Virology and Infection Overview of Enterovirus A71 (EV-A71) (Advanced)
- 演化之聲
- Mar 13
- 9 min read
The genus Enterovirus belongs to the family Picornaviridae. Within this genus there are 13 viral groups, which can be further divided into more than 300 serotypes. Among them, seven groups are known to infect humans, including polioviruses (PV), enteroviruses (EV), coxsackieviruses (CV), rhinoviruses (RV), and echoviruses. Viruses in the family Picornaviridae possess an icosahedral capsid composed of four structural proteins, VP1–VP4, with a diameter generally ranging from approximately 15 to 30 nm depending on the species. Their genetic material consists of positive-sense single-stranded RNA.
Members of the genus Enterovirus are capable of causing a wide variety of diseases. Examples of diseases associated with different enteroviruses include:
• Poliomyelitis: polioviruses (PV1, PV2, PV3)
• Acute flaccid paralysis and polio-like illnesses: enteroviruses (EV-68, EV-A71), echovirus 11
• Myopericarditis: coxsackieviruses (CV-A, CV-B)
• Acute hemorrhagic conjunctivitis: enterovirus D70 (EV-D70)
• Herpangina: coxsackievirus A16 (CV-A16), enterovirus A71 (EV-A71)
• Hand, foot, and mouth disease: coxsackievirus A16 (CV-A16), enterovirus A71 (EV-A71)
• Pneumonia: enterovirus 68 (EV-68), rhinoviruses
• Upper respiratory tract infection: enterovirus 68 (EV-68), rhinoviruses
• Aseptic meningitis: coxsackieviruses (CV-A9, CV-B), echoviruses, enterovirus A71 (EV-A71)
Because of the large number of viruses within this genus, it is not feasible to introduce them all individually. Among enteroviruses, the most extensively studied member is the poliovirus. However, it will not be discussed here. Instead, this article focuses on another relatively common member, Enterovirus A71.
Enterovirus A71 (EV-A71)
➊ Basic Virology:
Also referred to as Enterovirus-71, EV-A71 is the 71st serotype within Enterovirus A. Its RNA genome is approximately 7.4 kilobases in length. At the 5′ end, the genome is covalently linked to the viral protein VPg (viral protein genome-linked) through the hydroxyl group of the third tyrosine residue. VPg functions as a primer for RNA genome replication. Viral proteins with similar functions are also found in viruses belonging to the families Potyviridae, Astroviridae, and Caliciviridae. VPg additionally protects the viral RNA genome from recognition by cellular immune receptors such as RIG-I, MDA5, TLR3, and TLR7/8 (Goodfellow, 2011).
Immediately downstream lies the 5′ untranslated region (5′-UTR), which contains six stem-loop structures (stem loops I–VI). Among these, stem loops II–VI form the internal ribosome entry site (IRES), which serves as the binding and translation initiation site for ribosomes.
Following this region, nucleotides approximately 750 to 7300 constitute the coding region of the genome, which is divided into three segments: P1, P2, and P3. The P1 region encodes structural proteins in the order VP4, VP2, VP3, and VP1. The P2 and P3 regions encode non-structural proteins that participate primarily in viral translation and replication, including 2Apro, 2B, 2C, 3A, VPg (3B), 3Cpro, and 3Dpol.
These viral proteins are not translated individually. Instead, the genome is first translated into a single polyprotein precursor containing all viral proteins. The protease 2Apro first cleaves the junction between P1 and P2, and the protease 3Cpro subsequently processes the precursor to generate the eleven mature viral proteins.
The EV-A71 virion has a diameter of approximately 20–30 nm. The outer surface of the particle is formed by VP1, VP2, and VP3, while VP4 is located on the inner side of the capsid.
The primary host cell receptor identified for EV-A71 is SCARB2 (scavenger receptor class B member 2). Only a small number of viral strains can bind PSGL1 (P-selectin glycoprotein ligand 1). Several molecules have also been reported as potential co-receptors, including Annexin II, DC-SIGN, nucleolin, vimentin, heparan sulfate, and sialic acid.
The initial infection sites of EV-A71 commonly involve the digestive system and the skin. Once viral particles bind to host receptors, the cell internalizes the virus through endocytosis. During this process the viral capsid undergoes uncoating, releasing the RNA genome into the cytoplasm. Because Picornaviridae possess positive-sense RNA genomes, translation of the viral polyprotein can begin immediately after entry into the cytoplasm.
Certain non-structural viral proteins, including 2B, 2C, and 3A, interact with host proteins to induce the formation of replication organelles derived from membranes of the endoplasmic reticulum or Golgi apparatus. Viral RNA replication occurs within these membrane-associated compartments.
During replication, newly translated VPg proteins associate with viral 3Cpro and the RNA-dependent RNA polymerase 3Dpol at a small stem-loop structure called the cis-acting replication element (CRE) located within the coding region of the viral RNA genome. This forms the VPg uridylylation complex.
Because the CRE contains consecutive adenine residues (AAA), the RNA polymerase catalyzes the uridylylation of VPg by adding two uridine residues (UU) to the tyrosine residue of VPg. The modified protein is referred to as VPg-pUpU.
VPg-pUpU then serves as a primer at the poly-A tail located at the 3′ end of the positive-sense RNA genome, allowing synthesis of the negative-sense RNA genome. Once the negative strand is produced, host proteins such as heterogeneous nuclear ribonucleoprotein C (hnRNP-C) facilitate circularization of the RNA molecule by linking its ends. This circularized negative strand subsequently serves as a template for producing new positive-sense RNA genomes.
During synthesis of the new positive strands, the UU residues of VPg-pUpU again function as primers. This mechanism explains why the completed RNA genome ultimately carries a VPg protein at its 5′ end.
The newly synthesized positive-sense RNA genomes assemble with capsid proteins in the cytoplasm to form complete virions. These viral particles are typically released from the host cell through cell lysis, allowing infection of additional cells.
Some studies have suggested that during viral replication, autophagosomes generated by host cells may encapsulate viral particles. In such cases the virus may exit the cell via a non-lytic pathway (Baggen et al., 2018).
➋ Transmission and Infection:
Like most enteroviruses, EV-A71 infections tend to cause more severe symptoms in children younger than approximately 5–7 years of age.
Transmission primarily occurs through respiratory droplets, bodily secretions, and the fecal-oral route. Respiratory involvement in EV-A71 infection is generally less prominent, although a small number of pneumonia cases have been reported. Enterovirus 68 infections are more frequently associated with pulmonary disease.
The gastrointestinal tract is usually the primary site of initial infection. When the virus infects intestinal tissues, mild gastroenteritis may occur, producing symptoms such as vomiting and diarrhea (Akinnurun et al., 2023; Rao, 2021).
Severe intestinal symptoms caused by enteric pathogens often result from destruction of intestinal epithelial tissues and mucosa, as seen in rotavirus infections (Lundgren et al., 2001), or from toxin-mediated secretion of large amounts of water and ions leading to dehydration, as observed in Vibrio cholerae infections (Broeck et al., 2007).
In contrast, the reason why enteroviruses generally cause less severe intestinal damage remains unclear. Studies indicate that EV-A71 does not disrupt intestinal epithelial integrity nor reduce mucus secretion from goblet cells. Instead, intestinal tissues are capable of producing type III interferons (IFN-λs), which limit viral replication (Good et al., 2019). This may explain why severe intestinal symptoms are uncommon.
Beyond intestinal epithelial infection, enteroviruses can also infect gut-associated lymphoid tissue (GALT), including Peyer's patches. Within lymphoid tissues the virus may infect immune cells such as dendritic cells, macrophages, neutrophils, natural killer cells, and T cells (Noor et al., 2016).
Once viral replication in the intestine reaches a sufficient level, the virus may disseminate through the bloodstream or lymphatic system to infect other tissues. Infection of the skin can lead to hand, foot, and mouth disease, characterized by vesicles and inflammation on the hands, feet, and oral mucosa.
The virus can also invade the nervous system. In many cases, fatalities associated with enterovirus infections occur after the virus reaches the central nervous system. Associated neurological conditions include acute flaccid paralysis, polio-like illness, encephalitis, and meningitis.
The mechanism by which enteroviruses cross the blood–brain barrier (BBB) remains under debate, with several hypotheses supported by different studies.
One hypothesis suggests that EV-A71 initially infects the peripheral nervous system, such as motor neurons, and subsequently travels to the central nervous system through retrograde axonal transport (Chen et al., 2007), a mechanism similar to that used by the rabies virus.
Another study demonstrated that intracranial injection of the EV-A71 VP1 protein in mice increased the permeability of cerebral microvascular endothelial cells. This was accompanied by reduced expression of the tight junction protein claudin-5, weakening the integrity of the blood–brain barrier (Wang et al., 2020).
Additional research has found that small extracellular vesicles produced by infected cells may encapsulate viral components during EV-A71 infection. When these vesicles are internalized by brain microvascular endothelial cells of the blood–brain barrier, viral material may be released into the central nervous system (Gu et al., 2023).
More recent work suggests another possibility: immune cells infected within gut-associated lymphoid tissues may migrate through the circulatory or lymphatic systems and subsequently enter the central nervous system, carrying the virus with them (Gaume et al., 2024).
✿ Q&A
Why does severe disease occur when the virus reaches the brainstem rather than in the intestine?
The reason intestinal infection rarely causes severe damage remains uncertain. However, as mentioned earlier, intestinal cells infected by EV-A71 can secrete interferons λ1, λ2, and λ3 to suppress viral replication (Good et al., 2019). This antiviral response may prevent the virus from extensively damaging intestinal epithelial tissues.
Nevertheless, this suppression does not completely eliminate the virus. Clearance typically requires the adaptive immune system, and the efficiency of this response varies among individuals. This also explains why young children are more susceptible to severe disease: their immune systems are not yet fully mature, and viral clearance is less efficient than in adults. If viral replication is not effectively controlled, dissemination to other organs may occur.
Infection of the brainstem is inherently life-threatening. Even pathogens that cause only mild symptoms elsewhere can produce severe consequences once they successfully infect this region.
Several viruses are known to infect the brainstem, including herpes simplex virus type 1 (HSV-1), Japanese encephalitis virus, and rabies virus. When infections progress to encephalitis or meningitis, mortality rates increase substantially (Juntrakul et al., 2005; Marcocci et al., 2020; Mehta et al., 2021). EV-A71 is capable of infecting multiple regions of the central nervous system, including the brainstem.
Why does the virus spread to distant organs before causing severe disease?
Because the central nervous system is difficult for pathogens to access directly, viruses typically require an initial site of infection where replication can occur.
For example, in Japanese encephalitis, when mosquitoes transmit the virus into human skin, the virus first infects endothelial cells and immune cells in nearby microvasculature and lymphoid tissues. Viral replication increases the viral load before the virus spreads through circulation to other tissues and eventually reaches the central nervous system.
Rabies virus follows a different route. After an animal bite, the virus infects peripheral nerves at the wound site and then travels along neural pathways until it reaches the central nervous system.
Author: Shui-Ye You
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