The failure of Daudi cells to express the cellular prion protein is causedby a lack of glycosyl-phosphatidylinositol anchor formation

The cellular prion protein (PrPc) is a glycosyl-phosphatidylinositol (GPI)- linked cell surface protein, which is expressed at high density on nervous tissues and at lower levels on most other solid-organ tissues. It is also e xpressed on peripheral blood mononuclear cells (PBMC) of all lineages. In l ymphocytes, its level of expression is dependent upon the state: of cell ac tivation. and polyclonal anti-PrP antisera partially block lectin-induced T -cell activation, suggesting a functional role of the protein in this proce ss. Using the monoclonal antibody (mAb) 3F4 we examined PrPc surface immuno reactivity on leukaemic cell lines of T- and B-cell origin, and unexpectedl y observed a complete lack of PrPc cell-surface expression in Daudi cells. while all other cell lines displayed discernible reactivity. We demonstrate d the intracellular presence of PrP-specific mRNA and PrP protein. The lack of surface PrPc is unrelated to the well-known defect of beta (2)-microglo bulin (beta (2)m) expression in Daudi cells as other beta (2)m-deficient ce lls, such as the melanoma cell line F0-1 and spleen cells from beta (2)m ge ne-deleted mice, were not deficient in cell-surface PrPc. Daudi cells faile d to bind antibodies directed against all GPI-linked cell surface proteins. In somatic hybridization experiments using murine spleen cells as partners , we observed tic novo expression of human PrPc. CD55 and CD59. thus demons trating in Daudi cells the availability of these gene products for GPI link age and cell-surface expression.