Inhibition of antibody-binding as a radioimmunoassay for Plasmodium berghei infection in rats.

Sonicated red blood cells of rats infected with Plasmodium berghei (Pb) were used to coat plastic tubes, which were subsequently tested for capacity to bind anti-P, berghei antibodies. Binding was detected using radioiodinated Staphylococcus Protein A. Two antigenic preparations were used to make the solid-phase adsorbent (and also as inhibitors in the inhibition tests): sonicated red blood cells (SIRC), and a supernate of frozen-thawed, infected red blood cells (FTE). Treatment of the antigen-coated tubes with sera of immunized rats or by rabbits resulted in binding of antibodies to the tubes. The specificity of the binding was confirmed by inhibition tests. Preparations derived from infected blood but not from normal blood inhibited the binding of the anti-Pb antibodies. Using the inhibition tests, an infection corresponding to 0.00005% parasitemia could be detected. The sensitivity was higher when SIRC rather than FTE was used as antigen. The sensitivity of the inhibition test for the detection of parasites in the blood greatly surpassed that of the standard microscopical procedure. Parasites were demonstrated in blood samples 2 days after infection, 5 days before a microscopic diagnosis was positive. Repeated assays conducted with the same batch of reagents showed that the test was remarkable reproducible.