and CpG-1018 have been formulated in licensed vaccines for their adjuvant activity, and other TLR agonists are being developed for this purpose. The COVID-19 pandemic has also accelerated clinical research of vaccines containing TLR agonist-based adjuvants. In this paper, we reviewed the agonists for TLR activation and the molecular mechanisms associated with the adjuvants’ effects on TLR activation, emphasizing recent advances in the development of TLR agonist-based vaccine adjuvants for infectious diseases.

Lipopolysaccharide lipid A: A promising molecule for new immunity-based therapies and antibiotics

Lipopolysaccharides (LPS) are the main components of the external leaflet of the Gram-negative outer membrane and consist of three different moieties: lipid A, core oligosaccharide, and O-polysaccharide. The lipid A is a glucosamine disaccharide with different levels of acylation and phosphorylation, beside carrying, in certain cases, additional substituents on the sugar backbone. It is also the main immunostimulatory part of the LPS, as its recognition by the host immune system represents a fundamental event for detection of perilous microorganisms. Moreover, an uncontrolled immune response caused by a large amount of circulating LPS can lead to dramatic outcomes for human health, such as septic shock. The immunostimulant properties of an LPS incredibly vary depending on lipid A chemical structure, and for this reason, natural and synthetic variants of the lipid A are under study to develop new drugs that mimic or antagonise its natural effects. Here, we review past and recent findings on the lipid A as an antibiotic target and immune-therapeutic molecule, with a special attention on the crucial role of the chemical structure and its exploitation for conceiving novel strategies for treatment of several immune-related pathologies.

Anti-Viral Pattern Recognition Receptors as Therapeutic Targets

Pattern recognition receptors (PRRs) play a central role in the inflammation that ensues following microbial infection by their recognition of molecular patterns present in invading microorganisms but also following tissue damage by recognising molecules released during disease states. Such receptors are expressed in a variety of cells and in various compartments of these cells. PRR binding of molecular patterns results in an intracellular signalling cascade and the eventual activation of transcription factors and the release of cytokines, chemokines, and vasoactive molecules. PRRs and their accessory molecules are subject to tight regulation in these cells so as to not overreact or react in unnecessary circumstances. They are also key to reacting to infection and in stimulating the immune system when needed. Therefore, targeting PRRs offers a potential therapeutic approach for chronic inflammatory disease, infections and as vaccine adjuvants. In this review, the current knowledge on anti-viral PRRs and their signalling pathways is reviewed. Finally, compounds that target PRRs and that have been tested in clinical trials for chronic infections and as adjuvants in vaccine trials are discussed.

Self-Adjuvanting Cancer Vaccines from Conjugation-Ready Lipid A Analogues and Synthetic Long Peptides

Self-adjuvanting vaccines, wherein an antigenic peptide is covalently bound to an immunostimulating agent, have been shown to be promising tools for immunotherapy. Synthetic Toll-like receptor (TLR) ligands are ideal adjuvants for covalent linking to peptides or proteins. We here introduce a conjugation-ready TLR4 ligand, CRX-527, a potent powerful lipid A analogue, in the generation of novel conjugate-vaccine modalities. Effective chemistry has been developed for the synthesis of the conjugation-ready ligand as well as the connection of it to the peptide antigen. Different linker systems and connection modes to a model peptide were explored, and in vitro evaluation of the conjugates showed them to be powerful immune-activating agents, significantly more effective than the separate components. Mounting the CRX-527 ligand at the N-terminus of the model peptide antigen delivered a vaccine modality that proved to be potent in activation of dendritic cells, in facilitating antigen presentation, and in initiating specific CD8⁺ T-cell-mediated killing of antigen-loaded target cells in vivo. Synthetic TLR4 ligands thus show great promise in potentiating the conjugate vaccine platform for application in cancer vaccination.

Innate Immune Response against Hepatitis C Virus: Targets for Vaccine Adjuvants

Despite successful treatments, hepatitis C virus (HCV) infections continue to be a significant world health problem. High treatment costs, the high number of undiagnosed individuals, and the difficulty to access to treatment, particularly in marginalized susceptible populations, make it improbable to achieve the global control of the virus in the absence of an effective preventive vaccine. Current vaccine development is mostly focused on weakly immunogenic subunits, such as surface glycoproteins or non-structural proteins, in the case of HCV. Adjuvants are critical components of vaccine formulations that increase immunogenic performance. As we learn more information about how adjuvants work, it is becoming clear that proper stimulation of innate immunity is crucial to achieving a successful immunization. Several hepatic cell types participate in the early innate immune response and the subsequent inflammation and activation of the adaptive response, principally hepatocytes, and antigen-presenting cells (Kupffer cells, and dendritic cells). Innate pattern recognition receptors on these cells, mainly toll-like receptors, are targets for new promising adjuvants. Moreover, complex adjuvants that stimulate different components of the innate immunity are showing encouraging results and are being incorporated in current vaccines. Recent studies on HCV-vaccine adjuvants have shown that the induction of a strong T- and B-cell immune response might be enhanced by choosing the right adjuvant.

Synthetic Toll-like Receptors 7 and 8 Agonists: Structure-Activity Relationship in the Oxoadenine Series

Toll-like receptors 7 and 8 (TLR7/8) are broadly expressed on antigen-presenting cells, making TLR7/8 agonists likely candidates for the development of new vaccine adjuvants. We previously reported the synthesis of a new series of 8-oxoadenines substituted at the 9-position with a 4-piperidinylalkyl moiety and demonstrated that TLR7/8 selectivity and potency could be modulated by varying the length of the alkyl linker. In the present study, we broadened our initial structure-activity relationship study to further evaluate the effects of N-heterocycle ring size, chirality, and substitution on TLR7/8 potency, receptor selectivity, and cytokine (IFNα and TNFα) induction from human peripheral blood mononuclear cells (PBMCs). TLR7/8 activity correlated primarily to linker length and to a lesser extent to ring size, while ring chirality had little effect on TLR7/8 potency or selectivity. Substitution of the heterocyclic ring with an aminoalkyl or hydroxyalkyl group for subsequent conjugation to phospholipids or antigens was well tolerated with the retention of both TLR7/8 activity and cytokine induction from human PBMCs.

Comparison of adjuvants to optimize influenza neutralizing antibody responses

Seasonal influenza vaccines represent a positive intervention to limit the spread of the virus and protect public health. Yet continual influenza evolution and its ability to evade immunity pose a constant threat. For these reasons, vaccines with improved potency and breadth of protection remain an important need. We previously developed a next-generation influenza vaccine that displays the trimeric influenza hemagglutinin (HA) on a ferritin nanoparticle (NP) to optimize its presentation. Similar to other vaccines, HA-nanoparticle vaccine efficacy is increased by the inclusion of adjuvants during immunization. To identify the optimal adjuvants to enhance influenza immunity, we systematically analyzed TLR agonists for their ability to elicit immune responses. HA-NPs were compatible with nearly all adjuvants tested, including TLR2, TLR4, TLR7/8, and TLR9 agonists, squalene oil-in-water mixtures, and STING agonists. In addition, we chemically conjugated TLR7/8 and TLR9 ligands directly to the HA-ferritin nanoparticle. These TLR agonist-conjugated nanoparticles induced stronger antibody responses than nanoparticles alone, which allowed the use of a 5000-fold-lower dose of adjuvant than traditional admixtures. One candidate, the oil-in-water adjuvant AF03, was also tested in non-human primates and showed strong induction of neutralizing responses against both matched and heterologous H1N1 viruses. These data suggest that AF03, along with certain TLR agonists, enhance strong neutralizing antibody responses following influenza vaccination and may improve the breadth, potency, and ultimately vaccine protection in humans.

Identification and immunological evaluation of novel TLR2 agonists through structure optimization of Pam3CSK4

Toll-like receptor 2 (TLR2) is a bridge between innate immunity and adaptive immunity. TLR2 agonists have been exploited as potential vaccine adjuvants and antitumor agents. However, no TLR2 agonists have been approved by FDA up to now. To discover drug-like TLR2 selective agonists, a novel series of Pam₃CSK₄ derivatives were designed based on the crystal structure of hTLR2-hTLR1-Pam₃CSK₄ complex, synthesized and evaluated for their immune-stimulatory activities. Among them, 35c was identified as a murine-specific TLR2 agonist, while 35f was a human-specific TLR2 agonist. Besides, 35d (human and murine TLR2 agonist) showed TLR2 agonistic activity comparable to Pam₃CSK₄, which included: elevated IL-6 expression level (EC₅₀ = 83.08 ± 5.94 nM), up-regulated TNF-α and IL-6 mRNA expression and promoted maturation of DCs through activating the NF-κB signaling pathway. TLRs antibodies test showed that 35a and 35d were TLR2/1 agonists, while 35f was a TLR2/6 agonist.

Application of built-in adjuvants for epitope-based vaccines

Several studies have shown that epitope vaccines exhibit substantial advantages over conventional vaccines. However, epitope vaccines are associated with limited immunity, which can be overcome by conjugating antigenic epitopes with built-in adjuvants (e.g., some carrier proteins or new biomaterials) with special properties, including immunologic specificity, good biosecurity and biocompatibility, and the ability to vastly improve the immune response of epitope vaccines. When designing epitope vaccines, the following types of built-in adjuvants are typically considered: (1) pattern recognition receptor ligands (i.e., toll-like receptors); (2) virus-like particle carrier platforms; (3) bacterial toxin proteins; and (4) novel potential delivery systems (e.g., self-assembled peptide nanoparticles, lipid core peptides, and polymeric or inorganic nanoparticles). This review primarily discusses the current and prospective applications of these built-in adjuvants (i.e., biological carriers) to provide some references for the future design of epitope-based vaccines.

The Sulfated Polysaccharides of Brown Algae and Products of Their Enzymatic Transformation as Potential Vaccine Adjuvants

The review is devoted to critical analysis of literature data, deal with effects and mechanisms of action of sulfated polysaccharides (PSs) – fucoidans from brown algae and products of their enzymatic transformation as potential adjuvants for enhancement of anti-infective and antitumor immune response. Numerous experimental data indicate that sulfated PSs demonstrate properties of vaccine adjuvants. Application perspectiveness of fucoidans as vaccine adjuvants is defined by their high biocompatibility, low-toxicity, safety and good tolerance by macroorganism, and also mechanisms of their immunomodulatory action. In particular, fucoidans are agonists of receptors of innate immunity and strong inducers of cellular and humoral immune response. At presenting the data of structural - functional interrelations, attention focused to the defining role of degree of sulfation, uronic acids and polyphenols contents, and also molecular mass in actions of fucoidans to innate and adaptive immunity cells. Insufficiency of literary data on studying of correlation of structure – physicochemical characteristics with adjuvanticities of the sulfated PSs, and also the problem of standardization of their active fractions are noted. Special attention is paid to the analysis of immunomodulatory and adjuvant activity of fucoidan oligosaccharides. Presented here results of experimental trial indicate that, despite the difficulties due to preparation of highly purified structurally characterized fractions and complex structure of fucoidans, these substances can be used as safe and effective adjuvants in vaccines against various pathogens including viruses, and also in antitumor vaccines.

Recent Advances in Toll Like Receptor-Targeting Glycoconjugate Vaccines

Many malignant cell surface carbohydrates resulting from abnormal glycosylation patterns of certain diseases can serve as antigens for the development of vaccines against these diseases. However, carbohydrate antigens are usually poorly immunogenic by themselves, thus they need to be covalently coupled with immunologically active carrier molecules to be functional. The most well established and commonly used carriers are proteins. In recent years, the use of toll-like receptor (TLR) ligands to formulate glycoconjugate vaccines has gained significant attention because TLR ligands can serve not only as carrier molecules but also as built-in adjuvants to form fully synthetic and self-adjuvanting conjugate vaccines, which have several advantages over carbohydrate-protein conjugates and formulated mixtures with external adjuvants. This article reviews recent progresses in the development of conjugate vaccines based on TLR ligands. Two major classes of TLR ligands, lipopeptides and lipid A derivatives will be covered with more focus on monophosohoryl lipid A (MPLA) and related analogs, which are TLR4 ligands demonstrated to be able to provoke T cell-dependent, adaptive immune responses. Corresponding conjugate vaccines have shown promising application potentials to multiple diseases including cancer.

Recent advances in the discovery and development of TLR ligands as novel therapeutics for chronic HBV and HIV infections

INTRODUCTION

Toll-like receptor (TLR) ligands remain as promising antiviral drug candidates for the treatment of chronic viral infections. Basic research on the mechanisms of antiviral activity of TLR ligands in preclinical animal models and clinical testing of drug candidates have been carried out in recent years. Areas covered: This review provides an overview of the preclinical and clinical testing of TLR ligands in two major viral infections: hepatitis B virus (HBV) and human immunodeficiency virus (HIV). Recent results have further demonstrated the potent antiviral activity of various TLR ligands . A TLR7 agonist is in clinical trials for the treatment of chronic HBV infection while a HBV vaccine using a TLR9 ligand as an adjuvant has proven to be superior to conventional HBV vaccines and has been approved for clinical use. Generally, TLR activation may achieve viral control mainly by promoting adaptive immunity to viral proteins. Expert opinion: Recent research in this field indicates that TLR ligands could be developed as clinically effective drugs if the obstacles concerning toxicity and application routes are overcome. TLR-mediated promotion of adaptive immunity is a major issue for future studies and will determine the future development of TLR ligands as drugs for immunomodulation.

[Evaluation of adjuvant effects of fucoidane from brown seaweed Fucus evanescens and its structural analogues for the strengthening vaccines effectiveness]

The use of sulfated polysaccharides from brown seaweed Fucus evanescens as adjuvants (native fucoidan in combination with polyphenols, fucoidan without polyphenols, a product of enzymatic hydrolysis of fucoidan) stimulated the formation of specific antibodies to the surface antigen of the hepatitis B virus (HBs-AG). Immunization of mice with vaccine compositions containing HBs-AG and fucoidan samples resulted in increasing the serum level of the pro-inflammatory (TNF-a, IFN-g, IL-2) cytokines. Increased production of these cytokines was detected in the culture of splenocytes additionally stimulated in vitro by fucoidans or phytohemagglutinin. The adjuvant effect of fucoidan and its structural modifications was comparable to that of the traditional licensed adjuvant aluminum hydroxide. The obtained results indicate a promising use of sulfated polysaccharides from F. evanescens as vaccine adjuvants.

Characterization of innate responses induced by in ovo administration of encapsulated and free forms of ligands of Toll-like receptor 4 and 21 in chicken embryos

Toll-like receptors (TLRs) are a family of innate receptors that recognize pathogen-associated molecular patterns, including double-stranded RNA, CpG DNA and lipopolysaccharide (LPS). After interaction with their ligands, TLRs initiate innate responses that are manifested by activating cells and inducing expression of cytokines that help mediate adaptive immune responses. TLR ligands (TLR-Ls) have the potential to be used prophylactically (alone) or as vaccine adjuvants to promote host immunity. Encapsulating TLR-Ls in nanoparticles, such as Poly (d,l-lactic-co-glycolic acid), may prolong responses through sustained release of the ligands. PLGA nanoparticles protect encapsulated TLR-Ls from degradation and extend the half-life of these ligands by reducing their rapid removal from the body. In this study, encapsulated and free forms of LPS and CpG ODN were administered to embryonation day 18 (ED18) chicken embryos. Spleen, lungs and bursa of Fabricius were collected at 6, 18 and 48hour post-stimulation (hps) and cytokine gene expressions were evaluated using quantitative real-time PCR. Results indicate that both the free and encapsulated forms of LPS and CpG ODN induced innate immune responses in ED18 chicken embryos. Innate responses induced in embryos seem similar to those reported in mature chickens. Significant upregulation of cytokine genes generally occurred by 48hps. Further studies are needed to evaluate long term immunomodulatory effects of encapsulated TLR-Ls and their ability to mediate protection against pathogens of young chicks.

Targeting Pattern Recognition Receptors (PRR) for Vaccine Adjuvantation: From Synthetic PRR Agonists to the Potential of Defective Interfering Particles of Viruses

Modern vaccinology has increasingly focused on non-living vaccines, which are more stable than live-attenuated vaccines but often show limited immunogenicity. Immunostimulatory substances, known as adjuvants, are traditionally used to increase the magnitude of protective adaptive immunity in response to a pathogen-associated antigen. Recently developed adjuvants often include substances that stimulate pattern recognition receptors (PRRs), essential components of innate immunity required for the activation of antigen-presenting cells (APCs), which serve as a bridge between innate and adaptive immunity. Nearly all PRRs are potential targets for adjuvants. Given the recent success of toll-like receptor (TLR) agonists in vaccine development, molecules with similar, but additional, immunostimulatory activity, such as defective interfering particles (DIPs) of viruses, represent attractive candidates for vaccine adjuvants. This review outlines some of the recent advances in vaccine development related to the use of TLR agonists, summarizes the current knowledge regarding DIP immunogenicity, and discusses the potential applications of DIPs in vaccine adjuvantation.

Triggering Intracellular Receptors for Vaccine Adjuvantation

Immune adjuvants are components that stimulate, potentiate, or modulate the immune response to an antigen. They are key elements of vaccines in both the prophylactic and therapeutic domains. In the past decade substantial progress in our understanding of innate immunity has paved the way for the design of next-generation adjuvants that stimulate a wide range of receptors. Within the framework of vaccine adjuvant design, this review outlines the interest of targeting endosomal and intracellular receptors to enhance and guide the immune response. We present and compare the molecules as well as potential combinations which are currently in the spotlight. We emphasize how targeting the appropriate receptor can direct immunity towards the appropriate response, such as a cytotoxic or mucosal response.

Specific activation of the TLR1-TLR2 heterodimer by small-molecule agonists

Toll-like receptor (TLR) agonists activate both the innate and the adaptive immune systems. These TLR agonists have been exploited as potent vaccine adjuvants and antitumor agents. We describe the identification and characterization of a small molecule, N-methyl-4-nitro-2-(4-(4-(trifluoromethyl)phenyl)-1 H-imidazol-1-yl)aniline (CU-T12-9), that directly targets TLR1/2 to initiate downstream signaling. CU-T12-9 specifically induces TLR1/2 activation, which can be blocked by either the anti-hTLR1 or the anti-hTLR2 antibody, but not the anti-hTLR6 antibody. Using a variety of different biophysical assays, we have demonstrated the binding mode of CU-T12-9. By binding to both TLR1 and TLR2, CU-T12-9 facilitates the TLR1/2 heterodimeric complex formation, which in turn activates the downstream signaling. Fluorescence anisotropy assays revealed competitive binding to the TLR1/2 complex between CU-T12-9 and Pam3CSK4 with a half-maximal inhibitory concentration (IC50) of 54.4 nM. Finally, we showed that CU-T12-9 signals through nuclear factor κB (NF-κB) and invokes an elevation of the downstream effectors tumor necrosis factor-α (TNF-α), interleukin-10 (IL-10), and inducible nitric oxide synthase (iNOS). Thus, our studies not only provide compelling new insights into the regulation of TLR1/2 signaling transduction but also may facilitate future therapeutic developments.

Perspective of Peptide Vaccine Composed of Epitope Peptide, CpG-DNA, and Liposome Complex Without Carriers

The magnitude and specificity of cell-mediated and humoral immunity are critically determined by peptide sequences; peptides corresponding to the B- or T-cell receptor epitopes are sufficient to induce an effective immune response if delivered properly. Therefore, studies on the screening and application of peptide-based epitopes have been done extensively for the development of therapeutic antibodies and prophylactic vaccines. However, the efficacy of immune response and antibody production by peptide-based immunization is too limited for human application at the present. To improve the efficacy of vaccines, researchers formulated adjuvants such as alum, water-in-oil emulsion, and Toll-like receptor agonists. They also employed liposomes as delivering vehicles to stimulate immune responses. Here, we review our recent studies providing a potent method of epitope screening and antibody production without conventional carriers. We adopted Lipoplex(O), comprising a natural phosphodiester bond CpG-DNA and a specific liposome complex, as an adjuvant. Lipoplex(O) induces potent stimulatory activity in humans as well as in mice, and immunization of mice with several peptides along with Lipoplex(O) without general carriers induces significant production of each peptide-specific IgG2a. Immunization of peptide vaccines against virus-associated antigens in mice has protective effects against the viral infection. A peptide vaccine against carcinoma-associated antigen and the peptide-specific monoclonal antibody has functional effects against cancer cells in mouse models. In conclusion, we improved the efficacy of peptide vaccines in mice. Our strategy can be applied in development of therapeutic antibodies or in defense against pandemic infectious diseases through rapid screening of potent B-cell epitopes.

Toll-like receptor (TLR)-mediated innate immune responses in the control of hepatitis B virus (HBV) infection

The role of adaptive immune responses in the control of hepatitis B virus (HBV) infection is well accepted. The contribution of innate immune responses to the viral control is recognized but yet not fully understood. Toll-like receptors (TLRs) sense pathogen-associated molecule patterns and activate antiviral mechanisms including the intracellular antiviral pathways and the production of antiviral effectors like interferons (IFNs) and pro-inflammatory cytokines. Activation of the TLR3 pathway and the production of IFN-β represent one of the major mechanisms leading to the suppression of HBV replication in the liver, as shown in different in vitro and in vivo models. TLR4 signaling and TLR2 signaling result in the activation of intracellular pathways including MAPK and PI-3 K/Akt in hepatocytes and reduce HBV replication in an IFN-independent manner. HBV is able to counteract the actions of TLR3 and TLR2/4 through downregulation of TLR expression and attenuation of the cellular signaling pathways. Thus, TLR ligands are promising candidates as immunomodulators and therapeutics for the treatment of chronic HBV infection. Specific antiviral treatment against HBV could recover the TLR functions in chronic HBV infection and increase the effectiveness of therapeutic approaches based on TLR activation.

Immune Adjuvant Effect of Molecularly-defined Toll-Like Receptor Ligands

Vaccine efficacy is optimized by addition of immune adjuvants. However, although adjuvants have been used for over a century, to date, only few adjuvants are approved for human use, mostly aimed at improving vaccine efficacy and antigen-specific protective antibody production. The mechanism of action of immune adjuvants is diverse, depending on their chemical and molecular nature, ranging from non-specific effects (i.e., antigen depot at the immunization site) to specific activation of immune cells leading to improved host innate and adaptive responses. Although the detailed molecular mechanism of action of many adjuvants is still elusive, the discovery of Toll-like receptors (TLRs) has provided new critical information on immunostimulatory effect of numerous bacterial components that engage TLRs. These ligands have been shown to improve both the quality and the quantity of host adaptive immune responses when used in vaccine formulations targeted to infectious diseases and cancer that require both humoral and cell-mediated immunity. The potential of such TLR adjuvants in improving the design and the outcomes of several vaccines is continuously evolving, as new agonists are discovered and tested in experimental and clinical models of vaccination. In this review, a summary of the recent progress in development of TLR adjuvants is presented.

Modern subunit vaccines: development, components, and research opportunities

Traditional vaccines, based on the administration of killed or attenuated microorganisms, have proven to be among the most effective methods for disease prevention. Safety issues related to administering these complex mixtures, however, prevent their universal application. Through identification of the microbial components responsible for protective immunity, vaccine formulations can be simplified, enabling molecular-level vaccine characterization, improved safety profiles, prospects to develop new high-priority vaccines (e.g. for HIV, tuberculosis, and malaria), and the opportunity for extensive vaccine component optimization. This subunit approach, however, comes at the expense of decreased immunity, requiring the addition of immunostimulatory agents (adjuvants). As few adjuvants are currently used in licensed vaccines, adjuvant development represents an exciting area for medicinal chemists to play a role in the future of vaccine development. In addition, immune responses can be further customized though optimization of delivery systems, tuning the size of particulate vaccines, targeting specific cells of the immune system (e.g. dendritic cells), and adding components to aid vaccine efficacy in whole immunized populations (e.g. promiscuous T-helper epitopes). Herein we review the current state of the art and future direction in subunit vaccine development, with a focus on the described components and their potential to steer the immune response toward a desired response.

Preclinical development of TLR ligands as drugs for the treatment of chronic viral infections

INTRODUCTION

Toll-like receptors (TLRs) have been identified as key regulators of innate and adaptive immune responses in viral infection. Recent progress in this field revealed that there are significant interactions between the TLR system and pathogens in chronic viral infections. Therefore, TLR ligands have great potential for the treatment of chronic viral infections.

AREAS COVERED

This review provides an overview of the methodology for preclinical testing of TLR ligands for three major viral infections: hepatitis B virus (HBV), hepatitis C virus (HCV) and human immunodeficiency virus (HIV). TLR ligands have shown potent antiviral activity in different cell culture systems as well as animal models for these infections and induce the production of antiviral cytokines, modulated cellular immunological functions and antiviral effects in vivo.

EXPERT OPINION

The recent progress in this field demonstrated that activation of a large number of TLR ligands is effective against viral infections in cell culture systems and animal models. Exploring these models, further in-depth elucidation of the molecular and immunological mechanisms of the antiviral activity of TLR ligands will be necessary to develop them into clinical useful drugs.

Synthetic molecules and functionalized nanoparticles targeting the LPS-TLR4 signaling: A new generation of immunotherapeutics

Toll-like receptor 4 (TLR4), the receptor of bacterial endotoxins in mammalians, plays a pivotal role in the induction of innate immunity and inflammation. TLR4 activation by bacterial lipopolysaccharide (LPS) is achieved by the coordinate and sequential action of three other proteins, the lipopolysaccharide binding protein (LBP), the cluster differentiation antigen CD14, and the myeloid differentiation protein (MD-2) receptors, that bind LPS and present it in a monomeric form to TLR4 by forming the activated [TLR4·MD-2·LPS]2 complex. Small molecules and nanoparticles active in modulating the TLR4 signal by targeting directly the MD-2·TLR4 complex or by interfering in other points of the TLR4 signaling are presented in this paper. These compounds have great pharmacological interest as vaccine adjuvants, immunotherapeutics, anti-sepsis, and anti-inflammatory agents.

Monophosphoryl lipid A induced innate immune responses via TLR4 to enhance clearance of nontypeable Haemophilus influenzae and Moraxella catarrhalis from the nasopharynx in mice

Acute otitis media (AOM) is one of the most common infectious diseases in children. Nontypeable Haemophilus influenzae (NTHi) and Moraxella catarrhalis, Gram-negative bacteria, are considered major pathogens of AOM and respiratory tract infections. In this study, we used monophosphoryl lipid A (MPL) as a Toll-like receptor (TLR4) agonist to induce innate immune responses before challenge with NTHi and M. catarrhalis to enhance bacterial clearance from the nasopharynx. Mice were intranasally administered 40, 10, or 1 μg of MPL and challenged with NTHi and M. catarrhalis 12 and 24 h later. At 6 and 12 h after the bacterial challenge, the mice were killed and nasal washes were collected. The numbers of NTHi, M. catarrhalis, and inflammatory cells were quantitated. Inoculation of MPL produced a significant reduction in the number of bacteria recovered from the nasopharynx at 6 and/or 12 h after the bacterial challenge, when compared with control mice. The effect was dose dependent. MPL inoculation also induced the early accumulation of neutrophils in the nasopharynx after exposure to bacteria. MPL is effective for eliciting clearance of both NTHi and M. catarrhalis from the nasopharynx. These results indicate the possibility of a new strategy against Gram-negative bacterial infection that involves the stimulation of the innate immune system by TLR4 agonists such as MPL.

Targeting of Toll-like receptors: a decade of progress in combating infectious diseases

Toll-like receptors (TLRs) recognise highly conserved molecular structures, collectively known as pathogen-associated molecular patterns. In the past two decades, development and clinical implementation of TLR ligands-ie, chemically modified synthetic derivatives of naturally occurring ligands and fully synthetic small molecules-have been topics of intense research. Targeted manipulation of TLR signalling has been applied clinically to boost vaccine effectiveness, promote a robust T helper 1-predominant immune response against viral infection, or dampen the exaggerated inflammatory response to bacterial infection. Use of these new therapeutic molecules as adjuncts to conventional pharmacotherapy or stand-alone treatments might offer solutions to unmet clinical needs or could replace existing partly effective therapeutic strategies.

TLR-based immune adjuvants

This work describes the nature and strength of the immune response induced by various Toll-like receptor ligands and their ability to act as vaccine adjuvants. It reviews the various ligands capable of triggering individual TLRs, and then focuses on the efficacy and safety of those agents for which clinical results are available.

Advances in the design and delivery of peptide subunit vaccines with a focus on toll-like receptor agonists

Considerable success has been made with many peptide antigen formulations, and peptide-based vaccines are emerging as the next generation of prophylactic and remedial immunotherapy. However, finding an optimal platform balancing all of the requirements for an effective, specific and safe immune response remains a major challenge for many infectious and chronic diseases. This review outlines how peptide immunogenicity is influenced by the way in which peptides are presented to the immune system, underscoring the need for multifunctional delivery systems that couple antigen and adjuvant into a single construct. Particular attention is given to the ability of Toll-like receptor agonists to act as adjuvants. A survey of recent approaches to developing peptide antigen delivery systems is given, many of which incorporate Toll-like receptor agonists into the design.

Pathogen recognition and inflammatory signaling in innate immune defenses

The innate immune system constitutes the first line of defense against invading microbial pathogens and relies on a large family of pattern recognition receptors (PRRs), which detect distinct evolutionarily conserved structures on pathogens, termed pathogen-associated molecular patterns (PAMPs). Among the PRRs, the Toll-like receptors have been studied most extensively. Upon PAMP engagement, PRRs trigger intracellular signaling cascades ultimately culminating in the expression of a variety of proinflammatory molecules, which together orchestrate the early host response to infection, and also is a prerequisite for the subsequent activation and shaping of adaptive immunity. In order to avoid immunopathology, this system is tightly regulated by a number of endogenous molecules that limit the magnitude and duration of the inflammatory response. Moreover, pathogenic microbes have developed sophisticated molecular strategies to subvert host defenses by interfering with molecules involved in inflammatory signaling. This review presents current knowledge on pathogen recognition through different families of PRRs and the increasingly complex signaling pathways responsible for activation of an inflammatory and antimicrobial response. Moreover, medical implications are discussed, including the role of PRRs in primary immunodeficiencies and in the pathogenesis of infectious and autoimmune diseases, as well as the possibilities for translation into clinical and therapeutic applications.

Safety and immunogenicity of a CTL multiepitope peptide vaccine for HIV with or without GM-CSF in a phase I trial

There is an urgent need for a vaccine capable of preventing HIV infection or the development of HIV-related disease. A number of approaches designed to stimulate HIV-specific CD8+ cytotoxic T cell responses together with helper responses are presently under evaluation. In this phase 1, multi-center, placebo-controlled trial, we tested the ability of a novel multiepitope peptide vaccine to elicit HIV-specific immunity. To enhance the immunogenicity of the peptide vaccine, half of the vaccine recipients received recombinant granulocyte-macrophage colony stimulating factor (GM-CSF) protein as a coadjuvant. The vaccine was safe; tolerability was moderate, with a number of adverse events related to local injection site reactogenicity. Anti-GM-CSF antibody responses developed in the majority of GM-CSF recipients but were not associated with adverse hematologic events. The vaccine was only minimally immunogenic. Six of 80 volunteers who received vaccine developed HIV-specific responses as measured by interferon-gamma ELISPOT assay, and measurable responses were transient. This study failed to demonstrate that GM-CSF can substantially improve the overall weak immunogenicity of a multiepitope peptide-based HIV vaccine.

Clinical investigations of Toll-like receptor agonists

BACKGROUND

Toll-like receptors (TLR) represent a family of surface molecules that function as primary sensors of the innate immune system to recognize microbial pathogens. Ligand binding to TLR results in activation of cellular signaling pathways that regulate expression of genes involved in inflammation and immunity.

OBJECTIVE

Use of synthetic TLR ligands (agonists) for treatment and prevention of infectious and neoplastic diseases.

METHODS

Review of literature about clinical investigations of agonists of TLR 4, 7, 8, and 9.

RESULTS/CONCLUSIONS

Imiquimod was the first TLR agonist approved for treatment of anogenital warts, actinic keratosis and superficial basal cell carcinoma in humans. Several other agonists of TLRs 4, 7, 8 and 9 were also shown to be effective for treatment of infections and cancers and, furthermore, were used as adjuvants for vaccination. Based on safety and efficacy of the TLR agonists used to date, applications are likely to increase in the future.

The science of adjuvants

Adjuvants are substances that boost the immunogenicity of vaccines. However, most successful vaccines have been derived empirically and are capable of inducing robust T- and B-cell immunity without any adjuvant additives. Emerging evidence suggests that such live vaccines induce innate immune activation via a range of stimuli, including ligands specific for Toll-like receptors, which, in effect, serve as their own adjuvants. In contrast to these live vaccines, subunit vaccines need to be supplemented with adjuvants to boost their immunogenicity. However, there is a paucity of licensed adjuvants for clinical use and, thus, there is a critical need to develop safe and effective adjuvants. In this context, recent advances in innate immunity are beginning to offer new insights into how empiric vaccines and adjuvants mediate their efficacy. In this article, we review the latest progress and emerging concepts in adjuvant development, which includes novel findings in innate immune biology and their impact on vaccinology.

Update on toll-like receptor-directed therapies for human disease

Innate responses to microbes are mediated in large part by toll-like receptors (TLRs), which recognise a diverse range of molecules produced by viruses, bacteria and fungi. Great effort has been directed towards translating this knowledge into the development of new therapies for a wide spectrum of diseases, including infectious, malignant, autoimmune and allergic diseases. This review will provide a brief update on completed, ongoing and planned clinical trials of TLR ligand-based therapies for the treatment of diseases in humans.

The perfect mix: recent progress in adjuvant research

Developing efficient and safe adjuvants for use in human vaccines remains both a challenge and a necessity. Past approaches have been largely empirical and generally used a single type of adjuvant, such as aluminium salts or emulsions. However, new vaccine targets often require the induction of well-defined cell-mediated responses in addition to antibodies, and thus new immunostimulants are required. Recent advances in basic immunology have elucidated how early innate immune signals can shape subsequent adaptive responses and this, coupled with improvements in biochemical techniques, has led to the design and development of more specific and focused adjuvants. In this Review, I discuss the research that has made it possible for vaccinologists to now be able to choose between a large panel of adjuvants, which potentially can act synergistically, and combine them in formulations that are specifically adapted to each target and to the relevant correlate(s) of protection.

Therapeutic targeting of innate immunity with Toll-like receptor agonists and antagonists

The identification of the antigen recognition receptors for innate immunity, most notably the Toll-like receptors, has sparked great interest in therapeutic manipulation of the innate immune system. Toll-like receptor agonists are being developed for the treatment of cancer, allergies and viral infections, and as adjuvants for potent new vaccines to prevent or treat cancer and infectious diseases. As recognition grows of the role of inappropriate Toll-like receptor stimulation in inflammation and autoimmunity, significant efforts have begun to develop antagonists to Toll-like receptors as well.

Preparation of recombinant vaccines

Vaccination is one of the most efficient ways to eradicate some infectious diseases in humans and animals. The material traditionally used as vaccines is attenuated or inactivated pathogens. This approach is sometimes limited by the fact that the material for vaccination is not efficient, not available, or generating deleterious side effects. A possible theoretical alternative is the use of recombinant proteins from the pathogens. This implies that the proteins having the capacity to vaccinate have been identified and that they can be produced in sufficient quantity at a low cost. Genetically modified organisms harboring pathogen genes can fulfil these conditions. Microorganisms, animal cells as well as transgenic plants and animals can be the source of recombinant vaccines. Each of these systems that are all getting improved has advantages and limits. Adjuvants must generally be added to the recombinant proteins to enhance their vaccinating capacity. This implies that the proteins used to vaccinate have been purified to avoid any immunization against the contaminants. The efficiency of a recombinant vaccine is poorly predictable. Multiple proteins and various modes of administration must therefore be empirically evaluated on a case-by-case basis. The structure of the recombinant proteins, the composition of the adjuvants and the mode of administration of the vaccines have a strong and not fully predictable impact on the immune response as well as the protection level against pathogens. Recombinant proteins can theoretically also be used as carriers for epitopes from other pathogens. The increasing knowledge of pathogen genomes and the availability of efficient systems to prepare large amounts of recombinant proteins greatly facilitate the potential use of recombinant proteins as vaccines. The present review is a critical analysis of the state of the art in this field.