Criteria of cell killing in vitro.

The attachment of cells to plastic tissue culture plates is a widely used criterion of cell viability in microcytotoxicity assays. When cell suspensions of primary human colon carcinoma and melanoma cells which were of low initial viability (assessed by trypan blue exclusion) were allowed to adhere to tissue culture plates, many of the adhering cells did not satisfy other criteria of cell viability. They did not stain with fluorescein diacetate but did stain with propidium iodide and fluorescein-labelled antinuclear antibodies. Using complement-mediated cytotoxicity, relatively weak activity was inconsistently demonstrated against these cells. On the other hand, strong activity was always demonstrated against highly viable cells from one colon carcinoma and two melanoma cell lines. Immunologically mediated damage to these cells was demonstrated readily by loss of the ability to stain with fluorescein diacetate, but not by cell detachment.