Schistosoma mansoni: evidence that eosinophils and/or macrophages contribute to skin-phase challenge attrition in vaccinated CBA/Ca mice.

Naive CBA mice and mice vaccinated 4 weeks previously with gamma-irradiated cercariae were challenged percutaneously with normal cercariae and the skin site of challenge removed at various times to provide samples for histological examination. Neutrophil abscesses surrounding schistosomula as well as cast cercarial tails were identified at the skin surface in both naive/challenged and vaccinated/challenged mice. The dermis in both groups of animals became infiltrated with mononuclear cells and granulocytes. Neutrophils were the predominant granulocyte in naive/challenged skin but eosinophils replaced neutrophils by 48 h post-challenge in skin from vaccinated mice. Schistosomula with adherent mononuclear cells and granulocytes were identified preferentially in the dermis of vaccinated mice; such larvae frequently exhibited subtegumental vacuolation. A further major characteristic of vaccinated/challenged skin was the formation, from 18 h, of extensive subdermal inflammatory foci, consisting of up to 50% mononuclear cells and 50% eosinophils. These foci invariably surrounded one or more larval parasites which were seen by both optical and scanning electron microscopy to have a morphology typical of lung stage rather than skin stage schistosomula. Dead, infiltrated parasites were also recognized within the subdermal foci. Our data support the view that vaccine immunity in this mouse model is mediated primarily in the skin and indicate that challenge attrition involves immobilization and killing of larval parasites by effector cells within subdermal inflammatory foci.