Isotope-Release Cytotoxicity Assay With the Use of Indium-111: Advantage Over Chromium-51 in Long-Term Assays2

Abstract

The adaptation of indium-111-oxine (also known as 8-hydroxyquinoline) (¹¹¹InOx) chelate for long-term (18–48 hr) isotope-release assays of cell-mediated cytotoxicity (CMC) and its advantages over the use of⁵¹Cr are described. Labeling of DBA/2 P815 mastocytoma cells with¹¹¹InOx resulted in the incorporation of as many as a million counts per minute in 10⁵ cells with no reduction in cell viability.¹¹¹InOx labeled both mouse and human tumor cells.¹¹¹InOx, like⁵¹Cr, primarily labeled cytoplasmic constituents; up to 80% of the label existed in a releasable form.¹¹¹InOx was quantitatively released from labeled P815 in response to specifically sensitized C57BL/6 lymphocytes. The high labeling efficiency of¹¹¹InOx offered a significant advantage over⁵¹Cr in 18-to 48-hour assays for CMC by reducing the counting error and thus making the assay more precise. Because of its higher labeling efficiency,¹¹¹ InOx can be used in microcytotoxicity assays.¹¹¹InOx has the added advantage of a lower spontaneous release in culture than⁵¹Cr. This feature of¹¹¹InOx also makes the calculation of specific isotope release more accurate than that achieved with⁵¹Cr in long-term cytotoxic assays.