Lithium enhancement of megakaryocytopoiesis in culture: mediation via accessory marrow cells

To examine the effect of lithium (Li) on early megakaryocytopoiesis, murine marrow megakaryocytic (CFU-M) and granulocyte-macrophage (CFU-C) progenitors were assayed in vitro with and without addition of lithium chloride (LiCl) to culture. At 2 mM LiCl, the numbers of CFU-M- and CFU- C-derived colonies were increased to 146% +/- 8% and 128% +/- 6% of controls, respectively (p less than 0.005). Enumeration of megakaryocytes per colony showed a 78% increase of colonies (p less than 0.05) containing from 6 to 22 cells, suggesting an increased proliferative capacity of CFU-M in the presence of LiCl. Conditioned media from spleen cells cultured in the presence of both pokeweed mitogen (PWM-SCM) and 2 mM Li increased the numbers of CFU-M and CFU-C to 157% +/- 8% and 183% +/- 8%, respectively (p less than 0.025), compared to control cultures stimulated by PWM-SCM alone. Since the production of active colony-stimulating activities (CSA) from mitogen- stimulated conditioned media requires T lymphocytes, we hypothesized that the enhancement of the growth of early hematopoietic progenitors in marrow cultures was due to a Li-induced CSA production by accessory marrow cells, rather than a direct effect of Li on stem cells. To test this, cyclosporin-A (CyA), a T-lymphocyte function inhibitor known to suppress CSA production in PWM-SCM, was added to marrow cultures in the presence of 2 mM Li. CyA (3 micrograms/ml) abrogated the Li-induced enhancement of CFU-M and CFU-C growth, but had no effect on colony formation when added alone. The data suggest that the Li-induced enhancement of early megakaryocytopoiesis and granulocytopoiesis is due to local production of CSA(s) by an accessory cell population and requires the integrity of T-lymphocyte function.