Human cytomegalovirus (HCMV) is a β-herpesvirus that establishes a life-long presence in the infected host. The adaptive immune response is indispensable in controlling HCMV infection. Consequently, healthy individuals show no or mild symptoms following primary infection. In contrast, immunocompromised individuals who develop primary infection or recrudescence of HCMV can experience severe morbidity, and sometimes mortality. HCMV-specific T cell populations undergo changes in the architecture of their T cell receptor (TCR) repertoire following each episode of viral reactivation. A diverse TCR repertoire is thought to be required to provide the most efficient protection against virus infection. Perturbation to this repertoire, as can occur in immunocompromised individuals following transplantation, can lead to an increase risk of developing virus-associated clinical disease. Therefore, the study of factors influencing TCR selection is critically important in both healthy and immunocompromised individuals. To further understand the factors governing TCR selection in a persistent virus infection, the current thesis examined this process in different settings. CD8+ T cell responses to persistent viral infections are characterised by the accumulation of T cells exhibiting an oligoclonal T cell repertoire, with a parallel reduction in the naïve T cell pool. However, the precise mechanism for this phenomenon remains elusive. Here, we showed that HCMV-specific CD8+ T cells recognising distinct epitopes from the pp65 protein and restricted through an identical HLA class I allele (HLA B*3508) exhibited either a highly conserved public T cell repertoire, or a private, diverse T cell response, which was uniquely altered in each donor following in vitro antigen exposure. Selection of a public TCR was co-incident with an atypical peptide-MHC (pMHC) structure, whereby the epitope adopted a helical conformation that bulged from the peptide-binding groove, whilst a diverse TCR profile was observed in response to the epitope that formed a flatter, more ‘featureless’ landscape. Clonotypes with biased TCR usage demonstrated more efficient recognition of virus-infected cells, a greater CD8 dependency, and were more terminally differentiated in their phenotype when compared to the T cells expressing diverse TCR. These findings provide new insights into our understanding of how the biology of antigen presentation, in addition to the structural features of the pMHC, might shape the T cell phenotype and its corresponding repertoire architecture. Next, the role of HCMV in shaping the global and antigen-specific TCR repertoire in healthy donors was examined. First, exposure to HCMV led to an inflation of terminally differentiated CD57-expressing T cells. This effect was not seen in HCMV seronegative individuals who showed evidence of exposure to another persistent herpesvirus, Epstein-Barr virus (EBV). More importantly, these terminally differentiated CD8+ T cells in HCMV-exposed individuals displayed a highly skewed architecture of their peripheral blood T cell repertoire, with large monoclonal/oligoclonal expansions. However, ex vivo analyses of HCMV-specific T cells revealed a heterogeneous pattern of CD57 expression that showed no correlation to the antigenic source of its cognate epitope. Based on these observations, we proposed that exposure to HCMV drives the differentiation of not only the global T cell population, but select HCMV-specific T cell populations as well, and that expression of CD57 by these cells was co-incident with an oligoclonal T cell repertoire. Finally, the TCR repertoire was examined in a cohort of solid organ transplant (SOT) recipients, where primary infection or recrudescence of latent virus infection can be manifested either as asymptomatic or symptomatic disease. We examined 18 SOT recipients, and observed that symptomatic HCMV or EBV infection or recrudescence following solid organ transplantation was co-incident with a dramatic skewing of the TCR repertoire, with expansions of monoclonal/oligoclonal clonotypes. As the clinical symptoms resolved, the peripheral blood repertoire reverted to a more diverse distribution. In contrast, SOT recipients with asymptomatic or no HCMV/EBV infection or recrudescence showed minimal or no skewing of the TCR repertoire, and maintained TCR repertoire diversity. Interestingly, this disparate repertoire showed no correlation with levels of viral load in the peripheral blood. More importantly, we showed that large monoclonal/oligoclonal repertoire expansions was linked to the loss of antigen-specific T cell function observed in SOT patients undergoing symptomatic viral infection or recrudescence, while SOT recipients who maintained peripheral blood TCR repertoire diversity and functional antigen-specific T cell responses could resist clinical symptomatic disease in spite of high levels of viral load. Therefore, the work presented in this thesis provides additional evidence on the factors governing TCR selection in HCMV-exposed healthy individuals, as well as the consequences that perturbation to the TCR repertoire has on the functionality of the T cell compartment in immunocompromised individuals.
| Identifer | oai:union.ndltd.org:ADTP/253966 |
| Creators | Katherine Kay Wynn |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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