Effect of the nonreducing end of Shigella dysenteriae type 1 O-specific oligosaccharides on their immunogenicity as conjugates in mice

Endemic and epidemic shigellosis, an acute invasive disease of the lower intestines, afflicts millions of people worldwide with an estimated one million fatalities per annum at a low infectious dose. Our approach to vaccine development against Shigella is based on the hypothesis that serum IgG antibodies to the O-specific polysaccharide (O-SP) domains of the LPS of these organisms confer protection to infection. The synthetic oligosaccharides corresponding to the tetrasaccharide repeating unit of the O-SP of Shigella dysenteriae type 1 covalently linked to human serum albumin elicited O-SP-specific IgG in mice. The antibody levels were a function of both the saccharide chain length and their loading on the protein. These synthetic saccharide conjugates elicited significantly higher levels of IgG anti O-SP than conjugates prepared with the O-SP from the bacteria. Here, we evaluated the influence of the nonreducing terminal monosaccharide on the serum antibody response. To this end, we prepared synthetic oligosaccharides comprising hexa- to tridecasaccharide fragments of the native O-SP, having one of the four monosaccharide residues that constitute the repeating unit at their termini and bound them to BSA by a single-point attachment. The conjugates contained an average of 19 saccharide chains per BSA. The synthetic oligosaccharides inhibited the binding of serum raised against whole bacteria to its LPS to a similar extent but lower than the native O-SP. The highest anti-LPS levels were elicited by conjugates having N-acetylglucosamine (10-mer) or galactose residues (7- and 11-mers) at their nonreducing termini.