Donor risk factors for white blood cell antibodies associated with transfusion‐associated acute lung injury: REDS‐II Leukocyte Antibody Prevalence Study (LAPS)

The emergence of transfusion-associated acute lung injury (TRALI) as a leading cause of transfusion-associated morbidity and mortality has prompted the transfusion community to take active measures to reduce the risk of this complication. In this issue of TRANSFUSION, Eder and colleagues1 report that 70 to 75 percent of fatal TRALI cases recently reported to the American Red Cross are associated with female donors who have white blood cell (WBC) antibodies. The UK Serious Hazards of Transfusion data recently presented at the October 2006 AABB Annual Meeting in Miami, Florida (L. Williamson), showed that diversion of female plasma, starting in October 2003, has resulted in a major decline in reported TRALI cases and TRALI related mortality. On the heels of this information, the AABB issued a TRALI Association Bulletin on November 3, 2006, based on the recommendations of a recently convened AABB TRALI working group.2 The bulletin had three major recommendations: Blood-collecting facilities should implement interventions to minimize the preparation of high-plasma-volume components from donors known to be WBC-alloimmunized or at increased risk of WBC alloimmunization. Blood transfusion facilities should work toward implementing appropriate evidence-based hemotherapy practices to minimize unnecessary transfusion. Blood collection and transfusion facilities should monitor the incidence of reported TRALI and TRALI-related mortality. The recommendations were accompanied by a time frame for implementation: complete full implementation for plasma components and whole blood by November 2007, and apheresis platelet (PLT) components as soon as possible, but no later then November 2008. No specific interventions were recommended for lower-plasma-volume components, which include whole-blood PLTs, red cells (RBCs), or cryoprecipitate. Also, no specific preventive measures were proposed for TRALI caused by nonimmune mechanisms. Diversion of plasma from women has already begun at several US blood centers. This strategy for minimizing TRALI from plasma transfusions is manageable from the logistic and financial perspective for group A and O female donors, but may be problematic for group B or AB donors. In some regions of the country, there may not be sufficient male donors to meet the demand for these components. A diversion strategy is even more problematic for female apheresis PLT donors because both male and female donors are needed to meet current apheresis PLT demands. Clearly, other strategies for minimizing TRALI are necessary. Two likely possibilities are screening apheresis donors at risk for alloimmunization by history (of pregnancy or transfusion) and testing for WBC antibodies. There are a number of challenges in implementing these strategies.3 First, donors may not recount important immunizing events such as previous transfusions or miscarriages. Second, 15 to 25 percent of female donors are expected to have HLA antibodies from previous pregnancies,4, 5 and thus a substantial fraction of female apheresis donors could be lost. The impact on apheresis PLT supplies will likely be substantial and not easily replaceable. Third, the analytic methods used for measuring and interpreting HLA antibodies, such as flow cytometry, need to be validated in a high-throughput setting. For neutrophil antibodies, additional difficulties include inadequate standardization of method6 and labor-intensive manual assays that employ fresh human granulocytes, although newer methods with cloned antigens are being developed. Fourth, the prevalence of HLA antibodies in donors is not well defined since previous studies of the prevalence of HLA antibodies in blood donors with a history of pregnancy or transfusion or neutrophil antibodies in transfused and untransfused male donors did not use sensitive, state-of-the-art testing, or were hampered by insufficient sample sizes.4, 5, 7 Fifth, characteristics of HLA antibodies in donors with respect to specificity or titer using currently available sensitive tests are not known. For these reasons, a Leukocyte Antibody Prevalence Study (LAPS) is currently being conducted by the National Heart, Lung, and Blood Institute's (NHLBI) Retrovirus Epidemiology Donor Study-II (REDS-II) program. Contracts for REDS-II were awarded in August 2004 to six blood centers, Westat (Rockville, Maryland) as thecoordinating center and Blood Systems Research Institute as the REDS-II Central Laboratory. The blood centers include the American Red Cross New England Region, Dedham, Massachusetts; the American Red Cross Southern Region, Atlanta, Georgia; the Blood Center of Wisconsin, Milwaukee, Wisconsin; the Hoxworth Blood Center/University of Cincinnati Medical Center, Cincinnati, Ohio; the Institute for Transfusion Medicine, Pittsburgh, Pennsylvania; and the Blood Centers of the Pacific, San Francisco, California. The LAPS is a cross-sectional multicenter study designed to measure the prevalence of HLA and neutrophil antibodies in blood donors with or without a history of blood transfusion or pregnancy. Over a 6-month period, 7900 adult whole-blood and apheresis donors from the six REDS-II blood centers will be enrolled in the study. Donors will be asked to complete a short questionnaire relating to their transfusion history and, for female donors, their pregnancy history. Women who have been pregnant will be asked five additional questions about the number, outcome, and date of the last pregnancy. Male donors will be asked if they have ever had a transfusion and, if so, will be asked for the dates of prior transfusion episodes. Each donor will also be asked to provide a sample of blood to be tested for the presence, specificity, and titer of HLA Class I and Class II as well as certain endothelial cell antibodies using flow cytometry techniques (One Lambda, Inc., Canoga Park, CA). These data will be used to evaluate variations in HLA antibody prevalence and characteristics based on blood transfusion and pregnancy history and time since the last immunizing event (transfusion or pregnancy). This study will have sufficient power to establish with confidence the relationship between number of pregnancies and HLA antibody prevalence in US blood donors. Neutrophil-specific antibodies will be measured in all blood donors who have HLA antibodies because this group represents donors who are immune responders and also in a sample of donors without HLA antibodies. Specimens from donors found to have neutrophil antibodies will undergo further testing to determine their neutrophil phenotype using routine serologic and DNA methods, because individuals homozygous for certain neutrophil antigens are more prone to develop certain neutrophil antibodies. LAPS will also develop a repository of blood samples from these well-characterized blood donors for future evaluations of new HLA antibody screening assays and to better characterize the prevalence and types of neutrophil antibodies in the blood donor population. The repository will also include cellular blood samples from LAPS donors that will be available for studies designed to understand host genetic and immunologic determinants of alloimmunization. The data provided by LAPS will serve as one basis for calculating donor loss in the event that a TRALI prevention strategy is implemented and for answering questions such as: 1) is pregnancy history a reasonable surrogate for WBC antibody testing and 2) how much value is provided by HNA antibody testing over and above HLA antibody testing in apheresis donors? Such data are critical because any policy to restrict apheresis PLT donations could seriously impact the PLT supply. It is also desirable to determine the clinical significance of transfused WBC antibodies in recipients of blood components from these immunized donors. A lookback study of previous donations by LAPS donors is under consideration by the REDS-II study group. Alternative TRALI hypotheses, such as the role of monocyte or endothelial cell antibodies, can also be probed by testing the repository samples in conjunction with lookback of recipients. Thus, LAPS and the blood sample repository of the LAPS donors are expected to provide some key data in developing practical strategies for prevention of immune-mediated TRALI. Given the efforts under way to divert female plasma donations, this study will present a unique opportunity to study the impact of alloimmunization in transfusion recipients and perhaps supply scientific justification for applying the precautionary principle to control the risk of TRALI.