本帖最后由 StephenW 于 2011-11-2 09:52 编辑
表面抗原可以不依赖cccDNA单独合成? [Extracted from Serological Studies into the Natural History of Chronic Hepatitis B
A thesis submitted in fulfilment of the requirements for The degree of Doctor of Medicine
By Tin Quang Nguyen
t Department of Gastroenterology, St Vincent’s Hospital, Melbourne
And
Victorian Infectious Diseases Reference Laboratory
The University of Melbourne
February 2011]
Interest in quantitative HBsAg serology as a clinical biomarker has been based upon studies which showed a positive association with intrahepatic HBV cccDNA levels[103, 162, 167] and serum HBV DNA[103, 168]. Currently, HBVDNA quantification is the gold standard in selecting patients who are potential candidates for therapy, monitoring response to therapy, and detecting the emergence ofdrug resistance. Compared to HBV DNA, the assays for HBsAg quantification are far less expensive, and are fully automated with a high throughput capacity. However,the utility of HBsAg titres as a reliable surrogate for both HBV cccDNA and HBVDNA remains unclear, as other studies have also shown a poor correlation with HBV cccDNA[169], and only a positive correlation with HBV DNA in HBeAg positive CHB in our collaborative group[164]. An understanding of HBsAg titre changes throughout HBV infection may provide some potentially useful insights into hepatitis B pathogenesis and viral life cycle. The mechanisms linking HBsAg and viral replication during different phases of CHB are currently unclear. This study observed a modest correlation of serum HBsAg with HBV DNA in the IC phase of CHB (r = 0.77, p=0.0001). No correlation was observed in the IT, LR or ENH phases. Furthermore, the ratio of HBsAg to HBV DNA was significantly higher in the low replicative phase compared to all other phases (1.09 vs 0.55, 0.55, 0.69. p<0.0001), a finding which is in accordance with previous studies[162]. The apparent “disconnect” between HBsAg and HBV DNA at different phases may possibly be due to the expression of HBsAg from integrated viral envelope sequences, instead of HBsAg production off mRNA derived from the HBV cccDNA template. A second possible explanation is a difference in the immune regulation of viral replication during different phases of infection, resulting in altered ratios of HBV virion to sub-viral HBsAg particles[31].
HBsAg synthesis during the HBV viral life cycle is complex, and typically occurs at the endoplasmic reticulum (ER) (Figure1.3 and 3.11). The envelope open reading frame (ORF) contains three in-frame “start” codons which sub-divide it into preS1, preS2, and S domains. Envelope proteins are generated from two HBV mRNA transcripts, with subsequent translation resulting in production of the small (S), medium (Pre-S2+ S) and large surface envelope proteins(Pre-S1+Pre-S2+S); these are also known as S, M and L surface proteins, respectively. Figure 3.11. The two separate pathways of HBsAg and HBVDNA production.. RC-DNA, relaxed circular DNA; DSL DNA, double stranded linear DNA; cccDNA, covalently closed circular DNA; mRNA, messenger RNA; HBeAg, hepatitis B e antigen; HBsAg, hepatitis B surface antigen. Newly synthesized envelope protein interacts with mature HBV nucleocapsids at the ER prior to secretion from the hepatocyte. However, HBsAg production far exceeds that required for virion assembly, and excess surface envelope proteins are secreted as non infectious filamentous or spherical subviral particles[31]. These subviral particles may play a role in evading or subverting the host immune response[84], and may also co-exist with anti-HBs as part of circulating immune complexes (see Chapter 5 of thesis)[170]. It is important to appreciate that whilst HBsAg quantification detects all three forms of systemic HBsAg (part of HBV virion, spherical, filamentous), differentiation between the relative proportions is currently technically demanding, and not routinely performed, requiring either density gradient
Uncoating脱壳 ER内质网 Mature Nucleocapsid成熟核衣壳 Immature Nucleocapsid未成熟的核衣壳 Nuclear Transport核运输 RC-DNA TranscriptionRC - DNA转录 viral RNA HBV DNA Pathway途径 GOLGI高尔基体（Golgi apparatus） Translation翻译 DSL- DNA HBsAg乙肝表面抗原 Pre-S truncation前S截断 Viral Integration病毒集成 Spherical & Filamentous HBsAg球面及丝状乙肝表面抗原 Mature HBV virion成熟的乙肝病毒病毒粒子 Viral Integration Pathway病毒的整合途径 DSL-DNA Reverse Transcription转录 HBsAg Pathway乙肝表面抗原
centrifugation or non-denaturing gel electrophoresis for separation and immunoblot for initial detection and subsequent clarification. HBsAg may also be produced from HBV DNA integrated into the host genome. Although viral integration is an essential component of the life cycle of retroviruses such as HIV, it is not required for normal productive hepadnaviral infection. Rather, integration of HBV DNA occurs illegitimately through recombination mechanisms using host enzymes such as topoisomerases acting on the double-stranded linear (DSL) HBV DNA (Figure1.3 and 3.11)[32, 33]. In HBV infection, viral integration does seem to occur early in infection. Whilst HBV integration is believed to be a random event, a high preference for integration occurs at the direct repeat 1 (DR1) and DR2 sequences on the HBV genome[34]. Such integrated sequences cannot provide a template for productive viral replication as a complete genome is typically not present[36]. However, given that sequences of the S genes of the Enh I elements are often present in integrated segments, HBsAg may be produced[36]. The phase of CHB is currently determined by three main factors;HBeAg/anti- HBe status, HBV DNA titre and serum ALT level. This study demonstrated that HBsAg titres change during the natural history of CHB, and suggests that there may be HBsAg titre “set-points” within each phase. Further evaluation of baseline HBsAg titres in other cohorts of patients with CHB are required to confirm the findings of this study, and may help refine the current definition of the different phases of CHB. The status of a patient’s HBeAg/anti-HBe, HBV DNA and serum ALT are also the parameters which are currently used to assess the response to antiviral therapy. Sustained suppression of HBV replication as assessed by HBV DNA measurement currently represents the cornerstone of evaluation of antiviral efficacy. In the absence of HBsAg loss, long-term therapy with potent oral NAs is required to maintain effective suppression of HBV DNA. Thus, there is now a paradigm shift towards striving to achieve HBsAg loss and/or seroconversion. HBsAg loss is believed to be associated with both successful immunological control of HBV and durable suppression of viral replication, and consequently may represent an indication to cease oral NA therapy. Evaluation of HBsAg titres may allow determination of baseline levels which may be more predictive of HBsAg loss.Furthermore, assessment of on-treatment changes in HBsAg titres may facilitate new algorithms and future trials which are aimed at achieving this important endpoint.
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发表于 2011-11-2 07:09
