Human bone marrow cells were separated according to density by centrifugation on Ficoll-Hypaque gradients and then according to size by velocity sedimentation. This procedure resulted in fractions enriched for immature granulocytes, mature granulocytes, and lymphocytes. Cells in these fractions were analyzed for their expression of certain surface and functional differentiation markers and for their ability to respond to thymopoietin and ubiquitin with the expression of additional differentiation markers. A higher percentage of band form and segmented granulocytes than of more immature granulocytes expressed complement receptors on their surfaces. Thymopoietin and ubiquitin induced a significant percentage of the cells in the immature granulocyte fraction to express this marker. These data suggested that the complement receptor may be viewed as a differentiation marker on human granulocytes, the expression of which can be induced in vitro by thymopoietin and ubiquitin. Furthermore, fractions containing predominantly band form granulocytes were induced by ubiquitin (but not thymopoietin) to develop the capacity to respond to chemotactic agents, and cell fractions containing predominantly myelocytes and metamyelocytes were induced by thymopoietin and ubiquitin to develop the capacity to phagocytose latex particles. These findings indicated that thymopoietin and ubiquitin, two agents known to induce a number of stages of human and mouse lymphocyte differentiation, are also capable of inducing some stages of human granulocyte differentiation in vitro.

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