The Post-Transfusion Recovery and Survival of Red Blood Cells in Mice Is Affected by the Expression of Cellular Prion Protein

Three documented transfusion cases of vCJD underline the need of better insight in blood prion protein biology. Cellular prion protein (PrPc) plays key role in the pathophysiology of prion diseases. Its expression by cells is necessary for amplification of infectious prions and the disease process itself. Physiological function of PrPc remains obscure. Its clarification may provide important clues for the development of urgently needed blood test and effective disease treatment. PrPc is expressed on CD34+ hematopoietic stem cells and its expression is regulated during blood cell differentiation. Recently the importance of PrPc for self-renewal of long-term repopulating hematopoietic stem cells was suggested and other studies reported the protective function of PrPc against oxidative stress and apoptosis in various cell cultures. We previously demonstrated that human as well as mouse red blood cells (RBC) express approximately 200 PrPc molecules / cell (Holada et al., BJH 2000, 110, 472-80). To test if the PrPc expression plays a role in the post-transfusion recovery and survival of RBC we carried out transfusion study in mice. RBC isolated from blood of wild type (WT) and PrP knockout (KO) FVB mice were labeled in vitro by different levels of NHS-biotin. The labeling was optimized to allow simultaneous detection of both populations of RBC in mouse blood using flow cytometry. To exclude the influence of different level of cell biotinylation on the experiment outcome two mixtures of RBC were prepared. The first contained KO RBC labeled with high and WT RBC with low level of biotin and the second mixture contained cells labeled vice versa. Each mixture was injected via tail vein in a group of WT mice (n=5) and the survival of RBCs was followed. Samples were analyzed on day 1, 2, 3, 6, 9, 15, 21 and 29. The count of biotinylated RBC was measured in comparison to 100 000 nonlabeled recipient RBC. Simultaneously the expression of PrPc on RBC was monitored using flow cytometry with MAb 6H4. KO RBC displayed significantly higher first day post-transfusion recovery compared to WT RBC in both groups of mice (81±3 % vs. 74± 3 %, P<0.005 and 90±4 % vs. 80±4 %, P<0.005). The slope of the RBC survival curve in all individual mice during the initial 15 days was steeper for KO RBC (m_avg= - 3.44) than for WT RBC (m_avg = - 2.37) suggesting the protective role of PrPc in circulating RBC. The difference in the slope diminished during the 15 to 29 day period which was accompanied by a 50% decrease of PrPc surface expression on transfused WT RBC. To confirm our data the identical experiment was carried out in a group of KO mice (n=5) transfused with a mixture containing KO RBC labeled with low and WT RBC with high level of biotin. Again the first day post-transfusion recovery was higher for KO RBC (80±6 % vs. 75±6 %, P<0.05) and the initial slope of the KO RBC survival curve was steeper in all mice in the group. Our data suggest that PrPc expression plays role in the post-transfusion recovery and survival of RBC. The observation that WT RBC disappear from the circulation at lower rate than KO RBC until their level of surface PrPc reaches 50% is compatible with the protective role of PrPc expression on cells. Taken together our study demonstrates that physiological role of PrPc expression on RBC may lay in facilitating their longer survival in circulation. (GACR 310/04/0419, MSMT 0021620806).