CMV seronegative HCT recipients who receive a seropositive graft are at significant risk for CMV complications following hematopoietic cell transplantation. In our cohort, 20% of D+/R− patients developed CMV transmission during the first year of follow-up, and 3% developed invasive disease. The only risk factor associated with CMV transmission was a high graft TNC count. Other cellular components of the donor graft, including monocytes (CD14), did not appear to affect CMV transmission. No other pre- or posttransplant risk factors appeared to alter efficiency of CMV transmission, although the addition of CMV PCR testing did appear to detect CMV transmission earlier and identified several abortive infections.
Epidemiologic data have demonstrated that CMV seropositive recipients are at highest risk for the development of CMV complications. The majority of epidemiologic and intervention trials focus on this high risk population or combine D+/R− with R+ patients. Even studies focusing on risk related to donor CMV serostatus have centered on seropositive recipients [23
]. This distinction is important because CMV outcomes seen in seropositive recipients are because of different pathologic mechanisms than those that occur in D+/R− transplant recipients. In seropositive recipients, CMV complications primarily stem from reactivation of latent virus [25
], whereas in seronegative patients these events indicate transmission of virus. Risk factors for development of CMV in D+/R− patients may therefore be very different than those seen in seropositive recipients, depending less on posttransplant immunosuppression and more on total exposure to the virus [27
]. Furthermore, there are no studies that have focused on the smaller population of D+/R− patients and assessed risk factors for CMV acquisition that are specific to this population.
These data help to define the importance of the donor graft as the most frequent source of CMV transmission in D+/R− recipients. Previous studies have suggested that in infected individuals, bone marrow-derived hematopoietic cells, granulocyte-macrophage progenitors, and peripheral blood monocytes serve as reservoirs of latent virus from which reactivation occurs during immunosuppression or immunodeficiency [28
]. These cells can support active CMV replication and the allogeneic transplant process itself may stimulate reactivation [35
]. Still, the challenge of successfully reactivating CMV from naturally infected samples in the laboratory has limited the ability to translate these findings into clinically relevant predictors for transmission [27
]. Although determining a cell-specific reservoir of CMV latency has continued to be a source of ongoing research [10
], the lack of such models has hampered the development of prevention strategies in this population.
Donor PBSC as a graft source was also associated with CMV transmission in univariable analyses and is consistent with data from a recent randomized trial that demonstrated a trend toward higher rates of CMV complications in PBSC graft recipients [36
]. The association did not remain statistically significant in multivariable analyses, suggesting that the higher cellular content found in PBSC grafts when compared with bone marrow plays a more important role than the type of graft [37
]. We also analyzed other pretransplant risk factors considered to be associated with CMV transmission, including total-body irridiation dose, donor age, HLA matching, as well as posttransplant factors such as aGVHD [24
]. In our study, aGVHD itself, as well as other factors associated with the development of GVHD such as immunosuppressive prophylaxis [42
], were not associated with CMV transmission. HLA matching and conditioning with TBI also had no clear association with transmission. Similarly, although previously reported in the context of blood transfusion-associated CMV transmission [8
], we found no associations between transmission and number of RBC or platelet units transfused. Perhaps it is the allogeneic process itself, as has been suggested, that is the most important factor leading to the lytic phase of replication and subsequent detection [35
The close association with donor blood type B seen in univariate analyses is unclear. There are no known associations between ABO blood type and CMV, and this association did not appear to be linked to ABO mismatch between the donor and recipient. Risk of viral infections has been associated with ABO blood type. Interestingly blood type B has been associated with a decreased risk of Norovirus infection [44
] but an increased risk for HHV-8 infection [45
]. Although there continued to be a trend in multivariate analyses, this association did not remain significant. Additionally, the percentage of donors with blood type B (10.5%) was consistent with those reported in multicenter blood donor registry data, suggesting no baseline protection from CMV infection [46
]. Although it is possible that donor blood type B is more resistant to primary CMV infection, this needs further evaluation in larger cohort studies.
CMV transmission was more frequently detected and at earlier time points by PCR testing during the posttransplant period than with standard antigenemia testing; however, the effect of this improved detection was not statistically significant when controlling for other factors such as TNC. Also, in a subset of patients who were tested by both methods, PCR identified CMV transmission earlier than antigenemia but the rates of detection were similar (). The risk of CMV transmission in seronegative recipients in prior studies is reported to be between 9% and 21% [2
], although population differences (eg, pediatric versus adult) and variable prevention and screening strategies make comparisons between studies difficult. We found that approximately 19% of patients acquired CMV during the first year posttransplantation. Furthermore, the higher rates seen in those tested by PCR indicate that rates of CMV transmission in D+/R− transplant recipients may actually be higher than previously reported. However, some of those detected by PCR are likely abortive infections [17
]. The cases of abortive infection described in this study are plausible because viral load was low and patients did not progress to antigenemia or disease. Earlier detection through PCR screening did not appear to translate into lower CMV disease rates. This is not surprising, considering that there were only a small number of patients who were missed by antigenemia and relatively few that developed preengraftment CMV, a time period where PCR has advantages over antigenemia. Finally, CMV transmission was not associated with bacterial or fungal complications.
The strengths of our study include the size of our cohort, the uniformity of prevention strategies, and consistency of short and long-term clinical data available for review. As with all retrospective studies, these analyses are not without limitations. We included all patients with evidence of CMV infection from plasma or disease locations, but it is possible that we missed evidence of transmission through viral shedding at other sites. Urine and oral screening may have little relevance for disease outcomes [48
], suggesting that additional screening from nonblood sites would not have altered our results. Although not all patients were tested with both PCR and the antigenemia assay, we do not think that this affected that transmission rate significantly as both multivariate model and the head-to-head comparison of PCR versus antigenemia did not suggest a statistically significant increase in detecting CMV transmission (although PCR detected it earlier).
Although it is possible that seropositive recipients who were falsely seronegative on screening may have been included in our cohort, it is unlikely to have affected our results because misclassification of serostatus is a very uncommon event. Although we did not find an association with blood transfusions, the most likely alternate source during transplantation, we could not account for other possible sources of CMV acquisition not included in our risk factor analyses. The amount of intravenous immunoglobulin use is not available in this dataset and may have affected rates of CMV transmission in this cohort; however, the modest benefit seen with weekly CMV-specific intravenous immunoglobulin [51
] suggests that this would have had little impact on these analyses. Finally, the limited data on the specific cellular makeup of the seropositive graft and T cell reconstitution following transplantation do not allow for a complete in-depth analyses of other graft components and immune reconstitution on CMV outcomes.
In summary, we demonstrate a critical role for the cellularity of the donor graft in CMV transmission in D+/R− HCT recipients. Our improved preemptive strategies have helped to decrease the rate of CMV disease in HCT [52
], but it remains an important yet elusive goal to prevent primary transmission in the D+/R− population. These data make a strong case for the importance of the donor graft in primary CMV transmission to D+/R− patients, but suggest that until we understand primary CMV biology there may be limitations in our ability to intervene to prevent CMV transmission. As such, future studies aimed at understanding primary reservoirs for latent CMV within the donor graft and the role of viral load in transmission are needed. Assessments of transplant or immunologic factors, such as the role of concomitant transfer of donor-derived cellular and humoral immunity associated with conversion from latent to lytic replication, are also needed. Future studies addressing recipient target cell receptors and host genetics, or those evaluating donor immunity may provide additional predictors for viral transmission that could lead to important changes in donor selection or assist in risk stratification.