DISCUSSION 155 



Some recent findings of ours may be relevant to your own experi- 

 ments, Dr. Michie. When Billingham and I originally investigated 

 the age-dependence of tolerance we obtained a rapidly declining 

 curve — the number of tolerant mice declining very rapidly with age, 

 coming down to zero by about the fifth day after birth. In these ex- 

 periments we used a constant number of cells, an inoculum containing 

 5-10 milhon cells. Recently Gowland and I have been repeating this 

 kind of analysis, but using a weight-adjusted dose of cells; in other 

 words, we increased the number of cells with increasing age, to take 

 into account increases in body weight of the recipients. We find that 

 we get very nearly the same kind of curve, but the whole thing is 

 displaced by about 4 days so that a high proportion of tolerant animals 

 is obtained when animals are injected as late as 6 days after birth. But, 

 nevertheless, by about the 7th or 8th day the curve reaches zero and 

 after that a single weight-adjusted inoculum simply doesn't produce 

 any tolerance at all. All these injections were done intravenously. 

 Twelve-day-old mice do not become tolerant at all ; in fact, some be- 

 come sensitized. We have therefore tried to find out whether there is 

 any relationship between immunological tolerance and paralysis, by 

 injecting multiples of the weight-adjusted dose into 12-day-old mice; 

 we find that the greater the number of cells injected into 12-day-old 

 mice intravenously, the greater the sensitivity ! (We have injected up 

 to 300 million cells intravenously into 12-day-old mice.) 



We feel that tolerance probably can't be entirely accounted for in 

 terms of a sort of scaled-down version of immunological unresponsive- 

 ness in adult animals. Your experiments might help to explain these 

 results, in terms of a cellular changeover in developing animals. If 

 some mature cells are already present in newborn mice then that might 

 explain why tolerance production in newborn mice should be dosage- 

 dependent, which it is, and why it should be dosage-dependent in the 

 first few days of life, which it also is. We feel that, in order to eliminate 

 the mature cells (this is now following the stem-cell theory of tolerance), 

 a prolonged stimulus is required — the kind of stimulus that Shapiro, 

 Good and their colleagues applied by repeated inoculations of adult 

 mice with large numbers of cells. (Gowland and I are at present in- 

 vestigating this possibiUty.) But there is no doubt that one single 

 inoculum will not work, as it does so effectively in newborn mice. 



