Schistosoma mansoni: Excretory-Secretory Polypeptides Exhibit Selective Binding to Plasma Components of the Snail Biomphalaria glabrata

Abstract

Previous studies have shown that Schistosoma mansoni excretory-secretory polypeptides (ESP) inhibit various internal defense functions of hemocytes from Biomphalaria glabrata and that plasma also may exert a modulatory effect on hemocyte activity, To better understand how plasma may influence hemocyte-schistosome interactions in inbred strains of snails, biotinylated ESP (b-ESP) were used as probes to identify ESP-reactive plasma components and partially characterize the nature of their binding interactions. In a plasma binding assay, b-ESP bound in a dose-dependent fashion to immobilized snail plasma, although no quantitative differences in ESP reactivity to plasma of schistosome-susceptible (M-line) and resistant (13-16-R1) snail strains were detected. Moreover, co-incubation of b-ESP with homologous plasma or pretreatment of plasma with nonbiotinylated ESP in the plate assay significantly reduced b-ESP binding to immobilized plasma, indicating a specific interaction between ESP and plasma. Pretreatment of b-ESP with mannose, porcine stomach mucin, fetuin, and asialofetuin resulted in a significant inhibition of b-ESP binding to plasma, whereas pretreatment of immobilized plasma with a cocktail of monosaccharides (including mannose), porcine stomach mucin, and fetuin had no inhibitory effect. These data suggest the presence of a carbohydrate-binding protein(s) in sporocyst ESP that is targeted to plasma glycoconjugates. Based on the carbohydrate content of the major inhibitory glycoproteins, galactosyl and n-acetyl-galactosaminyl sugars may represent putative determinants for the lectin-like ESP molecule(s). However, ESP binding to plasma from M-line and 13-16-R1 B. glabrata strains exhibited similar sugar and glycoprotein inhibition patterns. SDS-PAGE and electroblot analyses further demonstrated that ESP bound to a subset of separated plasma polypeptides, most prominently to a doublet of 52 and 54 kDa, and a complex of proteins with molecular masses greater than 150 kDa. Other polypeptides exhibiting weaker binding interactions included components of 34, 60, 64, 86, and 125 kDa. Although both M line and 13-16-R1 snail strains contained a similar complement of ESP-binding plasma proteins, ESP binding to the 34- and 86-kDa proteins occurred at higher frequencies in susceptible M-line plasma. It is concluded that ESP, released during in vitro transformation of S. mansoni miracidia, contain carbohydrate-reactive proteins capable of selectively binding to components of snail plasma. Quantitative differences between snail strains in the occurrence of ESP-binding plasma molecules was documented.