Preparation and Preclinical Evaluation of a Novel Liposomal Complete-Core Lipopolysaccharide Vaccine

Abstract

ABSTRACT Our objective is to develop a prophylactic vaccine strategy that can be evaluated for surgical and other high-risk hospitalized patients. In this paper, we describe the preparation and preclinical evaluation of a liposomal complete-core lipopolysaccharide (LPS) vaccine that is nontoxic and broadly antigenic. Complete-core (Ra-chemotype) LPSs were isolated from four gram-negative bacterial strains ( Escherichia coli K-12, E. coli R1, Pseudomonas aeruginosa PAC608, and Bacteroides fragilis ), mixed together to form a cocktail of complete-core LPSs, and then incorporated into multilamellar liposomes consisting of dimyristoyl phosphatidyl choline, dimyristoyl phosphatidylglycerol, and cholesterol in a 4:1:4 molar ratio. The endotoxic activities of these LPS-containing liposomes were less than 0.1% of the endotoxicities of the original free LPSs as measured by the Limulus amoebocyte lysate assay. In vivo administration of liposomal complete-core LPS mixed with Al(OH) 3 to rabbits resulted in no pyrogenicity or overt toxicity over a 7-day period. In immunoblots, sera from rabbits following active immunization elicited cross-reactive antibodies to a large panel of rough and smooth LPSs from numerous clinically relevant gram-negative bacteria, including E. coli (serotypes O1, O4, O6, O8, O12, O15, O18, O75, O86, O157, and O111), P. aeruginosa (Fisher-Devlin serotypes 1, 2, and 3, which correspond to International Antigenic Typing Scheme types 6, 11, and 2, respectively), Klebsiella pneumoniae (serotypes O1, O2ab, and O3), B. fragilis , and Bacteroides vulgatus . Active immunization of mice with liposomal complete-core LPS provided protection against a lethal challenge with E. coli O18 LPS. The vaccine tested was nontoxic, nonpyrogenic, and immunogenic against a wide variety of pathogens found in clinical settings.