Immune responses of mice to inactivated rabies vaccine administered orally: potentiation by Quillaja saponin.

Administered orally, Quillaja saponin markedly potentiated the humoral immune responses of mice fed inactivated rabies vaccine, and significantly increased their resistance to subsequent intracerebral challenge with live rabies virus. Although mean serum neutralizing antibody titres were generally 8- to 16-fold higher when vaccine was given intraperitoneally, orally administered vaccine, with saponin, stimulated production of high protective antibody levels that were maintained for at least 6 months. The potentiating effect of saponin appeared to be mediated through several mechanisms, one of which was by increasing the permeability of the intestinal mucosa, allowing increased uptake of viral antigen. The potentiating effect was enhanced when saponin was administered in advance (up to at least 16 h) of the oral vaccine. Mice tolerated the effective saponin doses without any visible signs of distress or injury. In view of the many favourable physiological activities and low toxicity of orally delivered saponins, it is suggested that they may find more general applications in the immunopotentiation of oral vaccines.