Chronic viral infections often result in ineffective CD8 T-cell responses due to functional exhaustion or physical deletion of virus-specific T cells. However, how persisting virus impacts various CD8 T-cell effector functions and influences other aspects of CD8 T-cell dynamics, such as immunodominance and tissue distribution, remains largely unknown. Using different strains of lymphocytic choriomeningitis virus (LCMV), we compared responses to the same CD8 T-cell epitopes during acute or chronic infection. Persistent infection led to a disruption of the normal immunodominance hierarchy of CD8 T-cell responses seen following acute infection and dramatically altered the tissue distribution of LCMV-specific CD8 T cells in lymphoid and nonlymphoid tissues. Most importantly, CD8 T-cell functional impairment occurred in a hierarchical fashion in chronically infected mice. Production of interleukin 2 and the ability to lyse target cells in vitro were the first functions compromised, followed by the ability to make tumor necrosis factor alpha, while gamma interferon production was most resistant to functional exhaustion. Antigen appeared to be the driving force for this loss of function, since a strong correlation existed between the viral load and the level of exhaustion. Further, epitopes presented at higher levels in vivo resulted in physical deletion, while those presented at lower levels induced functional exhaustion. A model is proposed in which antigen levels drive the hierarchical loss of different CD8 T-cell effector functions during chronic infection, leading to distinct stages of functional impairment and eventually to physical deletion of virus-specific T cells. These results have implications for the study of human chronic infections, where similar T-cell deletion and functional dysregulation has been observed.Considerable evidence suggests that an effective CD8 T-cell response can control or eradicate viral infections. CD8 T cells appear to be particularly important in the immune response to chronic infections and in the long-term control of latent and reactivating viruses (2, 26). Indeed, potent human immunodeficiency virus (HIV)-specific CD8 T-cell responses correlate with acute viral control and long-term nonprogression (16,42,55,56,61,66). Further, rhesus macaques infected with simian immunodeficiency virus (SIV) show significantly higher viral loads if CD8 T cells are eliminated by antibody-mediated depletion (38). Evidence also suggests that CD8 T cells are important for the acute phase as well as long-term control of other persistent viruses, such as Epstein-Barr virus (EBV), cytomegalovirus, hepatitis B virus (HBV), and hepatitis C virus (HCV) (18,24,63,83). However, in many patients CD8 T cells fail to contain viral replication, particularly during HIV, HBV, and HCV infections. Recent studies suggest that functional impairment (exhaustion) and/or physical deletion of CD8 T cells can accompany ineffective viral control (44, 85). However, the impact of chronic viral infection on the induction and m...

We examined the regulation of virus-specific CD8 T cell responses during chronic lymphocytic choriomeningitis virus (LCMV) infection of mice. Our study shows that within the same persistently infected host, different mechanisms can operate to silence antiviral T cell responses; CD8 T cells specific to one dominant viral epitope were deleted, whereas CD8 T cells responding to another dominant epitope persisted indefinitely. These virus-specific CD8 T cells expressed activation markers (CD69hi, CD44hi, CD62Llo) and proliferated in vivo but were unable to elaborate any antiviral effector functions. This unresponsive phenotype was more pronounced under conditions of CD4 T cell deficiency, highlighting the importance of CD8– CD4 T cell collaboration in controlling persistent infections. Importantly, in the presence of CD4 T cell help, adequate CD8 effector activity was maintained and the chronic viral infection eventually resolved. The persistence of activated virus-specific CD8 T cells without effector function reveals a novel mechanism for silencing antiviral immune responses and also offers new possibilities for enhancing CD8 T cell immunity in chronically infected hosts.

The constraint of fitting a diverse repertoire of antigen specificities in a limited total population of lymphocytes results in the frequency of naive cells specific for any given antigen (defined as the precursor frequency) being below the limit of detection by direct measurement. We have estimated this precursor frequency by titrating a known quantity of antigen-specific cells into naive recipients. Adoptive transfer of naive antigen-specific T cell receptor transgenic cells into syngeneic nontransgenic recipients, followed by stimulation with specific antigen, results in activation and expansion of both donor and endogenous antigen-specific cells in a dose-dependent manner. The precursor frequency is equal to the number of transferred cells when the transgenic and endogenous responses are of equal magnitude. Using this method we have estimated the precursor frequency of naive CD8 T cells specific for the H-2Db–restricted GP33–41 epitope of LCMV to be 1 in 2 × 105. Thus, in an uninfected mouse containing ∼2-4 × 107 naive CD8 T cells we estimate there to be 100–200 epitope-specific cells. After LCMV infection these 100–200 GP33-specific naive CD8 T cells divide >14 times in 1 wk to reach a total of ∼107 cells. Approximately 5% of these activated GP33-specific effector CD8 T cells survive to generate a memory pool consisting of ∼5 × 105 cells. Thus, an acute LCMV infection results in a >1,000-fold increase in precursor frequency of DbGP33-specific CD8 T cells from 2 × 102 naive cells in uninfected mice to 5 × 105 memory cells in immunized mice.

Memory T cells can persist for extended periods in the absence of antigen, and long-term T cell immunity is often seen after acute infections. Paradoxically, there have been observations suggesting that T cell memory may be antigen-dependent during chronic infections. To elucidate the underlying mechanisms we have compared memory CD8 T cell differentiation during an acute versus chronic infection by using the mouse model of infection with lymphocytic choriomeningitis virus. We found that during a chronic infection virus-specific CD8 T cells failed to acquire the cardinal memory T cell property of long-term antigen-independent persistence. These chronically stimulated CD8 T cells were unable to undergo homeostatic proliferation, responded poorly to IL-7 and IL-15, and expressed reduced levels of the IL-7 and IL-15 receptors, thus providing a possible mechanism for the inability of these cells to persist long term in the absence of antigen. In striking contrast, virus-specific memory CD8 T cells that developed after an acute lymphocytic choriomeningitis virus infection could persist without antigen, were capable of self-renewal because of homeostatic proliferation, responded efficiently to IL-7 and IL-15, and expressed high levels of receptors for these two cytokines. Thus, memory CD8 T cells generated after acute infections are likely to have a competitive advantage over CD8 T cells that develop during chronic infections. These findings raise concerns about using vaccines that may persist and also suggest that there may be limitations and challenges in designing effective immunological interventions for the treatment of chronic infections and tumors.