Characterization of a native and recombinant Schistosoma haematobium serine protease inhibitor gene product

Immunologic screening of a Schistosoma haematobium cDNA library with species-specific human antisera identified a clone whose predicted amino acid sequence encodes a member of the serine protease inhibitor (serpin) gene family. This cDNA consists of 1397 bp with a single open reading frame that can encode a 409-amino acid protein of 46 261 Da. The native antigen is a 54–58-kDa glycoprotein and is located on the surface of adult worms. Sequence comparison with other serpins predicts the amino acid Phe at the putative reactive center of the protein. Phenylalanine is also found at the corresponding site of a vaccinia serpin that may contribute to the hemorrhagic phenotype of some strains of cowpox virus. Though the human parasites S. haematobium and Schistosoma mansoni demonstrate a close antigenic relationship, the S. haematobium serpin gene product demonstrates marked species-specific immunogenicity. By Northern blot hybridization, however, both species express a 1700-nucleotide mRNA that hybridizes with the S. haematobium serpin cDNA. The intensity of cross hybridization for the S. mansoni mRNA is 10-fold lower than that for S. haematobium. Southern blots of genomic DNA and gene titration experiments indicate that the S. haematobium gene is present in approximately 4–5 copies per haploid genome.