Immune exhaustion and immune senescence: two distinct pathways for HBV vaccine failure during HCV and/or HIV infection

Given the shared risk factors for transmission, co-infection of hepatitis B virus (HBV) with hepatitis C virus (HCV) and/or human immunodeficiency virus (HIV) is quite common, and may lead to increases in morbidity and mortality. As such, HBV vaccine is recommended as the primary means to prevent HBV super-infection in HCV- and/or HIV-infected individuals. However, vaccine response (sero-conversion with a hepatitis B surface antibody titer >10 IU/L) in this setting is often blunted, with poor response rates to standard HBV vaccinations in virally infected individuals when compared with the healthy subjects. This phenomenon also occurs to other vaccines in adults, such as pneumococcal and influenza vaccines, in other immunocompromised hosts who are really at risk for opportunistic infections, such as individuals with hemodialysis, transplant, and malignancy. In this review, we summarize the underlying mechanisms involving vaccine failure in these conditions, focusing on immune exhaustion and immune senescence--two distinct signaling pathways regulating cell function and fate. We raise the possibility that blocking these negative signaling pathways might improve success rates of immunizations in the setting of chronic viral infection.