Sheep pulmonary adenomatosis (SPA) is a naturally occurring contagious lung
tumour of sheep which has been associated aetiologically with a chimaeric type D/B
retrovirus known as jaagsiekte sheep retrovirus (JSRV). Studies on the aetiopathogenesis of this disease are extremely valuable in comparative pathology
because SPA represents a unique model of a naturally occurring lung cancer.
The role of JSRV in the aetio-pathogenesis of SPA is largely unknown. This is due to
several factors which have hampered research for a number of years, such as the lack
of a cell culture system for the propagation of JSRV and the lack of reagents and
techniques to detect JSRV. In addition, the presence in the sheep genome of 15 to 20
copies of JSRV-related endogenous sequences has impeded investigation at the
Studies directed at further understanding the biology of JSRV were undertaken in
this thesis. Initially, immunological techniques for the detection of JSRV were
developed and employed to establish JSRV sites of replication. In particular, a
blocking enzyme-linked immunosorbent assay (B-ELISA) was used to verify the
distribution of viral particles in several tissues collected from SPA-affected sheep
and unaffected controls. Immunohistochemistry was employed for the detection of
the JSRV major capsid protein at the cellular level. The results obtained at the end of
these studies pointed to epithelial tumour cells in the lungs of sheep affected by SPA
as major sites of viral replication. JSRV viral particles and viral proteins were never
detected in non-tumour tissues from SPA-affected sheep or in unaffected control
The relationship between the exogenous JSRV and the highly related endogenous
sequences present in the sheep genome (sheep endogenous retroviruses, enJSRV) was
then determined by RT-PCR coupled to sequencing and restriction enzyme analysis.
EnJSRVs were found to be transcriptionally active in a wide variety of tissues of
both healthy and SPA-affected sheep. By sequencing a fraction of the gag gene of
enJSRVs, a Seal restriction site was found to be a molecular marker for the
exogenous form of JSRV. JSRV provirus was detected in tumour genomic DNA of
SPA-affected animals but not in non-tumour tissues of the same animals or in
unaffected controls. These results demonstrated that JSRV was a horizontally
transmitted virus, specifically associated with SPA, and it was not an endogenous
retrovirus reactivated as a downstream event of neoplasia.
Finally, a highly sensitive exogenous-specific hemi-nested PCR was developed
utilising primers in the U3 region of the JSRV LTR, where major differences
between endogenous and exogenous sequences were found. Employing this test, it
was demonstrated that JSRV established a disseminated infection of the lymphoid
tissues of sheep affected by SPA, although the epithelial tumour cells are the main
sites of viral replication. This finding could be extremely important in future studies
aimed at understanding the interaction of JSRV with the host immune system.
In conclusion, the data in this thesis present compelling evidence to specifically
associate the exogenous form of JSRV with sheep pulmonary adenomatosis,
strengthening the view that this virus has a role in the aetiology of the tumour.