G. J. V. NOSSAL AND LOIS LARKIN 



and only one sort of antibody molecule. Antigenic stimulation in adult life 

 is followed by preferential multiplication of the cells of the clone capable of 

 responding to that antigen, and by increased synthetic activity. This results 

 in the outpouring of more of the relevant globulin, which we can now detect 

 by classical immunological techniques, and call the specific antibody. 

 When the same antigen is introduced a second time, there are far more cells 

 of the relevant clone ready to respond, and a typical secondary response 

 ensues. If a cell were confronted with its corresponding antigen while it was 

 still immature, it is postulated that a hypersensitivity reaction would follow, 

 and that the cell would die. By this means, the injection of large amounts of 

 an antigen during embryonic life could lead to the elimination of all the 

 cells of the clone capable of responding to it. Thus there would be a per- 

 manent impairment of the antibody response to this antigen; in other words, 

 immunological tolerance would result. 



The idea of a clonal mechanism of immunological memory has been 

 accepted in principle by Lederberg^ as a plausible genetic approach. 

 However, he has put forward a somewhat different concept, in which 

 mesenchymal cells are considered to be totipotent as regards antibody 

 formation, but can be genetically stabilized by contact with an antigen, 

 resulting in clonal expansion when the relevant antigen is injected a second 

 time. This is essentially a post-adaptive approach, in contrast to the pre- 

 adaptive approach of Burnet's formulation. 



EXPERIMENTAL 



In order to obtain experimental evidence bearing on these hypotheses, we 

 undertook studies on the transfer of immunologically competent cells to 

 host animals under a variety of conditions. The techniques involved are 

 simple. Spleens derived from immunized or non-immunized animals were 

 processed to give single-cell suspensions, and stimulated with a purified 

 flagellar antigen from Salmonella bacteria by incubation at 37°C for 30 min. 

 Excess antigen was then washed away, and the cells injected intraperito- 

 neally into recipient animals. 



Figure 1 indicates what happens when cells from an immunized rat are 

 secondarily stimulated in vitro, and injected into normal rats, either adult 

 or one-week old. It can be seen that in both situations, the transferred cells 

 make antibody, but in the case of the adult rats, the serum antibody 

 titres do not rise to such high levels, but begin to fall after about 7 days. 

 This is due to immunological rejection of the injected cells by the normal 

 defence mechanism of the host. On the other hand, the one-week-old rats, 

 which are incapable of rejecting the graft because they are immunologically 

 immature, harbour the transferred cells m.uch longer, and the serum anti- 

 body titres of the host animals therefore continue to rise for about two weeks. 



The controls in these experiments received injections of a cell-free homo- 

 genate of the same spleen. The antibody produced by the adult controls 

 was probably due to persistent antigen present in the cells, which caused a 

 primary type of response. 



Figure 2 gives the result of a similar experiment using sub-lethally irradiated 

 recipient rats. These rats received 500 r of whole-body irradiation on the day 

 before cell transfer. Control rats, injected with antigen after irradiation, made 



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