Release of erythropoietin from macrophages by treatment with silica

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Abstract
An erythropoietic stimulating factor (ESF) can be shown to be released from preincubated macrophage‐containing cell suspensions from mice by the macrophage‐specific, cytotoxic agent, silica. A concentrated silica treated spleen cell supernatant containing ESF is shown to cause a dose dependent increase in 59 Fe incorporation into red blood cells using the in vivo polycythemic mouse bioassay. The ESF from the same supernatant can also be neutralized by anti‐erythropoietin. A second concentrated supernatant fractionated using wheat germ lectin‐Sepharose 6MB and compared to either unfractionated or fractionated step 111 erythropoietin (Ep), tested in vitro using the erythroid colony‐forming technique and 12‐day fetal liver as target cells, indicates parallelism of all linear dose‐response lines. This, together with the in vivo data, strongly suggests that the ESF released from macrophages treated with silica is, in fact, Ep. Substituting Ca2+ ions for fetal calf serum in the preincubation procedure results in the same activity being released compared to the presence of 1% or 20% fetal calf serum.