Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-associated mortality in the United States and other countries. In most TRALI cases, human leukocyte antigen (HLA) class II antibodies are detected in implicated donors. However, the corresponding antigens are not present on the cellular key players in TRALI: neutrophils and endothelium. In this study, we identify monocytes as a primary target in HLA class II-induced TRALI. Monocytes become activated when incubated with matched HLA class II antibodies and are capable of activating neutrophils, which, in turn, can induce disturbance of an endothelial barrier. In an ex vivo rodent model, HLA class II antibody-dependent monocyte activation leads to severe pulmonary edema in a relevant period of time, whenever neutrophils are present and the endothelium is preactivated. Our data suggest that in most TRALI cases, monocytes are cellular key players, because HLA class II antibodies induce TRALI by a reaction cascade initiated by monocyte activation. Furthermore, our data support the previous assumption that TRALI pathogenesis follows a threshold model. Having identified the biologic mechanism of HLA class II antibodyinduced TRALI, strategies to avoid plasma from immunized donors, such as women with a history of pregnancy, appear to be justified preventive measures. (Blood. 2011;117(2):669-677) IntroductionTransfusion-related acute lung injury (TRALI) is the leading cause of transfusion-associated death in the United States and other countries. 1 It typically presents within 6 hours after transfusion as a clinical syndrome characterized by acute respiratory distress, hypoxemia, and a bilateral pulmonary edema on chest x-ray. 2 The incidence of TRALI has been estimated as 1/5000 for all blood components, and current mortality rates are in the range of 5%-25%. 3 All blood components have been implicated in TRALI, but those containing large amounts of plasma are mainly responsible. [3][4][5] A recently published literature review states that in 80% of all TRALI cases, white blood cell antibodies can be identified in the implicated blood donor, 6 and most implicated donors have been women with a history of pregnancy. Pregnancy results in alloimmunization against paternal white blood cell antigens in 21%-24% of women, 7,8 with 3 main antibody specificities: human leukocyte antigen (HLA) class I, HLA class II, and human neutrophil antigens (HNAs).The capability of antibodies recognizing either HNA 9-11 or HLA class I 12,13 to precipitate TRALI has been proved in different animal models. It is consensus opinion that neutrophil activation is the key mechanism by which antibodies to HNA and to HLA class I mediate TRALI, 4,14 be it by direct binding to the cognate antigen on the surface of the neutrophil [9][10][11]13 or, in the case of HLA class I antibodies, possibly by binding to HLA class I molecules on pulmonary endothelial cells, which leads to neutrophil trapping by the neutrophils' Fc receptors and subsequent neutrophil activation by receptor...

The aim of this study was to investigate which approach for serological testing of multiparous donors might be feasible and effective to reduce the risk of transfusion-related acute lung injury (TRALI). TRALI is a serious adverse event of blood transfusion. Antibodies to granulocytes and human leucocyte antigens (HLAs) are frequently detected in sera of implicated donors. These donors are often multiparous women. A general deferral of female plasma or screening strategies for leucocyte antibodies has been proposed to increase blood safety. A prospective study was initiated in 2003. Until 2006, serum samples from all female donors reporting three or more pregnancies (n = 229) were screened for the presence of antibodies against granulocytes and HLAs by immunofluorescence and agglutination tests as well as by a commercial HLA enzyme immunoassay. In total, 40% of all multiparous women were reactive in one of the assays. Twenty-nine percent of the reactive sera contained antibodies to granulocytes but not to HLAs. During the observation period, three TRALI reactions occurred in our hospital, two of which would have been prevented if the screening program had been extended to all previously pregnant donors. We conclude from these data that, not unexpectedly, the number of previous pregnancies is not a reliable indicator for the likelihood of inducing TRALI. More importantly, screening strategies for antibodies that might induce TRALI should probably not be reduced to HLA antibody screening. This finding awaits further research.

19 Introduction: Transfusion-related acute lung injury (TRALI) is the leading cause of transfusion-associated fatalities in developed countries. It has been demonstrated that antibodies to human neutrophil antigens (HNA) present in a blood product can precipitate a TRALI reaction in the blood recipient. In the majority of TRALI cases, however, HLA class II antibodies rather than HNA antibodies are detected in the implicated donor. HLA class II antigens are not present on neutrophils or endothelial cells, the cellular key players in TRALI pathophysiology. Whether or not HLA class II antibodies are capable of inducing TRALI is thus under discussion. Here, we investigated a possible mechanism of HLA class II antibody-induced TRALI via monocytes. Methods and Results: Plasma from blood donors containing HLA class II antibodies (H2+plasma) is capable of activating monocytes expressing cognate antigen(s) as demonstrated by the release of IL-8, GRO-alpha, and TNF-alpha in culture supernatants. H2+plasma does not activate monocytes that do not express the cognate HLA class II antigen(s). Culture supernatants from matched pairs, but not H2+plasma itself, induce a respiratory burst reaction in neutrophils. In Boyden chamber experiments, FITC-albumin transendothelial flux is significantly increased in the presence of H2+plasma, matched monocytes, and neutrophils, but unchanged in the presence of non-matched monocytes. Diphenyleneiodonium chloride inhibits this barrier breakdown. In the ex vivo rat lung model, supernatants from matched pairs, but not from non-matched pairs, are capable of inducing TRALI in the presence, but not in the absence, of neutrophils. H2+plasma precipitates a TRALI in this model if neutrophils and matched, but not non-matched, monocytes are present. IgG-depletion of H2+plasma abolishes this effect. Conclusions: HLA class II antibodies present in donor plasma can induce TRALI in a transfusion recipient via monocyte activation. Once activated, monocytes stimulate neutrophils to produce reactive oxygen species, which lead to an increase in endothelial permeability and subsequent lung edema. This cascade depends on a match between the donor's antibodies and the recipient's HLA class II antigens and can be reproduced in a rat lung model. Our findings have important impact on current strategies that aim to prevent TRALI by donor testing and/or exclusion. Disclosures: No relevant conflicts of interest to declare.