A novel serpin expressed by blood-borne microfilariae of the parasitic nematode Brugia malayi inhibits human neutrophil serine proteinases

Serine proteinase inhibitors (serpins) play a vital regulatory role in a wi de range of biological processes, and serpins from viruses have been implic ated in pathogen evasion of the host defence system. For the first time, we report a functional serpin gene from nematodes that may function in this m anner. This gene, named Bm-spn-2 has been isolated from the filarial nemato de Brugia malayi, a causative agent of human lymphatic filariasis, Polymera se chain reaction (PCR) and Western blot experiments indicate that Bm-spn-2 is expressed only by microfilariae (Mf), which are the long-lived blood-dw elling larval stage. A survey of the greater than 14,000 expressed sequence tags (ESTs) from B malayi deposited in dbEST shows that greater than 2% of the ESTs sequenced from Mf cDNA libraries correspond to Bm-spn-2, Despite its abundance in the microfilarial stage, Bm-spn-2 has not been found in an y other point in the life cycle. The predicted protein encoded by Bm-spn-2 contains 428 amino acids with a putative signal peptide. Antibodies to reco mbinant Bm-SPN-2 protein react specifically with a 47.5-kD native protein i n Mf extract. Bm-SPN-2 is one of the largest of the 93 known serpins, due t o a 22 amino acid carboxyterminal extension, and contains the conserved ser pin signature sequence. Outside these regions, levels of homology are low, and only a distant relationship can been seen to a Caenorhabditis elegans s erpin. The Bm-spn-2 gene contains 6 introns, 2 of which appear to be shared by both nematode species. The B malayi introns have an extended and conser ved 3' splice site and are relatively large compared with C elegans. A pane l of mammalian serine proteinases were screened and Bm-SPN-2 protein was fo und to specifically inhibit enzymatic activity of human neutrophil cathepsi n G and human neutrophil elastase, but not a range of other serine proteina ses. It is possible that Bm-SPN-2 could function as a stage-specific serpin in the blood environment of the microfilarial parasite in protection from human immunity and thus may be a good candidate for protective vaccine. (C) 1999 by The American Society of Hematology.