Blocking anti-Trichinella spiralis antibodies in chronically infected rats.

Specific anti-newborn larva antibodies present in the serum of rats chronically infected with Trichinella spiralis were incapable of inducing killing of newborn larvae by activation of normal peritoneal cells. These late antibodies blocked the cytotoxic reaction induced by early antibodies produced a few weeks after infection. Passive transference of late serum to normal mice failed to induce protective immunity against infection by newborn T. spiralis larvae. When late immunoserum was fractionated by gel filtration, blocking activity was found only in the fraction containing IgG subclasses. By indirect immunofluorescence assay and cytotoxic reaction it was shown that blocking antibodies were specific for newborn larvae and could not be adsorbed with muscle larvae. It is concluded that the synthesis of anti-newborn larva antibodies is modulated in the course of a chronic infection: early antibodies developing shortly after infection are cytotoxic, whereas blocking antibodies predominate in the late population. Furthermore, the results suggest that during a chronic infection, resistance to reinfection may be modified.