Chronic infections with hepatotropic viruses: Mechanisms of impairment of cellular immune responses

Chronic hepatitis B and C cause significant morbidity and mortality worldwide. Antiviral therapy suppresses but does not eliminate chronic hepatitis B virus (HBV) infection, and it is effective in only half of all hepatitis C virus (HCV)-infected patients. Because adaptive immune responses are associated with spontaneous resolution of acute HBV and HCV infection, therapeutic enhancement of immune responses has been proposed as alternative or supplementary therapy for chronic infection. However, all efforts have been hampered by poor proliferation and effector functions of HBV- and HCV-specific CD4 and CD8 T cells, which are thought to be due to T cell exhaustion, high antigenic load, and viral escape. Recent studies revealed that endogenous factors, such as regulatory T cells, immuno suppressive cytokines, and inhibitory receptors, also contribute to the impairment of virus-specific T cell responses in chronic infection, perhaps reflecting the host's attempt to protect itself against immune-mediated pathology. These endogenous mechanisms and potential avenues to revert them are the subject of this review.