Leishmania amastigotes: resistance to complement-mediated lysis is not due to a failure to fix C3

Amastigote forms of Leishmania major are sensitive to lysis by fresh serum, whereas those of L. donovani are resistant. To understand the basis for this resistance we have examined the interaction of complement with amastigotes of seven strains of leishmania. Complement activation was determined by measuring the ability of amastigotes to consume complement from normal serum and by identifying parasite surface-bound C3. All of the strains that were tested activated complement, including both those that are resistant and those that are susceptible to inactivation by fresh serum. Complement consumption by amastigotes was measured as a decrease in the ACH 50 titers of serum exposed to parasites. L. major, L. donovani, and L. mexicana mexicana (strain 1VLM) amastigotes decrease titers by 35.7, 33.5, and 40.3%, respectively. The binding of C3 to amastigotes was judged qualitatively by immunofluorescence and quantitatively by a C3 radiobinding assay. L. major amastigotes bind an average of 6.6 X 10(4) molecules of C3 per parasite. L. mexicana amazonensis, L. mexicana mexicana, and L. donovani bind an average of 3.9 X 10(4), 5.9 X 10(4), and 3.7 X 10(4) molecules, respectively. In all cases, C3 binding is the result of alternative pathway activation requiring Mg++ but not Ca++. Amastigotes of the disseminating strains of leishmania represent the first example of a group of protozoa that activate early complement components leading to fixation of C3, but that are resistant to inactivation by complement.