Recombinant Antigens Assessed for Serological Diagnosis of Syphilis

Syphilis is a chronic multisystemic disease caused by infection with the spirochete Treponema pallidum and acquired usually via sexual exposure or via vertical mother–fetus transmission or by blood transfusions. An estimated 18 million cases worldwide and 5.6 million new cases occur annually.

Currently, diagnosis remains primarily dependent on serologic tests due to the inability to culture T. pallidum in vitro. The serological tests for the diagnosis of syphilis are divided into non-treponemal tests and treponemal tests. Some of these tests are expensive, labor-intensive, time-consuming, highly operator-dependent, and not suitable for high-volume screening.

Biomedical Scientists at the Hengyang Medical School (Hengyang, China) and their colleagues collected human serum samples from patients with clinically diagnosed primary (n = 21), secondary (n = 78), latent (n = 228), and tertiary (n = 13) syphilis, healthy controls (n = 60), and hepatitis B (n = 56), tuberculosis (n = 20), rheumatoid arthritis (RA) (n = 40), systemic lupus erythematosus (SLE) (n = 40), between March 2019 and August 2020.

Two recombinant T. pallidum proteins Tp0100 and Tp1016 were expressed, purified, and identified by Western Blotting. A total of 600 clinical serum samples were tested with the Tp0100-based ELISA, the Tp1016-based ELISA, and the commercial LICA Syphilis TP kit (CHIVD Chemclin Diagnostics Corporation, Beijing, China). The sensitivities were determined by testing 340 samples from individuals with clinically diagnosed primary, secondary, latent, and tertiary syphilis. The specificities were determined by screening 260 samples from healthy controls and individuals with potentially cross-reactive infections. The team also performed indirect IgG enzyme-linked immunosorbent assays and light-initiated chemiluminescent assays (LICA).

The scientists used clinical syphilis diagnosis as the gold standard, Tp0100 exhibited an overall sensitivity of 95.6% and specificity of 98.1% for testing IgG antibody while Tp1016 demonstrated only an overall sensitivity of 75.0% and specificity of 79.6%. In contrast, the LICA Syphilis TP test revealed an overall sensitivity of 97.6% and specificity of 96.2%. In addition, the overall percent agreement and corresponding κ values were 96.7% (95% CI 95.6%–97.8%) and 0.93% for the Tp0100-based ELISA, 77.0% (95% CI 74.3%–79.7%) and 0.54% for the Tp1016-based ELISA, and 97.0% (95% CI 96.0%–98.0%) and 0.94 for the LICA Syphilis TP test, respectively.

The authors concluded that the recombinant T. pallidum protein Tp0100 showed promise as a novel diagnostic antigen in the serological tests for syphilis. The study was published on July 31 2022 in the Journal of Clinical Laboratory Analysis.

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