EFFECT OF 6-MERCAPTOPURINE ON THE BLOOD FORMING TISSUES OF YOUNG ADULT MICE

Abstract

Recently, 6-mercaptopurine (6-MP) was found to lead to suppresion of antibody formation to soluble protein antigens in the primary response, permit development of a state akin to true immunologic tolerance and supress homograft rejection. It appeared logical, therefore, to study the effects of this compound on hematopoietic tissues of intact animals, the thymolymphatic tissues closely related to the antibody formation in particular. The purpose of the present experimsnt was to examine by the application of quantitative technique this problem.Young adult male mice of a inbred Db strain, weighing 20-27g and of 60-75 days of age were used as experimeutal animals. Fresh solutions of 6-MP were preparsd by dissolving the powdered compound in 1N NaOH and then making appropriate dilutions with sterile isotonic saline. Injections of 6-MP were made intraperironeally. The controls consisted of untreated mice. The total number of nucleated cells in the thymolymphatic organs and bone marrows was estimated by the method of AWAYA et al. (1960, 1962). At the same time, histological and cytological studies of sections and imprints were also made of the blood-forming organs.The main findings were as follows.1. Effect of a single dose of 200mg per kg of body weight of 6-MP (Group I). There was found a slight decrease in red cell counts 1 to 7 days and a sharp reduction in blood leukocyte count 3 to 7 days following 6-MP administration (Fig. 1, 2). Ten days following 6-MP administration, there were found no significant changes in the number of nucleated cells in the blood-forming organs concerned (Fig. 3).2. Effect of a single dose of 250mg per kg of body weight of 6-MP (Group II). The higher doses of this drug (LD50) induced sharp reductions in the total number of nucleated cells in the thymolymphatic tissues, but on 5 days these values returned to the control level or exceeded it with the exception of thymus (Fig. 4, 5 and 6). There was no notable alteration in the number of nucleated cells per mg of bone marrow with the exception of one case on the 3rd day.3. Effect of a total dosage of 250mg per kg of body weight of 6-MP in 6 divided doses, given every 4 daye (50mg per kg at the first, then each 40mg per kg) (Group III). Five days after the last injection of this drug, namely, 25 days after the first injection, there were considerable reductions in the total number of nucleated cells per organ in the thymolymphatic tissues. But, in the spleen this value exceeded largely the control value on the 5th day (Fig. 7).4. Effect of a total dosage of 600mg per kg of body weight of 9-MP in 6 divided doses, given every 4 days (each 100mg per kg) (Group IV). In this group also, the quantitative analysis showed the similar results as observed in the group III (Fig. 8).5. Coincident with the reduction in numbsrs of nucleated cells, the destruction of cortical lymphocytes in the thymus, more or less cortical thinning with a decreased number of follicles in the mesenteric lymph nodes, marked reductions in amonnt of white pulp in spleen, and the parenchymal hypoplasia of bone marrow were observed. There appeared to be a transient, possibly significant, proliferation of erythroblasts in the red pulp. Reconstitution of these tissues were slower in the group received with 6 repeated injections of each 50-40mg per kg of body weight every 4 days than in the group injected with a single dose of 250mg per kg of body weinht of 6-MP. These histological findings were fully in accord with the above quantitative analysis.6. These findings show that the damage of 6-MP to the hematopoietic tissues is temporary and that repeated administrations in divided doses of this compound appear to be more effective for a long term than a single injection of the total dosage.7. The large lymphoblasts, cells considered to be immunologically competent (ANDRÉ et al., 1962)