Cytokines in viral hepatitis.

Cytokines play an important role in the defense against viral infections, both indirectly, through determination of the predominant pattern of host response, and directly, through inhibition of viral replication. However, in the context of an inflammatory response against a virus, cytokines may also lead to liver damage. The importance of this is best demonstrated in hepatitis B virus (HBV). In acute HBV infection, a vigorous polyclonal cellular immune response is critical; thus type 1 cytokine release is essential to initiating an effective immune response. The cytokines released by CD4+ and CD8+ cells also play an important role in downregulation of HBV replication, demonstrating that it is possible to control a viral infection without the death of infected cells. However, if there is a defect in the acute response, HBV becomes chronic; in that case, the presence of an ongoing suboptimal inflammatory response can activate the process of hepatic fibrosis. In hepatitis C infection, the role of cellular immune responses and cytokines is less clear. Hepatitis C may be resistant to inhibition by cytokines, so cytokines may have a more prominent role in liver damage than in controlling viral replication. Both hepatitis B and C may have specific mechanisms to inhibit cytokine production, highlighting the critical role of these molecules in recovery from infection.