Signalment:  

Adult (age unknown), female, Greyface sheep (Ovis aries)A mature Greyface ewe was culled due to prolonged respiratory distress. At necropsy, the only significant findings were in the lungs.

Gross Description:  

The lungs were almost diffusely much firmer and heavier than normal and failed to collapse. There were moderate to large amounts of frothy fluid in the tracheal and bronchial lumens. The cranioventral lung lobes and the caudal portions of the caudal lung lobes were expanded by fairly well demarcated, firm, ill-defined masses which were grey to pale purple. On cut section, the parenchyma in these areas was consolidated, firm and pale grey. (fig. 4-1). Both sections are from this sheep but the section on the left is relatively unaffected while the parenchyma in the section on the right is virtually replaced by slightly nodular, pale grey homogeneous tissue (fig. 4-1). Affected areas also exuded frothy fluid, especially on cut section.

Histopathologic Description:

Most of the normal lung parenchyma is replaced by multiple variably sized, nodular proliferations of well-differentiated cuboidal to low columnar epithelial cells (fig. 4-2). The nodules are non-encapsulated, infiltrative and often coalesce with each other. The epithelial cells form tubuloacinar structures with occasional papillary projections, supported by a fibrovascular stroma. Some of the tubuloacinar structures contain pink, proteinaceous material. Individual cells have variably distinct cell borders with centrally to basally located hypochromatic nuclei, moderate amounts of faintly granular eosinophilic cytoplasm and indistinct nucleoli. Nuclear pleomorphism is mild and mitotic figures rare. Rare nodules are centrally necrotic or infiltrated by necrotic and viable neutrophils and cellular debris. In many areas, the surrounding alveolar spaces are flooded by very large numbers of alveolar and epithelioid macrophages, with fewer neutrophils, lymphocytes and plasma cells and occasional multinucleated giant cells. The macrophages are very plump with homogeneous, pink cytoplasm which sometimes causes peripheralisation of the nucleus. Many bronchioles and bronchi are cuffed by large, discrete aggregates of large numbers of lymphocytes forming lymphoid follicles, many with distinct germinal centres. The bronchiolar and bronchial lining cells are variably hyperplastic or attenuated. 

Morphologic Diagnosis:  

1. Ovine pulmonary adenocarcinoma (OPA)
2. Severe, diffuse, histiocytic interstitial pneumonia with marked BALT hyperplasia

Condition:  

Pulmonary adenocarcinoma (jaagsiekte)

Contributor Comment:  

OPA has a number of synonyms, including sheep pulmonary adenomatosis and jaagsiekte (Afrikaans for driving sickness). It is a naturally occurring, transmissible disease characterized by the development of pulmonary neoplasia and caused by an exogenous betaretrovirus called jaagsiekte sheep retrovirus (JSRV)⁶. First recognized in South Africa, it now occurs worldwide, with the exception of the Antipodes. The incidence in the UK has been recorded to be as high as 30% and it can result in mortality rates of up to 50% in affected flocks⁴.

In this case, the gross changes were typical of the classical form of the disease. An atypical form occurs (although has apparently not been reported in Scotland) whereby the nodules are more discrete, harder and much drier. Lesions generally only occur in the lungs although metastasis to lymph nodes can arise and is one of the main features helping to classify the lung lesions as truly neoplastic, rather than simply proliferative. Extrathoracic metastases have also been reported⁴. Electron microscopy has confirmed that the alveolar proliferations are composed of type II pneumocytes, while those arising in bronchioles are composed of Clara cells. Both cell types are secretory, accounting for the copious amounts of frothy fluid produced, which tends to flood the respiratory passages in the classical form. This excessive fluid accumulation is absent from the atypical form⁶. Microscopically, the neoplasm is classified as a bronchioloalveolar carcinoma. The histological findings in this case were typical and tend to be identical between the two forms⁴.

The lung tropism of the JSRV and resultant neoplastic transformation of pulmonary epithelial cells is apparently unique in the retrovirus group. The exact mechanism of neoplastic transformation is the subject of much current research. Two genes appear to be important to this tropism and viral infectivity, the env gene and the long terminal repeat (LTR) gene. The env gene permits viral entry of cells because it encodes the viral glycoprotein which allows interaction with cell receptors. Thus, the virus can only infect cells which specifically express its receptor, although pulmonary epithelial cells are not the only cells to do so⁴. The LTR gene is integrated into the cellular genome after viral entry and induces viral expression by interacting with cellular transcription factors. It can activate proto-oncogenes via insertional mutagenesis, whereby provirus is inserted near a proto-oncogene and drives its overexpression. 

Of these two genes, the env gene is gaining favor as the more likely oncogenic agent since it has been shown to function as an oncogene, at least experimentally in mammalian fibroblast and epithelial cell lines. The mechanism of cell transformation is unclear although it is believed to involve the cytoplasmic tail of the envelope transmembrane protein as well as two downstream cell signaling pathways, H/N-Ras-MEK-MAPK and Akt-mTOR². The insertional mutagenesis theory seems less likely since the random nature and the infrequency of insertion in the desired place within the genome would not be efficient enough for proto-oncogene activation⁴.

The sheep genome also contains 15-20 copies of endogenous retrovirus which is very similar to the exogenous JSRV. The main difference lies in the LTR region of the genome such that the endogenous form of the virus does not have the same transcriptional efficiency in pulmonary epithelial cells as the exogenous, tumor-inducing form. The existence of the endogenous virus may explain the lack of an antibody response in OPA infected sheep, since the endogenous elements may promote immunotolerance during fetal development⁴.

There was marked BALT hyperplasia in this case, for which there could have been two main reasons. Firstly, it can occur in the atypical form of OPA. We felt this was less likely since the concomitant fibrosis and marked lymphoplasmacytic inflammation usually seen in the atypical form were not present⁶. Secondly, the possibility of concurrent maedi was considered since combined infections have been frequently recognized; no further testing was performed to confirm or refute this possibility⁵. There was also quite severe histiocytic inflammation. The widespread and marked infiltrate of plump macrophages is commonly found around neoplastic nodules; they are believed to be induced by the excessive surfactant protein production but their exact role in the pathogenesis is still unclear. Recent work suggests they reflect a cellular immune response to the presence of neoplastic cells. The apparent ineffectiveness of this response is believed to be due to putative immunosuppressive properties of the excess surfactant protein⁷. 

JPC Diagnosis:  

1. Lung: Carcinoma, Greyface sheep (Ovis aries), ovine.
2. Lung: Lymphofollicular hyperplasia, diffuse, moderate.
3. Lung: Pneumonia, interstitial, multifocal, mild.

Conference Comment:  

The contributor gives an excellent overview of retroviral- induced ovine pulmonary adenocarcinoma (OPA). This section also exhibits the typical histologic finding of abundant macrophages located at the periphery of the neoplasm in OPAs which are presumably attracted by the abundant surfactant secreted by the neoplasm. We agree with the contributor that there is likely at least one other disease process occurring in addition to OPA in the distributed section; ovine lentivirus pneumonia and a concomitant bacterial superinfection were also discussed in conference.

References:

1. Caporale M, Cousens C, Centorame P, Pinoni C, De las Heras M, Palmarini M: Expression of the Jaagsiekte sheep retrovirus envelope glycoprotein is sufficient to induce lung tumors in sheep. J Virol 80:8030-8037, 2006
2. Caswell JL, Williams KJ: Respiratory System. In: Jubb, Kennedy, and Palmers Pathology of Domestic Animals, ed. Maxie MG, 5th ed., vol. 2, pp. 621-622. Elsevier Limited, St. Louis, MO, 2007
3. Maeda N, Fu W, Ortin A, de las Heras M, Fan H: 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. J Virol 79:4440-4450, 2005
4. Palmarini M, Fan H: Retrovirus-induced Ovine Pulmonary Adenocarcinoma, an Animal Model for Lung Cancer. J Natl Cancer Inst 93:1603-1614, 2001
5. Rosadio RH, Sharp JM, Lairmore MD, Dahlberg JE, DeMartini JC: Lesions and retroviruses associated with naturally occurring ovine pulmonary carcinoma (sheep pulmonary adenomatosis). Vet Pathol 25:58-66, 1988
6. Salvatori D, De las Heras M, Sharp M: Ovine pulmonary adenocarcinoma: the story to date. In Practice 4:2004
7. Summers C, Norval M, De las Heras M, Gonzalez L, Sharp JM, Woods GM: An influx of macrophages is the predominant local immune response in ovine pulmonary adenocarcinoma. Vet Immunol Immunopathol 106:285- 294, 2005

A virtual slide is not available for this case.

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fig. 4-1 Lung

fig. 4-2 Lung

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