An immune responsive factor D-like serine proteinase homologue identified from the American dog tick, Dermacentor variabilis.

A Dermacentor variabilis cDNA encoding a clip-domain serine proteinase homologue with glycine replacing the catalytic serine was identified from tick haemocytes. The D. variabilis product was most similar to Tachypleus tridentatus haemocyte antimicrobial factor D and shared significant homologies with a number of immune-responsive gene products of arthropods, including insect prophenoloxidase-activating cofactors. Northern blotting analyses confirmed that the tick serine proteinase homologue expression levels were highest in haemocytes, and to lesser degrees in ovaries and then salivary glands whereas steady-state levels of expression in whole ticks were found to be slightly higher in fed versus unfed adults or eggs. Challenge of fed adults by Escherichia coli injection demonstrated that transcript abundance was significantly increased above those of naive controls in a temporal fashion. Additionally, an apparent orthologue of the D. variabilis clip-domain molecule was cloned, and expression detected, from a Dermacentor andersoni cell line indicating cross species conservation.