Virus-specific T cells are critical for controlling latency and restricting viral replication in hosts infected with human cytomegalovirus (HCMV). Primary HCMV infection in an immunocompetent individual is normally asymptomatic, however in patients with profound cellular immunodeficiency, such as solid organ transplant patients, it can cause severe disease and considerable morbidity and mortality. The development of ex vivo T cell assays and the mapping of numerous MHC class-I-restricted T cell epitopes has facilitated the enumeration and functional characterisation of antigen-specific T cells, and allowed investigation into the dynamics of the virus-specific T cell response in both the immunocompetent and immunocompromised host.
The classical paradigm for T cell dynamics suggests that the resolution of a primary acute viral infection is followed by the generation of a long-lived pool of memory T cells which is thought to be highly stable. Very limited alteration in this repertoire is expected until the immune system is re-challenged by reactivation of latent viruses or by cross-reactive pathogens. Contradicting this view, it has been shown in this study that the T cell repertoire specific for two genetically stable latent herpes viruses (HCMV and Epstein-Barr virus, EBV) in the peripheral blood, displayed significant contemporaneous co-fluctuations, albeit at lower levels than during the acute phase of infection, and involved both virus-specific CD8+ and CD4+ T cells. The co-ordinated responses to two different viruses suggested that these T cells were not driven by sub-clinical viral reactivation, but may instead by a more generalised ‘bystander’ effect. This contention was supported by the observation that whilst the absolute number of CD3+ T cells and their subsets and also the cell surface phenotype of antigen-specific T cells remained relatively constant over time, a loss of CD62L expression in the total CD8+ T cell population was co-incident with the expansion of tetramer-positive virus-specific T cells.
Human cytomegalovirus infection is one of the major infectious complications in transplant recipients. Pre-existing anti-viral immunity plays a crucial role in preventing viral pathogenesis in these patients, however pre-existing immunity and the observed homeostatic fluctuations are placed under intense pressure in the immunosuppressed transplant patient. The consequences of this pressure are episodes of symptomatic viral recrudescence, particularly in seronegative patients receiving a seropositive organ. Using ex vivo antigen-specific T cell analysis, HCMV-specific CD8+ T cell responses were longitudinally analysed in a cohort of solid organ transplant patients using a large panel of HLA class-I-restricted epitopes, and the relationship to the development of HCMV reactivation/disease determined. The impact of anti-HCMV prophylaxis on pre-existing HCMV-specific CD8+ T cell responses from seropositive solid organ transplant patients was also examined. These analyses revealed that symptomatic HCMV recrudescence in transplant recipients was co-incident with reduced expression of IFN-γ by virus-specific CD8+ T cells and an upregulation of CD38 expression on these T cells, although there was no significant change in the absolute number of virus-specific cells. In contrast, HLA class-I-matched transplant patients with asymptomatic viral recrudescence showed increased expansion of antigen-specific T cells and highly stable IFN-γ expression by epitope-specific T cells. Anti-viral prophylaxis was shown to have no effect on either the frequency or expression of IFN-γ by HCMV-specific CD8+ T cells from seropositive transplant patients. These studies in solid organ transplant patients suggested that a strong functional T cell response played a crucial role in defining the clinical outcome of acute viral recrudescence and provided a platform for the application of immune monitoring as a diagnostic tool for HCMV. Subsequently, the QuantiFERON®-CMV assay was developed; a novel diagnostic technology to monitor the HCMV-specific CD8+ T cell response which is based on the detection of secreted IFN-γ in the whole blood. Evaluation of the QuantiFERON®-CMV assay for healthy individuals revealed that this technology was at least as sensitive and with some HCMV epitopes more sensitive than the ELISPOT assay for detecting ex vivo IFN-γ. Results from the QuantiFERON®-CMV assays showed 97% (36/37 individuals) agreement with the anti-HCMV serology test from these healthy individuals. Furthermore, this technology was shown to effectively assess HCMV-specific T cell responses in solid organ transplant patients. Overall, the QuantiFERON®-CMV assay was shown to be a simple, reproducible and reliable test for the detection of IFN-γ in response to HCMV CD8+ T cell epitopes. It may therefore be a valuable diagnostic test for the detection of HCMV infection and a useful clinical tool for monitoring the immune response in immunosuppressed patients during therapy.