Serological and protective-antibody responses of rabbits to leptospiral antigens

The rabbit antibody response to leptospiral F4 antigen extracted from serovar pomona depended on the method of immunisation. Intravenous injection of whole leptospires stimulated F4 antibodies that were confined to the IgM class, but leptospires injected intramuscularly with adjuvant stimulated F4 antibodies in both the IgM and the IgM and the IgG classes. Both methods of immunisation stimulated agglutinins in both the IgM and IgG classes. F4 antigen in soluble form was immunogenci when injected intradermally with adjuvant, but not when given alone. The F4 antibodies were distinct from the agglutinins in that they did not protect hamsters from acute infection with homologous leptospires, nor did they kill leptospires in vitro although they reacted with leptospires. The hamster-protective capacity of rabbit sera depended on the level of agglutinin.