Development and Evaluation of Six Novel Recombinant GRA Proteins in Serodiagnosis of Human Toxoplasmosis

Toxoplasma gondii is a globally distributed protozoan parasite, and reliable serodiagnosis is essential for effective management of toxoplasmosis. Conventional assays rely on tachyzoite lysate antigen (TLA), which suffers from limited standardization and reproducibility. In this study, immunodominant fragments of six dense granule proteins-GRA29, GRA35, GRA36, GRA45, GRA54, and GRA64-were expressed in Escherichia coli, purified, and evaluated as candidate antigens in IgG ELISAs using human sera. This study represents the first assessment of their diagnostic utility. Initial screening identified GRA29, GRA45, and GRA54 as promising candidates, with AUC values of 0.9983, 0.8507, and 0.9323, respectively, while GRA35-, GRA36-, and GRA64-based ELISA showed poor discrimination between seropositive and seronegative samples. Extended evaluation of GRA29-based assay with a larger serum panel (n = 286) confirmed excellent diagnostic performance, yielding an AUC of 0.9942 and higher sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) compared with TLA-ELISA. A comparative analysis revealed that GRA29 produced stronger reactivity in positive sera and lower background in negatives. These findings highlight GRA29 as a promising recombinant antigen for the serodiagnosis of human toxoplasmosis and a potential standardized alternative to TLA.