A Comparison of Pleural Mesotheliomas Induced by Asbestos or SV40 Virus in Syrian Golden Hamsters

Pleural mesotheliomas are a well-known consequence of exposure to asbestos in both humans and animals. However, there are cases of mesothelioma in humans for which there is little or no known exposure to asbestos. Mesotheliomas have also been produced in hamsters infected with simian virus 40 (SV40), a contaminant of early polio vaccines, which was shown to replicate in individuals inoculated with the vaccine. Recently, wild-type SV40 has been detected in human mesotheliomas as well as other types of neoplasms. Because the tumors were induced with such dissimilar agents, we evaluated mesotheliomas produced in hamsters after intrapleural injection of SV40 compared to those produced after inhalation exposure to amosite asbestos to see if there were differences in the development and morphology of the tumors. Although the mesotheliomas produced by both agents were clearly of mesothelial origin based on standard morphological criteria, there were clear differences. The SV40-induced tumors occurred with a short latency period, were large multicentric lesions with pleural effusion that always caused death within 3 to 6 mo, and were largely composed of small round cells growing in a uniformly tubulopapillary pattern with many areas of sarcomatous change. They had a minimal amount of stroma and tended to invade the adjacent tissues. The unaffected pleura showed no evidence of fibrosis. In contrast, the mesotheliomas induced by asbestos occurred much later (most after 18 mo), were rarely (<10%) the cause of death, and were typically very small with little evidence of pleural effusion. While they also had a tubulopapillary pattern, they were composed of larger cells with more abundant cytoplasm and sarcomatous change was rare. The adjacent pleura typically showed marked evidence of fibrosis and local invasion was rarely encountered. Whether these striking morphological differences between SV40- and asbestos-induced mesotheliomas in hamsters have a correlate in humans is not known. It would be useful to conduct a similar comparison of mesotheliomas from humans known to have been exposed to high levels of asbestos to those with no known exposure to see if similar morphological differences exist.