Hepatitis B is one of five types of hepatitis viruses that infect the liver; the others are hepatitis A, C, D, and E. While historical references to jaundice occur throughout documented history, scientific characterization of these viruses only began in the late 1960s. A 2017 World Health Organization report estimated a global burden of 257 million cases of chronic hepatitis B (HBV) infection leading to 900,000 annual deaths (World Health Organization 2017). It is estimated that 29% of cirrhosis-related deaths and 39% of liver cancer deaths worldwide were due to hepatitis B in 2017, with hepatitis C virus, alcohol-associated liver disease, and non-alcoholic fatty liver disease also being key drivers, but at lower levels (Paik et al. 2020).
Ten HBV genotypes (A to J) and several subtypes of HBV have been identified. The genotype and subtype influence the course of the disease (Lin & Kao 2015). HBV infections fall into the acute category where the disease resolves within a few months, or the chronic state where HBV infection persists beyond six months. Healthy adults generally have an acute infection, many without symptoms, and tend to recover. However, around 5-10% of healthy adults will not clear the virus and will develop a chronic HBV infection. Chronic infection is also the outcome of most infections in newborns and accounts for approximately half of the HBV infections in children under five years old.
Acute HBV infection in the liver leads to the release of pathogen-associated molecular patterns (PAMPs). These are bound by the pattern recognition receptors (PRRs) of tissue-resident macrophages (Kupffer cells) which activate the nuclear factor kappa B (NF-kappaB) leading to a transient release of the proinflammatory cytokine interleukin-6 (IL-6), followed by IL-1beta, IL-8, and TNF-alpha. However, no interferon (IFN) response, associated with viral infections, occurs. IL-6 activates the mitogen-activated protein kinases (MAPK), extracellular signal-regulated kinase 1,2 (ERK1,2), and c-jun N-terminal kinase (JNK), which in turn inhibit the hypoxia-inducible factor-1 alpha (HIF-1α) and HIF-4α transcription factors. The latter two are essential for HBV gene expression and replication and they suppress viral replication. However, their activation also limits the activation of the adaptive immune response and enables HBV-infected hepatocytes to survive (Hösel et al. 2009). The lack of an IFN response and expression of interferon-stimulated genes (ISGs) was also shown in other studies indicating a limited role for the innate immune system in HBV infection (Wieland et al. 2004, Dunn et al. 2009). In chronic HBV infection, the data points to an even more inhibited innate immune system. Studies found that a range of ISGs (CXCL10, GBP1, IFITM1, IFNB1, IL10, IL6, ISG15, TLR3, SOCS1, and SOCS3) were strongly downregulated (Lebossé et al. 2017). In vitro work showed that the hepatitis B surface antigen (HBsAg) can inhibit IFN-alpha secretion in plasmacytoid dendritic cells, caused by TNF-alpha and IL-10 induction in monocytes (Shi et al. 2012). HBsAg could suppress activation of the NF-kappaB pathway by interfering with phosphorylation and polyubiquitination of TAK1 and TAB2 (Deng et al. 2021). Intriguingly, HBV does not interfere with the ability of the innate immune system to mount a response, since immune cells from liver resections of patients with chronic HBV were stimulated when exposed to other factors such as TLR3 stimulation or Sendai virus infection (Suslov et al. 2018).
CD8+ T cells seem to be the key effector population required to gain control of HBV infection. Depletion studies in animal models demonstrated that infection took longer to clear if CD8+ T cells were removed, in comparison to CD4+ T cells. HBV-specific CD4+ T cells collaborate in viral clearance by secreting IFN-gamma and supporting the function of CD8+ T cells, which clear the viral burden through noncytolytic and cytolytic effector functions (Thimme et al. 2003). This noncytolytic viral elimination was confirmed to work via IFN-gamma and TNF-alpha, reducing levels of HBV covalently closed circular DNA (cccDNA) in hepatocytes (Xia et al. 2016). Additionally, in acute HBV infection, the early anti-viral response is subjected to an IL-10 induced temporary attenuation of NK and T cell responses (Dunn et al. 2009) which then recovers. Chronic HBV infection is characterized by immune dysfunction and persistent infection.
The high level of viral antigens in the liver during chronic infection leads to T cell exhaustion. This can be measured by flow cytometry showing high-level expression of PD1, TIM3, CTLA4, and CD244 (Raziorrouh et al. 2010, Schurich et al. 2011, Nebbia et al. 2012, Bengsch et al. 2014). Additional barriers to resolving chronic HBV infections are the destruction of virus-specific CD8+ T cells through deletion. These T cells are eliminated by apoptosis through increased expression of BIM (Lopes et al. 2008), or increased levels of TRAIL-R2, which makes them a target of TRAIL+ liver NK cells.
While the characterization of immune responses to HBV has made progress and therapeutics and vaccines are available, no “functional cure” that removes the HBsAg fully, most likely through suppression of cccDNA, exists. Meaning more work is necessary to define immunoregulatory aspects holding back the immune response and seek better treatments (Meng et al. 2020). Additional research to understand the balance between immune tolerance and immune-mediated liver injury is equally important.
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