Lectin binding characteristics of haemocytes and parasites in the oyster, Crassostrea virginica, infected with Haplosporidium nelsoni (MSX).

Lectin-binding surface receptors on haemocytes from host oysters were compared with those on plasmodial stages of the ascetosporan parasite Haplosporidium nelsoni (MSX). Haemocytes were agglutinated, in descending order of strength, by WGA, HPA, LPA, ConA, and CFA. GMA, PHA, and RMA lectins failed to agglutinate at 100 micrograms/ml, the highest concentration tested. These results indicate that haemocytes contain surface receptors resembling N-acetyl-D-glucosamine and alpha-methylmannopyranoside. Fluorescent (FITC) labelled ConA and WGA also bound to H. nelsoni plasmodia, but parasites were commonly excluded from clumps of agglutinated haemocytes, except for those that were apparently trapped passively in large aggregates. Although seasonal variations existed, agglutination titres for all reacting lectins were 2- to 8-fold higher for cells from systemically infected oysters compared to control oysters not manifesting systemic infections. Preincubation of lectins in serum from control animals reduced agglutination titres 6- to 9-fold, whereas incubation in serum from systemically infected oysters reduced titres only 4- to 6-fold. The loss of lectin-like molecules from the serum of systemically infected animals, and the concurrent increase of lectin receptors in the haemocytes population, is probably related to known changes in haemocytes composition and the loss of serum glycoproteins in diseased animals. Antigenic similarities were detected between surface receptors on oyster haemocytes and those on H. nelsoni plasmodia, which may help explain the failure of oyster haemocytes to phagocytose this parasite.