Changes in Spleen Cell CFU Radiosensitivity Following Transplantation

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Abstract
Hematopoietic tissues contain a class of cells capable of forming spleen colonies (CFU) in heavily irradiated animals (1). McCulloch and Till (2) showed an in vitro and in vivo difference in radiation sensitivity of bone marrow CFU to cobalt-60 gamma rays, as measured by spleen colony-forming ability. Spleen-cell radiosensitivity also differed under in vitro and in vivo irradiation conditions (3). Siminovitch et al. (4) showed that colony-forming cells of different origins and past histories are dissimilar with respect to several properties, such as radiosensitivity. The radiosensitivity of transplanted spleen cells was studied at different time intervals following transplantation. Considerable changes in CFU radiosensitivity2 were found following the transplantation of the cells and observed during the period before CFU proliferation occurs (5). Inbred C3H(Z) male and female mice between two and four months of age were used in these experiments. Donors and recipients were from the same inbred colony and were of the same age and sex in each experiment. Spleens were excised from heparinized, freshly sacrificed donors, minced into small pieces, serially rinsed with ice-cold Hank's solution, and converted into single-cell suspensions. Nucleated cell density was determined by phase contrast microscopy following lysis of red blood cells with 1 per cent acetic acid. Cells for injection were prepared by dilution with Hank's solution (0–4°C) supplemented with 5 per cent fetal bovine serum. For each experiment, a single cell suspension was prepared at a known cell density. From it, samples were drawn for irradiation in vitro and/or in vivo. Recipients of spleen cells were exposed to x rays prior to cell injection (220 kVp; 15 mA; added filtration to give a half-value layer of 0.9 mm Cu). Surviving animals were sacrificed at eight to ten days, their spleens were excised and placed in Bouin's fixative, and nodules visible by eye were counted after fixation. Spleen cell radiosensitivity was determined in vivo by injecting graded numbers of unirradiated spleen cells into partially irradiated (610–950 rads) animals, which then received graded whole-body x-ray doses to a whole-body total irradiation dose of 900–1000 rads two hours after cell injection. In vitro radiosensitivity was determined by cell irradiation in plastic Petri dishes and assay in heavily irradiated recipients. Spleen cell CFU were more radiosensitive when irradiated in vitro (n = 0.9, D0 = 91 rads) than in vivo (n = 1.2, D0 = 109 rads) (3). To study the radiosensitivity transition, spleen cells were prepared and injected into partially irradiated animals (∼ 750 rads). Recipient animals were then irradiated with a whole-body x-ray dose of 185 rads at various times following the injection of the cells. The results of two separate experiments (Fig. 1) show that there was a decrease in the radiosensitivity of the cells beginning six to ten hours after transplantation.

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