IMMUNOGENETICS OF TUMOURS IN CHIMERAS I75 



subline SBLi, or with C57BL spleen cells, whereas simultaneous 

 transplantation of two SBLx grafts resulted in lethal takes of both. 

 Since experiments with resistant tumours showed that homografts 

 of such tumours grow progressively when grafted simultaneously 

 with strain-specific tumours of the same genetic origin (YafFe and 

 Feldman, 1959), it was thus suggested that the SBLx belongs to 

 the third category of the homotransplantable tumours. Further 

 experiments were carried out to test whether the antigenicity 

 of the tumour, measured by its capacity to elicit graft rejection, 

 is indeed low, or whether its action is merely delayed, and develops 

 after the tumour is already established. Two groups of C3H 

 mice were grafted subcutaneously, one with SBLx, the other 

 with SBLx. Seven days later, the tumours were extirpated, and 

 animals of both groups were grafted two days later with SBLx. 

 The results showed that a first graft of SBLx ehcited a homograft 

 reaction that prevented the growth of a second SBLx (x/x5), 

 whereas following a first SBLx graft, 10/15 of the animals showed 

 lethal takes of the tumour. It thus appears that the SBLx is highly 

 susceptible to the homograft reaction elicited by isoantigens of its 

 strain of origin. Yet its own antigenicity is very low. 



Mechanism of loss of strain-specificity 



In most of the experimental demonstrations of conversion of 

 strain-specific to non-specific tumours, long serial transplantations 

 have been involved. The only report on homotransplantability 

 acquired through transfers in genetically foreign X-irradiated 

 animals (Krebs, Thordarson and Harbo, 1942) is one in which 

 serial passages during 7 years were involved. It seems probable, 

 therefore, that in all these cases a gradual selection of " compatible" 

 cells took place. In our experimental system one transfer genera- 

 tion was sufficient to induce the loss of strain-specificity, which 

 makes the assumption of immunoselection rather improbable. 

 The conditions within the chimeras, which were found to be 



