Measurement of immunoglobulin binding to synovial fibroblast monolayers: comparison of staphylococcal protein A binding to cytotoxic assay methods.

We have studied the binding of rabbit antibody to cultured synovial fibroblasts by 3 methods: complement mediated cell lysis, 125I-labeled staphylococcal protein A ([125I]PrA) and antibody dependent cell mediated cytotoxicity (ADCC). The relative sensitivities of these 3 methods is 1 : 33 : 3300 respectively. The [125I]PrA binding method is shown to quantitatively detect cell bound IgG with a molar stoichiometry of approximately 1 : 1, protein A (PrA) to IgG if PrA is present in excess.