Further research into biomarkers for HBV complications may help to predict reactivation of CHB after the endpoint of drug treatment
Hepatitis B is one of the leading causes of death in China, though it is found all over the world. Around 2 billion people worldwide have been infected by HBV (1). Because of the severe complications caused by HBV infection, it is the 10th leading cause of death in the world (2). Sub-Saharan Africa, the East Asia and North America are the endemic regions with high occurrence of HBV infections (3). Hepatitis B disease is caused by the Hepatitis B Virus (HBV), an enveloped DNA virus (4). HBV horizontal transmission is primarily through the exchange of body fluids such as saliva, semen, and blood (3). HBV primarily infects hepatocytes and HBV infection can result in either acute or chronic infection. Furthermore, asymptomatic patients of HBV can be virus carriers that allow transmission of HBV. Many HBV infections occurred via transfusion of donated blood that contains HBV. Vertical transmission can occur through the placenta blood or intake of breast milk. Before the vaccination of infants was implemented extensively around the world, many children were infected with HBV during infancy, and 90% of HBV infected infants became chronically infected with HBV (1). HBV chronic infection can cause severe complications including cirrhosis, hepatocellular carcinoma (HCC) (5). According to Dr. Richard Hunt in the University of South Carolina School of Medicine, 80% of HCC worldwide are caused by HBV infections (3).
Acute Hepatitis B versus Chronic Hepatitis B:
The infection of HBV can be classified into two types: Acute Hepatitis B (AHB), and Chronic Hepatitis B (CHB). Acute Hepatitis B (AHB) is primarily a disease of adulthood that transmitted frequently by sexual exposure or intravenous drug use. However, with the introduction of the HBV vaccine during the 1980s, reported cases of AHB have dropped from 43.4% to 28.5% in Italy and Egypt from 1991 to 2002 (5). Among all the AHB patients, two-thirds of them are asymptomatic, causing cases of HBV transmission through contact of bodily fluids. However, in endemic HBV regions around Asian-Pacific continent, perinatal transmission remains to be the top leading cause of HBV infections. The elevation of serum alanine aminotransferase (ALT) and bilirubin are typical biochemical features of AHB (6). Patients often undergo symptoms such as fever, anorexia, nausea, and jaundice. These symptoms disappear after several months. However, fatigue in patients remains for a much longer time, indicating the immunosuppressed status. During this period, patients are susceptible to the presence of pathogens.
Worldwide, more than 350 million people have chronic Hepatitis B infections. In contrast to AHB, CHB patients can be divided into four phases with distinct symptoms, which are immune tolerance phase, immune clearance phase, immune control phase and immune escape phase. The immune clearance phase is also called the HBeAg-positive phase, showing that the e-antigen can be detected in the blood. This phase is highly relevant to cirrhosis, HCC and other complications of CHB. Similarly, the fourth phase — immune escape phase — also has the risk of complications. Therefore, wide ranges of treatments are used to reduce the risk of complications. Nevertheless, drug therapy is recommended to stop once the patients have achieved HBsAg seroclearance. After the treatment, HBV is not completely eliminated, but rather the viral replication is being suppressed to low levels by the drug, so the reactivation of HBV is still possible among CHB patients (6).
Serologic markers of HBV infection:
Through HBV serum test, several markers can show the presence and phase of infection. HBsAg, the surface antigen of HBV, is one diagnostic marker to test whether the patients are infectious. HBV DNA level is used to measure the viral replication and the disease progression. This is an important indicator to select patients for antiviral treatment and evaluate the risk of developing severe symptoms. The HBeAg level is related to both the HBV DNA level and the infectivity of patients. Anti-HBc demonstrates the persistence of cccDNA, and anti-HBs is an indicator to test the response of vaccine and monitor the treatment after liver transplantation (6).
Starting in 1988, the vaccination of HBV for newborns was implemented worldwide regardless of the HBV history of mothers. The vaccine has efficacy of 80% to 100% and duration of 20 years or more (1). The most common vaccine used is the recombinant HBsAg protein vaccine. The HBsAg gene of a CHB patient was extracted, cloned into the yeast DNA and grown in the yeast cells. The HBsAg is then collected, purified and added with adjuvants (1).
The treatment of HBV includes pegylated interferon (PEG-IFN) and nucleotide analogues. The IFN is a treatment with limited duration because of the occurrence of side effects and high cost of treatment. By enhancing adaptive immune response and inhibit the replication process of HBV in hepatocytes, this drug can sustain the suppression of virus for relatively long periods (7). However, the drug may cause severe side effects, so the use of this treatment cannot be applied to every CHB patient.
Entecavir is one of the most common treatment for CHB infection. This treatment has few side effects and causes less drug resistance of virus. It works by inhibiting the DNA polymerase and thus suppressing the DNA replication (7). The result of treatment includes the decrease in HBV DNA level and ALT level (7). Moreover, from Tang et al., the entecavir treatment can also reduce the incidence of complications such as HCC and complications after 20 months of monitoring.
The newest tenofovir treatment still targets to suppress the viral DNA replication. From the monitoring of treatment efficacy, this treatment can also reduce the effect of complications caused by CHB infection, and no drug resistance records are reported after 6 years of treatment (7).
Since humans are the only reservoir of HBV, the eradication of HBV is possible. However, due to the persistence of HBsAg and cccDNA, eliminating HBV from a CHB patient is difficult. Therefore, the suppression of virus amount and virus replication in patients is crucial for the control of HBV infections among the population (2).
Another big challenge of HBV eradication is the pantropic infection capacity in the body. Although the main target of HBV is the hepatocytes, it also infects the extrahepatic tissues. Therefore, the complete elimination of HBV is extremely difficult in CHB patients. In addition, because of the persistence of virus and the pantropic infection around organs, reactivation of HBV infection is common among cured patients. Patients who have had liver transplantation or discontinuation of drug treatment still have chances for the reactivation of infections (2).
The last difficulty is the economic cost and incomplete vaccination of infants. In order to promote the development of technology and treatment to better diagnose and treat the HBV infections, the expenditure remains to be high. Thus, some developing countries cannot afford the huge cost required for HBV breakthrough (2). Additionally, although the HBV vaccine is 80% to 100% efficacious and has already used worldwide among humans (1), the vaccination of HBV infection does not completely. In rural areas where the hospitals are not authenticated, not every newborn can receive the vaccines. Also, because the HBV vaccine requires injecting for three times, a small proportion of infants may not attend the complete vaccination series, so the memory cells for HBV antibodies may not reach the amount for long-term and effective protection.
For the reasons above, HBV reactivation is still prevalent. Due to the drug resistance and high cost of antiviral drugs, the therapy is recommended to stop once the endpoint has reached. Ideally, the endpoint of HBV treatment is the seroclearance of HBsAg, the surface antigen of Hepatitis B that can be found in serum in 30 to 60 days after HBV exposure, and studies have shown that it is correlated with the amount of cccDNA in the liver (8). However, the relapse rate of HBV infection remains to be as high as 25% to 50% among patients who have discontinued the treatments. In He et al., the researchers investigated the outcomes for the discontinuation of treatment and the appropriate endpoint of treatment. The result showed that HBeAg seroconversion can result in a more durable suppression of HBV. Although the risk of relapse of infection still exits, the relapse rate in the sample has decreased by 10% to 15% (9). In addition, 48 weeks is a marker for long-term suppression of HBV since all the relapses in this case happened in 48 weeks after discontinuation of Nucleotide Analogue treatment (9).
Notably, other factors can also influence HBV activation after stopping the treatment, including age and gender. One study showed that patients with age >50 are recommended to continue their treatment irrespective of their HBsAg levels because they have very high recurrence rates for 84.6% (10). Some studies suggested that patients with age