Mechanism of lethal effect of human serum upon Leishmania donovani
Pearson, R.D.; Steigbigel, R.T.
Journal of Immunology 125(5): 2195-2201
1980
ISSN/ISBN: 0022-1767 PMID: 7430624 Document Number: 167522
In order to gain a greater understanding of potential host defense mechanisms against L. donovani, the effect of nonimmune, human serum upon promastigotes and amastigotes was examined. Fresh sera were lethal for promastigotes but had no detectable effect on amastigotes. Serum-exposed promastigotes became immotile, did not take up neutral red dye, appeared disrupted and failed to recover after further incubation in fresh media. Heat labile components were required for promastigote killing since heat-inactivated serum (56.degree. C, 30 min) agglutinated but did not kill them. Sera that lacked the 5th or 6th complement (C) component had no effect when used alone, but when used together they were lethal, indicating that activation of the membrane attack complex (C5b-C9) was necessary for the lethal effect. The mode of C activation was determined by using serum with complete, selective deficiency of C2, and normal serum chelated with Mg-EGTA [ethylene glycol bis(.beta.-aminoethyl ether)tetraacetate]. The C2-deficient serum killed promastigotes only after the addition of purified C2, and Mg-EGTA chelated serum had no detectable lethal effect. Promastigotes appeared to activate C through the classical pathway. Human IgG and IgM, detected with 125I-anti-human antibody, bound to promastigotes. Removal of antibody from serum by absorption with promastigotes eliminated the lethal effect. The effect was restored by addition of heat-inactivated serum to absorbed serum. Promastigotes bind antibody and are killed by activation of the membrane attack complex of C through the classical pathway.