Ki‐1 (CD30) antigen is released by Ki‐1‐positive tumor cells in vitro and in vivo. I. Partial characterization of soluble Ki‐1 antigen and detection of the antigen in cell culture supernatants and in serum by an enzyme‐linked immunosorbent assay

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Abstract
An enzyme‐linked immunosorbent assay (ELISA) has been developed that allows the quantitative determination of the Ki‐1 (CD30) antigen in soluble form. Similar levels of sensitivity of this new Ki‐1 ELISA and the ELISA previously described for measuring the soluble 55‐kDa chain of the interleukin 2 receptor were seen. As assessed with this ELISA, the investigated Ki‐1 permanent cell lines released the Ki‐1 antigen into the culture supernatant. In culture supernatants of concanavalin A‐activated human peripheral blood lymphocytes, however, this antigen could not be detected. The released Ki‐1 antigen has an apparent molecular weight (Mr) of 85000, whereas the cell‐associated Ki‐1 antigen has an Mr of 105000. We investigated sera from 30 normal donors, 15 patients with systemic infections, and 63 patients suffering from lymphomas for soluble Ki‐1 antigen. In all sera from normal donors and patients with systemic infectious diseases, soluble Ki‐1 antigen was below the detection limit (i.e., < 70 pg). In contrast, high amounts of the soluble Ki‐1 antigen were found in sera from 18 malignant lymphomas containing Ki‐1 tumor cells. This finding demonstrates that the release of the Ki‐1 antigen takes place not only in vitro, but in vivo as well. Moreover, these results imply that the Ki‐1 antigen may be used as a serum tumor marker.