Rapid in vitro immobilisation of purified Treponema pallidum (Nichols strain), and protection by extraction fluids from rabbit testes.

The use of Percoll-purified treponemes in an assay similar to the Treponema pallidum Immobilisation test demonstrated that immobilisation of purified treponemes by seronegative normal human serum proceeded at a much higher rate than that of unpurified treponemes. This suggests that the removal of the testicular extract makes the treponemes more vulnerable to this action. A preincubation of the purified treponemes with the testicular extract from infected or uninfected testes delayed their rate of immobilisation to that demonstrated by the unpurified treponemes. This showed that substances produced during the infection are probably not responsible for the delay in immobilisation. Discrimination between the classical and the alternative pathway of complement activation, studied by the ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) method, showed that the classical pathway was responsible for the rapid immobilisation of the purified treponemes. However, the slow immobilisation in the EGTA-serum samples suggested a minor role of the alternative pathway in the immobilisation of the purified treponemes. Since the testicular extracts exerted an anti-complement activity, it needs to be investigated whether the protection offered to the purified treponemes by the testicular extracts is based on their deteriorating effect on the classical complement pathway or is due to a re-establishment of the protective cover around the treponemes.