Studies on the mechanism of lysis of Echinococcus granulosus protoscoleces incubated in normal serum

Brood capsules were obtained from freshly collected cysts of equine and ovine strains of Echinococcus granulosus . Protoscoleces were freed from brood capsules either by mechanical disruption or pepsin-HCl digestion. Preparations of protoscoleces studied included: mechanically released protoscoleces without further treatment, or incubated either in HCl pH 2·0 or in evaginating solution (containing Na taurocholate) for 24 h; pepsin-HCl released protoscoleces without further treatment or incubated in evaginating solution for 24 h or 7 days. Half of each preparation of ovine protoscoleces was fixed in absolute methanol. All fresh preparations of protoscoleces lysed rapidly when incubated in normal human serum. Studies with a fluorescein isothiocyanate (FITC) labelled sheep anti-human C 3 antiserum revealed the presence of C 3 on the surface of lysing protoscoleces. Antibody could not be detected on the surface of any of the preparations of fresh or methanol-fixed protoscoleces using direct or indirect fluorescent antibody tests suggesting that the classical pathway of complement activation was not involved in the lytic process. Strong evidence for lysis by the alternate pathway of complement activation was the lysis of protoscoleces which had been treated with pepsin-HCl and lysis of protoscoleces in guinea-pig serum deficient in C 4 component of complement.