| Inbred strains of the snail host, Biomphalaria glabrata , have been used as models for studying immune responses to Schistosoma mansoni, causative agent of human intestinal schistosomiasis. Snail host resistance to parasites depends on the ability of circulating phagocytic cells, hemocytes, to recognize and kill larval schistosomes by a multicellular response referred to as encapsulation. We hypothesized that integrin-like receptors (Arg-Gly-Asp (RGD)-binding) are present on the surface of snail hemocytes and that they are involved in immune defense responses in snails.;B. glabrata snail hemocytes from susceptible (S) or resistant (R) snail strains were isolated and the effects of an integrin-specific inhibitor peptide, Arg-Gly-Asp-Ser (RGDS), and control peptide, Arg-Gly-Glu-Ser (RGES), on hemocyte spreading responses were tested. Results showed that S and R hemocyte spreading was specifically inhibited by RGDS, suggesting that integrins are present on the surface of snail hemocytes and that RGD-binding receptors are involved in cell spreading. RGDS also inhibited spreading of B. glabrata embryonic (Bge) cells, indicating the presence of integrins on Bge cells. A b integrin subunit cDNA ( b Bge) was cloned by homology PCR and 5' RACE techniques. Sequence comparisons of b Bge identified it as an integrin homologue. Antibodies were generated to functional and conserved regions of the recombinant b Bge sequence. In western blot analysis, only one of four affinity-purified antibodies recognized a polypeptide having the predicted molecular mass of b Bge (88 kDa). Flow cytometric data showed that b Bge epitopes were expressed at the membrane surface of Bge cells, indicating that b Bge transcripts were translated and its proteins were transported to the plasma membrane.;Cross-reactivity of anti- b Bge epitopes was not detected on the surface or in extracts of B. glabrata hemocytes. However, using primers designed to the b Bge subunit in combination with first chain hemocyte cDNA, a partial snail hemocyte integrin sequence was amplified by PCR, and it shared 97% nucleic and 96% amino acid identity to b Bge. To date, it remains unknown if the proteins encoded by the molluscan integrin DNA sequences found are involved in cell-substrate or cell-cell adhesion events. |
