Assessment of <i>T. pallidum</i> recombinant proteins Tp0163 and Tp0971 as antigens for the diagnosis of syphilis by enzyme-linked immunosorbent assay

Background. Genomic sequencing of T. pallidum and the application of bioinformatics and immunoproteomics resulted in many recombinant proteins of T. pallidum that have been analyzed for their diagnostic value, and some of them are used as diagnostic antigens in commercial tests. However, in practice, there are often significant difficulties in the diagnosis of asymptomatic forms of syphilis, especially of the late forms, when the sensitivity of serological methods may differ significantly. Therefore, the search for more sensitive and specific antigens for serological diagnosis of syphilis is ongoing. Aim of study. Assessment of the potential of T. pallidum recombinant proteins Tp0163 and Tp0971 as candidate antigens for serum IgG determination in syphilis patients by enzyme-linked immunosorbent assay (ELISA). Methods. T. pallidum recombinant proteins Tp0163 and Tp0971 were produced by Cusabio (China). Serum samples from subjects with a confirmed diagnosis of primary, secondary, early latent and late latent syphilis, as well as serum samples from healthy subjects as controls were used for the tests with indirect ELISA. To characterize the antibody concentration, a positivity coefficient equal to the ratio of the absorbance obtained for each sample to the critical absorbance was determined. The positivity coefficient was used to determine the difference between groups of syphilis patients and healthy individuals using the Mann-Whitney test for unpaired samples, and to assess the clinical informativeness of laboratory test. The diagnostic efficacy of ELISA was assessed by calculating the values in groups of patients with established diagnosis of syphilis and in a group of healthy individuals in compliance with the standard assessment of the diagnostic informativeness of laboratory tests. Results. The overall diagnostic efficacy of the studied recombinant proteins for the detection of IgG class antibodies by ELISA is 65.4 and 66.7 % for Tp0163 and Tp0971, respectively, demonstrating high specificity and positive predictive value of the test. The sensitivity of ELISA to the studied forms of syphilis does not exceed 68.8 %. Conclusion. In order to improve treponema-specific diagnosis of syphilis, we believe it is important to continue working with these antigens to determine the efficacy of ELISA to detect IgG, IgM, or IgM/IgG antibody ratios with more clinical serum samples from subjects with syphilis, subjects without clinical and laboratory signs of syphilis, and healthy individuals.