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All three envelope proteins, large (L), middle (M) and small (S) share the same C-terminal domain, which contains the HBsAg, while the L and M proteins display progressive N-terminal extensions.5 The anti-HBs neutralisation domain has been mapped to amino acids 99–170 of the S-protein and is referred to as the α or ‘a’ determinant.10 Characteristic of HBV infection is the presence of non-infectious structures, the spherical and filamentous subviral particles (SVPs), which are exclusively composed of viral envelope proteins and host-derived lipids5 and are secreted in a 103–106-fold excess into the blood of infected individuals (figure 1).
HBV also produces and secretes a non-structural form of the nucleoprotein, the PC protein or HBeAg, which is not required for viral replication, but displays immune-modulating functions and hence contributes to viral persistence. The HBeAg is translated initially as the PC protein P25, and then undergoes cleavage of the N-terminal signal sequence in the ER/Golgi complex, producing a 22 kDa protein (P22), which can either undergo further processing to form the secreted 17 kDa HBeAg (P17), or traffic to the cytosol where it remains localised.11 Except for a 10-amino-acid N-terminal extension, which is highly conserved in all orthohepadnaviruses and contains a putative toll-interleukin-1 receptor (TIR) domain,12 HBeAg shares significant homology with the core protein (P21), which is translated using the core AUG codon of the same ORF.13
The first step in HBV infection involves a non-cell-type specific primary attachment which is then followed by an irreversible binding of the virus to a still unknown hepatocyte-specific receptor.5 14 Following viral entry, the capsids accumulate at the nuclear membrane, where interactions with the nuclear pore complex may favour the release of the HBV genome (rcDNA) into the cell nucleus.15 To establish a productive HBV infection host components of the cellular replicative machinery are needed to remove the covalently attached terminal protein and to complete the positive-DNA strand to form a super-coiled cccDNA molecule, which after being associated with histone and non-histone proteins, is incorporated into the host chromatin.6 9 16 Unlike the provirus DNA of retroviruses, the cccDNA does not need to be integrated into the host genome. Albeit integrations of HBV-DNA sequences do occur in infected hepatocytes, particularly in the presence of DNA damage17 and cell turnover,18 19 these are typically truncated.
Camouflaged as a minichromosome, and hence undetectable by innate immune defense mechanisms, the cccDNA utilises the cellular transcriptional machinery to produce all transcripts necessary for protein production and viral replication, which takes place in the cytoplasm after reverse transcription (rt) of a over-length pregenomic RNA (pgRNA).9 Viral transcription is under the control of two enhancer elements and four distinct viral promoters. Similarly to cellular chromatin, the transcription of the different viral genes is regulated by the activity and dynamic interplay of numerous cellular transcription factors, coactivators, corepressors and chromatin modifying enzymes.6
Experimental studies in the duck model indicated that the cccDNA can be formed from incoming virions and from newly synthesised nucleocapsids, which instead of being enveloped and secreted into the blood, are rather transported into the nucleus to ensure accumulation, and later maintenance, of the cccDNA pool9 20 (see figure 1).
Although a high cccDNA copy number is detected in chronically infected ducks and woodchucks,21 22 lower cccDNA intrahepatic loads are generally determined in human liver biopsies (median 0.1–1 cccDNA copy/cell)23–28 and in HBV chronically infected human liver chimeric uPA/severe combined immunodeficient (SCID) mice,29–32 suggesting that different viral and host mechanisms may control cccDNA dynamics and cccDNA pool size in human infected hepatocytes.6 In this regard, a recent study elegantly showed that HBV converted the rcDNA into cccDNA less efficiently than DHBV in the same human cell background.33 Although the cccDNA pool appears to be very stable in the absence of cell division, accumulating evidence indicates that hepatocyte proliferation can induce drastic reduction of the intrahepatic cccDNA loads in vivo.19 32 Since the cccDNA lacks centromere structures ensuring correct migration during mitosis, immune-mediated cell death and compensatory hepatocyte regeneration may facilitate cccDNA loss. This could represent a weak point in HBV persistence deserving further investigation as new therapeutic concepts are explored.32
The next crucial step in HBV replication is the packaging and rt of pgRNA within the newly formed capsids. After rt and concomitant degradation of the pgRNA, first a single-stranded DNA of minus polarity, whose 5′ end remains covalently linked to the terminal protein of the polymerase, and then a complementary plus-strand DNA are synthesised to form the rcDNA.9 34 Assembly and release of the DNA-containing nucleocapsids appears to require a balanced co-expression of the small and large envelope proteins to recruit the nucleocapsid to the site of budding. Although the role of the envelope proteins in regulating viral particle release and cccDNA amplification is not well understood, recent studies indicate that the lack of envelope protein expression increases cccDNA levels, while co-expression of the envelope proteins favours viral secretion with limited completion of the plus strand.35
Previous SectionNext SectionPhases of CHB
The natural history of CHB
The immunopathogenesis of hepatitis B depends on a complex interplay of host and viral factors. Factors such as age, gender and immune status are important. Perinatal and childhood infection results in chronic infection in 90–95% and 50% of cases respectively, whereas greater than 95% of immunocompetent adults with acute HBV spontaneously clear the infection. Furthermore, CHB infection is more common in immunocompromised hosts (eg, HIV infection). Viral factors such as HBeAg status are also important. Under normal circumstances viral replication is not cytopathic, and it is the host's immune response, which is typically ineffective and inappropriate, that causes much of the damage associated with CHB.
The natural history of CHB is generally regarded as consisting of four phases36: immune tolerant; HBeAg-positive CHB (immune clearance); immune control (low or non-replicative); and HBeAg negative CHB (immune escape). These phases have been identified on the basis of specific biochemical, serological and virological characteristics, including serum alanine aminotransferase (ALT) levels, HBeAg serostatus, HBV DNA titre, and more recently, HBsAg level (figure 2). It is important to note that these phases do not occur in all individuals with CHB and do not always occur sequentially.37 It is patients in either the immune clearance or immune escape phases that are potential candidates for the currently approved antiviral and immune modulatory treatments. |
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发表于 2012-7-2 15:08
