Hepatitis D Virus
1. Classification
a. Higher order taxa
Domain: Virus Family: Not Assigned (1) Genus: Deltaviridae (2) Species: Hepatitis D Virus
2. Description and significance
The Hepatitis D virus (HDV) is a unique deltavirus that causes severe liver disease in humans (2) (3). Hepatitis D represents the most chronic form of hepatitis, and there are currently no known cures for the infection (2). What sets HDV apart from other viruses is its compact single-stranded circular RNA genome, which contains only one open reading frame and encodes a single protein, the hepatitis delta antigen (HDAg) (4). HDV co-infects and relies on replication with the Hepatitis B virus (HBV) (5) (6), classifying HDV as a satellite virus. Though found worldwide, HDV infections are more prevalent in regions where HBV is endemic, including parts of Africa, the Middle East, and Asia (1) (4). Despite its simple genome, HDV exhibits a complex replication cycle that utilizes host RNA polymerase for genome replication (7). These unique characteristics, combined with the virus’s geographic spread and dependency on HBV, make it a critical public health concern in many regions. The mechanisms by which HDV accelerates liver disease remain unknown (8), hindering the development of treatments (9).
3. Genome structure
In 2015, the complete genome of a Hepatitis D strain was sequenced. The virus has approximately 1,700 nucleotides, 60% of the sequence containing GC base pairs (10) (11). HDV is a single-stranded and circular RNA genome (1). The genome forms a rod-like structure through internal base pairing (12). Hepatitis D is a negative sense virus, meaning its genome cannot directly serve as mRNA for protein synthesis: the virus must first be transcribed into a complementary positive sense strand before producing proteins (12). HDV encodes only one protein-coding gene, Hepatitis Delta Antigen (HDAg), due to its compact genome and one open reading frame (1) (4). The genome has no non-coding genes or multiple coding genes. Through RNA editing, HDAg is made into two forms: the small HDAg (S-HDAg) and the large HDAg (L-HDAg). S-HDAg is responsible for genome replication, while L-HDAg is essential for viral assembly and the packaging of the RNA (1) (4). S-HDAg is converted to L-HDAg when a stop codon in S-HDAg is converted into a tryptophan codon (1). HDV is further divided into eight genotypes (HDV-1 to HDV-8) based on genomic similarity, with HDV-1 being the most widespread globally and other genotypes showing more restricted geographic distributions, such as HDV-2 in Asia and HDV-3 in South America (13).
HDV replicates its genome using a “rolling circle” mechanism (7). The virus takes over an RNA-dependent synthesis mediated by a host RNA Polymerase II to transcribe its RNA genome. The genome of HDV contains regions that can interact with host machinery (5). As a result, promoters can recruit host RNA Polymerase II (7). This process happens in the nucleus of the host cell (5). HDV contains two self-cleaving ribozymes that help produce its RNA into functional units. These ribozymes, located in the antigenomic strands of the genome, facilitate the cleavage of multimeric RNA during the rolling circle mechanism (5).
4. Cell structure
HDV is a small, enveloped virus, approximately 35-37 nm in diameter (6). Its structure includes a lipid envelope derived from the host cell containing Hepatitis B surface antigen (HBsAg) proteins essential for viral entry into liver cells. A notable feature of HDV is its dependency on the Hepatitis B virus (HBV) (5) (6). The presence of HBV is necessary for developing the HDV envelope to form. Embedded in this envelope are the glycoproteins from the co-infection HBV, which are crucial for HDV’s ability to attach to and enter hepatocytes (liver cells). The HDV envelope interacts with sodium taurocholate cotransporting polypeptide (NTCP) receptors on hepatocytes, a key structural interaction required for viral entry into liver cells. NTCP serves as the primary receptor that facilitates the attachment and uptake of HDV, highlighting its critical role in the virus’s ability to infect the host (14) (15). The HDV virions are assembled, released, and dispersed with the aid of the HBsAg proteins (16). The hepatitis B surface antigen proteins form the viral envelope around HDV, which facilitates the virus’s entry into liver cells and helps it avoid detection by the host immune system. HBsAg is essential for HDV to complete its replication cycle and produce infectious viral particles (8) (9).
5. Metabolic processes
HDV is characterized as a satellite virus (8), which requires another virus to infect a cell. In the case of HDV, co-infection with Hepatitis B, which acts as the helper virus (16), is required. HDV does not have metabolic pathways for generating energy. Instead, HDV uses the host cell’s machinery to replicate its genome and survive (3).
HDV has no metabolic enzymes of its own but instead hijacks the host cell’s metabolic machinery, using the host cell processes to fulfill its energy requirements (17). There is only one protein synthesized by HDV, and it is the hepatitis delta antigen (HDAg) (1). HDAg exists in two isoforms, the small HDAg (S-HDAg) and the large HDAg (L-HDAg). S-HDAg is responsible for viral RNA replication, while L-HDAg’s function is to package the viral genome into new virions (8). In addition to its role in packaging, L-HDAg interacts with host transcription factors such as Smad3 and Twist, which alter the machinery of the host cell’s metabolic regulation by influencing pathways in processes such as cellular remodeling, energy use and replication machinery, which allow HDV to efficiently exploit resources of the host cell for survival (8). An additional protein that HDV takes advantage of is JAK1, which is involved with cellular signaling and the host’s immune response; JAK1 facilitates the phosphorylation processes that are required for HDV replication, indirectly influencing the energy use of the host cell and its machinery to support viral survival (9).
6. Ecology
HDV is found worldwide, but its prevalence varies depending on the region. Hepatitis D is more common in areas where HBV is widespread, since HDV requires HBV to survive and spread amongst the citizenry (1) (4). In regions with high rates of HBV infection, such as in Africa, the Middle East, or Asia, Hepatitis D infections are also more frequent (2). HDV thrives in environments where Hepatitis B is endemic, often in settings where there is high-risk sexual behavior or in areas with high intravenous drug use (2) (3) (4). The existence of many viral genotypes is one factor that affects the spread of hepatitis D. The most prevalent HDV genotype worldwide is Genotype 1, or HDV-1. Compared to other genotypes, this one frequently results in more severe cases of liver disease (13). In contrast, Genotype 2, or HDV-2, is found mainly in Eastern Asia and is associated with milder symptoms (13). Other genotypes, such as HDV-3, are found in particular regions like the Amazon Basin and are linked to varying disease severity (13).
Hepatitis D is mainly transmitted through contact with infected blood, with one of the most common routes being the sharing of needles among intravenous drug users (IDUs) (2). Given that both HBV and HDV depend on blood-to-blood transmission, this practice dramatically aids in the spread of both viruses in these communities (3). Although less frequent, vertical transmission from mother to child during childbirth occurs, especially when the mother has both HDV and HBV co-infections (3).
Hepatitis D replicates in hepatocytes (liver cells). Since Hepatitis D cannot make its own outer coat, it uses the Hepatitis B surface proteins to form new virus particles (16). This process only happens in individuals already infected with Hepatitis B (16). The virus relies on the same conditions in which HBV thrives, such as the presence of liver cells and the ability to use the host cells’ machinery to replicate (16). Efforts have been made to reduce Hepatitis D infection, with the most common focusing on preventing the infection of Hepatitis B in individuals. Since Hepatitis D can only infect those already carrying Hepatitis B, widespread vaccination Hepatitis B programs have been successful in reducing the number of cases of Hepatitis D infection in some countries (18). For instance, rates of both Hepatitis D and Hepatitis B dramatically dropped in nations where individuals are regularly vaccinated against Hepatitis B (18). However, HDV remains a significant public health concern in regions with limited access to healthcare and vaccination programs (18). Even among those already infected with Hepatitis B, testing specifically for HDV is not regular. This proves difficult when estimating the true prevalence of the virus in certain regions, such as in Slovakia, where regular testing was finite, even while the virus still posed a significant risk to individuals with chronic Hepatitis B (18).
7. Pathology
HDV is a pathogen that exclusively infects humans and is always associated with HBV. HDV can only infect individuals who are also infected with HBV, as the HBV surface antigen is required for its replication and the formation of new viral particles (4) (16). There are two main types of HDV infection: coinfection and superinfection (2) (3). Coinfection occurs when individuals are concurrently infected with both HBV and HDV. Superinfection occurs when an individual already chronically infected with HBV later acquires HDV (3). Superinfection is typically more severe and progresses more rapidly to liver cirrhosis and hepatocellular carcinoma (3) (8). Chronic HDV can accelerate liver damage and lead to severe complications, including fibrosis and cirrhosis, much faster than with just HBV monoinfection. This rapid progression is due to HDV’s ability to cause more severe liver inflammation and liver damage (3). HDV induces liver damage by promoting the transformation of liver cells through epithelial-mesenchymal transition (EMT), which is a critical mechanism in developing liver fibrosis. This transformation is driven by the large hepatitis delta antigen (L-HDAg), which interacts with host factors like Smad3 and Twist, promoting liver fibrosis and cirrhosis (8).
Although the precise mechanisms of the pathogenesis of Hepatitis D are still being researched, infection by HDV is known to entice a robust immunological response. Hepatitis D frequently progresses to chronic liver inflammation and a rise in the incidence of liver decompensation, fibrosis, and cirrhosis (5). HDV also influences the activity of cytokines, like TNF-α, which can further exacerbate liver damage (19).
A difficulty in understanding HDV pathogenesis lies in the lack of small animal models that can emulate human disease progression, which limits the capability to study the virus in preclinical settings. Research from 2018 worked on developing mouse models with human proteins such as NTCP to better study HDV pathogenesis and its response to medications (5). Other research focuses on understanding how HDV interacts with the host’s immune system and how the virus evades immune responses to maintain chronic infection (9) (16).
8. Current Research
Due to the severity of the virus, current research on HDV is advancing rapidly, focusing on developing new antiviral therapies. Currently, no treatment efficiently cures HDV patients (3). One significant breakthrough is Bulevirtide, also known as Myrcludex B. This drug blocks HDV entry into liver cells by inhibiting NTCP, a receptor crucial for the virus’s attachment (14) (15). Clinical trials demonstrated that Bulevirtide significantly reduces HDV RNA levels and improves liver function (14) (15). While Myrcludex B has opened new avenues for HDV treatment, existing therapies like pegylated interferon-alpha have shown limited success. Clinical trials combining Myrcludex B with interferon treatments have yielded the most significant reductions in HDV levels and liver damage, as indicated by decreased ALT (liver enzyme) levels (14).
HDV’s ability to evade the immune system contributes to its persistence and progression to severe liver diseases like cirrhosis and hepatocellular carcinoma (9). Although HDV triggers immune responses, it often progresses to chronic infection by manipulating host factors and signaling pathways to evade detection. One such target is JAK1, a protein involved in the body’s immune response and crucial for HDV replication. Inhibiting JAK1 can reduce HDV replication by disrupting viral processes within host cells (9). Medications commonly used for more prevalent health problems, such as arthritis, that target JAK1 were tested in infected cellular models, which demonstrated positive results in reducing, but not fully curing, HDV (9). Researchers continue to study how HDV manipulates host factors like JAK1, aiming to develop host-targeted antiviral therapies that leverage existing drugs to suppress HDV replication and improve the prognosis for chronically infected individuals (9).
Traditional animal models have been inadequate for mimicking human HDV progression (5). However, researchers have developed genetically humanized mouse models expressing the human NTCP protein, allowing HDV infection (5). These models are being used to test therapies such as Myrcludex B and Lonafarnib, the latter of which inhibits HDV replication (5).
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Edited by students of Jennifer Bhatnagar for BI 311 General Microbiology, 2024, Boston University.
