402 DISCUSSION 



(1956. Cancer Res.y 16, 426) have done experiments in which they 

 thymectomized mice before irradiation and then grafted thymuses 

 from syngeneic mice subcutaneously after total body irradiation. Lym- 

 phoid tumours arose in the grafted thymus and behaved genetically as 

 if derived from Hneal descendants of thymus cells which regenerated 

 from the donor thymus and which had not, therefore, been exposed to 

 direct irradiation. 



Medawar: Does the effect of thymectomy on susceptibility to leu- 

 kaemia suggest a hormonal interpretation or a cellular interpretation ? 



Miller: By using cytologically marked cells, it has been impossible to 

 uphold the theory that the thymic cells, themselves, are the cells which 

 are transformed to leukaemic cells. This is because, in at least 80 per 

 cent of cases of virus-induced leukaemias (Miller, J. F. A. P. [1962]. 

 aba Found. Symp. Tumour Viruses of Murine Origin, p. 262. London: 

 Churchill) or of spontaneous leukaemias (Law, L. W. [1952]. J. nat. 

 Cancer Inst., 12, 789), it is the host cells which undergo the leukaemic 

 change, presumably after colonizing the thymus graft. This certainly 

 would suggest a humoral interpretation or some sort of induction 

 mechanism. 



G. Klein: I wonder whether any of your animals survived long 

 enough for you to observe the formation of spontaneous tumours in 

 them. I am asking this because it has been postulated recently by 

 Burnet, by Prehn, and by us that one of the functions of the homograft 

 reaction may be to eUminate antigenically foreign neoplastic clones 

 that keep arising all the time. Several of the tumours induced by 

 chemical carcinogens have been shown to be strongly antigenic in the 

 syngeneic and in the autochthonous host, while several of the spontan- 

 eous tumours, such as mammary carcinomas, have not. There is some 

 reason to beUeve that the chemical carcinogens may hamper the homo- 

 graft response, and that is perhaps the reason why some of the tumours 

 induced by them are antigenic, and why the antigenic clones can grow 

 out. It appears to me that if the thymectomized mice lack homograft 

 reactivity, then tumours arising in them spontaneously, which are not 

 known to be antigenic otherwise, should be antigenic in the autoch- 

 thonous and the syngeneic host, if the concept is correct. 



Miller: That is a very interesting point. We are trying to get patho- 

 gen-free mice, hoping that they will live as long as possible after 



