Expression and function of CD95 (FAS/APO-1) in leukaemia-lymphoma tumour lines

Cross-linkage of the CD95 (FAS/APO-1) antigen is responsible for the induction of programmed cell death or apoptosis in a variety of normal and malignant cells of the haemopoietic system. In order to evaluate predominant expression of the CD95 gene in a cell lineage-specific manner, we have determined the CD95 expression patterns in cell lines of myeloid, T-, pre-B- or B-cell origin as well as those established from Hodgkin's disease (HD). Our results reveal constitutive transcriptional activation of the CD95 gene in all cell lines derived from the lymphoid and myeloid lineages. Despite the ubiquitous expression of CD95 transcripts in haemopoietic cells, the corresponding protein was undetectable in 2/5 cell lines derived from Burkitt lymphomas and 6/16 leukaemia cell lines of the megakaryocytic or monocytic lineage. In an effort to identify apoptosis-resistant cell lines resulting from mutations in the death-signalling domain of CD9 5 or from defects in the apoptotic pathway or in survival programmes, we applied a CD95-mediated apoptosis assay. However, 21/38 CD95-expressing cell lines were sensitive upon induction with an anti-CD95 antibody whereas the remaining cell lines (predominantly of myeloid derivation) were resistant to antibody-induced cell death. Resistance to CD95-mediated apoptosis was not due to mutations within the CD95 open reading frame as confirmed by a combined reverse transcription PCR sequencing method. Five myeloid out of 13 tumour lines with the apoptosis-resistance phenotype analysed showed programmed cell death, when protein synthesis was blocked by treatment with cycloheximide prior to CD95-mediated induction. These data suggest an active cellular mechanism for the maintenance of an apoptosis-resistant phenotype. Elucidating the steps in such an active process of resistance to apoptosis might be expected to provide new approaches for therapeutic intervention in certain tumours.