Author: Tara Kumar Bailkeri
Mentor: Dr. Ana-Maria Ortega-Prieto
Primus Public School, Bangalore
Hepatitis B is a small DNA virus with a diameter of approximately 42nm (Liang, 2009). Globally, around 296 million individuals are infected with the Hepatitis B Virus (HBV), and this infection often leads to the development of cancers and other diseases. Although treatment options exist for HBV, a complete cure is currently unavailable due to the stable nature of its genetic material within host cells, making elimination challenging. Additionally, the virus shares similarities with the human immunodeficiency virus (HIV) and studying these similarities can provide insights into the less understood aspects of HBV infection.
Hepatitis B Virus overview and relevance
Hepatitis B infection is caused by the Hepatitis B virus, which is a DNA virus with a partially double-stranded relaxed circular DNA (rcDNA). Its genetic material comprises four open reading frames, which code for seven viral proteins and does not have any non-coding sections. It belongs to the family Hepadnaviridae and possesses an outer envelope composed of lipids and surface antigens known as HBsAg (there are three forms of HBsAg: small S, middle S, and large S). The viral core is composed of core antigens, HBcAg. Another antigen, HBeAg, is closely associated with the viral nucleocapsid and can circulate in the blood since it is a soluble protein. At the 5’ end of the single-stranded section of DNA, there is an attachment to a viral RNA-dependent DNA polymerase (Aryal, 2015).
HBV infection can manifest as either chronic or acute. Only a small percentage, specifically 5%-10%, of adults who contract the virus progress to chronic hepatitis B. However, the risk of developing a chronic infection is significantly higher in individuals who acquire the infection at a younger age, with as many as 90% of infants infected with HBV likely to develop a chronic infection. Approximately 25% of chronic HBV cases advance to liver cancer, making HBV infection the primary cause of liver cancer worldwide (Fast Facts on Global Hepatitis B, 2022). Annually, approximately 820,000 people lose their lives due to HBV infections.
Hepatitis B is transmitted through exposure to blood and bodily fluids, including mother-to- child transmission, and primarily affects hepatocytes. The majority of HBV infections proceed without symptoms (asymptomatic), although an estimated 30% to 50% of infected individuals may experience symptoms such as joint pain, nausea and vomiting, dark urine, pale stool, jaundice, and upper right abdominal pain. Chronic HBV infections can result in liver cirrhosis due to inflammation and increase the risk of developing liver cancers (Aryal, 2015).
Hepatitis B can be diagnosed through a physical examination followed by blood tests, known as a hepatitis panel. These tests specifically check for hepatitis antibodies and antigens (Hepatitis Panel, 2022). A vaccine is available for HBV, and the World Health Organization (WHO) recommends its administration immediately after birth, followed by completion of the 3-dose vaccination series (Hepatitis B, 2022). The current treatment for Hepatitis B virus (HBV) infection focuses on suppressing viral replication and reducing liver inflammation. Antiviral medications, such as nucleoside and nucleotide analogues, are commonly used to achieve these goals. Interferon therapy, which stimulates the immune system, can also be employed. Combination therapy may be considered in certain cases. It is important to note that while treatment can effectively control the virus, it does not eradicate HBV from the body. Long-term or lifelong treatment may be necessary. Regular monitoring and adherence to medication and lifestyle modifications are essential. Individualized treatment plans should be determined in consultation with healthcare professionals.
Hepatitis B poses several challenges for cure. Once inside the cell, it forms a highly stable minichromosome that is difficult to eliminate. Additionally, the virus employs multiple immune evasion strategies to avoid detection by the body, hindering treatment efforts. The progression of the disease is still not fully understood, which makes preventing chronic infection a challenging task. In this context, we delve into the intricacies of these challenges, including the mechanisms of immune evasion, the difficulties associated with eradicating the HBV genome from infected cells, and the similarities between HBV and HIV infections.
Life Cycle of HBV
The entry of HBV into a cell is regulated by two surface receptors: HSPGs (Heparan Sulfate Proteoglycans) and NTCP (Sodium Taurocholate Co-transporting Polypeptide). HbsAg initially binds with low affinity to HSPGs and subsequently with high affinity to NTCP, facilitating the virus’s entry into the cell through receptor-mediated endocytosis. Once inside, the virus undergoes uncoating, and the nucleocapsid is transported to the nucleus via the cell’s microtubule system. The specific timing and location of uncoating remain a mystery.
The compact size of the entire HBV nucleocapsid (~42nm) enables it to enter the nucleus through a nuclear pore complex. Upon nuclear entry, the DNA and capsid separate, and the DNA is converted into covalently closed circular DNA (cccDNA). This cccDNA is highly stable and functions as a “mini chromosome,” being chromatinized, associated with proteins, and subject to transcriptional regulation (Van Damme, Vanhove, Severyn, Verschueren, & Pauwels, 2021). Even in patients who have fully recovered from HBV infection, detectable levels of cccDNA persist and can be reactivated under immunosuppression, highlighting its stability (Hong, Kim, & Guo, 2017).
In addition to encoding viral proteins, the cccDNA also serves as a template for the virus’s pregenomic RNA (pgRNA), which associates with the viral DNA polymerase and is encapsidated to form an immature virion. The pgRNA is subsequently reverse transcribed and matures into rcDNA. Mature virions can either exit the cell through exocytosis to infect other cells or re-enter the nucleus to contribute to the cccDNA pool. Immature virions are highly phosphorylated, preventing their exit from the cell. As they mature, the phosphate groups are removed, allowing the mature virions to move to the Golgi body for exocytosis (Schädler & Hildt, 2009).
Formation and regulation of HBV cccDNA
After entering the nucleus, the HBV rcDNA is separated from the viral polymerase. The partially double-stranded DNA undergoes completion, and its ends are covalently ligated by host DNA ligases, leading to the formation of covalently closed circular DNA (cccDNA) through a damage response pathway. The cccDNA becomes associated with both histone and non-histone proteins, resembling human chromatin (Xia & Guo, 2020), and it is transcribed by host RNA polymerase-II. The transcriptional state of cccDNA is regulated by modulating the acetylation states of the histones bound to cccDNA.
cccDNA contains three CpG islands (CGIs) where DNA methylation can occur. Methylation at CGI-1 is rare, while methylation at CGI-2 is associated with low viral loads due to reduced pgRNA synthesis and lower cccDNA replication. Methylation at CGI-3 is associated with the development of hepatocarcinoma (Hong, Kim, & Guo, 2017).
The transcriptional activity of HBV cccDNA is modulated by four promoters and two enhancers. Enhancer I controls the activation of HBx protein transcription, while enhancer II controls the transcription of all other genes (Xia & Guo, 2020). The viral HBx protein serves as a transactivator, promoting the transcription and expression of HBV proteins. The absence of HBx is associated with histone deacetylation and the accumulation of repressive markers in cccDNA (Hong, Kim, & Guo, 2017).
Immune evasion by HBV
Due to the persistent and highly stable nature of cccDNA, complete eradication of HBV from infected cells is challenging. Therefore, an effective approach to combat HBV infection is to eliminate the infected cells, which is accomplished through the immune system’s response upon viral detection.
However, HBV has developed strategies to evade detection by the host immune system. Upon HBV infection, the downregulation of class-I MHC molecules leads to the activation of natural killer (NK) cells. In chronic infection, NK cells remain chronically active and contribute to liver fibrosis. Kupffer cells (KCs) increase the expression of class-II MHC molecules to prime CD4+ and CD8+ T cells, but during chronic HBV infection, KCs exhibit abnormal function, possibly due to the downregulation of TLR-2. Additionally, during chronic HBV infection, dendritic cells (DCs) have impaired production of IFN-α.
To evade antibody detection, HBcAg and HBsAg are secreted in large amounts, effectively masking other antigens through extensive antibody binding. HBsAg has been found to downregulate TLR-3 signalling. STAT-1 and STAT-2 play a role in downregulating cccDNA activity and are critical in the interferon-induced JAK/STAT (Janus Activated Kinase/Signal Transduction and Activator of Transcription) pathway. The HBV polymerase prevents the entry of STAT-1/STAT-2 heterodimers into the nucleus, effectively inhibiting the interferon response. HBeAg inhibits TLR-2 expression in KCs, impairing their response. Additionally, various HBV proteins directly interact with and inhibit host cell immune activation. During chronic infection, HBV can selectively lose its HBeAg to conceal itself from immune detection (Ortega-Prieto & Dorner, 2017).
Similarity to HIV
HIV (Human Immunodeficiency Virus) is a retrovirus that primarily targets CD4+ T-cells. It encodes a reverse transcriptase and, similar to other retroviruses, integrates into the host cell genome with the assistance of integrase. HBV possesses an RNA-dependent DNA polymerase, functioning as a reverse transcriptase, enabling replication. Both viruses exhibit high mutation rates during reverse transcription due to the absence of exonuclease proofreading, resulting in hypermutability (Roberts, Bebenek, & Kunkel, 1988). Consequently, HIV and HBV exist as quasispecies, characterized by significant genetic heterogeneity (Nowak, 1992).
Due to the persistence of viral genetic material within the host cell nucleus, complete eradication of HIV or HBV has proven challenging. Current treatments focus on reducing the viral load to undetectable levels. Although most HBV patients achieve complete recovery with undetectable HBV DNA levels, HIV rarely leads to recovery. However, in cases of chronic HBV infection, both HIV and HBV can remain dormant for years before reactivation (Sharma, Saini, & Chawla, 2005).
Individuals with HIV have a higher susceptibility to various cancers, including “AIDS-defining cancers” like Kaposi’s Sarcoma, due to their compromised immune system (Hernández- Ramírez, Shiels, Dubrow, & Engels, 2017). Similarly, individuals with HBV are more prone to gastrointestinal and liver cancers due to genomic instability caused by cccDNA and the activity of HBx protein (Massimo & Zucman-Rossi, 2016). Upon HIV infection, immune evasion prevents the activation of interferon (IFN) responses (Guha & Ayyavoo, 2013). Similarly, in HBV infection, the JAK/STAT pathway is blocked, impairing the IFN response.
Every year, hepatitis B claims the lives of 820,000 individuals. Gaining a comprehensive understanding of the disease mechanisms and the challenges associated with finding a cure is crucial. Currently, it is known that the stability of HBV cccDNA, resembling a human chromosome, hinders its complete removal from infected cells. Furthermore, HBV employs a diverse array of immune evasion strategies, such as the overproduction of HBcAg and HBsAg, which enable it to persist undetected in the body for extended periods. Its resemblance to the deadly Human Immunodeficiency Virus suggests that insights from one may aid in finding a cure for the other.
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