Category Archives: Immunology

Fever!

September 11, 2012 David Esteban 6 Comments

Most viral infections start out with the same general symptoms: fever, malaise, aches. Those are usually the sign of your immune system starting to fight back. Fever is one of our defense mechanisms, and while it can be quite uncomfortable it is very rarely dangerous. One of the common questions I get asked in my classes is whether it is good or bad to take fever reducing medicines when you are sick. Well, Im not that kind of doctor so I can’t really answer that question, but I can tell you what research has been done on the benefits of fever.

The adaptive value of Saturday Night Fever, caused by listening to Disco, remains unknown

Presumably, by raising your body temperature, you can do a better job of fighting off an infection. The higher temperature may make it more difficult for the pathogen to grow because it grows optimally only at normal body temperature, or it may help the immune system work faster (or both). Some studies in animals have shown that reducing fever results in increased growth of bacteria or virus in the infected animal, and other studies have found increased proliferation, migration and activity of immune cells.

Whatever the mechanism, it is clear that fever must provide some advantage. There are many studies that demonstrate that fever is beneficial in overcoming infection. None of these studies alone is definitive, however taken together, they do seem to support a role for fever in fighting infection. For one, the febrile response is highly conserved in vertebrates (even “cold blooded” animals) and many invertebrates. Some lizards, for example, will seek warmer spots to rest and as a result, raise their body temperature when infected. Fever is also energetically very expensive, requiring 20% more energy to maintain a temperature even a few degrees above normal. It would be unlikely for such an expensive mechanism to be maintained by natural selection if it didn’t have some benefit.

In addition to the evolutionary perspective, several studies in animals show that a fever of a few degrees correlates with better survival rate from infection. Being correlations, we must be cautious in over-interpreting this. Another good way to test if something like fever is useful is to get rid of it. Infected animals can be treated with anti-pyretic (fever reducing) drugs to see what happens to their recovery in the absence of fever. These studies typically show that animals treated with anti-pyretic drugs take longer to recover, and in some cases even to increased mortality. There are some problems with anti-pyretic studies however, and one of the major problems is that many anti-pyretic drugs don’t just reduce fevers. They can have other effects on the body, not all of which are known, and so we can’t always be certain that fever reduction is the reason for the changes in morbidity and mortality.

Of course, I also like to look at the pathogens themselves for hints of what our immune system is doing. They are pretty good at defending themselves against our immune response, so if we look at their defenses we can learn more about how we attack them. The poxvirus Vaccinia encodes a protein that blocks fever. This protein interferes with the function of interleukin-1B, a component of our own immune system that regulates fever. Animals infected with Vaccinia lacking that protein develop fever, showing that when the viral protein is present, the virus can prevent fever. However, interleukin-1B does many other things too, not just regulation of fever, so it is possible that the virus is blocking interleukin-1B for a different reason.

So it is highly likely that fever is good for fighting off infections, but this is not to be taken as medical advice to avoid fever reducing medicines. In the case of naturally occurring infections in humans, we need much more research into fevers resulting from specific infections to decide when a fever is beneficial and should be left alone, whether the fever is dangerous, or if the fever is helpful but the risks of taking an anti-pyretic are worth alleviating the uncomfortable symptoms.

References:
Kluger, M.J. (1996) The adaptive value of fever. Infectious disease clinics of north america. 1(10):1-20.
Alcami, A. and Smith, G.L. (1996) A mechanism for the inhibition of fever by a virus. Proc Natl Acad Sci. 93:11029-11034.

Guest Blogger, Immunology, Pathogenesis

Mouse Pneumonia: Are We to Blame?

May 2, 2012 David Esteban 4 Comments

Contributed by guest blogger: Alix Zongrone ’12

Pneumonia virus of mice, or PVM, is the leading cause of pneumonia in laboratory mice; however, lack of evidence of PVM in wild rodents has left scientists in the dark with regards to the history and natural host of the virus. Because PVM is mostly found in captive settings (i.e. laboratories, pet shops, etc.) and PVM-neutralizing behavior has been observed in human cells, it has been suggested that human contact may play a pivotal role in the virus’s spread. Several studies have sought to investigate the prevalence of PVM in humans and its role in human respiratory infection; however, since PVM is closely related to human respiratory syncytial virus, or RSV, it is difficult to make sound conclusions based on this evidence alone.

Due to the evidence of PVM in humans, researchers inoculated two different non-human primate species with PVM to investigate replication activity of PVM in these mammals. They found that, over the course of twelve days, most of the samples exhibited viral replication as well as viral shedding. Although not all of the animals showed virus replication and shedding behavior, PVM antibodies were found in all test animals, suggesting that infection did take place, but replication was highly restricted. Though PVM was observed to not replicate well in non-human primates, human lung epithelial cells exhibited similar permissiveness of both PVM and RSV in vitro.
Controlling the interferon (IFN) immune response is a known mechanism of successful viral replication in the host. Researchers investigated the ability of PVM to block IFN response to further explore PVM host range restriction. The virus demonstrated an ability to block IFN response in these human epithelial cells thanks to the NS2 protein. However, a Western blot was used to compare proteins made from PVM and RSV and PVM-neutralizing activity specificity was also determined. Humans were tested for PVM antibodies to examine whether an immune response was triggered. No PVM antibodies were found in the human sera, and no reactivity between PVM proteins and observed PVM-neutralizing behavior was recorded. This demonstrates a lack of immune response in the human cells.
Although PVM was observed to replicate in vitro in human epithelial cells, the results remain inconclusive as to whether or not the virus should be considered a human pathogen. The lack of permissiveness in non-human primates suggests that the virus may not actually cause infection in humans. This is supported by the lack of reaction shown between PVM proteins and PVM-neutralizing activity in the Western Blot.
Questions remain as to the nature of the PVM-neutralizing activity in human serum as well the origin of PVM and its natural host. Is that which is categorized as PVM-neutralizing behavior not actually PVM-specific? What is causing PVM in captive laboratory mice but not in wild rodent species? Finally, what could possibly be the natural host of pneumonia virus of mice, if not mice?

Link: http://jvi.asm.org/content/86/10/5829.abstract?etoc

Alix Zongrone is a senior at Vassar College, majoring in biology.

Guest Blogger, Immunology, Vaccines

After many setbacks, cross-presentation provides new hope for a Herpes Simplex Virus 1 vaccine

April 27, 2012 David Esteban 4 Comments

Contributed by guest blogger: Stephanie Mischell ’12

Herpes simplex virus type 1 (HSV-1) is making news due to a paper by Jing et al identifying two promising new candidate antigens for a vaccine. HSV-1 is a widespread public health issue, infecting approximately 60% of Americans and causing symptoms, most likely cold sores or genital sores but on rare occasion blindness or fatal brain damage. Furthermore, finding a vaccine for HSV-1 has proved difficult, in part because of the vital but elusive role of CD8⁺ T-cells in the HSV-1 immune response. Mice studies suggest that a CD8-response could facilitate memory cell formation and ameliorate chronic disease caused by HSV-1, but human blood does not have many HSV-1 specific CD8⁺ T-cells and very few CD8 epitopes have been identified. Previous attempts at vaccines most recently using the HSV glycoprotein D (gD2), have focused on CD4⁺ T-cell specific epitopes. These attempts were unable to stimulate a CD8⁺ T-cell response, and the vaccine failed during clinical trials. A way to stimulate both CD4⁺ and CD8⁺ T-cell responses seems necessary to create an effective vaccine.

Jing et al’s work is significant because it harnesses properties originally used to study HSV-2 to identify HSV-1 epitopes recognized by CD8⁺ T-cells. An epitope, or antigenic determinant, is the part of an antigen that is recognized by the immune system; this interaction is what triggers a host immune response. Jing et al demonstrated previously that in vitro monocyte-derived dendritic cells (moDC’s), or antigen-presenting cells, can cross-present HSV-2 epitopes to create HSV-2 specific memory T-cells. In this paper, they harnessed this cross-reactivity of moDC’s and applied it to HSV-1, stimulating and identifying HSV-1 specific CD8⁺ T-cells. 45 distinct CD8⁺T-cell epitopes were identified. Furthermore, the genomes of host responder cells were cloned, and HSV-1 epitopes were analyzed for HLA restriction. Proteins from two genes, UL39 and UL46, were identified as most highly restricted, suggesting that they are most involved in the immunogenic response. PMBC assays confirmed these results quantitatively.

Jing et al conclude that the viral proteins coded by UL39 and UL46 are good candidate antigens for an HSV-1 vaccine because of their CD4⁺ and CD8⁺T-cell immunogenicity. However, they also acknowledge that their sample size is small and that subunit vaccines have not been successful vaccines for HSV-1. In fact, the large number of CD8⁺T-cell epitopes identified led the authors to conclude that a whole-virus vaccine may be more successful than subunits. Most of the failed vaccines showed similar promise until phase II or phase III of clinical trials, suggesting that the small amount of data from this study is just a start. This discovery is important but not a guaranteed vaccine.

While the identification of UL39 and UL46 are important steps in solving the public health issue posed by HSV-1, as is the identification of other CD8⁺T-cell epitopes, perhaps the most significant part of the study is the implications of their novel research methods on the study of viral vaccines. The enrichment techniques used could potentially make studying T-cell responses easier. The authors confirmed the applicability of their methods by using the same techniques to study the vaccinia virus, a microbe with a large genome of over 200 genes. This paper demonstrates a small advancement in HSV-1 research and control, but may have larger implications for this and other large viruses.

Link to original article: http://www.jci.org/articles/view/60556

Stephanie Mischell is a senior at Vassar College, majoring in biology.

Antiviral Drugs, Epidemics, Immunology, Molecular Virology

Luring HIV out of its latency may be the secret to developing an effective HIV cure

April 19, 2012 David Esteban 7 Comments

Contributed by guest blogger: Steven Chan ‘12

The emergence of highly active antiretroviral therapy (HAART) in the treatment of HIV-infected individuals has certainly changed the outlook of an HIV diagnosis today, compared to what such an outlook looked like in the earliest years of the epidemic. Such a treatment regimen, if strictly adhered to, has the potential to suppress the levels of active circulating HIV in the infected individual to a level that is manageable, essentially halting the progression of the disease. It soon became clear however, that these treatments could not effectively clear the body of all HIV particles—the virus manages to stow itself away within the cellular genome of the memory CD4⁺ T-cells, and remain transcriptionally silent indefinitely. These latent reservoirs of HIV-infected cells prove to be undetectable for these antiretroviral therapies, since antiretroviral drugs can only target HIV-infected cells when they are replicating. And so, memory cells, which replicate infrequently, cannot be effectively targeted, making it impossible to clear HIV-infected bodies of all HIV-particles. “We’re never going to cure anybody unless we go for this latent pool,” says Robert Siliciano, the researcher at Johns Hopkins University that first identified the latent HIV memory-T cells.

A great deal of HIV-therapy research over the past decade has focused on finding a way to coax these infected cells out of their latency to make them detectable by antiretroviral drugs. The problem that has been persistently hounding researchers has been the difficulty in luring these cells out of their latency without triggering the immune system in an inflammation response that would end up doing more harm than good. David Margolis, MD, and his research team at UNC Chapel Hill, who have been working on this problem for a while now, have found success with a set of histone deacetylase inhibitors called Zolinza (vorinostat), a chemotherapeutic cancer drug that has been found to stimulate gene expression within the latent HIV-infected cells without inducing an overwhelming immune response. HDAC inhibitors accomplish this by inhibiting the activity of histone deacetylase, which removes the acetyl groups from the lysine residues in the core histones, resulting in the formation of a condensed and transcriptionally silenced chromatin. By inhibiting this activity, the core histones become less compact, and the chromatin becomes more transcriptionally active. After initial success with in vitro tests in cell cultures and in blood tissues, six HIV-positive men were recruited in a clinical trial pairing this treatment alongside consistent antiretroviral therapy. Each of the study volunteers had already been taking part in a robust antiviral regimen for an average of four years, and displayed undetectable viral loads and stable CD4⁺ T-cell counts. Post-exposure to Zolinza, HIV-RNA levels—a marker of viral activity—in these patients increased by an average of 4.8 times, ranging from a 1.5-fold increase in one patient to a 10.0-fold increase in another. The drug took effect in as little as 8 hours, inducing a two-fold increase in cellular and chromatin-bound histone acetylation within that time span. Increased expression made these cells susceptible to detection and eradication by the antiretroviral drugs, which proceeds just as efficiently as usual.

Margolis addresses the significance of this advancement, “This study provides first proof of concept, demonstrating disruption of latency, a significant step toward eradication.” Just how effective this drug is in teasing out the latent cells still remains to be seen—with nearly a ten-fold difference in one trial participant compared to the other, the efficacy of such a drug remains questionable. The limited sample size in this initial trial also doesn’t give us too much to go on. There are also concerns that the drug could induce some serious side effects such as blood clots in the legs and lungs, diabetes, fewer platelets and RBC count, as well as dehydration from nausea and vomiting, but at least in this trial, there were only mild adverse effects at worst. Little is known about the potential adverse effects of long-term use of the drug. Margolis et al.’s study design made use of a single dose of Vorinostat, but it is likely that repeated intermittent doses would yield the most optimal effects. “Vorinostat may not be the magic bullet, but this success shows us a new way to test drugs to target latency and suggests that we can build a path that may lead to a cure,” says Margolis. Further studies to assess Vorinostat’s safety and effectiveness, and the way it interacts with other HAART treatments, would certainly be crucial before it can be deployed as a component in future HIV treatment regimen.

Links:

Archin N, Liberty A, Kashuba A, Choudhary S, Kuruc J, Hudgens M, Kearney M, Eron J, Hazuda D, and Margolis D. “Administration of Vorinostat Disrupts HIV-1 Latency in Patients on ART,” HIV Persistence, Latency, and Eradication at 19^th Conference on Retroviruses and Opportunistic Infections, March 8, 2012, http://www.retroconference.org/2012b/Abstracts/45315.htm

Contreras X, Schwenwker M, Chen CS, McCune JM, Deeks SG, Martin J, Peterlin BM. Suberoylanilide Hydroxamic Acid Reactivates HIV from Latently Infected Cells, J. Biol. Chem., January 9, 2009, http://www.jbc.org/content/284/11/6782.full

Horn T. “Pathway to a Cure: Cancer Drug Helps Purge HIV From Resting Cells,” AidsMeds, March 9, 2012, http://www.aidsmeds.com/articles/hiv_vorinostat_ cure_1667_22059.shtml

“Lymphoma Drug Wakes Up Dormant HIV,” AidsMeds, March 17, 2009, http://www.aidsmeds.com/articles/hiv_zolinza_latent_1667_16307.shtml

Steven Chan is a senior at Vassar College, majoring in Science, Technology, and Society

Guest Blogger, Immunology

The Viral Theory of Schizophrenia

April 17, 2012 David Esteban 21 Comments

Contributed by guest blogger: Hannah Ziobrowski ’12

Schizophrenia is a severely debilitating mental illness with no known cause or cure, although there is a strong genetic correlation. Interestingly, there is additionally a significant relationship between season of birth and the development of schizophrenia, as individuals born during late winter and spring have a significantly increased risk for developing schizophrenia. One hypothesis to explain this phenomenon is that this is due to prenatal viral infection, which is more likely to occur in the winter months. It is hypothesized that viral infections occurring during the third trimester of pregnancy result in the increased risk for developing schizophrenia. However, there is currently debate as to how this happens- is it due to a direct viral infection of the fetus, or due to maternal cytokines in response to infection?

A study by Faterni et al (2012) found that the placenta may be a site of pathology in viral infections. Using pregnant mice infected with a sublethal dose of influenza on the seventh day of pregnancy (E7), they found that viral infection resulted in many histological abnormalities in the placentae. These abnormalities included an absence of the labyrinth zone, the region of the maternal placenta in which nutrients and oxygen are exchanged between the maternal and fetal blood, the presence of thrombi, and an increased number of inflammatory cells. Additionally, microarray analyses revealed a significant upregulation of 77 genes and downregulaton of 93 genes in the placentae of infected mothers, compared to sham-infected mothers. 20% of these altered genes were involved in apoptotic or anti-apoptotic pathways, 10% were associated with immune response, 11% were involved with hypoxia, and about 11% were involved with inflammation. 9.4% were associated with major mental disorders including schizophrenia, bipolar disorder, major depression, and autism. All of these changes could potentially affect developing embryos. The deletion of a labyrinth zone could result in a reduction of oxygen delivered to the developing fetus and result in neural abnormalities, which may be ultimately caused by an inflammatory immune response.

The authors also analyzed gene expression in the hippocampus and prefrontal cortex of the offspring of infected mothers. Compared to offspring of sham-infected mothers, they found 6 upregulated and 24 downregulated genes in the prefrontal cortex at the first day after birth (P0), and 4 upregulated and 13 downregulated genes in the hippocampus at P0. Genes in the prefrontal cortex that showed a significant alteration in expression included glutamate receptor interacting protein I, platelet factor 4, contactin 1, and neurotrophic tyrosine kinase receptor type 3. Important genes in the hippocampus that showed altered levels of expression included paralemmin 2, and protein tyrosine kinase 2 beta. In total, 40 different genes showed altered expression in the two areas at P0 after infection at E7 (first trimester), compared to 39 at E9, 676 at E16, and 247 at E18 (as found in previous studies). These altered expression levels most likely reflect altered neural organization.

Importantly, HINI viral genes were not detected in either the placenta or brains of offspring whose mothers were infected at E7, suggesting that the virus did not cross the placenta to directly infect the offspring. This consequently implicates that the changes found in gene expression as well as the structural abnormalities of the placenta were most likely due to the production of maternal or fetal cytokines, most likely due to an increase in inflammatory cells in infected placenta.

These data overall illustrate that viral infections during pregnancy can lead to an inflammatory response and structural abnormalities in the placenta. These structural abnormalities may cause significant alterations in oxygen and nutrients delivered to the fetus, causing abnormalities in the overall development of the fetus, including the brain. Could these organizational neural abnormalities lead to an increased risk for developing schizophrenia later in life?

A next step for the authors would be to directly check the levels of cytokines in the placenta in order to assess the inflammatory response. Furthermore, are these results also found with infection of other viruses? Or are they more or less significant depending on the virus and time of infection?

Link: http://www.sciencedirect.com/science/article/pii/S0028390811000141

Hannah Ziobrowski is a senior at Vassar College, majoring in Neuroscience and Behavior.

Guest Blogger, Immunology, Molecular Virology

Natural Resistance: How Your Genes Can Determine The Severity of Your Influenza Infection

April 6, 2012 David Esteban 5 Comments

Contributed by guest blogger: Jared Saunders ’13

Every winter, the general public frantically agonizes over influenza prevention and protection. But is the purchase of hand sanitizer in bulk and tissue boxes by the dozen really necessary? After all, many people don’t even get sick during the winter months, and some just feel a little down for a couple of days. Why do some people catch “the flu” and end up in the hospital, fighting lung infections and plowing through boxes of tissues, while others just end up with a cough or runny nose? The answer may be come down to three letters. DNA.

Recent research by Everitt et al. at the Wellcome Trust Sanger Institute (WTSI) has revealed that a single gene found in humans can determine your fate when infected with a variety of the most common strains of the influenza virus. The gene encodes the important protein referred to as IFITM3, a member of the interferon-inducible transmembrane protein family. These IFITM proteins have been shown to potently restrict the replication of a variety of pathogenic viruses, and IFITM3 has been shown to greatly alter the course of influenza infection in both mice and humans.

Brass et al. previously identified IFITM3 through a functional genetic screen that indicated it mediated resistance to influenza A, dengue virus, and West Nile virus infection in vitro. This supported the hypothesis of the WTSI group (more than 30 authors!), that IFITM proteins are critical for intrinsic resistance to these viruses, and allowed them to proceed with determining the effects of IFITM3 in vivo using mice. IFITM3 knockout mice showed severe signs of clinical illness, including massive body weight loss, rapid breathing, and piloerection (also known as “goosebumps”) when infected with low-pathogenecity strains of influenza that do not usually cause such intense symptoms. Their presentation of infection was more consistent with high-virulence strains of influenza. Contrary to the knockout mice, the wild-type mice shed significantly less of their body weight before fully recovering.

With this significant data now being collected, the group moved on to testing their hypothesis that individuals who are homozygous dominant for the IFITM3 gene develop less virulent influenza infections. They sequenced the IFITM3 gene from 53 people who were hospitalized by the H1N1 or seasonal influenza infection during 2009 to 2010 to determine if they carried the wild-type gene or one with some polymorphism. Genetic analysis of a subset of these individuals showed no evidence of hidden population structure differences with respect to a 1000 genome control group from WTSI. In the hospitalized patients, the group found significant over-representation of a specific single nucleotide polymorphism (or SNP), referred to as SNP rs12252, that has a recessive C allele substituted for a normal dominant T allele. This leads to an ineffective IFITM3 variant lacking the first 21 amino acids of the protein. This recessive C variant leads to lower IFITM3 expression in the host and consequent increased susceptibility of the host to influenza infection, and is correlated with lower levels of IFITM3 protein expression.

The group’s work has shown conclusively that IFITM3 expression can act as a barrier to influenza A virus infection both in vitro and in vivo, and that in vivo it can lower the mortality and morbidity associated with infection by a variety of human influenza viruses. Discovery of this innate resistance factor in humans may explain why encounters with a novel strain that may cause severe infections in others that do not affect you or your family.

But can the IFITM3 gene be used to help develop treatments or vaccines for future influenza strain outbreaks? Is it possible to recover this gene, if an individual has an ineffective variant, through gene therapy so as to make someone more resistant to influenza? With more research being done on the genetic aspects of disease infection, many more questions will arise, and many more answers will as well!

Links:

http://www.nature.com/nature/journal/vaop/ncurrent/full/nature10921.html

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2824905/

Jared Saunders is a junior at Vassar College, majoring in biochemistry.

Epidemics, Immunology

HIV Microbicides and the Risks of Clinical Trials

March 28, 2012 David Esteban 7 Comments

Contributed by guest blogger: Julia Ding ’12

Once preliminary studies suggest that a drug is safe for human use, clinical trials are conducted in order to further investigate the effects and possible adverse reactions of the drug. The example of HIV microbicides has shown that caution and careful scrutiny is highly important for these trials. HIV microbicides are chemical entities which, when applied before vaginal or rectal intercourse, prevent the transmission of the virus. Of the potential microbicide agents that have been studied, two compounds classified as polyanions were thought to be promising for inhibiting HIV-1 transmission: carrageenan and cellulose sulfate (CS). However, these compounds were deemed in phase III clinical trials to be ineffective as microbicides.

In addition to that discovery, the more surprising and disturbing result of these trials was that the HIV microbicides appeared to actually enhance the rates of HIV infection. Pirrone and colleagues examined the validity of this claim in a study reassessing the in vitro activities of the compounds. Cells were infected with different strains of HIV-1 in the presence of three different polyanions: CS, λ-carrageenan (LC), and destran sulfate (DS). Resulting assays showed that all of these compounds exhibited antiviral activity against both R5 and X4 HIV-1 strains. However, further experiments also discovered that application and removal of polyanion microbicides prior to HIV exposure enhanced and increased the rates of HIV-1 infection. The compounds were added to cell cultures and washed out prior to HIV-1 infection to simulate the natural loss of the compound after vaginal application. In both HIV-susceptible cells and regular human cells, the results indicated an increase in the percentage of cells infected, unrelated to any change in cell viability. The level of enhancement was found to be dependent on the target cell, its co-receptor phenotype, the compound identity and concentration, and the timing of the viral challenge. While the mechanism through which HIV-1 transmission increased in the in vitro experiments is still unclear, these factors suggest that the nature of the host cell also plays a role in polyanion-dependent HIV-1 infection. This data provides a discouraging outlook on the use of these compounds as effective microbicides, while introducing new questions about its mechanisms of action.

This study provides us with many valuable insights about not only the microbicide technology itself, but also the risks and complications associated with clinical trials. The data suggested a significant increase in HIV-1 infection after the application and removal of the two microbicides. Furthermore, it emphasized the need for intense scrutiny of compounds prior to clinical trials, considering the dangers they may pose on human subjects. While previous studies supported the use of polyanion microbicides as a safe and possibly effective means of preventing HIV-1 transmission in women, the effects of the leakage and loss of the product over time was not taken into consideration, and significantly more women on the drug were found to have contracted HIV than if they had not taken it. The study also provides us with an example of the vital role clinical trials play in the testing of a drug, and how certain adverse effects may be missed through in vitro studies that only become apparent when applied to real world uses.

Links:

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3295645/

http://www.nejm.org/doi/full/10.1056/NEJMoa0707957#t=abstract

Julia Ding is a senior at Vassar College, with a major in Science, Technology and Society.

Guest Blogger, Immunology, Molecular Virology

Polydnavirus: Good for the Parasitic Wasp, Bad for the Host Caterpillar

March 23, 2012 David Esteban 1 Comment

Contributed by guest blogger: Jason Adler

An endoparasitoid wasp would disagree with the popular perception of viruses as malevolent. Parasitoids are organisms that spend a substantial portion of their life cycle in the host; unlike a parasite, a parasitoid usually kills or sterilizes the host. Endoparasitoid wasp oviposit into the body cavity of caterpillars. When the wasp larvae emerges, it then consumes the host as it develops.

Polydnaviruses (PDV), a family of double stranded DNA insect viruses, are symbiotic to some endoparasitoid wasps. Of two PDV genera, genus Ichnovirus is specific to ichneumoid wasps and Bracovirus to braconid wasps. The PDV genome is located on host wasp chromosomes in a segmented, proviral form. However, the integrated PDV genome is not fully functional as it cannot replicate independent of the wasp and capsid proteins are non-existent. It is unknown if PDV is derived from wasp genes or if ancestral wasps integrated a beneficial PDV into their genome with resulting loss of the genes responsible for capsid formation and virus replication.

As such, PDV only replicates at specific ovarian cells during the late pupal phase, where it acquires two viral envelopes. PDV integration does not occur in the viral life cycle; instead, the viral genome is vertically transmitted to wasp offspring during meiosis. When the female wasp injects her eggs into the lepidopteran host, virions are co-injected and result in infection. Although, PDV does not replicate in the host caterpillar, it does result in immunosuppression and alters the host development (i.e. prevents metamorphosis) and metabolism to favor the parasitoid larva. The normal response of lepidopteran larvae to small foreign material is phagocytosis, but larger pathogens must be encapsulated. This is accomplished through melanization, where certain hemocytes, invertebrate immune cells found in the hemolymph, secrete melanin, which surrounds the pathogen so that anti-microbial peptides can destroy it. When immune suppressed, host hemocytes do not destroy the wasp egg by forming hemocyte nodules. Thus, PDV and the wasp share a mutualistic relationship.

Cotesia plutellae, a braconid wasp, possesses a PDV – C. plutellae bracovirus (CpBV) – and parasitizes larvae of the diamond-back moth Plutella xylotsella. Recent research has found that CpBV encodes a viral histone H4 that shares high sequence homology with histone H4 on P. xylostella, except for the last 38 residues comprising the N-terminal tail. Additionally, this viral histone H4 N-terminal tail have been observed in other Cotesia-associated PDVs. It has been suggested that the N-terminal tail is altering gene expression regulation as viral H4 histones less easily detach from DNA than host H4 histones, thereby inhibiting transcription. Is the N-terminal tail of CpBV-H4 causing immunosuppression? The researchers hypothesized that the N-terminal tail is causing the suppression of antimicrobial peptide (AMP) genes.

To examine the effects of CpBV-H4, the researchers constructed two viral recombinants: a WT CpBV-H4 and a truncated CpBV-H4 that lacks the N-terminal tail. After injection of the viral vector into the host caterpillar, RT-PCR was used to look at the expression of putative AMP genes. Although basal expression levels were unchanged, when E. coli was introduced to the host to present an immune challenge CpBV-H4 inhibited inducible expression, while truncated CpBV-H4 did not. Additionally, by counting the number of melanized black nodules on the host caterpillar after injection of E. coli and the viral vector, the researchers assessed the immune response. While the larvae show hemocyte nodule formation in response to E. coli infection, transient expression of CpBV-H4 significantly suppressed the immune response by decreasing nodule formation, while truncated CpBV-H4 had no effect. Finally, the researchers examined a possible synergistic effect of CpBV-H4 and the entomopathogenic bacterium X. nematophila. Without CpBV-H4, X. nematophila infection resulted in low mortality; however, with CpBV-H4, there was significantly increased mortality with this synergistic effect lost if CpBV-H4 was truncated.

Based on these results, the researchers concluded that the N-terminal tail appears to be responsible for immunosuppression by inhibiting inducible expression of AMP genes, possibly by altering a normal epigenetic control. CpBV-H4 containing nucleosomes may less easily detach from DNA during transcription due to the increased positive charge resulting from the increased number of lysine residues in the N-terminal tail. By introducing a virus that expresses a viral H4 histone with a N-terminal tail, the parasitoid wasp is able to suppress the host immune system. This is important as without the immune suppression, the host hemocytes would encapsulate and destroy the wasp egg.

With 157 putative genes, CpBV is likely to have more than this one mechanism to suppress host immunity. Are there other mechanisms of CpBV immune suppression? How else is the complex ecological relationship of wasp, virus, and caterpillar host mediated at the molecular level?

Link:

http://www.sciencedirect.com/science/journal/0006291X/415/2

Jason Adler is a senior at Vassar College, majoring in biology.

Immunology, Vaccines

Can adenovirus be used to help cure a cocaine addiction?

April 27, 2011 David Esteban 7 Comments

Contributed by guest blogger: Jessica Hughes ’11

It is well known that drug addiction is a worldwide problem, and so finding a therapy or cure for this issue would be extremely valuable. Scientists have been trying to create a vaccine for people with drug addictions that would allow them to be rid of their chemical dependence, but there are several challenges they face in trying to do so. First, addictive drugs are small molecules that do not cause an immune response on their own. Furthermore, because of the extremely high level of drugs often found in the blood of a systemic drug user, there needs to be a way to create high-titer, high-affinity antidrug antibodies to address that extremely high drug concentration. This second challenge has limited the effectiveness of many attempts at anti-addiction active immunization strategies.

In a 2010 study, researchers looked at creating an anticocaine vaccine with the help of adenovirus. With the knowledge that inhaled cocaine could not reach its target receptors in the brain when exposed to anticocaine antibodies, researchers looked into the possibility that cocaine addiction could possibly be reversed with an anticocaine vaccine. Here’s where adenovirus came in. Researchers knew that adenovirus gene transfer vectors act as potent immunogens, which provoke adaptive immune responses. They predicted that if they coupled the adenovirus with a cocaine analog, they could elicit high-titer antibodies against cocaine and successfully prevent this drug’s access to the brain. Specifically, they used a disrupted E1-E3- adenovirus gene transfer vector, which means they were able to avoid viral gene products that would pose a risk of infection to the vaccine receiver but still have the benefit of the immunogenic property of the vectors. E1-E3- has been used many times in gene transfer applications, proving to be very safe.

In their experiment, once they created the vaccine (called dAd5GNC), they used mice to test its effects. Both naïve mice and vaccinated mice were given cocaine intravenously, and subsequently their locomotor activity was observed. The administration of cocaine caused hyperlocomotor activity in mice. These effects were completely and consistently reversed for the vaccinated mice. This is a promising result, and further studies obviously need to be done to continue looking into the possibility of using anti-addictive drug vaccines. Some questions to think about: Would an anticocaine vaccine work in the real-life scenario of preventing an addict from relapsing? Could there be dangers with taking these vaccines, such as accidental overdoses of someone trying to obtain the feeling he/she is used to getting from the drug?

Epidemics, Guest Blogger, Immunology, Vaccines

A possible new HIV vaccine target?

April 13, 2011 David Esteban 2 Comments

Contributed by guest blogger: Lydia Mendoza ’11

In 2009, it was estimated that 33.3 million people in the world were living with HIV/AIDS. Since the discovery of HIV, more than two decades ago, money has poured into research in the hopes that an effective vaccine might be developed. As of yet a vaccine remains elusive. One reason why it is so difficult to create a vaccine is because HIV is highly mutable and genetically diverse subtypes, or clades, have evolved. A vaccine needs to be able to offer protection from a range of HIV clades.

Normally viral vaccines are based upon neutralizing antibodies, which prevent infection of the host cell. The first attempts to develop neutralizing antibodies against HIV targeted gp120, which is known to play a role in HIV’s ability to enter and infect CD4 t-cells. These attempts have not been successful as of yet because of the gene’s high rate of mutations. However a recent paper has shown that the V3 loop of gp120 is a potential vaccine target.

The strand of protein known as the V3 loop was never thought to be an attractive vaccine target because it is not highly conserved. However, it appears to have conserved structural elements that are involved in interactions with coreceptors. To study whether V3 was a viable vaccine target, a human monoclonal antibody, HGN194 was used. HGN194 was isolated from memory B cells of a person infected with HIV-1 clade AG circulating recombianant form (CRF). HGN194 targets the V3 loop and has been previously shown to neutralize a broad range of neutralization-sensitive and resistant strains of HIV.

The study evaluated whether HGN194 was able to protect rhesus monkeys from an HIV model system. One group of monkeys was injected with HGN194 then they were challenged with a high dose of a clade C SHIV, which is a chimeric simian-human imunodeficiency virus encoding HIV envelope genes in a SIV backbone. The second group of monkeys was also given a high dose of SHIV but was not given the HGN194. The monkeys given the antibody were protected from SHIV infection, and those not given the antibody were infected. The researchers concluded that HGN194, isolated from an HIV-positive individual harboring a clade AG CFR, was able to confer complete cross-clade protection against clade C SHIV.

The antibody apparently latches onto the virus’s V3 loop and prevents the virus from invading cells. This does not mean that this antibody treatment technique is a vaccine for HIV. It does not create long-term protection because the antibodies do not remain active in the body for very long. This is only a first step. A vaccine target has been identified but now scientists must create an antigen that induces formation of an antibody similar in structure to HGN194. There is a lot of work left to be done but this finding hopefully brings researchers much closer to the development of a vaccine.

Virology and Microbiology at Vassar