Immunorecognition in the freshwater bivalve, Corbicula fluminea. I. Electrophoretic and immunologic analyses of opsonic plasma components.

Plasma (cell-free hemolymph) from the freshwater clam, Corbicula fluminea, has been found to possess opsonizing activity which could be completely eliminated by preabsorption of plasma with aldehyde-fixed rabbit red blood cells (RRBC). The detergents, SDS and Chapso, consistently dissociated several major proteins (Mr 20, 40, 66 and over 200 kd) from opsonized RRBC. Moreover, a partial recovery of opsonizing activity was attributed to this pool of desorbed proteins after removal of detergent by dialysis. Some of these proteins failed to bind to RRBC if the plasma was pretreated with heat, EGTA or preabsorbed with RRBC. These treatments were also found to eliminate plasma-opsonizing activity. A monoclonal antibody raised against opsonized RRBC bound to several of the same proteins seen in SDS-PAGE and could block opsonin-mediated RRBC phagocytosis by clam hemocytes. The blocking antibody also was crossreactive with epitopes expressed at the surface of washed hemocytes. Taken together these results suggest that the candidate opsonin(s) is a divalent cation-dependent, heat sensitive protein(s) with an Mr ranging from 20 to 66 kd (under reducing conditions). Furthermore, antigenically related forms of the molecules are found both in the plasma and associated with the surface membrane of circulating hemocytes.