Prevention of transfusion‐transmitted cytomegalovirus: reactivation of the debate?

Cytomegalovirus (CMV) is a persistent virus that can remain latent for long periods only to reactivate to a vigorous infection at later times. Fittingly, our interest in the biology and prevention of transfusion-transmitted CMV infection (TT-CMV) seems to follow a similar course, with long phases of relative inactivity punctuated by episodes of energetic debate. The article by Ziemann and colleagues1 in this issue of TRANSFUSION now presents some provocative data and suggestions for changes in clinical practice that may well spark another period of active interest in this enigmatic infectious complication of transfusion. TT-CMV, as defined here, is really only one form of “transfusion-related” CMV infection. TT-CMV specifically refers to transmission of replicating or latent CMV from an infected donor to a susceptible, CMV-naïve transfusion recipient. Active infection can alternatively occur if transfusion stimulates reactivation of preexisting latent virus in the recipient. In fact, Adler and colleagues2,3 reported that CMV-seropositive recipients had similar rates (approx. 10%) of CMV antibody increase whether they received blood from CMV-seronegative or -seropositive donors and concluded that viral reactivation, rather than a new infection, accounts for most active infections in CMV-seropositive transfusion recipients. Nonetheless, TT-CMV has garnered more transfusion medicine interest because of its lethal potential in CMV-naïve immunocompromised transfusion recipients. TT-CMV was once a focus of active investigation, and rightly so. In 1982, Ho4 summarized the preceding prospective studies and concluded that approximately 20 percent of CMV-seronegative recipients receiving multiple units of blood experienced primary infection and 45 percent of these were symptomatic. By 1988, however, Preiksaitis and others5 reported that transmission rates to nonimmunosuppressed seronegative recipients were closer to 1 percent, a reduction that may have resulted from less use of fresh whole blood.6 Although transmission rates remained higher in immunocompromised patients, the incidence of TT-CMV was markedly reduced after the efficacy of screening blood by CMV serology or leukoreducing blood by filtration was appreciated. Although the reduction in clinically apparent or severe TT-CMV cases has diminished research interest, a number of important questions concerning the mechanisms underlying TT-CMV and optimal approaches to its prevention remain unanswered. To appreciate these issues, one must first consider the fundamental biology of CMV infection, which is now fairly well understood.7-9 Viral exposure in a previously naïve host leads to a phase of acute infection characterized by high viral loads in peripheral blood white blood cells (WBCs; primarily monocytes) as well as plasma viremia. Subsequent development of the host antiviral immune response, which is usually detected clinically by the presence of CMV antibodies but also contains a prominent cellular immune component, leads to eventual clearance of plasma viremia and actively infected cells. Latent CMV is not eliminated, but persists long term in monocytes of remotely infected hosts (albeit at lower levels).9,10 Monocytes thus become a likely reservoir from which CMV can intermittently reactivate. Seroconversion typically does not occur until 6 to 8 weeks of infection,9 resulting in a seronegative window period in which viremia and high cell-associated viral loads may be prominent. Although the period of acute infection with active CMV replication probably extends somewhat beyond the end of the window period (i.e., past the advent of CMV immunity), “acute infection” and “window period” are used interchangeably here. The profiles of CMV replication and host immune response can be used to divide infected donors into acutely and remotely infected groups. The contribution of each group to the occurrence of TT-CMV is still unclear, however. CMV transmission as latent virus within WBCs of remotely infected donors, with subsequent activation to replicating virus by an allogeneic immunologic response in the recipient, is the leading theory. First proposed by Lang and Kummer in 1972,11 this hypothesis is supported by the observation that CMV has rarely if ever been cultured from the blood of healthy donors,9,12 suggesting that acute viremic infections are infrequent in this population. In addition, TT-CMV rates correlate with WBC content of transfused components. Although seronegative marrow transplant recipients receiving standard blood products have CMV infection rates that can exceed 25 percent, the rate declines to 1 to 4 percent with leukoreduction.13-16 Furthermore, there is a high frequency of TT-CMV after transfusion of seropositive blood enriched for WBCs,17-19 while there is no evidence for TT-CMV following infusion of fresh-frozen plasma, which contains negligible viable WBCs but would be expected to have plasma CMV if the donor was acutely infected and viremic.20 In contrast, there is also substantial evidence that blood from acutely infected donors produces TT-CMV. Although CMV is abundant in WBCs after initial infection,9 it is rarely detectable at other times. With CMV polymerase chain reaction (PCR) assays that were well validated and specific,21 we only detected CMV DNA in WBCs of 2 of 416 seropositive donors and in none of more than 500 seronegative donors.22 Additionally, window-phase donations may explain why leukoreduction is not fully protective but has been associated with a low residual risk of TT-CMV in both randomized controlled as well as observational studies.13,23 The residual infectivity of filtered blood may be due to the inability of filters to completely remove CMV when the blood components have high cell-associated and/or plasma viral loads, which can be seen in actively infected donors.24,25 Furthermore, the observation that transfusion of CMV-seronegative blood does not completely prevent TT-CMV also implicates the acutely infected window-phase donor in breakthrough cases of CMV transmission.13 Although donors with acute infections appear to contribute to the occurrence of TT-CMV, they also fortunately appear to be quite rare. Drew and coworkers26 showed that plasma CMV DNA (CMV DNAemia), as a marker for viremia and acute infection, could only be observed in donors following initial infection. They reported detection of CMV DNA in the plasma samples of 3 of 192 seroconverting donors (1.5%), but in none of 488 remotely infected seropositive donors.26 The interval between negative and positive serology results in the donors in their study, however, ranged from 8 weeks to several years, a very wide window. One viremic donor was in the seronegative window period. In this issue of TRANSFUSION, Ziemann and colleagues1 now confirm Drew's results and refine the viral kinetics. They were able to identify 82 seroconverting donors with the comparatively narrow interval of 1 year or less between negative and positive serology results. Among these donors, 44 percent demonstrated CMV DNAemia and 2 (2.9%) were in the seronegative window phase. Another result from this work also bears emphasis. Consistent with Drew's studies, Ziemann and colleagues only detected CMV DNAemia within 12 months of seroconversion; there was no evidence of viremia in 598 donors who were seropositive for 1 year or longer. Thus, these data not only demonstrate a restricted period of acute infection, but also show that CMV reactivation with viremia is vanishingly rare or absent in remotely infected donors. To better understand the implications of this work, first consider the drawbacks of current clinical practices for preventing TT-CMV. As CMV serology is currently used, negative results correctly identify blood from CMV-naïve donors as CMV-safe, whereas positive serology prevents the use of blood from remotely infected donors. The “soft left edge” of the window phase, however, which cannot be identified prospectively, means that there is a variable period of risk during which the donor may be infectious but serology is negative and thus does not interdict transfusion of these blood components. CMV nucleic acid testing may shrink the window period,21,26 but would add significant cost. After leukoreduction by filtration, components from both CMV-naïve and remotely infected donors should be CMV-safe. Leukoreduction, however, would not eliminate the risk of components from acutely infected donors because filters do not effectively capture free virus in plasma25 and cannot completely remove CMV if WBC-associated viral loads are extremely high.24 Furthermore, the “belt-and-suspenders” approach of using components that are CMV-seronegative and filtered may not be much safer than unfiltered seronegative blood, because it does not address the window period problem. Based on their data, Ziemann and coworkers suggest a practical and potentially more effective approach to prevent TT-CMV: transfuse at-risk recipients with leukoreduced components specifically selected from seropositive donors who seroconverted at least 1 year previously. This “reverse” application of serology takes advantage of their finding that there is a relatively “hard right edge” to seroconversion in that donors who are at least 1 year beyond seroconversion are unlikely to have plasma viremia (or, probably, high cellular viral loads). Leukoreduction can then be used to remove any small amounts of latent CMV that might persist in donor WBCs. This method may not only be safer, but could also reduce testing costs (because repeat donor serology would be unnecessary > 1 year after seroconversion) and improve availability of CMV-safe units in high seroprevalence communities. Furthermore, there would be few regulatory or logistic barriers to implementation of Ziemann's approach. One issue with the present work is that the authors tend to equate CMV DNAemia with the presence of infectious virus. Although plasma PCR is clearly a highly sensitive method to detect viremia, not all positive signals result from infectious CMV. The authors acknowledge a previous study that argues against equating DNAemia with viremia by showing that CMV DNA in the plasma samples of renal transplant recipients is highly fragmented. In a more convincing study of acutely infected nonimmunocompromised patients, however, cultures of WBC and plasma were frequently negative when CMV DNAemia was detected: 16 of 100 WBC and 7 of 50 plasma samples were CMV DNA–positive whereas only 1 of 54 WBC samples and no plasma samples were culture-positive.9 In the authors' defense, one should be careful not to put too much faith in viral culture because very low levels of viable and infectious CMV could be missed when a small aliquot of plasma or WBC is cultured, and even low levels of infectious virus can be lethal in immunocompromised patients. In conclusion, Ziemann and colleagues provide provocative data suggesting that a relatively minor logistical change in the selection of blood components may produce an important advance in CMV safety. This novel and practical proposal will hopefully serve as an impetus to reactivate interest in the biology of TT-CMV and the development of improved methods to prevent CMV transmission.