MicrobiologyBytes

Among the negative RNA viruses, ambisense RNA viruses occupy a distinct niche. Ambisense viruses contain at least one ambisense RNA segment, i.e. an RNA that is in part of positive and in part of negative polarity. Because of this unique gene organization, one might expect ambisense RNA viruses to borrow expression strategies from both positive and negative RNA viruses. However, they have little in common with positive RNA viruses, but possess many features of negative RNA viruses. Transcription and/or replication of their RNAs appear generally to be coupled to translation. Such coupling might be important to ensure temporal control of gene expression, allowing the two genes of an ambisense RNA segment to be differently regulated. Ambisense viruses can infect one host asymptomatically and in certain cases, they can lethally infect two hosts of a different kingdom. A possible model to explain the differential behavior of a given virus in different hosts could be that perturbation of the translation machinery would lead to differences in the severity of symptoms.

The ambisense coding strategy is an unusual way of encoding genes that presumably allows the virus to temporally control expression of the viral proteins, in particular if coupling of translation to transcription enhances the level of vc-encoded versus v-encoded protein expression. In any event, translation itself and/or translational control appear to play an important role in regulation of gene expression of ambisense viruses. Ambisense viruses have two hosts in which they can replicate. In their vector or reservoir host, infection is usually asymptomatic. However, in another host, multiplication of the virus can be lethal. Replication/transcription experiments in different host cell types would be helpful to shed further light on the differences observed in different hosts. At present, there are many complementary ways to study ambisense virus replication/ transcription such as cell culture, in vitro assays and reverse genetics systems. Since ambisense viruses are the meeting point of different viral families and are able to replicate in different hosts whether plants or animals and have different behaviors depending on the host, it would be particularly important to better understand the complex replicative cycle of ambisense viruses, in order to find the means to alleviate the lethal aspects of these pathogens.

Expression strategies of ambisense viruses. Virus Research 93: 141-150, 2003. doi: 10.1016/S0168-1702(03)00094-7

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• Negative sense RNA viruses
• MicrobiologyBytes: Arenaviruses
• MicrobiologyBytes: Bunyaviruses

The many millions of humans who have life-long virus infections represent a major health issue for the 21st century but also a unique opportunity for investigative virologists. For persistent virus infections to endure, two ingredients are essential. The first is a unique strategy of viral replication; that is, instead of killing its host cell, the pathogen causes little to no damage so it can continue to reside in those cells. The second requirement for persistent virus infection is an immune response that does not react to or remove virus-infected cells. Overall, our knowledge of how viral genes and cellular factors interact to allow persistence to occur is incomplete. Although our libraries contain volumes of facts on this subject, many physiologic functions and interrelationships of viral genes with host genes that establish persistence remain, in large part, unknown.

We do know that acutely infected cells express viral peptides, which, when attached to host major histocompatibility complex (MHC) molecules on their surfaces, signal the immune system to kill such cells. However, viruses apply numerous avoidance strategies to persist. One is direct selective pressure to suppress the infected host’s innate and/or adoptive immune system that would otherwise destroy them. For example, viruses can alter or interfere with the processing of viral peptides by professional antigen-presenting cells, thereby restricting expression of MHC/peptide complexes on cell surfaces, a requirement for activation and expansion of the T cells that normally remove infected cells. Additionally, viruses can downregulate co-stimulatory and/or MHC molecules also required for T cell signaling and expansion; they can inhibit the differentiation of antigen-presenting conventional dendritic cells (cDCs), and can infect effector T and B cells directly. Similarly, to persist in infected cells, viruses can disrupt the processing or migration of viral peptides or viral peptide/MHC complexes to the cells’ surface, thereby removing the recognition signals for activated killer T cells. Finally, viruses that persist frequently infect neurons, which have defects in TAP, a molecule required for the translocation of viral peptides to endoplasmic reticulum (ER). Perhaps neurons can also actively prevent cytotoxic T lymphocytes (CTLs) or natural killer (NK) cells from degranulating and thereby limit the activity of such virus-removing effector cells. Since neurons are essential to health but rarely regenerate when destroyed, Darwinian selection likely caused them to evolve mechanisms to avoid immunologic assault. Such events would allow infected neurons to escape immune recognition and live, as well as allow viruses to persist in a neuronal safe house.

Currently researchers are engaged in the discovery of additional negative immune regulators and their signaling pathway(s) using gene chip and forward genetics technology. These projects have a multitude of applications. Some examples are the development of pharmacologic small molecules as effective antagonists of negative immune regulators, the use of transient negative regulator blockers as an adjuvant approach to enhance both prophylactic and therapeutic vaccination, and the determination of how long during the course of persistent virus infection exhausted T cells can be rescued to become antiviral effector T cells. As always, the goal is to understand basic principles in viral pathogenesis and to extend results in the murine model to resolve persistent infections of humans.

Anatomy of Viral Persistence. PLoS Pathog 5(7): e1000523 doi:10.1371/journal.ppat.1000523

Related:

• Natural Regulatory T Cells and Persistent Virus Infection
• Antibodies against persistent viruses
• Unknown disease in South Africa identified as arenavirus infection

Persistent viruses such as hepatitis C virus (HCV) or HIV can defeat the body’s defense system and cause devastating epidemics worldwide. Recent attempts at vaccinating against HIV have relied on the induction of specific antiviral killer T lymphocytes but have failed to confer protection on the host. Better knowledge about how a successful defense should operate is therefore essential for developing and refining new vaccines. Researchers have recently used a mouse model to investigate basic defense mechanisms required to eliminate persisting viruses. Experiments in several genetically engineered mouse models show that contrary to common belief, not only antiviral killer T cells, but also antibodies (produced by B cells), are needed to prevent a virus from persisting in its host. These findings suggest that induction of antibodies, along with antiviral killer T lymphocytes, should be envisaged when devising new strategies for vaccinating against HIV or HCV.

Impaired antibody response causes persistence of prototypic T cell–contained virus. 2009 PLoS Biol 7(4): e1000080
CD8 T cells are recognized key players in control of persistent virus infections, but increasing evidence suggests that assistance from other immune mediators is also needed. Here, we investigated whether specific antibody responses contribute to control of lymphocytic choriomeningitis virus (LCMV), a prototypic mouse model of systemic persistent infection. Mice expressing transgenic B cell receptors of LCMV-unrelated specificity, and mice unable to produce soluble immunoglobulin M (IgM) exhibited protracted viremia or failed to resolve LCMV. Virus control depended on immunoglobulin class switch, but neither on complement cascades nor on Fc receptor c chain or Fc c receptor IIB. Cessation of viremia concurred with the emergence of viral envelope-specific antibodies, rather than with neutralizing serum activity, and even early nonneutralizing IgM impeded viral persistence. This important role for virus-specific antibodies may be similarly underappreciated in other primarily T cell–controlled infections such as HIV and hepatitis C virus, and we suggest this contribution of antibodies be given consideration in future strategies for vaccination and immunotherapy.

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On 12 September 2008, a tourist guide organising safari trips in Lusaka, Zambia, was evacuated in a critical condition to Johannesburg, South Africa. She was admitted to a clinic where she died on 14 September about 10 days after the onset of symptoms. The symptoms included a prodromal phase with fever, myalgia, vomiting, diarrhoea, followed by rash, liver dysfunction and convulsions. Cerebral oedema was detected on scan examination. No laboratory specimen was available for investigation. A total of four cases have now been reported.

On 12 October 2008, the National Institute for Communicable Diseases  in South Africa provided preliminary evidence that the causative agent of the disease was a virus from the Arenaviridae family. Specimens were shipped to the United States Centers for Disease Control and Prevention in Atlanta for additional investigations.

Arenaviruses are enveloped viruses (about 120 nm diameter) with a bi-segmented negative strand RNA genome. The typical image in electronic microscopy showing grainy ribosomal particles (from arena, the Latin for “sand”) inside the virions gave the name to this family of viruses. The prototype is the Lymphocytic Choriomeningitis (LCM) virus (LCMV), isolated in 1933 in North America from a human case with aseptic meningitis. Cases caused by LCMV occur worldwide. Other arenaviruses causing hemorrhagic fevers were reported in South America, causing sporadic cases or limited outbreaks: Junín virus in 1958 in Argentina, Machupo virus in 1963 in Bolivia, Guanarito virus in 1990-1991 in Venezuela, Sabia virus in 1990 in Brazil and more recently Chapare virus in 2004 in Bolivia. In West Africa, Lassa virus was identified in Nigeria in 1969. It causes thousands of cases each year in Sierra Leone, Liberia, Guinea and Nigeria. However, only limited data are available to assess the real incidence of Lassa fever in Africa.

Unknown disease in South Africa identified as arenavirus infection. Euro Surveill. 2008;13(42):pii=19008

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• A new arenavirus associated with hemorrhagic fever
• A new Arenavirus is responsible for deaths of three transplant recipients in Australia
• Negative sense RNA viruses

Four rodent-borne arenaviruses are known to cause hemorrhagic fever in the New World. These include Junin, Machupo, Guanarito, and Sabia viruses, which are found in rural areas of Argentina, Bolivia, Venezuela, and Brazil, respectively. In December 2003 and January 2004, a small number of hemorrhagic fever cases were reported in rural Bolivia in an area outside the known Machupo HF endemic zone, and sera from one fatal case was available for laboratory testing. The man had symptoms similar to those seen with other arenaviral hemorrhagic fever cases acute febrile illness beginning with headache, joint and muscle pain, and vomiting and rapidly progressed to shock, bleeding, and death at 14 days post onset of illness. Virus was isolated from two of the patient’s serum samples and identified as an arenavirus by reaction of virus infected cells with arenavirus-specific antibodies and by genetic detection techniques (PCR).

A team consisting of Bolivian health authorities, U.S. Navy health experts based in Lima, Peru, and the U.S. Centers for Disease Control and Prevention has now characterized Chapare arenavirus, a previously unrecognized arenavirus, discovered in serum samples from this patient in rural Bolivia. Named after the Chapare River in the eastern foothills of the Andes, the new Chapare arenavirus produces clinical hemorrhagic symptoms similar to those associated with other New World arenaviruses, such as the Junin, Machupo, Guanarito, and Sabia viruses. Genetically, however, Chapare is different from each. Junin, Machupo and Guanarito viruses have been associated with large outbreaks of hemorrhagic fever. Initial symptoms often include fever, malaise, muscle aches, nausea, vomiting, and anorexia, followed later by hemorrhagic symptoms. Untreated, more severe neurologic and/or hemorrhagic symptoms may develop, and death occurs in up to 30%. In this study, the authors first tested for yellow fever and dengue hemorrhagic fevers, but results were negative. Tests for Machupo and other related viruses also were negative. Sequence analysis of specific segments of the virus later confirmed it as a unique member of the Clade B New World Arenaviruses. Due to the remote nature of the region where the case occurred, only a limited description of a possible cluster of cases in the area was determined.

Further surveillance and ecological investigations should clarify the nature of the health threat posed by the Chapare virus, and give us better information on the source of human infection, says CDC virologist Tom Ksiazek of the Special Pathogens Branch. We need to learn more about this virus: how it is related to the other arenaviruses, how it causes disease, where it lives in nature, says Ksiazek. Together with our colleagues in Bolivia and Peru, we’re anticipating a more intensive investigation that improves our understanding of the virus, the disease it causes, and its ecology.

Chapare Virus, a Newly Discovered Arenavirus Isolated from a Fatal Hemorrhagic Fever Case in Bolivia
PLoS Pathog 4(4): 2008 e100004

Related:

• Negative sense RNA viruses
• A new Arenavirus is responsible for deaths of three transplant recipients in Australia

Scientists have identified a new virus which was responsible for the deaths of three transplant recipients who received organs from a single donor in Victoria, Australia. The previously unknown virus (a new Arenavirus, related to the rodent virus lymphocytic choreomeningitis virus, LCMV), was discovered using rapid sequencing technology at Columbia University with support from the National Institute of Allergy and Infectious Diseases. Normally, LCMV is harmless to humans, but has been implicated in a small number of cases of disease transmission by organ transplantation. The newly discovered virus is sufficiently different that it could not be detected using existing screening methods.