In the absence of IL-12, the induction of Th1-mediated protective immunity by the attenuated schistosome vaccine is impaired, revealing an alternative pathway with Th2-type characteristics.

Vaccination of mice with irradiated Schistosoma mansoni larvae confers high levels of immunity which is mediated by Th1-type lymphocytes. To investigate a possible role for IL-12 in the induction of protection, we have compared the immune response of IL-12 p40-deficient (KO) mice and their C57BL/6 (WT) counterparts following vaccination. Cultured lymph node cells from KO mice had markedly altered cytokine profiles with significantly decreased production of IFN-gamma increased IL-4. Correspondingly, KO mice had enhanced levels of IgE. After challenge, cells recovered from the lungs of KO mice secreted abundant IL-4 and IL-5 but little IFN-gamma, while flow cytometric and histological analysis of lung cell populations recorded a very high proportion of eosinophils. The levels of protection in KO mice were substantially lower than in their WT counterparts, demonstrating the importance of IL-12 and Th1-mediated immune responses. This conclusion is reinforced by the administration of rIL-12 to KO mice immediately after vaccination which led to increased IFN-gamma and the restoration of protective immunity. Nevertheless, the data also indicated that the limited levels of protection induced in KO mice occur via an IL-12-independent pathway, possibly mediated by Th2 cells.