Distribution of cell-associated prion protein in normal adult blood determined by flow cytometry.

Leucocyte subpopulations from normally healthy individuals were identified by recognized combinations of fluorochrome-conjugated antibodies to CD markers and stained by different monoclonal antibodies (MAb) to normal cellular prion protein (PrPC), including the 3F4 MAb. Cell preparations were examined by three-colour flow cytometry. All mononuclear leucocyte subpopulations and platelets expressed PrPC, but polymorphonuclear leucocytes and red blood cells expressed little or no PrPC. The amounts of PrPC expressed by the different cells were calculated by comparison to bead standards. Mononuclear leucocytes expressed 3000-4000 molecules of antibody-reactive PrPC per cell. Resting platelets expressed around 1400 molecules of PrPC per cell, whereas activated platelets expressed around 4800 molecules of PrPC per cell. Extrapolation of these values to the amounts of the various cells in whole blood showed that platelet PrPC accounted for at least 96% of cell-expressed PrPC in blood. The PrPC on mononuclear cells and platelets was sensitive to enzymatic treatment of cells by proteinase k and phosphatidylinositol-specific phospholipase C. Certain anti-PrPC MAbs which showed equivalent intensity of staining to MAb 3F4 on fresh cells showed relative reductions of staining compared to MAb 3F4 on stored cells, indicating possible structural alterations of PrPC under these conditions.