A novel C-type lectin secreted by a tissue-dwelling parasitic nematode

Many parasitic nematodes live for surprisingly long periods in the tissues of their hosts, implying sophisticated mechanisms for evading the host immu ne system. The nematode Toxocara canis survives for years in mammalian tiss ues, and when cultivated in vitro, secretes antigens such as TES-32. From t he peptide sequence, we cloned TES-32 cDNA, which encodes a 219 amino-acid protein that has a domain characteristic of host calcium dependent (C-type) lectins, a family of proteins associated with immune defence. Homology mod elling predicted that TES-32 bears remarkable structural similarity to mamm alian immune-system lectins. Native TES-32 acted as a functional lectin in affinity chromatography. Unusually, it bound both mannose- and galactose-ty pe monosaccharides, a pattern precluded in mammalian lectins by a constrain ing loop adjacent to the carbohydrate-binding site. In TES-32, this loop ap peared to be less obtrusive, permitting a broader range of ligand binding. The similarity of TES-32 to host immune cell receptors suggests a hitherto unsuspected strategy for parasite immune evasion.