A Quantitative Radioisotopic Assay for Hemolytic Antibody by Employing Technetium-99m-Labeled Erythrocytes

Abstract

Abstract We have developed a quantitative radioisotopic assay for hemolytic antibody with erythrocytes prelabeled with technetium-99m (99mTc), a high specific activity metastable gamma emitter. There was a linear relationship between the amount of 99mTc initially used for labeling and the amount of radioactivity taken up by and subsequently released from erythrocytes which were subjected to complete hemolysis. Hemolytic antibody titrations were carried out with sheep or mouse erythrocytes which had been labeled with varying amounts of either 99mTc or 51Cr and it was concluded that 1 mCi of 99mTc or 100 µCi of 51Cr was sufficient to label adequately the target cells. The same mean log2 endpoint titers were detected in parallel titrations with either mouse anti-sheep RBC serum or an H-2 alloantiserum although the absolute gamma counts of released radioactivity were up to 15 times greater when 99mTc erythrocytes were employed. In contrast to these data on the titration of hemolytic antibodies, it was not possible to use 99mTc-labeled murine thymocytes or lymphocytes as target cells in assays for cytolytic activity of high titer rabbit anti-mouse lymphocyte sera or an H-2 alloantiserum. The principal advantages of 99mTc compared to 51Cr are its high specific activity, low spontaneous release, and non-reutilizability. From our studies it can be concluded that 99mTc can be successfully employed for the quantitative determination of hemolytic antibodies and that its favorable physical characteristics and ready availability suggest that it may replace 51Cr as a label of choice for erythrocytes.