Our studies document the highly adaptable nature of T cell functions and show that CD4+
T cell effector and memory development are not hardwired processes but, instead, are continually modulated by signals from the environment. The discovery that T cell responses are alterable led to 4 findings. First, T cell function that is lost during persistent infection can be restored or preserved by either removing the T cells from the antigenic environment or administering antiviral therapy in vivo. Second, functional recovery is more difficult to achieve the longer CD4+
T cells remain in the presence of persistent viral replication. Third, the mechanisms that potentiate CD4+
T cell inactivation apparently differ from those that modulate CD8+
T cell function. That is, by decreasing viral titers in vivo, CD4+
T cell inactivation can be prevented, whereas CD8+
T cells still become largely nonresponsive. Fourth, CD4+
T cell recovery following early antiviral therapy correlates with an increased ability of APCs to costimulate T cells. Importantly, these collective findings provide what we believe to be the first insight into the mechanisms that induce T cell inactivation during persistent viral infection versus those that may maintain it (45
T cell responses are initially robust during persistent infection; however, T cells rapidly become nonresponsive in temporal association with the viral transition into persistence. Previous studies have shown that following in vitro priming, functional effector and memory T cell development is observed (15
). However, these studies focused on the outcome of a single antigenic encounter and did not address the role of extrinsic factors in T cell development or how the developmental program is modulated by repetitive stimulation. We show that unlike expansion and contraction, which are programmed during the initial priming interaction (15
T cell function is not hardwired but instead can be altered after priming. Functional alterations occur rapidly in response to persisting viral antigens, as virus-specific T cells primed during an acute infection (in a manner that would otherwise yield a functional response) become inactivated 4 days following transfer into an environment with persistent antigen and fail to develop into memory T cells. In contrast, CD4+
T cells primed in an environment with a heavy antigenic burden can recover from the seemingly inevitable inactivation and ultimately develop into memory T cells by transfer into an environment in which the virus is cleared acutely. The ability to rapidly adjust to the antigenic environment (although apparently detrimental in the case of persistent viral infections) explains how different T cell functions can be elicited depending on the type and duration of viral infection.
A fundamental problem preventing the eradication of persistent viral infections is the early loss and continued lack of effective T cell responses. During persistent viral infections of humans such as HIV, HCV, and HBV, the administration of antiviral therapy can decrease viral replication and correspondingly enhance T cell responses (6
). However, it is unclear from these studies whether the increased T cell activity was due to the functional restoration of nonresponsive T cells or to the generation of new effector T cells. To specifically distinguish between these 2 possibilities, we used traceable populations of virus-specific T cells that were present and primed at the time of infection and could be distinguished from new thymic emigrants. Our studies clearly indicate that virus-specific T cells still retain potential functional capacity despite periods of inactivation and, importantly, T cell activity can be fixed in antigen-experienced, nonresponsive T cells during a persistent viral infection. Although the ability to rescue T cell responses wanes over extended periods of inactivation, indicating a progressive imprinting on these cells, components of CD4+
T cell function remain intact and can be restored throughout persistent infection.
Our studies suggest that the early administration of therapies that decrease viral titers by as little as 1 log can have a substantial impact on the quality of the ensuing T cell responses, particularly CD4+
T cell activity, even in the face of ongoing levels of viral replication. Importantly, the early decrease in viral infection and enhanced T cell activity facilitated long-term control of viral replication well after the therapies were removed. Thus, we demonstrate that there is an optimal time for therapy during persistent viral infection. Administration of therapy late during the chronic stage of infection may not restore function to inactivated T cells, whereas, in contrast, it had a dramatic effect on sustaining function during the acute phase of infection. This finding is consistent with what is observed during HIV infection, where T cell responses are more readily restorable during the acute rather than the chronic phase of infection (23
), and suggests that an intrinsic functional defect likely accumulates in nonresponsive T cells with extended periods of inactivation. It is important to note that although therapy did not restore function during the chronic phase of infection, treatment at this time may still be beneficial when given in conjunction with agents that stimulate naive T cell activation, thereby facilitating productive stimulation of these cells. However, therapies to resurrect T cell activity appear to be most effective when administered early after the establishment of a persistent viral infection.
Excessive antigen stimulation has previously been reported to induce T cell nonresponsiveness (15
). However, hyperstimulation appears not to play a major role in our studies, since DCs from ribavirin-treated mice retained an ability to stimulate SMARTA cells similar to that of cells from untreated mice, and B cells actually became competent stimulators of naive CD4+
T cells following ribavirin treatment. A contributing cause of CD4+
T cell inactivation may be “incorrect” or insufficient stimulation by APCs during the effector phase of a Cl 13 virus infection. This is supported by our data showing that ribavirin-treated mice had elevated levels of costimulatory molecules on MHC class II–bearing cells. Since the signs of CD4+
T cell inactivation are not overtly manifest at day 5 after Cl 13 infection, our data further suggest that CD4+
T cells may still require costimulation during the effector phase to sustain their function. This is consistent with previous findings showing that prevention of costimulation during the chronic phase of Cl 13 infection diminishes antiviral CD8+
T cell responses and prevents control of viral replication (46
). Thus, costimulation by APCs may be required at all phases of infection to sustain productive CD4+
T cell responses and prevent inactivation. However, CD8+
T cell function is not fully restored by an early decrease in viral titers, indicating that different/additional signals, distinct from those of CD4+
T cells, likely modulate CD8+
T cell inactivation. We are currently determining the precise molecules that sustain T cell responses and the cell population(s) that display them in order to facilitate the development of therapeutics to prevent inactivation and restore antiviral responses to overcome persistent viral infections.