Identification of abundantly expressed novel and conserved genes from the infective larval stage of Toxocara canis by an expressed sequence tag strategy

Larvae of Toxocara canis, a nematode parasite of dogs, infect humans, causi ng visceral and ocular larva migrans, In noncanid hosts, larvae neither gro w nor differentiate but endure in a state of arrested development. Reasonin g that parasite protein production is orientated to immune evasion, we unde rtook a random sequencing project from a larval cDNA library to characteriz e the most highly expressed transcripts. In all, 266 clones were sequenced, most from both 3' and 5' ends, and similarity searches against GenBank pro tein and dbEST nucleotide databases were conducted. Cluster analyses showed that 128 distinct gene products had been found, all but 3 of which represe nted newly identified genes. Ninety-five genes were represented by a single clone, but seven transcripts were present at high frequencies, each compos ing >2% of all clones sequenced. These high-abundance transcripts include a mucin and a C-type lectin, which are both major excretory-secretory antige ns released by parasites. Four highly expressed novel gene transcripts, ter med ant (abundant novel transcript) genes, were found. Together, these four genes comprised 18% of all cDNA clones isolated, but no similar sequences occur in the Caenorhabditis elegans genome. While the coding regions of the four genes are dissimilar, their 3' untranslated tracts have significant h omology in nucleotide sequence, The discovery of these abundant, parasite-s pecific genes of newly identified lectins and mucins, as well as a range of conserved and novel proteins, provides defined candidates for future analy sis of the molecular basis of immune evasion by T, canis.