The first experimental Zika virus vaccine has been published, and in this episode of Virus Watch, I explain how it works – it’s a DNA vaccine – and I compare it with all the other vaccines out there.
The latest Zika virus news from the ConTWiVstadors, including a case of female to male transmission, risk of infection at the 2016 summer Olympics, a DNA vaccine, antibody-dependent enhancement by dengue antibodies, and sites of replication in the placenta.
You can find TWiV #399 at microbe.tv/twiv, or listen below.
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Viruses that replicate in the male or female reproductive tract are considered to be potential causes of human infertility. Several herpesviruses have been implicated in male infertility, and now human herpesvirus 6A (HHV-6A) has been found in endometrial cells of women with unexplained infertility (paper link).
HHV-6 was only recently discovered (1986) and is now known to occur as two variants, HHV-6A and HHV-6B. The latter is a major cause of exanthem subitum, a rash of infants, but no disease has been clearly associated with HHV-6A. These viruses are transmitted to infants early in life via saliva, from mother to child, from siblings, or from other infants at day care centers. Seroprevalence studies indicate that almost all children are infected with these viruses by 2 years of age.
To determine if HHV-6 might be a cause of infertility, a study (paper link) was conducted of 30 women with unexplained primary fertility, and 36 women with at least one previous pregnancy. HHV-6B DNA was detected in PBMC from both infertile and fertile groups (25 and 28%, respectively); HHV-6A DNA was not detected. In contrast, endometrial epithelial cells from 13/30 (43%) infertile women were positive for HHV-6A DNA; this viral DNA was not detected in endometrium of fertile women. When placed in culture, endometrial epithelial cells produced viral early and late proteins, suggesting the presence of infectious virus.
Presence of HHV-6A DNA in endometrial epithelial cells was also associated with an altered hormonal and immune environment. Estradiol levels were higher in infected versus uninfected infertile women. The authors suggest that higher levels of this hormone could be involved in allowing HHV-6A infection of the endometrium.
Levels of a specific type of uterine NK cell were lower in HHV-6A positive women, and IL-10 (a Th2 cytokine) was elevated while IFN-gamma (a Th1 cytokine) was decreased. There were no differences in the levels of these cells and cytokines in peripheral blood. These changes are consistent with an increase in the ratio of Th1/Th2 responses that has been documented in female infertility.
The authors also observed enhanced endometrial NK cell responses to HHV-6A in infected but not uninfected women, together with an increase in the number of these cells that are activated when cultured with HHV-6A infected cells.
I wonder what was the source of HHV-6A in the endometrium, as the virus was not detected in blood. Was the infection recently acquired, or did it occur years before, with the virus establishing a chronic infection in the uterus?
The results suggest that HHV-6A infection of the endometrium triggers an abnormal NK cell and cytokine profile, which in turn leads to a uterine environment that is not compatible with fertility. The results need to be confirmed with studies of additional fertile and infertile women. It would also be useful to have an animal model of HHV-6A infection of the endometrium, which could lead to mechanistic work to determine how virus infection causes infertility.
Image: Electron micrograph of HHV-6 (image credit)
Vincent speaks with Sandy Weller about her career and her work on the mechanisms of synthesis, maturation, cleavage and packaging of viral DNA genomes.
You can find TWiV 398 at microbe.tv/twiv, or listen/watch below.
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Andrew W. Gurman, M.D., President of the American Medical Association, has expressed disappointment in the failure of Congress to support the US public health response to Zika Virus:
At a time when concerns continue to mount about the nation’s readiness to protect the public from the Zika virus, the AMA is disappointed by Congress’ failure to pass legislation before adjourning for summer recess that would provide the resources necessary for our country to respond to this looming public health crisis.
Without ensuring there are sufficient resources available for research, prevention, control and treatment of illnesses associated with the Zika virus, the United States will be ill-equipped to deploy the kind of public health response needed to keep our citizens safe and healthy—especially since the spread of mosquito-borne illness is accelerated during the summer months.
I could not agree more with the AMA – Congress has so far failed to do the right thing with respect to Zika virus. Even if the virus does not spread within the continental United States, this country has an obligation to be a leader in scientific research that would benefit the entire world. Zika virus is clearly a threat to other parts of the globe, and by refusing to fund research on this virus, Congress is sending the message that it doesn’t care about the health problems of others.
Vincent speaks with Peter Palese about his illustrious career in virology, from early work on neuraminidases to universal influenza virus vaccines, on episode #396 of the science show This Week in Virology.
You can find TWiV #396 at microbe.tv/twiv, or listen below.
A battle is brewing between two research groups in Marseille, France that are involved in the discovery and study of giant viruses. Didier Raoul and colleagues believe that they have discovered a CRISPR-like, DNA based defense system in mimivirus that confers resistance to virophage (paper link). Claverie and Abergel disagree: they think that the defense system involves proteins, not nucleic acids (paper link).
Virophages are DNA viruses that can only replicate in cells infected by giant viruses like mimivirus. Their name, which means ‘virus eater’, comes from the observation that they inhibit mimivirus replication. A specific virophage called Zamilon was discovered that can inhibit the replication of lineage B and C mimivirus but not lineage A.
Examination of the DNA sequences of 60 different mimivirus strains revealed that the genomes of lineage A contained a 28 nucleotide sequence identical to Zamilon virophage. This sequence was not found in any lineage B mimivirus and in only one out of 19 lineage C mimiviruses. In addition, a 15 nucleotide subset of this sequence is repeated four times in the lineage B and C mimivirus genomes.
Near the 15 nucleotide Zamilon-derived repeated sequences in lineage B and C mimivirus genomes are genes encoding several proteins related to components of the bacterial CRISPR-Cas system. These include a nuclease, an RNAse, and an ATP-dependent DNA helicase.
The CRISPR system provides defense against invading DNA. When a foreign DNA, such as a bacteriophage genome, enters a bacterial cell, some is fragmented and integrated into the CRISPR locus as a ’spacer’ (sequences in the foreign DNA are called ‘protospacers’). Following transcription, CRISPR RNAs (crRNA) are processed by a multiprotein complex to produce ~60 nucleotide RNAs. When the spacer of a crRNA base pairs with a complementary sequence in an invading DNA molecule, CRISPR-associated endonucleases cleave the DNA. The integration of the sequences of the invading DNA into the host cell genome, from which they can be mobilized in the form of crRNAs, provides a form of “memory” and acquired immunity. It should be noted that there are six known types of CRISPR systems that differ in their components and mechanisms.
Because the CRISPR-Cas system is an adaptive immune system that protects bacteria and Archaea from virus infections and invasion of foreign DNA, the authors propose that they have discovered a new adaptive immune system that protects mimiviruses from virophage infection. They call this system mimivirus virophage resistance element, or MIMIVIRE.
The authors provide experimental support for their hypothesis by showing that silencing the genes encoding the endonuclease, the helicase, and the repeated insert using siRNA allows Zamilon replication in mimivirus-infected cells.
Claverie and Abergel think that Raoult and colleagues are wrong (paper link). They provide three reasons to dispute their findings, and ‘propose a simpler protein-based interaction model that explains the observed phenomena without having to extend the realm of adaptive immunity to the world of eukaryotic viruses, a revolutionary step that would require stronger experimental evidences.’
The first problem is that mimivirus and Zamilon virophage replicate in the same location in the infected cell, making a CRISPR-like defense system difficult to conceptualize. In contrast, CRISPR sequences reside in the bacterial genome, from which RNAs are produced that target the destruction of invading DNAs elsewhere in the cell.
The second problem is that the Zamilon sequences in the mimivirus genome are not regularly spaced or flanked by recognizable repeats, a hallmark of the CRISPR system (the name stands for ‘clustered regularly interspersed short palindromic repeats). However it should be noted that type VI CRISPR systems have no CRISPR locus and likely function via mechanisms that are different from other CRISPR systems.
Finally, Claverie and Abergel argue that there is no way for the proposed nucleic acid defense system to distinguish between the virophage and the virophage sequences in the mimivirus genome. In some CRISPR systems this discrimination is achieved by protospacer adjacent motifs (PAMs), short (2-5 nt) sequences next to the invader protospacer sequences that are recognized by the endonuclease complex guided by the crRNA. PAMs are not present in the bacterial genome, sparing it from endonucleolytic cleavage. Nevertheless, non PAM-based mechanisms of discriminating invader from host are known, for example, in the type III CRISPR system.
If Raoult and colleagues have not discovered a CRISPR-like mimivirus defense system, then why would silencing the genes encoding CRISPR-like proteins allow Zamilon replication? Claverie and Abergel think that it is not the 15 nucleotide Zamilon repeats that are important to mimivirus, but the encoded amino acids: Asp-Asn-Glu-Ser (DNES in one letter code). They believe that DNES is a motif present in proteins that block Zamilon replication by as yet unidentified mechanisms.
Many cellular proteins have been identified that interfere with virus replication, such as those encoded by interferon induced genes (ISGs). DNES containing proteins that inhibit Zamilon replication would be conceptually analogous, except that they are encoded by a virus, not the host.
Claverie and Abergele appear to have a strong case that mimivirus defense against Zamilon virophage is mediated by protein, not nucleic acid, but further experimentation is certainly needed to support their position. Nevertheless they recognize that the discovery by Raoult and colleagues “remains fascinating even if it falls short of demonstrating the existence of a CRISPR-Cas-like adaptive immune system”.
Vincent, Rich and Kathy speak with Stephen Russell about his career and his work on oncolytic virotherapy – using viruses to treat cancers. Recorded before an audience at ASV 2016 at Virginia Tech in Blacksburg, Virginia.
You can find TWiV #395 at microbe.tv/twiv, or listen/view below.
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Vincent and Alan speak with Erica Ollmann Saphire about her career and her work on understanding the functions of proteins of Ebolaviruses, Marburg virus, and other hemorrhagic fever viruses, at ASM Microbe 2016 in Boston, MA.
You can find TWiV #394 at microbe.tv/twiv, or listen or watch the video below.
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When we decided to work on Zika virus in February 2016, experiments in mice were certainly part of our plans. However, one does not simply walk into a mouse facility and start inoculating animals with viruses! Carrying out animal experiments requires approval of a detailed protocol by the Institutional Animal Care and Use Committee (IACUC). I have filed many IACUC protocols in the past 30 years, and to work on Zika virus in mice, we had to file a new one. Here is how the process works.
Read the remainder of this article at Zika Diaries.