A regulatory role of activated T-lymphocytes on human megakaryocytopoiesis in vitro.
Cellular interactions responsible for regulating in vitro megakaryocytopoiesis were studied using a microagar culture system which permits the simultaneous proliferation of human megakaryocytic progenitor cells (CFU-M) and T-lymphocytic colonies (CFU-TL). The proliferation of these colony types depends mainly on two factors: phytohaemagglutinin (PHA) and erythropoietin (EP). The direct addition of increasing PHA concentrations to the liquid overlayer resulted in a parallel increase of CFU-M and CFU-TL. If the T-lymphocytes were removed by an E-rosetting technique a marked diminution of CFU-M and CFU-TL numbers was observed. However, monocyte depletion resulted in a marked augmentation of CFU-M proliferation compared to unfractionated mononuclear cells. In order to confirm that the reduction of CFU-M proliferation observed after T-depletion was primarily mediated by the absence of T-lymphocytes, we have co-cultured different concentrations of previously removed autologous T-lymphocytes with a constant number of T-depleted bone marrow cells. A parallel increase of CFU-TL and CFU-M was found if 0.75 - 7.5 X 10(4) T-lymphocytes were added to the culture. In conclusion, our results indicate that activated T-lymphocytes augment proliferation of human bone marrow CFU-M and that monocytes are less important for the growth of megakaryocytic colonies.