Natural IgM antibodies in sera from various animals but not the cat kill <e1>Toxoplasma gondii</e1> by activating the classical complement pathway

Sera from swine, rabbit, and dog, that had never been exposed to <e1>Toxoplasma gondii</e1>, demonstrated significant killing of <e1>T. gondii</e1> tachyzoites <e1>in vitro</e1>, while cat serum did not. Swine and rabbit sera contained natural IgM antibody against the tachyzoites, and the classical complement pathway was activated by the binding of natural IgM antibody to the tachyzoites, leading to lysis. Anti-<e1>T. gondii</e1> antibodies, induced in swine or cat infected with <e1>T. gondii</e1>, had no killing effect by themselves but killed the tachyzoites in the presence of swine complement. However, the anti-<e1>T. gondii</e1> antibodies of swine or cat demonstrated a very low killing effect in the presence of cat complement. This suggests that <e1>T. gondii</e1> tachyzoites have an evasion mechanism to prevent lysis which is specific for cat complement.