Killing of Schistosomula of Schistosoma Mansoni Coated with Antibody and/or Complement by Human Leukocytes in Vitro: Requirement for Complement in Preferential Killing by Eosinophils

The capacity of human eosinophils, neutrophils, and mononuclear leukocytes to damage schistosomula coated with antibody and/or complement has been studied by using an in vitro assay in which helminth destruction was visualised directly. Schistosomula and leukocytes were incubated either with i) antibody (Ab) alone (as baboon anti-schistosomula serum, or serum from patients with schistosomiasis or its IgG fraction); ii) complement (C) alone (by incubating schistosomula with fresh serum that led to activation by the tegument of the alternate pathway of complement as shown by immunofluorescence with anti-C3, but not anti-C4, both with normal and C2-deficient serum); or iii) the combination of Ab and C. The efficiency of killing of schistosomula by granulocytes (i.e., mixtures of eosinophils and neutrophils) was 31 ± 7% for Ab alone, 52 ± 9% for C alone, and 70 ± 8% for Ab + C; control preparations were approximately 14%. Equivalent numbers of mononuclear leukocytes gave 22 ± 3%, 31 ± 5%, and 37 ± 4% killing with Ab alone, C alone, or Ab + C, respectively. Schistosomula killing was directly related to the concentration of antibody with Ab alone and to the dilution of complement with C alone and Ab + C. Damage by granulocytes or mononuclear leukocytes was dependent on the ratio of effector cells to schistosomula in the three experimental systems. Granulocyte-dependent damage was enhanced by the sequential addition, to schistosomula sensitized with antibody, of purified components of the classical pathway of complement and was directly related to the input of C3. This observation, together with the direct visualisation of C3 by immunofluorescence, suggests the participation of the classical pathway of complement in the Ab + C system.