by Brooke Napier
Devastating outbreaks of hand-foot-and-mouth disease have spread across the Asia-Pacific countries within the last decade, mostly targeting young children (under the age of 7 years old). The most common perpetrator (or cause) of hand-foot-and-mouth disease is the virus enterovirus 71 (EV71).
EV71 is a member of the human enterovirus genus, which also includes poliovirus, that cause gastrointestinal, respiratory, myocardial (heart), and central nervous system (CNS) disease. In millions of cases of hand-mouth-and-foot disease, EV71 has spread to the CNS, causing meningoencephalitis. Meningoencephalitis is infection or inflammation of the brain, and is associated with extremely high rates of mortality and morbidity.
Which brings us to our first question:
How does EV71 invade the CNS?
We can first look at what we know about invasion of the CNS by poliovirus, and see if any of these similarities can point us in the right direction for answering this question.
Poliovirus invasion occurs by the disruption of the blood-brain barrier (BBB) or via retrograde axonal transport (movement of the virus from the synapse to the cell body). As expected, EV71 has been seen to invade the CNS through retrograde axonal transport, however it seems that this might be a minor route of transmission. Additionally, these studies were done in mice, not humans; therefore, these data might not apply to what is happening during the human disease.
Hrm, next hypothesis?
Generally, immunocompromised individuals are afflicted with this disease, and it has been seen people with “deficient innate immunity” have extremely high levels of virus in the blood. These high levels of virus in the blood are the first step toward the secondary invasion of the CNS seen in EV71 infection. This makes sense, since young children (with relatively under-developed innate immunity) present the most severe CNS-related symptoms.
It seems that EV71 requires high levels of virus to invade the CNS, but it also needs to find a permissive cell type within the CNS to infect.
What cell types allow EV71 to invade the CNS?
Surprisingly, EV71 has not been studied directly from clinical samples during natural human infections. Due to the lack of knowledge in this field it’s unknown whether CNS invasion is caused by a secondary infection, steming from the primary infection OR if it is associated with specific viral adaptations within the human host.
Most people are familiar with viral adaptations in relation to HIV. HIV can continually mutate genes within its genome to evade host immune detection.
How does this apply to what we’re talking about?
EV71 might be mutating its genome to be able to invade neurons while in the host.
Cordey et al. began answering this question by analyzing the genomes of EV71 from different sites of infection in an immunocompromised host. By studying these genomes from different organs you can identify any inconsistencies in the genetic make-up of the virus – where these inconsistencies are identified might be where the viral adaptations (or mutations) might have occurred during natural human infection.
After sequencing these viral genomes, they observed very specific amino acid changes in viral proteins VP1 and 2B from cerebral spinal fluid (CSF) EV71 samples vs. upper respiratory tract EV71 samples. They believe these amino acid changes are the gateway into invading neurons. As if the virus needed to solve a puzzle before gaining access to the CNS and with time and environmental pressures it endured taught the virus how to answer the puzzle.
Five EV71 VP197Rmodel monomers arranged in capsid symmetry based on poliovirus capsid VP1 orientations (PDB 3epf). BC loops (green) and positions of residue 97 (red circles) and residue 167 (orange circles) are highlighted. (C) Side view of VP197R capsid assembly in B, rotated 80° on the plane of this page. The curvature and thickness of the capsid surface (based on PDB 3epf capsid assembly, VIPERdb) is represented as a light gray arc.
OR, structure of EV71 viral capsid, more specifically picture of amino acids (orange and red) that have changed during adaptation of the virus to invade neurons.
Furthermore, when these specific amino acid changes occur in the EV71 genome, the virus can more efficiently attach to neuroblastoma cells (SH-SYS5 cells). These data indicate that gaining these mutations gives the virus the key to the neuronal receptor, allowing EV71 to attach and (possibly) enter the cell.
Quantification of virus bound to neuroblastoma cells (SH cells) with either the new amino acid changes (white bar) vs. another virus without these adaptations (purple bar = less binding!).
These data are not conclusive, because although these mutations improve neuronal receptor-binding capacity in neuroblastoma cells, it cannot be ruled out that this amino acid change also confers advantages in other stages of infection (because surprise, they found better binding to vero – or fibroblast cells – too).
Inconclusive or not, its these types of studies, with real human disease applications, that are dominating the microbiology field – for good reason, they’re interesting and they tell us things we could never learn from mouse models. There is much work to be done in the field of EV71 invasion of neurons, however this is a brilliant start.
Samuel Cordey, Tom J. Petty, Manuel Schibler, Yannick Martinez, Daniel Gerlach, Sandra van Belle, Lara Turin, Evgeny Zdobnov, Laurent Kaiser, Caroline Tapparel (2012). Identification of Site-Specific Adaptations Conferring Increased Neural Cell Tropism during Human Enterovirus 71 Infection PLoS Pathogens