Toxocara canis: genes expressed by the arrested infective larval stage of a parasitic nematode

Toxocara canis is a widely distributed nematode parasite which reaches matu rity in dogs. However, eggs voided by canid animals are infective to a very wide range of paratenic hosts including humans. In noncanid hosts. infecti ve larvae emerge from the eggs and invade the soft tissues, often entering the brain and musculature. Such larvae may remain for many months or years in these tissues without further growth or differentiation, and yet appear to evade inflammatory reactions or other modes of immune attack. To underst and the ability of T. canis larvae to survive in the immunocompetent host, we have undertaken a molecular analysis of the major genes expressed at thi s stage. By a combination of protein sequencing, gene identification, and e xpressed sequence tag (EST) analysis we have characterised a range of poten tially important gene products from this parasite. Some of these are homolo gues of prominent mammalian proteins such as C-type lectins (represented by the secreted products TES-32 and TES-70), and mucins (TES-120), and additi onal products show strong similarities to known cysteine proteases, phospha tidylethanolamine-binding proteins and other ligands. A number of these pro teins include a conspicuous 36-amino acid motif containing six cysteines. T his domain (termed NC6 or SXC) appears to be an evolutionarily mobile modul e, which in T. canis is combined with a spectrum of diverse functional doma ins in different genes. In addition, we have identified a set of novel gene sequences that show no resemblance to any genes encoded by the free-living nematode C. elegans. Four of these are designated abundant novel transcrip ts, and collectively these account for nearly 20% of the cDNA isolated from the arrested infective stage. Such parasite-specific genes expressed at a high level by a stage that shows remarkable endurance may represent critica l products necessary for the success of the parasitic mode of life. (C) 200 0 Australian Society for Parasitology Inc. Published by Elsevier Science Lt d. All rights reserved.