Antigen antibody complexes are suspected to play a role in the pathogenesis of some of the lesions that result from schistosomiasis. To examine the effect of immune complexes on the immune system of mice experimentally infected with Schistosoma mansoni, we measured antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-mediated serum hemolysis in normal and infected animals. ADCC activity in infected mice was depressed compared to control mice. However, preincubation of spleen cells for 24 h in medium followed by washing restored ADCC activity. This suggested that a soluble factor(s), presumably immune complexes, was bound to the Fc receptors with resultant block in ADCC activity and this was removed in vitro during the 24-hour preincubation. Furthermore, the complement activity of mouse serum was markedly depressed in mice infected for 3 or 6 weeks. Again, the presence of immune complexes could explain this depression since immune complexes bind complement. We attempted to confirm and extend these findings with an immunoperoxidase-staining technique using antibody to S. mansoni antigen. Most of the granuloma formations identified in portal tracts and intestinal mucosa were composed of macrophages and epithelial cells surrounding a central nidus of schistosome egg. In addition, schistosome antigen was seen diffusely bound to some of the lymphoid elements in the lamina propria and many of these cells appeared plasmacytoid. Furthermore, large amounts of schistosome antigen were sequestered in the medullary cords of the mesenteric lymph nodes and in the Billroth cords of the spleen. This suggests that the antigen is conveyed to the lymph nodes and the spleen through the systemic circulation. Taken together, these findings suggest the circulating immune complexes are a feature of schistosomiasis and may well contribute to the associated immunopathology.

Schistosoma mansoni antigens play a crucial role in the induction of immunopathological processes and in modulating the host immune system. A polyclonal rabbit antiserum to an antigenic fraction of adult schistosoma worms was used to localize worm and egg antigens in tissue sections of infected mice. Most granuloma formations identified in paraffin sections of portal tracts and intestinal mucosa were vigorous with florid cellular composition, consisting of macrophages and epithelioid cells, surrounding a central nidus of S.mansoni egg. Schistosome pigments were demonstrated at the periphery of the granuloma, within sinusoidal Kupffer’s cells and within macrophages of intestinal mucosa. Large amounts of schistosome antigen were found sequestered in the mesenteric lymph nodes and in the spleen. A strong reaction for locally synthesized IgG was found. These findings suggest that the schistosome antigen-antibody complex found in the host lymphoreticular system plays a major role in the immunopathological process.