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Development of enterovirus 71 vaccines Expert Rev. Vaccines 9(2), 149–156 (2010)
Min-Shi Lee and Luan-Ying Chang† Author for correspondence Division of Pediatric Infectious Diseases, Department of Pediatrics, National Taiwan University Hospital, College of Medicine, National Taiwan University, No. 7 Chung-Shan South Road, Taipei 100, Taiwan Tel.: +886 223 123 456 ext. 71528 Fax: +886 223 147 450
[email protected] †
Enterovirus 71 (EV71) was first isolated in 1969 in California, USA. Several epidemic outbreaks with high mortality rates have occurred in European and Asian Countries (Bulgaria in 1975, Hungary in 1978, Malaysia in 1997, Taiwan in 1998 and China in 2008). EV71 CNS involvement may be associated with neurological sequelae, delayed neurodevelopment and reduced cognitive functioning. Since poliovirus was nearly eradicated by vaccination, EV71 is now considered as one of the top candidates for new vaccine development against human enteroviruses. Recently, several EV71 vaccine candidates, including live-attenuated virus, inactivated whole virus, recombinant viral protein, virus-like particle and DNA vaccines, have been evaluated in animals but no clinical trial has yet been conducted. Based on historical experiences with poliovirus vaccines and animal studies, the inactivated whole-virus vaccines are feasible and could be licensed readily, so these are targeted for preparing clinical trials in several organizations in Asia. Keywords : enterovirus 71 • epidemiology • genotype • sequelae • vaccine
Virology
Enterovirus 71 (EV71) was first described in 1974 after it was isolated from patients in California (USA) [1] and has been associated with sporadic cases or outbreaks of a wide spectrum of diseases, including hand, foot and mouth disease (HFMD), herpangina, aseptic meningitis, encephalitis, cerebellar ataxia, poliomyelitis-like syndrome and even fatal disease [2] . EV71 belongs to Picornaviridae, a family of single-stranded positive-sense RNA viruses. Serologic studies, using neutralization tests with animal hyperimmune antisera, have identified more than 100 human enterovirus serotypes: poliovirus serotypes 1–3, 23 coxsackie A virus serotypes, six coxsackie B virus serotypes, 31 echovirus serotypes, and the numbered enterovirus serotypes. Genetically, human enteroviruses can be classified into four species. EV71 belongs to species A and can be further classified into three genogroups (A, B and C) and 11 genotypes (A, B1–B5 and C1–C5) [2,3] . The EV71 genome encodes a long polyprotein with a single open reading frame followed by a polyA tract. The single polyprotein is flanked by untranslated regions at both the 5´ and 3´ ends, and can be divided into three different genomic regions (P1, P2 and P3). The P1 genomic region encodes the structural proteins VP1–VP4. The P2 and P3 genomic regions encode the nonstructural proteins 2A, 2B, 2C, 3A, 3B, 3C and www.expert-reviews.com
10.1586/ERV.09.152
3D [3] . The structural proteins are processed by the 3CD proteinase to form the right conformation for virus assembly and are the major antigens inducing neutralizing antibodies. However, the binding sites of neutralizing antibodies have not been fully identified. Most enteroviruses replicate in primate cells. The specific receptor on the surface of the host cell can be used to identify the host range and tissue tropism. Recently, two receptors for EV71 were identified: human P-selectin glycoprotein ligand-1 (CD162), a sialomucin membrane protein expressed on leukocytes that has a major role in the early stages of inflammation as a functional receptor for EV71 [4] and human scavenger receptor class B, member 2 (SCARB2, also known as lysosomal integral membrane protein II or CD36b-like-2) [5] . EV71 binds soluble SCARB2 or cells expressing SCARB2, and the binding is inhibited by an antibody to SCARB2. Transgenic mice bearing these cellular receptors will facilitate monitoring the immunogenicity and safety of EV71 vaccine candidates. Mode of transmission
In a previous study, we found that the isolation rate of EV71 was significantly higher in throat swabs (93%) than rectal swabs or feces (30%) in acute EV71 cases, but EV71 could be isolated from the stools of infected patients for up to 5 weeks [6] , which suggests that both oral–oral
© 2010 Expert Reviews Ltd
ISSN 1476-0584
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Lee & Chang
and oral–fecal routes are potential routes of transmission. Long periods of viral shedding may account for widespread transmission of enteroviral diseases, which is certainly the case for polio and coxsackievirus infections [7–9] . It is well documented that household transmission plays an important role in the spread of EV71, and EV71 household transmission rates are high for children, medium for parents and low for other adults [10–11] . Therefore, close household contact, higher viral load and long viral shedding periods may account for the high household transmission rate among children.
immunization with serum neutralizing antibodies provides protection against fatal challenges in mice [19,20] . In addition, human serological studies have documented that lack of EV71 serum neutralizing antibodies may account for the high incidence rates of EV71 infections in young Taiwanese children in 1998 [12,21] . Therefore, measurement of serum neutralizing antibodies was widely used for seroepidemiological studies of EV71 infections and evaluation of EV71 vaccine candidates [22–24] . Epidemiology
Clinical spectrum & long-term outcome
The EV71 household study showed that only 6% of infected children within the same household were asymptomatic, whereas 53% of infected adults were asymptomatic [11] . Interestingly, in a seroepidemiologic study, only 29% of the preschool children infected with EV71 developed HFMD/herpangina, indicating that approximately 70% of community-acquired infected children might be asymptomatic or subclinical [12] . According to previous clinical studies conducted in northern Taiwan [13–15] , symptomatic EV71 infection can progress through four stages: HFMD/herpangina (stage 1), CNS involvement (stage 2), cardiopulmonary failure (stage 3) and convalescence (stage 4) (Table 1) . Most EV71 cases in those studies stayed at stage 1, some progressed to stage 2 and a few would advance to the most severe condition, stage 3. Some survivors of stage 3 would have long-term sequelae during convalescence (stage 4). Table 1 shows the summary of clinical manifestations of different stages of EV71. Recent follow-up studies further demonstrated that EV71 infection could cause long-term sequelae, including neurological sequelae, delayed development and reduced cognitive function [16] . Correlate of protection
Based on tremendous poliovirus studies, the serum neutralizing antibody response is especially important for protection and lifelong immunity in humans [17,18] . For EV71, passive
Enterovirus 71 was first isolated in California, USA, in 1969 [1] . Since then, EV71 has been isolated globally [2–3,25] . Similar to other enteroviruses, EV71 is isolated throughout the year in tropical areas but is detected mainly in the summer and fall in temperate areas [25–29] . Globally, two patterns of EV71 outbreaks have been reported: small-scale outbreaks with few CNS-complicated cases and deaths, and large-scale outbreaks with frequent CNScomplicated cases and deaths. The latter pattern occurred in Bulgaria with 44 deaths in 1975 [30] , in Hungary with 45 deaths in 1978 [31] , in Malaysia with 29 deaths in 1997 [32–34] , in Taiwan with 78 deaths in 1998 [35–37] , in Singapore with five deaths in 2000 [38] , in Ho Chi Minh City, Vietnam with three deaths in 2005 [29] , in Brunei with three deaths in 2006 [39] , and recently in China with over 27 deaths in 2007, 126 deaths in 2008 and more than 255 deaths in 2009 (Table 2) [40–42] . Since EV71 mortality rates are heavily affected by healthcare accessibility and standards, a more relevant clinical definition, such as CNS complication, should also be used to monitor the disease burden of EV71 infections in prevalent areas. Seroepidemiological studies have demonstrated that EV71 infections were prevalent in Brazil, German, Japan, Singapore and Taiwan (Table 3) [12,22,43–45] but large-scale outbreaks of EV71related HFMD and neurological diseases were only documented in Taiwan and other Asian countries (Table 2) . The mechanism is not clear and needs multinational studies to clarify the reasons for more neurological and more severe disease in Asian countries.
Table 1. Clinical staging and manifestations of enterovirus 71 infection. Stage
Clinical manifestations Symptoms
1
HFMD/herpangina
HFMD (~80%), herpangina (~10%), pharyngitis and nonspecific febrile illness (~10%)
2
CNS involvement
Meningitis, encephalitis, polio-like syndrome or encephalomyelitis: myoclonic jerk during sleep, vomiting, headache and/or irritable crying, lethargy, sleepiness or coma, seizure attacks, ataxia, cranial nerve palsy, such as abducens palsy, facial palsy, dysphagia, upward gaze and nystagmus, asymmetric acute limb weakness plus decreased reflex without disturbance of limb sensation
3
Cardiopulmonary failure
3A
ANS dysregulation/ hypertension
Insomnia, profuse sweating, paralytic ileus, neurogenic bladder, panic or increased startle reflex, hyperthermia (fever >40°C), tachycardia (range: 135–250 beats per minute), transient hypertension and tachypnea
3B
Hypotension/ pulmonary edema
Progressive hypotension or shock, oliguria or anuria, and decreased levels of consciousness
4
Convalescence
Limb weakness and atrophy, dysphagia requiring tube feeding, and central hypoventilation requiring ventilator support, delayed neurodevelopment and reduced cognitive function, recurrent aspiration or ventilator-associated pneumonia
ANS: Autonomic nervous system; HFMD: Hand, foot and mouth disease.
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Development of enterovirus 71 vaccines
Review
Genetic & antigenic variation
Table 2. Enterovirus 71-related severe and fatal cases from 1997 Enterovirus 71 has tended to mutate more to 2008. quickly in the last 10 years and more genoYear Severe cases (n)* Deaths (n) Ref. types are now spreading globally [2–3,25] . Country Based on serum neutralization tests using Taiwan [35–37] 1998 405 78 hyperimmune animal antisera, EV71 is clas2000 152 25 2001 187 27 sified as a single serogroup. However, more 2002 55 7 genotypes were identified recently [3,25] . In 2003 44 4 sera collected from three monkeys immu2004 20 5 nized intravenously with live-attenuated 2005 82 7 EV71 vaccine, Arita et al. found significant 2008 347 14 antigenic differences (>tenfold) between [32–34] Malaysia 1997 No data 29 homologous (genotype A) and heterolo2000–2003 185 4 gous (genotypes B4 and C2) viruses [23] . 2006 436 6 Kung et al. did not detect significant anti[38] Singapore 2000 No data 5 genic differences between genotypes B4 [29] 2005 51 3 and C4 viruses using two acute-phase sera Vietnam [39] from hospitalized EV71 patients [46] . A Brunei 2006 No data 3 serosurvey in healthy children and adults China [40–42] 2007 No data >27 in Japan detected partial antigenic differ2008 1165 126 ences between genotype B5 and A viruses Jan–July 2009 10,509 255 (geometric mean titer [GMT]: 42 vs 15; *Different countries may have different definitions. p
