IMMUNOGENETICS OF TUMOURS IN CHIMERAS I73 



Had the immune pressure been the determining factor in the 

 CsH^CsH animals, it could be expected that the C3H^Fi(C3H 

 X C57BL) chimera would be a more active inducing host than 

 the C57BL->C3H, since both C3H^C3H and C3H->Fi(C3H x 

 C57BL) have been repopulated w^ith C3H, which is the immuno- 

 logically reactive element in the chimeras. And yet the C57BL-> 

 C3H induced a much more intense change in the tumour cell 

 population. Had any direct correlation existed between the degree 

 of the immune response which SBLi might elicit, and the degree 

 of loss of strain-specificity by the tumour cells, the order of 

 potencies of the inducing hosts would have been the following : 



C3H->C3H > C3H^Fi > C57BL->C3H > C57BL->Fi. 



However, the order of actual potencies found in the experiment 

 described was : 



C3H->C3H > C57BL-^C3H > C3H->Fi > C57BI^Fi. 



This order represents a gradient of antigenic components 

 within the chimera which are foreign to the SBLi tumour: 

 C3H components, which are highest in C3H->C3H and lowest in 

 C57BL->Fi. The C57BL antigens (isologous to the tumour), 

 seem to exert an inhibitory effect on the loss of strain-specificity. 

 Whether, in fact, the changes are due to interaction between 

 tumour cells of one genetic constitution and X-irradiated cells 

 of a different genetic constitution, is a possibility which must 

 await further experimental clarification. 



Properties of the adapted tumour 



The homotransplantabihty acquired by the adapted tumour 

 could be attributed to either of the following mechanisms : 



(i) Acquired resistance to the isoimmune response. Tumours 

 of this property will ehcit an intense homograft reaction, which, 

 however, they can resist (Feldman and Sachs, 1957). 



