Jaagsiekte sheep retrovirus is necessary and sufficient to induce a contagious lung cancer in sheep

Colostrum and milk can transmit jaagsiekte retrovirus to lambs

Ovine pulmonary adenocarcinoma (OPA) is a contagious disease caused by jaagsiekte sheep retrovirus (JSRV). In the three studies performed, we have obtained data of the importance of colostrum/milk (C/M) in the transmission of JSRV. In the first study, a group of sheep from a flock with a long history of OPA, samples from colostrum and peripheral blood leucocytes (PBLs) were collected. Two specific PCRs (U3-LTR and env of the JSRV) were carried out. Using U3PCR 8/34 sheep were positive in colostrum whereas with envPCR 7/34 were positive. From these animals only one was positive with U3PCR in the PBLs. Evidence of the transmission of JSRV infection by C/M was obtained in two more separate studies. In the second study, PBLs from five lambs from JSRV+ ewes and two from JSRV-ewes were tested by the U3PCR. They were fed C/M by their mothers during 3 months and slaughtered 7 months after birth. Three out of five lambs from the JSRV+ sheep become PBL positive at 3-4 months old and the other two were also positive at 4-6 months of age. One lamb of the JSRV-sheep became also PBL positive at an age of 3 months. In the third study, a group of lambs from JSRV negative mothers were fed with C/M from JSRV+ sheep and housed in separate unit. For comparison, another group of the same origin and maintained in another different unit, were fed with C/M containing a JSRV virus preparation. All lambs were blood sampled monthly and JSRV infection was detected as early as 15 days and several times onwards in both groups. Control groups fed with C/M from JSRV free flock and JSRV blood test negative sheep were always negative. Together these results indicate that suckling is an important natural transmission route for JSRV.

A paradigm for virus-host coevolution: sequential counter-adaptations between endogenous and exogenous retroviruses

Endogenous retroviruses (ERVs) are remnants of ancient retroviral infections of the host germline transmitted vertically from generation to generation. It is hypothesized that some ERVs are used by the host as restriction factors to block the infection of pathogenic retroviruses. Indeed, some ERVs efficiently interfere with the replication of related exogenous retroviruses. However, data suggesting that these mechanisms have influenced the coevolution of endogenous and/or exogenous retroviruses and their hosts have been more difficult to obtain. Sheep are an interesting model system to study retrovirus-host coevolution because of the coexistence in this animal species of two exogenous (i.e., horizontally transmitted) oncogenic retroviruses, Jaagsiekte sheep retrovirus and Enzootic nasal tumor virus, with highly related and biologically active endogenous retroviruses (enJSRVs). Here, we isolated and characterized the evolutionary history and molecular virology of 27 enJSRV proviruses. enJSRVs have been integrating in the host genome for the last 5–7 million y. Two enJSRV proviruses (enJS56A1 and enJSRV-20), which entered the host genome within the last 3 million y (before and during speciation within the genus Ovis), acquired in two temporally distinct events a defective Gag polyprotein resulting in a transdominant phenotype able to block late replication steps of related exogenous retroviruses. Both transdominant proviruses became fixed in the host genome before or around sheep domestication (∼ 9,000 y ago). Interestingly, a provirus escaping the transdominant enJSRVs has emerged very recently, most likely within the last 200 y. Thus, we determined sequentially distinct events during evolution that are indicative of an evolutionary antagonism between endogenous and exogenous retroviruses. This study strongly suggests that endogenization and selection of ERVs acting as restriction factors is a mechanism used by the host to fight retroviral infections.

The genome of all vertebrates is heavily colonized by “endogenous” retroviruses (ERVs). ERVs derive from retrovirus infections of the germ cells of the host during evolution, leading to permanent integration of the viral genome into the host DNA. Because ERVs are integrated in the host genome, they are transmitted to subsequent generations like any other host gene. The function of endogenous retroviruses is not completely clear, but some ERVs can block the replication cycle of horizontally transmitted “exogenous” pathogenic retroviruses. These observations lead to the hypothesis that ERVs have protected the host during evolution against incoming pathogenic retroviruses. Here, by characterizing the evolutionary history and molecular virology of a particular group of endogenous betaretroviruses of sheep (enJSRVs) we show a fascinating series of events unveiling the endless struggle between host and retroviruses. In particular, we discovered that: (i) two enJSRV loci that entered the host genome before speciation within the genus Ovis (∼ 3 million y ago) acquired, after their integration, a mutated defective viral protein capable of blocking exogenous related retroviruses; (ii) both these transdominant enJSRV loci became fixed in the host genome before or around sheep domestication (∼ 10,000 y ago); (iii) the invasion of the sheep genome by ERVs of the JSRV/enJSRVs group is still in progress; and (iv) new viruses have recently emerged (less than 200 y ago) that can escape the transdominant enJSRV loci. This study strongly suggests that endogenization and selection of ERVs acting as restriction factors is a mechanism used by the host to fight retroviral infections.

Specific in vivo expression in type II pneumocytes of the Jaagsiekte sheep retrovirus long terminal repeat in transgenic mice

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma, a transmissible lung cancer in sheep. Previous experiments in differentiated murine tissue culture cell lines suggested that the disease specificity of JSRV for secretory lung epithelial cells (type II pneumocytes an Clara cells) reflects transcriptional specificity of the viral long terminal repeat (LTR) for these cells. To test this in vivo, transgenic mice carrying the bacterial beta-galactosidase (beta-Gal) gene driven by the JSRV LTR were generated. Two transgenic lines showed beta-Gal expression in the lungs but not other tissues of F1 animals, although transgene silencing in subsequent generations was a major problem. The cells expressing the transgene were identified by two- and three-color immunofluorescence for marker proteins of type II pneumocytes (surfactant protein C [SPC]) and Clara cells (CC10) as well as for a T7 gene 10 epitope present in the beta-Gal reporter. F1 animals from both lines showed transgene expression in type II pneumocytes, but somewhat surprisingly not in Clara cells. Expression was not detected in bronchiolo-alveolar stem cells (BASCs) either. These results indicate that the JSRV LTR is specifically active in type II pneumocytes in the mouse lung, which is consistent with the fact that JSRV-induced OPA tumors in sheep largely have phenotypic markers of type II pneumocytes.

Jaagsiekte sheep retrovirus is not detected in human lung adenocarcinomas expressing antigens related to the Gag polyprotein of betaretroviruses

A proportion of human lung adenocarcinomas (hLACs) express an antigen related to the major capsid protein (CA) of Jaagsiekte sheep retrovirus (JSRV), a Betaretrovirus that causes a transmissible lung cancer in sheep. In this study, we have investigated whether JSRV or related betaretroviruses are expressed in hLACs. Results obtained indicate that JSRV is not associated with human lung adenocarcinomas. However, a proportion of hLACs reacted positively in immunohistochemistry with antibodies specific towards different domains of the JSRV Gag suggesting that a bona fide retrovirus antigen could be expressed in these tumours. Further studies will be necessary to ascertain whether the detection of antigens cross-reacting with betaretrovirus Gag antisera in some hLACs is due to expression of a human endogenous retrovirus or, more unlikely, of an uncharacterized exogenous retrovirus.

PCR examination of bronchoalveolar lavage samples is a useful tool in pre-clinical diagnosis of ovine pulmonary adenocarcinoma (Jaagsiekte)

Ovine pulmonary adenocarcinoma (OPA) is a contagious lung tumour of sheep caused by Jaagsiekte sheep retrovirus (JSRV). The disease is a particular problem in flocks in many parts of the world. The aim of the study was to assess screening methods for individual animals as a prelude to future eradication trials. Results of histological examination were used as the standard to evaluate the relative sensitivity and specificity of an established heminested polymerase chain reaction (PCR) test for JSRV proviral DNA from blood and bronchoalveolar lavage (BAL) samples. PCR results from tissue samples are included as control data. PCR testing of blood samples was found to have an estimated sensitivity of only 10% (95% confidence interval (CI) 3-20) while the sensitivity of the PCR test on BAL samples was 89% (CI 79-96) in comparison to the results of histological examination. We conclude that PCR testing of BAL samples is an effective confirmatory test for sheep with suspected clinical OPA. It is also a useful tool for the pre-clinical identification of individual infected sheep within an infected flock and therefore may prove beneficial in future control or eradication programmes.

A large animal model to evaluate the effects of Hsp90 inhibitors for the treatment of lung adenocarcinoma

Ovine pulmonary adenocarcinoma (OPA) is a naturally occurring lung cancer of sheep caused by Jaagsiekte sheep retrovirus (JSRV). The JSRV envelope glycoprotein (Env) functions as a dominant oncoprotein in vitro and in vivo. In order to develop the basis for the use of OPA as a lung cancer model, we screened a variety of signal transduction inhibitors for their ability to block transformation by the JSRV Env. Most inhibitors were not effective in blocking JSRV Env-induced transformation. On the contrary, various Hsp90 inhibitors efficiently blocked JSRV transformation. This phenomenon was at least partly due to Akt degradation, which is activated in JSRV-transformed cells. Hsp90 was found expressed in tumor cells of sheep with naturally occurring OPA. In addition, Hsp90 inhibitors specifically inhibited proliferation of immortalized and moreover primary cells derived from OPA tumors. Thus, OPA could be used as a large animal model for comprehensive studies investigating the effects of Hsp90 inhibitors in lung adenocarcinoma.

Comparison of LTR enhancer elements in sheep betaretroviruses: insights into the basis for tissue-specific expression

Jaagsiekte sheep retrovirus (JSRV), enzootic nasal tumor virus (ENTV), and endogenous sheep retroviruses (ESRVs) are highly related sheep betaretroviruses that display different expression profiles in vivo. JSRV and ENTV are expressed in lungs and nasal adenocarcinomas, respectively, while ESRVs are primarily expressed in the reproductive tract of ewes. Evidence suggests that the cell tropism of JSRV, ENTV, and ESRVs is due to the transcriptional specificity of the LTRs. We have previously found several enhancer elements in the JSRV LTR that are important for lung-specific expression, including binding sites for the lung-specific transcription factor HNF-3beta, as well as binding sites for the ubiquitously expressed transcription factors C/EBP and NF-I. In this study, we have aligned the U3 regions of JSRV, ENTV, and several ESRVs in order to compare the transcriptional enhancer elements of JSRV that are conserved or absent in ESRV and ENTV. All three JSRV U3 sequences examined contain two conserved HNF-3 binding sites, while the ENTV and ESRV U3 regions are not predicted to bind this transcription factor. In addition, the C/EBP binding site is interrupted in the ESRV LTRs, but conserved in the ENTV LTRs. Some enhancer elements are conserved between JSRV and ENTV, but a reporter vector carrying the ENTV-1 LTR showed less activity than a JSRV LTR-driven reporter vector in a lung epithelial cell line. These studies support the importance of LTR enhancer elements in the respective tissue specificities of these exogenous and endogenous betaretroviruses.

Mechanisms of late restriction induced by an endogenous retrovirus

The host has developed during evolution a variety of "restriction factors" to fight retroviral infections. We investigated the mechanisms of a unique viral block acting at late stages of the retrovirus replication cycle. The sheep genome is colonized by several copies of endogenous retroviruses, known as enJSRVs, which are highly related to the oncogenic jaagsiekte sheep retrovirus (JSRV). enJS56A1, one of the enJSRV proviruses, can act as a restriction factor by blocking viral particles release of the exogenous JSRV. We show that in the absence of enJS56A1 expression, the JSRV Gag (the retroviral internal structural polyprotein) targets initially the pericentriolar region, in a dynein and microtubule-dependent fashion, and then colocalizes with the recycling endosomes. Indeed, by inhibiting the endocytosis and trafficking of recycling endosomes we hampered JSRV exit from the cell. Using a variety of approaches, we show that enJS56A1 and JSRV Gag interact soon after synthesis and before pericentriolar/recycling endosome targeting of the latter. The transdominant enJS56A1 induces intracellular Gag accumulation in microaggregates that colocalize with the aggresome marker GFP-250 but develop into bona fide aggresomes only when the proteasomal machinery is inhibited. The data argue that dominant-negative proteins can modify the overall structure of Gag multimers/viral particles hampering the interaction of the latter with the cellular trafficking machinery.

In vivo tumorigenesis by Jaagsiekte sheep retrovirus (JSRV) requires Y590 in Env TM, but not full-length orfX open reading frame

Jaagsiekte retrovirus (JSRV) causes ovine pulmonary adenocarcinoma (OPA), a transmissible lung cancer of sheep. The envelope (Env) glycoprotein protein of JSRV functions as a dominant oncoprotein in vitro and in vivo. An SH2 binding domain (YXXM) in the cytoplasmic tail of the JSRV Env is one of the main determinants of viral transformation at least in vitro. In these studies, we report the first in vivo tests of site-specific mutants of JSRV in their natural host, the sheep. We show that, in vivo, JSRV(21) with the cytoplasmic tail YXXM mutated to DXXM did not cause disease nor detectable infection, indicating that this motif is absolutely required for virus replication and possibly transformation in vivo. In contrast, mutation of the JSRV open reading frame orfX, for which no function has yet been attributed, did not alter the disease induced by JSRV(21).

Jaagsiekte Sheep Retrovirus (JSRV): from virus to lung cancer in sheep

Jaagsiekte Sheep Retrovirus (JSRV) is a betaretrovirus infecting sheep. This virus is responsible for a pulmonary adenocarcinoma, by transformation of epithelial cells from the bronchioli and alveoli. This animal cancer is similar to human bronchioloalveolar cancer (BAC), a specific form of human lung cancer for which a viral aetiology has not yet been identified. JSRV interacts with target cells through the membrane receptor Hyal2. The JSRV genome is simple and contains no recognised oncogene. It is now well established that the viral envelope protein is oncogenic by itself, via the cytoplasmic domain of the transmembrane glycoprotein and some domains of the surface glycoprotein. Activation of the PI3K/Akt and MAPK pathways participates in the envelope-induced transformation. Tumour development is associated with telomerase activation. This review will focus on the induction of cancer by JSRV.

Lung cancer induced in mice by the envelope protein of jaagsiekte sheep retrovirus (JSRV) closely resembles lung cancer in sheep infected with JSRV

Background

Jaagsiekte sheep retrovirus (JSRV) causes a lethal lung cancer in sheep and goats. Expression of the JSRV envelope (Env) protein in mouse lung, by using a replication-defective adeno-associated virus type 6 (AAV6) vector, induces tumors resembling those seen in sheep. However, the mouse and sheep tumors have not been carefully compared to determine if Env expression alone in mice can account for the disease features observed in sheep, or whether additional aspects of virus replication in sheep are important, such as oncogene activation following retrovirus integration into the host cell genome.

Results

We have generated mouse monoclonal antibodies (Mab) against JSRV Env and have used these to study mouse and sheep lung tumor histology. These Mab detect Env expression in tumors in sheep infected with JSRV from around the world with high sensitivity and specificity. Mouse and sheep tumors consisted mainly of well-differentiated adenomatous foci with little histological evidence of anaplasia, but at long times after vector exposure some mouse tumors did have a more malignant appearance typical of adenocarcinoma. In addition to epithelial cell tumors, lungs of three of 29 sheep examined contained fibroblastic cell masses that expressed Env and appeared to be separate neoplasms. The Mab also stained nasal adenocarcinoma tissue from one United States sheep, which we show was due to expression of Env from ovine enzootic nasal tumor virus (ENTV), a virus closely related to JSRV. Systemic administration of the AAV6 vector encoding JSRV Env to mice produced numerous hepatocellular tumors, and some hemangiomas and hemangiosarcomas, showing that the Env protein can induce tumors in multiple cell types.

Conclusion

Lung cancers induced by JSRV infection in sheep and by JSRV Env expression in mice have similar histologic features and are primarily characterized by adenomatous proliferation of peripheral lung epithelial cells. Thus it is unnecessary to invoke a role for insertional mutagenesis, gene activation, viral replication, or expression of other viral gene products in sheep lung tumorigenesis, although these processes may play a role in other clinically less important sequelae of JSRV infection such as metastasis observed with variable frequency in sheep.

The transdominant endogenous retrovirus enJS56A1 associates with and blocks intracellular trafficking of Jaagsiekte sheep retrovirus Gag

The sheep genome harbors approximately 20 endogenous retroviruses (enJSRVs) highly related to the exogenous Jaagsiekte sheep retrovirus (JSRV). One of the enJSRV loci, enJS56A1, acts as a unique restriction factor by blocking JSRV in a transdominant fashion at a late stage of the retroviral cycle. To better understand the molecular basis of this restriction (termed JLR, for JSRV late restriction), we functionally characterized JSRV and enJS56A1 Gag proteins. We identified the putative JSRV Gag membrane binding and late domains and determined their lack of involvement in JLR. In addition, by using enJS56A1 truncation mutants, we established that the entire Gag protein is necessary to restrict JSRV exit. By using differentially tagged viruses, we observed, by confocal microscopy, colocalization between JSRV and enJS56A1 Gag proteins. By coimmunoprecipitation and molecular complementation analyses, we also revealed intracellular association and likely coassembly between JSRV and enJS56A1 Gag proteins. Interestingly, JSRV and enJS56A1 Gag proteins showed distinct intracellular targeting: JSRV exhibited pericentrosomal accumulation of Gag staining, while enJS56A1 Gag did not accumulate in this region. Furthermore, the number of cells displaying pericentrosomal JSRV Gag was drastically reduced in the presence of enJS56A1. We identified amino acid residue R21 in JSRV Gag as the primary determinant of centrosome targeting. We concluded that JLR is dependent on a Gag-Gag interaction between enJS56A1 and JSRV leading to altered cellular localization of the latter.

Envelope proteins of Jaagsiekte sheep retrovirus and enzootic nasal tumor virus induce similar bronchioalveolar tumors in lungs of mice

Jaagsiekte sheep retrovirus (JSRV) induces bronchioalveolar tumors in sheep and goats. Expression of the JSRV envelope (Env) protein in mouse airway epithelial cells induces similar tumors, indicating that Env expression is sufficient for tissue-specific tumor formation. Enzootic nasal tumor virus (ENTV) is related to JSRV but induces tumors in the nasal epithelium of sheep and goats. Here we found that ENTV Env can also induce tumors in mice but, unexpectedly, with a phenotype identical to that of tumors induced by the JSRV Env, indicating that factors other than Env mediate the tissue specificity of tumor induction by ENTV.

Tumors in mice transgenic for the envelope protein of Jaagsiekte sheep retrovirus

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a contagious lung cancer in sheep. Previous studies have shown that the JSRV envelope protein (Env) functions as an oncogene, in that it can morphologically transform rodent fibroblast and epithelial cell lines. To obtain a small animal model for JSRV-induced OPA, we generated a transgene expressing an epitope-tagged JSRV Env under control of the lung-specific Surfactant Protein A (SPA) promoter. Transgenic mice containing the SPA-Env-HA transgene showed low efficiency but specific expression in the lung. F1 male progeny from one transgenic founder developed subdermal lipomas that expressed the transgene. These results indicate that the JSRV Env protein is capable of inducing tumors in transgenic mice, and in other cell types besides lung epithelial cells.

Endogenous retroviruses regulate periimplantation placental growth and differentiation

Endogenous retroviruses (ERVs) are fixed and abundant in the genomes of vertebrates. Circumstantial evidence suggests that ERVs play a role in mammalian reproduction, particularly placental morphogenesis, because intact ERV envelope genes were found to be expressed in the syncytiotrophoblasts of human and mouse placenta and to elicit fusion of cells in vitro. We report here in vivo and in vitro experiments finding that the envelope of a particular class of ERVs of sheep, endogenous Jaagsiekte sheep retroviruses (enJSRVs), regulates trophectoderm growth and differentiation in the periimplantation conceptus (embryo/fetus and associated extraembryonic membranes). The enJSRV envelope gene is expressed in the trophectoderm of the elongating ovine conceptus after day 12 of pregnancy. Loss-of-function experiments were conducted in utero by injecting morpholino antisense oligonucleotides on day 8 of pregnancy that blocked enJSRV envelope protein production in the conceptus trophectoderm. This approach retarded trophectoderm outgrowth during conceptus elongation and inhibited trophoblast giant binucleate cell differentiation as observed on day 16. Pregnancy loss was observed by day 20 in sheep receiving morpholino antisense oligonucleotides. In vitro inhibition of the enJSRV envelope reduced the proliferation of mononuclear trophectoderm cells isolated from day 15 conceptuses. Consequently, these results demonstrate that the enJSRV envelope regulates trophectoderm growth and differentiation in the periimplantation ovine conceptus. This work supports the hypothesis that ERVs play fundamental roles in placental morphogenesis and mammalian reproduction.

Expression of the jaagsiekte sheep retrovirus envelope glycoprotein is sufficient to induce lung tumors in sheep

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA). The expression of the JSRV envelope (Env) alone is sufficient to transform a variety of cell lines in vitro and induce lung cancer in immunodeficient mice. In order to determine the role of the JSRV Env in OPA tumorigenesis in sheep, we derived a JSRV replication-defective virus (JS-RD) which expresses env under the control of its own long terminal repeat (LTR). JS-RD was produced by transiently transfecting 293T cells with a two plasmid system, involving (i) a packaging plasmid, with the putative JSRV packaging signal deleted, expressing the structural and enzymatic proteins Gag, Pro, and Pol, and (ii) a plasmid which expresses env in trans for JS-RD particles and provides the genomes necessary to deliver JSRV env upon infection. During the optimization of the JS-RD system we determined that both R-U5 (in the viral 5' LTR) and the env region are important for JSRV particle production. Two independent experimental transmission studies were carried out with newborn lambs. Four of five lambs inoculated with JS-RD showed OPA lesions in the lungs at various times between 4 and 12 months postinoculation. Abundant expression of JSRV Env was detected in tumor cells of JS-RD-infected animals and PCR assays confirmed the presence of the deleted JS-RD genome. These data strongly suggest that the JSRV Env functions as a dominant oncoprotein in the natural immunocompetent host and that JSRV can induce OPA in the absence of viral spread.

Mutational analysis of the cytoplasmic tail of jaagsiekte sheep retrovirus envelope protein

Jaagsiekte sheep retrovirus (JSRV) is the etiologic agent of a transmissible lung cancer in sheep, ovine pulmonary adenocarcinoma. JSRV is unique in that the envelope protein functions as an oncogene, since it can morphologically transform fibroblast and epithelial cells in culture and can induce lung tumors in mice. Previous studies indicated that the transmembrane (TM) protein is essential for transformation, and particular attention has focused on a YXXM motif in the cytoplasmic tail. In this study, we carried out systematic mutagenesis of the cytoplasmic tail of JSRV Env. Alanine scanning mutagenesis revealed four classes of mutants: mutants in which transformation was abrogated, those in which transformation was not affected, those with reduced transformation, and those with increased transformation (supertransformers). In general, the alanine mutations did not affect Env protein production or its localization to the plasma membrane. Three functional domains of the cytoplasmic tail were identified: an amphipathic helix at the N-terminal (juxtamembrane) side, a nonessential C-terminal region, and an internal region (including the YXXM motif) where mutations resulted in abrogation, decreases, or increases in transformation. Alanine mutations in the amphipathic helix in both the hydrophobic and hydrophilic faces generally abolished transformation. The mutation R591A showed partial transformation that was consistent with loss of signaling through the Akt-mTOR pathway and signaling predominantly through the Ras-Raf-MEK1/2-extracellular signal-regulated kinase 1/2 pathway. The supertransforming mutants generally showed increased signaling through Akt and reduced activation of p38 MAPK that is inhibitory for transformation. These mutants provide further insight into the role of the TM cytoplasmic tail in JSRV transformation.

Oncogenic transformation by the jaagsiekte sheep retrovirus envelope protein

Retroviruses have played profound roles in our understanding of the genetic and molecular basis of cancer. Jaagsiekte sheep retrovirus (JSRV) is a simple retrovirus that causes contagious lung tumors in sheep, known as ovine pulmonary adenocarcinoma (OPA). Intriguingly, OPA resembles pulmonary adenocarcinoma in humans, and may provide a model for this frequent human cancer. Distinct from the classical mechanisms of retroviral oncogenesis by insertional activation of or virus capture of host oncogenes, the native envelope (Env) structural protein of JSRV is itself the active oncogene. A major pathway for Env transformation involves interaction of the Env cytoplasmic tail with as yet unidentified cellular adaptor(s), leading to the activation of PI3K/Akt and MAPK signaling cascades. Another potential mechanism involves the cell-entry receptor for JSRV, Hyaluronidase 2 (Hyal2), and the RON receptor tyrosine kinase, but the exact roles of these proteins in JSRV Env transformation remain to be better understood. Recently, a mouse model of lung cancer induced by JSRV Env has been developed, and the tumors in mice resemble those seen in sheep infected with JSRV and in humans. In this review, we summarize recent progress in our understanding the molecular mechanisms of oncogenic transformation by JSRV Env protein, and discuss the relevance to human lung cancer.

In-situ Demonstration of Mitogen-activated Protein Kinase Erk 1/2 Signalling Pathway in Contagious Respiratory Tumours of Sheep and Goats

Ovine pulmonary adenocarcinoma (OPA) and enzootic nasal adenocarcinoma (ENA) are two contagious neoplastic diseases of secretory epithelial cells in the respiratory system of sheep and goats. Jaagsiekte sheep retrovirus (JSRV) is the aetiological agent of OPA, and enzootic nasal tumour virus (ENTV) is associated with ENA. The genomes of these retroviruses do not contain known oncogenes but products of the env gene are important in the generation of transforming stimuli. However, the cell signalling pathways activated in vivo are not completely understood. This study was based on the use of activation stage antibodies specifically detecting proteins of the extracellular signal regulated kinase Erk 1/2 cell signalling pathway and transcription factors. Tissue sections were collected from four natural cases of OPA, four experimentally induced OPA tumours, four ENA tumours in sheep, four ENA tumours in goats, two normal sheep lungs and two lungs with chronic inflammation. Routine immunohistochemical procedures with phosphorylation stage-specific antibodies were carried out. Representative proteins of the Erk1/2 pathway (Raf-1, Mek1/2 and p44/42MAPK) were activated in natural cases of OPA and ENA in sheep and goats and also in experimentally induced OPA. Transcription factors 90Rsk and Elk-1 were activated in OPA and ENA tumours. However, c-Myc was activated only in OPA tumours. In contagious respiratory neoplasms of sheep and goats the Erk1/2 pathway appears to be important for the in-vivo generation of the transforming stimuli.

Jaagsiekte sheep retrovirus-specific immune responses induced by vaccination: A comparison of immunisation strategies

Jaagsiekte sheep retrovirus (JSRV) is the aetiological agent of ovine pulmonary adenocarcinoma (OPA). No JSRV-specific immunological responses have been detected in clinical cases of OPA or in experimentally infected lambs. The aim of the present study was to induce immune responses in sheep against JSRV proteins using several immunisation strategies. The vaccines were administered subcutaneously and intradermally, or intranasally, in adjuvant. Antibodies were measured by ELISA and immunoblotting, and T cell responses by lymphoproliferation assay. Antibodies specific for JSRV-capsid protein were induced by inoculation of recombinant proteins in adjuvant, and transient JSRV-specific T cell responses by intranasal inoculation with inactivated virus. These results will help in the design of a protective vaccine against JSRV infection and the development of OPA.

In vivo and in vitro analysis of factor binding sites in Jaagsiekte sheep retrovirus long terminal repeat enhancer sequences: roles of HNF-3, NF-I, and C/EBP for activity in lung epithelial cells

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma, a contagious lung cancer of sheep that arises from type II pneumocytes and Clara cells of the lung epithelium. Studies of the tropism of this virus have been hindered by the lack of an efficient system for viral replication in tissue culture. To map regulatory regions important for transcriptional activation, an in vivo footprinting method that couples dimethyl sulfate treatment and ligation-mediated PCR was performed in murine type II pneumocyte-derived MLE-15 cells infected with a chimeric Moloney murine leukemia virus driven by the JSRV enhancers (DeltaMo+JS Mo-MuLV). In vivo footprints were found in the JSRV enhancers in two regions previously shown to be important for JSRV long terminal repeat (LTR) activity: a binding site for the lung-specific transcription factor HNF-3beta and an E-box element in the distal enhancer adjacent to an NF-kappaB-like binding site. In addition, in vivo footprints were detected in two downstream motifs likely to bind C/EBP and NF-I. Mutational analysis of a JSRV LTR reporter construct (pJS21luc) revealed that the C/EBP binding site is critical for LTR activity, while the putative NF-I binding element is less important; elimination of these sites resulted in 70% and 40% drops in LTR activity, respectively. Electrophoretic mobility shift assays using nuclear extracts from MLE-15 murine Clara cell-derived mtCC1-2 cells with probes corresponding to the NF-I or C/EBP sites revealed several complexes. Antiserum directed against NF-IA, C/EBPalpha, or C/EBPbeta supershifted the corresponding protein-DNA complexes, indicating that these isoforms, which are also important for the expression of several cellular lung-specific genes, may be important for JSRV expression in lung epithelial cells.

A Moloney murine leukemia virus driven by the Jaagsiekte sheep retrovirus enhancers shows enhanced specificity for infectivity in lung epithelial cells

Jaagsiekte sheep retrovirus (JSRV) is the etiologic agent of ovine pulmonary adenocarcinoma (OPA), a transmissible lung cancer in sheep. One of the unique features of this virus is that in infected animals, the only tissues that show expression of the virus are the tumor cells in the lung. We previously showed that the JSRV long terminal repeat (LTR) is preferentially active in murine lung epithelial cell lines (MLE-15 and mtCC1-2). To further explore the tissue specificity, we inserted the JSRV enhancer sequences from the U3 region of the LTR into a Moloney murine leukemia virus (M-MuLV) LTR lacking its own enhancer sequences, to give the chimeric LTR DeltaMo + JS. Transient transfection assays indicated that the DeltaMo + JS LTR is > 5-fold more active in lung epithelial cell lines than in non-lung lines, compared to the wild-type M-MuLV LTR. This was due to preferential activity of the JSRV enhancers in lung epithelial cells. Moreover, M-MuLV driven by the DeltaMo + JS LTR was > 3 logs more infectious in MLE-15 cells compared to non-lung cell lines. This chimeric virus may facilitate investigations of the tissue-specificity of JSRV.

A PCR technique for the detection of Jaagsiekte sheep retrovirus in the blood suitable for the screening of ovine pulmonary adenocarcinoma in field conditions

Ovine pulmonary adenocarcinoma (OPA) is a naturally occurring contagious lung neoplasia caused by jaagsiekte sheep retrovirus (JSRV). Although no specific circulating antibodies against the virus can be detected in infected sheep, JSRV proviral DNA sequences can be found in peripheral blood leukocytes (PBLs) in clinically affected and in a proportion of in contact animals. In this study, existing hemi-nested PCR procedure is compared with a new one-step PCR technique that was developed to minimise potential DNA contamination and reduce sample and reagent handling. Different blood preparations were assessed and the best results were achieved on DNA prepared from buffy coat. The sensitivity of this PCR was lower in JSRV infected sheep without lesions of OPA than in clinically affected sheep, which indicate that this PCR may not be not fully appropriate for screening of individual sheep, but rather to provide results at flock level. This PCR is the only currently available blood test for detection of JSRV infected sheep and may be useful in epidemiological studies and in control programmes of OPA.

Multiclonal pattern of Jaagsiekte sheep retrovirus integration sites in ovine pulmonary adenocarcinoma

Insertional mutagenesis and envelope (Env)-mediated oncogenesis are hypothesized mechanisms by which Jaagsiekte sheep retrovirus (JSRV) causes ovine pulmonary adenocarcinoma (OPA). Twenty-eight JSRV integration sites in lung tumors (LTs) from four sheep with OPA were cloned and sequenced by a multiple step gene walking technique. Using nested PCR, clonal expansion of these integration sites could be detected, if at all, only in the localized regions of LT from which the integration sites were derived. One sheep had a viral integration site in a sequence with 85 and 81% identity, respectively, over 100 bp to exon 2 of the human and mouse receptor protein tyrosine phosphatase gamma genes. Clonal integration of Jaagsiekte sheep retrovirus in this gene was demonstrated by nested PCR and Southern blot hybridization in the DNA sample from which the integration site was cloned, but not in other LT or kidney DNA samples from the same sheep. OPA may develop from multiple independent oncogenic events and a role for insertional mutagenesis cannot be ruled out.

Sheep Endogenous Betaretroviruses (enJSRVs) and the Hyaluronidase 2 (HYAL2) Receptor in the Ovine Uterus and Conceptus

The ovine genome contains approximately 20 copies of endogenous betaretroviruses (enJSRVs) that are highly related to two exogenous oncogenic viruses, Jaagsiekte sheep retrovirus (JSRV) and Enzootic nasal tumor virus. The cellular receptor for both JSRV and the enJSRVs is hyaluronidase 2 (HYAL2). In this study, we assessed expression of enJSRVs envelope (env) and HYAL2 mRNAs in the ovine uterus and conceptus (embryo/fetus and extraembryonic membranes) throughout gestation. By reverse transcription-polymerase chain reaction analyses, enJSRVs env were found to be expressed beginning in the Day 12 conceptus, whereas HYAL2 was expressed from Day 16. HYAL2 mRNA was detected throughout gestation in the placentome but not in the endometrium, whereas enJSRVs env expression was detected throughout gestation in endometrium and placentomes. The enJSRVs env mRNA was specifically expressed in the endometrial lumenal epithelium (LE) and glandular epithelium (GE) as well as the trophoblast giant binucleate cells (BNC) and multinucleated syncytia of the placenta. HYAL2 mRNA was only detected in the BNC and multinucleated syncytial plaques of the placentome. Partial sequencing of the transcriptionally active enJSRVs from sheep endometrium, placentomes, and placenta revealed expression of many enJSRV loci. Cloning of the expressed enJSRVs env mRNA from ovine uteroplacental tissues found sequences similar to the previously identified enJS5F16 and enJS56A1 gene with an intact open reading frame, although the polypeptides they encode were not studied. Collectively, results provide further support for our hypothesis that the enJSRVs Env have been beneficial to the host and are involved in protection of the uterus from viral infection and regulators of placental morphogenesis and function.

Inclusion of Jaagsiekte sheep retrovirus proviral elements markedly increases lentivirus vector pseudotyping efficiency

Retroviral pseudotyping for gene transfer applications endeavors to alter vector tropism and maintain a suitable titer. We investigated the compatibility of the Jaagsiekte sheep retrovirus (JSRV) envelope glycoprotein with the feline immunodeficiency virus (FIV) vector. A construct consisting of the minimal JSRV env coding region expressed from a standard mammalian expression plasmid generated FIV vector titers of approximately 10(4) TU/ml following standard triple transfection, collection of supernatants, and concentration by centrifuge. Interestingly, retention of the native proviral 5' and 3' flanking regions surrounding the JSRV env resulted in exceptional titers of approximately 10(8) TU/ml following the same viral preparation. To discern the regions necessary to achieve this 10,000-fold increase in titer, additional constructs were designed and tested. Our results indicate that the enhanced vector titer correlates with an increase in steady-state levels of envelope RNA that results from a combination of RNA splicing and stability, leading to increased envelope protein production. Expression of four other glycoproteins in an expression plasmid retaining the enhancing elements from the JSRV proviral sequence increased FIV vector titers from 0- to 100-fold. These novel data demonstrate that optimization of the envelope expression construct can profoundly influence titers for lentivirus vectors.

An influx of macrophages is the predominant local immune response in ovine pulmonary adenocarcinoma

Infection with a retrovirus, Jaagsiekte sheep retrovirus (JSRV), causes ovine pulmonary adenocarcinoma (OPA). The excess production of surfactant proteins by alveolar tumour cells results in increased production of pulmonary fluid, which is characteristically expelled through the nostrils of affected sheep. The immune response to JSRV and the tumour is poorly understood: no JSRV-specific circulating antibodies or T cells have been detected to date. The aim of the present study was to obtain phenotypic evidence for a local immune response in OPA lungs. Specific-pathogen free lambs were infected intratracheally with JSRV. When clinical signs of OPA were apparent, the lungs were removed at necropsy and immunohistochemistry (IHC) was performed on lung sections using a panel of mouse anti-sheep mAbs. No influx of dendritic cells, B cells, CD4, CD8 or gammadelta T cells was seen in the neoplastic nodules or in their periphery. MHC Class II-positive cells were found intratumourally, peritumourally and in the surrounding alveolar lumina. In the tumours, many of these cells were shown to be fibroblasts and the remainder were likely to be mature macrophages. In the alveolar lumen, the MHC Class II-positive cells were CD14-positive and expressed high levels of IFN-gamma. They appeared to be immature monocytes or macrophages which then differentiated to become CD14-negative as they reached the periphery of the tumours. A high level of MHC Class I expression was detected on a range of cells in the OPA lungs but the tumour nodules themselves contained no MHC Class I-positive cells. On the basis of these findings, it is proposed that the lack of an effective immune response in OPA could result from a mechanism of peripheral tolerance in which the activity of the invading macrophages is suppressed by the local environment, possibly as a consequence of the inhibitory properties of the surfactant proteins.

Infection of lung epithelial cells and induction of pulmonary adenocarcinoma is not the most common outcome of naturally occurring JSRV infection during the commercial lifespan of sheep

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA). In this study, we followed over a 31-month period the natural transmission of JSRV in adult sheep and in their offspring. We established groups derived from flocks with either a high or low incidence of OPA and monitored virus transmission, clinical disease and macroscopic/microscopic lung lesions at necropsy. Results obtained show that (i) JSRV infection can occur perinatally or in the first few months of life in lambs and in adult sheep; (ii) only a minority of JSRV-infected animals develop clinical disease during their commercial lifespan; and (iii) JSRV is more readily detectable in peripheral blood leucocytes and lymphoid organs than in the lungs. These data support a model of opportunistic JSRV infection and tumorigenic conversion of type II pneumocytes/Clara cells in the lungs, while lymphoreticular cells serve as the principal virus reservoir.

Roles of the Ras-MEK-mitogen-activated protein kinase and phosphatidylinositol 3-kinase-Akt-mTOR pathways in Jaagsiekte sheep retrovirus-induced transformation of rodent fibroblast and epithelial cell lines

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a transmissible lung cancer of sheep. The virus can induce tumors rapidly, and we previously found that the JSRV envelope protein (Env) functions as an oncogene, because it can transform mammalian and avian fibroblast cell lines. (N. Maeda, Proc. Natl. Acad. Sci. USA 98:4449-4454, 2001). The molecular mechanisms of JSRV Env transformation are of considerable interest. Several reports suggested that the phosphatidylinositol 3-kinase/Akt pathway is important for transformation of mammalian fibroblasts but not for chicken fibroblasts. In this study, we found that Akt/mTOR is involved in JSRV transformation of mouse NIH 3T3 fibroblasts, because treatment with the mTOR inhibitor rapamycin reduced transformation. We also found that H/N-Ras inhibitor FTI-277 and MEK1/2 inhibitors PD98059 and U0126 strongly inhibited JSRV transformation of NIH 3T3 fibroblasts, suggesting that the H/N-Ras-MEK-mitogen-activated protein kinase (MAPK) p44/42 pathway is necessary for the transformation. In RK3E epithelial cells, the MEK1/2 inhibitors also eliminated transformation, but FTI-277 only partially inhibited transformation. It was noteworthy that p38 MAPK inhibitors enhanced JSRV transformation in both fibroblasts and epithelial cells. Treatment of transformed cells with p38 inhibitors both increased levels of phospho-MEK1/2 and phospho-p44/42 and induced rapid enhancement of the transformed phenotype. Immunohistochemical staining of tumor tissues from naturally and experimentally induced OPA and naturally occurring enzootic nasal adenocarcinoma revealed strong activation of MAPK p44/42 in all cases examined. However, p38 activation was not generally observed. These results indicate that signaling through two pathways (in particular, H/N-Ras-MEK-MAPK and, to a lesser extent, Akt-mTOR) is important for JSRV-induced transformation and that p38 MAPK has a negative regulatory effect on transformation, perhaps via MEK1/2 and p44/42.

Gene Transfer to Respiratory Epithelia with Lentivirus Pseudotyped with Jaagsiekte Sheep Retrovirus Envelope Glycoprotein

A feline immunodeficiency virus (FIV)-based lentiviral vector was pseudotyped to identify envelope (env) glycoproteins that direct efficient gene transfer to pulmonary epithelia for the treatment or prevention of lung diseases. The envelope glycoprotein from the Jaagsiekte sheep retrovirus (JSRV) is a candidate under investigation. We utilized high titer FIV vector (>10(8) TU/ml) pseudotyped with the JSRV env glycoprotein (JSRVFIV) to study the transduction of polarized primary cultures of human airway epithelia and receptor/vector interactions. The reported receptor for JSRV, hyaluronidase 2 (HYAL2), is a GPI-linked protein. We expressed FLAG-tagged HYAL2 in polarized airway epithelia using an adenoviral vector and documented that the HYAL2 protein sorts predominantly to the apical surface. Of interest, the efficiency of gene transfer with apically applied JSRV-FIV was markedly less than FIV pseudotyped with VSV-G, even in Ad-HYAL2 complemented epithelia. The inefficient gene transfer with JSRV-FIV in HYAL2 complemented cells suggests that factors other than receptor abundance limit apical gene transfer efficiency with this envelope. JSRV-FIV transduced the distal lung epithelia of rabbits in vivo and transduced primary cultures of rabbit type II cells with 100-fold greater efficiency than primary cultures of rabbit tracheal cells. These data indicate that a lentivirus pseudotyped with the JSRV envelope glycoprotein transduces type II cells with greater efficiency than conducting airway epithelia and provides an example of glycoprotein-mediated cell-specific tropism within a tissue with a widely heterogeneous cell population.

Coexistence of enzootic nasal adenocarcinoma and jaagsiekte retrovirus infection in sheep

Ten sheep naturally affected with enzootic nasal adenocarcinoma (ENA), a disease associated with ovine enzootic nasal tumour virus (ENTV-1), were found also to be infected with jaagsiekte sheep retrovirus (JSRV), the causal agent of ovine pulmonary adenocarcinoma (OPA). Only one of the sheep showed OPA lung lesions. The animals belonged to 10 flocks located in a geographical area in which OPA is frequently seen. ENTV-1 was found in all the ENA tumours but only occasionally in extra-tumoral sites, confirming the results of a previous study. In contrast, JSRV had a disseminated tissue distribution, similar to that previously reported for animals infected with JSRV. However, the occurrence of JSRV in lymphoid tissues was clearly greater than in sheep infected with JSRV but with no lesions of ENA. The data suggested a synergistic relationship between ENTV-1 and JSRV, resulting in increased proliferation of JSRV.

Variable Regions 1 and 2 (VR1 and VR2) in JSRV gag Are Not Responsible for the Endogenous JSRV Particle Release Defect

Jaagsiekte sheep retrovirus (JSRV) is a betaretrovirus causing ovine pulmonary adenocarcinoma, a transmissible lung tumor of sheep. A very closely related endogenous retrovirus (enJSRV) occurs as 15 to 20 copies in the genome of all sheep, and is not known to be linked to pathogenesis. We previously localized a particle release defect of the full-length endogenous-derived expression construct pCMV2enJS56A1 to the amino-terminal region of gag that incorporates the two variable regions VR1 and VR2, which harbor the main sequence differences between endogenous and exogenous JSRV in this part of gag. Here, we tested the hypothesis that either or both of these variable regions are responsible for the observed particle release defect in enJS56A1. We found that the PPPPPPPS motif of the exogenous VR1 is neither necessary nor sufficient for particle release. Furthermore, the precise substitution of VR1 and VR2 in the exogenous JSRV expression plasmid pCMV2JS21, using their enJS56A1-derived counterparts, did not abrogate the ability of the resulting constructs to release particles. The particle release defect of enJS56A1 is therefore not determined exclusively by either VR1 or VR2. These results point to a small number of amino acids lying outside of VR1 and VR2 that may be responsible for the particle defect of enJS56A1 Gag.

Multiple domains of the Jaagsiekte sheep retrovirus envelope protein are required for transformation of rodent fibroblasts

Jaagsiekte sheep retrovirus (JSRV) is an exogenous retrovirus of sheep that induces a contagious lung cancer, ovine pulmonary adenocarcinoma. We previously showed that the gene encoding JSRV envelope protein (Env) appears to function as an oncogene, since it can transform mouse NIH 3T3 cells. The cytoplasmic tail of the Env transmembrane protein (TM) is necessary for the transformation. However, previous experiments did not exclude the involvement of the Env surface protein (SU) in transformation. In this study, we created a series of nested deletion mutants through the SU domain and assessed their ability to transform rodent fibroblasts. All SU deletion mutants downstream of the predicted signal peptide were unable to transform murine NIH 3T3 or rat 208F cells. Transport to the plasma membrane of selected deleted Env proteins was confirmed by confocal immunofluorescence microscopy of hemagglutinin-tagged versions. Additional sequential SU deletion mutants lacking 50-amino-acid (aa) blocks throughout SU also were unable to transform. Furthermore, minimal insertion mutants of two amino acids (Leu/Gln) at various positions in SU also abolished transformation. These data indicate that domains in SU facilitate efficient JSRV transformation. This could reflect a necessity of SU for appropriate configuration of the Env protein or independent activation by SU of a signaling pathway necessary for transformation. Complementation between SU and TM mutants for transformation supported the latter hypothesis. Cotransfection with DeltaGP Y590F (mutant in the TM cytoplasmic tail) with DeltaGP SUDelta103-352 (lacking most of SU) resulted in efficient transformation. The resulting transformants showed evidence for the presence and expression of both mutant plasmids.

Analysis of integration sites of Jaagsiekte sheep retrovirus in ovine pulmonary adenocarcinoma

Ovine pulmonary adenocarcinoma (OPA) is an infectious lung tumor of sheep caused by Jaagsiekte sheep retrovirus (JSRV). To test the hypothesis that JSRV insertional mutagenesis is involved in the oncogenesis of OPA, we cloned and characterized 70 independent integration sites from 23 cases of OPA. Multiple integration sites were identified in most tumors. BLAST analysis of the sequences did not disclose any potential oncogenic motifs or any identical integration sites in different tumors. Thirty-seven of the integration sites were mapped to individual chromosomes by PCR with a panel of sheep-hamster hybrid cell lines. Integration sites were found on 20 of the 28 sheep chromosomes, suggesting a random distribution. However, four integration sites from four different tumors mapped to chromosome 16. By Southern blot hybridization, probes derived from two of these sites mapped to within 5 kb of each other on normal sheep DNA. These sites were found within a single sheep bacterial artificial chromosome clone and were further mapped to only 2.5 kb apart, within an uncharacterized predicted gene and less than 200 kb from a mitogen-activated protein kinase-encoding gene. These findings suggest that there is at least one common integration site for JSRV in OPA and add weight to the hypothesis that insertional mutagenesis is involved in the development of this tumor.

Late viral interference induced by transdominant Gag of an endogenous retrovirus

The sheep genome harbors approximately 20 copies of endogenous retroviruses (enJSRVs) closely related to the exogenous and oncogenic Jaagsiekte sheep retrovirus (JSRV). One of the enJSRV loci, enJS56A1, has a defect for viral exit. We report a previously uncharacterized mechanism of retroviral interference. The defect possessed by enJS56A1 is determined by its Gag protein and is transdominant over the exogenous JSRV. By electron microscopy, cells transfected by enJS56A1, with or without JSRV, show agglomerates of tightly packed intracellular particles most abundant in the perinuclear area. The defect in exit and ability to interfere with JSRV exit could be largely attributed to the presence of tryptophan, rather than arginine, at position 21 of enJS56A1 Gag; C98 and V102 also contribute to these properties. We found that enJS56A1 or similar loci containing W21, C98, and V102 are expressed in sheep endometrium. enJS56A1 is a previously unrecognized example of a naturally occurring endogenous retrovirus expressing a dominant negative Gag acting at a late step of the viral replication cycle. Understanding the late blockade exerted by enJS56A1 could unravel fundamental aspects of retroviral biology and help to devise new antiretroviral strategies.

Endogenous betaretroviruses of sheep: teaching new lessons in retroviral interference and adaptation

The endogenous betaretroviruses of small ruminants offer an excellent model to investigate the biological relevance of endogenous retroviruses (ERVs). Approximately twenty copies of endogenous betaretroviruses (enJSRVs) are present in the genome of sheep and goats. enJSRVs are highly related to Jaagsiekte sheep retrovirus (JSRV) and the Enzootic nasal tumour virus (ENTV), the causative agents of naturally occurring carcinomas of the respiratory tract of sheep. enJSRVs interact/interfere at different levels both with the host and with their exogenous and pathogenic counterparts. enJSRVs blocks the exogenous JSRV replication by a novel two-step interference mechanism acting both early and late during the virus replication cycle. enJSRVs are highly active, they are abundantly and specifically expressed in the epithelium of most of the ovine female reproductive tract. The specific spatial and temporal expression of enJSRVs supports a role in trophoblast development and differentiation as well as conceptus implantation. In addition, enJSRVs are expressed during fetal ontogeny leading to the apparent tolerance of sheep towards the pathogenic JSRV. Thus, the sheep/enJSRVs system is a model that can be utilized to study many different aspects of ERVs and retrovirus biology. The impressive technologies developed to study the sheep reproductive biology, in conjunction with the knowledge gained on the molecular biology of enJSRVs, makes the ovine system an ideal model to design experiments that can functionally address the role of ERVs in mammalian physiology.

Jaagsiekte sheep retrovirus envelope efficiently pseudotypes human immunodeficiency virus type 1-based lentiviral vectors

Jaagsiekte sheep retrovirus (JSRV) infects lung epithelial cells in sheep, and oncoretroviral vectors bearing JSRV Env can mediate transduction of human cells, suggesting that such vectors might be useful for lung-directed gene therapy. Here we show that JSRV Env can also efficiently pseudotype a human immunodeficiency virus type 1-based lentiviral vector, a more suitable vector for transduction of slowly dividing lung epithelial cells. We created several chimeric Env proteins that, unlike the parental Env, do not transform rodent fibroblasts but are still capable of pseudotyping lentiviral and oncoretroviral vectors.

Transformation of mouse fibroblasts by Jaagsiekte sheep retrovirus envelope does not require phosphatidylinositol 3-kinase

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma, a transmissible lung cancer of sheep. The envelope of JSRV may have oncogenic properties, since it can morphologically transform mouse NIH 3T3 cells and other fibroblast lines. Recently, we found that the cytoplasmic tail of the envelope transmembrane (TM) protein is necessary for transformation, and in particular a consensus binding motif (YXXM) for phosphatidylinositol 3-kinase (PI3K) is important. Moreover, JSRV-transformed cells show phosphorylation (activation) of Akt/protein kinase B, a downstream target of PI3K. In these studies, we directly tested for the involvement of PI3K in transformation by JSRV. Contrary to expectations, four different experiments indicated that PI3K is not necessary for JSRV-induced transformation: (i) cotransfection with a dominant negative truncated form of the PI3K regulatory subunit (Deltap85) did not affect transformation frequency, (ii) cells stably expressing Deltap85 showed the same frequencies of transformation as parental NIH 3T3 cells, (iii) fibroblasts established from double-knockout mice lacking PI3K p85alpha and p85beta could be transformed with JSRV envelope, and (iv) incubation of cells with the PI3K inhibitor LY294002 did not specifically inhibit transformation, nor did the drug reverse transformation of JSRV-transformed cells. One alternate explanation for the lack of transformation by YXXM mutants could be that they were defective in intracellular trafficking. However, confocal microscopy of epitope-tagged envelope proteins of both wild-type and nontransforming YXXM mutants showed a cell surface or plasma membrane localization. While PI3K is not required for JSRV-induced transformation of NIH 3T3 cells, the downstream target Akt kinase was found to be activated (phosphorylated) in JSRV-transformed PI3K-negative cells. Therefore, JSRV envelope can induce PI3K-independent phosphorylation of Akt.

Chromosomal distribution of endogenous Jaagsiekte sheep retrovirus proviral sequences in the sheep genome

A family of endogenous retroviruses (enJSRV) closely related to Jaagsiekte sheep retrovirus (JSRV) is ubiquitous in domestic and wild sheep and goats. Southern blot hybridization studies indicate that there is little active replication or movement of the enJSRV proviruses in these species. Two approaches were used to investigate the distribution of proviral loci in the sheep genome. Fluorescence in situ hybridization (FISH) to metaphase chromosome spreads using viral DNA probes was used to detect loci on chromosomes. Hybridization signals were reproducibly detected on seven sheep chromosomes and eight goat chromosomes in seven cell lines. In addition, a panel of 30 sheep-hamster hybrid cell lines, each of which carries one or more sheep chromosomes and which collectively contain the whole sheep genome, was examined for enJSRV sequences. DNA from each of the lines was used as a template for PCR with JSRV gag-specific primers. A PCR product was amplified from 27 of the hybrid lines, indicating that JSRV gag sequences are found on at least 15 of the 28 sheep chromosomes, including those identified by FISH. Thus, enJSRV proviruses are essentially randomly distributed among the chromosomes of sheep and goats. FISH and/or Southern blot hybridization on DNA from several of the sheep-hamster hybrid cell lines suggests that loci containing multiple copies of enJSRV are present on chromosomes 6 and 9. The origin and functional significance of these arrays is not known.

Characterization of enzootic nasal tumour virus of goats: complete sequence and tissue distribution

The complete genome sequence of a new isolate of enzootic nasal tumour virus (ENTV-2), associated with enzootic nasal adenocarcinoma (ENA) of goats, was determined. The genome exhibits a genetic organization characteristic of beta-retroviruses. ENTV-2 is closely related to the retrovirus (ENTV-1) associated with enzootic adenocarcinoma of sheep, and to jaagsiekte retrovirus. The main sequence differences between these viruses reside in orfX, the U3 LTR, two small regions in gag and the transmembrane (TM) region of env. Sequence analysis of the TM region of env from several sheep and goats naturally affected by ENA suggested that ENTV-1 and ENTV-2 are distinct viruses rather than geographical variants. Although both viruses transform secretory epithelial cells of the ethmoid turbinate, the study of their tissue distribution using specific PCRs showed that ENTV-2 establishes a disseminated lymphoid infection whereas ENTV-1 is mainly confined to the tumour.

Relevance of Akt phosphorylation in cell transformation induced by Jaagsiekte sheep retrovirus

Expression of the JSRV envelope (Env) is sufficient to transform immortalized rodent fibroblasts. A putative docking site for the PI3-K kinase (Y(590)-X-X-M(593)) in the cytoplasmic tail of the transmembrane domain of the JSRV Env is a major determinant of viral-induced cell transformation. Akt is constitutively phosphorylated in rodent fibroblasts transformed by the JSRV Env. However, recent data suggest that Y590 and M593 are not necessary for JSRV Env-induced transformation of the immortalized chicken fibroblasts cell line DF-1. In this study we found that JSRV-induced transformation of DF-1 cells is Akt-independent. In addition, a replication-competent avian vector expressing the JSRV Env (RCASBP(A)+JE) was also able to induce transformation of primary chicken embryo fibroblasts (CEF). Vectors expressing JSRV Env Y590 mutants were still able to induce CEF cells transformation but not as efficiently as the vectors expressing the wild-type Env. In CEF cells, as in DF-1 cells, only the expression of the wild-type Env induced constitutive phosphorylation of Akt. Thus, in chicken cells, the degree of transformation induced by the JSRV Env is maximum in the presence of Y590 and Akt phosphorylation. We addressed the significance of Akt phosphorylation in rat 208F cells transformed by the JSRV Env and showed that Akt is indeed activated and shows kinase activity. Inhibitors of the PI-3K/Akt pathway reproducibly decreased the transformation efficiency of the JSRV Env. In vivo, we found phosphorylated Akt only in nasal tumors induced by the enzootic nasal tumor virus (ENTV), a JSRV-related beta-retrovirus. No evidence of Akt phosphorylation was found in lung tumor sections of sheep affected by pulmonary adenocarcinoma. As a whole, these results suggest that the activation of the PI-3K/Akt pathway contributes to the process of JSRV-induced cell transformation but most likely is not the primary determinant both in vitro and in vivo.

Putative phosphatidylinositol 3-kinase (PI3K) binding motifs in ovine betaretrovirus Env proteins are not essential for rodent fibroblast transformation and PI3K/Akt activation

Jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus (ENTV) are simple betaretroviruses that cause epithelial cell tumors in the lower and upper airways of sheep and goats. The envelope (Env) glycoproteins of both viruses can transform rodent and chicken fibroblasts, indicating that they play an essential role in oncogenesis. Previous studies found that a YXXM motif in the Env cytoplasmic tail, a putative docking site for phosphatidylinositol 3-kinase (PI3K) after tyrosine phosphorylation, was necessary for rodent cell transformation but was not required for transformation of DF-1 chicken fibroblasts. Here we show that JSRV and ENTV Env proteins with tyrosine or methionine mutations in the YXXM motif can still transform rodent fibroblasts, albeit with reduced efficiency. Akt was activated in cells transformed by JSRV or ENTV Env proteins and in cells transformed by the proteins with tyrosine mutations. Furthermore, the PI3K-specific inhibitor LY294002 could inhibit Akt activation and cell transformation in all cases, indicating that Akt activation and transformation is PI3K dependent. However, we could not detect tyrosine phosphorylation of JSRV or ENTV Env proteins or an interaction between the Env proteins and PI3K in the transformed cells. We found no evidence for mitogen-activated protein kinase activation in cells that were transformed by the JSRV or ENTV Env proteins. We conclude that ovine betaretrovirus Env proteins transform the rodent fibroblasts by indirectly activating the PI3K/Akt pathway.

Transformation of rodent fibroblasts by the jaagsiekte sheep retrovirus envelope is receptor independent and does not require the surface domain

Jaagsiekte sheep retrovirus (JSRV) is the etiological agent of a contagious lung cancer of sheep known as ovine pulmonary adenocarcinoma (OPA). Expression of the JSRV envelope protein (Env) is sufficient to transform immortalized and primary fibroblasts, but the precise mechanisms of this process are not known. The cellular receptor for JSRV is hyaluronidase 2 (Hyal-2), the product of a putative tumor suppressor gene that in humans maps to a chromosomal region frequently deleted in the development of lung and breast cancers. Here we report studies to determine whether the Hyal-2-JSRV Env interaction plays a role in virus-induced transformation of rodent fibroblasts. Chimeric Env proteins between JSRV and the unrelated murine retroviruses Moloney murine leukemia virus (MMuLV) and mouse mammary tumor virus (MMTV) showed cell surface expression comparable to that of wild-type MMuLV Env and rescued infection of MMuLV particle pseudotypes. Interestingly, an MMuLV-JSRV chimera in which the putative receptor binding domain (RBD) and proline-rich region (PRR) of JSRV Env were replaced by the RBD and PRR of MMuLV induced transformation of 208F, a rodent fibroblast line. Cell lines derived from foci of MMuLV-JSRV chimera-transformed 208F cells grew in soft agar and showed Akt activation, a hallmark of JSRV-transformed rodent fibroblasts. Transformation assays performed using proteins with amino-terminal deletion mutations showed that the carboxy-terminal 141 amino acids of the transmembrane subunit (TM) were sufficient to induce cell transformation when targeted to the membrane with a myristoylation signal. Thus, the JSRV TM is necessary and sufficient to transform rodent fibroblasts. Taken together these results indicate that the interaction with Hyal-2 at least is not an essential determinant of JSRV-induced transformation of fibroblasts and that the viral TM functions essentially as an oncoprotein.

A history of ovine pulmonary adenocarcinoma (jaagsiekte) and experiments leading to the deduction of the JSRV nucleotide sequence

Jaagsiekte (JS), a contagious cancer affecting the lungs of sheep has been called many names over the years. At a recent workshop in Missilac, France it was agreed that the disease would be called ovine pulmonary adenocarcinoma (OPA). The disease is caused by an infectious retrovirus called jaagsiekte sheep retrovirus (JSRV). This chapter focuses on the early research that led up to the isolation, cloning and sequencing of the exogenous infectious form of JSRV and the demonstration that it has an endogenous counter part that is present in all sheep. As there was no in vitro production source of the virus much of the early research focused on the in vivo production and purification of the virus to obtain sufficient material to use to identify the viral proteins and purify the viral genetic material. Typically, new born lambs were inoculated intra-tracheally with concentrated lung lavage from previously infected sheep lungs. The optimal purification involved the concentration of lung lavage of freshly slaughtered sheep, an extraction with organic solvent, and final purification by both rate zonal and isopycnic centrifugation. Monoclonal and polyclonal antibodies were made against the purified fractions. The polyclonal antibodies were not very specific and the monoclonal antibodies proved to be against antigens expressed in high concentrations in response to any lung pathology. The genomic RNA of the virus was isolated from ex vivo purified materials, and cloned as a collection of cDNAs. The full length sequence was assembled by walking through the cDNA clones. The genome of the exogenous virus is 7462 bases and has the classical gag, pol, env genome arrangement and is flanked by a long terminal repeat (LTR) on each end. An additional open reading frame (ORF) was observed in the viral genome and has been called orfX. A function has not been determined for this ORF. JSRV is classified as a betaretrovirus, with gag and pol closely related to D type retrovirus, whereas env is related to the B type viruses such as the human endogenous retrovirus HERV-K. An interesting finding was that the exogenous infectious virus had an endogenous counter part which is present in the genomes of all sheep and goats. It is estimated that there are between 15 and 20 endogenous loci per sheep genome. No circulating antibodies have been found in OPA-affected sheep. It is suggested that the endogenous JSRV transcripts are expressed at an early age and are cause for the clonal elimination of JSRV specific T cells during T-cell ontogeny. Histopathologically the sheep disease resembles human bronchiolar alveolar carcinoma and has been identified as a natural out bred animal model that could be used to study the human disease.

Molecular biology of jaagsiekte sheep retrovirus

Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine pulmonary adenocarcinoma (OPA), a contagious lung cancer of sheep. Until recently, research on JSRV/OPA was hampered by the lack of a tissue culture system for the propagation of the virus. Historically, pathological samples (lung fluid) collected from sheep affected by OPA were the only source of infectious JSRV. Thus studies on the JSRV/OPA system were conducted only where field isolates of OPA cases were readily available. In the past 10 years, the deduction of the JSRV sequence (York et al. 1991; York 1992), the isolation of an infectious and oncogenic JSRV molecular clone (JSRV21) (Palmarini et al. 1999a) and the establishment of a rapid method to produce infectious virus in vitro (Palmarini et al. 1999a) sparked many studies at the molecular level that strengthened past observations and revealed new properties of this unique virus. Here, we will review the data accumulated so far on the molecular biology of JSRV using the infectious and oncogenic JSRV21 molecular clone as virus of reference.

Transformation and oncogenesis by jaagsiekte sheep retrovirus

Jaagsiekte sheep retrovirus (JSRV) is an exogenous retrovirus of sheep that induces a contagious lung cancer, ovine pulmonary adenocarcinoma (OPA). JSRV is a potent carcinogen in the experimental setting, inducing end-stage tumors at around 6 weeks of age when newborn lambs are inoculated intratracheally. Despite this rapid oncogenesis, inspection of the JSRV genome sequence does not reveal any obvious viral oncogenes. In this review, recent advances in studies of JSRV oncogenic transformation are described. Molecular cloning of an infectious and oncogenic JSRV provirus was instrumental in the studies. DNA transfection of JSRV proviral DNA into mouse NIH3T3 cells results in morphological transformation, indicating that the JSRV genome carries an oncogene. Further experiments identified the JSRV envelope protein as the transforming gene, and a PI3 kinase docking site in the cytoplasmic tail of the transmembrane (TM) protein was shown to be necessary for transformation. Avian DF-1 cells infected with an avian retroviral vector (RCAS) expressing the JSRV envelope protein also undergo tumorigenic transformation. Possible mechanisms of transformation are discussed, and a cooperating role for insertional activation of proto-oncogenes in tumorigenesis is also considered. The transforming potential of the JSRV envelope protein may be necessary for JSRV infection and replication in vivo.

Enzootic nasal adenocarcinoma of sheep and goats

Enzootic nasal adenocarcinoma is a contagious tumour of the mucosal nasal glands affecting young adult sheep or goats. The disease occurs naturally in all continents except Australia and New Zealand. Clinical signs include continuous nasal discharge, respiratory distress, exophthalmos and skull deformations. The tumour is classified histologically as a low-grade adenocarcinoma. Nasal glands of both respiratory and olfactory muosal glands seem to be the origin of the neoplasia. It has been experimentally transmitted in sheep and goats using either tumour extracts or concentrated nasal fluids. Two distinct retroviruses are implicated in the aetiology of the neoplasia one in sheep (ONAV) and one in goats (CNAV). We suggest that jaagsiekte sheep retrovirus (JSRV), ONAV, CNAV, and their endogenous counterparts represent a unique family of retroviruses. The similarities between these viruses suggests that any control strategies, including vaccination, may be appropriate to both diseases. The differences, however, represent a unique resource for delineating the function of individual regions of the virus. It is intriguing that whilst ONAV and CNAV appear to be as different to each other as they are to JSRV, that they have very similar disease pathologies, distinct from that of OPA. Additionally, all three exogenous viruses manage to avoid instigating any apparent immune response. Whether this is indeed a result of tolerance induced by the endogenous counterparts or whether the viruses themselves have unique immunosuppressive properties will be an important finding.

Endogenous retroviruses related to jaagsiekte sheep retrovirus

Ovine betaretroviruses consist of exogenous viruses [jaagsiekte sheep retrovirus (JSRV) and enzootic nasal tumor virus, (ENTV)] associated with neoplastic diseases of the respiratory tract and 15-20 endogenous viruses (enJSRV) stably integrated in the ovine and caprine genome. Phylogenetic analysis of this group of retroviruses suggests that the enJSRV can be considered as 'modern' endogenous retroviruses with active, exogenous counterparts. Sequence analysis of JSRV, ENTV and enJSRV suggests that enJSRV do not directly contribute to the pathogenesis of ovine pulmonary adenocarcinoma (OPA) or enzootic nasal tumor through large-scale recombination events, but small-scale recombination or complementation of gene function cannot be excluded; experiments involving enJSRV-free sheep, which have not been found, would be needed to investigate this possibility. Evidence of expression of enJSRV structural proteins in tissues of the reproductive tract and lung implies that they do not have a primary role in disease. However, experimental exploitation of exogenous/endogenous retrovirus sequence differences by producing chimeras has been useful in establishing the determinants of JSRV Env-induced transformation. Even if enJSRV do not have a direct role in OPA, their expression during ontogeny or in neonatal life may impact the likelihood of exogenous JSRV infection and disease outcome via the induction of immunological tolerance. Aside from any role in disease, enJSRV loci may serve as useful genetic markers in the sheep and their frequent expression in the reproductive tract of the ewe may portend an important physiologic role in sheep.

Identification of Hyal2 as the cell-surface receptor for jaagsiekte sheep retrovirus and ovine nasal adenocarcinoma virus

Jaagsiekte sheep retrovirus (JSRV) and ovine nasal adenocarcinoma virus (ONAV) replicate in the airway and cause epithelial cell tumors through the activity of their envelope (Env) proteins. Identification of the receptor(s) that mediate cell entry by these viruses is crucial to understanding the oncogenic activity of Env and for the development of gene therapy vectors based on these viruses that are capable of targeting airway cells. To identify the viral receptor(s) and to further study the biology of JSRV and ONAV, we developed retroviral vectors containing Moloney murine leukemia virus components and the Env proteins of JSRV or ONAV. We used a new technique involving positional cloning by phenotypic mapping in radiation hybrid cells to identify and clone the human receptor for JSRV, Hyal2, which also serves as the receptor for ONAV. Hyal2 is a glycosylphosphatidylinositol-anchored cell-surface protein that has low hyaluronidase activity and is a member of a large family that includes sperm hyaluronidase (Spam) and serum hyaluronidase (Hyal1). Hyal2 is located in a region of human chromosome 3p21.3 that is often deleted in lung cancer, suggesting that it may be a tumor suppressor. However, its role in JSRV or ONAV tumorigenesis, if any, is still unclear. JSRV vectors are capable of transducing various human cells, and are being further evaluated for gene therapy purposes.