Diagnostic importance of Treponema pallidum Tp0971 in the serological assessment of treatment efficacy for syphilis

Over the last twenty years, there has been a global resurgence of infections caused by Treponema pallidum subsp. pallidum (T. pallidum), the bacterium responsible for syphilis. Presently, the T. pallidum IgG chemiluminescence immunoassay (CLIA) and T. pallidum particle agglutination (TPPA) are commonly employed for the investigation of potential syphilis cases through treponemal serological testing. The nontreponemal rapid plasma reagin (RPR) flocculation test is used to assess disease activity and test for cure or reinfection. Despite numerous limitations, RPR remains the best available serological standard for assessing the treatment effectiveness of patients at present. However, this method does not fully meet the requirements for treatment efficacy evaluation in syphilis patients because of the persistence of serofast reactions or spontaneous nontreponemal antibody titre decline. Therefore, a new and effective diagnostic specific marker that can also be used for monitoring treatment efficacy is urgently needed. We investigated the dynamic changes in Tp0971-specific antibodies in New Zealand rabbits via indirect ELISA and evaluated the diagnostic utility and treatment monitoring potential of Tp0971 in syphilis patients through combined ELISA and Western blot (WB) analyses. This study aims to establish a novel biomarker candidate for dual purposes of disease diagnosis and treatment response monitoring, thereby providing clinical references for the development of efficacy evaluation markers. We observed significant temporal dynamics in Tp0971-specific antibody levels and RPR titres between penicillin-treated and untreated New Zealand White rabbits. In the penicillin-treated cohort, both parameters demonstrated early elevation followed by marked late-phase decline (e.g., 30d: A450 nm = 1.217 [IQR: 0.940-1.494], RPR 1:16 [IQR: 1:8-1:32]; 312d: A450 nm = 0.4653 [IQR: 0.154-0.776], RPR negative [IQR: negative-1:2]). In contrast, the untreated group presented paradoxical findings: while late-phase RPR titres tended to decrease, leading to negative conversion, persistent elevation of Tp0971-specific antibodies was maintained throughout the observation period (e.g., 30d: A450 nm = 1.143 [IQR: 0.274-2.013], RPR 1:8 [IQR: 1:4-1:16]; 312d: A450 nm = 0.9317 [IQR: 0.185-2.048], RPR 1:1 [IQR: negative-1:4]). In patient samples, the absorbance of Tp0971-ELISA after treatment was significantly lower than that before treatment, and this difference could be observed within 4-6 months (pretreatment: A450 nm = 2.583 [2.376-2.790]; 4-6 m: A450 nm = 1.135 [0.451-1.819], p < 0.01). Moreover, among the collected RPR-negative primary syphilis and cerebral infarction syphilis or neurosyphilis samples, the Tp0971-ELISA results reached positive rates of 100% and 75%, respectively. In conclusion, Tp0971-ELISA may be used to evaluate the treatment efficacy of syphilis, and it has relatively high diagnostic value in patients with primary syphilis and cerebral infarction syphilis or neurosyphilis.