Serum regulation of in vitro lymphocyte responses in early experimental syphilis.

Sera from rabbits with early experimental syphilis were tested for their effect on in vitro lymphocyte transformation responses to related specific antigens (sonicated T. pallidum), unrelated specific antigens (sheep erythrocytes), and the T cell mitogen, concanavalin A. Results were compared with responses in preinfection sera and in sera from sham-infected rabbits. Titration experiments in which normal serum was used indicated that optimal lymphocyte responsiveness is obtained with a final serum concentration of 1%. Under these conditions, no differences in concanavalin A stimulation were observed in cultures with syphilitic sera. Responses to sonicated T. pallidum were inhibited, but only by 17 to 25% when compared with the response in preinfection sera. In cultures containing 10% serum, inhibition of lymphocyte proliferation to sonicated T. pallidum antigens was evident with sera from all syphilitic animals from day 10 (55% inhibition) through day 31 (80% inhibition) of infection. Responses to concanavalin A and sheep erythrocytes were significantly inhibited by day 10 sera; only 20% of the sera tested demonstrated substantial nonspecific inhibitory capacity. No differences were evident among sera from any of the sham-infected animals or among the preinfection sera from either group. Pooled serum with high inhibitory activity was fractionated by ammonium sulfate precipitation, DEAE ion exchange chromatography, and Sephadex G-200 gel filtration. Two separate inhibitors were identified: (i) a low-molecular-weight, ammonium sulfate-soluble, nonspecific inhibitory fraction containing albumin and alpha-globulins with the capacity to inhibit both antigen and mitogen responses and (ii) a high-molecular-weight, ammonium sulfate-precipitable, inhibitory fraction containing alpha-globulin and FTA-ABS-reactive immunoglobulin M which affected only the antigen-specific response to sonicated T. pallidum. Immunodiffusion failed to detect immunoglobulin or T. pallidum antigens in either fraction. DEAE-purified immunoglobulin G from immune serum was not inhibitory.