Design of highly immunogenic liposomal constructs combining structurally independent B cell and T helper cell peptide epitopes

Immunogenicity of Recombinant Lipid-Based Nanoparticle Vaccines: Danger Signal vs. Helping Hand

Infectious diseases are a predominant problem in human health. While the incidence of many pathogenic infections is controlled by vaccines, some pathogens still pose a challenging task for vaccine researchers. In order to face these challenges, the field of vaccine development has changed tremendously over the last few years. For non-replicating recombinant antigens, novel vaccine delivery systems that attempt to increase the immunogenicity by mimicking structural properties of pathogens are already approved for clinical applications. Lipid-based nanoparticles (LbNPs) of different natures are vesicles made of lipid layers with aqueous cavities, which may carry antigens and other biomolecules either displayed on the surface or encapsulated in the cavity. However, the efficacy profile of recombinant LbNP vaccines is not as high as that of live-attenuated ones. This review gives a compendious picture of two approaches that affect the immunogenicity of recombinant LbNP vaccines: (i) the incorporation of immunostimulatory agents and (ii) the utilization of pre-existing or promiscuous cellular immunity, which might be beneficial for the development of tailored prophylactic and therapeutic LbNP vaccine candidates.

USE OF ARTIFICIAL CELLS AS DRUG CARRIERS

Cells are the fundamental functional units of biological systems and mimicking their size, function and complexity is a primary goal in the development of new therapeutic strategies. Recent advances in chemistry, synthetic biology and material science have enabled the development of cell membrane-based drug delivery systems (DDSs), often referred to as "artificial cells" or protocells. Artificial cells can be made by removing functions from natural systems in a top-down manner, or assembly from synthetic, organic or inorganic materials, through a bottom-up approach where simple units are integrated to form more complex structures. This review covers the latest advances in the development of artificial cells as DDSs, highlighting how their designs have been inspired by natural cells or cell membranes. Advancement of artificial cell technologies has led to a set of drug carriers with effective and controlled release of a variety of therapeutics for a range of diseases, and with increasing complexity they will have a greater impact on therapeutic designs.

Clinical Relevance of Pre-Existing and Treatment-Induced Anti-Poly(Ethylene Glycol) Antibodies

Abstract

Poly(ethylene glycol) (PEG) is a nontoxic, hydrophilic polymer that is often covalently attached to proteins, drugs, tissues, or materials; a procedure commonly referred to as PEGylation. PEGylation improves solubility, circulation time, and reduces immunogenicity of therapeutic molecules. Currently, there are 21 PEGylated drugs approved by the Food and Drug Administration (FDA), and more in the developmental stage. In addition to the polymer's applications in the clinic, PEG is widely used as a solvent and emulsifying agent in the formulation of cosmetics, cleaning, and personal care products. Due to the ubiquitous presence of the polymer in everyday products, patients can develop antibodies against PEG (αPEG Abs) that can be problematic when a PEGylated drug is administered. These αPEG Abs can provoke hypersensitivity reactions, accelerated drug clearance, and decreased therapeutic efficacy. Herein, we review how the prevalence of PEG in everyday products has induced αPEG Abs within the general public as well as the effect of these Abs on the performance of PEGylated therapeutics. We will focus on clinical manifestations following the administration of PEGylated drugs.

Lay Summary

Poly(ethylene glycol) (PEG) is a polymer found in products including cosmetics, personal care products, cleaning agents, medicine, and food. Due to the prevalence of PEG, people can develop antibodies (αPEG Abs) against the polymer, which recognize PEG as foreign. Of note, PEG is frequently incorporated into drug formulations to improve therapeutic efficacy. Complications can arise when a patient receiving a PEGylated drug has previously developed αPEG Abs from interactions with PEG in everyday products. The presence of high concentrations of αPEG Abs in blood can result in decreased treatment efficacy and allergic reactions to a wide range of therapeutics.

Synthesis and Characterization of a New Bifunctionalized, Fluorescent, and Amphiphilic Molecule for Recruiting SH-Containing Molecules to Membranes

This study describes the synthesis and characterization of an amphiphilic construct intended to recruit SH-containing molecules to membranes. The construct consists of 1) an aliphatic chain to enable anchoring within membranes, 2) a maleimide moiety to react with the sulfhydryl group of a soluble (bio)molecule, and 3) a fluorescence moiety to allow the construct to be followed by fluorescence spectroscopy and microscopy. It is shown that the construct can be incorporated into preformed membranes, thus allowing application of the approach with biological membranes. The close proximity between the fluorophore and the maleimide moiety within the construct causes fluorescence quenching. This allows monitoring of the reaction with SH-containing molecules by measurement of increases in fluorescence intensity and lifetime. Notably, the construct distributes into laterally ordered membrane domains of lipid vesicles, which is probably triggered by the length of its membrane anchor. The advantages of the new construct can be employed for several biological, biotechnological, and medicinal applications.

Self-adjuvanting lipoimmunogens for therapeutic HPV vaccine development: potential clinical impact

The goal of therapeutic HPV vaccines is the induction of cytotoxic T lymphocyte immunity against HPV-associated cancers. Recombinant proteins and synthetic peptides have high safety profiles but low immunogenicity, which limits their efficacy when used in a vaccine. Self-adjuvanting lipid moieties have been conjugated to synthetic peptides or expressed as lipoproteins to enhance the immunogenicity of vaccine candidates. Mono-, di- and tri-palmitoylated peptides have been demonstrated to activate dendritic cells and induce robust cellular immunity against infectious diseases and cancer. Recently, a platform technology using the high-yield production of recombinant lipoproteins with Toll-like receptor 2 agonist activity was established for the development of novel subunit vaccines. This technology represents a novel strategy for the development of therapeutic HPV vaccines. In this review, we describe recent progress in the design of therapeutic HPV vaccines using lipoimmunogens.

Design considerations for liposomal vaccines: influence of formulation parameters on antibody and cell-mediated immune responses to liposome associated antigens

Liposomes (phospholipid bilayer vesicles) are versatile and robust delivery systems for induction of antibody and T lymphocyte responses to associated subunit antigens. In the last 15 years, liposome vaccine technology has matured and now several vaccines containing liposome-based adjuvants have been approved for human use or have reached late stages of clinical evaluation. Given the intensifying interest in liposome-based vaccines, it is important to understand precisely how liposomes interact with the immune system and stimulate immunity. It has become clear that the physicochemical properties of liposomal vaccines - method of antigen attachment, lipid composition, bilayer fluidity, particle charge, and other properties - exert dramatic effects on the resulting immune response. Here, we present a comprehensive review of the physicochemical properties of liposomal vaccines and how they influence immune responses. A discussion of novel and emerging immunomodulators that are suitable for inclusion in liposomal vaccines is also presented. Through a comprehensive analysis of the body of liposomal vaccine literature, we enumerate a series of principles that can guide the rational design of liposomal vaccines to elicit immune responses of a desired magnitude and quality. We also identify major unanswered questions in the field, pointing the direction for future study.

Antitumor activity of liposomal ErbB2/HER2 epitope peptide-based vaccine constructs incorporating TLR agonists and mannose receptor targeting

Synthetic and molecularly defined constructs containing the minimal components to mimic and amplify the physiological immune response are able to induce an efficient cytotoxic response. In the current study this approach was applied to the development of highly versatile liposomal constructs to co-deliver peptide epitopes in combination with TLR agonists in order to induce a specific anti-tumor cellular immune response against ErbB2 protein-expressing tumor cells. Liposomes containing ErbB2 p63-71 cytotoxic T lymphocyte (CTL) and HA307-319 T- helper (Th) peptide epitopes associated to innovative synthetic TLR2/1 (Pam(3)CAG) or TLR2/6 agonists (Pam(2)CAG and Pam(2)CGD), were injected in mice bearing ErbB2 protein-expressing tumor cells. Mannosylated ligands were also incorporated into the constructs to target antigen-presenting cells. We showed that the TLR2/6 agonists were more efficient than the TLR2/1 agonists for the eradication of tumors expressing ErbB2 protein. Furthermore, mannose-targeted liposomes displayed higher therapeutic efficiency against tumor allowing treatment with decreased quantities of both TLR ligands and peptide epitopes. Our results validated that antigen-associated mannosylated liposomes combined with efficient TLR ligands are effective vectors for vaccination against tumor. In this study we developed useful tools to evaluate the vaccination efficiency of various adjuvants and/or targeting molecules and their potential synergy.

Liposomes as delivery systems for nasal vaccination: strategies and outcomes

IMPORTANCE OF THE FIELD

Among the particulate systems that have been envisaged in vaccine delivery, liposomes are very attractive. These phospholipid vesicles can indeed deliver a wide range of molecules. They have been shown to enhance considerably the immunogenicity of weak protein antigens or synthetic peptides. Also, they offer a wide range of pharmaceutical options for the design of vaccines. In the past decade, the nasal mucosa has emerged as an effective route for vaccine delivery, together with the opportunity to develop non-invasive approaches in vaccination.

AREAS COVERED IN THIS REVIEW

This review focuses on the recent strategies and outcomes that have been developed around the use of liposomes in nasal vaccination.

WHAT THE READER WILL GAIN

The various formulation parameters, including lipid composition, size, charge and mucoadhesiveness, that have been investigated in the design of liposomal vaccine candidates dedicated to nasal vaccination are outlined. Also, an overview of the immunological and protective responses obtained with the developed formulations is presented.

TAKE HOME MESSAGE

This review illustrates the high potential of liposomes as nasal vaccine delivery systems.

Rational design and immunogenicity of liposome-based diepitope constructs: application to synthetic oligosaccharides mimicking the Shigella flexneri 2a O-antigen

We have designed chemically defined diepitope constructs consisting of liposomes displaying at their surface synthetic oligosaccharides mimicking the O-antigen of the Shigella flexneri 2a lipopolysaccharide (B-cell epitope) and influenza hemagglutinin peptide HA 307-319 (Th epitope). Using well controlled and high-yielding covalent bioconjugation reactions, the two structurally independent epitopes were coupled to the lipopeptide Pam(3)CAG, i.e. a TLR2 ligand known for its adjuvant properties, anchored in preformed vesicles. The synthetic construct containing a pentadecasaccharide corresponding to three O-antigen repeating units triggered T-dependent anti-oligosaccharide and anti-S. flexneri 2a LPS antibody responses when administered i.m. to BALB/c mice. Moreover, the long-lasting anti-LPS antibody response afforded protection against a S. flexneri 2a challenge. These results show that liposome diepitope constructs could be attractive alternatives in the development of synthetic carbohydrate-based vaccines.

Overcoming the innate immune response to small interfering RNA

Many types of nucleic acid, including canonical small interfering RNA (siRNA) duplexes, are potent activators of the mammalian innate immune system. Synthetic siRNA duplexes can induce high levels of inflammatory cytokines and type I interferons, in particular interferon-alpha, after systemic administration in mammals and in primary human blood cell cultures. These responses are greatly potentiated by the use of delivery vehicles that facilitate cellular uptake of the siRNA. Although the immunomodulatory effects of nucleic acids may be harnessed therapeutically, for example, in oncology and allergy applications, in many cases immune activation represents a significant undesirable side effect due to the toxicities associated with excessive cytokine release and associated inflammatory syndromes. The potential for siRNA-based drugs to be rendered immunogenic is also a cause for concern because the establishment of an antibody response may severely compromise both safety and efficacy. Clearly, there are significant implications both for the development of siRNA-based drugs and in the interpretation of gene-silencing effects elicited by siRNA. This review provides the background information required to anticipate, manage, and abrogate the immunological effects of siRNA and will assist the reader in the successful in vivo application of siRNA-based drugs.

Design of a liposomal candidate vaccine against Pseudomonas aeruginosa and its evaluation in triggering systemic and lung mucosal immunity

PURPOSE

To design and evaluate liposomal constructs capable of inducing a potent systemic and airway humoral response to Pseudomonas aeruginosa

METHODS

Liposomes contained a peptide derived from P. aeruginosa pilin protein as B epitope, a peptide derived from Influenza hemagglutinin protein as Th epitope, the TLR agonist Pam3CAG or Pam2CAG as adjuvant, and a mannosylated lipid as dendritic cell targeting agent. These constructions were administered to mice intraperitoneally (i.p.) or intranasally (i.n.). Their immunogenicity was evaluated by measuring B epitope-specific immunoglobulins in the serum and the airways by ELISA.

RESULTS

The B epitope, in its native form or after substitution of a cysteine by a serine, induced high systemic IgG titers when formulated in the presence of Pam3CAG or Pam2CAG and administered i.p.. No IgA response was observed in the airways upon injection of candidate vaccines by i.p. route, whatever the B epitope or the adjuvant. However, i.n. vaccination resulted in a significant local production of IgA. Finally, the production of IgG was more rapid when mannose was incorporated.

CONCLUSIONS

All liposomal candidate vaccines tested induced the production of IgG and/or IgA directed against an immunogenic peptide from P. aeruginosa. Liposomal constructs could be attractive in the vaccination against P. aeruginosa.

Immunization with mimotopes prevents growth of carcinoembryonic antigen positive tumors in BALB/c mice

PURPOSE

The carcinoembryonic antigen (CEA) is a glycoprotein that is overexpressed in nearly 50% of all human and veterinarian tumors. At present, anti-CEA antibodies are being tested in clinical studies as passive immunotherapeutics. This study aims to establish an active immunotherapy for the poorly immunogenic CEA glycoprotein by generating antigen surrogates.

EXPERIMENTAL DESIGN

We used the monoclonal anti-CEA antibody Col-1 and the biopanning method to generate peptide mimics (mimotopes) of the Col-1 epitope. The peptide showing the highest specificity and mimicry was synthesized as an octameric multiple antigenic mimotope (MAM). Subsequently, immunogenicity of the selected mimotope was examined in BALB/c mice. We assessed antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity mediated by the induced antibodies on CEA-expressing HT29 tumor cells. Furthermore, after immunization, the BALB/c mice were transplanted s.c. with Meth-A/CEA tumor cells.

RESULTS

When BALB/c mice were immunized with this MAM, they generated a specific humoral immune response against CEA. The mimotope-induced polyclonal and poly-isotypic antibodies induced antibody-dependent cellular cytotoxicity and complement-dependent cytotoxicity in vitro. Furthermore, when MAM-immunized mice were transplanted s.c. with Meth-A/CEA cells expressing human CEA, a suppressed tumor growth was observed.

CONCLUSION

From our results, we can conclude that the Col-1 epitope of the glycoprotein CEA can be translated into an immunogenic peptide mimic. The mimotope-induced antibodies recognize CEA and do effectively inhibit growth of CEA-positive tumors. Based on these finding, we suggest that the generated mimotopes are candidates for active immunotherapy of CEA-expressing tumors.

Synthesis of versatile chemical tools toward a structure/properties relationships study onto targeting colloids

As part of a drug delivery project, four aldehydes of the type Pam-Lys(Pam)-spacer-CO-CHO were synthesized to be included in targeting colloids. Though amphiphilic, they were obtained within reasonable yields (18-55%) and with high RP-HPLC purity ( approximately 90%). Parallely, six complementary targeting peptides of the type H(2)N-NH-CH(2)-CO-spacer-YGRGDSP-NH(2) were prepared to be anchored onto colloids. Isolated yields are related to the spacer length and nature. To easily and rapidly modulate the distance between the peptide and the vesicle, every partners were elaborated on solid phase and the expected constructions were obtained by hydrazone ligation. One possible application is presented here with multilamellar vesicles targeting HUVEC cells. Preliminary results prove that the fine-tuning of the spacer length permits to optimize the recognition toward the target cells.

Spleen plays an important role in the induction of accelerated blood clearance of PEGylated liposomes

It is well known that steric stabilization of the surface of liposomes by a polyethyleneglycol (PEG) conjugated lipid results in reduced recognition of the liposomes by the cells of the mononuclear phagocyte system and consequently extended circulation times of the liposomes (t1/2 approximately 20 h in rat). Recently, we reported on the "accelerated blood clearance (ABC) phenomenon", causing PEGylated liposomes to be cleared very rapidly from the circulation upon repeated injection. We also reported that abundant binding of IgM, secreted into the blood stream after the first dose and, to PEGylated liposomes, plays an essential role in the induction of the ABC phenomenon. Spleen is well known to play a central role in the immune reaction and to produce IgM following a bacterial infection. The aim of the present study was to determine whether spleen contributes to the induction of the ABC phenomenon and to unravel its role in the phenomenon. In rats that were splenectomized (surgical removal of spleen) prior to the first injection of liposomes (0.001 micromol phospholipids/kg), the ABC phenomenon was totally abolished. In these rats serum IgM concentrations as well as the amounts of IgM bound to PEGylated liposomes were substantially reduced. Splenectomy attenuated the ABC phenomenon when performed until 3 days post-first injection. Removal of the spleen 4 days post-first injection left the ABC phenomenon unchanged. This finding indicates that the immune reaction in the spleen against the PEGylated liposomes occurs during at least 2-3 days following the first administration and then IgM reactive to PEGylated liposomes is produced. The present study proves that the spleen plays a critical role in the induction phase of the ABC phenomenon. For effective clinical application, many liposomal drug formulations will require multiple injections. The ABC phenomenon described in this and several preceding papers therefore has important implications for the development and evaluation of therapeutically useful liposomal formulations requiring multiple-dose administration.

Tandem copies of a human rotavirus VP8 epitope can induce specific neutralizing antibodies in BALB/c mice

The VP8 subunit protein of human rotavirus (HRV) plays an important role in viral infectivity and neutralization. Recombinant peptide antigens displaying the amino acid sequence M(1)ASLIYRQLL(10), a linear neutralization epitope on the VP8 protein, were constructed and examined for their ability to generate anti-peptide antibodies and HRV-neutralizing antibodies in BALB/c mice. Peptide antigen constructs were expressed in E. coli as fusion proteins with thioredoxin and a universal tetanus toxin T-cell epitope (P2), in order to enhance the anti-peptide immune response. The peptide antigen containing three tandem copies of the VP8 epitope induced significantly higher levels of anti-peptide antibody than only a single copy of the epitope, or the peptide co-administered with the carrier protein and T-cell epitope. Furthermore, the peptide antigen containing three copies of the peptide produced significantly higher virus-neutralization titres, higher than VP8, indicating that a peptide antigen displaying repeating copies of the amino acid region 1-10 of VP8 is a more potent inducer of HRV-neutralizing antibodies than VP8 alone, and may be useful for the production of specific neutralizing antibodies for passive immunotherapy of HRV infection.

Hypersensitivity and loss of disease site targeting caused by antibody responses to PEGylated liposomes

The systemic application of nucleic acid drugs requires delivery systems that overcome the poor pharmacokinetics, limited biodistribution, and inefficient uptake of nucleic acids. PEGylated liposomes show considerable promise because of their intrinsic ability to accumulate at disease sites and facilitate transfection of target cells. Unlike many viral vectors, PEGylated liposomes are generally considered to be nonimmunogenic. We have developed a PEGylated liposome for the systemic administration of plasmid DNA that achieves high levels of selective gene expression at distal tumor sites. Here we report that the in vivo efficacy and safety of these systems can be severely compromised following repeat administration. This phenomenon is characterized by a loss of disease site targeting, accelerated clearance from the blood, and acute hypersensitivity. These effects are fully attributable to a surprisingly robust, long-lived antibody response generated against polyethylene glycol (PEG) that results from the strong adjuvant effect of the plasmid payload. Importantly, immunogenicity may be substantially reduced by modifying the alkyl chain of the PEG-lipid conjugate, thereby allowing successful repeat dosing of the modified plasmid formulations without adverse side effects. Immunogenicity is a relevant concern for a number of nonviral delivery systems given the potent immunostimulatory properties of many nucleic acid drugs.

Induction of effective and antigen-specific antitumour immunity by a liposomal ErbB2/HER2 peptide-based vaccination construct

Efficient delivery of tumour-associated antigens to appropriate cellular compartments of antigen-presenting cells is of prime importance for the induction of potent, cell-mediated antitumour immune responses. We have designed novel multivalent liposomal constructs that co-deliver the p63–71 cytotoxic T Lymphocyte epitope derived from human ErbB2 (HER2), and HA307–319, a T-helper (Th) epitope derived from influenza haemagglutinin. Both peptides were conjugated to the surface of liposomes via a Pam₃CSS anchor, a synthetic lipopeptide with potent adjuvant activity. In a murine model system, vaccination with these constructs completely protected BALB/c mice from subsequent s.c. challenge with ErbB2-expressing, but not ErbB2-negative, murine renal carcinoma (Renca) cells, indicating the induction of potent, antigen-specific immune responses. I.v. re-challenge of tumour-free animals 2 months after the first tumour cell inoculation did not result in the formation of lung tumour nodules, suggesting that long-lasting, systemic immunity had been induced. While still protecting the majority of vaccinated mice, a liposomal construct lacking the Th epitope was less effective than the diepitope construct, also correlating with a lower number of CD8⁺ IFN-γ⁺ T-cells identified upon ex vivo peptide restimulation of splenocytes from vaccinated animals. Importantly, in a therapeutic setting treatment with the liposomal vaccines resulted in cures in the majority of tumour-bearing mice and delayed tumour growth in the remaining ones. Our results demonstrate that liposomal constructs which combine Tc and Th peptide antigens and lipopeptide adjuvants can induce efficient, antigen-specific antitumour immunity, and represent promising synthetic delivery systems for the design of specific antitumour vaccines.

Polyelectrolyte multilayers functionalized by a synthetic analogue of an anti-inflammatory peptide, α-MSH, for coating a tracheal prosthesis

Polyelectrolyte multilayer films made of poly (L-lysine) (PLL) and poly (L-glutamic acid) (PGA) have been functionalized by covalent binding of a synthetic analogue of the anti-inflammatory peptide, alpha-melanocyte-stimulating hormone (alpha-MSH) to PGA to create biologically active coatings for tracheal prostheses. The morphology and in vivo stability of the films were investigated by atomic force microscopy and confocal laser scanning microscopy, respectively. For the in vivo evaluation, 87 rats were implanted and examined for a period superior to 3 months. Histological analysis, performed 1 month after implantation, showed a fibroblast colonization of the periprosthetic side and a respiratory epithelium type on the endoluminal side of the implant for all the polyelectrolyte coatings tested. However, for prostheses modified by PGA ending multilayer films, a more regular and less obstructive cell layer was observed on the endoluminal side compared to those modified by PLL ending films. Systemic anti-inflammatory IL-10 production was only detected in rats implanted with prostheses functionalized by alpha-MSH, demonstrating, in vivo, the anti-inflammatory activity of the embedded peptide into multilayer architectures.

The covalent coupling of HAV-VP3 (110-121) synthetic peptide to liposomes: physicochemical studies

In this work we have studied the conjugation of the immunogenic peptide sequence (110-121) belonging to the VP3 capsid protein of hepatitis A virus to the surface of preformed liposomes by means of an amide bond between the vesicles and the synthetic peptide. The surface activity of the conjugate at air/water interface was determined. Moreover, the interaction of the conjugate with lipids was also studied recording the pressure increases produced after the injection of the liposome-peptide preparation under dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG) and stearylamine (SA) monolayers at different initial surface pressures. As expected, due to the negative net charge of the liposome-peptide complex, the higher interaction was found with positive charge monolayers (SA). However, the conjugate was also able to incorporate to zwitterionic and anionic lipids. This behaviour was also confirmed performing compression isotherms of monolayers of these lipids spread on subphases containing the conjugate. These results suggest that the coupling of VP3 (110-121) to liposomes does not influence its ability to interact with membrane lipids such as DPPC and DPPG. Then it can be assumed that its immunogenicity will be preserved or even increased after this modification. All these results are also useful in the preparation of liposome-based synthetic peptide vaccines.

Synthesis of Thiol-Reactive Lipopeptide Adjuvants. Incorporation into Liposomes and Study of Their Mitogenic Effect on Mouse Splenocytes

Synthetic analogues of triacylated and diacylated lipopeptides derived from the N-terminal domain of respectively bacterial and mycoplasmal lipoproteins are highly potent immunoadjuvants when administered either in combination with protein antigens or covalently linked to small peptide epitopes. Because of their amphipathic properties, lipopeptides, such as S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-N-palmitoyl-(R)-cysteinyl-alanyl-glycine (Pam(3)CAG), can be conveniently incorporated into liposomes and serve as anchors for antigens that are linked to them. To design vaccination constructs based on synthetic peptides and liposomes as vectors. we have accordingly synthesized a series of lipopeptides that differ by the number (Pam(3)C vs Pam(2)C) and nature of the acyl chains (palmitoyl vs oleoyl) and by the presence at their C-terminus of thiol-reactive functions, such as maleimide or bromoacetyl. When incorporated into liposomes, these latter functionalized lipopeptides allow, in aqueous media, a well controlled chemoselective conjugation of HS-peptides to the surface of the vesicles. Using a BALB/c mice splenocyte proliferation assay ([(3)H]thymidine incorporation), we have measured the lymphocyte activation potency of the different lipopeptides. We found that, compared to their free (emulsified) forms, the liposomal lipopeptides were endowed with enhanced mitogenic activities; i.e., up to 2 orders of magnitude for Pam(3)CAG which was more potent than Pam(2)CAG. The impact of functionalization on the cellular activity of Pam(3)CAG was dependent on the thiol-reactive group introduced: whereas the bromoacetyl derivative retained its full activity, the presence of a maleimide group virtually abolished the lymphocyte activation of the lipopeptide. Finally, the substitution of saturated palmitoyl chains by unsaturated oleoyl chains was inhibitory. Thus, thiol-reactive Ol(3)CAG derivatives were the least active mitogens in our assay. Taken together, our findings are of importance for the further optimization of antigen-specific liposomal-based synthetic vaccines; the bromoacetyl derivative of Pam(3)CAG should be a promising lipopeptide derivative serving as an anchor for peptide epitopes while retaining its lymphocyte activation activity.

Membrane-active properties of alpha-MSH analogs: aggregation and fusion of liposomes triggered by surface-conjugated peptides

Reaction of the melanotropin hormone analogs [Nle(4),D-Phe(7)]-alpha-MSH and [Nle(4),D-Phe(7)]-alpha-MSH(4-10), which were extended at their N-terminus by a thiol-functionalized spacer arm, with preformed liposomes containing thiol-reactive (phospho)lipid derivatives resulted in the aggregation of the vesicles and in a partial leakage of their inner contents. This aggregation/leakage effect, which was only observed when the peptides were covalently conjugated to the surface of the liposomes, was correlated with the fusion of the vesicles as demonstrated by the observed decrease in resonance energy transfer between probes in a membrane lipid mixing assay. A limited fusion was confirmed by monitoring the mixing of the liposome inner contents (formation of 1-aminonaphthalene-3,6,8-trisulfonic acid/p-xylene bis(pyridinium bromide) complex). The membrane-active properties of the peptides could be correlated with changes in the fluorescence emission spectra of their tryptophan residue, which suggested that after their covalent binding to the outer surface of the liposomes they can partition within the core of the bilayers. A blue shift of 10 nm was observed for [Nle(4),D-Phe(7)]-alpha-MSH which was correlated with an increase in fluorescence anisotropy and with changes in the accessibility of the coupled peptide as assessed by the quenching of fluorescence of its tryptophan residue by iodide (Stern-Volmer plots). These results should be related to the previously described capacity of alpha-MSH, and analogs, to interact with membranes and with the favored conformation of these peptides which, via a beta-turn, segregate their central hydrophobic residues into a domain that could insert into membranes and, as shown here, trigger their destabilization.

Perspectives: towards a peptide-based vaccine against hepatitis C virus

Hepatitis C virus (HCV) is a widespread infectious disease in humans with the negative implication of becoming chronic in most persons. Patients infected with HCV are at risk of liver cirrhosis or hepatocellular carcinoma at later stages. In contrast to hepatitis A and hepatitis B, there is no immunization yet available, neither prophylactic nor therapeutic. Thus, there is an urgent need to develop a safe, protective vaccine against this fatal disease. Developing countries are even more at risk for HCV. There are currently a number of scientific approaches aimed towards solving this problem. Taking both risks and costs of immunization into consideration, a peptide-based vaccine may be a reasonable prophylactic protection. Also, it might be of therapeutic use in already infected patients by increasing a specific CTL response against HCV. In our lab, we are focusing on immunopotentiating reconstituted influenza virosomes (IRIVs) as carriers for immunogenic HLA-A2-restricted core epitopes to induce peptide-specific cytotoxic T lymphocytes (CTLs). The IRIVs are similar to liposomes, but in addition contain influenza-derived hemagglutinin and neuraminidase on their outer surface which makes them fusogenic, thus, permitting antigen delivery to host cells. So far, virosomes have been successfully used for vaccine development and as a result a virosomal vaccine against both influenza virus (Inflexal) BERNA) and hepatitis A virus (HAV) (Epaxal) BERNA) already exist on the market. This paper focuses on the importance of development of a successful vaccine against HCV and, more specifically, we discuss the use, advantages and disadvantages of a peptide-based vaccine. A brief report of our latest findings will be included.

Defined synthetic vaccines

Although vaccines have proven very successful in preventing certain infectious diseases, progress in the field has been slowed by the tediousness of developing classical vaccines consisting of whole pathogens. Thus, there is great need for improvement in several areas: firstly, the range of diseases which can be treated has to be expanded. Secondly, antigens have to be defined to make the use of whole pathogens as antigen obsolete. And thirdly, new adjuvants have to be developed which show low toxicity, high potency and are also able to drive the immune response in the desired direction. Ideally, a vaccine would only consist of well-characterized, synthetic materials. This review summarizes the different approaches for the development of completely defined synthetic vaccines.

Therapeutic applications of colloidal drug carriers

Colloidal drug carriers such as liposomes and nanoparticles can be used to improve the therapeutic index of both established and new drugs by modifying their distribution, and thus increasing their efficacy and/or reducing their toxicity. This is because the drug distribution then follows that of the carrier, rather than depending on the physicochemical properties of the drug itself. If these delivery systems are carefully designed with respect to the target and the route of administration, they may provide one solution to some of the delivery problems posed by new classes of active molecules, such as peptides and proteins, genes and oligonucleotides. They may also offer alternative modes for more conventional drugs, such as highly hydrophobic small molecules. This review discusses the use of colloidal, particulate carrier systems (25 nm to 1 µm in diameter) in such applications.

Differential reactivity of maleimide and bromoacetyl functions with thiols: application to the preparation of liposomal diepitope constructs

The comparative reactivity of maleimide and bromoacetyl groups with thiols (2-mercaptoethanol, free cysteine, and cysteine residues present at the N-terminus of peptides) was investigated in aqueous media. These studies were performed (i) with water-soluble functionalized model molecules, i.e., polyoxyethylene-based spacer arms that could also be coupled to lipophilic anchors destined to be incorporated into liposomes, and (ii) with small unilamellar liposomes carrying at their surface these thiol-reactive functions. Our results indicate that an important kinetic discrimination (2-3 orders of magnitude in terms of rate constants) can be achieved between the maleimide and bromoacetyl functions when the reactions with thiols are performed at pH 6.5. The bromoacetyl function which reacts at higher pH values (e.g., pH 9.0) retained a high chemoselectivity; i.e., under conditions where it reacted appreciably with the thiols of, e.g., HS-peptides, it did react with other nucleophilic functions such as alpha- and epsilon-amino groups or imidazole, which could also be present in peptides. This differential reactivity was applied to design chemically defined and highly immunogenic liposomal diepitope constructs as synthetic vaccines, i.e., vesicles carrying at their surface two different peptides conjugated each to a specific amphiphilic anchor. This was realized by coupling sequentially at pH 6.5 and 9.0 two HS-peptides to preformed vesicles containing lipophilic anchors functionalized with maleimide and bromoacetyl groups [Boeckler, C., et al. (1999) Eur. J. Immunol. 29, 2297-2308].