Dr. Slichter’s contributions to the field of transfusion medicine over 45 years have revolutionized platelet transfusion therapy. She has identified methods to significantly prolong the shelf life of platelets, determined approaches to prevent alloimmune platelet refractoriness, identified the minimum number of platelets required to maintain hemostasis, and provided insights into understanding platelet production and viability. These long-term studies have established and improved state of the art practice in transfusion medicine.
After earning her medical degree from George Washington University in Washington, DC, Dr. Slichter completed her residency at Parkland Hospital in Dallas. She went on to complete her fellowship in hematology and oncology at the University of Washington in 1968, and performed her extensive platelet research at Bloodworks Northwest (formerly Puget Sound Blood Center). Her lifelong determination, dedication, and major contributions to hematology continue to revolutionize the field.
Determining the Appropriate Dose of Prophylactic Platelets
Bloodworks Northwest is one of 17 trial sites across the U.S. that are part of the recently established Clinical Trials Network in Transfusion Medicine/Hemostasis funded by the National Institutes of Health. A prophylactic platelet dosage study in cancer patients, proposed by Dr. Slichter, has been selected as the first clinical trial to be initiated by the Network. In this study, cancer patients are randomly assigned to receive all their platelet transfusions at a low dose, a medium dose, or a high dose. The goals of this trial are to evaluate bleeding risk based on transfused platelet dose and how many total platelets are transfused in each study group. The primary objective is to determine the lowest dose of platelets that prevents bleeding. Although enrollment in the study has started, it will probably be another two years before enrollment is completed and the results of the study have been analyzed.
Once the minimal effective dose of prophylactic platelets that prevents bleeding is determined, this will be incorporated into patient care. We expect that the ongoing clinical trial will show that we can substantially reduce the number of platelets needed by cancer patients. This will not only decrease the need for platelet donors but should, more importantly, substantially reduce patient care costs.
Extending the Storage Time of Platelets
As platelets can currently only be stored for five days, as opposed to the 42 days possible for red cells, the need to collect platelets drives the donor collection system. Dr. Slichter’s lab is actively pursuing techniques to extend the storage time of platelets. Using a physiologic solution instead of residual plasma to store the platelets, the lab’s preliminary data suggests that platelets may be stored for up to 13 days. These studies have been done by storing a normal donor’s own platelets, tagging their platelets with a radioactive substance after storage, and transfusing the donor with their radioactively labeled platelets. By drawing serial blood samples from the donor after transfusion, the lab can determine the recovery and survival of the donor’s stored platelets by measuring the residual amount of circulating platelet radioactivity. Future studies will involve transfusing the extended stored platelets into patients with low platelet counts to determine their ability to circulate and prevent or control bleeding.
Having a longer shelf life for platelets will significantly reduce the current outdate rate for platelets as well as decrease the lab’s need for donors. It will also mean that the lab will be able to maintain a much larger inventory of platelets, thereby having a continuous supply of platelets readily available to meet patient needs.
Preventing Transfused Donor Platelets from Being Rejected
Red blood cell transfusions usually require matching for only two blood group systems between donors and recipients (ABO & Rh) to prevent the rejection of donor red cells. Unfortunately, platelets require matching for the very complex histocompatibility system that must also be matched between organ donors and recipients (such as stem cell or kidney transplants) to prevent graft rejection. Platelets are transfused from random donors until the patient shows signs of rejecting these platelets as evidenced by no increase in the patient’s post-transfusion platelet count. If rejection of random platelets occurs, platelets must be obtained from histocompatible matched donors by apheresis procedures to obtain enough platelets to constitute a transfusion dose.
The Blood Center maintains a registry of approximately 21,500 histocompatible typed donors to have a chance of finding one or more matched donors who will be able to provide compatible platelets for the genetically and racially diverse patient populations who require platelet transfusions. These platelet apheresis registry donors have agreed to be typed for histocompatibility markers and are on-call to provide platelets for patients who are rejecting platelets from random donors.
Using an animal model of transfusing platelets from random donors, Dr. Slichter’s lab has determined it is the white cells that contaminate the transfused platelets that signal the transfused recipient’s immune system that donor platelets are being transfused. These contaminating white cells activate the recipient’s immune system causing the rejection of the platelets from random donors. Unfortunately, the centrifugation procedure used to separate platelets from the other blood cells in a unit of whole blood or during an apheresis procedure does not completely remove all of the white cells.
In the lab’s animal transfusion model, preliminary data suggests that combining two different procedures may effectively prevent immune recognition of donor platelets. The first is to remove a large number of white cells by using a filter designed to allow the platelets to pass through, but not the white cells. If the filtered platelets are then gamma irradiated, this inactivates the residual white cells that the filter has not removed. Filtration and gamma irradiation procedures are both used routinely in blood centers. If it can be shown that combining these procedures is effective in preventing platelet rejection, these processes can easily be transferred to clinical application. However, additional animal studies are required to confirm these potentially very significant observations. Following the animal studies, transfusion studies in patients with low platelet counts will be used to determine the effectiveness of these approaches in patients.
Between 20 to 30 percent of chronically transfused cancer patients reject platelets from random donors. For these patients, it requires a substantial time commitment from platelet apheresis donors to support their platelet needs. For some patients, matched platelets are not available, which increases their bleeding risk. Therefore, preventing a transfused recipient’s immune system from recognizing donor platelets as foreign will be a significant benefit to both patients and platelet donors.
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