The Effect of Adrenalectomy on the Susceptibility of Rats to a Transplantable Leukemia*

Transplantable tumors, as a rule, develop more readily in young animals. A transplantable lymphatic leukemia of the rat, which has been under investigation in this laboratory (12), resembles the tumors in this respect. Animals from a susceptible strain, if inoculated under 5 months of age, prove susceptible in something over 95 per cent, while rats between 12 and 15 months of age develop the disease in less than 50 per cent. It would appear, therefore, that the lowered receptivity becomes evident even in the early middle age period, and is certainly acquired before there are definite senile changes. The parent strain (Wistar) from which our rats are derived has an estimated average life span of 33 months, with the menopause occurring between 15 and 18 months of age (4). The regression of the thymus is the most obvious change taking place with the aging process. According to Donaldson (4) the thymus reaches its largest size in the rat at about 2 89 months of age, which corresponds to the beginning of puberty. Thereafter the gland regresses, till at 1 year of age only a small fragment remains. The possibility that the underlying conditions responsible for the thymus regression may be of significance in determining susceptibility to inoculated malignant cells has not received experimental consideration. laffe (10) demonstrated some years ago that removal of the adrenals was followed by regeneration of the thymus in older rats and resulted in a definite stimulation of growth of the gland in young ones. A similar stimulation of the thymus reduced in size by x-ray has been noted by Gr~goire (8). In the present study advantage has been taken of this method of thymus stimulation and accompanying reactions following the removal of the adrenals on the natural resistance of old rats and induced resistance of young rats to transplantable lymphatic leukemia.