Characterization of serum proteins from Biomphalaria glabrata (Gastropoda) vector of the human blood fluke Schistosoma mansoni (Trematoda

Humoral factors play an important role in mediating the susceptibility of the gastropod mollusc, Biomphalaria glabrata, to the human blood fluke, Schistosoma mansoni. This study characterized and compared serum proteins from schistosome-resistant (10-R2) and susceptible (PR albino, M-line) strains of B. glabrata. Comparison of serum (by SDS-PAGE) revealed that both strains had similar polypeptide profiles with Hb (aMW = 160 kDa) and Hb subunits being dominant features. Protein assays showed that serum from M-line snails had more total protein than 10-R2 snails, however, 10-R2 snails had more low molecular weight proteins in a Hb-depleted fraction than M-line snails. Antibodies against whole 10-R2 or M-line serum reacted similarly (in immunoblot assays) when incubated with whole or Hb-depleted sera, although differences in staining intensities were observed. Antibodies against Hb reacted with many proteins in whole sera and Hb-depleted fractions from both strains, confirming that a majority of serum proteins were Hb-associated. Polypeptides in whole or Hb-depleted serum were modified by a reducing agent, suggesting a role for disulfide bonds in maintaining protein structure and function. Analysis of five polypeptides from serum using peptide mapping and HPLC revealed that all five had structural similarities to Hb. Infection with S. mansoni produced no detectable changes in the serum polypeptides of the 10-R2 snails when examined by SDS-PAGE and HPLC. The M-line snails showed significant changes in total protein concentration during infection, with an increase from 0 to 24 h and followed by a decrease to control levels by 336 h. SDS-PAGE profiles revealed time-related differences in staining intensities at 12 h and 336 h. Determination of which serum proteins bind to the surface of live S. mansoni sporocysts showed a differential uptake of labeled proteins by the parasite. A 66 kDa peptide bound to sporocysts incubated in 10-R2 serum but not to those incubated in M-line serum. These data show that serum proteins from these strains are very similar but that differences, quantitative as well as qualitative exist. Further investigation may reveal their significance in mediating the humoral response against S. mansoni and whether a single factor or a combination of many factors determine resistance or susceptibility.