DISCUSSION 349 



petent cells in the spleen are A-strain, and only a small proportion are 

 CBA. On the other hand, spleen cells from this chimera, unlike spleen 

 cells from a normal A-strain mouse, do not cause splenomegaly when 

 injected into newborn CBA mice. This investigation was prompted by 

 a question I asked at the meeting in Liege in 1959: are there such things 

 as tolerant cells? I wouldn't say this experiment proves there are 

 tolerant cells, but it does prove that the concept of tolerance appHes to 

 a population of cells at any rate. 



Nakic: Lacking the exact quantitative analysis of the chimeric state 

 in tolerant animals it is rather difficult to draw these conclusions. 



Silvers: After parabiosis, when the immunologically competent 

 system of one partner has apparently been replaced by that of the other, 

 have you ever challenged the animals with autografts, and are they 

 accepted ? Dr. P. KoUer and his associates have reported that homo- 

 logous, radiation-induced mouse chimeras can, indeed, reject their own 

 skin and we have also found this to be the case in similar experimental 

 situations. 



Nakic: We have intended to do that, but we haven't done it. 



Medawar: Dr. Nakic, could we just be clear on one point: does your 

 theory imply that adult cells which are not immunologically competent 

 couldn't produce tolerance ? 



Nakic: This is a rather delicate question. From the viewpoint of the 

 concept of the competitive replacement I would prefer that they could 

 not, but evidence has been presented that tolerance can be induced with 

 Fi hybrid cells — so either my theory is wrong or there are several 

 distinct phenomena, all fundamentally different, but all contributing to 

 the induction of tolerance to living tissue. 



