Investigations on the immunopathology of enterovirus 71 2

CHAPTER LITERATURE REVIEW 1.3 Clinical features 1.3.1 Mucocutaneous and respiratory HFMD is usually a mild childhood exanthema that is characterized by 3-4 days of fever, followed by the formation of papulovesicular rashes on the buccal mucosa, tongue, gums and palate, as well as on the palms, soles and buttocks (Ho et al, 1999; Shah et al, 2003). Other frequently encountered symptoms include poor appetite, vomiting and lethargy. In addition to HFMD, EV71 was also identified as a cause of herpangina which is an illness characterized by an abrupt onset of fever and sore throat, associated with the development of raised papular lesions on the mucosa of the anterior pharyngeal folds, tonsils, soft palate and uvula. HFMD is moderately contagious; spreading is through direct contact with nose and throat discharges, saliva, fluid from blisters, or the stool of the infected patients (Lin et al, 2002). This disease may be caused by a number of different enteroviruses like coxsackievirus (CV) group A4-A7, CV-A9, CV-A10, CV-A24, CV-B2 to B5, echoviruses 1, 4, 11, 18 and EV18. However, the two major etiological agents of this disease are CA16 and EV71, with the latter being associated with significant mortality (Huang et al, 1999; Lin et al, 2002). While older children commonly display a classic course of HFMD, those aged years and younger develop more widespread and atypical rashes (Chang et al, 2004). 18 CHAPTER LITERATURE REVIEW 1.3.2 Neurological and systemic manifestations HFMD and herpangina are common clinical syndromes of EV71 infection, but they are usually mild and self-limiting. In some occasions, EV71 infection can lead to severe neurological manifestations ranging from aseptic meningitis to acute flaccid paralysis and brainstem encephalitis (BE), which is associated with systemic features, such as severe PE and shock (McMinn, 2002). In a large prospective clinical study of several epidemics occurring over years in Sarawak, a subset (10–30%) of children hospitalized with EV71related HFMD also developed CNS complications, while these neurological complications were not presented in children with CVA16 infections (Ooi et al, 2007). The common presentations of CNS complications include brainstem and/or cerebellar encephalitis, accounting for 58% of neurological manifestations, followed by aseptic meningitis (36%) and BE with cardiorespiratory dysfunction (4%). Table 1.2 shows a brief description of the various neurological syndromes associated with EV71 infection. EV71-associated BE is usually clinically associated with a constellation of manifestations including myoclonic jerks, tremors, ataxia, limb weakness and cranial nerve palsies (Huang et al, 1999). In severe cases, these neurological symptoms can progress to include seizures, altered consciousness, and increased intracranial pressure. Neurogenic pulmonary oedema (PE), hemorrhage and acute respiratory distress syndrome (ARDS) might sometimes ensue BE, and are believed to be the main cause of mortality (Huang et al, 1999; Chan et al, 2000; 2003). Without intensive care, most children affected in this way will die before reaching hospital or within 24 hour (h) of admission. In the few studies where BE, magnetic resonance imaging (MRI) and post-mortem findings have correlated well, the diencephalon, pons, cerebellum and medulla, including brainstem respiratory and vasomotor centers are 19 CHAPTER LITERATURE REVIEW frequently destructed and inflamed, coupled with the presence of EV71 antigens detected within the neurons (Shen et al, 1999; Wong et al, 2000) Acute flaccid paralysis (AFP), the primary presenting feature in several neurological syndromes caused by EV71, may occur with encephalitis or as an isolated entity (McMinn, 2002). The virus antigens and pathology is typically limited to the spinal cord central gray matter, with an affinity for anterior horn cells, as in poliovirus-associated poliomyelitis. However, EV71 induced AFP appears to be milder and has a higher incidence of recovery compared to poliovirus infection. 20 CHAPTER LITERATURE REVIEW Table 1.3 Neurological syndromes associated with EV71 infection Frequency Purely neurological manifestations Encephalitis, especially brainstem Frequent Acute flaccid paralysis (anterior myelitis) Frequent Encephalomyelitis Frequent Aseptic meningitis Very Frequent Cerebella ataxia Infrequent Transverse myelitis Rare Neurological and systemic manifestation Brainstem encephalitis with cardiorespiratory failure Frequent Manifestations indicative of immune-mediated mechanisms Guillain-Barré syndrome Infrequent Opsoclonus-myoclonus syndrome Rare Benign intracranial hypertension Rare Reproduced from Ooi et. al., 2010 with permission. 21 CHAPTER LITERATURE REVIEW 1.3.3 Pathological observations Inflammation of the CNS has long been proposed as the underlying pathogenic mechanism of EV71-associated encephalitis, aseptic meningitis, BE and cardiopulmonary collapse (Lin et al, 2003; Shekhar et al, 2005). Studies on EV71 patients have shown that EV71-induced inflammation occurs predominantly in the grey matter of the spinal cord and the whole medulla oblongata, including the dorsal nucleus of the vagus, tractus solitarius, the nucleus, and reticular formation (Fig 1.4 A-C) (Shen et al, 1999; Wong et al, 2000). In addition, the hypothalamus, subthalamic and dentate nuclei, and to a lesser degree motor cortex of the cerebrum are also involved (Lum et al, 1998; Huang et al, 1999; Hsueh et al, 2000; Wong et al, 2008). No inflammatory changes were observed in the cerebellar cortex, thalamus, basal ganglia, peripheral nerve and autonomic ganglia (Fig 1.4D). Encephalitis induced by EV71 is generally similar to that observed in acute encephalitis caused by other viruses. Typically, widespread histopathological changes such as perivascular cuffing by mononuclear cells, variable oedema, neuronophagia and extensive amount of microglia nodules are observed in the specific regions of CNS (Yang et al, 2009). Yang et. al. reported that these inflammatory lesions included abundant amount of a distinctive neutrophil-like infiltrate mostly made up of CD68+ and CD15- cells. Using various detection methods other studies have also detected EV71 viral genome and antigens in the neurons, neuronal processes and associated inflammatory cells although no virus inclusion has been observed (Shieh et al, 2001; Yang et al, 2009). 22 CHAPTER LITERATURE REVIEW Figure 1.4 Distribution of inflammation in brain sections of EV71 patients. (A) Whole brain coronal sections of cerebral hemispheres with distribution of inflammation indicated by black dots (>half-field inflamed) and blue dots ( 0.05) between S41- and C2-infected mice. Together, the data indicate that high levels of IL-6 are produced in mice infected with lethal EV71 strains, which thus support our earlier hypothesis that sustained, high level of IL-6 is detrimental to the infected host. The present data also suggest that IL-6 may represent a biomarker of disease prognosis in this mouse model. However, more virus strains from various genogroups must be tested in order to validate this hypothesis. 174 CHAPTER INVESTIGATIONS ON EV71 VIRULENT DETERMINANTS A B C D Figure 6.4 Systemic cytokine levels in EV71-infected mice. Two-week-old AG129 mice were infected with the four different virus strains at 107 PFU/mouse. Naive mice were given PBS instead. Mice (n=5) were bled and euthanized at day 4, 8, or 12 PI. Serum levels of IL-6 (A), IFN-γ (B), IL-10 (C) and TNF-α (D) were measured by ELISA. Data are not available for S10-infected mice at day 12 PI due to earlier animals’ death. Data were expressed as mean ± SEM. *, P [...]... severe EV71 disease Further investigations into the mechanisms underlying the role of host genetic polymorphisms in the context of EV71 infection could provide important insights into the pathogenesis of the virus 1.4.3 Immunopathogenesis Most EV71 infections are controlled by the immune system which resolves the infection with minimal impact or without causing any substantial tissue damage The immune... well as the efforts in the field are accelerating The identification of the individual proteins employed by the viruses to disable the host anti-virus response including the innate immune system, is vital to provide information to develop specific therapy to treat EV71 Furthermore, the solving of the crystal structure of EV71 should facilitate the rational design and the development of novel antiviral... coupled with additional control measures such as issuing alert letters to affected institutions, conducting field investigations and mobilizing multiagency efforts to prevent further transmission of the disease have been shown to be effective in stemming the spread of infection (Ang et al, 20 09) However, others have argued that transmission of the virus within families rather than the peer-group at... pathogenesis of severe EV71 infection 36 CHAPTER 1 LITERATURE REVIEW 1.5 Control of viral infections 1.5.1 Virus surveillance and social distancing Over the past 12 years EV71 has become the cause of regular major epidemics across the Asia-Pacific region, and is now believed to be the most important threat to the world among the enteroviruses, now that poliovirus has been largely eradicated in most parts of the. .. to EV71 infection (Chang et al, 20 08) HLA-A33 is found more frequently in Asian populations (17-35%) than in Caucasian populations (0-1%), which may explain the frequent outbreaks of EV71 in Asian countries (Middleton et al, 20 03) In the same study, researchers noticed an apparent correlation between HLA-A2 and the development of EV71related cardiopulmonary failure Meanwhile, a polymorphism of another... individuals Together, these studies offer strong support to the hypothesis that EV71-associated neurological complications result from the synergistic effect between immunological disarray and CNS damage 32 CHAPTER 1 LITERATURE REVIEW 1.4.3 .2 Lymphocyte depletion The effector functions of lymphocytes are essential for viral clearance during EV71 infection (Lin et al, 20 09) However, a study with 73 EV71-infected... vaccine was able to prolong the survival of suckling mice after EV71 lethal challenge These results show the value of the inactivated virus vaccine for the effective control of EV71 A Vero-cell adapted virulent strain of EV71, YN3-4a was subsequently developed from the neu strain of EV71 and was characterized as suitable for use as an inactivated virus vaccine (Lin et al, 20 02) More recently, a microcarrier... CVA16-immune serum prolonged the survival rate of mice upon lethal EV71 challenge (Wu et al, 20 07) Based on the homology to poliovirus type I vaccine strain, a live attenuated strain of EV71, EV71 (S1-3), derived from the prototype strain BrCr was developed using genetic manipulations (Arita et al, 20 08) The live attenuated EV71 carries mutations in the 5’UTR, 3Dpol and 3’UTR regions, which resulted in... spread of the virus Intravenous inoculation of cynomolgus monkeys led to the production of antibodies against a broad spectrum of EV71 genotypes that protected against a lethal challenge with virulent EV71 (Ooi et al, 20 07; Arita et al, 20 07) However, the vaccine itself caused mild neurological symptoms, thus indicating that further work on attenuation is required Furthermore, certain issues including the. .. et al, 20 08), pyridanzinyl oxime ethers (Barnard et al, 20 04) and a suramin analog (Arita et al, 20 08) Table 1.4 provides a brief summary of the on- going antiEV71 drug-based strategies and their putative modes of inhibition 1.5.3 .2 Interferon therapy against EV71 infection IFNs are potent, selective mediators of cellular changes that induce a number of antiviral and immunological effects, all of which . (at the level of the splenium) with distribution of inflammation indicated by black and blue dots. (C) Horizontal section of cerebellum and midpons showing the distribution of inflammation (dots) effects of the systemic inflammatory response on the vascular endothelium. The key to further decipher the exact mechanisms for PE in EV71 encephalitis relies on the development of more sophisticated. severe EV71 disease. Further investigations into the mechanisms underlying the role of host genetic polymorphisms in the context of EV71 infection could provide important insights into the pathogenesis