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Reproducing forms of Trypanosoma lewisi isolated from X-irradiated rats and adult forms from intact rats were not lysed by fresh mammalian sera. Treating parasites with trypsin or chymotrypsin, but not with neuraminidase, under conditions which did not impair viability rendered the parasites sensitive to lysis by rat, mouse, rabbit, and human sera. Serum from animal strains or humans genetically deficient in complement component C3, C5, or C6 did not lyse protease-treated parasites. The lytic factors in serum displayed the heat sensitivity and the Mg2+ requirement characteristic of the alternate complement pathway. Lysis was resolved into two phases, Mg2+-dependent binding of serum factors to parasites and subsequent C5-dependent, Mg2+-independent lysis. Allowing protease-treated parasites to readsorb host proteins did not block lysis by serum. Protease-treated parasites regenerated components which prevented complement-mediated lysis during 2 h in culture at 37 degrees C. This regeneration was inhibited by cycloheximide but not by tunicamycin. Ten major components were resolved in radioautographs of sodium dodecyl sulfate-polyacrylamide gels of extracts of radioiodinated intact cells. Protease treatment before radioiodination reduced the amount of radioactivity associated with these components disproportionately. Components with apparent molecular weights of 102,000, 88,000, and 47,000 were strongly labeled in intact cells, poorly labeled after enzyme treatment, and again labeled in cells that were cultured at 37 degrees C after enzyme treatment. Cycloheximide blocked the reappearance of these components on cultured cells. The presence of these three components was therefore correlated with resistance to complement-mediated lysis.

infection and immunity

Externally disposed membrane polypeptides of intact and protease-treated Trypanosoma lewisi correlated with sensitivity to alternate complement pathway-mediated lysis