A cytoadherence assay was used to determine whether antibodies to plasma and hemocyte components of Schistosoma mansoni-susceptible (M-line) and -resistant (10-R2, 13-16-R1) strains of Biomphalaria glabrata affected attachment of hemocytes to chemically fixed schistosome sporocysts differentially. Experiments used purified, intact IgG and purified Fab fragments of each antibody. Indirect fluorescent antibody tests confirmed that the intact purified IgG to plasma and hemocytes from the 3 strains of snails bound to sporocysts. There was no qualitative difference in fluorescence among any of the antibodies to snail components, although the various antibodies affected adherence of hemocytes to the parasite differentially. Adherence assays revealed that antibodies to plasma components inhibited binding of hemocytes from the snail strain to which the antibody was generated. Additionally, antibody to plasma from resistant strains of B. glabrata inhibited binding of hemocytes from the homologous and heterologous resistant strains but not hemocytes from susceptible snails. Antibody to hemocytes from M-line snails did not inhibit binding of hemocytes from any of the snail strains. Since results of assays using intact IgG correlated well with assays using their Fab fragment counterparts, it was concluded that hemocytes from the 3 snail strains utilize specific antigen-binding sites on sporocysts. Results of these assays also indicate that, with regard to the mechanism of hemocyte binding to sporocysts, M-line and 13-16-R1 snails are more dissimilar than M-line and 10-R2 or 10-R2 and 13-16-R1 B. glabrata.
Previous work has indicated that injection of recombinant-human interleukin (rhIL)-1beta in Schistosoma mansoni-infected M-line Biomphalaria glabrata resulted in a significant reduction in the number of cercariae shed. The purpose of the present work was to determine if primary sporocysts were killed following rhIL-1beta injection in susceptible snails and, if so, to determine if killing was the direct result of hemocyte activity. Counting of primary sporocysts indicated a 50% reduction in the number surviving at 3 days PE in snails from 2 susceptible strains following injection. Histological analysis indicated that killing occurred with little-to-no observable hemocyte/parasite contact, whereas short-term culture of primary sporocysts with cell-free plasma (hemolymph) from injected snails rapidly initiated killing in vitro. Because levels of a snail IL-1-like molecule (SnaIL-1) drop significantly following schistosome exposure in M-line snails, because resistant snails maintain higher SnaIL-1 levels following infection, and because rhIL-1beta upregulates hemocyte cytotoxic mechanisms, these data support the contention that SnaIL-1 plays a role in determining resistance in B. glabrata. These data also indicate that schistosome death may be separated from parasite encapsulation by hemocytes and that an as yet unidentified humoral killing mechanism/factor may exist in B. glabrata. Lastly, these data further support the hypothesis that cytokine-like molecules are important, functionally conserved immunodefense mediators in both vertebrates and invertebrates.
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