318 H. B. ANDERVONT 



large doses of hydrocortisone but, in contrast to the latter tumors, those 

 altered by Xerosin did not revert to the typical tumor found in control birds. 

 These investigations, in which young chickens were used, showed that 

 extraneous factors affect the host-virus relationship. 



Cortisone and X-irradiation, alone or in combination, are known to lower 

 the resistance of animals to the growth of homologous or heterologous tumor 

 cells, and cortisone alters the susceptibility of animals to virus infections 

 (Andervont, 1957). The use of such measures should be explored with virus- 

 induced tumors to ascertain whether, as with the Rous virus, the viruses 

 change their affinities for tissues and species. The inhibition of growth of the 

 Rous sarcoma in young chickens by the administration of Xerosin or hydro- 

 cortisone could be used to test the suggestion of Duran-Reynals that the 

 Rous virus while under adverse conditions, is more conducive to change 

 which enables it to produce tumors in heterologous hosts. Cortisone could 

 also be used in attempts to lower the susceptibility of adults of a foreign 

 species to the Rous virus to ascertain whether they would then accept the 

 virus as readily as the more tolerant young animals. 



One handicap to early work with the Rous virus was the lack of genetically 

 homogeneous test animals. Perhaps one of the most serious questions facing 

 today's investigators is the advisability of a re-evaluation of factors affecting 

 the virus-host interactions through the use of chickens of known genetic 

 constitutions. In view of the proved usefulness of inbred animals in cancer 

 research, it may be assumed they would be of considerable assistance to 

 investigators working with the Rous virus. As one reads the case for the 

 influence of age upon the activity of the Rous virus, it suggests to those who 

 have used inbred animals that the effects attributed to age could also reflect 

 the susceptibility of the host. Also, the summation of serological studies with 

 the fowl tumors by Oberling and Guerin (1954) suggests that many of the 

 disconcerting results could be clarified through the use of inbred animals. 

 The best approach to the separation of genetic and other factors in host- 

 virus interactions is through the use of animals of known genetic 

 make-up. 



Inbred lines of chickens exhibiting differences in susceptibility to the Rous 

 virus have been developed by Waters (1945, 1951) and by Greenwood et al. 

 (1948). Bryan (1957) reported preliminary findings with two lines established 

 by Waters. The average effective dose of Rous virus was 10,000 times larger 

 for the resistant than for the susceptible line. Availability of such animals 

 enables the investigator to ascertain the response of animals of different 

 genetic constitutions to a standard amount of virus or vice versa. Information 

 of this sort may be of much assistance in studies of a tumor virus, which is 

 almost ideal for quantitative investigations because of its ability to elicit the 

 change to malignancy within a matter of days. 



