PURIFICATION OF A RECOMBINANT SCHISTOSOMA-JAPONICUM ANTIGEN HOMOLOGOUS TO THE 22-KDA MEMBRANE-ASSOCIATED ANTIGEN OF SCHISTOSOMA-MANSONI, A PUTATIVE VACCINE CANDIDATE AGAINST SCHISTOSOMIASIS

We describe the cDNA cloning, overproduction and purification of a 22. 6-kDa antigen from the human blood fluke Schistosoma japonicum. A 777- bp cDNA (C32) was isolated from a S. japonicum lambda ZAPII cDNA expre ssion library immune-screened with hyperimmune rabbit serum (HRS) rais ed against soluble adult S. japonicum proteins. The open reading frame of C32 encodes a protein of 191 amino acids (aa) which exhibits 71% i dentity to a 22.6-kDa membrane associated antigen of S. mansoni, a put ative vaccine candidate for schistosomiasis. We have identified a sequ ence motif known as an EF-hand calcium-binding domain in both the S. j aponicum and S. mansoni aa sequences, suggesting that the 22.6-kDa ant igens are able to bind Ca2+. Further, we have, for the first time, obt ained the 22.6-kDa antigen in purified, non-denatured, recombinant for m, and in sufficient quantity to assess the protective value of the mo lecule in vaccination/challenge experiments. This was achieved by synt hesizing the schistosome antigen with a short polyhistidine tag fused to the N-terminus which was then used for subsequent affinity purifica tion. The recombinant protein was purified under non-denaturing condit ions using nickel-chelate affinity chromatography.