A Radioactive Antigen Microprecipitin (RAMP) Assay for Schistosomiasis

Abstract

An assay for measuring antigen binding by antibody in schistosomiasis was developed using a fraction of Schistosoma mansoni antigen labeled with radioactive iodine. This assay which is not de- signed for the routine serological diagnosis of schistosomiasis was designated radioactive antigen micro- precipitin (RAMP). It is highly reproducible, quantitative, and may serve as an indicator of the immune response in schistosomiasis. Serum samples from patients with proven schistosomiasis and other diseases were assayed for their ability to precipitate 125I-labeled S. mansoni antigen. Eighty-one per cent of 104 sera from schistosomiasis patients were reactive in the RAMP assay; no reactions were obtained with 90 sera from healthy persons. A few nonspecific reactions were obtained in other disease states. The RAMP assay and the passive cutaneous anaphylaxis (PCA) tests were compared using 85 serum specimens from infected schistosomiasis patients reactive in the soluble antigen fluorescent antibody (SAFA) technique. The RAMP assay and the PCA test were both reactive with 48 of these specimens. Three specimens in which reaginic antibodies had been detected failed to react in the RAMP assay and 17 sera which reacted in the RAMP assay gave negative results in the PCA test. Fractions of pooled human anti-S. mansoni sera were tested in the RAMP assay and the results were compared with those obtained with the same frac- tions in the SAFA and PCA tests. The first fraction from DEAE-A-25 contained the major amounts of IgG and IgA immunoglobulins and all of the detectable SAFA reaction, while fraction 5 contained RAMP and PCA activity but no detectable SAFA antibodies. Heating or reduction and alkylation of the immune serum destroyed or markedly reduced the antibody reactions in the RAMP assay and PCA test, but did not affect the reactivity of the SAFA technique. When rabbit S. mansoni antiserum was absorbed with goat antirabbit IgE serum, the reactivity of the serum was greatly reduced in the RAMP assay, elimi- nated in the PCA test, and was unchanged in the SAFA technique. The RAMP assay and PCA test cor- related closely in demonstrating the time-course development of antibodies in animals infected with schis- tosomiasis, but the antibodies detected by the SAFA technique followed an entirely different time course. These studies indicate that the RAMP assay can reliably measure antibodies in all immunoglobulin classes and particularly in IgE, and this assay is a means of demonstrating primary binding of antigen by antibody in schistosomiasis.