Enhanced antibody response to liposome-associated protein antigens: preferential stimulation of IgG2a/b production

A peptide-CpG-DNA-liposome complex vaccine targeting TM4SF5 suppresses growth of pancreatic cancer in a mouse allograft model

Background

Patients with pancreatic cancer have a poor prognosis and are usually diagnosed at a late stage. Because TM4SF5 is known to be overexpressed in hepatocellular carcinoma, colon cancer, and pancreatic cancer, it is considered as one of the candidate molecular targets for an anticancer strategies.

Purpose

The purpose of this study was to evaluate possible utility of TM4SF5 to treat pancreatic cancer using a mouse allograft model.

Materials and methods

We analyzed expression of TM4SF5 in pancreatic cancer tissues using immunohistochemistry. We established a mouse pancreatic cancer cell line stably expressing TM4SF5 and identified the effect of TM4SF5 expression in vitro. We used the CpG-DNA-peptide-liposome complex as a peptide vaccine and investigated antitumor effects of the vaccine in a mouse model with TM4SF5 expressing pancreatic cells. To investigate the function of produced antibody, we evaluated effects of the anti-TM4SF5 monoclonal antibody in vitro in terms of cell growth and migration properties.

Results

Immunohistochemical analysis showed that 36.4% of pancreatic cancer tissue samples expressed TM4SF5. Expression of TM4SF5 induced increased cell proliferation and motility in vitro. Injection of the TM4SF5 peptide vaccine induced the production of anti-hTM4SF5 antibodies and reduced the growth of pancreatic tumors in mice established by subcutaneous injection of the TM4SF5-expressing mouse pancreatic cancer cell line. The treatment of TM4SF5-expressing cells with the anti-hTM4SF5 monoclonal antibody reduced cell growth, modulated the expression of the epithelial–mesenchymal transition markers Vimentin and E-cadherin, and decreased cell motility in vitro.

Conclusion

Our results showed that the TM4SF5 peptide vaccine had a protective effect against pancreatic tumors expressing TM4SF5, and this effect was mediated, at least in part, by the production and suppressive function of the anti-TM4SF5 antibodies. Therefore, we suggest that targeting TM4SF5 could be a novel strategy to prevent or treat pancreatic cancer.

Biological evaluation of liposome-encapsulated hemoglobin surface-modified with a novel PEGylated nonphospholipid amphiphile

Traumatic injury is often associated with hemorrhagic shock. Liposome-encapsulated hemoglobin (LEH) is being developed as an artificial oxygen carrier to address post-hemorrhage oxygen and volume deficit. Here, we report a new composition of LEH based on the use of polyethylene glycol (PEG2K ) conjugated with nonphospholipid hexadecylcarbamoylmethylhexadecanoate (HDAS) to modify the surface of LEH particles. LEH was manufactured by the high-pressure homogenization method using dipalmitoylphosphatidylcholine (∼38 mol%), cholesterol (∼38 mol%), HDAS (∼20 mol%), and highly purified stroma-free human hemoglobin. HDAS-PEG2K was postinserted into the resultant LEH to generate HDAS-PEG2K -LEH. We investigated the potential immune response to HDAS-PEG2K -LEH in a mice model. At the same time, the preparation was tested in a rat model to study the effect of repeated HDAS-PEG2K -LEH injection over 4 weeks. We found that HDAS-PEG2K modification substantially reduced the circulating levels of anaphylatoxins C3a and C5a, as well as plasma levels of thromboxane B2, in mice. Repeated injections of HDAS-PEG2K -LEH in rats did not appear to alter its clearance profile after 4 weeks of treatment. No antibody response against human hemoglobin or PEG was detected in rat plasma. Histological observations of lung, liver, spleen, and kidney were not significantly different between saline-treated rats and HDAS-PEG2K -LEH-treated rats. Immunohistochemical staining for rat heme oxygenase-1 (HO-1) did not show induced expression of HO-1 in these organs. These results suggest that the new surface modification of LEH is immune-neutral and does not adversely affect histology even after repeated administration.

Protein flexibility and conformational states of Leishmania antigen eIF-4A: identification of a novel plausible protein adjuvant using comparative genomics and molecular modeling

Recent homology modeling studies have identified specific residues (epitope) of the Leishmania RNA helicase protein (LmeIF) that stimulates production of IL-12 cytokine. However, question remains concerning how LmeIF's N-terminal moiety initiates adjuvant effects. Extensive molecular modeling combining the normal mode analysis (NMA) and molecular dynamics simulations, in the present study, has demonstrated that the LmeIF structure may exist in two different forms corresponding to the extended and collapsed (closed) states of the entire structure. The computational results showed that the two domains of the LmeIF structure tend to undergo large fluctuations in a concerted fashion and have strong effect on the solvent accessible surface of the epitope situated on the N-terminal structure. The conformational freedom of the C-terminal domains may explain why the entire LmeIF protein is not as active as the N-terminal moiety. Thereafter, a comparative genome analysis with subsequent homology modeling and molecular electrostatic potential (MEP) techniques allowed us to predict a novel and plausible RNA helicase (LI-helicase) from the Listeria source with adjuvant property as observed for the Leishmania eIF-4A protein. The structural folding and MEP maps revealed similar topologies of the epitope of both LmeIF and LI-helicase proteins and striking identity in the local disposition of the charged groups. An animated Interactive 3D Complement (I3DC) is available in Proteopedia at http://proteopedia.org/w/Journal:JBSD:7.

Leishmania major: Immune response in BALB/c mice immunized with stress-inducible protein 1 encapsulated in liposomes

Protection against leishmaniasis is depending upon generation of a Th1 type of immune response. Field trials of first generation Leishmania vaccine showed a limited efficacy even with multiple doses mainly due to lack of an appropriate adjuvant. In this study, susceptible BALB/c mice were immunized with rLmSTI1 encapsulated in liposomes to explore the extent of protection induced by Leishmania antigen encapsulated in the liposomes against challenge with Leishmania major. The results showed that s.c. immunization of BALB/c mice with liposomal rLmSTI1 induced a significant protection against challenge and a significant lower parasite burden in spleen up to 14 weeks after challenge. The protected animals showed a significantly smaller footpad thickness after challenge, and a higher level of anti-SLA IgG antibodies before and after challenge with a predominant IgG2a titer. The data supports the possibility of using liposomal Leishmania antigens as a vaccine.

Fusogenic potential of sperm membrane lipids: Nature's wisdom to accomplish targeted gene delivery

The membrane-membrane fusion during fertilization of oocyte by spermatozoa is believed to be mainly mediated by so called "fusion proteins". In the present study we have tried to demonstrate that beside the proteins, lipid components of membrane may play an important role in fusion of oocyte with spermatozoa. Conventional membrane-membrane fusion assays were used as means to demonstrate fusogenic potential of human sperm membrane lipids. The liposomes (spermatosomes) made of the lipids isolated from sperm membrane were found to undergo strong membrane-membrane fusion as evident from fluorescence dequenching and resonance energy transfer assays. Furthermore, the fusion of these liposomes with living cells (J774 A.1 macrophage cell line) was demonstrated to result in an effective transfer of a water-soluble fluorescent probe (calcein) to cytosol of the target cell. Lastly, the liposomes were demonstrated to behave like efficient vehicles for the in vivo cytosolic delivery of the antigens to target cells resulting in elicitation of antigen specific CD8(+) T cell responses.

Encapsulation of Fluorescent Molecules by Functionalized Polymeric Nanocontainers: Investigation by Confocal Fluorescence Imaging and Fluorescence Correlation Spectroscopy

Nanocontainers (NCs) were prepared from amphiphilic triblock copolymers, having an average molecular weight of around 8000 g/mol, by using previously published preparation methods consisting of dispersing the polymer in an aqueous buffer solution containing molecules for encapsulation. A small molecular weight fluorophore, sulforhodamine B, as well as the fluorescent protein avidin labeled with Alexa 488 were encapsulated, and the resulting nanocontainers were characterized using fluorescence correlation spectroscopy (FCS) and fluorescence cross-correlation spectroscopy (FCCS). Nanocontainer size determination by FCS is very robust and compares well with results obtained from photon correlation spectroscopy: the measured diameters of the polymeric nanocontainers vary between 140 and 172 nm. Encapsulation of fluorescent molecules was determined by evaluating the molecular brightness of nanocontainers with an encapsulated fluorescently labeled protein (avidin-Alexa 488). Results indicate that the number of encapsulated avidin-Alexa 488 molecules corresponds well with the initial concentration of the fluorescently labeled protein and the encapsulated volume. A nanocontainer binding assay was developed using biotinylated fluorescently labeled nanocontainers. Binding of biotinylated nanocontainers to fluorescently labeled streptavidin was followed by fluorescence cross-correlation spectroscopy. The intrinsic dissociation constant, K(d), of labeled streptavidin to the ligand-modified nanocontainers is 1.7 +/- 0.4 x 10(-8) M, and about 1921 +/- 357 molecules of labeled streptavidin are bound to each nanocontainer.

Generating neutralizing antibodies, Th1 response and MHC non restricted immunogenicity of HIV-I env and gag peptides in liposomes and ISCOMs with in-built adjuvanticity

For enhancing immunogenicity and develop vaccine strategies using peptide based constructs against HIV-1, a chimeric peptide containing V3 loop and transmembrane sequence of gp41 with two glycine motifs as spacer was constructed. The V3-gp41, gp41 peptide and p17 and p24 peptides separately or in a cocktail were entrapped with or without MA729 as an immunoadjuvant in liposomes or ISCOMs. The immunogenicity, antigen induced T-cell proliferation and cytokine profiles of various formulations were studied in four different inbred strains of mice of H-2^d, H-2^b, H-2^k and H-2^q haplotypes, keeping alum as a control adjuvant. Both liposomes and ISCOM preparations elicited high titer and long lasting antibody response (60 days and above). When compared to the alum formulation, the liposomes co-entrapped with MA729 produced high antibody levels, comparable with that induced by ISCOMs. Peptide in alum, liposomes and ISCOMs enhanced both antigen specific IgG2a and IgG2b isotypes and high T-cell stimulation index. Peptide formulations also induced antibodies with high affinity and in vitro neutralizated the formation of HIV-1 syncytia. T-cell supernatants contained high levels of IFN-γ and IL-2. Thus formulation in these adjuvants induced a predominant Th1 like response with MA729 as a versatile novel delivery vehicle for stimulating the appropriate arm of the immune response that can selectively modulate MHC class I or MHC class II response. The above peptide can be of wide vaccination interest as a means to improve immune responses to several other HIV-1 antigens and may serve as candidates for vaccine development.

The influence of repeated injections on pharmacokinetics and biodistribution of different types of sterically stabilized immunoliposomes

Sterically stabilized immunoliposomes (IL) with diameters of about 135 nm carrying mouse IgG, either coupled directly to the liposome surface, or linked to the terminal ends of grafted poly(ethylene glycol) (PEG) chains by a recently described conjugation procedure (Cyanur-PEG-PE), were intravenously injected into rats and the elimination kinetics and biodistribution were determined and compared with control liposomes. The amounts of conjugated antibodies were about 30 microg/micromol total lipid for all IL. In naive rats, plain pegylated liposomes displayed the longest blood circulation time, whereas the terminal-coupled IL exhibited the fastest elimination. Liposomes containing the underivatized anchor molecules circulate nearly as long as plain pegylated liposomes, indicating that the fast elimination of the IL can be attributed to the presence of antibodies.A second injection of identical liposomes 14 days after the first injection had a considerable influence on the pharmacokinetic parameters of the liposomes. The circulation time of plain pegylated liposomes drastically dropped by half and their uptake by the liver increased concomitantly, indicating that the PEG, upon repeated injection, ceases to function as an efficient barrier reducing opsonization and/or immune reactions. The circulation time of conventional IL was moderately reduced upon a second injection, whereas that of the terminally coupled IL was nearly unaffected. These differences among the IL demonstrate that the pharmacokinetic behavior of IL is strongly dependent on the antibody conjugation site on the liposome. The observed effects of repeated injections were similar for liposomes of 90-nm diameter. The phenomena described may have important implications for the repeated application of IL as drug carriers.

Fusogenic potential of prokaryotic membrane lipids. Implication in vaccine development

Development of protective immunity against many pathogens, particularly viruses, requires fine orchestration of both humoral- and cell mediated-immunity. The immunization of animals with soluble antigens usually leads to the induction of humoral immune responses. In contrast, the activation of a cell-mediated immune response against exogenous antigens has always been a challenge, requiring special strategies to expose them to the proteasome, a multifunctional protease complex in the cytosol of the target cells. The degradation of the protein by the cytosolic proteolytic system forms a cardinal step for the induction of cytotoxic T lymphocytes (CTLs). In the present study, we report that a potent primary CTL response against a soluble protein, ovalbumin, can be induced in mice by encapsulating it in the liposomes comprised of Escherichia coli membrane lipids. These lipids were shown to induce strong membrane-membrane fusion as evident from resonance energy transfer and content mixing assays. Furthermore, the fusion of these liposomes with living cells (J774 A1) was demonstrated to result in effective transfer of a fluorescent lipid probe to the plasma membrane of the cells. Moreover, ricin A, a protein synthesis inhibitor that does not cross plasma membrane, was demonstrated to gain access to the cytosol when it was encapsulated in these liposomes. Finally, the liposomes were demonstrated to behave like efficient vehicles for the in vivo delivery of the antigens to the target cells resulting in the elicitation of antigen reactive CD8+ T cell responses.

New age adjuvants and delivery systems for subunit vaccines

The dramatic advancements in the field of vaccinology has led to the formulation of chemically well defined vaccines composed of synthetic peptides and recombinant proteins derived from the immunologically dominant regions of the pathogens. Though these subunit vaccines are safer compared to the traditional vaccines they are known to be poorly immunogenic. This necessitates the use of adjuvants to enhance the immunogenicity of these vaccine formulations. The most common adjuvant for human use is alum. Research in the past has focused on the development of systemic immunity using conventional immunization protocols. In the present are, the emphasis is on the development and formulation of alternative adjuvants and delivery systems in generating systemic as well as mucosal immunity. This review mainly focuses on a variety of adjuvants (particulate as well as non-particulate) used with protective antigens of HIV, malaria, plague, leprosy using modified delivery vehicles. The experience of our laboratory and other researchers in this field clearly proves that these new age adjuvants and delivery systems undoubtedly generate enhanced immune response-both humoral and cell mediated. The choice of antigens, the nature of adjuvant used and the mode of delivery employed have a profound effect on the type of immune response generated. Besides the quantity, the quality of the antibodies generated also play a vital role in protection against these diseases. Some of the adjuvants and delivery systems used promoted high titre and affinity antibodies, which were shown to be cytophilic in nature, an important criteria in providing protection to the host. Thus the studies on these adjuvants/delivery systems with respect to various infectious diseases indicate their active role in efficient modulation of immune response along with safety and permissibility.

Improving vaccine performance with adjuvants

New vaccines are presently under development and in testing for the control of infectious diseases, including human immunodeficiency virus (HIV) and tuberculosis. Several of these vaccines are composed of synthetic, recombinant, or highly purified subunit antigens. Subunit vaccines are designed to include only the antigens required for protective immunization and to be safer than whole-inactivated or live-attenuated vaccines. However, the purity of the subunit antigens and the absence of the self-adjuvanting immunomodulatory components associated with attenuated or killed vaccines often result in weaker immunogenicity. Immunologic adjuvants are agents that enhance specific immune responses to vaccines. Formulation of vaccines with potent adjuvants is an attractive approach for improving the performance of vaccines composed of subunit antigens. Adjuvants have diverse mechanisms of action and should be selected for use on the basis of the route of administration and the type of immune response (antibody, cell-mediated, or mucosal immunity) that is desired for a particular vaccine.

Pathways for potentiation of immunogenicity during adjuvant-assisted immunizations with Plasmodium falciparum major merozoite surface protein 1

Vaccine adjuvants exert critical and unique influences on the quality of immune responses induced during active immunizations. We investigated the mechanisms of action of immunological adjuvants in terms of their requirements for cytokine-mediated pathways for adjuvanticity. Antibody responses potentiated by several adjuvants to a Plasmodium falciparum MSP1-19 (C-terminal 19-kDa processing fragment of MSP1) vaccine were studied in gamma interferon (IFN-gamma) or interleukin (IL-4) knockout mice. The levels of anti-MSP1-19 antibodies and the induction of Th1- and Th2-type antibodies were analyzed. Results revealed a spectrum of requirements for cytokine-mediated pathways in the potentiation of immunogenicity, and such requirements were influenced by interactions among individual components of the adjuvant formulations. One adjuvant strictly depended on IFN-gamma to induce appreciable levels of anti-MSP1-19 antibodies, while some formulations required IFN-gamma only for the induction of Th1-type antibodies. Other formulations induced exclusively Th2-type antibodies and were not affected by IFN-gamma knockout. There were three patterns of requirements for IL-4 by various adjuvants in the induction of Th2-type anti-MSP1-19 antibodies. Moreover, the induction of Th1-type anti-MSP1-19 antibodies by adjuvants showed two distinct patterns of regulation by IL-4. The utilization of an IL-4 regulated pathway(s) for the induction of Th2-type antibodies by the same adjuvant differed between mouse strains, suggesting that animal species variability in responses to vaccine adjuvants may be due, at least in part, to differences in the utilization of immune system pathways by an adjuvant among animal hosts.

Clearance properties of liposomes involving conjugated proteins for targeting

This review examines methods of protein conjugation onto liposomes and the effects of surface bound protein on the liposomes' biological behavior. It is evident that the presence of a conjugated protein significantly alters the attributes of targeted liposomes. Specifically, protein conjugation can result in dramatic increases in liposome size, enhanced immunogenicity, and increased plasma elimination. Techniques are discussed for preventing some of the physical (size) and biological (immunogenic) alterations involving the use of PEG-lipids and drug loaded liposomes. In addition, the advantages of conjugating antibodies via carbohydrate moieties, to minimize changes in antibody binding and tertiary structure as well as effectively decreasing plasma elimination, are also discussed. It is, however, apparent that the accessibility of targeted liposomes to extravascular sites is a key step that will require further study and it is, therefore, anticipated that with the development of novel ligands and novel ligand-liposome interactions, the therapeutic utility of targeting strategies will likely be realized.

Flow cytometric analysis of cellular changes in mice after intradermal inoculation with a liposome-iscom adjuvanted vaccine

As it is not known what changes to leucocyte homeostasis are mandatory for effective adjuvant action, the biological relevance of systemic changes elicited by different vaccine formulations can only be interpreted in the context of the immunological outcomes. We used flow cytometry to quantify the changes in leucocyte subsets induced in mice intradermally immunized with SAMA4 (adjuvant group), outer membrane proteins (OMP) purified from Actinobacillus pleuropneumoniae (OMP antigen group), SAMA4 adjuvanted OMP (OMP vaccine group), or phosphate-buffered saline (PBS: control group). This approach allowed direct comparisons to be made between the effects of antigen, adjuvant or antigen-adjuvant complexes on immune effector cell populations. Antigens complexed with the liposome-iscom hybrid adjuvant, SAMA4, generated strong antibody responses and cytotoxic T-cell activity in animals immunized intradermally, reflecting remobilization and recruitment of specific cell populations. Splenomegaly, due to granulocytosis, monocytosis and megakaryocytosis, was most prominent in the OMP vaccine group. Histological examination of spleen sections confirmed that these changes were due primarily to splenic haematopoiesis. Circulating numbers of granulocytes and monocytes increased significantly (P < 0.05) in the blood of the OMP vaccine group, as did granulocyte numbers in the lungs (P < 0.05). No changes in T- and B-cell numbers were detected by flow cytometry in the spleens, lungs or blood over the 28-day period in any treatment group. Thymocyte numbers (predominantly CD4+CD8+ cells) in the OMP vaccine group fell by 95% within 3 days of immunization. Identical cellular responses were obtained when an innocuous antigen, ovalbumin, was complexed with SAMA4 instead of OMP, thus demonstrating that the adjuvant effects of SAMA4 were due to synergistic interaction between antigen and adjuvant and not due to the presence of toxic components. The association of strong adaptive immune responses with such complex changes in leucocyte homeostasis induced by complexing adjuvant and antigen suggested that the changes were important for effective vaccination and were not purely circumstantial.

Differential biodistribution of encapsulated and surface-linked liposomal antigens

The biodistribution of liposomal antigens either encapsulated in or surface-linked to liposomes of similar composition was studied over time following intravenous injection and the results analyzed in relation to adjuvanticity. The two formulations were shown to behave very differently in vivo. While encapsulated antigen was rapidly focused to liver and spleen as expected, surface-linked antigen exhibited a more disseminated distribution which parallels that of the free protein. In dual-labelling experiments, it was also shown that encapsulated antigen remains associated with its liposomal vehicle in contrast to surface-linked antigen which is rapidly dissociated. This dissociation was apparently neither due to an exchange with plasma lipoproteins nor to a direct action of blood constituents. Besides, it was found that surface-linked antigen was rapidly accumulated in the carcass. We propose that the retention of the surface-linked antigen in the carcass results from a pre-processing of the protein involving more probably mononuclear phagocytes. This pre-processing might in turn favor the dissociation of the protein from the liposomes in a form that allows its dissemination in the whole organism and its interaction with more efficient antigen presenting cells such as for example Langerhans or dendritic cells.

Adjuvant properties of non-phospholipid liposomes (Novasomes) in experimental animals for human vaccine antigens

Non-phospholipid liposomes composed of dioxyethylene cetyl ether, cholesterol and oleic acid were evaluated as adjuvants with human vaccine antigens, tetanus toxoid (TT) and diphtheria toxoid (DT), in mice and rabbits. Antigens encapsulated in or mixed with liposomes elicited antitoxin levels similar to those elicited by antigens given with Freund's adjuvant or adsorbed onto aluminum phosphate. All liposomal antigen preparations, antigen given with Freund's adjuvant or adsorbed onto aluminum phosphate, elicited significantly higher IgG antibodies and antitoxin levels than soluble antigens in mice after a single injection and in rabbits after each of three injections. TT encapsulated in liposomes elicited sustained anti-TT IgG antibody levels in mice after a single injection as compared to TT mixed with liposomes. TT mixed with or encapsulated within liposomes containing monophosphoryl lipid A/squalene or squalene alone, as well as aluminum phosphate adsorbed TT elicited greater primary responses in mice than TT mixed with or encapsulated within plain liposomes. Liposomal TT preparations produced a slightly higher anamnestic response in mice than aluminum phosphate adsorbed TT. Subclass analysis of anti-TT antibodies showed that the majority of the antibodies belong to IgG1 subclass. Liposomal TT preparations, particularly those with encapsulated monophosphoryl lipid A/squalene or squalene alone, consistently elicited higher levels of anti-TT IgG2a and IgG2b than aluminum phosphate adsorbed or soluble TT. None of the preparations elicited IgG3 or IgM antibodies. It appears that non-phospholipid liposomes are as potent adjuvants as the currently employed adjuvant for human vaccines (aluminum phosphate) or a benchmark adjuvant for experimental immunology (Freund's adjuvant), and may be able to modulate the immune response towards the Th1 type.

Murine IgG subclass antibodies to antigens incorporated in liposomes containing lipid A

The IgG subclass responses to antigens incorporated in liposomes containing lipid A were investigated using a synthetic malarial antigen (SPf66) and cholera toxin (CT). The antigen-specific IgG subclass response was determined in BALB/c mice immunized with either: (a) SPf66 encapsulated in liposomes containing lipid A, (b) CT bound to the surface of liposomes containing lipid A, or (c) both encapsulated SPf66 and surface-bound CT in the same liposomes. In each case the antibodies to SPf66, CT and lipid A demonstrated an IgG2a predominance. Liposomes containing lipid A not only increased the magnitude of the antibody response to liposomal antigens but elicited predominantly IgG2a subclass antibodies as well.

Immunologic adjuvants for modern vaccine formulations

Optimization of the immunogenicity of many new-generation vaccine formulations, including combination vaccines, will require the use of immunologic adjuvants other than the aluminum compounds in today's licensed vaccines. The selection of adjuvants for use in vaccine formulation may be as critical as the choice of the vaccine antigens themselves in providing optimal efficacy for the target populations, vaccine compliance, and cost. Adjuvants have diverse mechanisms of action and must be selected for use based on the immune responses desired for a particular candidate vaccine. Recent advances in the number and variety of adjuvants available for clinical evaluation coupled with the increased understanding of their mechanisms of action encourage the inclusion of adjuvants as part of rational vaccine design. Finally, the proposed standardized methods to evaluate adjuvant safety should be implemented for human candidate vaccines formulated with novel adjuvants.

Immunogenicity of immunoliposomes: reactivity against species-specific IgG and liposomal phospholipids

Immunoliposomes with surface-linked avidin-biotinylated mouse IgG2a were prepared from dipalmitoylphosphatidylcholine (DPPC), dimyristoylphosphatidylglycerol (DMPG) and biotinylated dipalmitoylphosphatidylethanolamine (DPPE-biotin) in the molar ratio 10:1:0.1 with or without 5 mol% poly(ethylene glycol) dipalmitate (PEG-(C18)2). The ability of IgG2a immunoliposomes to elicit anti-IgG2a antibodies in mice was compared with alum and N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP). IgG2a 5 microgram/mouse) did not elicit an IgG1 antibody response after 4 s.c. injections. Alum-adsorbed IgG2a elicited 2.1 +/- microgram IgG1 antibody/ml serum, whereas MDP elicited 24.3 +/- microgram/ml serum. IgG2a immunoliposomes elicited 12.4 +/- 3.7 microgram IgG1 antibody/ml serum, while immunoliposomes containing lipophilic PEG-(C18)2 elicited 21.4 +/- 5.1 microgram IgG1 antibody/ml serum. Elicited antibodies were specific for IgG2a, with no cross-reactivity with IgG2b. Anti-DPPC or anti-DMPG IgG antibody levels did not change during immunization. Anti-DPPE IgG antibody levels were slightly but significantly elevated during immunization, and there was a significant increase in the level of anti-DPPE-biotin antibodies. These results demonstrate that immunoliposomes prepared with species-specific antibody are immunogenic and induce significant levels of isotypespecific antibody upon repeated injection.

The role of adjuvants in retroviral vaccines

The global HIV epidemic continues unchecked. Reports to the World Health Organization's Global Programme on AIDS indicate that more than 14 million persons have become infected with HIV and more than two million have died with AIDS. The spread of AIDS has generated a worldwide mandate for the development of safe and effective vaccines against HIV. Vaccines have been the most effective defense against other viral diseases such as polio and smallpox. However, the development of a vaccine against HIV-1 is a formidable task due to the variation of the virus, inadequate animal models of HIV disease, and the lack of correlates of protective immunity. Several candidate HIV vaccines are composed of synthetic, recombinant, or highly purified subunit antigens. Vaccines composed of subunit antigens generally are considered to be safer than traditional whole-killed or live-attenuated vaccines. However, purified subunit vaccines often are inherently less immunogenic than traditional vaccines. Immunologic adjuvants are agents that act generally to enhance specific immune responses to vaccine antigens. Formulation of experimental HIV vaccines with potent immunologic adjuvants is an attractive approach for amplifying and directing immune responses to highly purified antigens. Alum adjuvants, consisting of aluminum salts, first described in the 1920s, remain the only adjuvants in U.S.-licensed vaccine formulations. Novel adjuvants now undergoing preclinical and clinical testing with experimental subunit vaccine include detoxified lipid A, adjuvant emulsions, liposomes, biodegradable microspheres, muramyl peptides, and saponins. Adjuvants have been shown to elicit cytotoxic T-cell responses as well as antibody to subunit antigens.(ABSTRACT TRUNCATED AT 250 WORDS)

Liposomal adjuvanticity: effect of encapsulation and surface-linkage on antibody production and proliferative response

The immunoadjuvanticity of liposomal antigens, namely encapsulated and surface-linked conalbumin, was studied at different levels of an immune response including immunoglobulin production, blastogenic response and lymphokine production in sensitization conditions compatible with vaccine designs. The results demonstrated that both liposomal formulations stimulate all properties analyzed with respect to free antigen but significantly differ in some of their inductive capabilities, suggesting that they follow different routes in the immune network. Thus, although both liposomal antigens are capable of inducing potent humoral responses characterized by increased production of IgM and IgG2a, covalently linked antigen stimulates a quasi-polyclonal blastogenic response accompanied by the simultaneous secretion of IL-2 and IFN gamma, while encapsulated antigen which is less blastogenic mainly induces IL-2 secretion. We can conclude that: first, both antigenic formulations induce a Th1 type of activation and might therefore potentiate cell-mediated immunity, but surface-linkage favors a more rapid maturation of the response and a much more intense help induction. Second, although a strong adjuvanticity can be observed whatever the route of sensitization, namely intraperitoneally, intravenously or subcutaneously, the intravenous injections induce the better potentiation. The in vitro data were all compared to those obtained with naïve mice, allowing dissociation of the contribution of the in vitro rechallenge from that of the in vivo immunization. These differences observed between the liposomal antigens might be taken advantage of while formulating vaccines specifically suited to meet required needs and suggest that covalently linked antigen might be particularly useful in situations where induction of cell-mediated immunity is of prime importance.

Modulation of murine macrophage nitric oxide synthesis by liposomal phospholipids: correlation with liposome immune adjuvant activity

The influence of alum and liposomal phospholipids on interferon-gamma-(IFN-gamma), IFN-gamma/N-acetylmuramyl-L-alanyl-D-isoglutamine- (MDP) or IFN-gamma/tumor necrosis factor-alpha- (IFN-gamma/TNF-alpha) induced macrophage nitric oxide (NO) synthesis has been investigated. IFN-gamma induced NO synthesis in a dose-dependent manner. TNF-alpha and MDP did not induce NO synthesis, but interacted synergistically with sub-optimal doses of IFN-gamma. Alum strongly inhibited IFN-gamma-induced NO synthesis (ID50 25 microgram/ml). Liposomes composed of dipalmitoylphosphatidylcholine (DPPC) had no effect on IFN-gamma-induced NO synthesis. IFN-gamma-induced NO synthesis was stimulated by DPPC/dimyristoylphosphatidylglycerol (DMPG) liposomes (9:1 mol ratio, ED50 45 nmol phospholipid/ml), and inhibited by DPPC/dipalmitoylphosphatidylethanolamine (DPPE) liposomes (9:1 mol ratio, ID50 > 500 nmol phospholipid/ml), and DPPC/phosphatidylserine (PS) liposomes (7:3 mol ratio, ID50 150 nmol phospholipid/ml). Alum, DPPC/PE and DPPC/PS liposomes also inhibited IFN-gamma/MDP- and IFN-gamma/TNF-alpha-induced NO synthesis. Neither alum or the liposome preparations had significant toxicity towards macrophages in vitro at concentrations that induced maximal inhibition or stimulation of IFN-gamma-induced NO synthesis. Immunization of mice with alum-adsorbed and liposome-incorporated bovine serum albumin (BSA) demonstrated that enhancement or reduction of both IgG antibody and the proportion of IgG2a/IgG2b was correlated with stimulation or inhibition of IFN-gamma-induced NO synthesis.

Liposomal presentation of antigens for human vaccines

Liposomes are considered prime candidates to improve the immunogenicity of both antigens with hydrophobic anchor sequences and soluble, nonmembrane proteins or synthetic peptides. During the 20 years since liposomes were first demonstrated to have adjuvant potential, studies have shown that variation in liposomal size, lipid composition, surface charge, membrane fluidity, lipid-protein composition, anchor molecules, and fusogenicity can significantly influence results. In addition, antigen location (e.g., whether it is adsorbed or covalently coupled to the liposome surface or encapsulated in liposomal aqueous compartments) may also be important. Analysis of these variables as well as a comparison of the various techniques used to ensure the efficacy, stability, homogeneity, and safety of liposomal vaccine have been discussed.

Veterinary vaccines

Vaccination of animals for the prevention of infectious diseases has been practised for a number of years with little change in product composition. Recent advances in molecular biology, pathogenesis and immunology have laid the groundwork for the development of a new generation of veterinary vaccines based on pure subunits as well as live vectored bacteria and viruses. Along with novel methods of antigen preparation, the use of new adjuvants and delivery systems will permit targeting of the appropriate immune response as well as offering flexibility in terms of vaccination protocols. These new technologies are also being applied to the development of vaccines to enhance animal productivity and to control reproduction.

The immunological adjuvant and vaccine carrier properties of liposomes

Animal immunization studies by numerous laboratories have shown that liposomes promote humoural and cell-mediated immunity to a wide spectrum of bacterial, protozoan and viral antigens as well as tumour cell antigens, venoms and allergens. Adjuvanticity depends on liposomal structural characteristics which determine vesicle fate in vivo and, thus, the mode of antigen interaction with antigen-presenting cells. Adjuvanticity is further promoted by receptor mediated targeting of liposomes to macrophages, or the presence of other adjuvants including cytokines. The immunoadjuvant function of liposomes is supplemented by their ability to act as a carrier for co-entrapped B and T-cell epitopes, thus eliminating the need for a carrier protein. Recently, a technique has been developed for the entrapment of live or attenuated microbial vaccines into giant liposomes under conditions which retain their viability. Liposomes containing microbial vaccines (together with other soluble antigens or cytokines) could be used as carriers of vaccines in cases where there is a need to prevent interaction of vaccines with maternal antibodies or preformed antibodies to vaccine impurities.

Comparison of conventional adjuvants and 'adjuvant-free' monoclonal antibody targeting for stimulating antibody responses against a conjugate of luteinizing hormone releasing hormone and avidin

A conjugate of luteinizing hormone releasing hormone (LHRH) and avidin was used for immunizing mice and sheep. In mice, a range of adjuvants co-administered with the conjugate was shown to affect antibody titre and isotype. The commercially available Montanide and Ribi adjuvant emulsions, polyadenylic-polyuridylic acid, Quil A and Freund's complete adjuvant all produced high antibody titres to LHRH (reciprocal titre range 8000-20,000) and avidin (range 9000-35,000). Alhydrogel, Pluronic gel, Immunostim, DEAE-Dextran and muramyl dipeptide were weak to moderately effective (reciprocal titre ranges of 1300-5000 for LHRH and 3000-6000 for avidin, respectively). With Quil A, the response to avidin in mice was found to consist of roughly equal proportions of the IgG1, IgG2a and IgG2b isotypes. In contrast, the response to avidin using the other adjuvants and to LHRH consisted predominantly of the IgG1 isotype. An 'adjuvant-free' immunization strategy was attempted by targeting the LHRH-avidin conjugate to class II major histocompatibility complex (MHC) determinants on cells of the immune system. Using an immunoconjugate formed by linking LHRH-avidin to an antibody specific for mouse and sheep class II molecules, a twofold augmentation of titres relative to controls was obtained in mice. These antibodies were almost exclusively of the IgG1 isotype. In contrast, monoclonal antibody targeting in sheep resulted in a significantly enhanced immune response (reciprocal titres of 15,000 and 20,000 for LHRH and avidin, respectively) which was comparable to that achieved by immunizing with Quil A as adjuvant. Both the monoclonal antibody targeting and Quil A treatments tended to favour the production of antibody isotype IgG1 over IgG2.(ABSTRACT TRUNCATED AT 250 WORDS)

New carriers and adjuvants in the development of vaccines

Different approaches are being followed to enhance the immunogenicity of established and newly developed vaccines. Some of them foresee the utilization of carrier molecules recognized by the immune systems of a large proportion of the human population. Others are directed mainly towards the development of improved adjuvants and delivery systems.

Impaired stimulation of anti-bovine serum albumin IgG antibodies by vaccine adjuvants in murine acquired immunodeficiency syndrome

The effect of three adjuvants - alum, N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP), and liposomes - on the IgG antibody isotype response to bovine serum albumin (BSA), was determined in normal and LP-BM5 retrovirus infected C57BL/6 mice. Alum and MDP induced comparable levels of IgG antibodies in normal mice (predominantly IgG1 (greater than 90%)), whereas liposomes induced IgG1 (60%), IgG2a/b (30%) and IgG3 (10%) antibodies. IgG antibody levels using liposomes as adjuvant were five-fold higher than those observed with alum or MDP. Immunization after LP-BM5 infection significantly reduced the effectiveness of alum and MDP, IgG antibody levels being reduced by 80 and 90% at 3 or 7 weeks respectively. The adjuvant activity of liposomes was reduced by 55 and 65% when immunization was started 3 or 7 weeks post LP-BM5 infection. Boosting of pre-immune mice with BSA and alum, MDP or liposomes 3 weeks after LP-BM5 infection showed that, while the magnitude of the antibody response and isotype distribution was not affected, the persistence of the response was severely diminished compared to control, non-infected mice. The reduced immunoadjuvant activity correlated with a reduction in the frequency of splenic Thy1.2+/CD4+ T cells. These results demonstrated that liposomes were more effective than alum or MDP in inducing IgG antibodies, and that immunoadjuvant activity for prophylactic or therapeutic immunization for all 3 adjuvants was significantly impaired by retroviral infections.

Synthetic immunoadjuvants: application to non-specific host stimulation and potentiation of vaccine immunogenicity

It is well recognized that immunoadjuvants mainly play two roles; non-specific stimulation of host resistance against infections and cancer, and the potentiation of vaccine immunogenicity. This article reviews the recent results of the development of synthetic immunoadjuvants in our laboratory with special reference to muramyldipeptide (MDP), trehalose dimycolate (TDM), lipid A, chitin and their related compounds. The usefulness of MDP derivative MDP-Lys(L18), which has recently gone on the market as a haematopoietic agent for restoration of leukopenia in cancer patients treated with radiotherapy and chemotherapy, is reviewed. The various approaches to application of synthetic immunoadjuvants to the potentiation of vaccine immunogenicity, including adjuvant formulation, are also discussed.