Synthesis and characterization of a polyvalent Escherichia coli O-polysaccharide-toxin A conjugate vaccine

Abstract

A 12-valent Escherichia coli O-polysaccharide (O-PS)-toxin A conjugate vaccine was formulated. Nonpyrogenic, low-molecular-weight O-PS was derived from lipopolysaccharides (LPS) of the following serotypes: O1, O2, O4, O6, O7, O8, O12, O15, O16, O18, O25, and O75. Individual O-PS were covalently coupled to Pseudomonas aeruginosa toxin A using adipic acid dihydrazide as a spacer molecule and carbodiimide as a coupling agent. On a weight basis, the final multivalent vaccine was composed of 43% O-PS and 57% toxin A. The vaccine was nontoxic and nonpyrogenic in standard animal tests. Immunization of rabbits engendered a marked rise (6–74-fold) in anti-LPS immunoglobulin G (IgG) antibody titers. When passively transferred to mice, immune rabbit IgG conferred statistically significant ( p < 0.05) protection against a challenge with 9 of the 12 vaccine serotypes. For two serotypes, although the mortality rate declined by ≥ 50% in the passively immunized versus the control group, the difference did not reach statistical significance. The degree of protection provided by passively transferred IgG was influenced by both the anti-LPS antibody levels in the IgG preparation and the virulence of the challenge strain. Active immunization of mice with either conjugate vaccine or killed E. coli whole cells did not confer protection. This was most probably due to the fact that these antigens induced a meagre anti-LPS IgG antibody response.