Humoral immunity in experimental syphilis. II. The relationship of neutralizing factors in immune serum to acquired resistance.

Evidence for a humoral mechanism in immunity to experimental syphilis was provided by the demonstration of immune rabbit serum factor(s) capable of inactivating virulent Treponema pallidum, Nichols strain, in an in vitro-in vivo neutralization test. After intratesticular infection, rabbits were bled periodically and their resistance to reinfection was determined by challenge with T. pallidum. The results of challenge showed that resistance to reinfection begins to develop by 11 days after infection, becomes complete by 3 months, and persists for at least 2 years. In the neutralization test, a mixture of treponemal suspension and serum from the infected animals was incubated anaerobically at 34 degrees C and the virulence of the treponemes was determined by intradermal inoculation into normal rabbits. Complete inactivation of treponemes by immune serum required heat-stable and heat-labile (56 degrees C, 30 min) serum components and 16 hr of incubation, and was accelerated by pre-incubation of the treponemes for 4 hr with nonimmune serum but not by 100 mug/ml of added lysozyme. Serum-neutralizing activity, first demonstrable 1 month postinfection, was quantitated by a neutralizing endpoint (NEP). A relatively close quantitative correlation was shown between the development of resistance to symptomatic reinfection and the appearance and persistence of both TPI antibody and neutralizing serum factor(s). The nature of the serum factor(s), the mechanism of treponemal inactivation, and the application of the test in assessing the immune status are discussed.