The cellular and humoral immune response to Schistosoma mansoni infections in inbred rats : III. Development of optimal protective immunity following natural infections and artificial immunizations

The effects of a variety of exposure regimens were assessed for their ability to induce optimal, immunologically mediated resistance against subsequent homologous reinfection by Schistosoma mansoni. Following exposure, Fischer rats modulated their subsequent worm burden, through as yet undefined internal mechanisms, and in a manner which resulted ultimately in a relatively consistent worm burden. The magnitude and rapidity of development and the longevity of maintenance of resistance was related to the intensity and pattern of exposure to normal cercariae. The modulatory effects tended to favor the development of optimal resistance over a wide variety of exposure patterns. Optimal resistance was most efficiently stimulated and maintained by immunogens which were associated with early stages of infection. Antihelminthic drug therapy adversely affected both the development and maintenance of resistance. Rats were also exposed to irradiated cercariae and soluble surface membrane preparations obtained from cercariae, adult worms, or eggs. Animals exposed to irradiated cercariae or soluble cercarial immunogen developed resistance to approximately 80 and 35%, respectively, of the levels shown by the naturally infected animals. The mechanism of this variable resistance was investigated using a number of in vivo and in vitro assessments and is the subject of a subsequent report.