Endotoxin-induced serum factor kills malarial parasites in vitro.

We investigated the possibility that malarial parasites may be killed by nonspecific soluble mediators, such as those in tumor necrosis serum, that are obtained from mice given macrophage-activating agents like Corynebacterium parvum or Mycobacterium bovis BCG, followed by endotoxin. Such sera killed parasites in vitro after overnight incubation; killing was measured directly by using an in vivo infectivity assay. Parasite infectivity was not decreased by incubation in sera from mice given C. parvum or BCG alone (no endotoxin) or by incubation in sera from normal mice given endotoxin. Plasmodium yoelii, its lethal variant, and Plasmodium berghei were equally susceptible to inactivation. Sera obtained from mice given endotoxin during the course of infection with these parasites also contained parasite-killing factor. The activity of this factor appeared to be proportional to parasitemia in that it was higher in the sera from mice infected with the lethal parasites than in the sera from mice with infections which resolved either spontaneously or after vaccination.