In view of the currently available data, prevention of alloimmunization requires filters with higher efficiency to achieve a reduction in the number of leukocytes below 10 million per transfusion. Two versions (Pall PL-100 and PL-50) of the new generation leukocyte-depletion filters were studied. Single donor (SDPC)- and pooled multiple donor (MDPC) platelets were run in parallel. At a flow rate of 10 ml/min, the PL-100 filter was shown to effectively reduce the number of residual leukocytes to far below the critical immunogenic threshold of 10 million in all SDPC units and in 77% of MDPC units. Apheresis platelets appear not only to be better depleted than pooled multiple donor platelets, but also to have a better post-filtration platelet recovery (96% versus 84%). The efficiency of the smaller version of the filter (Pall-50) was higher than that of the Pall-100 filter for both single and pooled multiple donor platelet concentrates (PC). Leukocytes were absent in more than 92% of units in both types of concentrates. The maximal number of detected leukocytes was 2.2 million in a pool of 6 units. The outcome of filtration of 5-day-old pooled platelets was less favorable than filtration of 1- or 2-day-old pooled platelets, indicating that filtration soon after preparation is preferred to filtration after storage. Post-filtration platelet integrity, activation state, function, and morphology were all well preserved in both single and multiple PCs.

Platelet concentrates were pretreated with a stable synthetic prostacyclin analogue (Iloprost) at two different concentrations before the second centrifugation step (pelleting step) of preparation. This resulted in loss of platelet sensitivity to aggregating agents. To mimic the situation after transfusion and to assess the reversibility of platelet inhibition, platelets were washed during and after storage and resuspended in fresh-frozen autologous plasma. The Iloprost-treated and washed platelets exhibited an increased sensitivity to the aggregating agents, compared with the control platelets (p less than 0.01). Post-storage recovery of the synergistic aggregation was more than 80% of prestorage aggregation. Beta-thromboglobulin (beta TG) release and thromboxane B2 (TXB2) formation were significantly inhibited in Iloprost-treated platelets (p less than 0.01). After the second centrifugation step, beta TG release was 0.7% +/- 0.3%, compared with 2.7% +/- 0.9% for the controls. TXB2 was 99 +/- 91 pg/ml, compared with 495 +/- 356 pg/ml for the controls. Platelet morphology and ultrastructure were well preserved during 5-day storage. In addition, Iloprost exerted a cytoprotective effect, as evidenced by the significant reduction in lactate dehydrogenase leakage. Post-storage LDH was 378 +/- 159 and 415 +/- 239 U/l respectively by the two Iloprost concentrations, compared with 1180 +/- 937 U/l for the control platelets. The inhibitory and cytoprotective effects of Iloprost were sustained throughout storage, in contrast to the effect of PGE1 (Prostin) which was limited to the early phase of storage.

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"Leukocyte-depleted" platelet concentrates (LD-SDPC) were prepared by cotton-wool and by polyester filtration of "leukocyte-poor" PC collected by the elutriation technique. The storage characteristics of LD-SDPC were comparable to those of the filtered pools of multiple donor platelet concentrates (LD-MDPC). However, LD-SDPC were less activated and less damaged than LD-MDPC over a 7-day storage period, as evidenced by beta-thromboglobin percent release and lactic dehydrogenase percent leakage in both products. LD-SDPC were prophylactically transfused to 12 thrombocytopenic patients; the mean bleeding time was shortened from 12 min 20 sec to 4 min 39 sec. Corrected count increment (CCI) was 22.8 at 1 hr and 14.1 at 24 hr compared to 15.5 and 10.0, respectively, with standard PCs.

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