Abstract: Objective   To investigate the early response of immunoglobulin G （IgG） antibody responses to Schistosoma japonicum infection in mice by using the recombinant proteins，S. japonicum leucine aminopeptidase （rSjLAP） and S. japonicum fructose-1, 6-bisphosphate aldolase （rSjFBPA）, and evaluate the potential of rSjLAP and rSjFBPA in diagnosis as well as in assessment of therapeutic efficacy in human schistosomiasis.  Methods   rSjLAP or rSjFBPA was induced from Escherichia coli BL21 strain transfected with the expression vectors，pET-28a-rSjFBPA/BL21 or pET-28a-rSjLAP/BL21 using isopropyl-β-D-thiogalactoside （IPTG），and purified by Ni-NTA His Bind resin. 88 BALB/c female mice，inbred and 6 to 8 weeks old，were randomly divided into 4 groups. Groups A，B and C each made up of 21 mice and group D comprised 25 mice. Groups A，B and C were infected with 5，15 and 25 S. japonicum cercariae respectively. As control，mice in group D were left uninfected. 3 mice from each of groups A，B and C were sacrificed and sera collected on days 3，7，10，14，20，30，and 60 post infection. All the 25 mice in group D were sacrificed on the first day of the experiment for serum collection. rSjLAP and rSjFBPA were screened and used in ELISA to test the antibody response of the serum samples. Also，sera of 38 acute patients，96 chronic patients with schistosomiasis japonica，90 healthy donors and patients with other parasite infections including Clonorchis sinensis （33 cases），Paragonimus westermani （40） and hookworms （37） were tested using the recombinant protein-based ELISA. In addition，36 sera each from the acute and chronic patients 12 months after treatment with praziquantel and 64 of the chronic patients in more than 2 years post-treatment of praziquantel were tested. The dosage of praziquantel for both acute and chronic patients was 60 mg/kg，2 times/d×2 d.  Results   IgG antibody response was first detected at day 10 post infection by rSjLAP，rSjFBPA or the combined antigen assay. The mean absorbance （A450） on this day were 0.535±0.053，0.595±0.033，0.696±0.104 for group B;  0.548±0.060，0.608±0.063，0.621±0.090 for group C; and 0.415±0.038，0.455±0.056，0.498±0.077 for group A for rSjLAP，rSjFBPA and the combined assay respectively （P<0.05）. Early antibody level to both antigens was significantly higher in mice infected with 15 or 25 cercariae than those with 5 cercariae （P<0.05）. However，ELISA results in patients with confirmed schistosomiasis revealed positive rates of 97.4% （37/38） and 87.5% （84/96） for acute and chronic schistosomiasis with rSjLAP ，94.7% （36/38） and 88.5% （85/96） for acute and chronic schistosomiasis with rSjFBPA and 94.7% （36/38） and 85.4%（82/96） with both rSjLAP and rSjFBPA respectively. Statistical analysis showed no significant difference in the positive rate （P＞0.05）. Also，rSjLAP and combined antigens showed a specificity of 96.7% （87/90） while that of rSjFBPA was 97.8% （88/90）. There was a general decrease in the antibody titer of the patients after treatment. In 12 months after treatment it was 0.236±0.212 with rSjLAP，0.287±0.191 with rSjFBPA，and 0.235±0.120 with both antigens respectively for acute cases；For chronic patients, it was 0.266±0.124，0.261±0.143 and 0.265±0.140 in 12 months post-treatment, and 0.204±0.074，0.176±0.074，and 0.176±0.073 in 2 years, respectively. For healthy control, it was 0.188±0.056, 0.173±0.45， and 0.184±0.051, respectively. No significant difference on antibody titer was found between treated patients and control （P>0.05）. The cross reaction with C. sinensis was 15.2%(5/33） for rSjLAP，12.1% （4/33） for rSjFBPAand 9.2% （3/33） for combined antigens. With P. westermani, it was 15.0% （6/40），12.5% （5/40） and 15.0% （6/40）, respectively, and 8.1% （3/37） with hookworm infection.  Conclusion  The study showed a satisfactory sensitivity and specificity of rSjLAP and rSjFBPA by ELISA which is promising for the immunological diagnosis of schistosomiasis.

Key words: Schistosomiasis japonica, Immunodiagnosis, Leucine aminopeptidase, Fructose-1, 6-bispho-sphate aldolase