IMMUNOGENETICS OF TUMOURS IN CHIMERAS I77 



immunogenic dose. A qualitative change can be visualized as a 

 change in the antigenic structure, which could confer on the H- 

 antigens of the SBLx cells properties of haptens. Like haptens, 

 such antigens will be devoid of immunogenic properties, but will 

 be susceptible to the antibody response produced by the "com- 

 plete" antigens of the SBLi or C57BL spleen cells. 



It may be pointed out that changes in the immunogenetic 

 behaviour of haemopoietic cells, following their exposure to the 

 foreign isoantigens of the X-irradiated host, have recently been 

 reported. KoUer and Doak (i960) found that BALB/c bone 

 marrow cells, injected into lethally irradiated C57BL, and then 

 retransplanted into secondary and tertiary C57BL hosts, showed 

 adaptation to their foreign hosts, manifested in an increased 

 survival of the secondary and tertiary homologous chimeras. A 

 similar conclusion was drawn by Ilbery (i960) and Ford, Ilbery 

 and Winn (1961) from serial transplantation of foetal and spleen 

 cells in homologous hosts. It appears that the duration of exposure 

 of donor cells to the isoantigens of the irradiated host might deter- 

 mine the "adaptation". In our experiments we found that the 

 tumours must grow for more than 25 days before they reveal the 

 loss of strain-specificity . In bone marrow experiments it has been 

 shown that a short exposure of donor cells to the foreign iso- 

 antigens of the host results in a secondary response when such 

 cells are reinoculated to secondary irradiated homologous hosts 

 (Feldman and Yaffe, 1958; Popp, 1961). If, however, the foreign 

 bone marrow cells are exposed for a prolonged period (more than 

 30 days) and then retransplanted, an adaptation to the foreign 

 host is observed (Popp, 1961). These processes of adaptation, 

 unlike the changes described in tumour cells, seem to be host- 

 specific: if the "adapted" cells are inoculated to a third, unrelated 

 host, they produce the usual secondary disease (Popp, personal 

 communication). The two processes of change, in bone marrow 

 and tumour cells, seem to be of a different nature. One is a 

 specific suppression of antigen production, while the other is a 



