An immunosuppressive factor in the serum of baboons (Papio anubis) infected with Schistosoma mansoni.

Serum taken from baboons infected with Schistosoma mansoni was able to suppress in vitro reactivity of normal baboon lymphocytes. The concanavalin A response was significantly suppressed by such serum, whereas the specific suppression of the phytohaemagglutinin response was minimal. Serum from S. mansoni infected donors also depressed the mixed lymphocyte reactions to xenogeneic targets, but did not affect the specific transformation of lymphocytes stimulated with a parasite Ag. Significant suppressive activity occurred in the baboon serum from 4 to 11 weeks after the initial infection. Serum from animals with a chronic infection of 6-42 months, did not suppress in vitro cell-mediated immunity. The suppressive factor was heat-stable, non-dialysable and, following ultracentrifugation of the suppressive serum, was found to be present in the high mol. wt fraction. From these studies, it is suggested that the immunosuppressive factors are immune complexes, which appear in the serum of the baboons following their infection with this blood parasite.