Decreasing immunogenicity of developing schistosome larvae.

Cercariae, skin stage schistosomula, and lung stage schistosomula of Schistosoma mansoni were attenuated by gamma irradiation and tested for their ability to induce protective immunity against cercarial challenge in C57Bl/6J mice. The highest levels of resistance were induced by cercariae administered either percutaneously or intramuscularly. Skin stage schistosomula inoculated intramuscularly gave less protection while lung stage schistosomula from syngeneic donors were the least immunogenic. A similar ranking in immunogenicity was observed when the anti-skin stage schistosomular antibody responses induced by the different parasite stages were compared. In contrast, none of the immunization protocols were found to stimulate antibodies capable of recognizing lung stage schistosomula. These results suggest that, as schistosome larvae mature from the cercarial to the lung stage, they undergo a substantial loss in immunogenicity. This change may help explain the failure of older larvae to be immunologically destroyed in infected hosts.