Threatened abortion and/or threatened preterm labor have been shown to be significant risk factors.11 All those who are infected in utero become HBsAg carriers, but the natural

history of this kind of HBV-infected persons remains unclear. Once the infection becomes chronic, HBsAg carriage is refractory; the average annual incidence of loss CDK inhibitor of serum HBsAg is 0.6% in children.12 Although the low rate of serum HBsAg clearance persists in adulthood, cumulated clearance rate can reach 40% after 25 years of follow up.13 Whether this “seroclearance” of HBsAg means that serum HBsAg decreases to the detection limit of the assay or is a real clearance of HBV from the host remains to be seen. The clinical course of chronic HBV infection is not monotonous; it actually evolves from a replicative phase to a non-replicative phase. Replication of HBV is a prerequisite for hepatic injury. The natural course of chronic HBV infection can be divided into the following phases (Fig. 1).1,8,14,15 In this phase, the host’s immune system is tolerant Ku0059436 to the virus, and the attempts to eliminate the virus are weak or absent. The virus replicates very actively as can be seen from the extremely high serum HBV DNA levels

and the presence of HBeAg as well as the abundant levels of HBsAg in the serum and hepatitis B core antigen (HBcAg) in the hepatocytes. The virus tolerance phase is especially evident in subjects who contract HBV infection through perinatal mother-to-infant see more transmission.1,8,14 In this phase, hepatic injury is minimal, because of the lack of host immune responses against the virus. The disease activity begins to appear or increase after 2–3 decades of virus tolerance. The reason why the previously tolerant host begins to exert efforts to get rid of the virus is unclear. Perhaps due to prolonged carriage of HBV, viral replication starts to wane and the state of immune

tolerance is no longer maintained. Non-cytolytic intracellular inactivation of HBV by certain inflammatory cytokines released by activated lymphomononuclear cells may have an important role in clearing the virus.16 HBcAg/HBeAg-specific cellular immune responses result in lysis of the infected liver cells17,18 (reviewed in 18). The liver then begins to have active disease as revealed by the presence of lobular hepatitis. The previously symptomless HBV carrier may then start to have symptoms of acute hepatitis. However, many remain asymptomatic despite active hepatitis. After a variable period of time, usually decades, the host eventually gets rid of active viral replication and only residual HBV genome is found integrated into the host chromosomes. In both virus tolerance phase and virus clearance phase, HBV replicates actively in the hepatocytes. HBV replication is unique in that in the replication cycle, the viral RNA is reversely transcribed to DNA. This is the target of current antiviral therapy.