Comparison of the protective resistance induced by 60Co-irradiated cercariae and schistosomula of the WFFS and NMRI strains of Schistosoma mansoni.

Mice, CBA/HT6T6 and C57BL/10, were vaccinated with 1 X 350 or 1 X 500 Schistosoma mansoni cercariae or schistosomula attenuated with 20 or 56 krad 60Co irradiation and challenged with 200 cercariae. Protective resistance against homologous strain challenge was compared using the Winches Farm Field Station (WFFS) and Naval Medical Research Institute (NMRI) strains of S. mansoni. Maximal resistance to challenge was obtained in both strains of mice with cercariae or schistosomula of either WFFS or NMRI strain attenuated with 20 krad. Protection using organisms attenuated with 56 krad was significantly lower. Since previous studies with the two parasite strains have shown that the biological effects of irradiation are similar, the difference in the immunogenicity of the 56 krad-irradiated NMRI strain in this study from earlier studies must be due either to different local conditions for irradiation or to adaptation of the NMRI strain to a new laboratory environment. This finding may have important implications for vaccination studies and investigations of the mechanisms of immunity where radiation-attenuated parasites are used.