NEW EVIDENCE ON THE MECHANISM OF RADIATION LEUKAEMOGENESIS 55 



does not, except inside the blood vessels. Now that is why I ask if you have done any 

 experiments with perfused brain? 



UPTON: I think the question of the source of the leukaemogenic virus is a very important 

 question. Schwartz, I am told, used brain because he thought there would be the least 

 possibility of antibodies which might bind virus present in the brain. Most virologists 

 with whom I have discussed this are not very impressed by this argument. Prof. 

 Berenblum's data suggest that the agent is present in every tissue although he has not 

 perfused them. I don't know whether other lymphoma viruses have been extensively 

 enough studied to tell us which tissues are richest in the virus. I think this is a very 

 important question because if we are attemptmg to detect leukaemia viruses or tumour 

 viruses in tumours we may be looking in precisely the wrong place. With polyoma virus, 

 for example, the animals become viraemic shortly after inoculation, then begin to develop 

 antibodies to the virus, and when the tumour is finally apparent the virus can often no 

 longer be detected. So attemptmg to work from the tumour backwards in pathogenesis 

 when you are dealing with a tumour virus may be a very difficult way to go at it. I should 

 like to ask Prof. Berenblum whether a serial analysis of these activated tissues might 

 disclose differences. 



berenblum: We have so far tested crude tissues, in vitro irradiated crude tissue, high- 

 speed centrifuged extract and filtered extract. We have not done much beyond that. 

 Incidentally, in all these experiments, both donors and recipients were adult animals 

 between 6 and 8 weeks old. 



UPTON: I should think it is very important to give your prospective recipients urethane 

 before inoculation of the activated tissue to determine whether the interaction of urethane 

 plays a role. 



ALEXANDER: I have been thuiking of an alternative scheme and the reason that I really 

 want to have an alternative scheme is that I think one is loading up this poor simple 

 formula of urethane with too many functions. We've got it as a complete carcmogen, as 

 an initiator, as a promoter, as a cytotoxic agent, and God knows what. I just wonder 

 whether in these particular strains of mice one would normally get small tumours as a 

 result of irradiation with perhaps quite low doses, but in general this is prevented because 

 repair from the bone-marrow prevents this carcinogenic stimulus. This would explain 

 why one has to give several repeated doses in Kaplan's experiments and why bone- 

 marrow stops it. Now, if the answer were that the urethane so damages the thymus 

 that it is not responsive to repair by bone-marrow, perhaps only for a short period 

 of time, then I would say that, in the absence of bone-marrow repair, 50 r is sufficient 

 to induce a high incidence of tumours. Is it not also possible that you had a laboratory 

 contaminant virus? Perhaps work on polyoma virus began in your laboratory at about 

 this time? 



BERENBLUM : There are two points you raised. One is the theory I mentioned earlier. I 

 quite agree that we haven't excluded the alternative possibility of urethane acting on the 

 thymus itself. As for the second point — the idea of a virus floating around in the animal 

 colony — this is of course difficult to exclude with certamty. I can only say that in our 

 animal colony of 12,000 mice, the pattern of spontaneous tumours is not what one might 

 expect if polyoma virus were present. The spontaneous incidence of leukaemia in our 

 C57BL mice is to 1%; the incidence in urethane-treated C57BL mice is 2 to 7%. 

 Other spontaneous tumours in this strain are extremely rare in our colony. 

 GRAY: Doesn't the evidence from the in vitro experiments suggest that this postulated 

 virus is present almost immediately. You irradiated the tissue in vitro and then you 

 inoculated it into the animal and you got your tumours. This shows that it must have 

 been a rather immediate effect of the irradiation of these tissues. 



