A modification of the JAM test is necessary for a correct determination of apoptosis induced by FasL⁺ adherent tumor cells

Tumor cells from several organs including colon have recently been shown to express Fas ligand (FasL) in vitro and in vivo. The expression, which in some tumours occurs de novo, was suggested to facilitate immune escape of malignant cells by killing tumor-infiltrating lymphocytes via Fas-FasL- induced apoptosis. An argument to support this hypothesis is the detection of tumor cell-induced apoptosis in Jurkat cells (as model T cells) by means of the widely used JAM test. In the present work the validity of this test for the analysis of colon carcinoma cell-mediated apoptosis in Jurkat cells was scrutinized in detail. The presented data show that the JAM test as described previously is prone to false-positive detection of apoptosis, when adherent epithelial cells are used as effectors. Furthermore, three lines of evidence indicated that several FasL⁺ colon carcinoma cell lines did not induce detectable apoptosis in Jurkat cells in vitro. We conclude that: (1) The JAM test must be modified for testing DNA fragmentation induced through adherent effector cells and (2) FasL⁺ colon carcinoma cells may be unable to induce apoptosis in vitro.