Liposomal vaccines: clinical status and immunological presentation for humoral and cellular immunity

Immunomodulatory properties of nanostructured systems for cancer therapy

Based on statistical data reported in 2020, cancer was responsible for approximately 10 million deaths. Furthermore, 17 million new cases were diagnosed worldwide. Nanomedicine and immunotherapy have shown satisfactory clinical results among all scientific and technological alternatives for the treatment of cancer patients. Immunotherapy-based treatments comprise the consideration of new alternatives to hinder neoplastic proliferation and to reduce adverse events in the body, thereby promoting immune destruction of diseased cells. Additionally, nanostructured systems have been proven to elicit specific immune responses that may enhance anti-tumor activity. A new generation of nanomedicines, based on biomimetic and bioinspired systems, has been proposed to target tumors by providing immunomodulatory features and by enabling recovery of human immune destruction capacity against cancer cells. This review provides an overview of the aspects and the mechanisms by which nanomedicines can be used to enhance clinical procedures using the immune modulatory responses of nanoparticles (NPs) in the host defense system. We initially outline the cancer statistics for conventional and new treatment approaches providing a brief description of the human host defense system and basic principles of NP interactions with monocytes, leukocytes, and dendritic cells for the modulation of antitumor immune responses. A report on different biomimetic and bioinspired systems is also presented here and their particularities in cancer treatments are addressed, highlighting their immunomodulatory properties. Finally, we propose future perspectives regarding this new therapeutic strategy, highlighting the main challenges for future use in clinical practice.

Axial pharmaceutical properties of liposome in cancer therapy: Recent advances and perspectives

Evaluation of axial properties including preparation, surface functionalization, and pharmacokinetics for delivery of pharmacologically active molecules and genes lead to pharmaceutical development of liposome in cancer therapy. Here, analysis of effects of the axial properties of liposome based on cancer treatment modalities as individually and coherently is vital and shows deserving further investigation for the future. In this review, recent progress in the analysis of preparation approaches, optimizing pharmacokinetic parameters, functionalization and targeting improvement and modulation of biological factors and components resulting in a better function of liposome in cancer for drug/gene delivery and immunotherapy are discussed. Here, recent developments on liposome with vaccines and immunoadjuvant carriers, and antigen-carrier system to cancer immunotherapy are introduced.

Protection induced by Leishmania Major antigens and the imiquimod adjuvant encapsulated on liposomes in experimental cutaneous leishmaniasis

There is a need for new, effective, and less expensive and toxic treatment for Leishmaniasis. It seems that the use of a suitable adjuvant and a delivery system is effective in inducing immune reactions for protection. Liposomes can be applied as immunoadjuvants to trigger immune reactions to different antigens. The adjuvant effects of imiquimod using DSPC liposomes containing SLA (soluble Leishmania antigens) were studied on the type and intensity of the produced immune reaction to the challenge of Leishmania major in BALB/c mice. Liposomes were produced by the lipid film procedure. BALB/C mice were immunized subcutaneously, three times at 2-week intervals and with various formulations. Lesion development and the parasite burden in the spleens and feet after the challenge with Leishmania major, Th1 cytokine (IFN-γ), and the IgG isotype titration were assessed to evaluate the induced immune reaction and the protection level. The group of mice immunized with Liposome DSPC +Imiquimod +SLA revealed less severe footpad swelling, being significantly different (P < .05) from other groups. A higher level of IgG2a and IFN-γ secretion was observed in the mice immunized with Liposome DSPC +Imiquimod +SLA than the control group. These observations imply that the DSPC liposome containing imiquimod induces the Th1 immune response that is protective against the challenge of Leishmania major.

The immune system of the gut and potential adverse effects of oral nanocarriers on its function

There is substantial effort in modern pharmacotherapy to use nanoparticle-based drug delivery systems (nDDS) for improving the oral absorption of drugs. An often neglected circumstance regarding this approach is that the gut is a major part of the immune system that may be vulnerable for immune-cell toxicity, or mediate humoral immune response against various components of nDDS, recognized as foreign. This review recapitulates the structure and function of gut-associated lymphoid tissue (GALT), i.e., the enteral section of mucosa-associated lymphoid tissue (MALT) and reminds how virus-like nDDS may potentially induce immunogenicity just as attenuated or killed viruses do in oral vaccines. Furthermore, we present examples for immune toxicities of emulsifiers and polymer-containing micelles, manifested in complement activation-related pseudoallergy (CARPA). A major message of the review is that early testing of immunogenicity or other adverse immune effects of nDDS in appropriate test systems or models may be prudent to recognize the risk of rare immune problems that may surface in late-stage clinical trials or after marketing of nDDS.

Liposomal adjuvants for human vaccines

INTRODUCTION

Liposomes are well-known as drug carriers, and are now critical components of two of six types of adjuvants present in licensed vaccines. The liposomal vaccine adjuvant field has long been dynamic and innovative, and research in this area is further examined as new commercial products appear in parallel with new vaccines. In an arena where successful products exist the potential for new types of vaccines with liposomal adjuvants, and alternative liposomal adjuvants that could emerge for new types of vaccines, are discussed.

AREAS COVERED

Major areas include: virosomes, constructed from phospholipids and proteins from influenza virus particles; liposomes containing natural and synthetic neutral or anionic phospholipids, cholesterol, natural or synthetic monophosphoryl lipid A, and QS21 saponin; non-phospholipid cationic liposomes; and combinations and mixtures of liposomes and immunostimulating ingredients as adjuvants for experimental vaccines.

EXPERT OPINION

Liposomes containing monophosphoryl lipid A and QS21 have considerable momentum that will result soon in emergence of prophylactic vaccines to malaria and shingles, and possible novel cancer vaccines. The licensed virosome vaccines to influenza and hepatitis A will be replaced with virosome vaccines to other infectious diseases. Alternative liposomal formulations are likely to emerge for difficult diseases such as tuberculosis or HIV-1 infection.

Differential immune responses to HIV-1 envelope protein induced by liposomal adjuvant formulations containing monophosphoryl lipid A with or without QS21

Liposomes have shown promise as constituents of adjuvant formulations in vaccines to parasitic and viral diseases. A particular type of liposomal construct, referred to as Army Liposome Formulation (ALF), containing neutral and anionic saturated phospholipids, cholesterol, and monophosphoryl lipid A (MPLA), has been used as an adjuvant for many years. Here we investigated the effects of physical and chemical changes of ALF liposomes on adjuvanted immune responses to CN54 gp140, a recombinant HIV-1 envelope protein. While holding the total amounts of liposomal MPLA and the gp140 antigen constant, different liposome sizes and liposomal MPLA:phospholipid molar ratios, and the effect of adding QS21 to the liposomes were compared for inducing immune responses to the gp140. For liposomes lacking QS21, higher titers of IgG binding antibodies to gp140 were induced by small unilamellar vesicle (SUV) rather than by large multilamellar vesicle (MLV) liposomes, and the highest titers were obtained with SUV having the MPLA:phospholipid ratio of 1:5.6. ALF plus QS21 (ALFQ) liposomes induced the same maximal binding antibody titers regardless of the MPLA:phospholipid ratio. ALF MLV liposomes induced mainly IgG1 and very low IgG2a antibodies, while ALF SUV liposomes induced IgG1≥IgG2a>IgG2b antibodies. Liposomes containing QS21 induced IgG1>IgG2a>IgG2b>IgG3 antibodies. ELISPOT analysis of splenocytes from immunized mice revealed that ALF liposomes induced low levels of IFN-γ, but ALFQ induced high levels. ALF and ALFQ liposomes each induced approximately equivalent high levels of IL-4. Based on antibody subtypes and cytokine secretion, we conclude that ALF liposomes predominantly stimulate Th2, while ALFQ strongly induces both Th1 and Th2 immunity. When CN54 gp140 was adjuvanted with either ALF or ALFQ liposomes, antibodies were induced that neutralized two HIV-1 tier 1 clade C strain pseudoviruses.

Anthrax vaccine antigen-adjuvant formulations completely protect New Zealand white rabbits against challenge with Bacillus anthracis Ames strain spores

In an effort to develop an improved anthrax vaccine that shows high potency, five different anthrax protective antigen (PA)-adjuvant vaccine formulations that were previously found to be efficacious in a nonhuman primate model were evaluated for their efficacy in a rabbit pulmonary challenge model using Bacillus anthracis Ames strain spores. The vaccine formulations include PA adsorbed to Alhydrogel, PA encapsulated in liposomes containing monophosphoryl lipid A, stable liposomal PA oil-in-water emulsion, PA displayed on bacteriophage T4 by the intramuscular route, and PA mixed with Escherichia coli heat-labile enterotoxin administered by the needle-free transcutaneous route. Three of the vaccine formulations administered by the intramuscular or the transcutaneous route as a three-dose regimen induced 100% protection in the rabbit model. One of the formulations, liposomal PA, also induced significantly higher lethal toxin neutralizing antibodies than PA-Alhydrogel. Even 5 months after the second immunization of a two-dose regimen, rabbits vaccinated with liposomal PA were 100% protected from lethal challenge with Ames strain spores. In summary, the needle-free skin delivery and liposomal formulation that were found to be effective in two different animal model systems appear to be promising candidates for next-generation anthrax vaccine development.

Lipid nanocapsule as vaccine carriers for his-tagged proteins: evaluation of antigen-specific immune responses to HIV I His-Gag p41 and systemic inflammatory responses

The purpose of this study was to design novel nanocapsules (NCs) with surface-chelated nickel (Ni-NCs) as a vaccine delivery system for histidine (His)-tagged protein antigens. Ni-NCs were characterized for binding His-tagged model proteins through high-affinity non-covalent interactions. The mean diameter and zeta potential of the optimized Ni-NCs were 214.9 nm and -14.8 mV, respectively. The optimal binding ratio of His-tagged Green Fluorescent Protein (His-GFP) and His-tagged HIV-1 Gag p41 (His-Gag p41) to the Ni-NCs was 1:221 and 1:480 w/w, respectively. Treatment of DC2.4 cells with Ni-NCs did not result in significant loss in the cell viability up to 24h (<5%). We further evaluated the antibody response of the Ni-NCs using His-Gag p41 as a model antigen. Formulations were administered subcutaneously to BALB/c mice at day 0 (prime) and 14 (boost) followed by serum collection on day 28. Serum His-Gag p41-specific antibody levels were found to be significantly higher at 1 and 0.5 μg doses of Gag p41-His-Ni-NCs (His-Gag p41 equivalent) compared with His-Gag p41 (1 μg) adjuvanted with aluminum hydroxide (AH). The serum IgG2a levels induced by Gag p41-His-Ni-NCs (1 μg) were significantly higher than AH adjuvanted His-Gag p41. The Ni-NCs alone did not result in the elevation of systemic IL-12/p40 and CCL5/RANTES inflammatory cytokine levels upon subcutaneous administration in BALB/c mice. In conclusion, the proposed Ni-NCs can bind His-tagged proteins and have the potential to be used as antigen delivery system capable of generating strong antigen-specific antibodies at doses much lower than with aluminum-based adjuvant and causing no significant elevation of systemic pro-inflammatory IL-12/p40 and CCL5/RANTES cytokines.

Activation of complement by therapeutic liposomes and other lipid excipient-based therapeutic products: prediction and prevention

Some therapeutic liposomes and lipid excipient-based anticancer drugs are recognized by the immune system as foreign, leading to a variety of adverse immune phenomena. One of them is complement (C) activation, the cause, or major contributing factor to a hypersensitivity syndrome called C activation-related pseudoallergy (CARPA). CARPA represents a novel subcategory of acute (type I) hypersensitivity reactions (HSR), which is mostly mild, transient, and preventable by appropriate precautions. However, in an occasional patient, it can be severe or even lethal. Because a main manifestation of C activation is cardiopulmonary distress, CARPA may be a safety issue primarily in cardiac patients. Along with an overview of the various types of liposome-immune system interactions, this review updates the experimental and clinical information on CARPA to different therapeutic liposomes and lipid excipient-based (micellar) anticancer drugs, including PEGylated liposomal doxorubicin sulfate (PLD, Doxil®) and paclitaxel (Taxol®). The substantial individual variation of in vitro and in vivo findings reflects an extremely complex immune phenomenon involving multiple, redundant pathways of C activation, signal transduction in allergy-mediating cells and vasoactive mediator actions at the effector cell level. The latest advances in this field include the proposal of doxorubicin-induced shape changes and aggregation of liposomes in Doxil as possible contributing factors to CARPA caused by PLD, and the finding that Doxil-induced immune suppression prevents HSR to co-administered carboplatin, a significant benefit of Doxil in combination chemotherapy with carboplatin. The review evaluates the use of in vitro C assays and the porcine liposome-induced cardiopulmonary distress model for predicting CARPA. It is concluded that CARPA may become a frequent safety issue in the upcoming era of nanomedicines, necessitating its prevention at an early stage of nanomedicine R&D.

A novel liposomal vaccine improves humoral immunity and prevents tumor pulmonary metastasis in mice

Basic fibroblast growth factor (bFGF) is an important stimulator of angiogenesis involving in neovascularization progression. The aim of this study is to evaluate whether a liposomal vaccine (MLB) based on xenogeneic human bFGF plus monophosphoryl lipid A (MPLA) could effectively induce cross-reaction immunity in mice and increase antitumor activity. Sera of mice were analyzed and IgG antibody titer in MLB group was obviously higher than other groups including the mice immunized with liposomal bFGF vaccine, bFGF plus Freund's adjuvant, empty liposome and PBS. Furthermore, tumor metastasis was significantly inhibited in MLB group, compared with L and PBS group. The IFN-γ production of cultured splenocytes in vitro was evidently up-regulated meanwhile IL-4 production sustained in a low level, revealing that this vaccine stimulated Th1 immunity response preferentially. Taken together, these findings suggested that this novel bFGF vaccine could effectively induce humoral immunity through cross-reaction, mediate Th1 immune response preferentially and enhance antitumor activity in vivo.

Liposome-encapsulated HIV-1 Gag p24 containing lipid A induces effector CD4+ T-cells, memory CD8+ T-cells, and pro-inflammatory cytokines

Liposomal lipid A is an effective adjuvant for the delivery of antigens and for the induction of both cellular and humoral immunity. In this study, we demonstrate that following the third immunization with HIV-1 Gag p24 encapsulated in liposomes containing lipid A [L(p24+LA)], central memory CD8+ T-cells were localized in the spleen and lymph nodes of mice while effector memory CD8+ T-cells and effector CD4+ T-cells were found in the PBMC. Effector CD4+ T-cells were also detected in the spleen and lymph nodes. The predominant cytokine secreted from splenic lymphocytes and lymph nodes was IFN-gamma. In contrast, IL-6 and IL-10 were the major cytokines produced from PBMC. The peptide stimulation indicated that the cytokine responses observed were T-cell specific. The results demonstrate the importance of the adjuvant liposomal lipid A for the induction of HIV-1 Gag p24 -specific CD8+ T-cells, effector CD4+ T-cells, and cytokines with a Th-1 type profile after immunization with L(p24+LA).

Nanotechnology in vaccine delivery

With very few adjuvants currently being used in marketed human vaccines, a critical need exists for novel immunopotentiators and delivery vehicles capable of eliciting humoral, cellular and mucosal immunity. Such crucial vaccine components could facilitate the development of novel vaccines for viral and parasitic infections, such as hepatitis, HIV, malaria, cancer, etc. In this review, we discuss clinical trial results for various vaccine adjuvants and delivery vehicles being developed that are approximately nanoscale (< 1000 nm) in size. Humoral immune responses have been observed for most adjuvants and delivery platforms while only viral vectors, ISCOMs and Montanide™ ISA 51 and 720 have shown cytotoxic T cell responses in the clinic. MF59 and MPL® have elicited Th1 responses, and virus-like particles, non-degradable nanoparticles and liposomes have also generated cellular immunity. Such vaccine components have also been evaluated for alternative routes of administration with clinical successes reported for intranasal delivery of viral vectors and proteosomes and oral delivery of a VLP vaccine.

Cationic liposomal lipids: from gene carriers to cell signaling

Cationic lipids are positively charged amphiphilic molecules which, for most of them, form positively charged liposomes, sometimes in combination with a neutral helper lipid. Such liposomes are mainly used as efficient DNA, RNA or protein carriers for gene therapy or immunization trials. Over the past decade, significant progress has been made in the understanding of the cellular pathways and mechanisms involved in lipoplex-mediated gene transfection but the interaction of cationic lipids with cell components and the consequences of such an interaction on cell physiology remains poorly described. The data reported in the present review provide evidence that cationic lipids are not just carriers for molecular delivery into cells but do modify cellular pathways and stimulate immune or anti-inflammatory responses. Considering the wide number of cationic lipids currently available and the variety of cellular components that could be involved, it is likely that only a few cationic lipid-dependent functions have been identified so far.

Coencapsulation of CpG oligodeoxynucleotides with recombinant Leishmania major stress-inducible protein 1 in liposome enhances immune response and protection against leishmaniasis in immunized BALB/c mice

CpG oligodeoxynucleotides (CpG ODN) have been shown to have potent adjuvant activity for a wide range of antigens. The purpose of this study was to determine the potential benefit of using liposomes as a delivery vehicle to enhance the adjuvant activity of CpG ODN with Leishmania major stress-inducible protein 1 (LmSTI1) antigen in induction of the Th1 response in a murine model of leishmaniasis. BALB/c mice were immunized subcutaneously three times in 3-week intervals with liposomal recombinant LmSTI1 (Lip-rLmSTI1), rLmSTI1 coencapsulated with CpG ODN in a liposome (Lip-rLmSTI1-CpG ODN), rLmSTI1 plus CpG ODN in phosphate-buffered saline (PBS), rLmSTI1 plus non-CpG ODN in PBS, rLmSTI1 in PBS, empty liposome, or PBS. The intensity of infection induced by L. major promastigote challenge was measured by footpad swelling. A significant (P < 0.001) inhibition of infection in mice immunized with Lip-rLmSTI1-CpG ODN was shown compared to the other groups, and no parasite was detected in the spleens of this group 14 weeks after challenge. The highest immunoglobulin G2a (IgG2a) titer and the highest IgG2a/IgG1 ratio were also shown in the sera of mice immunized with Lip-rLmSTI1-CpG ODN before and 14 weeks after challenge. The results indicated the superiority of CpG ODN in its liposomal form over its soluble form to induce the Th1 response when used in association with rLmSTI1 antigen. It seems that using a liposome delivery system carrying CpG ODN as an adjuvant coencapsulated with Leishmania antigen plays an important role in vaccine development strategies against leishmaniasis.

The role of CpG ODN in enhancement of immune response and protection in BALB/c mice immunized with recombinant major surface glycoprotein of Leishmania (rgp63) encapsulated in cationic liposome

CpG oligodeoxynucleotides (CpG ODN) are known to be a potent immunoadjuvant for a wide range of antigens. The aim of this study was to evaluate the role of CpG ODN co-encapsulated with rgp63 antigen in cationic liposomes (Lip-rgp63-CpG ODN) in immune response enhancement and protection in BALB/c mice against leishmaniasis. Lip-rgp63-CpG ODN prepared by using dehydration-rehydration vesicle (DRV) method significantly inhibited (P<0.001) Leishmania major infection in mice measured by footpad swelling compared to Lip-rgp63, rgp63 alone, rgp63 plus CpG ODN, PBS or control liposomes. The mice immunized with Lip-rgp63-CpG ODN also showed the lowest spleen parasite burden, highest IgG2a/IgG1 ratio and IFN-gamma production and the lowest IL-4 production compared to the other groups. The results indicate that co-encapsulation of CpG ODN in liposomes improves the immunogenicity of Leishmania antigen.

Review: novel nonviral delivery approaches for interleukin-12 protein and gene systems: curbing toxicity and enhancing adjuvant activity

It has become increasingly apparent that the ability to generate an optimal host immune response requires effective cross talk between the innate and adaptive components of the immune system. Pro-inflammatory cytokines, in particular those that can induce a danger signal, often called signal 3, are crucial in this role of initiating and augmenting the presentation of exogenous antigen to T cells by dendritic cells. Interleukin-12 (IL-12) in particular has been defined as a "signal 3" cytokine required for the antigen cross priming. Given this unique interactive function, a significant amount of work has been performed to define possible therapeutic applications for IL-12. Systemic IL-12 administration can clearly act as a potent adjuvant for postvaccination T cell responses in a variety of diseases. As an example, in the cancer setting, systemic IL-12 is capable of suppressing tumor growth, metastasis, and angiogenesis in vivo. IL-12, however, has been associated with significant dose- and schedule-dependent toxicity in early clinical trials, results that have proven to be a major obstacle to its clinical application. Recent research has focused on decreasing the toxicity of IL-12 using different delivery approaches, including virus-based and gene-modified cell-based delivery. Although effective, these approaches also have limitations, including the generation of neutralizing antibodies, in addition to lacking the simplicity and versatility required for universal clinical application. Thus, there is a significant interest in the development of alternative delivery approaches for IL-12 administration that can overcome these issues. Several nonviral delivery approaches for IL-12 protein or gene expression vectors are being defined, including alum, liposomes, and polymer-based delivery. These developing approaches have shown promising adjuvant effects with significantly lessened systemic toxicity. This article discusses the potential capabilities of these nonvirus-based IL-12 delivery systems in different disease settings, including allergy, infection, and cancer.

Immune response and protection assay of recombinant major surface glycoprotein of Leishmania (rgp63) reconstituted with liposomes in BALB/c mice

In this study the ability of recombinant gp63 entrapped in liposomes to induce immune response and protection against L. major infection in susceptible BALB/c mice was studied. Liposomes containing rgp63 (Lip-rgp63) were prepared from egg lecithin and cholesterol using detergent solubilization method. Immunization of BALB/c mice with rgp63 alone conferred a partial protection while entrapment of rgp63 in liposomes significantly increased the rate of protection (P<0.05). The parasite burden of spleen in mice challenged with L. major was significantly (p<0.001) lower in group of mice immunized with rgp63 alone or Lip-rgp63, however, the least parasite burden was seen in Lip-rgp63 group. Both rgp63 alone and Lip-rgp63 elicited significant delayed-type hypersensitivity (DTH) response compared to controls (p<0.01), however, the DTH response of PBS-rgp63 was less than the Lip-rgp63. Titration of anti-Leishmania IgG isotypes (IgG2a/IgG1) showed a preferential Th1 type of immune response only in mice immunized with Lip-rgp63. The results indicate that liposomes might be used as a suitable immunoadjuvant for development of Leishmania vaccine.

Development of a Virosome Vaccine Against Avian Metapneumovirus Subtype C for Protection in Turkeys

An avian metapneumovirus (aMPV) virosome vaccine was prepared and tested for protection of turkeys by aMPV challenge. The vaccine was produced using a detergent-based (Triton X-100) extraction of aMPV subtype C followed by detergent removal with SM2 Bio-Beads. Western blot and virus-neutralization analysis confirmed that the aMPV virosomes contained both the fusion and attachment glycoproteins. Specific-pathogen-free turkeys were immunized either intranasally (i.n.) or intramuscularly (i.m.) with two doses of the aMPV virosome vaccine. Vaccination decreased clinical signs of disease following virulent challenge, and IN vaccination was superior to i.m. vaccination in reducing clinical signs. Decreases in viral load in the respiratory tract were observed in turkeys receiving i.n. vaccination with aMPV virosomes compared to unvaccinated poults. Increased virus-neutralizing antibody levels against aMPV were observed in birds vaccinated with virosomes. These results demonstrate that immunization of turkeys with aMPV virosomes can be an effective strategy for control of disease.

Development of a Virosome Vaccine for Newcastle Disease Virus

In an effort to protect chickens against Newcastle disease (ND), a nonreplicating virosome vaccine was produced by solubilization of Newcastle disease virus (NDV) with Triton X-100 followed by detergent removal with SM2 Bio-Beads. Biochemical analysis indicated that the NDV virosomes had similar characteristics as the parent virus and contained both the fusion and hemagglutinin-neuraminidase proteins. To target the respiratory tract, specific-pathogen-free chickens were immunized intranasally and intratracheally with the NDV virosome vaccine. This vaccine was compared with a standard NDV (LaSota) live-virus vaccine for commercial poultry. Seroconversion (> or = four fold increase in hemagglutination inhibition [HI] antibody titers) was achieved in all birds vaccinated with the virosome vaccine. Upon lethal challenge with a velogenic NDV strain (Texas GB), all birds receiving either vaccination method were protected against death. Antibody levels against NDV, as determined by enzyme-linked immunosorbent assay and HI titer, were comparable with either vaccine and increased after virus challenge. These results demonstrate the potential of virosomes as an effective tool for ND vaccination.

Adjuvants in veterinary vaccines: modes of action and adverse effects

Vaccine adjuvants are chemicals, microbial components, or mammalian proteins that enhance the immune response to vaccine antigens. Interest in reducing vaccine-related adverse effects and inducing specific types of immunity has led to the development of numerous new adjuvants. Adjuvants in development or in experimental and commercial vaccines include aluminum salts (alum), oil emulsions, saponins, immune-stimulating complexes (ISCOMs), liposomes, microparticles, nonionic block copolymers, derivatized polysaccharides, cytokines, and a wide variety of bacterial derivatives. The mechanisms of action of these diverse compounds vary, as does their induction of cell-mediated and antibody responses. Factors influencing the selection of an adjuvant include animal species, specific pathogen, vaccine antigen, route of immunization, and type of immunity needed.

Novel synthetic LPS receptor agonists boost systemic and mucosal antibody responses in mice

Safe and cost-effective adjuvants are a critical requirement for subunit vaccine development. We report here the in vivo activity of a series of fully synthetic LPS receptor agonists that have been shown to activate NF-kappaB signaling through the Toll-like receptor 4 (TLR4). These compounds boost antibody responses to protein antigens when coadministered at microgram doses in mice. At these dosage levels no adverse effects are observed. Antibody responses are largely IgG1, with enhanced IgG2a, and down-regulated IgE as compared to alum adjuvanted immunization. Stimulation of Th1 is confirmed by enhanced gamma-interferon production after in vitro antigen restimulation of spleen cells from mice immunized with the synthetic adjuvants. The adjuvants are active by both subcutaneous and intranasal routes of vaccine administration, and in the latter case can amplify both serum IgG and serum and mucosal IgA responses. The compounds must be administered at the same site with antigen to boost anti-vaccine antibody. These fully synthetic ligands of the innate immune system offer the potential for use as effective, safe, and nonbiologically-derived adjuvants.

Effect of liposomal antigens on the priming and activation of the immune system by dendritic cells

Dendritic cells (DCs) are recognized as the sole professional antigen-presenting cells capable of priming naive T cells of the helper and cytotoxic phenotypes. This property is presently exploited with success in vaccinal strategies against pathogens or tumor cells that otherwise escape immune recognition, but the repeated infusions of ex vivo expanded and sensitized DCs are usually required to achieve protection. In this paper, we demonstrate that liposomal antigens can efficiently relay and propagate the action of DCs, inducing a strong long-term response against their associated antigen. Their effect is mainly achieved by improving the ex vivo loading of DCs and by efficiently channeling the activation stimulus into the induction of effector function. This is demonstrated by the sustained immunoglobulin production as well as by the sustained lymphoproliferation and the increased cytokine secretion that can be achieved upon restimulation of DC-primed immune cells with limited amount of liposomal antigenic material. Being well-tolerated and easily prepared, liposomal antigens could therefore be expected to significantly contribute to the efficiency and to a more general utilization of the highly promising but rather cumbersome DC-based immunotherapies.

Enhanced immune protection by a liposome-encapsulated recombinant respiratory syncytial virus (RSV) vaccine using immunogenic lipids from Deinococcus radiodurans

The radiation-resistant bacterium, Deinococcus radiodurans contains a variety of phospho-, glyco- and phosphoglycolipids, the structures of which appear to be largely unique in nature. We show here that such lipids are immunogenic when administered as liposomes intranasally in mice, as evidenced by the induction of serum antibodies which recognize D. radiodurans lipids but not other lipids by thin layer chromatographic immunostaining. By modifying a liposomal vaccine against respiratory syncytial virus (RSV) we find that vaccine efficacy is markedly enhanced by the inclusion of lipids isolated from D. radiodurans. Using dioleoylphosphatidylcholine (DOPC) or D. radiodurans lipids, liposomes were prepared which encapsulated a soluble fragment of the RSV G protein (G(128-188)) fused with a portion of the bacterial thioredoxin (Trx) protein. Mice immunized intranasally with D. radiodurans liposomes showed markedly greater protection against RSV challenge over mice immunized with DOPC liposomes. Enhanced vaccine efficacy was achieved using liposomes prepared from either whole D. radiodurans lipids or from a single isolated phosphoglycolipid previously identified as alpha-galactosylphosphatidylglyceroylalkylamine (lipid 7). Mice immunized and protected against RSV challenge were free of pulmonary eosinophilic infiltration, an undesirable consequence of many RSV vaccines. The results provide further support for liposome-based vaccines for RSV and underline the importance of lipid composition in liposome formulations.

Developments in liposomal drug delivery systems

Liposomes are the leading drug delivery systems for the systemic (iv.) administration of drugs. There are now liposomal formulations of conventional drugs that have received clinical approval and many others in clinical trials that bring benefits of reduced toxicity and enhanced efficacy for the treatment of cancer and other life-threatening diseases. The mechanisms giving rise to the therapeutic advantages of liposomes, such as the ability of long-circulating liposomes to preferentially accumulate at disease sites including tumours, sites of infection and sites of inflammation are increasingly well understood. Further, liposome-based formulations of genetic drugs such as antisense oligonucleotides and plasmids for gene therapy that have clear potential for systemic utility are increasingly available. This paper reviews the liposomal drug delivery field, summarises the success of liposomes for the delivery of small molecules and indicates how this success is being built on to design effective carriers for genetic drugs.

Design, characterization and preclinical efficacy of a cationic lipid adjuvant for influenza split vaccine

We prepared a series of cationic lipid vesicles comprising a cationic cholesterol derivative, DC-Chol with or without a neutral phospholipid, DOPC or DOPE. The vesicles were tested for their ability to bind and adjuvant split inactivated influenza vaccines. We found that DC-Chol-containing liposomes are capable to strongly bind influenza vaccine antigens upon simple mixing with the vaccine. The resulting formulations induced robust anti-influenza immune responses both after s.c. and i.n. administration in BALB/c mice while neutral Cholesterol/DOPC liposomes displayed virtually no stable antigen binding and no adjuvant effect. The parenteral adjuvant effect of DC-Chol on trivalent split influenza vaccines was then confirmed in outbred mice and monkeys. Among the most potent formulations tested, a simple mixture of the vaccine with a microfluidized dispersion of DC-Chol in an aqueous buffer is being considered for further development to produce an improved influenza vaccine.

Tuftsin-bearing liposomes in treatment of macrophage-based infections

The use of liposomes as drug carriers in treatment of various diseases has been explored extensively for more than 20 years. 'Conventional' liposomes, when administered in vivo by a variety of routes, rapidly accumulate in the mononuclear phagocyte system (MPS). The inherent tendency of the liposomes to concentrate in MPS can be exploited in enhancing the non-specific host defence against infections by entrapping in them the macrophage modulators, and as carriers of antibiotics in treatment of intracellular infections that reside in MPS. This must further be enhanced by grafting on the liposome surface the ligands, e.g. tuftsin, that not only binds specifically to the MPS cells but also enhances their natural killer activity. Keeping this in view, we designed and developed tuftsin-bearing liposomes as drug carriers for the treatment of macrophage-based infections and outline these studies in this overview.

Evaluation of the pulmonary and systemic immunogenicity of Fluidosomes, a fluid liposomal-tobramycin formulation for the treatment of chronic infections in lungs

In previous studies, we have developed a fluid bactericidal liposomal formulation containing tobramycin, called Fluidosomes, which has been shown to be highly bactericidal both in in vitro and in in vivo studies against Pseudomonas aeruginosa and other related and unrelated bacteria. One foreseeable application of these Fluidosomes is the treatment of chronic pulmonary infections in cystic fibrosis patients colonized with P. aeruginosa and other related bacteria. Considering the capacity of some liposomal preparations to play an adjuvant role in vaccines, the non-immunogenicity of Fluidosomes has to be demonstrated. The systemic and local immunogenicity of Fluidosomes were assessed by effectuating repeated intraperitoneal (i.p.) and intratracheal (i.t. ) immunizations in BALB/c mouse. No significant mucosal and serum immune responses against Fluidosomes and/or tobramycin were detected as compared with preimmune sera. These data suggest that Fluidosomes could be administered repeatedly without adverse immune responses to control chronic pulmonary infections in cystic fibrosis.

Design and development of adjuvant-active nonionic block copolymers

Nonionic block copolymers are surfactants synthesized using propylene oxide and ethylene oxide, and they can be designed so that individual copolymers have unique vaccine adjuvant properties. We have designed and produced nonionic block copolymers based on high molecular weight (MW), 9-15 kDA, cores of poly(oxypropylene) (POP) coupled with smaller poly(oxyethylene) (POE) end blocks. Copolymers synthesized with less than 10% (w/w) POE will spontaneously assemble into 300 nm-3 microm micelles or microparticles in aqueous solutions at physiological pH, and when formulated with protein, complex microparticles consisting of both the protein and copolymers are formed. The adjuvant activity of nonionic block copolymers is influenced by both size and POE content; maximal activity is associated with low POE content, 5-10%, and a molecular size of 11-12 kDa. The type of immune response produced is also influenced by the POE content. Copolymers with 10% POE significantly augmented Type 2 helper T-lymphocyte responses whereas copolymers with lower POE contents augmented both Type 1 and Type 2 helper T-lymphocyte responses. This property allows for vaccines to be "customized" by using adjuvant-active nonionic block copolymers that will augment the most appropriate types of immune responses.

Liposomes, a potential immunoadjuvant and carrier for a cryptococcal vaccine

Mice immunized with a cryptococcal culture filtrate antigen (CneF) emulsified in complete Freund's adjuvant (CFA) develop an anticryptococcal cell-mediated immune response (CMI). CMI is detected by delayed-type hypersensitivity (DTH) reactions and by enhanced clearance of Cryptococcus neoformans from infected tissues. The objective of this research was to evaluate anticryptococcal DTH reactivity and clearance of cryptococci from groups of mice immunized with CneF encapsulated into liposomes (CneF-liposome) and compare the results to results from mice immunized with CneF-CFA. CBA/J mice were injected subcutaneously with vaccines or control formulations (saline-liposome or saline-CFA). Six days later the mice were footpad tested to assess their DTH response to CneF or the animals were challenged intravenously with 10(5) viable C. neoformans to determine clearance of infection. Clearance was evaluated 7 days later by enumeration of cryptococcal colony forming units (CFU) in lungs, spleens, livers, and brains of the infected mice. The CneF-liposome formulation induced a positive anticryptococcal DTH response and elicited increased clearance of C. neoformans from tissues as compared to mice treated with saline-liposome. Even though the CneF-liposome preparation did not induce as strong of a DTH response or as much protection as did CneF-CFA, our results indicate that liposomes are promising carriers for immunization with cryptococcal antigen and that such immunization can provide some protection to subsequent infection with C. neoformans.

Covalent polymyxin B conjugate with human immunoglobulin G as an antiendotoxin reagent

Polymyxin B (PMB) is a cyclic decapeptide antibiotic which also binds and neutralizes endotoxin. Unfortunately, PMB can be considerably nephrotoxic at clinically utilized doses, thereby limiting its utility as a therapeutic antiendotoxin reagent. We sought to change the pharmacokinetics and toxicity profile of PMB by covalently linking it to a human immunoglobulin G (IgG) carrier. Conjugates of PMB with IgG were prepared by EDAC [1-ethyl-3-(3-dimethylaminopropyl) carbodiimide]-mediated amide formation. Analysis by dot enzyme-linked immunosorbent assay with an anti-PMB monoclonal antibody showed that the purified conjugate contained bound PMB. The IgG-PMB conjugate reacted with lipid A and J5 lipopolysaccharide in Western blot assays in a manner comparable to that of whole antiserum with anti-lipid A reactivity; unconjugated IgG had no reactivity. The PMB bound in the conjugate retained its endotoxin-neutralizing activity compared to that of unbound PMB as evidenced by its dose-dependent inhibition of tumor necrosis factor release by endotoxin-stimulated human monocytes in vitro; unconjugated IgG had no activity. By this assay, the PMB-IgG conjugate was determined to have approximately 3.0 microg of bound functional PMB per 100 microg of total protein of conjugate (five molecules of PMB per IgG molecule). The PMB-IgG conjugate was also bactericidal against clinical strains of Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae relative to unconjugated IgG with MBCs of <4 microg of conjugate per ml for each of the tested strains. The conjugate appeared to be nontoxic at the highest doses deliverable and provided statistically significant protection from death to galactosamine-sensitized, lipopolysaccharide-challenged mice in a dose-dependent fashion when administered prophylactically 2 h before challenge. However, neither free PMB nor the PMB-IgG conjugate could protect mice challenged with endotoxin 2 h after administration. This suggests that these reagents can play a role in prophylaxis but not in therapy of sepsis. These experiments demonstrated that the PMB-IgG conjugate retains bound yet functional PMB as evidenced by its endotoxin-neutralizing activity both in vitro and in vivo. Further work is required to define the role that this or related conjugate compounds may play in the prophylaxis of endotoxin-mediated disease.

Outer membrane protein of Neisseria meningitidis as a mucosal adjuvant for lipopolysaccharide of Brucella melitensis in mouse and guinea pig intranasal immunization models

A mucosal vaccine against brucellosis consisting of the lipopolysaccharide (LPS) of Brucella melitensis complexed with the outer membrane protein (GBOMP) of group B Neisseria meningitidis was tested in small-animal models of intranasal immunization. Mice given two doses of the vaccine developed high levels of immunoglobulin G (IgG) and IgA antibodies specific for B. melitensis LPS in lung lavages and specific IgG and IgA antibody-secreting cells in the lungs and spleen. Similarly, in guinea pigs immunized twice intranasally, IgG and IgA LPS-specific antibodies were detected in lung lavages, and specific antibody-secreting cells were isolated from the spleen and cervical nodes. In mice immunized with LPS only, pulmonary responses consisted mostly of IgM antibodies, while guinea pigs given LPS alone developed local antibody of all three isotypes, but at lower levels compared to animals given the complex vaccine. Both mice and guinea pigs also developed high levels of serum IgG and moderate levels of IgA as a result of intranasal immunization with the complex vaccine. The serum antibodies in both cases were found to cross-react with the LPS of B. abortus, which shares an immunogenic epitope with B. melitensis LPS. In mice given the complex vaccine, there was a prominent serum IgG1 response that was absent in the mice given LPS alone. In conclusion, the N. meningitidis GBOMP was an effective mucosal adjuvant for secretory IgA and IgG responses in the lungs of both mice and guinea pigs. The IgG1 subclass response in mice suggests that GBOMP may have favored a Th2 type of response to the LPS. A vaccine capable of stimulating high levels of antibody at local sites has the potential to protect against brucellae, since these pathogens gain entry to the host via mucosal routes.