Mouse cytomegalovirus infection as a model for persistent viral infections in mice and humans.

Background: CD8+ T cells are critical not only for the termination of acute viral infections but also for the long-term control of viruses capable to establish persistent infection such as Human Immunodeficiency Virus (HIV), Hepatitis B and C virus (HBV and HCV), Epstein Barr Virus (EBV) and Cytomegalovirus (CMV). The level of viral persistence (or antigen load) has an important impact on the magnitude and the quality of virus-specific T cells and vice versa. Usually, high antigen load during a prolonged period of time has a damaging influence on the fimctionality of antiviral T cell responses. However, it is less clear whether low level antigen persistence is beneficial or detrimental for the maintenance of virus specific T cells and ultimately for protective T cell memory. This has obvious implications for vaccine design. Hypothesis: I speculated that antigen persistence at a low level is beneficial for the long term maintenance of virus specific CD8+ T cells and for their protective capacity against challenge infections. Methods: Mice were infected with mouse cytomegalovirus (MCMV), which leads to a low level persistent or latent infection with the potential for viral reactivation. To compare the long-term dynamics of virus specific CD 8+ T cells after persistent (MCMV) or transient infections [Vaccinia virus (W), Influenza A virus (Influenza)], I used recombinant MCMV expressing T cell epitopes derived from heterologous viruses. Thereafter, I longitudinally characterized the magnitude, the phenotype and the functionality of CD8+ T cells specific for native and recombinant epitopes in detail and tested their long-term protective capacity against viral challenge. Results: This longitudinal analysis of the native MCMV-specific CD8+ T cell response revealed a most unusual pattern of T cell dynamics: after initial expansion and limited contraction MCMV-specific memory CD8+ T cells steadily accumulated over time, with 20% of all CD8+ T cells being specific for a single MCMV-derived T cell epitope at one year after infection. Accumulation of effector memory T cells was seen in all organs tested and was accompanied by a gradual restriction in T cell receptor Vp chain usage over time. The pattern of accumulation, which I have termed ‘memory inflation’, was observed only in two out of five epitopes tested. Responses against other tested epitopes followed more ‘classical’ dynamics with initial expansion and contraction to stable memory T cell levels. Challenge with recombinant W more than 6 months after MCMV-infection provoked vigorous expansion of these ‘inflated’ effector memory T cells, which mediated immediate protection against challenge. Accumulation of effector memory T cells was also seen after infection with recombinant MCMV and long-term protective immunity, based on recombinant T cell epitopes, was maintained more efficiently than after transient infection with non-persisting viruses, which expressed identical epitopes. Conclusions: My results have two major implications: 1) ‘Memory inflation’ is a previously unrecognised pattern of an antiviral T cell response, which is most likely driven by continuous or repetitive antigen encounter during latent or low-level persistent MCMV-infection. Recent cross-sectional studies in humans suggest that similar virus-host interactions might exist for human viral infections particularly with CMV and EBV. 2) Effector memory T cells accumulating during low-level persistent infection provide efficient long-term protective T cell immunity without the need for boosting. These results demonstrate that low-level antigen persistence is beneficial for protective T cell memory and they provide the first ‘proof-of-principle’ evidence to encourage further exploration of persistent ‘self-boosting’ vectors for the development of successful T cell based vaccines.