Recent developments in synthetic oligosaccharide-based bacterial vaccines.

Synthetic advances made possible chemical assembly of complex oligosaccharide fragments of polysaccharide domains on the surface of human pathogenic bacteria. These oligosaccharides may be recognized by antibodies raised against high molecular weight, native, polysaccharides. In addition to their antigenicity, synthetic oligosaccharides can also function as haptens in their protein conjugates that can elicit not only oligo- but also polysaccharide-specific IgG antibodies in animal models and in humans. A major milestone in the development of new generation vaccines was the demonstration that protein conjugates of synthetic fragments of the capsular polysaccharide of Haemophilus influenzae type b are as efficacious in preventing childhood meningitis and other diseases as is the corresponding licensed commercial vaccine containing the bacterial polysaccharide. The lessons learnt in this and other endeavors described herein are manifold. For example, they teach us about the significance of the oligosaccharide epitope size, the number of their copies per protein in the conjugate, the possible effect of the spacer on anti-saccharide immune response, and the proper choice of the carrier protein combined with the selection of the animal model. The H. influenzae b story also teaches us that that the synthetic approach can be commercially viable.