DISCUSSION 401 



Hildemawi: If the immune response capacity of these thymecto- 

 mized mice is so seriously impaired that they are unable to make anti- 

 bodies against bacteria and viruses, one wonders why they don't all 

 promptly die from infectious diseases. 



Miller: The mortahty is extremely heavy and, at about 4 to 6 months 

 of age, 70 per cent of mice have died, possibly from infection. Neo- 

 natally thymectomized mice may have some antibody-forming cells or 

 other means of combating infection by viruses, but they appear to be 

 incapable of reacting immunologically with antigenic substances that 

 appear in the higher forms of life, such as tissue histocompatibihty 

 antigens. 



Barnes: Were you repopulating only with infantile or neonatal thy- 

 muses, or did you use adult thymuses ? 



Miller: I always repopulate with infantile thymuses. I don't really 

 know at what stage, in the life of the donor, thymuses will no longer be 

 capable of repopulating when grafted to neonatally thymectomized 

 mice. 



Billingham: Dr. Miller, earher you asked what happened to the 

 thymuses in runt disease, and I mentioned in our analysis in the rat, 

 when they die at 14-16 days of age of the acute form, the thymus is just 

 a few fibrotic remnants, but I failed to state that there was tremendous 

 hyperplasia of all the lymph nodes in the body, and the spleen; you get 

 enlargement by a factor of 10. This mystified us considerably and I 

 would value your opinion on this. 



Miller: I can only speculate. It is possible that the thymus is over- 

 active in this case and is trying to put out as many lymphoid cells as 

 possible to repopulate those areas which are destroyed. And in the 

 course of doing so exhaustion atrophy takes place. 



Medawar: Do you or Dr. Loutit know if any experiments analogous 

 to spleen shielding during whole-body irradiation have been done on 

 the thymus ? 



Loutit: Not analogous, because with spleen shielding you exteriorize 

 the spleen, and pack it round with lead. I know of no way of exterior- 

 izing the thymus and packing it with lead. If you just take the intact 

 animal and put a lump of lead over the thymic area you protect other 

 things besides the thymus — the sternum and so on. 



Miller: H. S. Kaplan, M. B. Brown, B. B. Hirsch and W. H. Games 



