A History of Ovine Pulmonary Adenocarcinoma (Jaagsiekte) and Experiments Leading to the Deduction of the JSRV Nucleotide Sequence

Abstract

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.