Ultrastructural changes of adult Schistosoma mansoni worms recovered from C57BL/6 mice passively immunized with normal and vaccinated rabbit sera in vivo

The purpose of the study is to explore the role of humoral immunity against Schistosoma mansoni infection in C57BL/6 mice using highly resistant rabbits that had been exposed to three separate immunizations with ultraviolet (UV)-irradiated cercariae (8,000 attenuated cercariae/ rabbit in each immunization), and their sera were tested for their ability to transfer protection against S. mansoni challenge. The present study showed the reduction in challenge worm burden had reached 32.76-43.64% when compared with recipients of normal serum or no serum. The surface topography of the worms collected from immunized mice with either normal rabbit sera (NRS) or vaccinated rabbit sera (VRS) revealed severe tegumental alterations, especially in the VRS group. Worms collected from groups that were immunized by NRS or VRS postinfection (200 normal cercariae/mouse) by day 42. Worms from group immunized with NRS showed damage in the tegument, characterized by severe swelling or erosion of tegumental folds, accompanied by changes in tubercles, swelling, shortening, and loss of spines in male worms. The alteration in female tegument was characterized by swelling of tegumental folds, atrophy of ventral sucker, damage of sensory papillae along all the body, severe peeling in some regions, and appearance of few small blebs. VRS induced more severe tegumental damage than NRS in both male and female worms. Severe shrunken vesicles were protruded from the surface between the two suckers. The tegument of the male showed a collapse of tubercles followed by the appearance of vesicles on their surfaces, fusion, erosion, and superficial focal peeling of tegumental folds. In the female worms, severe damage to the oral sucker, the surface between the two suckers, extensive peeling, severe swelling of the tegument, and damage of sensory papillae. In conclusion, the present study support the hypothesis that high levels of antibodies were developed in rabbit sera after multiple exposures to attenuated cercariae of S. mansoni. Furthermore, immunization might have transferred protection against the infection, indicated by severe morphological alterations, a sign of elimination of the worms. Further investigation is being carried out to reveal the molecular mechanisms underlying the transfer of protection.