Soluble factors from peripheral blood T-cells of patients with polycythemia vera stimulate normal BFU-E

T cells and monocytes from patients with polycythemia vera (PV) were isolated and grown in culture. The conditioned medium was tested for the presence of soluble factors that promote proliferation of erythroid colonies from the blood of healthy donors. We show that T cells from all 14 PV patients that were examined secrete factor/s that stimulate the proliferation of erythroid burst-forming units (BFU-E) in the absence of an external source of erythropoietin and BPA. Addition of cyclosporin A to the culture did not inhibit the production of this activity. The conditioned medium from monocytes of PV patients can also stimulate normal BFU-E but to a much lesser extent than T-cell conditioned medium. Such stimulation was not observed with control T cells or monocytes. We observed that the fraction of DR-positive T cells was significantly higher in PV patients comparing to normal. These results suggest that PV patients possess an abnormally high level of circulating activated T cells which may in turn be the source of the putative factor that facilitates uncontrolled erythroid differentiation.