AbstractErythropoietic activity of spleen cell grafts was measured (Fe59uptake) in X‐irradiated recipient mice under conditions in which these grafts were engaged in homograft reactions against allogeneic target cells or in graft‐versus‐host reactions. Such Fe59incorporation was greatly reduced at 7 to 10 days after graft implantation relative to that of control grafts. This reduced erythropoiesis did not occur when the spleen cell graft was immunologically incompetent. Transplantation of bone marrow‐lymph node cell mixtures also resulted in a relative decline in Fe59uptake, but only when minimal numbers (105to 106) of marrow cells were injected. The incorporation of I125UdR in the spleen of irradiated recipients was used to assess cellular proliferation. Incorporation of this label was reduced when measured 7–10 days after implantation of the lympho‐hemopoietic cell graft, but reached a peak at five days—the latter indicating stimulated lymphopoiesis. These data are consistent with the concept of depletion of a pluripotent stem cell pool (limited in size under these experimental conditions) due to excessive and concurrent functional demands for erythropoiesis and lymphopoiesis. An alternative explanation would involve cytotoxic effects on hemopoietic elements present in the milieu of the immunol
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