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

Sheep pulmonary adenomatosis (SPA) is a contagious and experimentally trans missible lung cancer of sheep resembling human bronchiolo-alveolar carcinom a. A type D retrovirus, known as jaagsiekte sheep retrovirus (JSRV), has be en associated with the etiology of SPA, but its exact role in the induction of the tumor has not been clear due to the lack of (i) a tissue culture sy stem for the propagation of JSRV and (ii) an infectious JSRV molecular clon e. To investigate the role of JSRV in the etiology of SPA, we isolated a fu ll-length JSRV proviral clone, pJSRV(21), from a tumor genomic DNA library derived from a natural case of SPA. pJSRV(21) was completely sequenced and showed open reading frames in agreement with those deduced for the original South African strain of JSRV. In vivo transfection of three newborn lambs by intratracheal inoculation with pJSRV(21) DNA complexed with cationic lip ids showed that pJSRV(21) is an infectious molecular clone. Viral DNA was d etected in the peripheral blood mononuclear cells (PBMCs) of the transfecte d animals by a highly sensitive JSRV-U3 heminested PCR at various time poin ts ranging from 2 weeks to 6 months posttransfection. In addition, proviral DNA was detected in the PBMCs, lungs, and mediastinal lymph nodes of two l ambs sacrificed 9 months posttransfection, but no macroscopic or histologic al SPA lesion was induced. We prepared JSRV particles by transient transfec tion of 293T cells with a JSRV construct (pCMV2JS(21)) in which the upstrea m U3 was replaced with the cytomegalovirus early promoter. Four newborn lam bs were inoculated with JSRV(21) particles produced in this manner, and two of them showed the classical signs of SPA 4 months postinfection. The resu lting turners were positive for JSRV DNA and protein. Thus, JSRV(21) is an infectious and pathogenic molecular clone and is necessary and sufficient t o induce sheep pulmonary adenomatosis.