Leukocytes are normally present in varying numbers in units of red blood cells (RBCs) and platelet concentrates. In the last decade, a growing body of evidence has linked the leukocytes present in transfusable cellular blood components to complications such as febrile transfusion reaction, HLA alloimmunization, viral disease transmission, graft-versus-host disease (GVHD), and immune modulation. However, a variety of specially prepared blood products such as leukocyte-reduced, cytomegalovirus (CMV)-screened, and irradiated blood components are now available to prevent or reduce the adverse effects of contaminating leukocytes (1-5). Unfortunately, these specially prepared blood components are expensive, and the clinical indications for their use are not entirely defined and are still evolving (6). This review examines the benefits of, indications for, and controversies surrounding the transfusion of such components.
Leukocyte-Reduced Blood Components
The average unit of RBCs contains approximately 1-2 billion leukocytes; each unit of random-donor platelets, about 50 million leukocytes; and each unit of platelets prepared by apheresis, between 1 million and 1 billion leukocytes. Over the years, various techniques have been used to reduce the number of leukocytes in transfused blood. During the 1960s and 1970s, the principal technique for depleting leukocytes from RBC units was to remove buffy coat from blood by sedimentation or centrifugation. The technique was not very efficient, however, yielding leukocyte depletions of 60-80% but RBC losses of 20-30% (7). Another method, namely, washing RBCs, could remove 50-95% of leukocytes with a 15% loss of RBCs (8). In the 1970s, a freezing, deglycerolizing process was widely used to prepare RBCs for use in kidney transplant and dialysis patients. Even though the deglycerolized RBCs contained substantially fewer residual leukocytes (on the order of 30-50 million) than blood depleted of leukocytes by other methods available at the time, the cost and the complexity of the process prevented its being widely used solely to reduce leukocyte counts in RBC units.
Another method, still widely used today, is filtration. Since the 1960s, when the first blood filter was licensed in the United States for use in preventing febrile transfusion reaction, filter technology has significantly improved. A new generation of blood filters developed over the last few years can reduce the leukocyte content of cellular blood components by 2 to 3 logs. The reduction of leukocytes to 5 million or less in cellular blood components, a level apparently sufficient to prevent alloimmunization, is now easily achieved using commercially available filters (2).
The rationale behind depleting leukocytes is to alleviate their detrimental side effects. For instance, it is well known that leukocytes in transfused blood can cause fever and chills. The frequency of febrile transfusion reactions in transfused patients ranges from 0.5% to 5% (9). However, in patients who experience repeated febrile reactions that cannot be controlled by antipyretics, transfusion of leukocyte-reduced blood components is indicated since there is ample evidence that leukocyte depletion can reduce the frequency and severity of febrile reactions (2,6,10).
Another crucial consequence of the infusion of RBCs and platelet concentrates contaminated with leukocytes, more crucial than fever and chills, is the development of refractoriness and alloimmunization in multiply transfused patients (11-14). A recent study suggests that platelet refractoriness due to alloimmunization occurs in 30-50% of multiply transfused acute leukemia patients (14). There are currently two major approaches to preventing alloimmunization: (i) reduce leukocytes in blood components to less than 5 million or (ii) inactivate contaminating leukocytes by ultraviolet (UV) irradiation (15). The use of UV irradiation is still at the experimental stage because of technical problems and limited clinical data demonstrating its efficacy and safety. On the other hand, a number of clinical studies have demonstrated that leukocyte-reduced blood products may significantly decrease the incidence of alloimmunization. In several randomized studies, the incidence of clinical refractoriness was 11-21% for patients receiving leukocyte-reduced blood products versus 19-50% for control subjects (2). Likewise, anti-HLA antibodies were detected in 6-20% of patients receiving leukocyte-reduced components versus 13-50% of controls.
Although promising, these results leave many questions unresolved. Most studies have reported that alloimmunization was significantly reduced when leukocyte-reduced blood components were used but that a significant percentage of patients still developed refractoriness and produced alloantibodies. Thus, prevention is by no means complete using the currently available technology. Also, it remains to be seen whether further reduction of residual leukocyte levels much below the 5 million level will more effectively prevent alloimmunization. Moreover, because state-of-the-art blood filters are expensive, a program to support multiply transfused patients exclusively with leukocyte-reduced blood products is extremely costly. At the same time, the percentage of patients who could possibly benefit from such a program is relatively small since many multiply transfused patients do not become alloimmunized regardless of the leukocyte content of transfused blood (2,14).
Additional studies are needed to determine what patients would benefit most from the use of leukocyte-reduced blood products. Large, multicenter, controlled clinical trials are in progress to further and conclusively substantiate the role of leukocyte reduction in preventing alloimmunization. Currently at M. D. Anderson Cancer Center, patients who routinely require leukocyte-depleted cellular blood products include those who experience repeated febrile reactions, those who are candidates for bone marrow transplants, and those who are expected to require a large number of platelet transfusions. Quality control of the performance of the filters used at M. D. Anderson shows that units of leukocyte-depleted blood or platelets contain less than 1 million white cells/unit of blood product, far below the accepted standard of less than 5 million.
Besides febrile transfusion reaction and alloimmunization, viral disease transmission, especially in immunocompromised patients, is also linked to leukocytes transfused with cellular blood components. Substantial additional data suggest that leukocyte-reduced blood products can significantly prevent transmission of some viral infections. The virus whose transmission appears to be most affected by leukocyte reduction is CMV (3,16,17). This is important since the prevention of transfusion-transmitted CMV infection is a major concern for selected patient populations.
Many clinical studies have investigated the frequency of transfusion-induced CMV infection in patients receiving leukocyte-reduced blood components. In several studies, no seroconversion was observed in 184 patients supported with leukocyte-reduced blood components, in contrast to a seroconversion rate of 25% (75/303) in a control group (16). These impressive results were obtained by either state-of-the-art filtration or an apheresis method that reduces the number of residual leukocytes to less than 5 million. It should be emphasized, however, that these methods of preventing CMV transmission are highly dependent on the consistent efficiency of leukocyte depletion and require rigorous quality control.
CMV-Screened Cellular Blood Components
An alternative approach to preventing transfusion-transmitted CMV infection is to transfuse blood components from donors who have been screened for CMV antibodies. Clinical studies have shown that the risk of primary CMV infection in transfusion recipients can be reduced by using CMV-seronegative blood products (16,17). Yet, despite the proven clinical effectiveness of such products in high-risk patient groups, there are problems with this approach. Because of the high prevalence of CMV antibodies among healthy blood donors, maintaining an inventory of blood components screened for CMV is a major undertaking for blood centers and hospital transfusion services. Worldwide, the prevalence of CMV antibodies among blood donors ranges from 30% to 100% (18). Within the United States, significant regional variations in donor antibody positivity have been observed, ranging from 30-40% in the northern states to above 65% in the southern states (19). Hence, finding CMV-seronegative donor units could be a serious problem in some geographic areas.
Furthermore, multiply transfused patients sometimes develop alloantibodies to RBC, leukocyte, or platelet antigens. In such instances, finding compatible blood components that are also negative for CMV antibody is almost impossible. Current screening tests such as the latex agglutination test and enzyme-linked immunosorbent assays measure CMV-specific total or IgG antibodies. Although positive results with such tests indicate previous infection, it is not possible to identify donors who are actually infectious. Studies suggest that only about 10% of seropositive donors can actually transmit infection (20).
Because of their limited availability, CMV-screened blood products should be reserved for those patient groups whose risk of transfusion-transmitted CMV infection is well established. These include CMV-seronegative pregnant women, premature infants born to CMV-seronegative mothers, CMV-seronegative recipients of allogeneic bone marrow transplants from CMV-seronegative donors, and CMV-seronegative patients with acquired immunodeficiency syndrome (17).
Irradiated Cellular Blood Components
Transfusion-associated GVHD (TA-GVHD) is an intensive immunologic reaction mediated by transfused immunocompetent lymphocytes directed against an immunocompromised host. Compared with GVHD following bone marrow transplantation, TA-GVHD has a rapid onset, severe symptoms, and a poor response to treatment. The mortality rate of TA-GVHD is over 90% (4,21). Very rare in immunocompetent patients, TA-GVHD occurs most often in immunocompromised transfusion recipients (22). Patients at highest risk of developing TA-GVHD are those who recently have undergone bone marrow transplantation and patients with severe congenital immunodeficiency syndromes. Other patient groups that appear to be at intermediate risk include premature infants undergoing exchange transfusions and patients with leukemia and lymphoma during the period of bone marrow suppression induced by chemotherapy.
Prophylactic gamma irradiation of cellular blood components is currently the most efficient and reliable way to prevent TA-GVHD. Yet, it is not known whether leukocyte reduction in blood components can reliably decrease the risk of GVHD (4,21,23).
Studies have shown that exposing lymphocytes to 1,500 rad and 5,000 rad can cause 90% and 97% reductions, respectively, in mitogen-stimulated blast formation (24). Controversy, however, surrounds the effects of irradiation on platelets and granulocytes. Whereas RBCs appear to be quite resistant to radiation damage even up to 10,000 rad, the effects on platelets and granulocytes are more variable. Other studies have shown that a dose of 5,000 rad can affect platelet survival by 33% and impair the ability of platelets to correct aspirin-induced prolongation of bleeding time (25). The effects of ionizing radiation on granulocyte function are especially controversial. Chemotaxis and bactericidal capacity do not seem to be affected by radiation doses up to 5,000 rad (24,26). However, a dose less than 5,000 rad can impair superoxide production and oxidative metabolism of granulocytes (27,28).
To alleviate the potentially deleterious effects of TA-GVHD, all cellular blood components issued by the Transfusion Service at M. D. Anderson for infusion are irradiated with 3,000 rad (30 Gy) at the time of issue. Although it is not clear that all cancer patients require irradiated blood products, the current policy is logistically simple enough not to require maintenance of special lists of patients who should receive irradiated blood products.
In conclusion, there are benefits in using any of the three types of blood components reviewed here. Blood components depleted of leukocytes by filtration can help prevent febrile reactions, alloimmunization, and CMV transmission; but leukocyte depletion is costly and must be done under strict quality control and with consistent efficiency. CMV-screened cellular blood components are an alternative to leukocyte-reduced blood components in preventing CMV transmission, but the availability of such products is often limited. Either leukocyte-reduced or CMV-screened blood components should be used for those groups of patients who could benefit most from such components. Irradiated cellular blood components are particularly useful in preventing TA-GVHD; consequently, all cellular blood components transfused by M. D. Anderson's Transfusion Service are now irradiated at the time of their issue.
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