The aim of this work was to develop a liposomal formulation which could act as a carrier for allergens during oral desensitization therapy. A model protein, ovalbumin, was associated with negatively charged, multilamellar vesicles of various compositions and their stability in the presence of synthetic intestinal media (bile salt, pancreatic enzymes and their combination) was investigated. Liposomes containing soya phosphatidylcholine as the main lipid, regardless of their cholesterol content (20-40%), were unable to protect ovalbumin against the combined action of pancreatic enzymes and bile salt. In contrast, liposomes prepared from distearoylphosphatidylcholine and cholesterol (6:3.5 molar ratio) were more stable: about 50% of the lipid remained as liposomes after a 4-h incubation at 37 degrees C and intact ovalbumin could be demonstrated therein by immunoblotting. The immunomodulating properties of liposomes were tested by following changes in serum IgE levels (by passive cutaneous anaphylaxis) in Balb/C mice sensitized to ovalbumin, after feeding various preparations. In this model, free ovalbumin was able to provoke a premature fall in IgE levels, and liposomes, whatever their composition, contributed no further effect.
Desensitization therapy for type I allergy is now current practice. Liposomes have been proposed as a support for allergens to improve safety and effectiveness. The aim of this work was to optimize liposomal formulations of three different standardized allergen extracts and to test their allergenicity in vitro and in a preclinical trial. Allergen extracts (Dactylis glomerata, Dermatophagoides pteronyssinus, and cat hair and dander) were associated with multilamellar liposomes of varying compositions at different pH. Liposome-bound allergens were quantified by RAST inhibition after ultracentrifugation, and analyzed qualitatively by SDS-PAGE followed by immunoblotting. Their allergenicity was assessed by basophil degranulation in vitro, as compared with the allergenicity of an aqueous extract, and by skin tests in allergic subjects. The best association, about 50% of the added allergen, was obtained with negatively charged liposomes, when the pH of the allergen solution was adjusted so as to impart a net positive charge to the proteins. One-third of the liposome-associated allergens was located on the surface of the liposomes and was free to interact with antibody, as shown by RAST inhibition assays and the basophil degranulation test; the remaining two-thirds was encapsulated within the liposomes. All the major immunoreactive proteins in the extract were included. These liposomes could be readily freeze-dried and reconstituted without changing their properties. This study reveals the allergenic characteristics of liposomes and suggests their potential use in the treatment of allergic patients.
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