Developing a multi-antigen fusion protein (Tp15-17-47) serological test to improve neurosyphilis diagnosis

OBJECTIVES: Our previous research identified a luciferase immunosorbent assay (LISA) for detecting serum antibodies against Tp17 and Tp47 as a potential tool for diagnosing neurosyphilis (NS). In this study, we developed a serological test based on a multi-antigen fusion protein (Tp15-17-47) to simplify the detection process and improve the diagnostic accuracy for NS. METHODS: The Tp15, Tp17, and Tp47 genes were amplified separately by polymerase chain reaction (PCR) and assembled into a fusion gene coding for a tripartite fusion protein antigen using overlap PCR. This recombinant Tp15-17-47 antigen was used retrospectively to detect serum anti-Tp15-17-47 IgG antibodies in 434 HIV-negative patients suspected of NS from cohorts in Beijing and Guangzhou. Two diagnostic prediction models were developed using stepwise logistic regression. RESULTS: Serum anti-Tp15-17-47 IgG antibodies demonstrated moderate diagnostic capability for NS in the development cohort, with an area under the curve (AUC) of 0.791 (95% CI: 0.735-0.846). An optimized NS prediction model substituting anti-Tp15-17-47 antibodies for TRUST/RPR outperformed the base NS diagnostic model. The optimized model achieved an AUC of 0.877 (95% CI: 0.835-0.919) compared to 0.843 (95% CI: 0.795-0.890) for the base model in the development cohort (p = 0.035). This superior performance was confirmed in the validation cohort (AUC: 0.880, 95% CI: 0.827-0.933 vs. 0.792, 95% CI: 0.726-0.859; p < 0.001). Decision curve analysis (DCA) revealed that the optimized model provided greater net benefit than the base model across threshold probabilities ranging from 0.10 to 0.95 in both cohorts. CONCLUSIONS: Quantitative profiling of serum IgG targeting the recombinant treponemal antigen Tp15-17-47, along with its diagnostic model, exhibited robust discriminatory power for neurosyphilis diagnosis, which supports the viability of replacing lumbar puncture with this less invasive serological strategy for neurosyphilis ascertainment.