Studies on the immunity and pathogenesis of important virus infections in Taiwan- Long term immunity induced by hepatitis B vaccine and molecular pathogenesis of enterovirus 71

• Main Authors: Chun-Yi Lu, 呂俊毅
• Other Authors: Li-Min Huang
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/28121790489572006649

博士 === 國立臺灣大學 === 臨床醫學研究所 === 96 === Universal neonatal and infant hepatitis B (HB) vaccination program has been launched in Taiwan in 1984 (1) and has resulted in a significant reduction in the prevalence of chronic HBV infections(2), fulminant HB(19), and hepatocellular carcinoma (3). Booster vaccinations are not recommended for immunocompetent subjects 15 years after neonatal vaccination. Nevertheless, there have also been reports of cases of chronic HBV infections that occurred after vaccine-induced protecting antibodies had disappeared [13]. Uncertainties about whether the protection conferred by HB vaccination can truly span 15 years and the need for a booster dose during adolescence or adulthood exist. We aimed to study the duration of HB vaccine-conferred protection and indications for boosters. In different study cohorts, we measured antibody to HB core antigen (anti-HBc), HB surface antigen (HBsAg), and pre- and post-booster titers of HBsAg antibody (anti-HBs) 13-18 years after primary neonatal immunization with plasma-derived or recombinant HB vaccines. In the first study (Chapter 2), we found anti-HBs was undetectable (antibody titer <10 mIU/mL) in 29.5% of 78 children who were born to HB e antigen–positive HBsAg carrier mothers and had developed protective levels of anti-HBs antibodies (>10 mIU/mL) following HB immunization. After a single booster dose of HB vaccine, 2.7% remained anti-HBs-negative. This finding raised the concern about the risk of breakthrough infection. One or more booster immunizations might be needed in seronegative subjects by at least 15 years following neonatal immunization with plasma-derived HB vaccine. In the second part of the study (Chapter 3), we aimed to investigate long-term HB immunity in adolescents who have received plasma-derived HB vaccines in their infancy. In 2004–2005, 6,156 high school students (15–21 years old) who had been vaccinated with plasma-derived HB vaccine as infants were recruited for HB seromarker screening. The immune response to an HB vaccine booster was evaluated in 872 subjects who were seronegative. HB surface antibody (anti-HBs) titers and levels of HB surface antigen (HBsAg)-specific interferon (IFN)-γ- or interleukin (IL)-5-secreting peripheral blood mononuclear cells (PBMCs; measured by enzyme-linked immunospot assay) were determined 4 weeks later. Our results confirmed the vaccine highly efficacious in reducing the HBsAg positivity rate. However, 63.0% of the vaccinees had no protective anti-HBs. After the booster, anti-HBs remained undetectable in 28.7% (158/551) of the subjects who had received complete HB vaccination (4 doses) during infancy. We estimated that at least 10% of the total population had lost their HB vaccine-conferred booster response. A notable proportion of fully vaccinated adolescents had lost immune memory conferred by a plasma-derived HB vaccine 15–18 years later. This decay of immune memory also raised concerns about the need for a booster vaccine for high-risk groups in the long run. We have followed up the study subjects for 3 years (Chapter 4). The incidence of HBV infection (manifested as newly converted anti-HBc) in this age group was about 0.33%. Under such a low incidenc, a 3-year follow-up was not long enough to detect any benefit of booster vaccination in adolescents in this age group. We then shifted our focus to the long-term immunity of recombinant HB vaccine. Similarly, we recruited 933 middle school students and tested their HB seromarkers (Chapter 5). The HBsAg positive rate was as low as 0.3%. 71.9% of study subjects harbored none of HBsAg, anti-HBs, or anti-HBc. A booster vaccination was given to 573 of these subjects. After that, 26.4% of subject remained negative of anti-HBs. A second booster dose raised the positive rate to 93.8%. These results confirmed that immune memory coferred by the recombinant HB vaccine decayed in around 20% (93.8%-73.6%) of vaccinees 13-14 years after the primary vaccination in infancy. In 1998, an epidemic of hand-foot-and-mouth disease and herpangina caused by enterovirus 71 occurred in Taiwan, leaving many fatalities and severely handicapped survivors in its wake. The reasons this rather common pathogen would cause such a large-scale epidemic remain unknown. We performed a seroepidemiological survey to elucidate the epidemiological characteristics of this outbreak, including its incidence and case-fatality rates (Chapter 6). Microneutralization tests for antibodies against enterovirus 71 were used to screen several collections of serum samples. The results showed EV71 was an endemic in Taiwan even before 1998. The EV71 seropositive rates increase with age. Approximately half of all children aged 6 years or older were enterovirus 71 seropositive. In children aged 0.5 to 3 years, seropositive rates were significantly higher in 1999 than in 1997. The incidence of enterovirus 71 infection during the epidemic was estimated to be 13 to 22 percent, with the higher rates in younger children. The case-fatality rate was highest (96.96 per 100,000) in infants aged 6 to 11 months, and declined in older children. Our results showed that enterovirus 71 is endemic in Taiwan. The apparent lack of large-scale enterovirus 71 activity in the three years prior to 1998 might have been the prelude to the epidemic’s appearance in 1998, and suggests that enterovirus 71 infection will reappear every few years. The lack of a protective antibody in younger children may account for the high incidence and case-fatality rate in this age group. Enteroviruses have ability to protect host cells from apoptosis induced by various cytotoxic chemicals. The molecular mechanism by which enteroviruses executes its anti-apoptosis function is largely unknown. In Chapter 7, we found a novel molecular mechanism by which enterovirus 71 (EV71) executes its anti-apoptotic activity. By a yeast two-hybrid screen, myeloid cell leukemia-1 (Mcl-1) was identified as an interacting protein of EV71 2B protein. Both in vitro and in vivo binding studies further substantiate the interaction between EV71 2B and Mcl-1. Expression of EV71 2B protein increases stability of Mcl-1, correlating with increased resistance to apoptosis and increment of cell survival. Together, our findings provide a novel mechanism by which EV71 controls survival of its host cell via regulating Mcl-1 stability.