The adjuvant effect of liposomes in eliciting anti-galactosyl antibodies

Protective effect of antiganglioside antibodies against experimental Trypanosoma brucei infection in mice

Liposome-associated ganglioside antigens (ganglioside GM1 or bovine brain gangliosides) were prepared to facilitate the potential protective efficacy for Trypanosoma brucei. Mice were immunized with liposome-associated ganglioside GM1 or bovine brain gangliosides intraperitoneally (i.p.). After immunization, significantly higher antigen-specific IgG and IgM antibodies were detected in sera than in the nonimmunized control group. When sera from immunized mice were analyzed for isotype distribution, antigen-specific IgG1, IgG2a, and IgG3 antibody responses were also noted. After immunization, mice were challenged i.p. with 1 x 10(2) cells of T. brucei. Sixty percentage of liposome-associated ganglioside GM1-immunized mice survived the infection, and all the mice immunized with bovine brain gangliosides-containing liposomes survived. However, all control mice died within 7 days after infection. These data demonstrate that liposomes containing ganglioside antigens have the potential usefulness for the induction of a protective immune response against T. brucei infection and suggest the possibility of developing vaccines that may ultimately be used for the prevention of trypanosomiasis.

Induction of partial protection against Leishmania donovani by promastigote antigens in negatively charged liposomes

Negatively charged liposomes, proposed as potential vaccine adjuvants, have been extensively studied in association with various antigens. In the present study, we investigated the adjuvanicity of negatively charged liposomes to enhance the protective immunity of membrane antigens of Leishmania donovani promastigotes (LAg). In comparison to the control mice immunized with phosphate-buffered saline and empty liposomes, immunization with free LAg led to significant levels of protection against infection with virulent promastigotes. Encapsulation of LAg in liposomes also induced effective protection. However, the level of protection by LAg-liposome was not significantly different from that induced by free LAg. Investigation of the immune responses showed, in contrast to free LAg, that immunization with LAg-liposome elicited strong antibody responses. IgG isotype analysis revealed the presence of all 4 isotypes. However, the titer of IgG1 was significantly higher than IgG2a, IgG2b, and IgG3. Following infection, stimulation of IgG and IgG isotypes did not differ in the different immunization groups. Delayed-type hypersensitivity (DTH) analysis after immunization showed significant induction by LAg and LAg-liposomes, in comparison to controls. With infection, again, the level of DTH in all the groups became almost comparable. Stimulation of insufficient cellular response, as reflected by DTH and potentiation of IgG1 over IgG2a, IgG2b, and IgG3 suggest a dominance of Th2 response with this liposome-antigen formulation, resulting in weak protection against visceral leishmaniasis.

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.

Adjuvants--a balance between toxicity and adjuvanticity

Adjuvants have been used to augment the immune response in experimental immunology as well as in practical vaccination for more than 60 years. The chemical nature of adjuvants, their mode of action and the profile of their side effects are highly variable. Some of the side effects can be ascribed to an unintentional stimulation of different mechanisms of the immune system whereas others may reflect general adverse pharmacological reactions. The most common adjuvants for human use today are still aluminium hydroxide, aluminium phosphate and calcium phosphate although oil emulsions, products from bacteria and their synthetic derivatives as well as liposomes have also been tested or used in humans. In recent years monophosphoryl lipid A, ISCOMs with Quil-A and Syntex adjuvant formulation (SAF) containing the threonyl derivative of muramyl dipeptide have been under consideration for use as adjuvants in humans. At present the choice of adjuvants for human vaccination reflects a compromise between a requirement for adjuvanticity and an acceptable low level of side effects.

Production of oligosaccharide-binding monoclonal antibodies of diverse specificities by immunization with purified tumor-associated glycolipids inserted into liposomes with lipid A

Two species of neutral glycosphingolipids purified from rat colon carcinoma tissue, isoglobotetraosylceramide [GalNAc(beta 1----3)Gal(alpha 1----3)Gal(beta 1----3)Glc(beta 1----1)Cer] and a related 6-sugar "analogue" were inserted into liposomes together with lipid A (from bacterial lipopolysaccharide) and used for immunization of mice and monoclonal antibody production. The yield of hybridomas producing glycolipid-specific antibody was 5-10% using a high-dose booster schedule with liposome-inserted glycolipid. In contrast the frequency was below 0.1% (no glycolipid-binding antibodies were found) when using the previously described method of immunizing with glycolipid coated on the surface of acid-treated S. minnesota. Monoclonal antibodies were screened on the purified glycolipids used for immunization and selected for differential reactivity to the two glycolipids. A diversity of specificities was demonstrated by binding to the purified antigens, in a thin-layer chromatogram binding assay and in binding tests to tumor and normal target cells.

Characterization of anti-N-acetyl-D-glucosamine antibodies elicited through haptenated liposomes

Antiserum was raised in rabbits against p-aminophenyl 2-acetamido-2-deoxy-beta-D-glucoside covalently coupled to phosphatidylaminoethanol-containing lecithin liposomes. The affinity-purified antibodies were found to be IgM. The properties of the antibodies were studied by ligand inhibition of quantitative precipitation, fluorescence titration with 2-acetamido-2-deoxy-D-glucose, and complement lysis of haptenated liposomes.

Binding of anti-galactosyl antibodies to galactosylated liposomes

The binding characteristics of rabbit anti-galactosyl antibodies elicited through galactosylated liposomes were studied. The association constants (13 X 10(5) M-1 and 21 X 10(5) M-1 of Fabmu and Fab fragments with the galactosylated liposomes were determined by measuring the binding of radioiodinated antibodies. The effect of free ligand (methyl-beta-D-galactopyranoside) and the ligand coupled to the liposomal surface (galactosylated liposomes) on the structure of antibodies was investigated using circular dichroism (CD) measurements. Changes were observed in both the secondary and tertiary structure of antibodies upon binding to galactosylated liposomes. In contrast, the free ligand induced alterations only in the near-UV region of the CD spectra, indicating a change in tertiary structure only.

Preparation of anti-polysaccharide antiserum using liposomes as immunological adjuvant

Bovine serum albumin (BSA) was coupled to dextran by controlled periodate oxidation followed by sodium borohydride reduction. The conjugate was entrapped in negatively charged, multilamellar phosphatidylcholine (lecithin) liposomes in which the polysaccharide remained surface-exposed, at least partially. Liposome-entrapped conjugate elicited in rabbits an appreciable anti-dextran response when compared with the sera raised in saline or in Freund's adjuvant. The anti-dextran antibodies belonged to both the IgM and IgG classes. The precipitin reaction of dextran with the antiserum raised in liposomes was determined.

Immunogenicity of galactosylated liposomes

The kinetics of the immune response to galactosylated liposomes in saline was studied in rabbits. The antibody level in the secondary immunization was found to be higher compared to that in the primary response. The antibodies were purified on an immunoadsorbent column of galactosylated bovine serum albumin. Anti-galactosyl antibodies belonged to both IgM and IgG classes, the former being the major fraction. Both failed to agglutinate normal or neuraminidase treated human or rabbit erythrocytes but they agglutinated trypsinized cells.