Two antigenically different types of colony-stimulating activities in sera of patients with aplastic anemia

Sera obtained from seven normal volunteers and 11 patients with aplastic anemia were assayed for two types of colony-stimulating activity (CSA) using monolayer agar cultures containing human unfractionated and monocyte-depleted bone marrow cells. Sera obtained from normal volunteers had low CSA for unfractionated bone marrow cells and no CSA for monocyte-depleted bone marrow cells. On the other hand, sera obtained from patients with aplastic anemia (patients' sera) had moderate CSA for both unfractionated and monocyte-depleted bone marrow cells. The patients' serum CSA titer for unfractionated bone marrow cells rose markedly with the addition of a small amount of diluted control rabbit serum (control serum) into the CSA assay system, while it did not rise with addition of the same amount of diluted rabbit antiserum against partially purified human urinary colony-stimulating factor (CSF) (antiserum). The patients' serum CSA titer for monocyte-depleted human bone marrow cells rose with the addition of control serum as well as antiserum. These findings indicate that the addition of rabbit serum enhances colony formation by human bone marrow cells in the presence of aplastic anemia sera as a source of CSA, and that sera of patients with aplastic anemia contain two antigenically different types of CSA: one that is active on human unfractionated bone marrow cells and partially neutralized by the addition of antiserum, and another that is active on human monocyte-depleted bone marrow cells and is not neutralized by the addition of antiserum.