THE HLA CLASS I DEFICIENT BURKITT LYMPHOMA CELL LINE DAUDI FAILS TO EXPRESS GPI-ANCHORED CELL SURFACE PROTEINS

Abstract

267 The Burkitt lymphoma-derived Daudi B cell line is the best known HLA class I deficient human cell line and has extensively been used as a target in cell mediated cytolysis and other functional assays. During a routine screening of an extensive panel of leukemic cell lines for cell surface expression of the cellular prion protein (PrPc) we noted a complete absence of this protein on the surface of Daudi cells. It was the purpose of this study to characterize in detail the nature of this unexpected defect. We used RT-PCR to demonstrate transcripts of equal size and specificity in human brain tissue, EBV-transformed JY25 and Daudi B cells. Permeabilization with L-α-lysophosphatidyl choline of cell membranes of Daudi, JY5 B cells and CEM C7E2A T cells, respectively, permitted binding of the PrP-specific mAb 3F4 to intracellular PrPc thus demonstrating translation of transcripts into immunoreactive PrPc protein. We speculated that the lack of PrPc expression might be linked to the known β2m gene defect. However, other β2m deficient cells (human melanoma line FO-1, murine T cell line R1E/TL8x.1, splenocytes from β2m gene-deleted mice) displayed unequivocal cell surface expression of PrPc. Transfection of a functional β2m gene into Daudi cells restored HLA class I but not PrPc cell surface expression thus refuting our hypothesis. PrPc belongs to the family of GPI-anchored cell surface proteins. Therefore, we examined the expression of other GPI-linked proteins on Daudi cells. We were unable to demonstrate antibody binding to CD55 and CD59 on Daudi and control B cells with known defects in GPI anchor formation. We finally used somatic hybridization of Daudi cells with murine spleen cells to successfully demonstrate de novo cell surface coexpression of HLA class I and PrPc, CD55 and CD59, respectively. We demonstrate here a hitherto unknown defect of GPI anchor synthesis in Daudi cells. Since Daudi cells are commonly used in cellular assays experimental results must be interpreted in light of this novel finding. Finally, Daudi cells may be useful in future studies of GPI anchor synthesis.