Evidence that autologous idiotypic regulation of anti-arginine-glycine-aspartic acid autoantibodies may influence development and progression of syphilitic lesions in infected rabbits.

The 83-kDa receptor protein of Treponema pallidum (TpN83) recognizes and binds fibronectin (Fn) at the amino acid sequence RGD site. By using experimentally infected animals, we have demonstrated that immunoglobulin G antibodies to this antigen and autoantibodies to the RGD site of Fn are putative components of immune complexes. This, and other findings, led us to initially hypothesize that anti-idiotypes (anti-Id) of an anti-TpN83 response are autoantibodies to RGD. Alternatively, we reasoned that if anti-Fn autoantibodies played a role in the pathogenesis of syphilis, then down-regulation of such a response by the Id network might directly affect progression of the disease. To test the hypothesis, rabbits were immunized with either affinity-purified TpN83 antigen or the synthetic Fn-7 peptide, KYGRGDS, and subsequently challenged with T. pallidum. Compared with results obtained with unimmunized, control rabbits, accelerated lesion development was noted in the rabbits immunized with TpN83. Pronounced, though unexpected, differences with respect to lesion development and progression were noted in the animals immunized with Fn-7 and then challenged intravenously; a minimal number of lesions appeared with a delayed onset. These lesions, like the localized chancres seen following intradermal challenge, were smaller and minimally ulcerated, and they healed rapidly. The Fn-7-immunized rabbits all differed from the controls in that anti-Id to anti-RGD F(ab')2 were demonstrable within 4 weeks following infection; decreases in anti-Fn autoantibody levels were associated with concomitant increases in anti-Id levels. Immunoglobulin Gs (anti-Id) from these animals following elution from anti-RGD F(ab')2 immunoaffinity columns also reacted with affinity-purified TpN83 antigen in immunoassays. These results suggest that down-regulation of autoreactive clones by manipulation of the idiotypic network in experimental syphilis warrants further investigation.