T cell and monocyte-derived burst-promoting activity directly act on erythroid progenitor cells.

While the terminal stages of erythroid differentiation are regulated by the hormone erythropoietin, the early stages of proliferation and differentiation of immature erythroid progenitor cells also depend on cellular factors functionally defined as burst-promoting activity (BPA). Thus, in vitro there is suboptimal development of primitive erythroid progenitor cells (burst-forming units--erythroid, BFU-E) into colonies unless a source of BPA is added. It has been demonstrated that T cells and monocytes produce BPA. Monocytes may represent the main source of BPA and the major role of T cells may be to augment BPA production by monocytes. Irradiated bone marrow cells, which contain T cells, monocytes and other BPA-producing cells, also promote BFU-E colony formation. As these studies used crude BFU-E populations as target cells, it was not possible to define which of the accessory cell products act directly on the progenitor cell. Here we have used a panel of monoclonal antibodies to purify BFU-E from peripheral blood. We demonstrate that BPA produced by both a monocyte and a T-cell line acts directly on the erythroid progenitor cell and can support colony formation by single BFU-E.