The influenza virus vaccine is frequently updated to ensure that it protects against infection with circulating virus strains. In some years the vaccine matches the circulating strains, but in others, there is a mismatch. The result is that the vaccine is less effective at protecting from infection. During the 2014-15 influenza season there was a mismatch due to growing the vaccine in embryonated chicken eggs.
Lynda Coughlan joins the weekly virtual bus companions for a discussion of a host defense peptide from frogs that destroys influenza virus, and mouse models for acute and chronic hepacivirus infection.
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Show notes at microbe.tv/twiv
Flaviviruses are unusual because antibodies that cross-react with different viruses can enhance infection and disease. This property, called antibody-dependent enhancement or ADE, has been documented to occur among the four serotypes of dengue virus. It has implications for infection with or vaccination against Zika virus or dengue virus.
Earlier this year (virology blog link) it was shown that antibodies to dengue virus – in the form of serum from infected patients, or two human monoclonal antibodies – bind to Zika virus and can enhance infection of Fc-receptor bearing cells (Fc receptors bind the antibody molecule, allowing uptake into cells – illustrated). When the antibodies to dengue virus were omitted, Zika virus barely infected these cells. The conclusion is that dengue antibodies enhance infection of cells in culture by Zika virus.
This early work was first published as a preprint on the bioRxiv server – which lead some to criticize me for discussing the work before peer review. However, I subjected the paper to my own peer review, of which I am entirely capable, and decided it was worthy of discussion on this blog.
The results have now been confirmed by an independent group (paper link). Sera from patients that were infected with dengue virus, as well as dengue virus specific human monoclonal antibodies, were shown to bind Zika virus and enhance infection of Fc receptor bearing cells. These are the same findings of the group who first published on bioRxiv. That paper still has not been published – apparently it is mired in peer review, with many new experiments requested. I do hope that none of the authors of the second paper are involved in delaying its publication – something that happens all too often in science. As a colleague once remarked, ‘the main function of peer review is to prevent your competitors from publishing their work’.
Whether or not antibodies to dengue virus enhance Zika virus disease in humans is an important unanswered question.
If you are wondering whether antibodies to Zika virus can enhance dengue virus infection, the answer is yes (paper link). Monoclonal antibodies were isolated from four Zika virus-infected patients, and shown to enhance infection of Fc receptor bearing cells with either Zika virus or dengue virus. Furthermore, administration of these antibodies to mice before infection with dengue virus led to severe disease and lethality, a demonstration of antibody-dependent enhancement in an animal model.
Of interest is the finding that ADE mediated lethality in this mouse model can be completely prevented by co-administering the same antibody that has been modified to block binding to Fc receptors on cells. This result suggests a modality for treating patients with enhanced disease caused by either dengue virus or Zika virus.
These observations suggest that we need to be careful when deploying vaccines against Zika virus or dengue virus – it is possible that the antibody response could enhance disease. Recently a dengue virus vaccine called Dengvaxxia was approved for use in Brazil, Mexico, and the Philippines. However, the vaccine is not licensed for use in children less than 9 years of age because in clinical trials, immunization lead to more severe disease after infection compared with non-immunized controls. Analysis of the clinical trial data (paper link) indicates that seronegative individuals of all ages were at increased risk for developing severe disease that requires hospitalization. The authors suggest that severe disease is a consequence of enhancement of infection caused by antibodies induced by the vaccine (see CIDRAP article for more information).
These observations lead to the question of whether immunization against dengue and Zika viruses might enhance disease caused by either virus. Could a solution to this potential problem be to use a vaccine that combines the four serotypes of dengue virus with Zika virus? If so, the dengue virus component should not be Dengvaxia, but possibly another vaccine (e.g. TV003 – virology blog link) that does not induce disease enhancing antibodies.
Four virologists discuss our current understanding of Zika virus biology, pathogenesis, transmission, and prevention, in this special live episode recorded at the American Society for Microbiology in Washington, DC.
You can find TWiV #392 at microbe.tv/twiv, or listen/watch below.
The results of recent structural studies have given us the ability to display the structure of Zika virus and of the viral E protein bound to antibody. In this video from Virus Watch I explain how the Zika virus particle is built, and how it interacts with an antibody that blocks infection, in beautiful three dimensional imagery.
Preprint servers, the structure of an antibody bound to Zika virus, blocking Zika virus replication in mosquitoes with Wolbachia, and killing carp in Australia with a herpesvirus are topics of episode #388 of the science show This Week in Virology, hosted by Vincent, Dickson, Alan, and Kathy.
You can find TWiV #388 at microbe.tv/twiv, or listen below.
You can find TWiV #387 at microbe.tv/twiv, or listen below.
On episode #345 of the science show This Week in Virology, the TWiVonauts review how the weather affects West Nile virus disease in the US, benefit of B cell depletion for ME/CFS patients, and an autoimmune reaction induced by influenza virus vaccine that leads to narcolepsy.
You can find TWiV #345 at www.microbe.tv/twiv.
Did you ever wonder what different virus infections you have had in your lifetime? Now you can find out with just a drop of your blood and about $25.
Immune defense systems of many hosts produce antibodies in response to virus infections. These large proteins, which are generally virus specific, can block or inhibit virus infection, and persist at low levels for many years after the initial infection. Hence it is possible to determine whether an individual has had a virus infection by looking for anti-viral antibodies in the blood. Up to now the process of identifying such antibodies has been slow and limited to one or a few viruses. A new assay called VirScan allows unbiased searches for all the virus antibodies in your blood, providing a picture of all your past infections.
To identify the human antivirome, DNAs were synthesized encoding proteins from all viruses known to infect humans – 206 species and over 1000 strains. These DNAs were inserted into the genome of a bacteriophage, so that upon infecting bacteria, the viral peptides are displayed on the phage capsid. These ‘display’ phages were then mixed with human serum, and those that were bound by antibodies were isolated. The DNA sequence of the phage genomes were then determined to identify the human virus bound by the antibodies.
This method was used to assay samples from 569 humans. The results show that each person had been exposed to an average of 10 viruses, with a range from a few to over 20 (two individuals had antibodies to 84 different virus species!). The most frequently identified viruses included herpesviruses, rhinoviruses, adenoviruses, influenza viruses, respiratory syncytial virus, and enteroviruses. The overall winner, found in 88% of samples, is Epstein-Barr virus.
These results are not unexpected: all of us are infected with at least a dozen viruses at any time, and the viruses identified in this study known to infect much of the human population. What was surprising is the absence of some common viruses, such as rotaviruses, and the ubiquitous polyomaviruses. According to serological surveys, the most common human viruses are the small, single-stranded DNA containing anelloviruses. Yet the related torque teno virus was only found in 1.7% of samples. These differences are likely due to a combination of technical and biological issues (e.g., failure of antibodies to certain viruses to persist in serum).
This new assay may one day become a routine diagnostic tool that is used along with complete blood counts and chemistries to know if a patient’s signs and symptoms might be attributable to a past virus infection. VirScan technology is not limited to virus infections – it can be used to provide a history of bouts with bacteria, fungi, and parasites.
VirScan might also allow us to determine which virus infections are beneficial, and which contribute to chronic diseases such as autoimmune or neurodevelopmental disorders or cancer. The assay can be used to conduct unbiased population-based studies of the prevalence of virus infections and their possible association with these diseases. Such connections were not previously possible with antibody assays that search for one virus at a time. This approach was not only inefficient, but required guessing the responsible virus.
Some other findings of this study are noteworthy. As expected, children had fewer virus infections than adults. HIV-positive individuals had antibodies to more viruses than HIV-negative individuals, also expected given the damage done by this virus to the immune system. Frequencies of anti-viral antibodies were higher outside of the United States, possible due to differences in genetics, sanitation, or population density. In most samples, there was a single dominant peptide per virus, although there were occasional differences among populations. This information might be useful for improving vaccines, or tailoring them to specific countries or regions.
Update: It would be very informative to use VirScan to search for antibodies against viruses that are not known to infect humans. Other animal viruses, plant viruses, insect viruses: to which do a significant fraction of humans respond? The information might identify other viruses that replicate in humans and which might constitute future threats (or present benefits).
ZMapp, a mixture of three antibodies against Ebola virus, became a household name after it was used to treat two Americans who were infected while working in Liberia. The structure of these antibodies bound to the Ebola virus glycoprotein suggest how they inhibit infection and ways to improve ZMapp.
The three monoclonal antibodies that comprise ZMapp (called c13C6, c2G4, and c4G7) were produced by immunizing mice with a recombinant vesicular stomatitis virus in which the glycoprotein was replaced with that from Ebola virus. Antibodies that bound the viral glycoprotein and protected mice from infection were identified, and three were made to resemble human antibodies and produced in tobacco plants. Ecco Zmapp!
Embedded in the membrane of the filamentous Ebola virus particle are many copies of the glycoprotein, seen as club-shaped spikes in the image to the right (image credit: ViralZone). The viral glycoprotein is essential for entry of the virus into cells. The antibodies in ZMapp are directed against the viral glycoprotein.
To determine how the antibodies bind the virus particle, they were individually mixed with purified Ebola virus glycoprotein, and the structures were determined by electron microscopy and image reconstruction. The results, shown in the illustration, indicate precisely where each antibody binds to the Ebola virus glycoprotein. The individual antibodies colored red (c2G4), yellow (c4G7), and purple (c13C6) are bound to a single Ebola virus glycoprotein in white, with the viral membrane below (Image credit).
The structures reveal that c13C6 (purple) binds at the tip of the viral glycoprotein, perpendicular to the plane of the viral membrane. The other two antibodies (red, yellow) bind at the base of the viral glycoprotein. Their binding sites overlap but are not identical (the Ebola virus glycoprotein is a trimer, and in the image, the yellow and red antibodies are shown binding to different subunits for clarity). Two other antibodies that block Ebola virus infection also bind at the base of the glycoprotein.
Antibody c13C6, which binds to the tip of the viral glycoprotein, does not neutralize viral infectivity. Nevertheless, it can protect animals from Ebola virus infection. This observation suggests that the c13C6 antibody may work in concert with complement, a collection of serum proteins, to block virus infection. It is not known why c13C6 antibody is non-neutralizing, but one possibility is that it binds to a part of the viral glycoprotein that is removed by an endosomal protease, cathepsin, before receptor binding in late endosomes.
Antibodies c2G4 and c4G7, which bind to the membrane-proximal part of the viral glycoprotein, neutralize viral infectivity. How they do so is not known, but one possibility is that they prevent structural changes of the viral protein that are essential for the fusion with the endosomal membrane, a process that delivers the viral nucleic acid into the cell cytoplasm.
These studies reveal two general areas of the Ebola virus glycoprotein that are important targets for antibodies that protect animals from Ebola virus infection. Those directed at the base of the glycoprotein neutralize infectivity while those that bind the tip do not. This information can now be used to isolate additional antibodies that bind either site. These can be used in animal protection studies to design mixtures that are even more potent than ZMapp.