Larval Schistosoma mansoni excretory-secretory glycoproteins (ESPs) bind to hemocytes of Biomphalaria glabrata (Gastropoda) via surface carbohydrate binding receptors

Flow cytometric analysis of circulating blood cells (hemocytes) of Biomphal aria glabrata, molluscan intermediate host of Schistosoma mansoni, revealed the presence of 2 overlapping hemocyte subpopulations, designated R1 and R 2. R1 hemocytes are characterized by their smaller size. reduced granularit y, and the presence of the BGH(1) surface epitope, whereas R2 cells are lar ger. more granulated, and generally lack the BGH, cell marker. Both hemocyt e subpopulations bound fluorescent dye (Oregon Green)-conjugated excretory- secretory glycoproteins (fESPs), although the specific fESP binding signal (geometric mean value). after correction for cellular autofluorescence, was greater in the RI hemocyte subpopulation compared to that of the R2 subset . Partial inhibition of fESP binding to hemocytes consistently was achieved using various glycoconjugates (mucin, asialo-mucin, asialo-fetuin, heparin ) and polysaccharides (fucoidan, dextran sulfate 8000), suggesting the invo lvement of hemocyte carbohydrate-binding receptors (CBRs) in reactions with ESP-associated glycans. Although sulfation of carbohydrate ligands contrib uted significantly to ESP blocking activity of some inhibitory polysacchari des and heparin, other sulfated proteoglycans (chondroitins A and B. hepara n sulfate) were noninhibitory, indicating that charge alone was not solely responsible for the observed inhibition of hemocyte binding by fESPs. A sim ilar blocking effect by desialylated glycoproteins (asialo-mucin, asialo-fe tuin) further supports the contention that ESP-hemocyte interactions are me diated primarily through CBRs. The glycoconjugate inhibitors of ESP binding were only partially effective over a range of concentrations and their gly can moieties (oligosaccharides or long-chain polymers) comprised a diversit y of major sugar groups, suggesting that hemocyte CBRs and S. mansoni larva l ESPs likely represent a multiple receptor-ligand system. Previously repor ted findings of differential effects of ESPs on a variety of in vitro hemoc yte functions are consistent with such a hypothesis.