Developmentaland ComparativeImmunology, Vol. 13, pp. 9-16, 1989 0145-305X/89 $3.00 + .00 Printed in the USA. All rights reserved. Copyright © 1989 Pergamon Press plc PHAGOCYTOSIS OF YEAST BY BIOMPHALARIA GLABRATA: CARBOHYDRATE SPECIFICITY OF HEMOCYTE RECEPTORS AND A PLASMA OPSONIN Sarah E. Fryer, Charles J. Hull, and Christopher J. Bayne Department of Zoology, Oregon State University, Corvallis, OR 97331-2914 E]Abstractmln vitro phagocytosis of yeast cells by hemocytes of the mollusc Biomphalaria gla- brata can occur in the absence of plasma, but is enhanced by opsonization in plasma from cer- tain snail strains. We have investigated the carbohydrate specificity of the hemocyte- bound receptor for phagocytosis and the free plasma opsonin using two dominant carbohy- drate components of the yeast cell wall (lami- narin and mannan). Phagocytic uptake in the absence of plasma of both untreated and op- sonized yeast is strongly inhibited by lami- narin, but is unaffected by mannan. In contrast, laminarin does not interfere with opsonization whereas mannan blocks the process, blocking the binding of a 57 kD plasma component as detected by Western immunoblot. These re- sults suggest that the opsonic lectin in plasma is not simply a free form of the hemocyte- bound receptor for yeast phagocytosis. [~Keywords--Biomphalaria; Lectins; Phagocytosis; Opsonins; Hemocyte receptors; Invertebrate immunology. Introduction We have recently demonstrated (1) the presence of an opsonin for heat killed yeast (Saccharomyces cerevisiae) in the plasma of two strains of Biomphalaria glabrata which are resistant to infection with the PR-1 strain of Schistosoma mansoni (13-16-R1 and 10.R2). This op- sonin is absent in susceptible M-line snails, although hemocytes from all three strains were able to phagocytose unopsonized yeast at a low level. In ad- dition, opsonization in resistant strain plasma increased subsequent phagocy- tosis by all hemocytes. The basis of compatibility of B. gla- brata with S. mansoni is still poorly un- derstood. Although plasma from resis- tant snails alone is not toxic, host plasma factors have been demonstrated to influence the outcome of sporocyst encounters with hemocytes. In vitro the presence of plasma from resistant strains of B. glabrata facilitates cytotoxic de- struction of sporocysts by normally be- nign susceptible strain hemocytes (2,3). In vivo, passive transfer of resistance by injection of plasma from resistant to sus- ceptible individuals has been demon- strated (4). Identification of plasma com- ponents involved in the internal defense system of B. glabrata which differ be- tween resistant and susceptible strains may lead to a greater understanding of this host-parasite system. Recognition and phagocytosis of for- eign particles in molluscs may involve lectins with specific carbohydrate binding properties. Both cell membrane- bound and humoral lectins are thought to be involved, with interactions be- tween these leading to the efficient clearance of invasive organisms (5). In- volvement of both hemocyte-bound and humoral lectins has been indicated in the clearance of foreign particles from Helix pomatia (6) and was clearly demon- strated for phagocytosis of yeast by My- tilus edulis in vitro (7). The main components of yeast cell walls are mannans and 13-1,3 linked glucans (8). In the present investigation we have utilized these carbohydrates to determine the molecular basis for both