Identification of a new C-type lectin, TES-70, secreted by infective larvae of Toxocara canis, which binds to host ligands

Infective larvae of the dog roundworm Toxocara canis survive in the tissues of their hosts for extended periods in a state of developmental arrest, su ccessfully evading immune destruction. This survival strategy is thought to be mediated by T. canis excretory/secretory (TES) products which downregul ate or divert the immune response. We purified one of the major TES product s, TES-70 and gained amino acid sequence from 4 tryptic peptides. These pep tides were matched to a predicted protein from a cDNA that was isolated by expression screening a T. canis cDNA library with mouse anti-TES serum. The predicted protein (Tc-CTL-4) is similar to, but larger than, Tc-CTL-1, a 3 2-kDa C-type lectin secreted by T. canis larvae. Tc-CTL-4 has a signal pept ide, 2 Cys-rich domains and a C-terminal calcium-dependent C-type lectin do main that shares sequence similarity with host immune cell receptors such a s macrophage mannose receptor and CD23. The lectin domain was expressed in bacteria and antiserum to the purified recombinant protein was used to conf irm that Tc-ctl-4 did encode the native TES-70 glycoprotein. TES-70 selecti vely bound to ligands on the surface of Madin-Darby Canine Kidney cells in vitro in a calcium-dependent manner, inhibitable by mammalian serum, indica ting that a host glycan is the native ligand for this new parasite lectin.