Comparison of irradiated-cercaria schistosome vaccine models that use 15- and 50-kilorad doses: the 15-kilorad dose gives greater protection, smaller liver sizes, and higher gamma interferon levels after challenge.

The protection and immune response to infection caused by the parasite Schistosoma mansoni were studied by comparison of two murine irradiated-vaccine models. Mice were exposed from 1 to 4 times to infective-stage cercariae attenuated with either a moderate dose (15 kilorads) or a high dose (50 kilorads) of radiation. Seven weeks after challenge infection, the mice were assessed for resistance, liver size, and lymphokine responses to parasite antigens. Both vaccine models showed high levels of protection, but the moderate-dose model proved superior in that mice in those groups achieved higher levels of resistance in fewer exposures. Additionally, the mice exposed three times and four times to moderately irradiated cercariae all had significantly lower liver weights independent of worm burden. Assessment of lymphokine production by the spleen cells at the time of perfusion showed that gamma interferon was the only lymphokine of those measured that was differentially produced in the two models and correlated with a decrease in size of in vitro granulomas. The findings suggest that a selected vaccine regimen may lead to greater resistance and decreased liver pathology, the latter of which appears to be mediated by induction of gamma interferon.