OBJECTIVETo study the immunogenicity and immunoprotection of the recombinant expressing product (rSpaO) of S. paratyphi A spaO gene, and to demonstrate the frequencies of spaO gene carrying and expressing in S. paratyphi A isolates.

METHODSThe spaO gene of a clinical S. paratyphi A strain JH01 was amplified and then cloned. After sequencing of the cloned spaO gene, a prokaryotic expression system of the gene was constructed. SDS-PAGE were applied to examine the rSpaO expression. Ni-NTA affinity chromatography was performed to collect rSpaO. Immunogenicity of rSpaO was determined by Western blot assay. A PCR assay and an ELISA were established to respectively detect the carrying and expressing frequencies of the spaO genes in 98 S. paratyphi A isolates. The immunoprotective effects of rSpaO in S. paratyphi A strain 50001 infected mice were observed.

RESULTSIn comparison with the reported corresponding sequences, the nucleotide and putative amino acid sequence homologies of the cloned spaO gene were 99.45%-99.89% and 99.01%-100%, respectively. The expression output of rSpaO was approximately 75% of the total bacterial proteins. S. paratyphi A antiserum could recognize as well as combine with rSpaO. rSpaO could efficiently induce rabbits to produce specific antibody. 94.9% (93/98) of the S. paratyphi A isolates had spaO gene and 91.4% (85/93) of the spaO+ strains could express SpaO. 58.3% and 50.0% of the mice that oral-taken or subcutaneous injected with 500 microg of rSpaO for immunization were survival after challenged by lethal dose of S. paratyphi A strain 50001. When co-immunized with 5 microg rLTB, the survival rates of the mice increased to 88.3% and 75.0%, respectively.

CONCLUSIONThe S. paratyphi isolates had relatively high carrying and expressing frequencies of spaO gene. rSpaO showed a fine immunogenicity and a certain immunoprotective effect, which could be used as an antigen candidate for developing genetic engineering vaccine of S. paratyphi.