Killing of Schistosoma mansoni sporocysts in Biomphalaria glabrata implanted with amoebocyte-producing organ allografts from resistant snails.

Schistosome-susceptible National Institutes of Health (NIH) albino Biomphalaria glabrata were implanted with the amoebocyte-producing organ (APO) from 4 types of donors: (1) exposed-resistant (eR), i.e., schistosome-resistant 13-16-R1 snails that had been exposed to miracidia of Schistosoma mansoni 30 or more days previously in order to verify their resistance, (2) exposed-susceptible (eS), i.e., NIH albino snails that had been similarly exposed to miracidia, (3) unexposed-resistant (uR), and (4) unexposed-susceptible (uS). Allograft recipients, along with unimplanted NIH albino and 13-16-R1 controls (cS and cR, respectively), were then challenged with 100 miracidia each of Schistosoma mansoni at 14-15 days postimplantation. Histological sections of tentacles fixed at 3 days postchallenge (PC) showed significantly fewer normal sporocysts and more numerous developmentally retarded sporocysts in cR snails than in the other 5 treatment groups, and significantly more killed sporocysts in both cR snails and recipients of eR APOs than in the other 4 groups. In addition, the histological condition of eR allografts in both unchallenged (at 1, 3, 7, 10, and 14 days postimplantation) and schistosome-challenged (at 3 days PC) NIH albino recipients was examined. Viable hematopoietic cells were found in 96% of implants, and in 86% of implants low numbers of mitotic figures were found among these cells, although no increased mitotic activity occurred in challenged recipients. These data suggest that lowered susceptibility to infection with S. mansoni in recipients of APO allografts results primarily from hemocyte-mediated resistance.