Retroviridae

Chapter 65


Retroviridae


Retroviruses (Latin retro, backwards) are spherical, labile, enveloped RNA viruses with a diameter of 80 to 100 nm (Fig. 65.1). Two subfamilies comprising seven genera are currently recognized within the family (Fig. 65.2): Alpharetrovirus, Betaretrovirus, Gammaretrovirus, Deltaretrovirus, Epsilonretrovirus and Lentivirus in the subfamily Orthoretrovirinae while the subfamily Spumaretrovirinae contains a single genus, Spumavirus. The family name refers to the presence in the virion of the enzyme reverse transcriptase which is encoded by the pol gene. The envelope is acquired from the plasma membrane of the host cell. It surrounds an icosahedral capsid which contains two identical linear, positive-sense, single strands of RNA and a number of core proteins including the enzymes reverse transcriptase and integrase. Historically, based on their appearance using electron microscopy, retroviruses were described as A-type, B-type, C-type and D-type particles. Retroviruses are sensitive to heat, lipid solvents and detergents. However, on account of their diploid genomes, they are relatively resistant to UV light.




Reverse transcriptase functions as an RNA-dependent DNA polymerase, transcribing from RNA to DNA. The four major genes are 5′-gag-pro-pol–env-3′. The gag (group-specific antigen) gene encodes internal structural proteins. The pro (protease) gene encodes the enzyme protease while the pol (polymerase) gene encodes the enzymes reverse transcriptase and integrase. The env (envelope) gene encodes surface (SU) and transmembrane (TM) envelope glycoproteins. Cell entry follows attachment of an envelope glycoprotein to specific cell receptors. Double-stranded DNA copies of the viral genome are synthesized in the cytoplasm of the host cell under the direction of the reverse transcriptase. During this process, repeat base sequences, containing several hundred base-pairs are added to the ends of the DNA transcripts. These sequences are called long terminal repeats (LTR). The DNA transcripts are integrated into the chromosomal DNA at random sites under the direction of the viral integrase and referred to as provirus. The nature and extent of host cell changes following infection are determined by the sites of proviral integration. Important promoter and enhancer sequences are located within the LTRs and these are involved in the transcription of mRNA and virion RNA from provirus. Release of mature virions typically occurs by budding from cell membranes. Insertion of a provirus of certain retroviruses close to host cell genes responsible for the regulation of cell division (insertional mutagenesis) may result in an increase in the rate of mitosis and give rise to an increased tendency for the cell to transform.


A high mutation rate is a feature of retroviral replication with errors occurring relatively frequently during reverse transcription. In addition, reverse transcriptase can transfer from the RNA template of one virus to that of another giving rise to recombination between retroviral genomes in multiply infected cells. As a consequence, antigenically distinct retroviruses or quasispecies emerge at frequent intervals making precise classification of species and subtypes difficult.


Retroviruses may be described as endogenous or exogenous retroviruses. Endogenous retroviruses occur widely among vertebrates. They are the result of infection of germline cells at some time in the past and are transmitted only as provirus in germ cell DNA from dam to offspring. The provirus genes are usually silent and are under the control of cellular genes. Endogenous retroviral genomes may contribute env genes to produce recombinant feline leukaemia viruses and avian leukosis viruses. It is sometimes possible to reactivate endogenous retrovirus elements by irradiation, mutagens or carcinogens, resulting in the completion of replication and the production of new virions. There are fears that endogenous retroviruses of pigs may pose a danger to humans receiving xenotransplants.


Retroviruses in the genera Alpharetrovirus, Betaretrovirus, Gammaretrovirus, Deltaretrovirus and Epsilonretrovirus are frequently referred to as oncogenic retroviruses because they can induce neoplastic transformation in host cells. Exogenous oncogenic retroviruses are described either as slowly transforming (cis-activating) viruses or as rapidly transforming (transducing) viruses depending on the interval between exposure to the virus and tumour development. For example, slowly transforming retroviruses induce B cell, T cell or myeloid tumours after long incubation periods. The provirus must be integrated into the host cell DNA close to a cellular oncogene (c-onc, proto-oncogene), resulting in interference with the regulation of cell division (cis-activation) or alternatively the proviral LTR may increase the rate of mitosis and enhance the risk of neoplasia. Insertion of the provirus is largely random and it may be some time before the provirus is inserted in a position in the cell genome that gives rise to transformation of the cell. Rapidly transforming retroviruses, which can induce tumour formation after short incubation periods, contain viral oncogenes (v-onc). Viral oncogenes are cellular oncogenes which have been acquired by the virus through recombination. In the case of Rous sarcoma virus the oncogene has been integrated into the viral genome without loss of replicative virus genes. This retrovirus is described as replication-competent. However, it is more common for existing viral sequences necessary for replication to be deleted as a consequence of cellular oncogene integration into the viral genome, These retroviruses are referred to as replication-defective retroviruses, because they cannot multiply without helper viruses (related replication-competent viruses). They are rarely transmitted under normal field conditions but may cause a rapidly developing neoplastic disease in the host animal in which they arise. A third method of tumour induction by retroviruses has been described. Bovine leukaemia virus possesses a tax gene which encodes for a protein capable of up-regulating both viral LTR and cellular promoter sequences. This up-regulation can occur even when the provirus is integrated into a different chromosome from the one containing the up-regulated cellular sequences (transactivation).


The important oncogenic retroviruses of veterinary species are presented in Table 65.1. The genus Epsilonretrovirus contains viruses associated with neoplasia in fish. Lentivirus (Latin lentus, slow) infections are characterized by diseases with long incubation periods and insidious protracted courses. Examples of important animal and human lentivirus diseases include acquired immunodeficiency syndrome (AIDS), feline immunodeficiency, equine infectious anaemia and maedi-visna (Table 65.2). Spumaviruses (Latin spuma, foam) have been described in primates, cattle, horses and cats. They cause vacuolation of cultured cells, but are not associated with clinical disease.





Avian leukosis


Both replication-competent and replication-defective retroviruses make up the avian leukosis virus (ALV) group. These viruses are associated with neoplastic conditions in chickens including lymphoid, erythroid and myeloid leukoses, fibrosarcoma, haemangiosarcoma and nephroblastoma. Lymphoid leukosis, a B cell lymphoma, is the most common and economically important. Viral envelope glycoproteins determine virus neutralization properties, viral interference patterns and host range. Avian leukosis viruses can be divided into ten subgroups (A to J) on the basis of differences in these glycoproteins. Subgroups A, B, C, D, E and J contain the isolates from chickens. Isolates from outbreaks of disease in chickens generally belong to subgroup A. Endogenous avian leukosis viruses, subgroup E, are commonly present in chickens and are transmitted vertically in the germline cells. Subgroup J isolates are associated with myeloid leukosis in broilers and have arisen from recombination of a novel family of endogenous viruses (ev/J) and exogenous avian leukosis viruses (Benson et al. 1998).


There is usually an incubation period of months to years between natural infection with ALV and the development of neoplasia, because of the time required for the genetic events to occur that lead to transformation of cells to malignancy. Neoplastic conditions associated with ALV include lymphoid leukosis, myeloid leukosis, sarcomas, osteopetrosis and renal tumours. The generation of recombinant, rapidly transforming viruses, which have incorporated a cellular oncogene into their genome, often occurs on the pathway to transformation. Examples of such viruses isolated from tumours include avian erythroblastosis virus, avian myeloblastosis virus and Rous sarcoma virus.


Transmission of exogenous ALV occurs both vertically, through virus present in egg albumen, and horizontally, by close contact via saliva and faeces. The chicks that hatch from infected eggs are usually immunotolerant and persistently viraemic. As a result they represent the principal on-going source of virus in a flock. In contrast, the chicks infected after hatching usually develop a transient viraemia before they produce neutralizing antibodies. Some horizontally infected chicks may become persistently infected, particularly if maternal antibodies are absent and the birds are exposed very early in life. Neoplasms occur most frequently in the congenitally infected birds. In general, natural exposure of adult birds to infection does not result in virus shedding. Virus-neutralizing antibodies, passed from antibody-positive hens in the yolk sac to their chicks, provide protection for the first few weeks of life.



Pathogenesis


Lymphoid leukosis has an incubation period of four months or more. Following infection the virus spreads throughout the body with most tissues supporting viral replication. Avian leukosis virus transforms B cells in the bursa of Fabricius by integrating as provirus close to the c-myc gene which becomes abnormally expressed as a result of the influence of the viral LTR promoter. The developing lymphoma metastasizes to the viscera. Less commonly ALV has been associated with erythroblastosis where the c-erbB gene in an erythroid cell is activated by insertional mutagenesis. Rapidly transforming viruses arise as a rare event in individual birds by the transduction and modification of a cellular proto-oncogene in transforming avian retroviruses. Multiple insertions of the v-onc gene into a host cell genomes give rise to excessive gene expression and over-production of a transformation-associated protein which may act as a hormone or growth factor receptor, a transcription factor in the nucleus or a kinase in a signal transduction pathway.


Disease tends to be sporadic in infected flocks. Affected birds become weak and emaciated with pale wattles. The liver and bursa of Fabricius may be enlarged. Subclinical infections are associated with depressed egg production and fertility, decreased hatchability and growth rate, and increased death rates. The increased deaths in egg-laying and breeding birds between five and nine months of age accounts for most of the economic loss associated with ALV infection.



Diagnosis




• Post mortem findings and histopathological determination of tumour type are often sufficiently characteristic to be diagnostic.


• Differentiation from Marek’s disease is important. It is based on the age of affected birds, the presence of bursal tumours, an absence of thickening of peripheral nerves and a histological assessment of neoplastic cell types.


• Virus isolation is difficult and generally not attempted.


• Commercial ELISA kits for the detection of ALV p27 group-specific antigen are available. However, ALVs are widespread in poultry flocks and may be present in the absence of tumour formation. Detection of ALVs in flocks is particularly useful in control programmes aimed at the elimination of ALV-positive birds from breeder flocks.


• The presence of flock infection can be demonstrated by detecting antibodies in serum or egg yolk. Suitable assays include virus neutralization, ELISA and indirect immunofluorescence.


• The RT-PCR assay has been developed for the detection of ALV (Smith et al. 1998, Cavanagh 2001). Sequencing of the amplicon can be used to determine the infecting subgroup and to differentiate endogenous ALV from exogenous ALV (Pham et al. 1999). Alternatively primers specific for particular subgroups may be used (Garcia et al. 2003, Silva et al. 2007).



Enzootic bovine leukosis


This worldwide, retroviral disease of adult cattle is characterized by persistent lymphocytosis and the development of B cell lymphosarcoma in a number of infected animals. A number of countries have eradicated enzootic bovine leukosis (EBL), while other countries are embarking on eradication programmes. Bovine leukaemia virus (BLV) is labile and intimately cell-associated. Transmission occurs by direct contact or transplacentally through the transfer of blood or secretions such as colostrum and milk containing infected lymphocytes. Iatrogenic transmission is important. Transplacental transmission is not particularly efficient with less than 10% of calves born to infected dams being infected at birth. In addition, calves are protected for several months by maternally derived antibody. Typically animals are infected between six months and three years of age (Hopkins & DiGiacomo 1997). The prevalence of infection is higher in dairy cattle than in beef cattle. Susceptibility to infection is influenced by the animal’s genotype.




Diagnosis


Differentiation of EBL from sporadic bovine leucosis, which usually affects calves and young adult cattle, is necessary. Formerly, blood lymphocyte counts were used for laboratory diagnosis and for the eradication of EBL. However, lymphocytosis is not present in all cases and serological testing for virus-specific antibody is now generally used for diagnosis and eradication.



• Several serological tests such as AGID, ELISA and radioimmune assay can be used for the detection of antibodies to BLV, usually antibodies directed against the gp51 and p24 of the virus. Both indirect and blocking ELISAs are available commercially and may be designed for use with bulk milk samples or serum samples. Antibodies present in calves less than six months of age may be colostral in origin.


• Virus isolation, by cultivation of peripheral blood lymphocytes, is not performed routinely. Infected cells are usually co-cultivated with an indicator cell line such as bovine lung cells and infectious virus production is encouraged by the use of mitogens. The virus causes syncytial development in the cell sheet.


• The polymerase chain reaction has been utilized for the detection of provirus in peripheral blood lymphocytes (Ballagi-Pordany et al. 1992, Belak & Ballagi-Pordany 1993). It is useful as a confirmatory test in individual cases or as an adjunct to large-scale serological testing.



Jaagsiekte


Jaagsiekte (Afrikaaner word meaning ‘panting sickness’) is caused by jaagsiekte sheep retrovirus (JSRV), also known as ovine pulmonary adenocarcinoma virus. The disease is also called ovine pulmonary adenomatosis and is a slowly progressing neoplastic disease of adult sheep. With the exception of Australasia and Iceland, jaagsiekte has a worldwide geographical distribution. Infection occurs rarely in goats. About 20 copies of related endogenous betaretroviruses (enJSRV) occur in the genome of sheep and goats. Expression of these endogenous viruses during foetal ontogeny may be the reason for the apparent immune tolerance and lack of immune response in mature sheep to exogenous JSRV (Palmarini et al. 2004). Transmission of JSRV occurs by the respiratory route and close contact facilitates spread of infection. The disease incidence in an infected flock may be up to 20%, it is influenced by breed and the type of flock management.



Pathogenesis


The incubation period is highly variable ranging from several months up to two years. Virus replication occurs in two types of pulmonary cells, type II pneumocytes and non-ciliated bronchial cells. Tumours arising from these cell types progressively replace normal lung tissue. A viral oncogene has not been detected and the mechanism of neoplastic transformation is unclear. Studies have confirmed the transformation potential of the envelope protein in sheep (Caporale et al. 2006). Affected animals are usually three to four years of age. They are in poor bodily condition and display mouth breathing, particularly after exercise. By raising the hind legs and lowering the head (wheelbarrow test), a clear fluid can be seen to flow from the nostrils. At any one time only a single animal in an infected flock may be clinically affected. The course of the disease may extend over weeks or months and secondary pasteurellosis is common.

< div class='tao-gold-member'>

Only gold members can continue reading. Log In or Register to continue

Related posts:

  1. Astroviridae
  2. Bunyaviridae
  3. Glucose non-fermenting, Gram-negative bacteria
  4. Campylobacter, Arcobacter and Helicobacter species

Stay updated, free articles. Join our Telegram channel
Tags:
Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Retroviridae

Full access? Get Clinical Tree
Get Clinical Tree app for offline access