330 H. B. ANDERVONT 



do not develop breast tumors and the virus disappears from them within a 

 few generations; others accept the virus, develop tumours, and transmit it 

 through successive generations. This implies that, as with other virus 

 infections, the host plays an important role in the host-virus relationship. 

 Inbred strains of rabbits are not available at present, but races of rabbits have 

 been established which display striking differences in morphological and 

 physiological characteristics (Sawin, 1955). 



Greene has applied techniques of tissue transplantation to a study of the 

 papilloma virus and the results have a direct bearing upon the problem of 

 variant viruses. The skin from rabbit embryos was infected with the virus 

 and placed into the brains of adult rabbits; the virus from embryo skin was 

 then carried through 3 serial passages in domestic rabbits (Greene, 1953a). 

 Papillomas developed in infected embryonic rat skin, following transplanta- 

 tion into the brain or subcutaneous tissues of adult rats (Greene, 1953b). 

 This finding presents an opportunity to ascertain whether the virus becomes 

 modified through residence in a foreign species. In another paper, Greene 

 (1954) included experiments designed to compare the infectiousness of the 

 virus from cottontail and domestic papillomas. Cottontail papillomas, when 

 transplanted into the brain of domestic rabbits or to the subcutaneous tissues 

 of hamsters, retained infectious virus for domestic rabbits, but papillomas 

 from domestic rabbits did not contain infectious virus after exposure to brain 

 tissues of cottontails. Exposure of cottontail papillomas to resistant species 

 (adult rats, mice, and guinea pigs) failed to influence the infectiousness of the 

 virus for domestic rabbits. Greene concluded; "Loss of infectiousness is a 

 function of factors resident in the epidermal cells of the domestic rabbit." 

 In other experiments, Greene (1955a,b) was concerned with problems of 

 immunity and concluded that immunity to the virus was humoral in nature 

 and embryonic rabbit skin retained its susceptibility to the virus after 

 exposure to tissues of naturally resistant hamsters and mice. 



These experiments supplied the foundation for a later paper (Greene, 1955c) 

 which included findings pertinent to the present discussion concerned with 

 the adaptability of the virus. Embryonic, newborn, and adult skin from 

 domestic rabbits were exposed to the virus obtained from natural cottontail 

 lesions and then transplanted into the brain of domestic rabbits or the 

 subcutaneous spaces of hamsters. One month later the transplanted tissues 

 were removed and tested for virus by administration to the skin of domestic 

 rabbits. Embryonic skin retained the virus most effectively, and papillomas 

 induced in domestic rabbits with this virus supplied a virus which was carried 

 through 8 serial passages in domestic rabbits. Likewise, when papillomas 

 from cottontail rabbits were implanted directly into hamsters and mice, the 

 virus recovered from the papillomas was found to be infectious through 5 

 transfer generations in domestic rabbits. 



