Serum from CBA/Ca mice vaccinated with irradiated cercariae of Schistosoma mansoni protects naive recipients through the recruitment of cutaneous effector cells.

Passive transfer experiments showed that 76% of the resistance induced in CBA/Ca mice by exposure to radiation-attenuated cercariae of Schistosoma mansoni could be transferred to naive recipients by administration of donor serum. The level of protection achieved depended on the volume of serum administered and immunity was demonstrated most consistently with serum harvested from thrice vaccinated donors. The serum was twice as effective when given to the recipients at the time of cercarial challenge as compared to administration 5 days after challenge, a result which indicates that serum-dependent challenge elimination is probably accomplished in the cutaneous tissues. This view was confirmed by the observation that passive protection could be ablated by administration of a monoclonal antibody which we have shown elsewhere to deplete the cutaneous inflammatory reaction to cercarial penetration. Histopathological studies revealed that vaccine serum induced subdermal inflammatory reactions in challenged recipient mice which were identical both in induration and kinetics to those seen in conventionally vaccinated individuals; on days 4 and 5 post-challenge, the reactions comprised 60% mononuclear cells and 40% eosinophils. Challenge larvae, which had transformed from skin-stage to lung-stage parasites, became trapped within such reactions and eventually showed generalized vacuolation consistent with the onset of damage. Some foci were seen to contain degenerated leucocytes, free eosinophil granules and debris which is thought to represent the remnants of dead parasites. Small focal reactions were identified on occasion in naive challenged mice and in recipients of normal mouse serum, but these reactions comprised predominantly mononuclear cells and were rarely seen to encompass challenge parasites. These data show that serum-transferred resistance in the vaccinated CBA/Ca mouse model involves the induction of a cutaneous inflammatory response in the recipients.