Broadly reacting precipitating and agglutinating antigen of leptospirae.

A saprophytic Leptospira biflexa strain of equine origin was found which cross-reacts with immune rabbit antisera to 14 pathogenic Leptospira interrogans serotypes. Sera from goats experimentally inoculated with the saprophyte showed multiple low-level cross-agglutination reactions against a battery of live L. interrogans serotypes. Sonically treated and saline-extracted suspensions of the L. biflexa strain and serotypes canicola, icterohaemorrhagiae, and pomona yielded a common precipitating protein antigen that was detected by immunodiffusion and immunoelectrophoresis with all of the antileptospiral sera examined. In cross-absorption and gel diffusion tests, the precipitinogen from each of the strains was shown to be identical. Formaldehyde treatments and heating at 100 degree C suggest that the cellular location of the common antigen is either somatic or subsurface, and Sephadex G-200 gel filtration enabled the isolation of the active fraction of the L. biflexa antigen. Monoprecipitin sera against the common antigen of L. biflexa were produced by immunizing rabbits with specific precipitates in agar. In gel diffusion and immunoelectrophoresis tests the antisera with each of the soluble antigens developed a single precipitin formation, and the antisera agglutinated formolized and heated whole-cell suspensions of serotypes canicola, icterohaemorrhagiae, and pomona at low dilutions. The soluble L. biflexa antigen was evaluated as an immunogen and in passive immunity tests for protection against death and kidney infection in hamsters. No cross-protection occurred when the hamsters were challenged with virulent leptospires. In contrast, the animals vaccinated or administered hamster immune serum before challenge died earlier than the control animals.