Immunization with nontreponemal antigen alters the course of experimental syphilis in the rabbit model

The role of nontreponemal antibodies in the Treponema pallidum infection course is unclear. We investigated the effect of immunization with nontreponemal antigen on T. pallidum-challenged rabbits. Nontreponemal antigen was injected intravenously into rabbits in the nontreponemal group (n = 12) to elicit antibodies (≥1:64), and normal saline-injected rabbits were used as controls (n = 12). Then, rabbits were challenged with 10⁶T. pallidum per site along their back. Lesion development was observed, and the injection sites were biopsied for mRNA analysis every week. Six rabbits from both groups were euthanized at 14 d and 28 d. The popliteal lymph nodes were extracted to assess infectivity using a rabbit infectivity test. The maximum lesion diameters were not different between the two groups (12.4 ± 0.9 mm in the nontreponemal group vs. 12.5 ± 1.0 mm in the control group, P = 0.386), but the time to maximum diameter appearance was delayed by approximately 4 d in the nontreponemal group (14.4 ± 1.6 d vs. 10.8 ± 1.9 d, P = 0.000). There were no significant differences in the proportions of lesions (58/60 (96.7%) vs. 59/60 (98.3%), P = 0.500) or ulcers (55/60 (91.7%) vs. 57/60 (95.0%), P = 0.359) between the two groups. An ulcer development delay of 5 d was observed in the nontreponemal group (19.3 ± 2.0 d vs. 14.0 ± 1.8 d, P = 0.000). IL-2 and IFN-γ mRNA expression in the nontreponemal group was significantly higher than that in the control group at 7 d and 14 d post-challenge. flaA mRNA expression and the rabbit infectivity test positive rate were not different between the two groups. Immunization with nontreponemal antigen altered the syphilis course in rabbits, resulting in delayed maximal lesion diameter and ulcer development, but it could not inhibit the spread of T. pallidum from primary lesion sites to viscera.