Proerythroblast stimulating activity: its purification from mouse serum and its effect on mouse erythroid cell proliferation in vitro

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Abstract
Summary. A factor in mouse serum which stimulates proerythroblast proliferation in in vitro culture (proerythro‐blast stimulating activity‐PSA) was purified by green A dye ligand and high performance liquid chromatography (HPLC). As judged by HPLC, PSA obtained after these two steps appeared to be a homogenous protein of Mr 100 000. It increased the proliferation of proerythroblasts when added alone to a liquid culture of bone marrow which was treated with anti‐RBC antibody to remove haemoglobin containing erythroid cells. PSA functioned synergistically with erythro‐poietin (Ep) so that when added to culture together proliferation was 10‐fold higher than when added to culture alone. PSA also increased CFU‐E number but only when added together with Ep. Dose–response studies indicated that PSA increased CFU‐E number when Ep remained constant and vice versa. PSA addition to culture could be delayed by as much as 12 h without any decrease in the number of CFU‐E colonies that developed. When PSA was added to BFU‐E cultures at the time of culture initiation no increase in BFU‐E was seen. If, however, PSA was added 5 d after culture initiation BFU‐E colonies were significantly increased. These results demonstrate that there is a factor (PSA) which can be purified from mouse serum which is not Ep. Its major site of action appears to be the more mature erythroid precursors that have the capacity to divide.