Apoptosis in leucodepleted packed red blood cells

Packed red blood cells (pRBCs) contain apoptotic white cells. We studied apoptotic cells in pRBCs after filtration and at various time-points during storage.

To maintain the same subset of cells, seven pRBC units were pooled in a single bag and divided equally into seven aliquots. Two series of five experiments were performed: in the first we utilized the Biofil R01 Max filter, and in the second the Pall BPF4 filter was used. One aliquot was immediately leucodepleted while the others were stored at 4 degrees C and filtered on days 3, 7, 10, 14, 21 and 42 of storage. The postfiltration leucocyte counts and apoptotic evaluations were performed by using the Nageotte chamber and flow cytometry.

The absolute number of residual leucocytes was always less than 0.5 x 106 in each experiment. Nageotte chamber counts showed a greater number of white blood cells than flow cytometry during the 42 days of storage. On day 0, the percentage of apoptotic cells in non-leucodepleted pRBCs was 1.1 +/- 0.4 and 1.2 +/- 0.4, while in filtered pRBCs it was high from day 0, at 53.5 +/- 16.3 and 52 +/- 18.5, respectively, with Biofil and Pall filters. On day 10 of storage, apoptotic cells reached a percentage of 42.5 +/- 15.8 and 41.6 +/- 18.6 in non-leucodepleted pRBCs, while in filtered units an average value of approximately 90% was found with both filters.

The percentage of apoptotic cells was higher in leucodepleted than in non-leucodepleted pRBCs. After filtration, the degree of apoptosis was already high on day 0, and reached a mean of approximately 90% by day 10. The difference in residual WBC counts between the Nageotte chamber and flow cytometry could be related to the presence of a high percentage of apoptotic cells in filtered blood components, and to the method used to distinguish viable from apoptotic cells.