A NEW IG-SUPERFAMILY MEMBER, MOLLUSCAN DEFENSE MOLECULE (MDM) FROM LYMNAEA-STAGNALIS, IS DOWN-REGULATED DURING PARASITOSIS

To survive the attack of the internal defence system (IDS) of their ho st, parasites have developed various strategies to manipulate the IDS. We present evidence that the avian schistosome parasite Trichobilharz ia ocellata affects gene expression in the granular cells, a cell type of the IDS of the intermediate host, the mollusc Lymnaea stagnalis. F rom a differential screening, a clone was isolated encoding a protein named molluscan defence molecule (MDM), which encompasses five C2-like immunoglobulin (Ig) domains. The protein shares a domain organization and high amino acid sequence identity with hemolin, an Ig-family memb er of the insect IDS. Interestingly, both MDM and hemolin have highest sequence identity with neural cell adhesion molecules, but lack the t ypical fibronectin repeats and motifs for membrane anchors. We find th at the expression of the MDM gene is gradually down-regulated during t he course of parasitosis to similar to 21% compared to the non-parasit ized level, 8 weeks post-infection. Based on our finding, we suggest t he MDM is involved in the proper functioning of the Lymnaea IDS, and t hat down-regulation of MDM is part of the parasite-induced disabling o f non-self recognition.