CMV is the most common viral infection after solid organ transplantation (SOT) and is associated with significant morbidity1. Without prophylaxis, most CMV disease occurs in the first 3 months post-transplant during the period of intense of immunosuppresion.2 SOT recipients at the greatest risk are those that are seronegative recipients of organs from seropositive donors (CMV D+/R-).3, 4 Antiviral agents have proven to be useful in the prevention of CMV infection and disease in SOT recipients, including the high-risk D+/R- patients.5-9 Upon completion of prophylaxis however, CMV infection and disease occurs in about 50% and 25-30% respectively of D+/R- SOT recipients within the first year after transplant [30% rate of investigator treated CMV disease in PV16000].9 The incidence of CMV infection following prophylaxis in D+/R- lung transplant recipients may be as high as 80%.10

As CMV disease occurring after prophylaxis will continue to impact morbidity and mortality in SOT recipients, it would be desirable to be able to predict which patient will develop this complication. Currently, there are no reliable methods that are routinely available to predict the risk of CMV infection or disease in an individual patient. CMV viral load testing after prophylaxis in D+/R- patients has been shown to have poor predictive value for subsequent CMV disease.11 CMV serology (a measure of humoral immunity) was also shown to be only of marginal use in predicting the risk of late onset disease.12 Cell mediated immunity (CMI) is known to be more important than humoral immunity in controlling CMV infection. CMV infection elicits a strong virus specific CD4+ and CD8+ T-cell response. CD8+ T-cell responses to the virus often contain multiple antigen-specific reactivities including to viral pp65 or IE-1 antigens as well as pp50, glycoprotein B, and IE-2 and other antigens.13 CD4+ T cells also play a part in CMV control via promotion of priming, expansion and maintenance of CD8+ CMV-specific CTLs.14 Measuring an individual's CMI response to CMV may be a useful predictor of the risk of CMV infection or disease after prophylaxis. Patients with a poor CMI response (especially a poor CD8+ T-cell response) could then be targeted with longer courses of antiviral prophylaxis.

Cell mediated immunity testing Most of the previous studies have focused on CTL responses to the CMV phosphoprotein pp65.15-17 However, since CD8+ T-cell responses to CMV often contain multiple antigen-specific reactivities, measurement of CMI using epitopes restricted to a single protein may not yield adequate results. In conjunction with Dr. Rajiv Khanna (Queensland Institute of Medical Research, Australia), and Cellestis Ltd (Sydney, Australia), we have done preliminary validation and assessment of a CMI (Quantiferon-CMV) assay in which we measure the IFN-γ responses to a range of T-cell epitopes of CMV viral proteins including pp65, pp50, the glycoprotein gB, and the immediate early IE-1 antigen that are specific for a wide range of HLA class I specificities [See Appendix Table 1]. The assay employs a peptide pool for stimulation of whole blood and is suitable for routine clinical use and evaluation in multicenter studies. The Quantiferon-CMV assay has been compared to an ELISPOT assay in a study involving 37 healthy volunteers and 25 SOT recipients.18 In this study, the Quantiferon-CMV assay was at least as sensitive as the ELISPOT for some CMV epitopes, and more sensitive for other CMV epitopes. In addition, the Quantiferon-CMV results highly correlated with the CMV serostatus, in both healthy volunteers and transplant recipients. In another study,19 the Quantiferon-CMV assay was used in HIV-infected individuals with and without a history of CMV disease. The CMV specific immune response measured by the Quantiferon-CMV assay was higher in patients without a history of CMV disease, suggesting that a positive result of the Quantiferon-CMV may predict the likelihood for developing a protective immune response against CMV.