During acute hepatitis B virus (HBV) infection viral loads reach high levels (Ϸ10 10 HBV DNA per ml), and nearly every hepatocyte becomes infected. Nonetheless, Ϸ85-95% of infected adults clear the infection. Although the immune response has been implicated in mediating clearance, the precise mechanisms remain to be elucidated. As infection clears, infected cells are replaced by uninfected ones. During much of this process the virus remains plentiful but nonetheless does not rekindle infection. Here, we analyze data from a set of individuals identified during acute HBV infection and develop mathematical models to test the role of immune responses in various stages of early HBV infection. Fitting the models to data we are able to separate the kinetics of the noncytolytic and the cytolytic immune responses, thus explaining the relative contribution of these two processes. We further show that we need to hypothesize that newly generated uninfected cells are refractory to productive infection. Without this assumption, viral resurgence is observed as uninfected cells are regenerated. Such protection, possibly mediated by cytokines, may also be important in resolving other acute viral infections.immune response ͉ mathematical modeling ͉ viral kinetics H epatitis B virus (HBV) is a small (Ϸ3.2 kb) partially dsDNA virus that infects hepatocytes (1). There are Ͼ350 million chronic HBV carriers worldwide, and infection with HBV is the cause of significant morbidity and mortality in countries of high prevalence (2). During acute infection, HBV viral loads can reach high levels, up to 10 10 HBV DNA copies per ml of plasma, which last for several weeks, coincident with HBV infection of a large percentage of hepatocytes (3-6). Subsequently, viral loads decrease, and in 85-95% of acutely infected adults the infection is cleared (7). Patients that clear the virus tend to have stronger and more diversified CD4 ϩ and CD8 ϩ T cell responses (8, 9). In addition, clearance of the virus in acutely infected patients is usually accompanied by an alanine aminotransferase (ALT) flare. Elevated levels of ALT are indicative of liver damage and an active cell-mediated immune response. Although the immune response plays a crucial role in decreasing the viral load, the precise mechanisms are not fully understood (10).Studying the immune response to HBV during acute infection of humans is difficult, and the field has progressed by the study of experimental infection in chimpanzees, woodchucks, and ducks and transgenic mice that express HBV genes. These studies demonstrated the importance of the immune response, because acute infection in chimpanzees depleted of CD8 ϩ T cells results in delayed HBV clearance and recovery (11). However, chimpanzee studies have also shown that the initial reduction in HBV viral load occurs much earlier than any detectable cytolytic immune response, liver T cell infiltration, or liver damage (3), suggesting that some form of noncytolytic response is involved. The importance of a noncytolytic response has been conf...

Mathematical models have been used to understand the factors that govern infectious disease progression in viral infections. Here we focus on hepatitis B virus (HBV) dynamics during the acute stages of the infection and analyze the immune mechanisms responsible for viral clearance. We start by presenting the basic model used to interpret HBV therapy studies conducted in chronically infected patients. We then introduce additional models to study acute infection where immune responses presumably play an important role in determining whether the infection will be cleared or become chronic. We add complexity incrementally and explain each step of the modeling process. Finally, we validate the model against experimental data to determine how well it represents the biological system and, consequently, how useful are its predictions. In particular, we find that a cell-mediated immune response plays an important role in controlling the virus after the peak in viral load.

Hepatitis B is a DNA virus that infects liver cells and can cause both acute and chronic disease. It is believed that both viral and host factors are responsible for determining whether the infection is cleared or becomes chronic. Here we investigate the mechanism of protection by developing a mathematical model of the antibody response following hepatitis B virus (HBV) infection. We fitted the model to data from seven infected adults identified during acute infection and determined the ability of the virus to escape neutralization through overproduction of non-infectious subviral particles, which have HBs proteins on their surface, but do not contain nucleocapsid protein and viral nucleic acids. We showed that viral clearance can be achieved for high anti-HBV antibody levels, as in vaccinated individuals, when: (1) the rate of synthesis of hepatitis B subviral particles is slow; (2) the rate of synthesis of hepatitis B subviral particles is high but either anti-HBV antibody production is fast, the antibody affinity is high, or the levels of pre-existent HBV-specific antibody at the time of infection are high, as could be attained by vaccination. We further showed that viral clearance can be achieved for low equilibrium anti-HBV antibody levels, as in unvaccinated individuals, when a strong cellular immune response controls early infection.

Understanding the mechanisms governing host-pathogen kinetics is important and can guide human interventions. In-host mathematical models, together with biological data, have been used in this endeavor. In this review, we present basic models used to describe acute and chronic pathogenic infections. We highlight the power of model predictions, the role of drug therapy, and advantage of considering the dynamics of immune responses. We also present the limitations of these models due in part to the trade-off between the complexity of the model and their predictive power, and the challenges a modeler faces in determining the appropriate formulation for a given problem.

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