Evaluation of LIPS (luciferase immunoprecipitation system) for serodiagnosis of Toxoplasmosis

Development of reliable, quantitative technologies for serodiagnosis of Toxoplasma gondii infection remains desirable. The luciferase immunoprecipitation system (LIPS) is a relatively simple, highly sensitive, and rapid quantitative immunoassay. The major advantages of this assay over ELISA are a wider dynamic range, shorter overall assay time, and less sample volume. In this study, we aimed to use this method for the serodiagnosis of toxoplasmosis. Recombinant Toxoplasma antigens (dense granule antigens GRA6, GRA7, and GRA8 and bradyzoite antigen BAG1) fused with nanoluciferase (Nluc, a small luciferase enzyme) were expressed in Escherichia coli, purified, and tested in LIPS assays with sera from experimental mice infected with T. gondii and a WHO standard anti-Toxoplasma human immunoglobulin (TOXM). In the experimentally infected mice, LIPS assays detected antibodies against Nluc-GRA6, Nluc-GRA7, and Nluc-GRA8 as early as day 14, whereas antibodies against Nluc-BAG1 remained undetected until day 21 and then showed significant elevation on day 60. In TOXM sera, LIPS assays with each Nluc recombinant protein produced reliable standard curves with a coefficient of determination (R2) of 0.980-0.989 for GRA6, 0.986-0.990 for GRA7, 0.998-0.999 for GRA8, and 0.942-0.987 for BAG1. The detection limits were estimated to be 3.9, 2, 1, and 1 IU/ml for rGRA6, rGRA7, rGRA8, and rBAG1, respectively. The LIPS assay for toxoplasmosis could detect antibodies against T. gondii in the mouse and human sera with a reasonably high sensitivity. We consider the LIPS assay to be a promising alternative tool for screening, diagnosing, and monitoring toxoplasmosis. In particular, detection of antibodies against BAG1 may be useful for a longitudinal seroprevalence study in suspected high-risk areas on the basis of its elevated serum concentration in the chronic phase.