Cytotoxicity of Human Serum for Leishmania donovani Amastigotes: Antibody Facilitation of Alternate Complement Pathway-Mediated Killing

摘要

Mechanisms that mediate recovery from leishmanial infection have not been fully characterized but are generally believed to involved interactions between T lymphocytes and macrophages. A major role for serum-mediated effector mechanisms in the protection of humans from reinfection with Leishmania, however, has not not been ruled out. Amastigotes of L. donovani were incubated with dilutions of serum from normal subjects and from patients with kala-azar. Normal serum was cytotoxic for parasites at a dilution of 1:20. Cytotoxicity did not occur in the presence of EDTA, was abolished by heating serum to 56 C for 30 min, and was not diminished by prior adsorption of normal serum with parasites at 0 C. Killing proceeded normally in the present of magnesium-ethylene glycol-bis (beta-aminoethyl ether)-N,N-tetraacetic acid, however, and was fully effected by C2-deficient serum. These studies indicated that killing of amastigotes, unlike that of promastigotes, was mediated via the alternate pathway of serum complement. In further studies, cytotoxicity of normal serum was enhanced three-to fivefold by factors in patient serum. This enhanced cytotoxicity were characterized as parasite-specific immunoglobulin G: they eluted with immunoglobulin G on column chromatography, were adsorbed by immobilized staphylococcal protein A, and were not removed from the parasite surface by extensive washing.