Erythropoiesis in spleens of lethally irradiated Lewis rats grafted with 4–35 X 10(6) syngeneic marrow cells was inhibited or delayed during the test period of 5 days; this was in marked contrast to observation in irradiated mice. The mechanism of this inhibition was the subject of this study. Pretreatment of recipients 9 days prior to irradiation with the cytotoxic drugs cyclophosphamide (CY), busulfan (BUS), or dimethylmyleran (DMM), or the induction of iron deficiency with anemia in recipients reversed this delayed erythropoiesis. However, neither iron-deficiency anemia nor pretreatment with BUS or DMM affected the ability of irradiated recipients to reject 20 to 50 X 10(6) allogeneic marrow cells. The administration of commercial preparations of erythropoietin to hosts stimulated erythropoiesis moderately. However, proliferation of syngeneic marrow cells was not enhanced when infused into lethally irradiated Spontaneous Hypertensive (SH) inbred-strain rats which have high levels of endogenous erythropoietin. Finally, plasma from irradiated rats treated with phenylhydrazine to produce severe anemia was rich in erythropoietin but failed to stimulate erythropoiesis in the cell transfer system. Two hypotheses are considered: (1) Irradiation inhibits the secretion of a factor (not erythropoietin) responsible for initiating early stages in differentiation of transplanted stem cells; iron-deficiency anemia and cytotoxic drugs stimulate the secretion of this factor. (2) Normal rats secrete a factor which suppresses erythropoiesis; iron-deficiency anemia and cytotoxic drugs inhibit the production or function of this factor. Cellular rather than humoral factors may by involved.

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