The TWiV team takes on an experimental plant-based poliovirus vaccine, contradictory findings on the efficacy of Flumist, waning protection conferred by Zostavax, and a new adjuvanted subunit zoster vaccine.
You can find TWiV #403 at microbe.tv/twiv, or listen below.
Become a patron of TWiV!
Although the use of the live, attenuated (Sabin) poliovirus vaccines has been instrumental in nearly eradicating the virus from the planet, the rare reversion to virulence of these strains has lead to the World Health Organization to recommend their replacement with inactivated poliovirus vaccine (IPV). Unfortunately IPV is also not without shortcomings, including high cost, failure to induce intestinal immunity, and the need to keep the vaccine at low temperatures. An experimental poliovirus vaccine produced in plants could overcome these problems.
A new vaccine candidate was made by producing the poliovirus capsid protein VP1 in the chloroplast of tobacco plants (nuclear-directed antigen synthesis is often inefficient). VP1 was fused to the cholera toxin B (CTB) subunit which allows good transmucosal delivery of the protein. Leaves were freeze dried, ground to a powder, mixed with saline and fed to mice after subcutaneous inoculation with IPV. The results show that boosting with the plant-derived VP1-CTB protein lead to higher antibody neutralizing titers (against all three poliovirus serotypes) both in the blood and in fecal extracts, compared with mice inoculated with IPV alone.
The VP1-CTP protein within lyophilized plant cells was stable for 8 months at ambient temperatures. If immunogenicity is maintained under these conditions, it would eliminate the need for a cold chain to maintain vaccine potency, an important achievement.
The authors propose that plant-produced VP1-CTP protein could substitute for IPV once the use of OPV is discontinued. Whether this suggestion is true depends on confirmation, by clinical trial, of these findings in humans. Furthermore, oral administration of VP1-CTP plant cells alone produces no serum neutralizing antibodies, and whether VP1-CTP boosts immunity in OPV recipients remains to be determined. Because VP1-CTP does not provide protection in children who have never received IPV or OPV, it cannot be used if poliovirus circulation continues indefinitely in the face of a growing cohort that has not been immunized with IPV or OPV. Nevertheless the technology has promise for the development of other vaccines that are inexpensive and do not need low temperature storage.
Polio returns to Nigeria, Zika virus spreads in Miami, and virus infection of plants attracts bumblebees for pollination, from the virus gentlepeople at TWiV.
You can find TWiV #400 at microbe.tv/twiv, or listen below.
Become a patron of TWiV!
Volatile organic compounds that are emitted by plants – which play important roles in attracting insects – can be altered by virus infection. Infection by cucumber mosaic virus (CMV) increases the emissions that attract aphids, but the virus-altered plant juices are not so tasty, so the insect quickly leaves to spread virus to other plants (listen to TWiV #70 for a full discussion of this finding). Plant pollinators can also respond to plant virus-induced volatile compounds (paper link)
Cucumber mosaic virus is a (+) strand RNA virus that is a pathogen of tomato plants. The virus is spread by aphids, and bumblebees increase fruit yield by buzz-pollination (tomato plants are self-pollinating but bumblebees cause increased seed production).
To study the attraction of bumblebees to tomato plants, the authors encased single tomato plants in a tower topped with a screen and a small cup of 30% sucrose (shown in photo: credit). This arrangement allowed volatile compounds to waft through the screen above the plant.
Bumblebees clearly preferred to visit the CMV-infected over the uninfected plants. They also shunned plants infected with a mutant of CMV that cannot antagonize the RNA interference system of the plant – a major defense against viruses. The results suggest that CMV somehow alters plant produced volatile compounds to attract bumblebee pollinators, and that production of these compounds is regulated by microRNAs.
These findings – that CMV infected plants attract bees via the production of volatiles under the control of microRNAs – were confirmed using Arabidopsis, a model plant which can be genetically manipulated.
Chemical analysis of the volatiles produced by infected and uninfected tomato plants revealed that virus infection reduced the levels of two terpenoids, 2-carene and beta-phellandrene. These compounds are known to repel bumblebees, leading to the suggestion that their reduction may explain why bees prefer infected plants.
Is attraction of bumblebees by viral infection random, or does it have a purpose? CMV infection of tomato plants decreases seed production, an effect that is reversed when bumblebees are attracted to the plants and pollinate them.
The results show that CMV infection of tomato plants causes the emission of volatile compounds that attract pollinating bumblebees, which negate the inhibition of seed production by infection.
Bumblebees do not transmit CMV, so how did this non-fatal attraction come about? One can imagine that at one time CMV infection of plants did not attract bumblebees, and that infection severely depressed plant production. Then a random viral mutant arose that could induce the proper volatiles to attract bumblebees. This change provided a selective advantage because plant seed production was restored by bumblebee pollination. This is called a ‘payback’ hypothesis: the virus has a place to replicate, and the plant lives on due to virus-induced volatile compounds.
I have a bit of trouble with this payback idea, because at the base of it are human-like intentions. Why do we assume that what drives evolution are forces that make sense to us? The plant-centric view is that it was not a CMV mutant, but a plant mutant that arose, which could make the proper bumblebee attractants upon CMV infection. The selective force to maintain such a plant is very clear and involves no anthropocentric ‘payback’.
Whatever the origin of this non-fatal attraction, it merits further study, not only for developing better pollination methods, but for understanding the viral-host symbiosis.
On this episode of Virus Watch we cover three Zika virus stories: the first human trial of a Zika virus vaccine, the first local transmission of infection in the United States, and whether the virus is a threat to participants in the 2016 Summer Olympic and Paralympic Games.
Zika virus spreads in the USA, a Zika virus DNA vaccine goes into phase I trials, and how mosquito bites enhance virus replication and disease, from the friendly TWiFolk Vincent, Dickson, Alan, and Kathy.
You can find TWiV #401 at microbe.tv/twiv, or listen below.
Become a patron of TWiV!
Mosquito saliva, which is injected into the host as a mosquito probes for a blood vessel, contains a collection of chemicals which include anticoagulants to prevent blood clotting, vasodilators to keep blood vessels wide, and anesthetics to prevent us from sensing the mosquito. Saliva also contains components that enhance viral replication, dissemination, and pathogenesis by inducing an inflammatory response that inadvertently promotes infection by providing new cell targets for infection (paper link).
To separate the bite from virus inoculation, mice were first exposed to Aedes aegyptii mosquitoes, and then infected at the bite site with two different mosquito transmitted viruses, Semliki Forest virus or Bunyamwera virus. Mosquito bites caused more virus replication at the inoculation site, greater dissemination of virus, and more lethality compared with control mice that received only virus.
How does mosquito saliva enhance virus replication and dissemination? Part of the story is that as the mosquito probes for a blood vessel, it causes damage that leads to vascular leakage and accumulation of fluid (edema) which inhibits movement of virus to draining lymph nodes.
But delaying dissemination of virus alone does not promote infection and disease. Mosquito bites cause an infiltration of neutrophils (a type of white blood cell) into the bite site. The edema at the bite site is enhanced by neutrophils, because depleting these cells from mice greatly reduced edema. This depletion also returned viremia to levels observed in unbitten control mice, and restored dissemination of virus to draining lymph nodes. Neutrophils are not susceptible to infection with Semliki Forest virus, and therefore cannot explain the increase in virus replication at the bite site.
Enhanced virus replication in the skin occurs because the neutrophils elaborate chemokines that attract macrophages, which can be infected by Semliki Forest virus and Bunyamwera virus. One of the chemokines produced by neutrophils that is a macrophage attractant – CCL2 – binds a receptor on macrophages. Mice lacking the gene encoding the CCL2 receptor are protected from bite enhancement of Semliki Forest virus enhancement.
When a mosquito bites a host, it delivers saliva along with a virus. The saliva induces an inflammatory response and attracts neutrophils into the bite site. The resulting edema holds virus at the bite site until chemokines produced by neutrophils attract macrophages, which are then infected. The virus produced disseminates widely, reaching secondary tissues and causing disease.
It seems likely that the ability to replicate in macrophages that are recruited to the bite site is a property that was selected during evolution of mosquito-transmitted viruses. By replicating in macrophages, the amount of virus in the blood is increased, as well as the likelihood that the virus will be picked up by another mosquito and transmitted to a new host – a powerful selection mechanism. The down side – increased disease in the mammalian host – is an accidental side effect.
Think about that the next time you are scratching that raised bump on your skin caused by a mosquito bite.
The TWiV team is together in New York City for a conversation with Nobel Laureate Harold Varmus about his remarkable career in science.
You can find TWiV #400 at microbe.tv/twiv, or listen/watch below.
Become a patron of 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.