Application of a radiometric ear assay for studies of adjuvant arthritis in rats

A radioisotopic method, originally developed for measuring the cellular response in delayed hypersensitivity lesions in mice, has been evaluated in adjuvant arthritic rats. Focal accumulation of 5-iodo-2'-deoxyuridine-125I (125IUdR) at a site of antigen challenge (left pinna) was measured and expressed as increased radioactivity in the challenged (left) over the unchallenged (right) ear (L/R ear ratio). Immunologic specificity of the assay was established with anti-lymphocyte globulin (ALG)-treated arthritic rats. ALG significantly inhibited the 125IUdR L/R ear ratio; normal rabbit globulin had no effect on this parameter. A significant negative correlation was observed between the 125IUdR ear ratios and subjective arthritic scores in established adjuvant disease. Certain characteristics of the 125IUdR radiometric ear assay in rats were established in several types of experiments: disappearance of the isotope from blood, whole body irradiation studies in turpentine-injected rats, and cyclophosphamide pretreatment in a sheep erythrocyte antigenic system. The results of this study support the utility of the 125IUdR ear assay to quantify cellular accumulation at a site of antigen challenge in adjuvant arthritic rats and possibly other antigenic systems in this species.