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DISCUSSION 



ALEXANDER: What role do you think contamination may play in the finding of viral 

 agents in the radiation-induced leukaemias in your laboratory? You've got the Gross 

 virus in one animal house, and it may appear in passage materials since tumour cells 

 are an ideal growth medium for aU sorts of viruses. 



UPTON: I think that experiments of the type that Dr. Berenblum mentioned, in which 

 one irradiates an animal and then can detect in the animal filterable leukaemogenic 

 activity, strongly suggest that such agents are in fact present in subliminal amounts in 

 non-irradiated individuals. From the early work of Gross, and subsequent investigators, 

 I have the impression that the amounts of virus recoverable from animals which have 

 developed leukaemia spontaneously or as a result of irradiation are usually disappoint- 

 ingly small. We badly lack today a potent assay system, and I think that one of the 

 encouraging features of Dr. Berenblum's work is that he may have shown us a better 

 way to assay small amounts of virus. I shall admit that as cells are passaged the likeli- 

 hood of contamination is appreciable. Whether success in getting a good preparation 

 after many serial passages means that virus has entered in the course of passaging or 

 whether an agent that was there at the outset has simply become more potent, I don't 

 think we can yet decide with certainty. 



ALEXANDER: One reason why I brought this up is that viruses were long sought in 

 radiation-induced leukaemias and were not found. Then other viruses, among them 

 leukaemia viruses, became popular; many laboratories adopted them, and then suddenly, 

 many more virus-induced tumours crop up. Is Gross's radiation-induced vu-us distin- 

 guishable from his standard virus? 



MOLE: There are three examples now of radiation-induced leukaemias associated with 

 a virus; Dr. Upton's, Gross's and that of Lieberman and Kaplan. All three have come 

 from laboratories where leukaemogenic viruses have in fact been in use before this 

 demonstration was made. People don't usually report negative results so there are those 

 who have failed to find viruses in radiation-induced leukaemia. This does seem to be a 

 serious criticism and how you meet it and what kind of experiments or precautions you 

 ought to take I don't quite know. 



UPTON: I am not sure whether in Kaplan's laboratory there were leukaemogenic viruses 

 in the environment before he did his work with C57BL mice, certainly in our laboratory 

 we have raised AKR and RF mice in proximity. One of the things we need to do now to 

 make headway with this and related problems in radiation virology is to maintain 

 rigorously isolated animal stocks. One of the things we are hoping to do is to derive 

 germ-free lines by caesarean section, and keep them behind absolute barriers. I think 

 that as yet it has not been possible to culture the leukaemia viruses involved, so that many 

 of the immunological tests which could be brought to bear to distinguish antigenic differ- 

 ences have yet to be applied. 



