The humoral immune response in rabbits infected with Treponema pallidum: Comparison of antibody levels measured by the staphylococcal protein A-IgG (SPA-TP) microassay with VDRL, FTA-Abs, and TPI antibody responses during the development of acquired resistance to challenge.

The ontogeny of the immunoglobulin G (IgG) immune response in experimental syphilis was determined by use of an antitreponemal microassay (SPA-TP) in which radioiodinated staphylococcal protein A (SPA) was used. Results were compared with those obtained in the same rabbits by use of the Venereal Disease Research Laboratory (VDRL), fluorescent treponemal antibody-absorption (FTA-Abs), and Treponema pallidium immobilization (TPI) tests. Of the four serologic tests, only the SPA-TP and TPI assays gave results that correlated significantly over the entire 17-month period of infection. Preliminary evidence indicated that the antibody detected by the SPA-TP microassay may correlate quantitatively with the state of host immunity as determined by in vivo challenge; if this finding is confirmed, the SPA-TP microassay could be applied for assessment of the immune status in syphilis.